ROLE OF VAGAL FEEDBACK FROM THE LUNG IN HYPOXIC-INDUCED TACHYCARDIA IN HUMANS

We assessed the cardiovascular responses to systemic normocapnic hypox ia in five normal subjects, five double lung transplant patients with lung denervation and intact hearts, and five patients with denervated hearts. Progressive normocapnic hypoxia was induced over 10-15 min and maintained for 2-3 min each at 90, 87, 84, and 80% arterial O-2 satur ation (Sa(o2)). Normal subjects showed the most pronounced mean increa se in heart rate (dHR/dSa(o2) = 0.86 +/- 0.13 beat/min per 1% Sa(o2)). Three lung-denervated subjects had normal tachycardiac responses (1.6 , 0.9, and 0.69 beats/min per 1%), whereas the other two had distinctl y lower responses (0.34 and 0.39 beat/min per 1%). Most of the lung-de nervated subjects also showed a significant tachycardia with even mild hypoxia; none showed a bradycardiac response to any level of hypoxia. In the heart-denervated group, hypoxic tachycardia was significantly lower than normal (0.29 +/- 0.13 beat/min per 1%). We conclude that va gal feedback from the lungs is not required for the normal chronotropi c response to hypoxia in humans; however, this mechanism may contribut e significantly to the marked variability in hypoxic-induced tachycard ia found among human subjects. These data in humans contrast with the progressive bradycardiac response to hypoxia reported in vagally dener vated (or nonhyperpneic) dogs and cats.