Influence of controlled breathing with diminished tidal volume on hypoxic heart rate response in man.

In 12 normal healthy males, the effect of preventing augmentation in tidal volume (VT) on the heart rate (HR) was examined during isocapnic progressive hypoxia. The amount of diminished tidal volume (delta VT) was 0.61 +/- 0.36 liters from the maximal VT value attained in the free breathing hypoxia. Hypoxic HR response in terms of arterial oxygen saturation (SaO2)/HR response slope (delta HR/delta SaO2) was significantly decreased from the free to controlled breathing (p less than 0.05). The effective VT range for eliciting such a HR change appeared to be more than 1,000 ml as has been demonstrated in the Hering-Breuer reflex on ventilation in humans. However, the magnitude of delta HR/delta SaO2 decrement was not significantly correlated with delta VT, suggesting that the command signal from the higher nervous center interfered with the expected response. Contrary to the HR response, the hypoxic ventilatory and occlusion pressure responses were not significantly influenced by the controlled breathing. We conclude that the hypoxic HR response is significantly depressed by the diminished VT breathing.