Heart rate variability (HRV), usually performed from 24 hour ambulatory ECG recordings, measures the variability of the heart rate. Alterations in autonomic balance and baroreceptor function, increased plasma norepinephrine levels, and reduced HRV have been associated with progressive myocardial failure. Not only has reduced HRV been associated with the degree of myocardial failure, possibility of prognostic significance, but also increased sudden death risk in both ischemic and nonischemic cardiomyopathy in humans. Heart rate variability analysis in the dog is confounded by pronounced sinus arrhythmia, and whether or not this technique has clinical utility in the dog remains to be demonstrated.