Heart rate variation with the respiratory cycle, known as respiratory sinus arrhythmia (RSA), is a well-documented physiological phenomenon in humans and many vertebrates. It reflects the interaction between respiration and autonomic regulation of the heart.

How Heart Rate Changes During Respiratory Phases

• Inspiration: Heart rate increases. This occurs primarily due to a decrease in vagal (parasympathetic) tone during inspiration, leading to less inhibition of the sinoatrial (SA) node.

• Expiration: Heart rate decreases because of restoration of vagal tone, resulting in increased parasympathetic influence that slows the heart rate.

Physiological Mechanism

The mechanism behind RSA involves:

• The central respiratory centers modulate vagal motor neurons controlling the heart.

• During inspiration, these neurons are inhibited, reducing vagal output to the SA node.

• Reduced parasympathetic activity causes heart rate acceleration.

• Conversely, during expiration, vagal activity increases, slowing the heart rate.

This leads to a cyclical pattern of heart rate variation synchronized with breathing.

Evaluating the Options

1. Heart rate accelerated during expiration, no change during inspiration — Incorrect.

2. Heart rate accelerated during inspiration and decelerated during expiration — Correct.

3. Heart rate accelerated during expiration and decelerated during inspiration — Incorrect.

4. Heart rate accelerated during inspiration and no change during expiration — Incorrect; heart rate decreases during expiration.

Summary

The correct description of heart rate variation during respiratory phases is:

(2) The heart rate is accelerated during inspiration and decelerated during expiration.

Clinical Relevance

• RSA is a marker of cardiac vagal tone and autonomic nervous system health.

• It typically decreases with age or certain cardiovascular diseases but is considered a normal, healthy finding in young individuals.

• RSA improves efficiency by matching heart rate to pulmonary ventilation, optimizing gas exchange.