Bradycardia

Articolo redatto e revisionato del Dott. Luca Moretti

Bradycardia is a heart rate lower than expected for age, activity and the physiological condition of the body. In adults it is commonly described as a rate below 60 beats per minute, but this convention alone does not identify a disease. Sleep and physical training may be accompanied by low, well-tolerated rates; a rate that appears less markedly reduced may instead be inadequate during exertion or associated with a dangerous conduction disorder. Its significance depends on the origin of the rhythm, the response to metabolic demand and the stability of perfusion.

The assessment must distinguish slowed impulse formation, failure of conduction to the ventricles and phenomena that mimic a low rate, such as nonperfused ectopic beats or blocked premature beats. Treatment decisions are based on the diagnosis and its relationship with symptoms, with important exceptions for some high-risk atrioventricular blocks. This monograph provides the general framework; the characteristics of sinus bradycardia and the individual forms of atrioventricular block are discussed separately.

Heart rate, cardiac output and physiological adaptations

Cardiac output depends on the product of heart rate and stroke volume. A reduction in heart rate can be compensated for by greater filling and a higher stroke volume, provided that the heart has sufficient reserve. This capacity varies with contractility, diastolic function and circulating volume. A low rate in a trained individual therefore does not have the same significance as a similar rate in the presence of heart failure, hypovolemia or loss of the atrial contribution.

Heart rate must be verified with a reliable measurement. In ectopic bigeminy, some electrical beats generate weak or absent peripheral pulses and the pulse may appear very slow. An optical sensor may undercount because of movement or hypoperfusion. The ECG distinguishes a true reduction in ventricular activations from a mechanical pulse deficit. This distinction prevents a low value recorded by a home device from being automatically attributed to the sinus node.

During sleep, vagal influence often increases and the sinus rate decreases. In athletes, autonomic adaptation and intrinsic changes in the pacemaker system also contribute; interpretation requires consideration of age, training level and rhythm behavior during activity. An appropriate response to exercise and the absence of symptoms support a physiological adaptation. Very low rates, atypical pauses or associated abnormalities should nevertheless be assessed in context rather than using athlete status as a universal explanation.

The distinction between normal and pathological values cannot be entrusted to a single threshold. Documents addressing sinus node dysfunction also use rates below 50 beats per minute and pauses longer than three seconds as descriptors, but these isolated findings do not automatically define the syndrome or an indication for a pacemaker. What matters is the ability to maintain adequate circulation during different activities. The absence of complaints must be verified because a patient may have unconsciously reduced the level of exertion.

A low atrial rate and a low ventricular rate have different implications. In sinus bradycardia with intact conduction, the atria and ventricles maintain a coordinated sequence. In complete heart block, ventricular rate depends on a subsidiary pacemaker and its reliability. Two patients with the same number of QRS complexes per minute may therefore have very different risks depending on the site of the impulse generator, the stability of the escape rhythm and disease of the conduction system.

Impulse formation, conduction and major causes

Sinus node dysfunction includes inability to maintain an adequate rate, pauses and disorders of chronotropic response. It may result from degenerative and fibrotic changes, heart disease, infiltrative processes or less common predispositions. Alternation with atrial tachyarrhythmias characterizes some patients with bradycardia-tachycardia syndrome. Slowing after termination of a tachycardia requires differentiation among transient recovery, drug effect and true impairment of sinus node function.

In atrioventricular blocks, atrial activity may remain normal or accelerated while some or all impulses fail to reach the ventricles. A nodal or infranodal site influences prognosis. A wide QRS, bundle branch disease and worsening as the rate increases may point to distal involvement, but no single sign always localizes the block. A 2:1 conduction ratio alone does not permit classification as Mobitz I or Mobitz II.

Drugs are a frequent and sometimes modifiable cause. Beta-blockers, verapamil, diltiazem, digoxin and several antiarrhythmic drugs may slow impulse formation or conduction. Renal impairment, interactions and dose changes can enhance the effect. A medication taken for a long time is not automatically innocent if its elimination changes or another therapy is added. The review should include the indication and clinical necessity, avoiding uncoordinated discontinuation of essential treatments.

Other causes include hypothyroidism, hypothermia, potassium abnormalities, ischemia and inflammation. Inferior myocardial infarction may be associated with a vagal component and nodal block, whereas other ischemic syndromes may involve the conduction system more extensively. Infections or infiltrative diseases warrant further investigation when the history and age are suggestive. The presence of a correctable metabolic disorder does not exclude the possibility that it has unmasked a pre-existing substrate.

Sleep apnea may be accompanied by nocturnal bradyarrhythmias and warrants evaluation when snoring, respiratory pauses or sleepiness coexist. Treatment of the respiratory disorder may reduce the episodes, whereas bradycardia limited to sleep is not by itself an indication for a pacemaker. Congenital, postoperative and familial forms also require dedicated pathways: age at onset, neuromuscular disease and family history may point to selective genetic investigation.

Symptoms, clinical presentation and ECG interpretation

Manifestations include fatigue, reduced exercise tolerance, dyspnea, dizziness, presyncope and syncope. Vulnerable patients may develop confusion or worsening heart failure. These symptoms are nonspecific and may arise from causes independent of the rhythm. Temporal correlation is therefore essential: documenting a low heart rate on a different day from the episode does not prove that it caused the symptoms. Likewise, a normal ECG between events does not exclude intermittent block.

The initial assessment looks for hypotension with signs of low perfusion, altered consciousness, ischemia or pulmonary edema. Unstable bradycardia requires prompt treatment while the cause is being clarified. Recent syncope with suspected advanced block may be significant even if the patient is currently asymptomatic. The instantaneous rate does not summarize the risk of a subsequent pause or loss of the escape rhythm, especially in the presence of distal conduction disease.

On the ECG, atrial activity is identified first. Regularly conducted sinus P waves point toward slowing of the sinus node; P waves not followed by QRS complexes require analysis of intervals and sequences. Pauses without visible atrial activity may result from sinus arrest, sinoatrial block or a premature atrial wave hidden within repolarization. Comparison with the baseline cycle is useful, but sinus variability limits interpretations based solely on mathematical multiples of the PP interval.

Atrioventricular dissociation is not automatically equivalent to complete heart block. A junctional rhythm may compete with a slowed sinus rhythm, rendering atria and ventricles independent even though conduction may still be preserved. The presence of captures and the temporal sequence help identify interference. Conversely, in true complete heart block no atrial impulse is conducted during the observed period. This distinction profoundly changes the assessment of the need for pacing.

Pseudobradycardia caused by ectopy must be excluded. Blocked premature atrial beats may produce long slow sequences, whereas concealed junctional ectopic beats may interfere with conduction of a sinus impulse and mimic block. The search for T-wave deformation, prematurity and associated aberrant beats may provide clues. In doubtful cases, specialist review of the tracing is preferable to automatically accepting the diagnosis generated by the Holter system.

Diagnostic pathway and identification of high-risk forms

The clinical history covers the pattern of symptoms, usual activity, medications, recent infections and previous procedures. Events during exertion, at rest or during sleep are sought and previous ECGs are compared. Physical examination and blood pressure measurement help identify hypoperfusion or alternative causes such as orthostatic hypotension. A reduced heart rate and a drop in blood pressure may coexist without the former fully explaining the latter; treatment should address the components that are actually responsible.

Rhythm monitoring is adapted to the frequency of episodes. Prolonged recordings increase the likelihood of documenting intermittent disorders but also produce incidental findings. The report should distinguish daytime and nighttime events, duration, morphology and correlation with the diary. If syncopal episodes are rare and remain unexplained, an implantable monitor may be appropriate within the specialist diagnostic pathway. The absence of events during brief monitoring does not constitute definitive exclusion.

Exercise testing can assess the chronotropic response and the behavior of conduction. Failure to accelerate must be interpreted in relation to the actual workload achieved, medications, age and protocol, avoiding diagnoses based solely on a formula for theoretical maximum heart rate. Worsening of a block as the rate increases may suggest infranodal disease. The test requires appropriate selection and supervision when a potentially unstable disorder is suspected.

Blood tests are directed toward electrolytes, renal function, thyroid function and other plausible causes. Echocardiography assesses heart disease and ventricular function; advanced imaging may be useful when inflammation, infiltration or genetic disease is suspected. Not every asymptomatic bradycardia requires magnetic resonance imaging or an invasive study. An electrophysiological study may instead contribute when it is necessary to localize a disorder or clarify syncope in a patient with conduction disease.

In inherited forms, genetic testing should be guided by the phenotype and by the possibility of changing management or family screening. A progressive disorder at a young age, family history or extracardiac signs may justify further evaluation. In children, heart rate must be interpreted according to age and device indications take anatomy, growth and natural history into account. Mechanical application of adult threshold values may result in both overdiagnosis and failure to recognize an inadequate rate.

Emergency treatment and indications for pacing

In bradycardia with circulatory compromise, breathing and perfusion are supported, the rhythm is monitored and pacing measures are prepared as needed. In the European adult pathway, atropine is administered at a dose of 500 micrograms intravenously, repeatable every 3–5 minutes up to 3 mg, when appropriate for the mechanism. The algorithm does not make the drug useful in every situation: the site of the block, transplantation and other specific conditions modify the choice.

In advanced blocks with a wide QRS, atropine may be ineffective and European recommendations advise against its use, favoring a strategy appropriate to distal conduction disease. If the pharmacological response is insufficient, chronotropic infusions and temporary pacing are considered. Transcutaneous pacing may serve as a bridge, with verification of electrical capture and mechanical response; the monitor appearance alone does not prove effective cardiac output. In a conscious patient, pain and sedation require proportionate management.

Temporary transvenous pacing is considered when reliable protection is required in an unstable patient or a patient at risk. It should be accompanied by investigation and correction of the cause, for example ischemia, electrolyte imbalance or drug toxicity. Antidotes and specific treatments depend on the exposure; not every drug-induced bradycardia responds to the same intervention. Lack of an initial response does not justify indefinitely repeating an ineffective measure while delaying the necessary electrical protection.

In sinus node dysfunction, a permanent pacemaker is generally considered when symptoms are attributable to bradycardia or chronotropic incompetence and the condition is not explained by correctable causes. There is no minimum heart rate or pause duration that, by itself, mandates implantation in every patient. Therapy may also be necessary to allow indispensable medications that would otherwise cause symptomatic bradycardia, after assessment of alternatives and expected benefit.

In acquired Mobitz II, advanced or complete atrioventricular blocks not caused by reversible or physiological factors, an indication for pacing may exist regardless of symptoms. Diagnostic distinction is therefore decisive. Device selection takes into account atrial rhythm, ventricular function and the expected burden of pacing; conduction system pacing or cardiac resynchronization may be appropriate in specific settings. Preventing the effects of pacing-induced dyssynchrony is part of treatment planning, not merely of subsequent follow-up.

Prognosis, reassessment and quality of life

Prognosis depends mainly on the mechanism and substrate. A low sinus rate in a healthy, appropriately assessed individual may require no intervention; an infranodal block may instead progress to unstable escape rhythms and asystole. Resolution of bradycardia after correction of a factor does not always prove that the conduction system is intact: drugs or metabolic disorders may have unmasked pre-existing disease. Follow-up is adapted to this possibility without assuming progression in every case.

In patients managed without a device, it is necessary to clarify which symptoms should be reported and when medications and activity should be reassessed. New syncope, reduced exercise tolerance or ECG changes may alter management. Observation is an active clinical choice when the diagnosis is reassuring and risk is low; it does not mean ignoring an abnormal numerical value. It should include a comprehensible explanation of the finding and of the limitations of the assessment that was performed.

After implantation, device function, lead integrity, programming and clinical response are checked. Persistent fatigue or syncope requires investigation of causes not corrected by pacing, such as hypotension, anemia or heart disease. Normalization of heart rate does not guarantee that every symptom will disappear. The response during exercise may also require adjustments, especially when the main problem was an inadequate chronotropic increase.

In patients with a high percentage of ventricular pacing, cardiac function is followed according to risk and device type. Development of dysfunction may require reassessment of the pacing strategy and underlying heart disease. Decisions should take into account benefit, procedural complications, age and preferences. A more complex device is not automatically better: the problem to be corrected should match the functions that are actually required.

Management of physical activity, work and other activities is individualized according to symptoms, diagnosis and treatment. Generic restrictions for every low heart rate may be inappropriate, whereas episodes of loss of consciousness require specific advice until the risk has been clarified. Good communication avoids both alarm caused by physiological bradycardia and false reassurance related to the temporary absence of symptoms in a significant conduction disorder.