Case Presentation: A 67-year-old female with hypertension, hyperlipidemia, diabetes, chronic kidney disease, sarcoidosis in remission, and rheumatic fever in childhood, presented to emergency department (ED) with right ear fullness and bloody discharge. During ED evaluation, she was found to have junctional rhythm at 50 beats per minute without evident P-wave, prompting admission for observation and further evaluation. System review was notable for long standing complaints of fatigue, poor functional capacity, and dyspnea on exertion. She also reported occasional bilateral leg swelling, but no chest pain, orthopnea or paroxysmal nocturnal dyspnea. She denied smoking, alcohol or recreational drug use history. Physical examination was unremarkable except for bradycardia and crusted blood in right ear canal. Laboratory evaluation revealed leukocytosis, hypomagnesemia and mild hyponatremia. Troponin, brain natriuretic peptide and thyroid function were within normal range. Chest X-ray was normal for age.Patient remained in junctional rhythm during admission, but blood pressure remained relatively stable on home regimen of Valsartan and Hydrochlorothiazide. Review of other home medications did not reveal any rate controlling medication, such as beta-blockers or non-dihydropyridine calcium channel blockers. Hypomagnesemia resolved with replacement, while mild hyponatremia resolved without intervention. Right ear infection was treated with Amoxicillin/Clavulanate. Echocardiogram revealed only mild aortic valve sclerosis with moderate aortic valve stenosis and borderline pulmonary pressure. A modified exercise stress test demonstrated underlying sinus rhythm that changed to junctional rhythm with maximum heart rate of 62 beats per minute with exercise (compared to 122 that corresponded to 80% age predicted maximal heart rate), consistent with chronotropic incompetence. A dual chamber pacemaker was subsequently placed, resulting in notable improvement in exercise tolerance. She was discharged in relative stable health and was scheduled for outpatient cardiology follow up and cardiac rehabilitation.

Discussion: Chronotropic incompetence is the inability to adequately increase heart rate to meet increase in metabolic demand. It is an important cause of exercise intolerance and can occur in many conditions, including sinus node dysfunction, autonomic dysfunction, heart failure, ischemic heart disease, atrial fibrillation, heart block, some medications and hypothyroidism. Diagnosis is made on the basis of inability to achieve at least 80% of age predicted maximal heart rate with exercise testing, preferably in conjunction with respiratory gas analysis.

Conclusions: Chronotropic incompetence is a common cause of exercise intolerance that is often overlooked in clinical practice, as many conditions may present similarly. It is also an independent predictor of major cardiovascular and all cause mortality, and increased screening may assist in more effective risk stratification and prognosis. It is easily diagnosed with widely available exercise testing methods and rate-adaptive pacing can improve chronotropic response and exercise capacity in appropriate cases.