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 mathematical 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 distribution is not a function of time. For lung units with different impedances but equal time constants (τ), the distribution of volume will depend only on the ratio of compliances or resistances for both modes of ventilation. For different τ but equal resistances, CFI gives more uniform volumetric expansion and possibly lower risk of barotrauma than CPI. For different τ 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.
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|Published - 1994