Cardiac Autonomic Neuropathy (CAN) is a serious medical condition that often creates instability in heart rate control as well as complications with central & peripheral vascular dynamics. Cardiac autonomic neuropathy has been linked to a significantly greater risk of mortality due to autonomic performance of the heart. Patients with CAN often experience incidence of asymptomatic (silent) ischemia, myocardial infarction with a decreased likelihood of survival after myocardial infarction.
Cardiac autonomic neuropathy can be found in the elderly (age induces autonomic decline) but CAN is most common in patients with diabetes. CAN is known to occurs in approximately 17% of patients with type 1 diabetes and approximately 22% of those with type 2. An additional 9% of type 1 patients and 12% of type 2 patients have borderline autonomic dysfunction where close investigation is recommended. In a review of several epidemiological studies among individuals with diabetes, the 5-year mortality rate is five times higher for individuals with cardiac autonomic neuropathy than for individuals without CAN. The stronger association observed in studies defining cardiovascular autonomic neuropathy by the presence of two or more abnormalities may be due to more severe autonomic dysfunction in these patients or a higher frequency of other co-morbid complications that contributed to their higher mortality risk.
Symptoms of Cardiac Autonomic Neuropathy (CAN)
- Orthostatic hypotension fall in systolic blood pressure of greater than 30 mm Hg upon standing.
- Resting Tachycardia
- Neurocardiogenic Syncope
- Postural Orthostatic Tachycardia Syndrome (POTS)
- Exercise Intolerance
- Silent Myocardial Ischemia
ANS Testing for Cardiac Autonomic Neuropathy (CAN)
Autonomic nervous system (ANS) testing with Heart rate variability (HRV) is considered the earliest indicator and most frequent finding in symptomatic cardiovascular autonomic dysfunction. ANS testing uses beat by beat blood pressure, ECG or pulse wave form, and heart rate variability (HRV) to assess R-R intervals & blood pressure changes with several challenges. The main challenges of ANS testing include resting heart rate & blood pressure, valsalva maneuver, deep breathing & the sit to stand challenge.
ANS testing reveals that diabetic patients often have an increased resting heart rate most likely due to the vagal cardiac neuropathy with no balancing cardiac sympathetic activity. Patients with cardiac autonomic neuropathy often demonstrate lower cardiac ejection fraction, decreased diastolic filling and systolic dysfunction. Further, exercise intolerance may be present due sympathetic and parasympathetic dysfuntion that normally increase cardiac output and redirect peripheral blood flow.
ANS testing has demonstrated a prolonged corrected QT interval and QT dispersion (the difference between the longest and shortest QT interval) indicates an imbalance between right and left sympathetic innervation. CAN patients with a regional sympathetic imbalance and QT interval prolongation may be at greater risk for arrhythmias. Autonomic denervation in the myocardium combined with altered vascular responsiveness (in diabetic autonomic neuropathy) may a precursor to lethal arrhythmias and sudden cardiac death.
Diabetic patients have a high rate of coronary heart disease, which may be asymptomatic suggesting cardiac autonomic neuropathy. Silent ischemia is significantly more frequent in patients with CAN than in those patients without CAN (38% versus 5%). The cause of silent myocardial ischemia may be controversial, but a reduced ability to detect (feel) ischemic pain may affect the patient response or delay the appropriate care thus leading to greater incidences of mortality.
In non-diabetic patients, acute myocardial infarction has a circadian variation with a significant morning peak. The characteristic diurnal variation in the onset of myocardial infarction is altered in diabetic patients, with a lower morning peak and a higher percentage of infarction during evening hours. The blunted morning surge of incidence of myocardial infarction results from altered autonomic balance in patients with cardiac autonomic neuropathy. Mortality rates after a myocardial infarction are also higher for diabetic patients than for non diabetic patients. It is known that autonomic insufficiency increases the incidence of ventricular arrhythmias and cardiovascular events post myocardial infarction.