<unk>  <unk>  refers to the normal slowing down of heart rate during expiration (breathing out) and speeding up of heart rate during inspiration.  Heart rate is normally controlled by centers in the medulla oblongata.  One of these centers, the nucleus ambiguus, increases parasympathetic nervous system input to the heart via the vagus nerve.  The vagus nerve decreases heart rate by decreasing the rate of SA node firing.  Upon expiration, the cells in the nucleus ambiguus are activated and heart rate is slowed down.  In contrast, inspiration triggers inhibitory signals to the nucleus accumbens and consequently the vagus nerve remains unstimulated.
Respiratory sinus arrhythmia
Sinus arrhythmia
Supraventricular arrhythmias
Ectopic atrial rhythm
Wide complex tachycardia
Wandering atrial pacemaker
Sinoatrial arrest
Fascicular VT