[The autonomous nervous system and pulmonary circulation. Stimulation of chemoreceptors and ventilation/perfusion...

The existence of neurogenic pulmonary vasomotrocity and of vasomotor reflexes elicited by stimulation of peripheral chemoreceptors has been demonstrated in different animal species. There are efferent noradrenergic and cholinergic nerve endings in the walls of musculo-pulmonary arteries. Stimulation of sympathetic efferents and exogenous noradrenaline cause vasoconstriction of pulmonary arteries when initial vasomotor tone is normal. There are also beta-adrenergic and cholinergic vasodilation pathways whose effects oppose the above mentioned vasoconstrictor ones, are blocked by the corresponding inhibitors, and can be demonstrated when initial vasomotor tone is high. The role of pulmonary vasomotor tone in the distribution of ventilation perfusion ratios is unknown. The stimulation of peripheral chemoreceptors causes a rise in pulmonary vascular resistance. Inversely, the stimulation of peripheral chemoreceptors by hypoxia prevents the local vasoconstrictor effect of localised alveolar hypoxia in sheep. There are thus theoretical reasons to think that the effect on regional pulmonary resistance of nervous stimulation may differ in cases of inhomogeneous lung disease, according to the local state of the pulmonary vessels. Consequently, it is not possible to anticipate the effect of stimulating chemoreceptors on the distribution of ventilation/perfusion ratios.