We observed a consistent paradoxical reduction of arterial pCO2 in patients with Adult Respiratory Distress Syndrome (ARDS) whenever we minimized minute ventilation with respiratory rates as low as 5/min using 4:1 inverse inspiratory/expiratory (I:E) mode ventilation. This effect was counter-intuitive and its magnitude was a qualitative departure from behavior anticipated by deadspace timing effects or other ventilation/perfusion (V/Q) mechanisms. We wondered whether incomplete CO2 equilibration between the end-capillary and gas-containing compartments could account for our observation.
We developed a rudimentary two compartment steady-state model of diffusion-limited gas exchange. The model determines the mixed-venous pCO2 value required in the pulmonary capillary to produce a diffusion flux across the boundary into the gas compartment equal to the metabolic production of CO2. The arterial pCO2 is in turn determined by stable metabolic production and cardiac output to be some constant amount less than this mixed-venous value. As opposed to V/Q-based analysis, there is no restriction that CO2 equilibration be complete between pulmonary end-capillary (arterial) blood and ‘alveolar’ gas. Our model maintains diffusion across the alveolar boundary proportional to the partial pressure difference between the mixed-venous blood compartment and the gas compartment. The gas compartment oscillates in an instantaneous fashion, partially emptying between end-inspiratory and end-expiratory positions analogous to the ventilatory cycle of the lung. The model addresses effects of airway dimension by allowing diffusion only during the maximal volume portion of each oscillation. A reconfiguration time corrects for the non-square-wave character of physiologic ventilation.
Using a low diffusion co-efficient, inputs of I:E, respiratory rate and tidal volume yield output patterns which demonstrate the efficacy of inverse I:E ventilation and reproduce the phenomenon of paradoxical ventilation.
Paradoxical ventilation may be a consequence of a diffusion-limited gas exchange regimen.
By directing ventilator strategy in ARDS to maximize diffusion transfer, under certain circumstances much lower ventilator frequencies can be theoretically justified. Lower frequencies might limit ventilator-induced lung injury by minimizing lung movement.
Michael Perry, None.