Mean alveolar pressure during constant-flow and constant-pressure inflation of diseased lungs

An important goal in managing mechanical ventilation is optimizing key variables such as mean alveolar pressure (PA).

Determine the effects of lung mechanics on PA during constant-flow inflation (CFI) and constant-pressure inflation (CPI) in models of nonhomogenous lung disease.

We postulated a mathematical lung model consisting of 2 parallel lung units with the airways and the chest wall. Analysis was performed for step inputs of inspiratory flow and pressure while maintaining the same tidal volume, respiratory frequency, and positive end-expiratory pressure. The analysis was performed for purely restrictive, purely obstructive, and mixed lung disease.

When simulating purely restrictive lung disease, PA was always higher in healthy alveoli than in diseased alveoli, with constant inspiratory flow or constant inspiratory pressure. However, PA in each alveoli was always greater during CPI than during CFI. For purely obstructive lung diseases, PA was always lower in diseased alveoli than in healthy alveoli, with constant inspiratory flow or constant inspiratory pressure. However, PA in each alveoli was always greater during CPI than during CFI. For mixed lung diseases with equal time constants, PA was always higher in diseased alveoli than in healthy alveoli, with constant inspiratory flow or constant inspiratory pressure. However, PA in each alveoli was always the same during CPI as during CFI.

For the same tidal volume, the mean alveolar pressure in different alveoli depends on the type of disease. The difference in mean alveolar pressure between a normal and a diseased alveolus depends on the difference in time constant between those alveoli, regardless of the mode of ventilation.