Types of Chronotropic Effects
- Negative chronotropic effect: A decrease in heart rate, clinically presenting as bradycardia. It is observed in hypothyroidism and under the action of \(\beta_1\)-blockers due to reduced SA node automaticity.
- Positive chronotropic effect: An increase in heart rate (tachycardia). It is observed during \(\beta_1\)-adrenergic receptor stimulation and in hyperthyroidism; hyperthyroidism can cause a subjective sensation of palpitations even at rest.
Physiological Significance of the Negative Effect
A reduced heart rate combined with shortened systole leads to a prolonged diastole. This ensures the following processes:
- Restoration of resources: Cardiomyocyte energy reserves are replenished.
- Improved trophics: The period of coronary blood flow is extended, as myocardial perfusion occurs primarily during diastole.
- Shift to an "economy mode": The heart operates with increased efficiency while minimizing the increase in myocardial oxygen demand.
Endocrine Regulation
Heart rate is closely linked to thyroid gland function:
- Hyperthyroidism produces a positive chronotropic effect: tachycardia and a subjective sensation of palpitations even at rest.
- Hypothyroidism leads to a negative chronotropic effect (bradycardia) accompanied by generalized slowing, lethargy, and fatigue.
Thyroid hormones also increase basal metabolic rate, raise tissue oxygen demand, enhance cardiac workload, and increase both heart rate and cardiac output.
Pharmacological Regulation and Clinical Significance
Several drug classes alter heart rate and associated cardiac performance metrics:
- \(\beta\)-blockers (atenolol, metoprolol, bisoprolol, propranolol): Block cardiac \(\beta_1\)-adrenergic receptors, producing negative chronotropic and inotropic effects. This lowers cardiac output, reduces cardiac work, and decreases myocardial oxygen demand. \(\beta\)-blockers are drugs of choice for stable angina pectoris.
- Cardiac glycosides (digoxin): Used for inotropic support; particularly important in atrial tachyarrhythmias. They slow the heart rate, prolonging diastole and improving ventricular filling and output.
- Quinidine: Reduces automaticity and conduction velocity, suppressing subsidiary pacemakers. It also has vagolytic action, increasing SA nodal automaticity and accelerating AV conduction to counteract its direct depressant effect.
- Antiarrhythmic drugs: May cause adverse effects, including negative inotropic and chronotropic effects, carrying the risk of exacerbating heart failure and inducing bradycardia.
- Orciprenaline: Stimulates \(\beta_2\)- and \(\beta_1\)-receptors; via \(\beta_1\)-stimulation, it produces a positive chronotropic effect and more pronounced tachycardia compared to selective \(\beta_2\)-agonists.
- Dobutamine: Produces a marked positive inotropic effect; heart rate, automaticity, and conduction velocity increase to a lesser extent than contractile force.
Related Cardiac Regulatory Effects
The chronotropic effect is one of several fundamental properties governing cardiac regulation. Alongside it, the following are distinguished:
- Inotropic effect — change in myocardial contractility.
- Bathmotropic effect — change in excitability.
- Dromotropic effect — change in conduction velocity.
- Tonotropic effect — change in myocardial tone.