Types of chronotropic effect
- Negative chronotropic effect — a decrease in heart rate, clinically manifested as bradycardia. It is observed in hypothyroidism and under the influence of $\beta_1$-blockers due to reduced sinus node automaticity.
- Positive chronotropic effect — an increase in heart rate (tachycardia). It is observed upon stimulation of $\beta_1$-adrenoceptors and in hyperthyroidism; in hyperthyroidism, a subjective sensation of palpitations may occur even at rest.
Physiological significance of the negative effect
The decrease in heart rate along with the shortening of systole leads to the formation of a prolonged diastole. This ensures the following processes:
- Resource restoration: energy reserves of cardiomyocytes are replenished.
- Improved trophism: the period of coronary blood flow is prolonged, since heart nutrition occurs during diastole.
- Transition to an "economical mode": the heart operates with increased efficiency at a minimal increase in oxygen demand.
Endocrine regulation
Heart rate is linked to thyroid function:
- In hyperthyroidism, a positive chronotropic effect occurs: tachycardia and a subjective sensation of palpitations even at rest.
- In hypothyroidism, a negative chronotropic effect (bradycardia) is observed against the background of prevailing inhibition: lethargy and general weakness.
Thyroid hormones also enhance basal metabolism, increase tissue oxygen demand, boost cardiac workload, and increase HR and cardiac output.
Pharmacological regulation and clinical significance
A number of drugs alter HR and related cardiac performance parameters:
- $\beta$-blockers (atenolol, metoprolol, bisoprolol, propranolol): block cardiac $\beta_1$-adrenoceptors, causing negative chronotropic and inotropic effects. This reduces cardiac output, decreases cardiac workload, and lowers myocardial oxygen demand. $\beta$-blockers are drugs of choice in stable angina pectoris.
- Cardiac glycosides (digoxin): used for inotropic support; of particular importance in atrial tachyarrhythmia. They cause a slowing of the rhythm, which lengthens diastole and improves filling and output.
- Quinidine: reduces automaticity and conduction, suppressing pacemakers. At the same time, it has a vagolytic effect: it increases sinoatrial node automaticity and accelerates AV conduction, counteracting the direct depressant effect.
- Antiarrhythmic drugs: may cause side effects, including negative inotropic and chronotropic effects; this is associated with the risk of worsening heart failure and causing bradycardia.
- Orciprenaline: stimulates $\beta_2$- and $\beta_1$-adrenoceptors; via $\beta_1$-stimulation, it causes a positive chronotropic effect and more pronounced tachycardia compared to selective $\beta_2$-agonists.
- Dobutamine: causes a pronounced positive inotropic effect; HR, automaticity, and conduction increase to a lesser extent than the force of contraction.
Related effects of cardiac regulation
The chronotropic effect is one of the effects of cardiac regulation. Alongside it, the following are distinguished:
- Inotropic effect — change in contraction force.
- Bathmotropic effect — change in excitability.
- Dromotropic effect — change in conduction velocity.
- Tonotropic effect — change in myocardial tone.
Class and exam questions
What is a positive chronotropic effect?
This is an increase in heart rate (tachycardia). It occurs upon stimulation of cardiac $\beta_1$-adrenoceptors and in hyperthyroidism.
Why is the negative chronotropic effect of $\beta$-blockers beneficial in stable angina pectoris?
Blockade of cardiac $\beta_1$-adrenoceptors reduces HR and cardiac contraction force. As a result, cardiac workload and myocardial oxygen demand decrease.
How do $\beta$-blockers affect the chronotropic effect?
Drugs of this group block $\beta_1$-adrenoceptors, reducing sinus node automaticity. This leads to a negative chronotropic effect — a decrease in heart rate.
What else to study on this topic
- Inotropic effect
- Dromotropic effect
- Bathmotropic effect
- Beta-adrenoceptors
- Regulation of cardiac activity
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