Cardiac responses to hypoxia and hypercapnia in spinal man

The purpose of this study was to evaluate the effect of interruptionof the descending supraspinal sympathetic outflow on heart ratecontrol during exposures to chemical stimuli. We investigatedthe heart rate responses to progressive isocapnic hypoxia andhyperoxic hypercapnia using the rebreathing technique and quantifiedthe relationship between heart rate (HR), oxygen saturation(SaO₂), alveolar PCO₂ (P_ACO₂), and minute ventilation (V_E) in16 chronic tetraplegic subjects with low cervical spinal cordtransection. The HR responses were determined from the linearslopes of HR on SaO₂ and HR on P_ACO₂. We found that mean restingheart rate was within normal range; 66 ±3 (SEM) beatsmin^–1. HR increased as oxygenation fell or CO₂ tensionrose. The mean tetraplegic HR/SaO₂ was 0.83 ± 0.14 beatsmin^–1 per 1% fall in SaO₂ and that of HR/P_ACO₂ was 0.30± 0.13 beats min^–1 per mmHG rise in P_ACO₂. TheHR and V_E responses to either hypoxia or hypercapnia were relatedin the tetraplegic subjects. We conclude that the stimulatoryHR reponses to chemical stimuli are not suppressed by cervicalspinal cord transection. Thus, the descending sympathetic activitydoes not underlie the HR acceleration by chemical stimuli.