BASICS OF WAVEFORM INTERPRETATIONRET 2284Principles of Mechanical Ventilation

ObjectivesIdentify graphic display options provided by mechanical ventilators.

Describe how to use graphics tomore appropriately adjust the patient ventilator interface.

IntroductionMonitoring and analysis of graphic display of curves and loops during mechanical ventilation has become a useful way to determine not only how patient are being ventilated but also a way to assess problems occurring during ventilation.

Uses of Flow, Volume, and Pressure Graphic Display Confirm mode functions Detect auto-PEEP Determine patient-ventilator synchrony Assess and adjust trigger levels Measure the work of breathing Adjust tidal volume and minimize overdistension Assess the effect of bronchodilator administration Detect equipment malfunctions Determine appropriate PEEP level

Uses of Flow, Volume, and Pressure Graphic DisplayEvaluate adequacy of inspiratory time in pressure control ventilationDetect the presence and rate of continuous leaksAssess inspiratory termination criteria during Pressure Support VentilationDetermine appropriate Rise Time

Measured ParametersFlowPressureVolumeTime

Most Commonly Used Waveforms (Scalars)Pressure vs. Time

Flow vs. Time

Volume vs. Time

Pressure vs. Time Curve

Pressure-Time Curve12345620SecPawcmH2OPressure VentilationVolume Ventilation

Patient Triggering12345630SecPawcmH2O-10

Adequate Flow During Volume-Control Ventilation30Time (s)-1012awPcmH2OAdequate flow3

Inadequate Flow During Volume-Control Ventilation30Time (s)-1012awPcmH2OAdequate flow3

Patient/Ventilator SynchronyVolume Ventilator Delivering a Preset Flow and VolumeAdequate Flow123456-20SecPawcmH2O

Patient/Ventilator SynchronyThe Patient Outbreathing the Set FlowAir Starvation123456-20SecPawcmH2O

Plateau TimeInadequate plateau time-2012345630SECPawcmH2O

Adequate Plateau Time-2012345630SECPawcmH2OPlateau Time

Flow vs.Time Curve123456SEC120120EXHINSPV.LPMInspiration

Flow vs.Time Curve123456SEC120120EXHINSPV.LPMInspirationExpiration

Flow vs.Time Curve 123456SEC120120EXHINSPInspirationV.LPMConstant FlowDescending Ramp

Flow-Time Curve123456SEC120120EXHINSPInsp. PauseExpirationV.LPM

Inspiratory Time Short Normal Long

123456SEC120-120V.LPMExpiratory Flow Rate and Changes in Expiratory Resistance

A Higher Expiratory Flow Rate and a Decreased Expiratory Time Denote a Lower Expiratory Resistance123456SEC120120V.LPM

Obstructed LungDelayed flow return

Pressure-Time and Flow-Time Curves

12345620SecPawcmH2OExpirationV.Volume Ventilation

Pressure-Time and Flow-Time CurvesDifferent Inspiratory Flow Patterns12345620SecPawcmH2OExpirationV.Volume VentilationInspiration

20Pressure-Time and Flow-Time Curves123456SecPawcmH2OV.Pressure VentilationInspiratory TimeVolume Ventilation

Rise Time

How quickly set pressure is reached

TimeMinimal Pressure OvershootPressure ReliefSlow rise Moderate rise Fast risePV.Flow Acceleration Percent Rise Time

Patient / Ventilator Synchrony Volume Ventilation Delivering a Preset Flow and VolumeAdequate Flow12345630-20SecPawcmH2O

Air Starvation12345630-20SecPawcmH2OPatient -Ventilator Synchrony The Patient Is Outbreathing the Set Flow

If Peak Flow Remains the Same, I-Time Increases: Could Cause AsynchronyLPM123456SEC120-120V.

Changing Flow Waveform in Volume Ventilation: Effect on Inspiratory Time123456SEC120-120V.LPM

Increased Peak Flow: Decreased Inspiratory Time 123456SEC120-120V.LPM

Note: There can still be pressure in the lung behind airways that are completely obstructedDetecting Auto-PEEPLPMZero flow at end exhalation indicates equilibration of lung and circuit pressure123456SEC120-120V.

Detecting Auto-PEEPThe transition from expiratory to inspiratory occurs without the expiratory flow returning to zero 123456SEC120120V.LPM

Volume vs.Time Curve

Volume vs.Time CurveExpirationSEC800 ml234561VT

Typical Volume Curve123456SEC1.2-0.4VTLitersI-TimeE-TimeABA = inspiratory volumeB = expiratory volume

Leaks123456SEC1.2-0.4VTLitersAA = exhalation that does not return to zero

123456SEC123456600 cc 120120SEC0450 ccSetting Appropriate I-Time

Setting Appropriate I-Time500 cc450 ccLost VT123456SEC123456600 cc 120120SEC0

Loops

Pressure-Volume Loops

Flow-Volume Loops

Pressure-Volume Loop

Mandatory Breath

Mandatory Breath Expiration02040602040-600.2LITERS0.40.6PawcmH2OInspirationVT Counterclockwise

Spontaneous BreathInspiration02040602040-600.2LITERS0.40.6PawcmH2OVTClockwise

Spontaneous BreathInspirationExpiration02040602040-600.2LITERS0.40.6PawcmH2OVTClockwise

Work of Breathing0204060-20-40-600.20.40.6LITERSPawcmH2OVT

Assisted Breath02040602040-600.2LITERS0.40.6PawcmH2OAssisted BreathVT

Assisted BreathInspiration02040602040-600.2LITERS0.40.6PawcmH2OAssisted BreathVT

Assisted BreathInspirationExpiration02040602040-600.2LITERS0.40.6PawcmH2OAssisted BreathVT Clockwise to Counterclockwise

Pressure-Volume Loop Changes0204060-20-40-600.20.40.6LITERSPawcmH2OVT

Changes in CompliancesIndicates a drop in compliance (higher pressure for the same volume) 02040602040-600.20.40.6LITERSPawcmH2OVT

OverdistensionBA0204060-20-40-600.20.40.6LITERSPawcmH2OCA = inspiratory pressure B = upper inflection point C = lower inflection pointVT

Lung Overdistension

Normal Flow-Volume Loops

Flow -Volume Loops Volume ControlFlowVolumeTidal VolumeInspiration Expiration

Flow -Volume Loops Volume ControlFlowVolumePeak Expiratory FlowPeak Inspiratory FlowTidal VolumeInspiration Expiration

ETT or Circuit Leaks

Obstructive Pattern

Bronchodilator Response211233VLPS.BEFOREVLPS.

Bronchodilator Response211233VLPS.BEFOREAFTERWorse211233VLPS.

Bronchodilator Response211233VLPS.VTINSPEXHBEFOREAFTERWorseBetter211233VLPS.

Remember!Waveforms and loops are graphical representation of the data generated by the ventilator.Typical Tracings Pressure-time, Flow-time, Volume -timeLoops Pressure-Volume Flow-Volume

Assessment of pressure, flow and volume waveforms is a critical tool in the management of the mechanically ventilated patient.

****A good way to identify an adequate plateau time is to observe the pressure-time curve. This slide shows an inadequate plateau time; no plateau has occurred. This could lead to an inaccurate estimation of plateau pressure.*Here, we see that a plateau has occurred, as evidenced by the flattening of the pressure curve at the arrow. ***This slide depicts a long expiratory phase. Note the low expiratory flow rate and extended exhalation phase. This could be caused by a number of clinical situations: bronchospasm, COPD, expiratory filter contamination, secretions or water in the tubing. Watching for changes in expiratory flow helps judge the efficacy of any intervention.

*A higher expiratory flow rate and a decreased expiratory time denote a lower expiratory resistance. A decrease in expiratory resistance may also be observed after the patient receives a bronchodilator (e.g. MDI or aerosolized neb tx). Monitoring the duration of the therapys effect can help determine the indicated frequency of therapy.*Exhalation, seen here in yellow, occurs when the tidal volume has been delivered or the high pressure limit has been reached. Flow ceases and exhalation begins. This is a passive process caused by the elastic recoil of the lung.*Lets take a look at the pressure curve resulting from a volume-based breath with a decelerating flow pattern. As you can see in green here, pressure increases more rapidly from the PEEP level when the decelerating flow curve is used. This pressure curve is starting to look more like a pressure-based breath.*In pressure-based ventilation, once the Pinsp has been reached, the pressure then remains constant for the Tinsp set on the ventilator. Flow decelerates towards end inspiration and then remains at or near zero base line until the set inspiratory time is met. Note the pressure curves are similar. The difference is in the ventilators response to changes in resistance, compliance, or patient demand.*Did anybody say rise time or flow acceleration percent? Literature suggests that inappropriate flow rate, too high or too low at any time during the inspiratory phase in PCV or PSV, may result in increased inspiratory muscle effort or work of breathing. It can also increase the likelihood of patient discomfort and patient/ventilator asynchrony. FAP allows the clinician to sculpt the shape of the rise to pressure to meet patient demand or comfort. Slow, moderate, and aggressive rise to pressure curves are shown. What is happening at the two arrows?

On the pressure-time curve, it is a minimal pressure overshoot caused by an aggressive rise to pressure. On the flow curve, it is a pressure relief that occurs with an active exhalation valve. What happens when there isnt an active exhalation valve? *All right, now lets get on with the fun stuff: detecting abnormalities on waveforms. When your patient begins to fight the ventilator and becomes asynchronous, your job as a clinician is to determine why. We all know that many things can cause the patient to become out of synch. It could be caused by pain, frustration from trying to communicate, or their spouse may have just told them they are filing for divorce. But on a more serious note, patient ventilator dysychrony can be caused when the patient outstrips the peak flow set on the ventilator. Lets take a look at this.

This is a normal pressure curve in volume ventilation with an adequate setting for peak flow.*What we see here is a patients inspiratory flow demand greater than the peak flow set on the ventilator, which can lead to patient/ventilator dysynchrony. What are we going to do to amend this situation? *However, remember what we talked about earlier. If the peak flow is left at the same setting when we switch to a decelerating flow pattern, the inspiratory time will increase. A decreased expiratory time may have the potential to cause patient/ventilator dysynchrony in and of itself. Perhaps a different mode of ventilatory support may be more appropriate, such as PCV or PS.*As we discussed earlier, both square and decelerating flow patterns are commonly used in clinical practice. We will not debate the clinical application of these flow patterns, but will point out the impact of changing from a square to decelerating flow curve in volume ventilation without changing the set peak flow. Note the increase in inspiratory time with the decelerating flow pattern. *If the goal is to maintain a similar inspiratory time, this can be accomplished by increasing peak flow to approximate the same inspiratory time with the decelerating flow pattern that existed with the square wave pattern. This could decrease the potential for developing Auto-PEEP.*We can look at our flow-time curve. If there is zero flow at the end of exhalation, it would indicate an equilibration of the lung and circuit pressure.

Note: There can still be pressure in the lungs behind airways that are completely obstructed.*On the other hand, if the transition from exhalation to inspiration occurs without the expiratory flow returning to zero, you have Auto-PEEP present.*The volume-time curve shows the gradual changes in the volume that is delivered during inspiration. Volume is typically measured in milliliters. Pictured in green is the inspiratory phase, in which volume increases continuously until the set tidal volume is achieved or the high pressure alarm limit has been reached, or I-Time has expired.*During expiration, seen in yellow here, the transferred volume decreases, again due to the passive recoil of the lung. Generally, what goes in comes out, unless you have a leak in the patient circuit or the patient, or gas is trapped in the lung.*Using both the volume- and flow-time curves provides insight to set the appropriate PIP and I-Time in PCV. For example, the physician orders PCV and tells you that he wants a VT of 500 cc. PCV is initiated with a PIP of 20 cm, resulting in a VT of 450 cc. Before increasing the inspiratory pressure to obtain additional VT, maximize inspiratory time. As shown here at the arrow, inspiratory flow does not return to zero before cycling into expiration. This could result in a lesser delivered volume. *Pictured in blue here is potentially lost VT. Increasing inspiratory time to allow the flow to return to baseline may increase VT without increasing PIP. *We have reviewed the normal components of the three standard time curves: Flow, Pressure, and Volume. Now, lets investigate the normal components of the pressure-volume loop. Instead of plotting one parameter against time, the pressure-volume loop plots the interaction between pressure, on the horizontal axis, and volume, on the vertical axis. *On a ventilator-initiated mandatory breath, or VIM, the movement of the PV Loop is counterclockwise, starting with inspiration, shown here in green. During inspiration, the lung begins to fill and normally there is a simultaneous increase in both pressure and volume.*When the inspiration criteria are met, exhalation begins as pictured in yellow here. Normally, this curve resembles a football.*During a spontaneous, non-pressure-supported breath, the rotation is clockwise; inspiration and then expiration. *During a spontaneous, non-pressure-supported breath, the rotation is clockwise; inspiration and then expiration. *The arrow indicates patient work of breathing. Inspiratory effort to the left of the vertical axis translates into increased inspiratory workload for the patient. This is commonly addressed by instituting flow triggering.*When a patient-initiated mandatory (PIM) breath is triggered, you will initially see a clockwise rotation like a spontaneous breath; then the ventilator takes over and delivers the mandatory breath. At the point marked with the white arrow, it changes to the classic counterclockwise rotation seen with a VIM breath. *When a patient-initiated mandatory (PIM) breath is triggered, you will initially see a clockwise rotation like a spontaneous breath; then the ventilator takes over and delivers the mandatory breath. At the point marked with the white arrow, it changes to the classic counterclockwise rotation seen with a VIM breath. *When a patient-initiated mandatory (PIM) breath is triggered, you will initially see a clockwise rotation like a spontaneous breath; then the ventilator takes over and delivers the mandatory breath. At the point marked with the white arrow, it changes to the classic counterclockwise rotation seen with a VIM breath. *The pressure-volume loop changes, flattening out and moving to the right. What could cause this to happen? *Did anybody say decrease in compliance? The difference between the white arrow and the red arrow represents a change in compliance as indicated by an increase in pressure without a corresponding increase in tidal volume.*Overdistention is caused by a combination of PEEP and too much volume or pressure. A is the peak inspiratory pressure; B is the upper inflection point; C is the lower inflection point. The lower inflection point identifies the level of PEEP where the lung is more compliant. This is also referred to as critical opening pressure. The upper inflection point indicates where the lung becomes less compliant and illustrates where overdistension starts to occur. Decreasing the volume or pressure may help avoid barotrauma in this situation.*This example shows before and after flow-volume loops that indicate a response to bronchodilators. The loop at the far left (before) is the control. Compare the three peak expiratory flow rates and the lower half of each loop. In the center loop, the relatively low expiratory flow rate (A) and the scalloped shape (B) near end exhalation indicates a negative response to treatment. At the far right, the higher expiratory flow rate and the flatter shape near end exhalation indicate a positive response.*This example shows before and after flow-volume loops that indicate a response to bronchodilators. The loop at the far left (before) is the control. Compare the three peak expiratory flow rates and the lower half of each loop. In the center loop, the relatively low expiratory flow rate (A) and the scalloped shape (B) near end exhalation indicates a negative response to treatment. At the far right, the higher expiratory flow rate and the flatter shape near end exhalation indicate a positive response.*This example shows before and after flow-volume loops that indicate a response to bronchodilators. The loop at the far left (before) is the control. Compare the three peak expiratory flow rates and the lower half of each loop. In the center loop, the relatively low expiratory flow rate (A) and the scalloped shape (B) near end exhalation indicates a negative response to treatment. At the far right, the higher expiratory flow rate and the flatter shape near end exhalation indicate a positive response.*