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Nov 2006Kishore P.
Critical Care Conference
Why ventilate?
Improve oxygenation Increase/maintain minute ventilation and
help CO2 clearance Decrease work of breathing Protect airway
Origins of mechanical ventilation
• Negative-pressure ventilators (“iron lungs”)
• Non-invasive ventilation first used in Boston Children’s Hospital in 1928
• Used extensively during polio outbreaks in 1940s – 1950s
• Positive-pressure ventilators
• Invasive ventilation first used at Massachusetts General Hospital in 1955
• Now the modern standard of mechanical ventilation
The era of intensive care medicine began with positive-pressure ventilation
The iron lung created negative pressure in abdomen as well as the chest, decreasing cardiac output.
Iron lung polio ward at Rancho Los Amigos Hospital in 1953.
Modes of Mechanical Ventilation
Spontaneous/Controlled/Dual Controlled Mechanical Ventilation (CMV) Assist Control (AC)/Volume Control (VC) Intermittent Mandatory Ventilation (SIMV) Pressure Control (PCV) Pressure Support Ventilation (PSV)
Nov 2006Kishore P.
Critical Care Conference
Patient system interaction
Assist control Assisted-pressure supportControlled MV
Spontaneous
SIMVIMV
+PEEP
CPAP
Breath Types
1.Spontaneous Breath • Inspiration is both initiated and
terminated by the patient.
2.Mandatory Breath • Inspiration is either initiated or
terminated by the ventilator.
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Breath Patterns
1.Continuous Mandatory Ventilation • CMV • All breaths mandatory
2.Intermittent Mandatory Ventilation • IMV or SIMV • Mandatory and spontaneous
breaths
3.Continuous Spontaneous Ventilation • All breaths spontaneous
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Phase Variables
TRIGGER starts inspiration Example: pressure drop when patient
sucks in
LIMIT preset inspiratory value Example: preset maximum inspiratory
flow
CYCLE stops inspiration Example: preset inspiratory time
><
VOLUME-CYCLED VENTILATION
the controlled variables of tidal volume and inspiratory flow determine airway pressure and inspiratory time
Variations in airway resistance or lung compliance alter airway pressures but do not affect minute ventilation
There are three methods of initiating the inspiratory phase in volume-cycled mechanical ventilators: controlled, assist-control, and intermittent mandatory ventilation (IMV)
Nov 2006Kishore P.
Critical Care Conference
Modes of ventilationPressure controlled
Ventilator applies a predefined target pressure to the airway during inspiration
Adv.- decreased risk of barotrauma
Disadv.- with decreasing compliance or increasing resistance, tidal volume and minute ventilation fall
Comparison chart
Volume Pressure Flow I-time
Spontaneous
variable
variable
variable
variable
VCV FIXED variable FIXED FIXED
PCV variable FIXED
variable FIXED
PSVvariable FIXED
Variable
variable
MODES OF PPV
C M VA /C V o lu m e
A /C P ressu re
S IM VS IM V V o lu m e
S IM V P ressu re
S P O N TA N E O U SC P A P w /P S V
C P A P w /o P S VO th er
3 C a teg ories o f P P V
Controlled mechanical ventilation (CMV)
minute ventilation is completely dependent upon the rate and tidal volume set on the ventilator. Any respiratory efforts made by the patient do not contribute to minute ventilation
Controlled ventilation is the required ventilatory mode in patients who are making no respiratory effort (eg, spinal cord injury or drug overdose and those who have been subjected to pharmacologic paralysis).
Controlled Mechanical Ventilation Advantages: rests muscles of respiration Disadvantages: requires sedation/neuro-
muscular blockade, potential adverse hemodynamic effects
Advantages: rests muscles of respiration Disadvantages: requires sedation/neuro-
muscular blockade, potential adverse hemodynamic effects
Controlled Mode (Pressure-Targeted Ventilation)
Controlled Mode (Pressure-Targeted Ventilation)
Pressure
Flow
Volume
(L/min)
(cm H2O)
(ml)
Time (sec)
Time-Cycled
Set PC level
Time Triggered, Pressure Limited, Time Cycled Ventilation
Pressure Control Ventilation C = VT / PC
Pressure Control Ventilation C = VT / PC
Flow
Pressure
VolumeCl
Cl
Set PC level
Time (sec)
(L/min)
(cm H2O)
(ml)
Controlled Mode Volume TargetedControlled Mode Volume Targeted
Flow
Pressure
Volume
Time (sec)
(L/min)
(cm H2O)
(ml)
ASSIST-CONTROL MECHANICAL VENTILATION
In the assist-control (A/C) mode, the ventilator senses an inspiratory effort by the patient and responds by delivering a preset tidal volume. Every inspiratory effort that satisfies the ventilator's demand valve trigger threshold initiates delivery of the preset tidal volume
Patient work is therefore required to trigger the ventilator and continues during inspiration
A control mode back-up rate is set on the ventilator to prevent hypoventilation
Assist-Control Ventilation
Volume or time-cycled breaths + minimal ventilator rate
Additional breaths delivered with inspiratory effort
Order: AC Vt 500, RR12, 100% FiO2, 5 PEEP
Assist-Control Ventilation
Advantages: reduced work of breathing; allows patient to modify minute ventilation
Disadvantages: potential adverse hemodynamic effects or inappropriate hyperventilation
INTERMITTENT MANDATORY VENTILATION
With intermittent mandatory ventilation (IMV), the degree of ventilatory support is determined by the selected IMV rate. At regular intervals, the ventilator delivers a breath based upon a preset tidal volume and rate. In addition, the patient is allowed to breathe spontaneously through the ventilator circuit at a tidal volume and rate determined according to need and capacity.
Most present day ventilators synchronize the intermittent ventilator breaths with inspiratory effort by the patient, a modality termed synchronized IMV or SIMV. However, this modification requires a trigger modality
Synchronized Intermittent Mandatory Ventilation (SIMV)
Potential advantages More comfortable for some
patients Less hemodynamic effects
Potential disadvantages Increased work of breathing
SIMV(Volume-Targeted Ventilation)
SIMV(Volume-Targeted Ventilation)
Spontaneous Breaths
Flow(L/m)
Pressure(cm H2O)
Volume(mL)
Pressure
Flow
Volume
(L/min)
(cm H2O)
(ml)
SIMV + PS (Pressure-Targeted
Ventilation)
SIMV + PS (Pressure-Targeted
Ventilation)
PS Breath
Set PS levelSet PC level
Time (sec)
Time-Cycled Flow-Cycled
PRESSURE SUPPORT VENTILATION Pressure support ventilation (PSV) is flow-
cycled in that, once triggered by a demand valve, the preset pressure is sustained until the inspiratory flow tapers, usually to 25 percent of its maximal value [22]. PSV tends to be a comfortable ventilatory modality because the patient has greater control over ventilator cycling and flow rates. Close monitoring is required whenever PSV is used alone because neither tidal volume nor minute ventilation is guaranteed. PSV can be added during full or partial support with SIMV to overcome endotracheal tube and ventilator circuitry resistance encountered during spontaneous breaths
Pressure-Support Ventilation Pressure assist during spontaneous inspiration
with flow-cycled breath Pressure assist continues until inspiratory
effort decreases Delivered tidal volume dependent on
inspiratory effort and resistance/compliance of lung/thorax
Order: PS 10, PEEP 0, 50% FiO2
Pressure-Support Ventilation Potential advantages
Patient comfort Decreased work of breathing May enhance patient-ventilator synchrony Used with SIMV to support spontaneous
breaths
Pressure-Support Ventilation
Potential disadvantages Variable tidal volume if pulmonary
resistance/compliance changes rapidly
If sole mode of ventilation, apnea alarm mode may be only backup
Gas leak from circuit may interfere with cycling
CPAP + PSV CPAP + PSV
Set PS level
CPAP level
Time (sec)
Flow(L/m)
Pressure(cm H2O)
Volume(mL)
Flow Cycling
VENTILATOR SETTINGS
Ventilatory support requires consideration of trigger mode and sensitivity, respiratory rate, tidal volume, flow rate, flow pattern, and the fraction of inspired oxygen (FiO2).
Nov 2006Kishore P.
Critical Care Conference
Key concepts
Determinants of CO2 clearance
- Ventilator factors* Rate* Tidal volume* Anatomical dead space
- Patient factors* Physiological dead space* CO2 production
Alveolar minute ventilation
Nov 2006Kishore P.
Critical Care Conference
Key concepts Determinants of Oxygenation
- Ventilator factors:* FiO2 ( fraction of oxygen in inspired air)
* Mean airway pressure* PEEP ( positive end expiratory pressure)
- Patient factors* V/Q (ventilation/ perfusion) mismatch* Shunt* Diffusion defect* Reduced mixed venous oxygen
Nov 2006Kishore P.
Critical Care Conference
Adjust FiO2 and PEEP according to PaO2 and SpO2
Adjust TV and rate according to PCO2 and pH
POSITIVE END-EXPIRATORY PRESSURE
PEEP: an elevation in alveolar pressure above atmospheric pressure at the end of exhalation
Extrinsic PEEP (ePEEP): applied through a mechanical ventilator
ACV without PEEP
ACV with PEEP
Auto-PEEP Detection viathe Flow Waveform
Auto-PEEP Detection viathe Flow Waveform
Normal
--------------------------------------------------------
Flow does not return
to zero- Auto-PEEP
Air TrappingAir Trapping
Inspiration
Expiration
NormalPatient
Time (sec)
Flo
w (
L/m
in)
Air TrappingAuto-PEEP
}
Flow-Volume LoopFlow-Volume Loop
Volume (ml)
PEFR
FRC
Inspiration
Expiration
Flo
w (
L/m
in)
PIFR
VT
Air TrappingAir TrappingInspiration
Expiration
Volume (ml)
Flow (L/min)
Does not returnto baseline
NormalAbnormal