The Bainbridge reflex (aka, atrial reflex) occurs when the heart rate increases in response to a rise in atrial pressure.
This is a compensatory mechanism since increased right atrial pressures frequently result from elevated left heart pressures from decreased cardiac output. Elevating the heart rate should increase the cardiac output.
The Bainbridge reflex acts in opposition to the carotid baroreceptor reflex which increases heart rate when the stretch is decreased in states of hypotension or hypovolemia.
Sinus arrhythmia may be explained by the Bainbridge reflex, as venous return increases during inhalation causing a brief increase in heart rate.
An ECG tracing of sinus arrhythmia