Components of carbon monoxide transfer at different alveolar volumes during mechanical ventilation in pigs

We studied the effect of increasing alveolar volume on pulmonary carbon monoxide transfer (DLCO) and its components, i.e. membrane diffusing capacity (D-M) and capillary blood volume (Q(c)), during mechanical ventilation in eight anaesthetized and paralysed healthy pigs (mean weight 11.2 kg). We used an inspiratory pause procedure for simulation of the single-breath technique, and inflated 15, 20, 25 and 30 mi kg(-1) in random order. D-M and Q(c) were derived using the Roughton-Forster equation. Per litre BTPS increase in effective V-A, DLCO (inspiratory oxygen fraction 0.30) decreased on average by 11.8 mu mol s(-1) kPa(-1), D-M slightly increased by 2.7 mu mol s(-1) kPa(-1) and Q(c) decreased by 241 ml. The increase in D-M was much smaller than might be expected from the increase in VA, which we ascribe to a loss of the alveolar capillary membrane for gas transfer because of the concomitant decrease in Q(c). The decrease in ec may be explained by a squeezing effect of the intrapulmonary pressure rise on the alveolar wall and by stretching of lung tissue.