Background: High rates of inflation energy delivery coupled with transpulmonary tidal pressures of sufficient magnitude may augment the risk of damage to vulnerable, stress-focused units within a mechanically heterogeneous lung. Apart from flow amplitude, the clinician-selected flow waveform, a relatively neglected dimension of inflation power, may distribute inflation energy non-uniformly among alveoli with different mechanical properties. Our primary objectives were to develop a mathematical model of intracycle inflation power that allows comparisons among the flow modes commonly encountered in clinical practice: constant, linearly decelerating, exponentially decelerating (pressure control), and spontaneous (sinusoidal).
Methods: We first tested the predictions of our mathematical model of passive inflation with the physical performance of a mechanical ventilator-lung system that simulated ventilation to three types of patients: normal, severe ARDS, and severe airflow obstruction. After verification, model predictions were then generated for 5000 ‘virtual ARDS patients’. Holding constant the tidal volume and inflation time between modes, the validated model varied the flow profile and quantitated intensity and timing of potentially damaging ‘elastic’ energy and intracycle power (pressure-flow product) developed in response to random combinations of machine settings and severity levels for ARDS.
Results: Our modeling indicates that while the varied flow patterns ultimately deliver similar total amounts of alveolar energy during each breath, they differ profoundly regarding the potentially damaging pattern with which that energy distributes over time during inflation. Pressure control imposed relatively high maximal intracycle power.
Conclusions: Flow amplitude and waveform may be relatively neglected and modifiable determinants of VILI risk when ventilating ARDS.