Respiratory System Behavior during Mechanical Inflation with Constant Inspiratory Pressure and Flow

Abstract

Background

Constant-pressure inflation (CPI) and constant-flow inflation (CFI) are widely used forms of mechanical ventilation applied to critically ill patients. The relative advantages of these two approaches to ventilation are controversial. The purpose of this study was to determine the theoretical factors that affect the distribution of volume in a mathemati-cal model of the lung.

Methods

We postulated a lung model composed of two parallel lung units. Analysis of model response was performed for step inputs of pressure (ie, CPI) and flow (ie, CFI) using the Laplace transform.

Results

For lung units with equal impedances, both CPI and CFI result in equal distribution of volume between the two lung units and distribu-tion is not a function of time. For lung units with different impedances but equal time con-stants (t), the distribution of volume will depend only on the ratio of compliances or resis-tances for both modes of ventilation. For different t but equal resistances, CFI gives more uni-form volumetric expansion and possibly lower risk of barotrauma than CPI. For different t but equal compliances, CPI gives more uniform volumetric expansion and possibly lower risk of barotrauma than CFI.

Conclusions

We conclude that in the absence of auto-PEEP lung time constants are the primary factors that determine the distribution of volume between lung units and that the relative superiority of one ventilatory mode over another depends on the underlying lung pathology that dictates the components of the time constant.