[A central mechanism augments ventilation of the dependent lung on the lateral decubitus position]

Spontaneously increased ventilation of the dependent lung on the lateral decubitus position compensates for the gravitational shift of the pulmonary blood flow to this side. The present study indicates that the hemilateral predominant ventilation is centrally controlled, though it has previously been attributed to the peripheral mechanisms with simple changes in the mechanical properties between the two lungs. A series of chest roentgenograms taken at every 0.3 seconds during normal breathing in healthy 6 volunteers showed that the diaphragmatic movement was significantly larger on the dependent side (20.4 +/- 7.5 mm, M +/- SD) than on the other (12.1 +/- 4.4 mm). Thoracic or mediastinal movement was minimal on either side. Expired minute ventilation (VE), peak inspiratory pressure (PIP)and airway occlusion pressure (P0.1) were measured on each of the lungs separated with an endobronchial tube in 14 subjects. At light levels of halothane anesthesia (0.2-0.4% in oxygen). VE of the dependent lung was larger than that of the nondependent lung by 56%. PIP and P0.1 were also larger on the dependent side by 61% and 36%, respectively. At deeper levels of anesthesia (halothane 1.4-2.2%), the predominant ventilation of the dependent lung was absent and VE, PIP or P0.1 no longer differed significantly between the lungs. Augmented P0.1 on the dependent side seemed to be essential to produce larger PIP and VE. It is considered that the more widely stretched diaphragm on the dependent side sends stronger muscle afferent signals to increase excitability of the phrenic neurons producing augmented inspiratory driving force.(ABSTRACT TRUNCATED AT 250 WORDS)