Physiology of Heart Rate and Breathing

Study Notes

The autonomic nervous system regulates vital functions like blood pressure, heart rate, temperature regulation, and respiration.
Autonomic function tests during exercise measure how different body systems controlled by the autonomic nervous system respond to stimulation. These tests include heart rate variability during respiration, heart rate variation during postural change, heart rate response to the Valsalva maneuver.

Elevated heart rate characterized by impulses exceeding 100 beats per minute in a typical adult.
Physiological causes include exercise and stress.
Pathological causes can be fever or hyperthyroidism.
Often asymptomatic unless very rapid; symptoms may include palpitations, a racing heartbeat, or a feeling of "fluttering" in the chest.
Treatment often involves addressing the underlying cause. Beta-blockers can be beneficial if the cause is sympathetic overactivity.

Bradycardia is a potentially serious condition where the heart doesn't pump enough oxygen-rich blood to the body, potentially leading to insufficient oxygen supply to vital organs and tissues.
Symptoms can include near-fainting, fainting (syncope), fatigue, chest pains, and exhaustion during physical activity.
Possible causes include vagal stimulation, acetylcholine, digitalis, and beta-blockers.
A slow heart rate can be normal for well-trained athletes.

Exercise alters action potential duration, conduction velocity, and contractile velocity, which impact heart rate.
Heart rate increases linearly with exercise intensity up to a maximum heart rate.
Failure of heart rate to increase or an abnormally slow increase during exercise (chronotropic incompetence) might indicate electrical conduction pathway disease or drug-induced effects like beta-blockers or calcium channel antagonists.
ECG changes during exercise include decreased RR interval, minimal shortening of QRS complex, and rate-related shortening of the QT interval.
Sometimes ECG abnormalities are only detectable during exercise (stress test or treadmill test).

Heart rate rises during inhalation and falls during exhalation.
This fluctuation in heart rate is termed respiratory sinus arrhythmia (RSA).
It's a normal physiological phenomenon that couples the heart's rhythm with breathing.
RSA improves pulmonary gas exchange at rest and its value during exercise increases due to increased ventilation.

Increased heart rate during inhalation is due to decreased cardiac vagal activity.
Decreased heart rate during exhalation is due to increased vagal activity.
Respiratory sinus arrhythmia is an indication of cardiac vagal function.

Electrodes are attached for recording lead II.
Subjects breathe deeply at a six breaths per minute pace (5-second cycles of inhalation/exhalation).
Record maximum and minimum heart rates during each respiratory cycle.
Average these three rates.
Normal heart rate variation is greater than or equal to 15 beats per minute; borderline values range from 11 to 14 beats per minute; abnormal values are less than or equal to 10 beats per minute.