Pulmonary vasomotor responses to neural activation in man

Passive adaptation to changes in flow is an established mechanism of regulation of pulmonary vasomotility. The role of the autonomic nervous system is not well defined in man. To separate the neural from the mechanical component (variation in flow) the venous return and the lung blood flow of six normal men (while undergoing haemodynamic diagnostic procedures) were impeded by a balloon tipped catheter placed in the inferior vena cava. Mental arithmetic and cold pressor tests were used as sympathetic activators. When the venous return was impeded cardiac output was reduced by a mean of 540 ml compared with baseline. During the arithmetic test the rise in cardiac output fell from 2140 ml when venous return was unimpeded to 890 ml when venous return was impeded. This stimulus changed from being a slight pulmonary vasodilator to being an unequivocal vasoconstrictor. Cold stimulation had little effect on cardiac output and caused an increase in pulmonary arteriolar resistance. This effect was more than doubled when flow through the lungs was impeded. These observations suggest that a neural regulation of the pulmonary circulation exists in man, which is disclosed (arithmetic test) or potentiated (cold pressor test) when the influence of mechanical factors is prevented or reduced.