A 48-year-old man, who engaged in regular physical exercise, went to see his physician because of recurrent headaches. Physical examination revealed that the patient had a mean heart rate of 55 beats/min. His physician noted that the patient's cardiac rhythm varied substantially with the phases of respiration; the heart rate increased during inspiration and decreased during expiration. QUESTION: What changes in cardiac sympathetic and parasympathetic activity take place during the respiratory cycle? QUESTION: Are the respiratory fluctuations in heart rate produced by the rhythmic changes in sympathetic activity, in parasympathetic activity, or both?
A 48-year-old man, who engaged in regular physical exercise, went to see his physician because of recurrent headaches. Physical examination revealed that the patient had a mean heart rate of 55 beats/min. His physician noted that the patient's cardiac rhythm varied substantially with the phases of respiration; the heart rate increased during inspiration and decreased during expiration.
QUESTION: What changes in cardiac sympathetic and parasympathetic activity take place during the respiratory cycle?
QUESTION: Are the respiratory fluctuations in heart rate produced by the rhythmic changes in sympathetic activity, in parasympathetic activity, or both?
The physician diagnosed this patient's headaches as migraine. He advised the patient to take propranolol, a β-adrenergic receptor antagonist, to relieve the headaches. The physician noted that after the patient had taken the propranolol, the mean heart rate diminished very slightly, and the respiratory fluctuations in heart rate were not appreciably different from those observed before the propranolol was taken.
QUESTION: Does the failure of propranolol to induce a substantial change in mean heart rate or in the respiratory fluctuations in heart rate necessarily signify that the patient's cardiac sympathetic neural activity was negligible at the time he was being examined? Explain.
Step by step
Solved in 3 steps with 2 images
