From the Guidelines
Isoprenaline should be used with caution in tachybrady syndrome due to its potential to increase myocardial oxygen demand and decrease coronary perfusion, which may be harmful in settings with concern for coronary ischemia. The mechanism of action of isoprenaline in tachybrady syndrome involves stimulating beta-1 receptors in the heart, increasing sinoatrial node automaticity and atrioventricular conduction, thereby preventing symptomatic bradycardia 1.
Key Points
- Isoprenaline is a non-selective beta-adrenergic receptor agonist that increases heart rate during bradycardic episodes.
- It works by stimulating beta-1 receptors, increasing intracellular cAMP levels, enhancing calcium influx into cardiac cells, and strengthening cardiac contractility while accelerating heart rate.
- The typical dosage ranges from 1-20 mcg/min administered as an intravenous infusion, titrated to achieve the desired heart rate response 1.
- Isoprenaline is generally used as a bridge therapy until a permanent pacemaker can be implanted, which is the definitive treatment for tachybrady syndrome.
Considerations
- Patients should be continuously monitored for potential side effects, including tremor, palpitations, headache, and rarely, angina in susceptible individuals.
- Two RCTs of isoproterenol as adjunctive therapy in the setting of cardiac arrest did not show improved return of spontaneous circulation or survival to hospital discharge 1.
- A trial of 82 patients presenting with unstable bradycardia refractory to intravenous fluid bolus and atropine randomized to transcutaneous pacing or dopamine showed no difference in survival to hospital discharge or serious adverse events 1.
From the Research
Mechanism of Action of Isoprenaline in Tachy-Brady Syndrome
- Isoprenaline is a beta-adrenergic drug used to increase heart rate and facilitate the induction of supraventricular and ventricular tachycardias 2.
- In tachy-brady syndrome, isoprenaline can exert paradoxical effects, including a decrease in heart rate, which is a rare phenomenon 2.
- The mechanism of isoprenaline's effects on cardiac pacemaking and driving in sick sinus syndrome is controversial and unresolved, but mathematical models suggest that it can increase the amplitude of action potential and diminish the bradycardic effect of mutations 3.
- Isoprenaline can also increase the heart rate and improve symptoms in patients with unstable third-degree atrioventricular block, with a more favorable pharmacological profile compared to other drugs 4.
Effects of Isoprenaline on Heart Rate and Rhythm
- Isoprenaline infusion can increase the heart rate to 100-140 bpm in most patients, but can also induce bradycardia in some cases, including sinus or junctional bradycardia and second-degree atrioventricular block 2.
- The occurrence of paradoxical bradycardia during isoprenaline infusion is a rare finding, but can be a sign of hypervagotonia or underlying cardiac conduction disturbances 2.
- Isoprenaline can also induce arrhythmias, including supraventricular and ventricular tachycardias, but its use can be beneficial in certain cases, such as in the management of unstable third-degree atrioventricular block 4.
Clinical Implications of Isoprenaline Use
- Isoprenaline can be a useful drug in the management of tachy-brady syndrome, particularly in patients with sick sinus syndrome or unstable third-degree atrioventricular block 3, 4.
- However, its use requires careful consideration of the potential risks and benefits, including the risk of inducing arrhythmias or worsening underlying cardiac conduction disturbances 2, 4.
- Further studies are needed to fully understand the mechanisms of isoprenaline's effects on cardiac pacemaking and driving, and to determine its optimal use in clinical practice 3, 4.