Ventilator Waveforms and the Physiology of Pressure ...rc.rcjournal.com/content/respcare/50/2/166.full.pdf · The Ventilator Trigger PSV is patient-triggered. If the patient becomes

Conference Proceedings

Ventilator Waveforms and the Physiologyof Pressure Support Ventilation

Dean R Hess PhD RRT FAARC

IntroductionThe Ventilator TriggerThe Equation of Motion As It Applies to PSVMuscle PressureAirway Pressure During PSVFlow and Volume Delivery During PSVThe Cycle From Inhalation to ExhalationPressure Support With a SighSummary

Pressure support ventilation (PSV) is a commonly used mode. It is patient-triggered, pressure-limited, and (normally) flow-cycled. Triggering difficulty occurring during PSV is usually due tointrinsic positive end-expiratory pressure. The airway pressure generated at the initiation of inha-lation is determined by the pressure support setting and the pressure rise time (pressurization rate)settings on the ventilator. The rise-time setting is clinician-adjustable on many current-generationventilators. Flow delivery during PSV is determined by the pressure support setting, the pressuregenerated by the respiratory muscles, and respiratory system mechanics. The delivered tidal vol-ume is determined by the area under the flow-time curve. Patient-ventilator dyssynchrony mayoccur during PSV if the flow at which the ventilator cycles to exhalation does not coincide with thetermination of neural inspiration. The newer generation ventilators offer clinician-adjustable flow-termination during PSV. Ventilator waveforms may be useful to appropriately adjust the ventilatorduring PSV. Key words: acute respiratory failure, mechanical ventilation, pressure support ventilation.[Respir Care 2005;50(2):166–183. © 2005 Daedalus Enterprises]

Introduction

Pressure support ventilation (PSV) is patient-triggeredand pressure-limited. It is primarily flow-cycled, meaningthat the ventilator normally cycles to exhalation when the

flow decreases to a ventilator-determined level. PSV is acommonly used ventilation mode. One survey reportedthat PSV was used as a weaning mode with 45% of pa-tients in the United States.1 In an international survey ofmechanically ventilated patients, PSV was used with 21%of patients during weaning from mechanical ventilation.2

PSV is generally considered a simple ventilation mode. Inthis paper I will use ventilator waveforms to illustrate thetechnical and clinical characteristics of PSV.Dean R Hess PhD RRT FAARC is affiliated with the Department of

Respiratory Care, Massachusetts General Hospital, and Harvard MedicalSchool, Boston, Massachusetts.

Dean R Hess PhD RRT FAARC presented a version of this article at the34th RESPIRATORY CARE Journal Conference, Applied Respiratory Phys-iology: Use of Ventilator Waveforms and Mechanics in the Managementof Critically Ill Patients, held April 16–19, 2004, in Cancun, Mexico.

Correspondence: Dean R Hess PhD RRT FAARC, Respiratory Care,Ellison 401, Massachusetts General Hospital, 55 Fruit Street, Boston MA02114. E-mail: [email protected].

166 RESPIRATORY CARE • FEBRUARY 2005 VOL 50 NO 2

The Ventilator Trigger

PSV is patient-triggered. If the patient becomes apneic,a modern ventilator will detect the apnea, initiate backupventilation, and alarm to alert the clinician. However, theuse of PSV assumes that the patient is capable of initiatingan inspiratory effort. Usually the patient’s effort is de-tected by a pressure trigger or a flow trigger.

Pressure-triggering requires patient effort sufficientto decrease airway pressure from the end-expiratory levelto a threshold setting (pressure sensitivity) on the ven-tilator (Fig. 1). Pressure sensitivity settings from �0.5cm H2O to �2.0 cm H2O are safe and effective withmost patients. With flow-triggering, breath initiation isbased on a flow change in the ventilator circuit beyondsome predetermined threshold (flow sensitivity) (see Fig.1). Flow sensitivity settings of 1–5 L/min are typical.Sassoon et al have described triggering in detail.3– 6 Dur-ing the pre-trigger phase, the patient generates effort

prior to ventilator response. This delay may producepatient-ventilator dyssynchrony if prolonged. The trig-ger sensitivity settings and the responsiveness of theventilator to the trigger affect the pre-trigger phase. Thepost-trigger phase, however, is affected by other venti-lator settings (rise time and the pressure support set-ting).

A number of studies have compared pressure triggeringand flow triggering.7 With older generations of ventilatorsa common finding was that flow-triggering was superior topressure-triggering. However, Tutuncu et al,8 using theSiemens Servo 300 ventilator, and Goulet et al,9 using thePuritan-Bennett 7200 ventilator, reported similar patientresponses with flow and pressure-triggering during PSV.Aslanian et al10 reported a modest benefit from flow-trig-gering with PSV, but suggested that the benefit may be toosmall to affect clinical outcomes (Fig. 2). Moreover, theyreported that differences between pressure triggering andflow triggering were related primarily to the post-trigger

Fig. 1. Pressure trigger (left) and flow trigger (right). Note that with pressure-triggering the pressure-supported breath is triggered when thepressure drops below baseline. With flow-triggering the pressure-supported breath is triggered when the flow rises above baseline,indicating an inspiratory effort from the patient.

Fig. 2. Representative breaths during pressure-triggered and flow-triggered pressure support ventilation. The decrease in airway pressure(Paw) during the trigger phase is smaller with flow-triggering. There is less muscle effort with flow-triggering, with no change in the Paw

contour. Pdi � transdiaphragmatic pressure. (From Reference 10, with permission.)

VENTILATOR WAVEFORMS AND THE PHYSIOLOGY OF PRESSURE SUPPORT VENTILATION

RESPIRATORY CARE • FEBRUARY 2005 VOL 50 NO 2 167

Fig. 3. Flow and pressure measured at the proximal airway (Paw) and esophageal pressure (Pes) recorded from a patient receiving pressuresupport of 16 cm H2O, positive end-expiratory pressure (PEEP) of 7 cm H2O, trigger sensitivity of �2 cm H2O. The down-pointing arrowsrepresent patient efforts and the up-pointing arrows represent ventilator triggers. Note that the patient is breathing at 48 breaths/min butthe ventilator is only triggering at 20 breaths/min. This patient’s difficulty triggering the ventilator is due to intrinsic PEEP (auto-PEEP). Notethat the patient must generate an inspiratory effort � 5 cm H2O to trigger the ventilator, suggesting an auto-PEEP of about 5 cm H2O. Whenthe patient’s efforts are insufficient to overcome the level of auto-PEEP, the ventilator does not recognize the patient’s effort. Note the effectof failed trigger efforts on the flow waveforms. (From Reference 16, with permission.)

Fig. 4. Top panel: Esophageal pressure (Pes), airway pressure (Paw), and flow at the tracheostomy. The patient’s inspiratory efforts areidentified by the negative Pes swings. The positive end-expiratory pressure (PEEP) is set at zero. Paw appropriately drops to zeroduring expiration, demonstrating little circuit or valve resistance. Note that there is one triggered breath for every 3– 4 efforts.Prolonged expiratory flow is due to airflow limitation. Pes swings have little effect in retarding the expiratory flow and even less effecton Paw. Bottom panel: PEEP is increased to 10 cm H2O. There is persistent flow at end-expiration; thus auto-PEEP is still present.Trigger dyssynchrony has improved, with 1 breath triggered every 2–3 inspiratory efforts. There is less limitation of expiratory flow,and the Pes swings are more effective in retarding the persistent expiratory flow. Note the effect of failed trigger efforts on the flowwaveform. (From Reference 17, with permission.)



phase. Most important, they found that the pressure trig-gers of current-generation ventilators are superior to thosein older ventilators.

In patients with flow limitation, the presence of intrinsicpositive end-expiratory pressure (auto-PEEP) is an impor-tant impediment to triggering.11–13 To trigger the ventila-

tor, the patient’s effort must first overcome auto-PEEPbefore a pressure (or flow) change will occur at the prox-imal airway to trigger the ventilator. In those patients theeffort to overcome auto-PEEP is much greater than theeffort to trigger the ventilator. The addition of appliedPEEP may counterbalance the auto-PEEP and improve thepatient’s ability to trigger. Several studies14,15 have re-ported advantages of flow-triggering in patients with auto-PEEP, which is probably due to the base flow that causesa small increase in airway pressure due to resistance throughthe expiratory limb of the ventilation circuit. That increasein expiratory airway pressure counterbalances auto-PEEPand improves triggering.

Careful inspection of ventilator waveforms may allowdetection of failed triggering efforts due to auto-PEEP.Fabry et al16 observed trigger dyssynchrony in 9 of 11patients recovering from acute respiratory failure with theapplication of PSV (Fig. 3). Chao et al17 reported thattrigger dyssynchrony commonly occurs in long-term me-chanically ventilated patients, occurs more commonly inpatients with a diagnosis of chronic obstructive pulmonarydisease (COPD), and is associated with a poor outcome.Moreover, they found that adjusting the trigger sensitivityor changing from pressure triggering to flow triggeringrarely affected the degree of trigger dyssynchrony. How-ever, the addition of PEEP decreased the amount of (butoften did not eliminate) trigger dyssynchrony. Those find-ings are consistent with auto-PEEP as the cause of triggerdyssynchrony (Fig. 4). Nava et al18 reported that ineffec-tive trigger efforts were likely in patients with COPD (Fig.5). They also found that ineffective trigger efforts were

Fig. 5. Airway pressure (Paw), transdiaphragmatic pressure (Pdi), flow, and tidal volume during pressure support ventilation in a patient withchronic obstructive pulmonary disease (COPD). Note the presence of several ineffective efforts (between the arrows). (From Reference 18,with permission.)

Fig. 6. Shape signal. A shape signal is produced by offsetting theactual patient flow signal by 15 L/min and delaying it by 300 ms.The intentional delay causes the ventilator-generated shape signalto be slightly behind the patient’s flow rate. A sudden change inpatient flow will cross the shape signal, causing the ventilator totrigger to the inspiratory phase or cycle to the expiratory phase.No evaluations of this have been reported. IPAP � inspiratorypositive airway pressure. EPAP � expiratory positive airway pres-sure. (Courtesy of Respironics.)



reduced by PEEP that did not exceed the level of auto-PEEP.

A relatively new form of triggering is Auto-Trak, whichis available with the Respironics Vision and S/T-D 30ventilators. A shape signal is produced by offsetting theactual patient flow signal by 15 L/min and delaying it by300 ms. This causes the ventilator-generated shape signalto be slightly behind the patient’s flow rate (Fig. 6). Asudden change in patient flow will cross the shape signal,causing the ventilator to trigger to the inspiratory phase orcycle to the expiratory phase. Prinianakis et al19 evaluatedthe effect of the shape-signal triggering method on patient-ventilator interaction during PSV. They compared trigger-ing with the Respironics Vision ventilator, which used theshape signal, to the Drager Evita 4 ventilator, which usedflow-triggering at 2 L/min. They studied 12 patients and 3levels of pressure support. They found that shape-signaltriggering improved the ventilator function and decreased

the patient effort during the triggering phase (Fig. 7). How-ever, they also found that shape-signal triggering increasedthe number of auto-triggers.

One solution to triggering issues during PSV may beto couple the ventilator trigger to diaphragm electro-myographic activity (Fig. 8).20 In that way, ventilatortriggering is tied to neural respiratory-center output.The neural trigger (ie, diaphragmatic electromyogram)is not affected by auto-PEEP and therefore does notrequire application of external PEEP for triggering pur-poses. When positive pressure is applied at the onset ofdiaphragmatic activity, the delay from the onset of in-spiratory effort and mechanical assistance is shortened,the esophageal pressure deflection is reduced, and theWOB is decreased. In situations where conventionaltriggering cannot provide ventilator support in synchronywith the patient’s neural inspiratory drive, neural trig-gering has the potential to improve patient-ventilator

Fig. 7. Flow, respiratory muscle pressure (Pmus) and airway pressure (Paw) as a function of time in a patient during pressure supportventilation delivered with (A) a Respironics Vision ventilator and (B) a Drager Evita 4 ventilator. With the Vision ventilator the breathwas triggered with the shape signal method. The beginning of neural inspiration was defined as zero time. Triggering of the ventilator(arrows) occurred 120 and 200 ms after the beginning of neural inspiration, respectively, with the Vision and Evita 4. Note that thedrop in Paw during the triggering phase was considerably less with the Vision that with the Evita 4. The better performance of theshape signal triggering method occurred despite the fact that with the Vision ventilator the expiratory flow at zero time was higher(0.33 vs 0.24 L/s), whereas inspiratory effort was comparable between ventilators. The dotted line represents the flow shapesignal. The dashed line represents the electronic signal rising in proportion to actual inspiratory flow. (From Reference 19, withpermission.)



interaction. However, this approach is investigational atpresent and its clinical usefulness remains to be deter-mined.

More sensitive settings may result in auto-triggeringdue to signal noise, such as leaks, patient movement, waterin the ventilator circuit, and cardiac oscillations. Imanakaet al21 found that auto-triggering caused by cardiogenicoscillation was common in post-cardiac-surgery patientswhen flow-triggering was used (Fig. 9). Auto-triggeringoccurred more often in patients with more dynamic circu-lation and caused respiratory alkalosis and hyperinflationof the lungs.

The lack of a backup rate with pressure support maybe problematic. Parthasarathy and Tobin22 evaluated theeffect of ventilation mode on sleep quality among 11critically ill patients. Sleep fragmentation was greaterduring PSV than during continuous mandatory ventila-tion (Fig. 10). Central apneas occurred during PSV in 6patients, and heart failure was more common in those 6patients than in the 5 patients without apneas. Changesin sleep-wakefulness state caused greater changes inend-tidal PCO2

during PSV than during continuous man-datory ventilation. The authors concluded that PSV

causes hypocapnia, which, combined with the lack of abackup rate and wakefulness drive, can lead to centralapneas and sleep fragmentation, especially in patientswith heart failure.

The Equation of Motion As It Applies to PSV

The interactions between the ventilator and the patientcan be described by the equation of motion, which statesthat the pressure required to deliver a volume of gas intothe lungs is determined by the elastic and resistive prop-erties of the respiratory system. With PSV the pressure isthe sum of the pressure that the ventilator applies to theairway (Paw) and the pressure generated by the respiratorymuscles (Pmus). The elastic properties of the respiratorysystem are determined by compliance (C) and tidal vol-ume (VT), and the resistive properties of the lungs aredetermined by flow (V) and airways resistance (R). Theequation of motion thus becomes:

Paw � Pmus � VT/C � V � R (1)

Fig. 8. With a conventional pressure trigger (left), mechanical ventilatory support starts when airway pressure decreases by a preset amount. Thebeginning of inspiratory effort (solid vertical line) precedes inspiratory flow by several hundred milliseconds. That delay is due to intrinsic positiveend-expiratory pressure and occurs despite externally applied PEEP. There is also a further delay from the onset of inspiratory flow (verticaldashed line) to the rise in positive airway pressure, due to the mechanical limitation of the ventilator trigger. With neural triggering (right), theventilator provides support as soon as diaphragmatic electrical activity exceeds a threshold level. The delay to onset of inspiratory flow andincrease in airway pressure is almost eliminated. Pes � esophageal pressure. Paw � airway pressure. (From Reference 20, with permission.)



During PSV, Paw is fixed by the ventilator. If the patientgenerates an inspiratory effort during PSV (ie, greater Pmus),flow and volume delivery increase. Pmus is determined byrespiratory drive and respiratory muscle strength.

Note that an increase in pressure support will notaffect flow and VT if there is a subsequent decrease inrespiratory drive, which results in a lower Pmus. Changesin pressure support might result in one of several effectson VT. If Pmus remains constant when pressure supportis changed, then the VT will change. However, if Pmus

changes in response to the change in pressure support,then the VT might not change. For example, an increasein pressure support may unload respiratory muscles,producing a decrease in Pmus and little change in VT

(Fig. 11).

Muscle Pressure

The time course of Pmus can be approximated by thesecond-order polynomial function:23,24

Pmus(t) � �d � (t � Ti)2 � d � Ti2 (2)

in which d is a constant, and Ti is defined as the timebetween the onset of the increase in inspiratory Pmus andthe start of its decline. It is assumed that Pmus reaches itsmaximum and becomes flattened at the end of the neuralinspiratory effort. Thus, the maximum Pmus�max can beexpressed as:

Pmus�max � d � Ti2 (3)

Substituting into the above equation yields:

Pmus(t) � �Pmus�max � (1 � t/Ti)2 � Pmus�max (4)

in which 0 � t � Ti. This is illustrated in Figure 12.

Airway Pressure During PSV

Figure 13 shows a schematized airway pressure wave-form during PSV.25 The initial airway-pressure change

Fig. 9. Flow, volume, esophageal pressure (Pes), airway pressure (Paw), and blood pressure (BP) waveforms from a patient who underwentmitral valve replacement and tricuspid annuloplasty for mitral stenosis, tricuspid regurgitation, and aortic regurgitation. With triggeringsensitivity set at 1 L/min (left), pressure support ventilation was activated between 2 synchronized intermittent-mandatory-ventilationbreaths. When trigger sensitivity was changed to 4 L/min (right), pressure support breaths disappeared and there were marked oscillationsin flow, Paw, and Pes. Cardiogenic oscillation was evaluated as the peak inspiratory flow fluctuation (A), amplitude in the flow oscillation (B),amplitude in airway pressure (C), and amplitude in esophageal pressure (D). Also note that the baseline Pes was elevated when autotrig-gering occurred, suggesting hyperinflation of the lungs. (From Reference 21, with permission.)



Fig. 10. Polysomnography waveforms during assist-control ventilation and pressure support of a representative patient. From top tobottom, the waveforms show electroencephalogram (C4-A1, O3-A2), electrooculogram (ROC, LOC), electromyograms (chin and leg),integrated tidal volume (VT), rib-cage (RC), and abdominal (AB) excursions on respiratory inductive plethysmography. Arousals and awak-enings, indicated by horizontal bars, were more numerous during pressure support than during continuous mandatory ventilation (assist-control). (From Reference 22, with permission.)

Fig. 11. Airway pressure, esophageal pressure, flow, and tidal volume in a patient with 0, 10, and 20 cm H2O of pressure applied to theairway. Note the decrease in esophageal pressure as airway pressure is increased. There is only a small increase in tidal volume with theincrease in pressure support. In this case the principal effect of pressure support is to provide respiratory muscle unloading. PSV � pressuresupport ventilation.



during PSV can be described mathematically. Once theventilator is triggered, Paw increases exponentially to thepressure support level with a ventilator time constant (�v)and then stays at that level until the termination of theinspiratory phase:24

Paw(t) � PPS � (1 � e�t/�v) (5)

in which PPS is the pressure support setting, e is the baseof the natural logarithm, and t � 0. Figure 14 showsairway pressure waveforms for several levels of PPS and�v.

In previous generations of ventilators, �v was presetin the engineering of the ventilator. Many current-gen-eration ventilators allow the clinician to adjust �v. From

Fig. 12. Changes in the pressure generated by the respiratory muscles (Pmus) with 3 levels of maximum Pmus (Pmus�max) and 2 levels ofneural inspiratory time. Note that a higher Pmus�max and a shorter neural inspiratory time translate clinically into a greater respiratory drive.

Fig. 13. Characteristics of a pressure supported breath. In thisexample, baseline pressure (ie, positive end-expiratory pressure[PEEP]) is set at 5 cm H2O and pressure support is set at 15 cmH2O. The inspiratory pressure is triggered at point A by a patienteffort, resulting in an airway pressure decrease. The rise to pres-sure (line B) is provided by the initial flow into the airway. If theinitial flow is excessive, initial pressure exceeds set level (B1). Ifthe initial flow is low, a slow rise to pressure occurs (B2). Theplateau of pressure support (line C) is maintained by control offlow. A smooth plateau indicates appropriate flow responsivenessto patient demand. Termination of pressure support occurs atpoint D and should coincide with the end of neural inspiration. Ifbreath termination is delayed, the patient may actively exhale (risein pressure above plateau) (D1). If breath termination is premature,the patient may have continued inspiratory efforts (D2). (From Ref-erence 25, with permission.)

Fig. 14. The effect of rise time (�) on the pressurization rate at theinitiation of the inspiratory phase. Illustrated are 2 pressure sup-port levels: 20 cm H2O (upper panel) and 10 cm H2O (lower panel).



an operational standpoint, this becomes the rise-timesetting on the ventilator. The term “rise time” refers tothe time required for the ventilator to reach the pressuresupport setting at the onset of inspiration; it is the rateof pressurization at the initiation of the inspiratory phase.The rise time should be adjusted to patient comfort, andventilator waveforms may be useful to guide this set-ting. The rise-time adjustment effectively allows theclinician to set the flow at the onset of the inspiratoryphase during PSV. Note that a fast rise time (one inwhich the ventilator reaches the pressure support setting

quickly) is associated with high flow at the onset ofinhalation (Fig. 15). On the other hand, a slow rise time(one in which the ventilator reaches the pressure sup-port setting slowly) is associated with a lower flow atthe onset of inhalation. Theoretically, patients with ahigh respiratory drive should benefit from a fast risetime, whereas those with a lower respiratory drive mightbenefit from a slower rise time.

MacIntyre and Ho26 found that the optimal rise timefor some patients is at a high setting, whereas the op-timal rise time for others is at the slow setting (Fig. 16).

Fig. 15. Flow and pressure waveforms for 3 rise times (pressurization rates) at a pressure support of 20 cm H2O. Note the effect of rise timeon flow at the initiation of the inspiratory phase.

Fig. 16. Examples of airway pressure waveforms from 2 patients using different rise times during pressure support ventilation. The maximumrise time is with the waveforms labeled #1. The minimum rise time is with the waveforms labeled #7. Note that the optimal rise time for thefirst patient (top 3 waveforms) is at a high setting, whereas the optimal rise time for the second patient (bottom 3 waveforms) is at a slowsetting. Paw � airway pressure. VT � tidal volume. (From Reference 26, with permission.)



Branson et al27 also found that individual patient titra-tion of the rise time was necessary to optimize the ef-ficacy of PSV. Bonmarchand et al28,29 reported the low-est work of breathing (WOB) with the fastest rise time(ie, the one that reached the pressure support setting in0.1 s, compared to 0.5 s, 1 s, and 1.5 s) in patients withrestrictive lung disease and COPD. Mancebo et al30

reported that a slower rise time increased the WOB,although it did not affect the VT and respiratory rate. In

patients with a high respiratory drive, Uchiyama et al31

reported that a fast rise time was as effective as increas-ing the level of pressure support (note that either afaster rise or higher pressure support setting will in-crease the flow at the onset of inhalation). Chiumello etal32 studied the effects of different rise times duringPSV on breathing pattern, WOB, gas exchange and pa-tient comfort in patients with acute lung injury. Theyfound that the lowest pressurization rate (slow pressure

Fig. 17. An example of flow, esophageal pressure (Pes), and airway pressure (Paw) at 5 different rise times. Note the effect of rise-time settingon flow and Pes swing. Also note that the lowest Pes-swing occurs at the intermediate rise-time setting. (From Reference 32, withpermission.)

Fig. 18. Airway-flow waveforms during pressure support ventilation with 2 types of lung mechanics. The waveforms on the left illustrate theconditions of restrictive lung disease (eg, acute lung injury) and those on the right illustrate conditions of obstructive lung disease. Alsoillustrated are 3 levels of pressure support and 2 rise times. Note that flow at the beginning of inhalation increases with a faster rise timeand higher pressure support setting, with either set of lung mechanics. R � resistance. C � compliance. Pmus � pressure generated by therespiratory muscles. PS � pressure support.



Fig. 19. Effect on respiratory mechanics of cycling of pressure support from inhalation to exhalation. Pressure support is set at 20 cm H2O,rise time (�) is 0.01 s, and the maximum pressure generated by the respiratory muscles (Pmus�max) is 10 cm H2O. Flow-termination is setat 25% of the peak pressure, as illustrated by the broken line. The upper panel represents the respiratory mechanics of a patient withrestrictive lung disease. The lower panel represents the respiratory mechanics of a patient with obstructive lung disease. In each case, theneural inspiratory time is 1.0 s. Note that the breath terminates prematurely in the patient with restrictive lung disease, but the breath isprolonged in the patient with obstructive lung disease. Also note that the peak flow is greater in restrictive lung disease and the pressuredecrease is more rapid in restrictive lung disease. R � resistance. C � compliance.

Table 1. Cycle Criteria for Some Commonly Used Adult Mechanical Ventilators

Ventilator Flow Cycle Pressure Cycle Time Cycle

Puritan-Bennett 7200 5 L/min PEEP � pressure support � 1.5 cm H2O 3 sPuritan-Bennett 840 Adjustable (1–80% of peak flow) PEEP � pressure support � 1.5 cm H2O 3 sPuritan-Bennett 740/760 10 L/min or 25% of peak flow PEEP � pressure support � 3 cm H2O 3.5 sServo 900C 25% of peak flow PEEP � pressure support � 3 cm H2O 80% of set cycle timeServo 300 5% of peak flow PEEP � pressure support � 20 cm H2O 80% of set cycle timeServoi Adjustable (1–40% of peak flow) High-pressure limit � 2.5 s, based on

flow-cycle setting*Drager Evita 4 25% of peak flow High-pressure limit 4 sBear 1000 25% of peak flow High-pressure limit 5 sHamilton Veolar 25% of peak flow High-pressure limit 3 sHamilton Galileo Adjustable (10–40% of peak flow) High-pressure limit 3 sInfrasonics Star 4 L/min PEEP � pressure support � 3 cm H2O 3.5 sBird 8400 and TBird 25% of peak flow High-pressure limit 3 sPulmonetic LTV Adjustable (10–40% of peak flow) High-pressure limit Adjustable (1–3 s)Viasys Avea Adjustable (5–45% of peak flow) High-pressure limit Adjustable (0.2–5.0 s)Newport E500 Variable, based on time constant

and pressure above pressuresupport setting

High-pressure limit 3 s

PEEP � positive end-expiratory pressure*Flow drops to a range between 25% of peak flow and flow-cycle criteria, and time in this range exceeds 50% of the time before entering this range.



rise) caused the lowest VT, highest respiratory rate, andhighest WOB (Fig. 17). The other pressurization ratesproduced no differences in breathing pattern or WOB.Patient comfort was worse at the lowest and highestpressurization rates. In patients with COPD recoveringfrom acute hypercapnic respiratory failure and receiv-ing noninvasive positive-pressure ventilation, Prini-anakis et al33 reported that the greatest reduction in thepressure-time product of the diaphragm occurred withthe highest pressurization rate (fast pressure rise), butthat was accompanied by substantial air leaks and poortolerance.

There are several potential drawbacks to a high inspira-tory flow at the onset of inspiration (such as might resultfrom a higher pressurization rate).34 First, if the flow ishigher at the onset of inspiration, the inspiratory phasemay be prematurely terminated if the ventilator cycles to

the expiratory phase at a flow that is a fraction of the peakinspiratory flow. Second, several studies have suggestedthe existence of a flow-related inspiratory terminating re-flex.35–39 Activation of this reflex causes a shortening ofneural inspiration, which could result in brief, shallowinspiratory efforts (particularly at low pressure supportsettings). The clinical effects of this inspiratory-flow-ter-minating reflex during PSV remains to be determined. Atthe least, it suggests that manipulation of rise time duringPSV may result in a complex interaction between venti-lator function and physiology.

Flow and Volume Delivery During PSV

During PSV, inspiratory flow is determined by the pres-sure applied to the airway (ie, pressure support setting),the pressure generated by the respiratory muscles (Pmus),the airways resistance, and the time constant:

Fig. 20. Flow, airway pressure, and transversus abdominis elec-tromyogram (EMG) from a mechanically ventilated patient withchronic obstructive pulmonary disease receiving pressure supportventilation at 20 cm H2O. The onset of expiratory muscle activity(vertical dotted line) occurred when mechanical inflation was onlypartly completed, as indicated by the onset of expiratory muscleactivity. Also note that active exhalation causes an increase in airwaypressure at end-exhalation, causing the ventilator to pressure-cyclerather than flow-cycle. (From Reference 41, with permission.)

Fig. 21. An example of delayed termination of inhalation duringpressure support ventilation in a patient with chronic obstruc-tive pulmonary disease. The patient was ventilated with a Pu-ritan-Bennett 7200 ventilator, using pressure support ventila-tion at 12 cm H2O and positive end-expiratory pressure of 3 cmH2O (inspiratory pressure of 15 cm H2O), which has a flowtermination of 5 L/min during pressure support. Note that theventilator cycles at a flow of 18 L/min. The pressure increaseabove the set level of pressure support causes the ventilator topressure-cycle in response to the patient’s active exhalation.(From Reference 42, with permission.)



V � (�P/R) � (e�t/�) (6)

in which �P is the sum of pressure support and Pmus, R isairways resistance, e is the base of the natural logarithm, tis the elapsed time after initiation of the inspiratory phase,and � is the product of airways resistance and respiratorysystem compliance (the time constant of the respiratorysystem). This is illustrated in Figure 18.

The area of the flow-time curve is the delivered VT:

VT � � V dt (7)

Thus, VT during PSV is determined primarily by thepressure support setting, the inspiratory effort of the pa-tient, airways resistance, respiratory-system compliance,and inspiratory time. The delivered VT will also be af-fected by auto-PEEP. An increase in auto-PEEP effec-tively decreases the driving pressure gradient, and thus theVT decreases. Theoretically, the delivered VT will be zeroif the auto-PEEP equals the pressure support setting. Thus,auto-PEEP will affect breath delivery in 2 ways duringPSV. First, it increases the effort required to trigger theventilator. Second, it decreases the delivered VT.

The Cycle From Inhalation to Exhalation

During PSV, the ventilator is normally flow-cycled. Theflow at which the ventilator cycles can be a fixed absolute

Fig. 22. Examples of flow-termination criteria of 10%, 25%, and 50%, using a Puritan-Bennett 840 ventilator with pressure support 15 cmH2O and positive end-expiratory pressure of 5 cm H2O. Lung model settings were: resistance 5 cm H2O/L/s, compliance 0.05 L/cm H2O.

Fig. 23. Flow, volume, airway pressure (Paw), and esophageal pres-sure (Pes) waveforms with flow-termination criteria of 5% and 45%.With flow termination of 5%, inspiratory flow terminated simulta-neously with the cessation of inspiratory effort, estimated by Pes.In contrast, premature termination with double-triggering occurredwith flow termination of 45%. (From Reference 46, with permis-sion.)



flow, a flow based on the peak inspiratory flow, or a flowbased on peak inspiratory flow and elapsed inspiratorytime (Table 1). In some cases, the cycle is quite sophisti-cated. The Respironics Vision ventilator, for example, usesthe shape signal to cycle the ventilator (see Fig. 6).

Ideally, the ventilator should cycle to exhalation atthe end of the neural inspiratory time. If the breathterminates before the end of neural inhalation, the pa-tient may double-trigger the ventilator. If breath deliv-ery continues into neural exhalation, the patient mayactively exhale, causing the ventilator to pressure-cyclerather than flow-cycle. The inspiratory time during PSVis determined by lung mechanics and the flow cyclecriteria (Fig. 19).

Several studies have reported dyssynchrony with PSVin subjects who have airflow obstruction (eg,COPD).40 – 42 With airflow obstruction the inspiratoryflow decreases slowly, the flow cycle criteria may notbe reached at the end of neural inhalation, and thisstimulates active exhalation to pressure-cycle the breath(Figs. 20 and 21). This can be seen on the ventilatorwaveforms as a rise in pressure at end-exhalation thatexceeds the pressure support setting on the ventilator.This problem increases with higher levels of pressuresupport and with higher levels of airflow obstruction.Mathematical and laboratory analyses by Hotchkiss etal43– 45 showed that PSV in the setting of airflow ob-struction can be accompanied by marked variations in

Fig. 24. Examples of flow, airway pressure (Paw), and esophageal pressure (Pes) waveforms at various inspiratory rise-time and flow-cyclecriteria combinations. RT � rise time. OC � off criteria (the flow that terminates the inspiratory phase). (From Reference 47, with permission.)

Table 2. Effect of Pressure Support Setting, Pmus, and Time Constant on the Appropriate Flow-Termination Setting During Pressure SupportVentilation*

Resistance(cm H2O/L/s)

Compliance(L/cm H2O)

� (s)10 cm H2O Pmus�max 30 cm H2O Pmus�max

PS 10 PS 20 PS 30 PS 10 PS 20 PS 30

20 0.8 1.14 79 70 65 85 82 7920 0.4 0.66 58 48 43 70 64 5820 0.2 0.36 30 22 19 44 36 305 0.8 0.29 21 15 12 34 26 215 0.4 0.17 8 5 4 16 11 85 0.2 0.09 3 2 1 6 4 3

PS � pressure support setting*Note that the flow-termination ranges from as low as 1% of peak flow to as high as 85% of peak flow.Pmus�max � maximum pressure generated by the respiratory muscles� � time constant(Data from Reference 24.)



VT and auto-PEEP, even when the subject’s effort isunvarying. The mechanism underlying this observed in-stability is “feed forward” behavior mediated by oscil-latory elevations in auto-PEEP. Approaches to correctthis problem during PSV include: (1) administer bron-chodilators and clear secretions to decrease airways re-sistance, (2) use a lower level of pressure support, (3)use pressure-controlled ventilation with the inspiratorytime set short enough that the patient does not contractthe expiratory muscles to terminate inspiration (eg, 0.8 –1.2 s) or the inspiratory time can be adjusted by observ-ing patient comfort and avoiding a period of zero flowat the end of inspiration, (4) adjust the flow at which theventilator cycles (Fig. 22).

Several studies also examined flow termination duringPSV in patients recovering from acute lung injury. To-kioka et al46 reported that higher levels of flow terminationin that patient population resulted in a lower VT, higherrespiratory rate, and increased WOB. Premature breathtermination with double-triggering often occurred witha higher flow-termination setting (Fig. 23). Chiumelloet al47 evaluated rise time and flow termination in pa-tients recovering from acute lung injury, and receivingPSV. They found that the fastest rise time reduced theWOB, and the lowest cycling flow reduced the respira-tory rate and increased the VT with no change in theWOB (Fig. 24).

Some new-generation ventilators allow adjustment ofthe flow at which the ventilator cycles during PSV (seeTable 1). Modifications of flow-cycle criteria may needto be carefully adjusted during PSV, and waveformsmay assist in adjusting flow termination to a level ap-propriate for the patient. Using a mathematic model,

Yamada and Du24 showed that the ratio of the flow atthe end of patient inspiratory effort to peak inspiratoryflow is a function of the patient’s respiratory mechanicsand the pressure support setting (Table 2). They sug-gested that the flow-termination criteria during PSVshould not be fixed and its setting should be automatedso that it varies breath-to-breath, as appropriate, to al-low the ventilator to cycle in synchrony with the pa-tient’s neural inspiration.24,48 Tassaux et al validated themodel of Yamada and Du in 28 intubated patients un-dergoing PSV.49

Another issue with PSV is the presence of leaks (eg,bronchopleural fistula, cuffless airway, mask-leak withnoninvasive ventilation). This may be particularly prob-lematic when providing noninvasive ventilation for pa-tients with obstructive lung disease.44,45,50 If the leakexceeds the termination flow at which the ventilatorcycles, either active exhalation will occur to terminateinspiration or a prolonged inspiratory time will be ap-plied. With a leak, either pressure-controlled ventilationor a ventilator that allows an adjustable flow-termina-tion should be used.

Pressure Support With a Sigh

Sighs in conjunction with PSV may counteract thetendency for lung collapse associated with low VT andthus improve gas exchange. This was studied by Pa-troniti et al.51 They applied sighs in conjunction withpressure support in 13 patients and reported that sighswere associated with PaO2

improvement, an increase inend-expiratory lung volume, an increase in respiratory-

Fig. 25. An example of the waveforms from use of a sigh breath in conjunction with pressure support ventilation. The patient was ventilatedwith a Drager Evita 4 ventilator (in PCV� mode). Paw � airway pressure. (From Reference 51, with permission.)



system compliance, and a decrease in respiratory drive.Sighs can be used with PSV on the Puritan-Bennett 840(bi-level mode) and the Drager Evita 4 (PCV� mode).The sigh rate is set at 1– 4 breaths/min, the pressureduring the sigh is set at 25–30 cm H2O, and the sighduration is 2– 4 s (Fig. 25).

Summary

PSV has been effectively used to ventilate many pa-tients. However, it has become increasingly appreciatedthat pressure support may not be a simple mode of venti-lation. Issues related to triggering, rise time, and cyclingduring PSV should be appreciated, and ventilator wave-forms may assist with the proper setting of those.

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Discussion

Benditt: Thanks for focusing usback on the patient, which I alwaysthink is critical. There’s been a lot ofeffort focused on the ventilator andwhat it can do, but how it integrateswith the patient is the critical thing. Itgets back to the BiCore monitor orother methods for looking at the workof breathing. We are still missing themonitor of the patient effort, comfort,and so forth. You could say, why notgo to the bedside and just assess it?

Hess: Use the “eyeball test.”

Benditt: Right; we’ll adjust the ma-chine by looking at the patient. But isthere something we could be measur-ing to try to get to this really bottomline, especially in the weaning pro-cess?

Hess: What you’re asking for maynot be easily attainable with presenttechnology.

Benditt: Maybe what we need is anEMG [electromyogram] of the abdom-inal muscles or something?

Nilsestuen: There have been a num-ber of articles about using the EMG asthe ideal signal to evaluate patient-ventilator synchrony.1–4 The EMG isthe variable most aligned with trueneural effort, and is not hindered bythe delay times associated with otherkinds of transducers. So the EMG isthe ultimate signal, and the question iswhether we can develop techniques tomeasure the EMG in a less invasiveway that is stable and comfortable forthe patient. If such a technique wasavailable, that would definitely be theway to do it.

REFERENCES

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4. Javaheri S, Vinegar A, Smith J, DonovanE. Use of a modified Swan-Ganz pacingcatheter for measuring Pdi and diaphrag-matic EMG. Pflugers Arch 1987408(6):642–645.

Hess: Sinderby et al1 used a gastrictube with sensors at the diaphragm andused diaphragmatic EMG to triggerthe ventilator. Maybe we could usesomething like that and a surface elec-



trode on the abdomen. You could usethat setup to both trigger and cycle thebreath, though the cycling part has notbeen looked at, as far as I know.

REFERENCE

1. Sinderby C, Navalesi P, Beck J, Skrobik Y,Comtois B, Friberg S, et al. Neural controlof mechanical ventilation in respiratory fail-ure. Nat Med 1999;5(12):1433–1436.

MacIntyre: Yours is one of the bestdescriptions I’ve seen of the cyclingproblem with pressure support. Werecognized this problem several yearsago, thanks to the work of Martin To-bin’s group.1 In our institution wedon’t use pressure support that muchanymore, for just that reason. We’rereally concerned that cycling is an is-sue; it is difficult to set it right, andthese tools you’ve described are notreadily available. We’ve switched towhat we call pressure-assist—a modethat has been around for 20 years. It’sthe pressure control mode on most ven-tilators—and if you set the rate to verylow or zero, patients can still triggerthose breaths. These breaths begin justlike a pressure support breath: you seta pressure target; they’re patient-trig-gered; they have all the rise time char-acteristics you described. The only dif-ference between it and the pressuresupport breath is that you, the clini-cian, have control over the inspiratorytime. You have to set it.

REFERENCE

1. Tobin MJ, Jubran A, Laghi F. Patient-ven-tilator interaction. Am J Respir Crit CareMed 2001;163(5):1059–1063.

Hess: But you’ve got to set it right!

MacIntyre: Yes, you’ve got to setit right. But let me return to what JoshBenditt said, and point out that we’vegotten fairly comfortable looking atthe way a patient breathes, and look-ing for those little pressure spikes inthe waveform that say that the pres-sure breath is going too long, or thelittle sucking that occurs at the end

that says a breath is too short. I wouldsubmit to you that maybe we’ve gotsomething simple right now to use, ifwe’re smart enough to recognize andwatch the patient’s inspiratory and ex-piratory efforts, that we might be ableto set the inspiratory time and use pres-sure-assist to achieve all the goals andsolve the cycle synchrony issue. I’m alittle nervous saying that, since I’msitting next to the guru of patient-ven-tilator synchrony [Jon Nilsestuen], butI thought I’d stick my neck out andthrow that out as a possible option toaddress the cycling issue.

Hess: Like you, we also use pres-sure control as an alternative to pres-sure support in some patients with cy-cle dyssynchrony. My problem withthat is that as clinicians we have to getthe inspiratory time setting just right,or we have all the same problems thatwe have with pressure support. I thinkthat what you say can be done, but asclinicians we have to be able to adjustthe inspiratory time to go shorter orlonger as necessary. Another way wecould do that is to use a ventilator thatallows adjusting the flow cycle-off cri-teria, and adjust that up or down dur-ing pressure support as the mechanicschange. But what you said we do inpractice, so I agree with you, but I stillam a little nervous about it at times.

MacIntrye: I think adjusting theflow cycling is even more problem-atic, because if the patient changes hisefforts, that can change a lot. The cy-cle-off time with the flow cycle wouldchange even more than the inspiratorytime.

Hess: Point well made. One of theproblems you get into is that if you ad-just the rise time, that will adjust theflow at the beginning of ventilation, andthen for the same fraction of cycle-offcriteria it’ll adjust the cycle-off flow.

MacIntyre: But the inspiratory timestays the same. The setting will stay thesame if you’re using pressure assist.

Hess: Point taken.

Sanborn: In 1999 Magdy Younes’sgroup1 looked at proportional-assistventilation. The pressure support partwas fascinating. They studied the fre-quency of breath triggering under 2conditions: low support pressure andhigh support pressure. At the lowersupport pressures there were no pres-sure (or corresponding flow) pertur-bations during exhalation. But as thesupport pressure was raised, both peakinspiratory flow and inspiratory timeincreased. Then there were pressureand flow perturbations during exhala-tion. On the assumption that those per-turbations reflected failed inspiratorytrigger efforts, they plotted the inter-val between the inspiratory trigger andthe following perturbation against thecorresponding pressure signals froman esophageal balloon. As I recollect,the correlation was excellent. Thestudy demonstrated that over-supportcan cause patient-ventilator dyssyn-chrony. The point was that a lot ofclinicians don’t pay enough attentionto the pressure support level, and theynecessarily increase inspiratory time,which disrupts the normal cycle fre-quency of the patient’s respiratory con-trol center, causing the diaphragm tocontract during exhalation. And in theend you get patient-ventilator dyssyn-chrony. But if you reduce the pressuresupport level, maybe that’s comingback to what you’re doing—you bet-ter promote patient-ventilator har-mony.

REFERENCE

1. Giannouli E, Webster K, Roberts D, YounesM. Response of ventilator-dependent pa-tients to different levels of pressure supportand proportional assist. Am J Respir CritCare Med 1999;159(6):1716–1725.

Benditt: I think this is also a bigproblem in noninvasive ventilation,which we use a lot. Ventilator-patientsynchrony is so crucial to the patient’scomfort and acceptance of noninva-sive ventilation at home. The Quan-



tum noninvasive machine was aheadof its time; 10 years ago it had the risetime setting, which made a tremen-dous difference in patient acceptance.The BiPAP machine did not have ad-justable rise time, and for a lot of ourneuromuscular patients it was very dif-ficult to tolerate.

Hess: But the Respironics BiPAPVision now has the rise time adjust-ment.

Benditt: Yes. Now almost all ofthem have adjustable rise time, and ithas much improved patient acceptanceof these machines at home.

Hess: I agree. Getting back to Neil’spoint, some of the newer BiPAP ma-chines also allow you to set the max-imum inspiratory time, which also im-proves patient-ventilator synchrony,particularly in the case of leaks.

Pierson:* The term “rise time” hastroubled me ever since the first time Isaw it. It should be “rise per time”because when we say a faster rise time,we mean faster rate of rise. When wesay slower rise time, we mean a moregradual rate of rise. But people areusually using the term in a way that’stechnically opposite of its intendedmeaning. More time means slowerrise, and faster rise means less time. Idon’t think the words are used the waythey really should be used.

Hess: I take your point, althoughthe way that the terms are sometimesused—the way I think about it—is thatrise time is the amount of time that isrequired to reach the pressure.

Pierson: The term ought to be mod-ified.

Hess: It really has to do with slope;and in fact the way that I modeled itmathematically, it really is slope.

Pierson: It’s like trigger sensitivity,which we’ve always been drawn to. Themore sensitive, the less effort it shouldtake, but that’s not the way we use it.

Hess: It becomes very confusing be-cause there’s no consistency amongmanufacturers, and making it a biggernumber on some machines means ittakes more time to reach the pressure,and on others it means it takes less.Warren, you were going to add some-thing to that?

Sanborn: In defense of good lan-guage, we wrestled with this mightilyand came up with “flow accelera-tion”—that’s what it really is—and ev-erybody hated it. They just crucifiedit, so we went back to rise time.

Hess: That’s what it is, and I triedto make the point that the value of thatis how it affects the flow, not by howit affects the pressure. But cliniciansthink about it as the pressure rise, orpressure rise per time.

Nilsestuen: I suppose it depends alittle bit on what kind of valve you’retalking about, but in some mechanicalventilators it’s related to the rate atwhich the valve opens. So when I givelectures or teach students, I use therate of valve opening as a way to de-scribe it, because if it opens quicklythen the gas flow increases rapidly; ifthe valve opens slowly, then gas flowincreases gradually. That seems to befairly clean, at least in terms of theconcept.

Shrake:* I just want to echo JoshBenditt’s comments that at some pointwe have to look at the patient, and

we’ve got some great tools. You’vebeen in the field long enough thatyou’ve ventilated patients without anyof these tools. When they teach pilotshow to fly by instruments, they tellthem, “Always trust the instruments;never trust your senses, because yoursenses will kill you.” How frequentlydo you see a difference between yourclinical assessment and what thesetools are telling you, and what do youtrust, and why?

Hess: I guess I’m going to weaselon this one a little bit and say that weneed both. I think that if we have thesetools—waveforms and graphics and soforth—and we have an astute clini-cian at the bedside who can do a goodpatient examination, then I think wehave the best of both worlds. In mypractice I look a lot at the graphicsand the waveforms, but Scott Harrisand Luca Bigatello will tell you that Ialso look at the patient. They’ve seenme put my stethoscope on the chest.

Sanborn: What did you call that de-vice? Stethoscope?

Hess: In one of my noninvasive ven-tilation lectures I talk about this. I showa slide that has nice graphics on a non-invasive ventilator, and I say that whenI initiate noninvasive ventilation, Ilook at the graphics but I also still likethe good old-fashioned eyeball test,and there’s a picture of my eye.

Nilsestuen: Your Figure 17 showedthat as they increased the rate at whichthe valve opened, they reached a pointwhere it was optimal for the patient,and then they went past that, to whereit opened so fast that the esophagealpressure or the Pmus actually gotgreater again. Do you have anythoughts about that? I think maybethat’s a feedback response from thelung, that it might be irritant receptorsor something, but I didn’t know if any-body has an explanation for why, ifyou give it too fast all of a sudden thepatient goes into this new zone where

* David J. Pierson MD FAARC, Division ofPulmonary and Critical Care Medicine, Univer-sity of Washington, Seattle, Washington.

* Kevin Shrake MA RRT FAARC, Chief Op-erating Officer, American Association for Re-spiratory Care.



it’s more work rather than less work,which doesn’t make sense from theperspective of gas physics.

MacIntyre: That slide, Jon [Nilses-tuen], indicates not that they did moreinspiratory work; what we found isthat they would fight against it andthe tidal volumes plummeted and wegot a lot of expiratory activity. Thosewaveforms Dean showed are differentfrom what we observed. We observedpatients actually fighting against thevery rapid flow. They didn’t like itcoming in so fast and they prematurelyterminated the breath. So we didn’tsee what you did.

Bigatello: There is one thing thatmight help in answering your ques-tion. I interpreted it as dyssynchrony.It represents the point at which thepatient does not like what he is get-ting from the ventilator. It doesn’t to-tally explain that, but in that study wealso looked at a subjective patient-comfort score, and very high rise timewas not the most comfortable. Themost comfortable rise time was in themiddle. With too slow a rate of rise,patients were not getting enough flow,so you might think that the highestrise time would be more comfortable,but that wasn’t true. They were mostcomfortable in the middle, which isalso where they have the least-nega-tive deflection of esophageal pressure.In the last 2 panels are where the pa-tients are not comfortable, and some-how they must be dyssynchronouswith the ventilator, and that’s why theyare making their own efforts.

Hess: Let me propose somethingelse. With the increasing pressuriza-tion rate—“rise per time” for Dr Pier-son—the flow is very high, the ven-tilator cycles off sooner, and theinspiratory time is shorter. That low-ers the tidal volume, so if the patientis going to defend his tidal volume, he

has to make a greater inspiratory ef-fort. Feel free to debate!

Bigatello: You have to set the in-spiratory time!

Nilsestuen: Dean, I guess I like thatexplanation a little better; the reasonbeing that if the patient’s discomfortcauses them to resist the inspiratoryflow, their esophageal pressure shouldgo the opposite way. It should becomesharply positive to resist the flow, andthat’s not what this [Figure 17] shows.This shows that the esophageal pres-sure continues to decline, indicatingmore inspiratory effort, and that’s whatseems so counterintuitive.

Hess: That would highlight the pointabout setting the inspiratory time,rather than having it flow-cycle, becauseby changing the pressurization rate—the rise time—it changes the flow, andthen that changes where the ventilatorcycles off if the cycle-off criterion is afixed percentage of peak flow.

Harris: The only thing about that isthat it assumes that somehow the re-spiratory center knows that it’s not go-ing to get enough tidal volume, be-cause if you look at the esophagealpressure, it’s actually—

Hess: But I don’t think these arebreaths in sequence.

Bigatello: So those aren’t actuallymatched in time?

Hess: Yes, they are matched in time,but they are not one breath after theother, and because they’re not onebreath after the other, if the tidal vol-ume drops, the PCO2

will go up a bit,and that will increase the respiratory-center output. There will be more res-piratory-muscle contraction, esopha-geal pressure deflection, and tidal

volume flow, all in an attempt to lowerthe PCO2

.

Harris: So it would have to be aphysiologic change that has alreadyoccurred, and the respiratory center issensing that and then responding to it.

Hess: Right.

Dhand: One piece of informationthat would help in that respect is toknow what happened to the fre-quency of breathing in those patients.I think that what Neil is referring tois possible—that when the Hering-Breuer reflex gets activated, thatshortens the inspiration. Because in-spiration and expiration are linked,then expiration gets shortened too,and that tends to increase the fre-quency of breathing, and the respi-ratory drive. When the respiratorydrive is increased, you could get amore negative deflection on theesophageal pressure waveform.

Benditt: I want to clarify one point.When you say neural inspiratory time,how do you measure that? What isthat?

Hess: That’s the time of the respi-ratory-center output, and it’s not easyto measure. Some investigators havespent a lot of time trying to measurethat, but essentially what it means isthe amount of time that there is anoutput from the respiratory center.

Dhand: You can measure it if youare looking at the diaphragmaticEMG. The time for the activity ofthe diaphragm gives you an idea ofthe neural inspiratory time, and that’sreally where a lot of the problemswith pressure support arise, becauseone controller is in the patient’s brainand then the other controller is inthe ventilator, and the two are notmatching.



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Ventilator Waveforms and the Physiology of Pressure ...rc.rcjournal.com/content/respcare/50/2/166.full.pdf · The Ventilator Trigger PSV is patient-triggered. If the patient becomes