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In respiratory arrhythmia, during inspiration, what will happen to heart rate?

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Respiratory (Sinus) Arrhythmia - Effect of Inspiration on Heart Rate

During inspiration, heart rate INCREASES.
This is a normal, physiological phenomenon - not a pathological arrhythmia despite its name.

What happens across the full respiratory cycle?

PhaseHeart RateMechanism
InspirationIncreasesWithdrawal of vagal (parasympathetic) tone
ExpirationDecreasesReturn/increase of vagal tone

Mechanism

The key driver is vagal (parasympathetic) tone modulation:
  1. During inspiration, lung inflation activates pulmonary stretch receptors, which inhibit the cardioinhibitory center in the medulla. This causes a withdrawal of vagal tone to the sinoatrial (SA) node - with less parasympathetic braking, the SA node fires faster, increasing heart rate.
  2. During expiration, vagal tone is restored, slowing the SA node back down.
This reflex-mediated variability is sometimes called heart rate variability (HRV) and is considered a marker of healthy autonomic function. Deepening respirations exaggerates these cyclic changes.
As Goldman-Cecil Medicine explains:
"With inspiration, a withdrawal of vagal tone increases heart rate; by comparison, expiration is associated with a drop in heart rate."

Key Points

  • Sinus arrhythmia is normal and adaptive - not pathological
  • It is most prominent in young, healthy individuals and diminishes with age
  • Reduced or absent respiratory sinus arrhythmia can indicate autonomic neuropathy (e.g., in diabetes)
  • Clinically, the deep breathing test (6 breaths/min) is used to quantify vagal function - a heart rate variation of less than 10-15 bpm suggests vagal dysfunction
  • The irregularity seen on ECG reflects changing R-R intervals in sync with the respiratory cycle
  • Medical Physiology (Boron & Boulpaep), p. 730
  • Goldman-Cecil Medicine, Chapter 51
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