Key result
Muscarinic blockade with glycopyrrolate attenuated the progressive increase in heart rate sensitivity to hypoxia over 8 hours (0.10 vs 0.29 beats/min/%), but did not affect ventilation.
Why the study?
Does muscarinic blockade with glycopyrrolate affect the progressive increases in ventilation and heart rate during 8 h of isocapnic hypoxia in humans?
Does muscarinic blockade with glycopyrrolate affect the progressive increases in ventilation and heart rate during 8 h of isocapnic hypoxia in humans?
Muscarinic mechanisms mediate much of the progressive increase in heart rate, but not ventilation, during 8 hours of hypoxia in humans.
Answers are grounded in this paper and cite their sources.
May guide autonomic research in sustained hypoxia; leaves open any therapeutic role for muscarinic blockade in clinical hypoxemia.
This study examined the role of muscarinic parasympathetic mechanisms in generating the progressive increases in ventilation (V(E)) and heart rate previously reported with 8 h exposures to hypoxia. The sensitivities of V(E) (G(p)) and heart rate (G(HR)) to acute variations in hypoxia, and V(E) and heart rate during acute hyperoxia were assessed in 10 subjects before and after two 8 h exposures to isocapnic hypoxia (end-tidal P(O2) = 50 mmHg). The responses were measured during muscarinic blockade with glycopyrrolate (0.015 mg kg(-1)) and without glycopyrrolate, as a control. There were significant increases in G(p) (P < 0.01) and V(E) during hyperoxia (P < 0.01) following hypoxic exposure, but these were unaffected by glycopyrrolate. G(HR) increased significantly by 0.29 +/- 0.08 beats min(-1) %(-1) (mean +/- S.E.M.) following exposure to hypoxia under control conditions, but only non-significantly by 0.10 +/- 0.08 beats min(-1) %(-1) with glycopyrrolate. This difference was significant. Changes in heart rate during hyperoxia were slight and inconclusive. We conclude that muscarinic mechanisms play little 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. Experimental Physiology (2001) 86.4, 529-538.
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Clar et al. (2001) studied this question. Muscarinic blockade with glycopyrrolate vs. Without glycopyrrolate was evaluated on Sensitivities of ventilation and heart rate to acute variations in hypoxia (p=<0.01). Muscarinic blockade with glycopyrrolate attenuated the progressive increase in heart rate sensitivity to hypoxia over 8 hours (0.10 vs 0.29 beats/min/%), but did not affect ventilation.
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