ABSTRACT
From the previous discussion it becomes apparent that the pressure required to ventilate either spontaneously or mechanically is affected by compliance, resistance, volume and flow. These variables are related by the law of motion, which states that the pressure needed to ventilate a patient is equal to volume (V) divided by compliance plus resistance times flow:
Peak ventilating pressure is a result of the combined effects of resistance and compliance whereas PPLAT is solely a reflection of the pressure to overcome compliance and the difference between P1P and PPLAT the pressure to overcome
airways resistance. A change in compliance would change the slope of the airway pressure curve from zero pressure to point A (Figure 1.6 ) and the magnitude of both A and B, whereas a change in resistance would
change the magnitude of the difference between A and B and the magnitude of A. Similarly, a change in volume or flow would alter this relationship. It is essential to realize that the pressure generated during ventilation is solely affected by these factors regardless of whether pressure or volume ventilation is used or assisted or
Total respiratory systema (RS)0.08-0.1 l cm-1 H2O
Lunga0.16-0.2 l cm-1 H2O
Thoraxa0.16-0.2 l cm-1 H20
Specific compliancea 0.08 (no units)
RS medically ventilated 60-80 ml cm-1 H2O
RS ARDS <40 ml cm-1 H20
Airway resistancea (R)0.6-2.4 cm H2O l –1 s-1 at a flow of 0.5 l
s-1
R-mechanically ventilated normal <5 cm H2O difference P IP-PPLAT, constant flow
R-mechanically ventilated marked obstruction
>10 cm H2O difference P IP-PPLAT, constant flow
Figure 1.6 An inspiratory positive pressure waveform with an end inspiratory pause. Gas flow is delivered with a constant flow. A, peak airway pressure; B, end inspiratory plateau pressure.
