Expansion
Intrinsic sinoatrial rate is about 100; resting vagal tone brings it down to about 70
Expansion
- The intrinsic rate of the isolated sinoatrial node is about 100 beats per minute
- Resting heart rate is about 70, because continuous vagal tone suppresses it
- Removing vagal tone with atropine raises the rate towards the intrinsic value
- A transplanted (denervated) heart therefore has a resting rate of around 100 and responds slowly to exercise, relying on circulating catecholamines
Vagal effects are also fast in onset and offset, acting through directly gated potassium channels, whereas sympathetic effects are slower, acting through a second messenger cascade. This is why beat-to-beat variation in rate is largely vagal.
Sinus arrhythmia, the quickening of the heart on inspiration, reflects respiratory modulation of vagal tone. It is a normal finding in the young and its loss is a marker of reduced heart rate variability.