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

Different pressure-time waveforms using a ‘square

wave’ flow pattern

This is a normal pressure-time waveformWith normal peak pressures ( Ppeak) ;

plateau pressures (Pplat )and airway resistance pressures (Pres)

time

pres

sure

Ppeak

Pres

Pplat

Pres

Scenario # 1

Normal values:Ppeak < 40 cm H2OPplat < 30 cm H2O Pres < 10 cm H2O

Waveform showing increased airways

resistance

This is an abnormal pressure-time waveform

time

pres

sure

Ppeak

Pres

Pplat

Pres

Scenario # 2

The increase in the peak airway pressure is drivenentirely by an increase in the airways resistance

pressure. Note the normal plateau pressure.

e.g. ET tubeblockage

Waveform showing high airway resistance

due to high flow rates

This is an abnormal pressure-time waveform

time

pres

sure

Ppeak

Pres

Pplat

Pres

Scenario # 3

The increase in the peak airway pressure is drivenentirely by an increase in the airways resistance

pressure caused by excessive flow rates. Note the shortened inspiratory time and high flow

e.g. high flowrates

Normal (low) flow rate

Waveform showing decreased lung compliance

This is an abnormal pressure-time waveform

time

pres

sure

Ppeak

Pres

Pplat

Pres

Scenario # 4

The increase in the peak airway pressure is drivenentirely by the decrease in the lung compliance.

Increased airways resistance is often also a part of this scenario.

e.g. ARDS

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 shortInspiratory Inspiratory time)time)

Low CLow CLL::e.g. ARDSe.g. ARDS

NormalPIP

NormalPplat

HighPIP

HighPIP

HighPplat

NormalPplat

NormalPplat

HighPIP

pre

ssure

time