What is the mechanism and physiological significance?
A decrease in heart rate (HR) is described as a negative chronotropic effect.
- Electrophysiological mechanism — slowing of the spontaneous diastolic depolarization process in the membranes of pacemaker cells (P-cells) of the sinoatrial node.
- Formation of prolonged diastole — consists of the shortening of systole and the overall slowing of the rhythm.
- Physiological significance — prolongation of diastole is important because the nutrition of the heart (coronary blood flow) occurs precisely during this phase. This ensures the restoration of the energy resources of cardiomyocytes. As a result, the heart switches to an "economy mode" of operation: an increase in performance is observed with a minimal increase in oxygen demand.
Indicators and criteria
- Physiological norm — regular sinus rhythm with an HR of less than 60 beats per minute (typically 40–60 bpm at rest).
- Hemodynamically significant bradycardia — a condition requiring medical intervention. Criteria include HR < 40 bpm and/or the presence of pauses greater than 300 ms (clinically significant pauses are usually greater than 3000 ms, or 3 seconds).
- In the treatment of chronic heart failure — a decrease in HR < 50 bpm while taking $\beta$-blockers is an indication to reduce their dose by half (complete discontinuation is rarely required, only in case of marked deterioration).
Why does bradycardia occur?
Slowing of the rhythm can be either a variant of the norm or the result of various pathologies or external influences.
- Physiological norm — found in practically healthy individuals. The mechanism is associated with increased vagal tone (vagotonia) against the background of reduced sympathetic nervous system influence.
- Pathological conditions — decreased thyroid function (hypothyroidism, including cretinism in children, where bradycardia is combined with arterial hypotension); anorexia nervosa (accompanied by hypotension and hypothermia); hemorrhagic fever with renal syndrome (HFRS) in the oliguric phase (the appearance of tachycardia at this moment is an unfavorable sign indicating myocardial damage and risk of complications).
- Fetal pathology — bradycardia acts as a risk factor for periventricular leukomalacia (along with acidosis and hypotension).
- Iatrogenic (drug-induced) causes — administration of antiarrhythmic drugs, particularly amiodarone, and $\beta$-blockers.
- Intoxications — poisoning with organophosphorus compounds (OPCs), whose action is opposite to atropine, as well as poisoning with sleeping pills.
Clinical significance and management
With the development of hemodynamically significant sinus bradycardia or atrioventricular (AV) block with an inadequate escape rhythm, the main goal is hemodynamic stabilization.
- Mandatory actions — ECG recording to exclude blocks and conduction disturbances, review of concomitant therapy affecting HR, and determining the cause of the rhythm slowing.
- Drug therapy (first line) — intravenous administration of agents with a positive chronotropic effect. Atropine (at a dosage of 0.6–1.0 mg) and epinephrine (adrenergic and dopaminergic agents) are used.
- Temporary cardiac pacing (TCP) — indicated when atropine is ineffective and marked bradycardia persists. The method involves temporary endocardial pacing.
Class and exam questions
In which phase of the cardiac cycle does myocardial nutrition occur, and how does bradycardia affect it?
Heart nutrition (coronary blood flow) occurs during diastole. In bradycardia, diastole is prolonged, which ensures the restoration of the energy resources of cardiomyocytes and switches the heart to an "economy mode".
What electrophysiological process underlies sinus-node bradycardia?
The basis is the slowing of the process of spontaneous diastolic depolarization of the membranes of pacemaker cells (P-cells) in the sinoatrial node.
What drugs are used for emergency care in hemodynamically significant bradycardia?
As a first-line treatment, intravenous administration of agents with a positive chronotropic effect is used: atropine (at a dose of 0.6–1.0 mg) or epinephrine. If atropine is ineffective and marked bradycardia persists, temporary cardiac pacing (TCP) is used.
What else to study on this topic
- Spontaneous diastolic depolarization
- Sinoatrial node (SAN)
- Atrioventricular (AV) block
- Influence of the autonomic nervous system on the heart
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