Chronotropic incompetence: inability to increase heart rate
Chronotropic incompetence implies that the heart is not capable of increasing the heart rate adequately. For example, during physical exercise. Individuals with chronotropic incompetence experience reduced exercise capacity, as well as pronounced fatigue and dyspnea during physical activity.
Chronotropic incompetence is an independent predictor of overall and cardiovascular mortality. The condition is very common among individuals with heart failure. Beta-blockers actually increase chronotropic competency in heart failure, despite its negative chronotropic effect. Among persons who do not suffer from heart failure, chronotropic incompetence may be caused by beta-blockers, amiodarone, digitalis. Moreover, sinus node dysfunction is a common cause of chronotropic incompetence.
Chronotropic incompetence is defined as failure to reach 80% of the expected maximum heart rate (age-adjusted). To determine this it is necessary to perform an exercise stress test, during which it is fundamentally important that the patient performs maximally. Only two variables are needed to determine whether chronotropic incompetence exists, namely age and heart rate. The following equation is used:
The numerator is the achieved increase in heart rate and the denominator is the expected increase in heart rate (220 – age estimates the age-adjusted max heart). Thus, the formula yields how large (%) the increase in heart rate was in relation to the expected increase. The cut-off for chronotropic incompetence is 80% (i.e less than 80% diagnoses chronotropic incompetence)