Isoprenaline and Dopamine Infusions for Bradycardia

Definition and Pathophysiology

Bradycardia and conduction disorders arise from dysfunction within the sinus node, the atrioventricular node (AVN), or the His-Purkinje system. The underlying pathophysiology varies widely and may involve intrinsic cardiac disease, extrinsic systemic conditions, or pharmacologic effects. Inflammatory heart disease, caused by infections (viral, bacterial, protozoal, fungal, parasitic), autoimmune processes (giant cell myocarditis, sarcoidosis, rheumatic heart disease, connective tissue disease, eosinophilic myocarditis), toxins (alcohol, cocaine, cancer therapies), or physical reactions (radiation therapy), frequently involves the conduction system. In these conditions, involvement of the AVN and the conduction system occurs more often than sinus node involvement, and atrioventricular block (AVB) may indicate septal involvement in the inflammatory process.

A broad spectrum of pharmacologic agents can induce bradycardia or conduction disorders. These include beta-blockers, non-dihydropyridine calcium channel blockers, antiarrhythmic drugs (amiodarone, dronedarone, sotalol, flecainide, propafenone, procainamide, disopyramide, adenosine, digoxin, ivabradine), psychoactive and neuroactive drugs (lithium, phenothiazine, phenytoin, carbamazepine), and various chemotherapeutic agents.

In the acute setting, bradyarrhythmic cardiac arrest or asystole may result from reversible systemic insults, including hypovolemia, hypoxia, cardiac tamponade, tension pneumothorax, preexisting acidosis, drug overdose, hypothermia, and hyperkalemia. When bradycardia occurs in the context of inflammatory heart disease, it often reflects myocardial pathology that may also reduce myocardial contractility and cause ventricular fibrosis, thereby increasing the risk of ventricular arrhythmias.

Clinical Presentation and Symptoms

Patients with bradycardia or intermittent conduction disorders frequently present with presyncope or syncope. Diagnosing transient or paroxysmal AV block in these individuals can be challenging. A high suspicion of intermittent infrahisian block should be maintained, particularly in older patients and those exhibiting a bundle branch block (BBB) or an intraventricular conduction defect. In severe cases, bradycardia may progress to hemodynamic instability, presenting as shock or cardiac arrest manifesting as bradyarrhythmic arrest, asystole, or pulseless electrical activity (PEA).

Evaluation and Physical Examination

The source material does not provide specific details regarding the physical examination maneuvers for patients with bradycardia.

Diagnostics

Electrocardiography and Ambulatory Monitoring

Because bradycardia and AV block can be paroxysmal, extended ambulatory monitoring is often required to capture symptomatic events. A typical 24- to 48-hour Holter monitor is frequently insufficient. Monitors that offer real-time data transmission, such as mobile cardiac outpatient telemetry devices, are generally preferred over devices that require post-completion processing. If symptoms remain elusive and extended monitoring exceeding 3 weeks is necessary, an implantable loop recorder is essential to accurately establish the diagnosis.

Electrophysiology

In patients with presyncope or syncope—especially older individuals with BBB or intraventricular conduction defects—an electrophysiology study (EPS) may be helpful if ambulatory monitoring fails to capture an event. EPS can thoroughly assess AV conduction and may include the infusion of agents to stress the AV node and His-Purkinje system, such as isoproterenol and/or procainamide.

EPS with isoproterenol is also recommended for risk stratification in specific populations, such as asymptomatic patients with pre-excitation who have high-risk occupations or participate in competitive athletics. In the context of Brugada syndrome (BrS), pharmacologic challenge with sodium channel blockers (ajmaline or flecainide) is utilized for diagnostic unmasking; if ventricular tachycardia/ventricular fibrillation (VT/VF) is induced during these tests, intravenous isoproterenol is administered as part of the acute management.

Biomarkers and Laboratory Findings

The source material does not provide information on biomarkers or routine laboratory findings for bradycardia.

Treatment and Management

General Principles

In acute, life-threatening settings, bradycardia should be treated similarly across different patient populations, including during pregnancy. Reversible causes of bradyarrhythmic arrest or PEA—such as hypovolemia, hypoxia, cardiac tamponade, tension pneumothorax, acidosis, drug overdose, hypothermia, and hyperkalemia—must be promptly identified and treated.

Drugs cannot be relied upon to reliably increase heart rate for more than several hours to several days without producing significant side effects. Therefore, temporary or permanent pacemaker insertion is the definitive therapy for symptomatic AV block. Pharmacologic infusions serve primarily as a bridge to adequate pacing therapy or in scenarios where the block is expected to be temporary.

Isoproterenol Infusion

Isoproterenol is a catecholamine that provides pure beta-adrenergic agonist activity (B1 and B2), making it useful for increasing heart rate and improving cardiac output. It can be used transiently to treat heart block at any site. In the setting of cardiac arrest, isoproterenol may be used to treat primary or post-defibrillation bradycardia when heart rate control is the primary goal. Intravenous isoproterenol infusions at doses up to 15 to 20 µg/min have been used to elicit spontaneous electrical activity or increase the rate of bradycardia, though success is limited.

Isoproterenol is also indicated for the treatment of bradyarrhythmias, especially in torsade des pointes and Brugada syndrome. In BrS patients suffering electrical storm, isoproterenol infusion is recommended. If VT/VF occurs during sodium channel blocker challenge for BrS, intravenous isoproterenol is administered alongside intravenous sodium bicarbonate.

In inflammatory heart disease complicated by bradycardia, intravenous isoprenaline can serve as a backup to specific therapy or temporary pacing. However, isoproterenol must be used with extreme caution or avoided entirely in patients with acute myocardial infarction (MI); in these circumstances, transcutaneous or temporary transvenous pacing is preferable. During pregnancy, isoproterenol can be used when the benefits outweigh the risks, though it is unknown if it is excreted in human milk.

Dopamine Infusion

Dopamine is a catecholamine with dose-dependent receptor binding (alpha-1, beta-1, beta-2, and dopamine receptors). It is indicated for shock (vasodilatory and cardiogenic) and for symptomatic bradycardia unresponsive to atropine or pacing. In the management of heart block, when the chronotropic effect of epinephrine is undesirable, dopamine is preferable to norepinephrine for its inotropic effect. Higher dosages of dopamine stimulate alpha adrenoceptors, elevating aortic diastolic pressure through peripheral vasoconstriction, which increases cerebral and myocardial flow during resuscitative efforts.

Pacing Strategies

For short-term therapy, when AV block is likely temporary, vagolytic agents (atropine) are useful for AV nodal disturbances. However, for symptomatic AV block or high-grade AV block (e.g., infrahisian, type II AV block, third-degree heart block not caused by congenital AV block), permanent pacemaker placement is the treatment of choice.

There is growing evidence that patients with AV block and pre-existing left ventricular dysfunction may benefit from more physiologic pacing (biventricular or conduction system pacing) rather than right ventricle-only pacing to prevent the development or progression of heart failure symptoms. In inflammatory heart disease, if bradycardia does not resolve within a clinically reasonable period or cannot be expected to resolve (e.g., after radiation therapy), permanent pacing is indicated. Before choosing a device type, the potential need for an implantable cardioverter-defibrillator (ICD) or cardiac resynchronization therapy (CRT) should be considered, as the underlying inflammatory process may also reduce contractility and cause ventricular fibrosis.

Pacing indications (temporary and permanent) do not differ between pregnant and non-pregnant women. Pacemakers can be safely implanted during pregnancy using standard methods with minimal fluoroscopy or non-fluoroscopic methods. To date, no clear benefit has been established for transcutaneous pacing in the setting of bradyarrhythmic or asystolic arrest.

Drugs, Doses, and Practical Considerations

Isoprenaline (Isoproterenol)

Practical Considerations: Isoproterenol should be used with extreme caution or not at all in patients with acute MI. It is utilized during EPS to stress the AV node and His-Purkinje system and is the agent of choice for managing electrical storm in Brugada syndrome.

Dopamine

Practical Considerations: At lower doses (2-4 µg/kg/min), dopamine provides inotropy and vasodilation. At higher doses (>5 µg/kg/min), it provides inotropy and vasoconstriction. Higher dosages stimulate alpha adrenoceptors, which is important during definitive resuscitative efforts to elevate aortic diastolic pressure and improve myocardial and cerebral flow.

Other Relevant Vasoactive and Chronotropic Agents

Guideline Recommendations

Specific guideline recommendations regarding the use of isoproterenol and related therapies include:

• EPS with isoproterenol is recommended (Class I, Level B) to risk stratify individuals with asymptomatic pre-excitation who have high-risk occupations or participate in competitive athletics.

• Isoproterenol infusion should be considered (Class IIa, Level C) in Brugada syndrome patients suffering electrical storm.

• Sodium channel blocker tests are not recommended (Class III, Level C) in patients with a prior type I Brugada pattern.

• In all patients with Brugada syndrome, avoidance of drugs that induce ST-segment elevation in right precordial leads, avoidance of cocaine and cannabis, and treatment of fever with antipyretics are recommended (Class I, Level C).

Prognosis and Follow-up

The prognosis of bradycardia depends heavily on the underlying etiology. For instance, in inflammatory heart disease, AV block is a predictor of adverse outcome and may indicate septal involvement. If bradycardia is secondary to a reversible cause or acute inflammatory process, it may resolve spontaneously or with immunosuppressive therapy. However, if it does not resolve within a clinically reasonable period—or if it is irreversible, such as after radiation therapy—permanent pacing is required.

In conditions such as inappropriate sinus tachycardia (IST), the prognosis is benign, and treatment is aimed at symptom reduction. For patients requiring pharmacologic rate control, ivabradine should be co-administered with a beta-blocker when possible, though it should be avoided during pregnancy and breastfeeding and used cautiously with cytochrome p450 (CYP) 3A4 inhibitors or inducers. Catheter ablation is not recommended for routine management of IST due to limited and disappointing evidence.