Local gas transport in eucapnic ventilation: Effects of gravity and breathing frequency

The effects of body position and respiratory frequency (f) on regional gas transport during eucapnic conventional ventilation (CV) and high-frequency ventilation (HFV) were assessed from the washout of nitrogen 13 (¹³NN) using positron-emission tomography. In one protocol, six dogs were ventilated with CV or HFV at f = 6 Hz and tidal volume (VT) selected supine for eucapnia. A coronal cross section of the lung base was studied in the supine, prone, and right and left lateral decubitus positions. In a second protocol, six dogs were studied prone: apical and basal cross sections were studied in CV and in HFV with f = 3 and 9 Hz at eucapnic VT. Regional alveolar ventilation per unit of lung volume (spV̇(r)) was calculated for selected regions and analyzed for gravity-dependent cephalocaudal and right-to-left gradients. In both CV and HFV, nonuniformity in spV̇(r) was highest supine and lowest prone. In CV there were vertical gradients in spV̇(r) in all body positions: nondependent less ventilated than dependent regions, particularly in the supine position. In HFV there was a moderate vertical gradient in spV̇(r) in addition to a preferentially ventilated central region in all body positions. Overall lung spV̇ was unaffected by body position in CV but in HFV was highest supine and lowest prone. Nonuniformity in eucapnic prone HFV was unaffected by f and always higher than in CV. The preferentially ventilated central area during HFV, unaffected by body position or frequency, and the finding of the most efficient overall gas transport in the presence of the least uniform spV̇(r) distribution, further support the hypothesis that direct convection of fresh gas into central areas of the lung plays a major role in the transport of respiratory gases during eucapnic HFV.