basic concepts in adult mechanical ventilation pa
TRANSCRIPT
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Basic Concepts in Adult Mechanical Ventilation
Pa
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Points of Discussion
1.Basic respiratory dynamics
2.Basic ventilator parameters
3.Control variables: pressure, volume
4.Phase variables: trigger, limit and cycle
5.Conditional variables
6.Breath types
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Inspiration Inspiration
Spontaneous Breathing
Exhalation Exhalation
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Precondition of Inspiration
Pa < PbSpontaneous
breathPb > Pa
Mechanical ventilation
PbPb
Pa
Gas Flow
PaPa << PbPb
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PleuralPressure
Three - Dimensional Spring
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Inspiration
Mechanical Breath
Spontaneous Breath
Pressure
Time
Alveolar Pressure Changes
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Spontaneous Inspiration
Volume Change
Gas Flow
Pressure Difference
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Mechanical Ventilation
Pressure Difference
Volume Change
Gas Flow
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“The Feature of the Tube”
R = P
F
Airway Resistance
Pressure Difference = Flow Rate x Resistance of the Tube
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Volume Change = Pressure Difference x Compliance of the Balloon
Volume Change = Pressure Difference x Compliance of the Balloon
Volume
Pressure
V
P
C = V P
Compliance
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Compliance
Measures compliance of the lung and thorax Tidal volume / Plateau-PEEP Units = ml/cmH20
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C= V P
R = PF
Compliance and Resistance
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Resistance Measures airway resistance
Length Viscosity Flow Radius4
Peak-plateau / Flowrate Units = cmH20/Lps
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Peak and Plateau Pressures
Peak airway pressure reflects Baseline (PEEP) Pressure due to compliance (L+T) Pressure due to resistance
Plateau pressure (breath hold) reflects Baseline (PEEP) Pressure due to compliance (L+T) (alveolar distending pressure)
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Peak Alveolar and Transpulmonary PressuresP(t) = VT/CR+ Flow x RR + PEEP tot
meanPaw
External PEEPIntrinsic PEEP
Plateau pressure
Peak Airway Pressure+
_
_ _
_ _++
++ Palveolar
Ppleural
Ptranspulmonary = Palveolar - Ppleural Pplat = Maximum Palveolar
Alveolar Pressure
Transpulmonary pressure is a key determinant of alveolar distension.
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Basic Ventilator ParametersFiO2
Fractional concentration of inspired oxygen delivered expressed as a % (21-100)
Breath Rate (f)The number of times
over a one minute period inspiration is initiated (bpm)
Tidal volume (VT)The amount of gas that
is delivered during inspiration expressed in mls or Liters. Inspired or exhaled.
FlowThe velocity of gas
flow or volume of gas per minute
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Terminology
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Terminology: PIP & MAP
A=A1+A2+…+AnPressure
Inspiration + Exhalation
A
Mean Airway Pressure
Peak Inspiratory PressurePeak Inspiratory Pressure
Time
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Terminology: PEEP, I:E Ratio
Pressure
Time
Positive EndExpiratory Pr.
T insp. T exp. I : E = 1 : 2 I : E = 1 : 2
I : E = 4 : 1 I : E = 4 : 1
PEEP
PIP
PIP
PEEPT insp.
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Terminology: Flow and Volume
Minute Ventilation = Tidal Volume x Breath RateMinute Ventilation = Tidal Volume x Breath Rate
Pressure
Flow
Time
Tidal Volume Tidal Volume
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Volume
Flow Rate
Time
Volume = Flow X Time
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PEEPDefinition
Positive end expiratory pressureApplication of a constant, positive pressure such
that at end exhalation, airway pressure does not return to a 0 baseline
Used with other mechanical ventilation modes such as A/C, SIMV, or PCV
Referred to as CPAP when applied to spontaneous breaths
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PEEPIncreases functional residual capacity (FRC) and
improves oxygenationRecruits collapsed alveoliSplints and distends patent alveoliRedistributes lung fluid from alveoli to perivascular
space
5 cm H5 cm H22O O
PEEP PEEP
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ClassificationControl variable
Flow (volume)Pressure
Phase variableTrigger, limit, cycle, baseline
Conditional variablePatient effort and time
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“Control Variable”
Which parameter remains constant despiteWhich parameter remains constant despite changes in pulmonary mechanics?changes in pulmonary mechanics?
Pressure Generated BreathPressure Generated Breath
Pressure
Flow
Time
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Control variablesVolume Ventilation Pressure Ventilation
Pressure
Flowtime
Pressure
Flowtime
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If compliance decreases the pressure increases to maintain the same Vt
11 22 33 44 55 66
SECSEC
11 22 33 44 55 66
PPawaw
cmHcmH 2200
6060
-20-20
120120
120120
SECSEC
INSPINSP
EXHEXH
FlowFlowL/minL/min
Volume Control Breath Types
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As compliance changes - flow and volumes change
1 2 3 4 5 6
SEC
1 2 3 4 5 6
PawcmH20
60
-20
120
120
SEC
INSP
EXH
FlowL/min
Volume Targeted (Pressure Controlled)
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Pressure then raises to assure that the set tidal volume is delivered
1 2 3 4 5 6
SEC
1 2 3 4 5 6
PawcmH20
60
-20
120
120
SEC
INSP
EXH
FlowL/min
New Volume Targeted Breath Pressure Variability is Controlled
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Phase Variables
A. Trigger mechanismWhat causes the breath to
begin? Patient (assisted)Machine (controlled)
B. Limit variableWhich parameter is sustained
at a preset level during the breath?
FlowPressure
C. Cycle mechanismWhat causes the breath to end?VolumeTime
A
B C
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Trigger Variable- Start of a BreathTime - control ventilationPressure - patient assistedFlow - patient assistedVolume - patient assistedManual - operator control
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Inspiratory Trigger MechanismTime
Controlled Mechanical VentilationPressureFlowFlowChest impedanceChest impedanceAbdominal movementAbdominal movement
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Inspiratory - delivery limitsMaximum value that can be reached
but will not end the breath-VolumeFlowPressure
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Breath TypesMandatory
Ventilator does the workVentilator controls start and stop
SpontaneousPatient takes on workPatient controls start and stop
AssistedPatients triggers the breathThe ventilator delivers the breath as per control variable
SupportedPatients triggers the breathVentilator delivers pressure support Breath cycles at set flow
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