What is chronotropic incompetence?

What is chronotropic incompetence?

Chronotropic incompetence is the inability of the heart to increase its rate according to the body’s demand, mostly with increased activity. Chronotropic incompetence can reduce exercise tolerance and impair quality of life. It is an independent predictor of adverse cardiovascular events and all-cause mortality [1]. Though increase in stroke volume and oxygen extraction increases with exercise, the predominant contribution to increase in oxygen consumption (VO₂) is by an increase in heart rate. A reduced exercise heart rate is cited as the reason for age related decline in aerobic work performance [2].

Though the intrinsic rate of the sinoatrial node is around 100/min, resting heart rate is generally in the range of 60-80/min due the influence of the parasympathetic nervous system. Increased resting heart rate would mean either increased sympathetic activity or reduced parasympathetic tone. Increased resting heart rate has been associated with increased cardiovascular death, coronary artery disease and sudden cardiac death. Chronotropic incompetence may manifest as failure to achieve the predicted maximal heart rate, inadequate submaximal heart rate or as heart rate instability during exercise. Chronotropic incompetence may be noted in sick sinus syndrome, atrioventricular block, coronary artery disease and heart failure [3].

Heart rate recovery after exercise is due to withdrawal of sympathetic activity. Attenuated heart rate recovery after exercise has been shown to be a predictor of mortality independent of angiographic severity in coronary artery disease. This was from a study of 2,935 consecutive patients who underwent symptom limited exercise testing for suspected coronary artery disease and then had a coronary angiogram within 90 days. Follow up period was 6 years. A cut off value of 12 beats/min or less during the first minute after exercise has been given. In those undergoing stress echocardiography, cutoff was 18 beats/min or less [4]. Reactivation of vagal activity is an important mechanism for heart rate recovery after exercise. It is blunted in patients with chronic heart failure while it is accelerated in athletes [5].

Chronotropic incompetence can be quantified by the chronotropic index. First the age predicted maximal heart rate is estimated using the Astrand’s formula as 220 – age in years [6]. Then the chronotropic index is calculated as follows [7]:

Chronotropic index = (peak heart rate– resting heart rate)/(age-predicted maximal heart rate – resting heart rate)

References

1. Brubaker PH, Kitzman DW. Chronotropic incompetence: causes, consequences, and management. Circulation. 2011 Mar 8;123(9):1010-20. doi: 10.1161/CIRCULATIONAHA.110.940577. PMID: 21382903; PMCID: PMC3065291.
2. Higginbotham MB, Morris KG, Williams RS, Coleman RE, Cobb FR. Physiologic basis for the age-related decline in aerobic work capacity. Am J Cardiol. 1986 Jun 1;57(15):1374-9. doi: 10.1016/0002-9149(86)90221-3. PMID: 3717040.
3. Orso F, Baldasseroni S, Maggioni AP. Heart rate in coronary syndromes and heart failure. Prog Cardiovasc Dis. 2009 Jul-Aug;52(1):38-45. doi: 10.1016/j.pcad.2009.05.006. PMID: 19615492.
4. Vivekananthan DP, Blackstone EH, Pothier CE, Lauer MS. Heart rate recovery after exercise is a predictor of mortality, independent of the angiographic severity of coronary disease. J Am Coll Cardiol. 2003 Sep 3;42(5):831-8. doi: 10.1016/s0735-1097(03)00833-7. PMID: 12957428.
5. Imai K, Sato H, Hori M, Kusuoka H, Ozaki H, Yokoyama H, Takeda H, Inoue M, Kamada T. Vagally mediated heart rate recovery after exercise is accelerated in athletes but blunted in patients with chronic heart failure. J Am Coll Cardiol. 1994 Nov 15;24(6):1529-35. doi: 10.1016/0735-1097(94)90150-3. PMID: 7930286.
6. ASTRAND I. Aerobic work capacity in men and women with special reference to age. Acta Physiol Scand Suppl. 1960;49(169):1-92. PMID: 13794892.
7. Nadruz W Jr, West E, Santos M, Skali H, Groarke JD, Forman DE, Shah AM. Heart Failure and Midrange Ejection Fraction: Implications of Recovered Ejection Fraction for Exercise Tolerance and Outcomes. Circ Heart Fail. 2016 Apr;9(4):e002826. doi: 10.1161/CIRCHEARTFAILURE.115.002826. PMID: 27009553; PMCID: PMC4807736.