Theoretical study of inspiratory flow waveforms during mechanical ventilation on pulmonary blood flow and gas exchange

A lumped two-compartment mathematical model of respiratory mechanics incorp orating gas exchange and pulmonary circulation is utilized to analyze the e ffects of square, descending and ascending inspiratory flow waveforms durin g mechanical ventilation. The effects on alveolar volume variation, alveola r pressure, airway pressure, gas exchange rate, and expired gas species con centration are evaluated. Advantages in ventilation employing a certain ins piratory flow profile are offset by corresponding reduction in perfusion ra tes, leading to marginal effects on net gas exchange rates. The descending profile provides better CO2 exchange, whereas the ascending profile is more advantageous for O-2 exchange. Regional disparities in airway/lung propert ies create maldistribution of ventilation and a concomitant inequality in r egional alveolar gas composition and gas exchange rates. When minute ventil ation is maintained constant, for identical time constant disparities, ineq ualities in compliance yield pronounced effects on net gas exchange rates a t low frequencies, whereas the adverse effects of inequalities in resistanc e are more pronounced at higher frequencies. Reduction in expiratory air no w (via increased airway resistance) reduces the magnitude of upstroke slope of capnogram and oxigram time courses without significantly affecting end- tidal expired gas compositions, whereas alterations in mechanical factors t hat result in increased gas exchanges rates yield increases in CO2 and decr eases in O-2 end-tidal composition values. The model provides a template fo r assessing the dynamics of cardiopulmonary interactions during mechanical ventilation by combining concurrent descriptions of ventilation, capillary perfusion, and gas exchange. (C) 1999 Academic Press.