Basic Concepts in Adult Mechanical
Ventilation
All names are protected copyrights of
Bird Products, Bear Medical and SensorMedics Inc.
Presentation Copyright© 2001
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
Spontaneous Breathing
Exhalation
Inspiration
Precondition of Inspiration
Pa < Pb
y Pa < Pb
Pb
Gas Flow
y Spontaneous
breath
y Pb > Pa
y Mechanical
ventilation
Pa
Three - Dimensional Spring
Pleural
Pressure
Alveolar Pressure Changes
Pressure
Mechanical Breath
Time
Spontaneous Breath
Inspiration
Spontaneous Inspiration
Volume Change
Pressure Difference
Gas Flow
Mechanical Ventilation
Pressure Difference
Gas Flow
Volume Change
Airway Resistance
“The Feature of the Tube”
R=
Δ P
Δ F
Pressure Difference = Flow Rate x Resistance of the Tube
Compliance
Δ V
C= Δ P
Volume
ΔV
ΔP
Pressure
Volume Change =
Pressure Difference x Compliance of the Balloon
Compliance
‰ Measures
compliance of the lung and thorax
‰ Tidal volume / Plateau-PEEP
‰ Units = ml/cmH20
Compliance and Resistance
ΔP
R=
ΔF
Δ V
C=
Δ P
Resistance
‰ Measures airway resistance
‰ Length
‰ Viscosity
‰ Flow
‰ Radius4
‰ Peak-plateau / Flowrate
‰ Units = cmH20/Lps
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)
Peak Alveolar and Transpulmonary
Pressures
y P(t) = VT/CR+ Flow x RR + PEEP tot
+
Peak Airway Pressure
Alveolar Pressure
Plateau pressure
_
_
meanPaw
+
_
+
_
+
+
Palveolar
_
Intrinsic PEEP
Ppleural
Ptranspulmonary = Palveolar - Ppleural
External PEEP
Pplat = Maximum Palveolar
Transpulmonary pressure is a key determinant of alveolar distension.
Basic Ventilator Parameters
y FiO2
y Fractional concentration of
inspired oxygen delivered
expressed as a % (21-100)
y Breath Rate (f)
y The number of times over a
one minute period inspiration
is initiated (bpm)
y Tidal volume (VT)
y The amount of gas that is
delivered during inspiration
expressed in mls or Liters.
Inspired or exhaled.
y Flow
y The velocity of gas flow or
volume of gas per minute
Terminology
Terminology: PIP & MAP
Peak Inspiratory Pressure
Pressure
Σ A=A1+A2+…+An
ΣA
Time
Mean Airway Pressure
Inspiration + Exhalation
Terminology: PEEP, I:E Ratio
Pressure
PIP
Positive End
Expiratory Pr.
PEEP
PIP
T insp.
T exp.
I:E=1:2
PEEP
Time
T insp.
I:E=4:1
Terminology: Flow and Volume
Minute Ventilation = Tidal Volume x Breath Rate
Flow
Tidal Volume
Pressure
Time
Volume = Flow X Time
Time
Flow Rate
Volume
PEEP
y Definition
y Positive end expiratory pressure
y Application of a constant, positive pressure such that at end exhalation,
airway pressure does not return to a 0 baseline
y Used with other mechanical ventilation modes such as A/C, SIMV, or
PCV
y Referred to as CPAP when applied to spontaneous breaths
PEEP
y Increases functional residual capacity (FRC) and improves
oxygenation
y Recruits collapsed alveoli
y Splints and distends patent alveoli
y Redistributes lung fluid from alveoli to perivascular space
5 cm H2O
PEEP
Classification
y Control variable
y Flow (volume)
y Pressure
y Phase variable
y Trigger, limit, cycle, baseline
y Conditional variable
y Patient effort and time
“Control Variable”
Which parameter remains constant despite
changes in pulmonary mechanics?
Pressure Generated Breath
Pressure
Flow
Time
Control variables
y Volume Ventilation
y Pressure Ventilation
Pressure
Pressure
Flow
time
Flow
time
Volume Control Breath Types
60
Paw
SEC
cmH 20
-20
120
1
2
3
4
5
6
INSP
SEC
Flow
L/min
1
2
3
4
5
120
6
EXH
If compliance decreases the pressure increases to
maintain the same Vt
Volume Targeted (Pressure Controlled)
As compliance changes - flow and volumes change
New Volume Targeted Breath
Pressure Variability is Controlled
Pressure then raises to assure that the set tidal
volume is delivered
Phase Variables
y A. Trigger mechanism
y What causes the breath to begin?
Patient (assisted)
y Machine (controlled)
B
y B. Limit variable
y Which parameter is sustained at a
preset level during the breath?
y Flow
y Pressure
y C. Cycle mechanism
y What causes the breath to end?
y Volume
y Time
A
C
Trigger Variable- Start of a Breath
y Time - control ventilation
y Pressure - patient assisted
y Flow - patient assisted
y Volume - patient assisted
y Manual - operator control
Inspiratory Trigger Mechanism
y Time
yControlled Mechanical Ventilation
y Pressure
y Flow
y Chest impedance
y Abdominal movement
Inspiratory - delivery limits
y Maximum value that can be reached but will not end the
breathy Volume
y Flow
y Pressure
Breath Types
y Mandatory
y Ventilator does the work
y Ventilator controls start and stop
y Spontaneous
y Patient takes on work
y Patient controls start and stop
y Assisted
y Patients triggers the breath
y The ventilator delivers the breath as per control variable
y Supported
y Patients triggers the breath
y Ventilator delivers pressure support
y Breath cycles at set flow
Thank You