A patient undergoing mechanical ventilation has a tidal volume of 500 mL and specific pressures during breaths. What is this patient's respiratory system compliance?

To understand why the correct answer is 25 mL/cm H2O, it is important to grasp the relationship between tidal volume, pressure changes during ventilation, and compliance.

Respiratory system compliance is defined as the change in volume divided by the change in pressure. In the context of mechanical ventilation, this relationship can be expressed mathematically as:

[ \text{Compliance} = \frac{\Delta V}{\Delta P} ]

Where:

• ΔV is the tidal volume (the volume of air that is inhaled or exhaled in each breath),

• ΔP is the pressure change during that breath.

In this scenario, the tidal volume is given as 500 mL. To compute compliance, one needs to know the corresponding change in pressure during ventilation, which would typically be provided or inferred from the specific pressures noted during breaths.

If we assume that the change in pressure (ΔP) during these breaths is approximately 20 cm H2O (a common pressure change in mechanically ventilated patients), we can plug the values into the formula:

[ \text{Compliance} = \frac{500 \text{ mL}}{20 \text{ cm H2O}} = 25 \text{ mL