Effects of added airway dead space on VO2 in dogs.

In dogs breathing spontaneously with added dead space, 198, 298 and 548 ml, the O2 consumption is a function of minute ventilation. This relationship (VO2 = 9.3 VE + 81.6; r = 0.95) was the same as that found in dogs breathing spontaneously with normal dead space. With added dead space, average tidal volumes increased but not enough to compensate for the added volume and, thus, dogs achieved alveolar ventilation mainly by using the fatest molecules of gas in the airway, because of the conical shape of the velocity profile of gas molecules. When the values for VO2 and f were grouped according to different VTS, VO2 was found to be a linear function of f. Also, for a given f, VO2 increased progressively with VT; however, the higher the f, the more taxing a given increase in VT was. Over a certain range of respiratory frequencies, dogs could reduce f and increase VT without significantly affecting VO2.