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1. •A basic ventilator circuit diagram
•Airway pressures
•The equation of motion for the respiratory
system
•The pressure-time waveform

2. Different pressure-time waveforms using a
‘square wave’ flow pattern
This is a normal pressure-time waveform
With normal peak pressures ( Ppeak) ;
plateau pressures (Pplat )and
airway resistance pressures (Pres)
time
pressure
Ppeak
Pres
Pplat
Pres
Scenario # 1
Normal values:
Ppeak < 40 cm H2O
Pplat < 30 cm H2O
Pres < 10 cm H2O

3. Waveform showing increased airways
resistance
This is an abnormal pressure-time waveform
time
pressure
Ppeak
Pres
Pplat
Pres
Scenario # 2
The increase in the peak airway pressure is driven
entirely by an increase in the airways resistance
pressure. Note the normal plateau pressure.
e.g. ET tube
blockage

4. Waveform showing high airway resistance
due to high flow rates
This is an abnormal pressure-time waveform
time
pressure
Ppeak
Pres
Pplat
Pres
Scenario # 3
The increase in the peak airway pressure is driven
entirely by an increase in the airways resistance
pressure caused by excessive flow rates.
Note the shortened inspiratory time and high flow
e.g. high flow
rates
Normal (low)
flow rate

5. Waveform showing decreased lung
compliance
This is an abnormal pressure-time waveform
time
pressure
Ppeak
Pres
Pplat
Pres
Scenario # 4
The increase in the peak airway pressure is driven
entirely by the decrease in the lung compliance.
Increased airways resistance is often
also a part of this scenario.
e.g. ARDS

6. The same pressure-time tracings
using a ‘decelerating ramp’ flow pattern
High Raw:High Raw:
(e.g. asthma)(e.g. asthma)
NormalNormal
High flow:High flow:
(Note short(Note short
InspiratoryInspiratory
time)time)
Low CLow CLL::
e.g.e.g.
ARDSARDS
Normal
PIP
Normal
Pplat
High
PIP
High
PIP
High
Pplat
Normal
Pplat
Normal
Pplat
High
PIP
pressure
time

7. The same pressure-time tracings
using a ‘decelerating ramp’ flow pattern
High Raw:High Raw:
(e.g. asthma)(e.g. asthma)
NormalNormal
High flow:High flow:
(Note short(Note short
InspiratoryInspiratory
time)time)
Low CLow CLL::
e.g.e.g.
ARDSARDS
Normal
PIP
Normal
Pplat
High
PIP
High
PIP
High
Pplat
Normal
Pplat
Normal
Pplat
High
PIP
pressure
time