Autonomic Responses During Exercise

Autonomic Response During Exercise

• The lecture covers autonomic responses during exercise, specifically focusing on heart rate variations.

Heart Rate Variation During Postural Change

• Heart rate (HR) is continuously monitored for 30 seconds before and 60 seconds after standing.
• In healthy individuals, HR reflexively accelerates around 15 seconds after standing and gradually returns to near-supine levels.
• Failure of HR to increase with orthostatic hypotension indicates potential autonomic dysfunction.

Causes of Autonomic Dysfunction

• Diabetes Mellitus: A common cause of autonomic dysfunction.
• Nutritional Deficiencies: Vitamin B12 deficiency can contribute to the problem.
• Toxic/Metabolic Causes: Conditions like porphyria can affect the autonomic nervous system.
• Central Nervous System Issues: Cerebral vascular accidents, central hemorrhages, and syringomyelia can be contributing factors.
• Primary/Idiopathic Degeneration: Degeneration of postganglionic autonomic fibers can also lead to dysfunction.

Heart Rate Response to Valsalva Maneuver

• The Valsalva maneuver involves forced exhalation against resistance.
• This maneuver creates complex changes in cardiovascular function.
• It affects intrathoracic and intra-abdominal pressure, and subsequently blood pressure (BP).
• The maneuver is a simple, relatively inexpensive, non-invasive and reproducible method.
• The procedure requires maintaining resistance for 15 seconds, with intrathoracic pressure around 40 mmHg.
• Jugular vein distension is a crucial sign of proper maneuver execution.

Phases of Valsalva Maneuver

• Phase I (onset of strain): Blood pressure slightly increases due to increased intrathoracic pressure.
• Phase II (continued strain): Arterial pressure decreases, heart rate increases. Impeding venous return with decreased cardiac output causes a drop in arterial pressure. The reduced pressure is detected by arterial baroreceptors. This triggers increased sympathetic activity, leading to tachycardia.
• Phase III (release): Further blood pressure drop due to sudden reduction in intrathoracic pressure, and heart rate increase is sustained.
• Phase IV (recovery): Associated with increased cardiac output, resulting in a transient hypertension, and subsequently reflex bradycardia. Venous return restores diastolic heart filling, improving cardiac function and leading to increased stroke volume and cardiac output.

Pressure Overshoot

• During phase IV (recovery), blood pressure typically rises 20-40 mmHg above baseline values.
• In some individuals, it can even rise to 80 mmHg above the control value. This is due to continuing noradrenaline circulation, which prevents vessel dilation, and also the inertia of constricted vessels, and high blood volume ejection from the left ventricle, leading to significant increase in arterial pressure.

Reflex Bradycardia

• The increased pressure stimulates arterial baroreceptors, initiating a reflex slowing of the heart rate.
• While the Valsalva maneuver is a standard method for autonomic testing, it can have negative effects.
• It can lead to increased intra-ocular pressure potentially resulting in retinal or macular hemorrhage.

This quiz explores autonomic responses, particularly heart rate variations during postural changes in healthy individuals. It also addresses the causes of autonomic dysfunction, including diabetes and nutritional deficiencies. Test your knowledge on how these factors influence heart rate during exercise.