Cardiovascular responses at the onset of static exercise in patients with dual-chamber pacemakers

Parasympathetic neural activity accounts for the lowering of exercise heart rate at high altitude

BACKGROUND

In chronic hypoxia, both heart rate (HR) and cardiac output (Q) are reduced during exercise. The role of parasympathetic neural activity in lowering HR is unresolved, and its influence on Q and oxygen transport at high altitude has never been studied.

METHODS AND RESULTS

HR, Q, oxygen uptake, mean arterial pressure, and leg blood flow were determined at rest and during cycle exercise with and without vagal blockade with glycopyrrolate in 7 healthy lowlanders after 9 weeks' residence at >/=5260 m (ALT). At ALT, glycopyrrolate increased resting HR by 80 bpm (73+/-4 to 153+/-4 bpm) compared with 53 bpm (61+/-3 to 114+/-6 bpm) at sea level (SL). During exercise at ALT, glycopyrrolate increased HR by approximately 40 bpm both at submaximal (127+/-4 to 170+/-3 bpm; 118 W) and maximal (141+/-6 to 180+/-2 bpm) exercise, whereas at SL, the increase was only by 16 bpm (137+/-6 to 153+/-4 bpm) at 118 W, with no effect at maximal exercise (181+/-2 bpm). Despite restoration of maximal HR to SL values, glycopyrrolate had no influence on Q, which was reduced at ALT. Breathing FIO(2)=0.55 at peak exercise restored Q and power output to SL values.

CONCLUSIONS

Enhanced parasympathetic neural activity accounts for the lowering of HR during exercise at ALT without influencing Q. The abrupt restoration of peak exercise Q in chronic hypoxia to maximal SL values when arterial PO(2) and SO(2) are similarly increased suggests hypoxia-mediated attenuation of Q.

Facial cooling-induced bradycardia: attenuating effect of central command at exercise onset

Facial cooling (FC) elicits a marked bradycardia at rest that appears to be reduced during exercise. This study was done to delineate the effects of exercise mediated central command from those of muscle afferent feedback and sympathetic stimulation on the attenuation of the bradycardic effect of FC during the onset of exercise. Ten healthy subjects (26 +/- 2 yr) were exposed to FC under five different conditions: 1) seated rest on the cycle ergometer, 2) onset of mild exercise (resting HR + 20 beats.min-1), 3) onset of moderate exercise (resting HR + 50 beats.min-1), 4) seated rest on the ergometer during electrical stimulation, and 5) seated rest on the ergometer during a cold immersion test (CT) (one hand immersed in an ice slurry at 0 degree C). The two exercise intensities were presumed to provide different degrees of central command. Electrical stimulation of the quadriceps was assumed to provide isolated muscle afferent feedback, while the CT served as a sympathetic stimulus. Beat-by-beat data were recorded for HR and mean arterial blood pressure for the duration of each test (50 s), and a rating of perceived pain was taken after each FC. FC elicited significant increases in mean arterial pressure during mild and moderate exercise compared with resting control (P < 0.05) and during moderate exercise compared to exercise without FC (P < 0.05). Mean decreases in HR during FC were similar for resting control (-12 +/- 3 beats.min-1), electrical stimulation (-10 +/- 3 beats.min-1), and CT (-9 +/- 3 beats.min-1). The HR response to FC during mild exercise (-7 +/- 2 beats.min-1) was significantly different (P < 0.05) from the rest condition; however, there was no significant bradycardia (-2 +/- 2 beats.min-1; P > 0.05) during onset of moderate exercise. These findings suggest that the magnitude of cold face-induced bradycardia may be attenuated at exercise onset by neural signals related to the higher levels of central motor command associated with heavier exercise.