Mechanisms mediating the heart rate response to hypoxemia

Abstract

We studied the effect of phasic pulmonary afferent information on heart rate (HR) during a progressive reduction in oxygen saturation (SaO2). The Hering-Breuer reflex was evaluated with the use of the ratio of apnea duration after lung inflation to the preceding expiratory time (dT). Phasic afferent activity was stopped in anesthetized, paralyzed dogs by constant-flow ventilation (CFV), a technique that removes cyclic changes in lung volume. During normocapnic (PaCO2 = 36.4 ± 1.1 mm Hg) spontaneous breathing, there was a wide variability in HR response, with a mean ΔHR/ΔSaO2 (± SE) of 0.62 ± 0.27 beats/min/% (values greater than 0 indicate a tachycardiac response). There was a good correlation between ΔHR/ΔSaO2 and dT (r = .79). Mean ΔHR/ΔSaO2 for the combined normocapnic and hypercapnic studies during CFV was lower (- 1.32 ± 0.19 bpm/%) than that during spontaneous breathing (0.23 ± 0.19, p < .0001). We suggest that the HR response to hypoxemia is strongly related to the strength of the Hering-Breuer reflex, which may explain the large interdog variability in HR responses.