Effects of 8 h of isocapnic hypoxia with and without muscarinic blockade on ventilation and heart rate in humans

This study examined the role of muscarinic parasympathetic mechanisms in ge nerating the progressive increases in ventilation (V-E) and heart rate prev iously reported with 8 h exposures to hypoxia. The sensitivities of V-E (G( p)) and heart rate (GHR) to acute variations in hypoxia, and V-E and heart rate during acute hyperoxia were assessed in 10 subjects before and after t wo 8 h exposures to isocapnic hypoxia (end-tidal Po-2 = 50 mmHg). The respo nses were measured during muscarinic blockade with glycopyrrolate (0.015 mg kg(-1)) and without glycopyrrolate, as a control. There were significant i ncreases in G(p) (P < 0.01) and V-E during hyperoxia (P < 0.01) following h ypoxic exposure, but these were unaffected by glycopyrrolate. G(HR) increas ed significantly by 0.29 +/- 0.08 beats min(-1) %(-1) (mean +/- S.E.M.) fol lowing exposure to hypoxia under control conditions, but only non-significa ntly by 0.10 +/- 0.08 beats min(-1) %(-1) with glycopyrrolate. This differe nce was significant. Changes in heart rate during hy conclude that muscarin ic mechanisms play tittle role in the progressive ventilatory changes that occur over 8 h of hypoxia, but that they do mediate much of the progressive increase in heart rate.