Expansion
Heart rate rises on inspiration and falls on expiration through varying vagal tone
Expansion
- Inspiration - heart rate increases
- Expiration - heart rate decreases
Several mechanisms contribute:
- Inspiratory inhibition of vagal outflow by the respiratory centre, the dominant factor
- Reduced intrathoracic pressure increasing venous return, stretching the right atrium and triggering the Bainbridge reflex
- Transient fall in stroke volume and blood pressure unloading the baroreceptors
The phenomenon is greatest in children and young adults and diminishes with age.
Loss of sinus arrhythmia is one of the earliest signs of cardiac autonomic neuropathy, notably in diabetes, and reduced heart rate variability more generally is an adverse prognostic marker after myocardial infarction and in heart failure.