Effects of acute hypoxia on the normal and diseased sinus node

We have studied the effects of breathing 12% oxygen in three groups of patients with sinus bradycardia: 11 with intrinsic sick sinus syndrome, 11 with extrinsic sick sinus syndrome and 11 without any evidence of sinus dysfunction. During hypoxia, the heart rate increased by an average of 6.7 beats/min in the patients with intrinsic sick sinus syndrome, 15.7 beats/min in the patients with extrinsic sick sinus syndrome and 14.3 beats/min in those with normal sinus node function. The difference in response of the heart rate between the two groups with sick sinus syndrome was highly significant (P less than 0.001). There was no difference between the three groups in the response of the blood pressure, ventilation, and partial pressures of oxygen and carbon dioxide. The increased heart rate which accompanies hypoxia is thought to be mediated through sympathetic mechanisms consequent on hyperventilation. In the intrinsic sick sinus syndrome, there is evidence that the sympathetic tone is already increased and this may limit the extent of the tachycardia due to hypoxia. Hypoxia may be a useful clinical test to distinguish between the two forms of sick sinus syndrome.