The Determinants of C02SB
In a mechanistic sense, weaning from mechanical ventilation is a test of the load response of the respiratory pump. The finding that most patients with weaning-induced respiratory distress retained C02SB compared with C02RT suggests that in them the net effect of intrinsic loading on alveolar ventilation was greater than the effect of mechanical feedback inhibition on C02RT. At the end of the weaning trial, seven of nine subjects in group 2 had a minute ventilation less than would have been necessary to maintain C02SB within 2 mm Hg of C02RT. Only one patient (subject 8) had already developed respiratory acidosis by the time he met predefined weaning failure criteria.
Our findings raise the question whether a 3 mm Hg increase in C02SB above C02RT heralds impending respiratory pump failure or is simply a sign of a steady-state compensation to a large intrinsic load. Because tachypnea and dyspnea are nonspecific manifestations of respiratory compromise, our data do not provide a clear answer. The only evidence in favor of impending respiratory pump failure as a cause of load-induced hypercarbia in group 2 patients is based on the observation that awake normal subjects do not retain C02 during breathing against sustainable extrinsic loads.