Transcutaneous pacing and cardioversion are often confused because both procedures use external electrical energy delivered through pads placed on the chest to treat a heart rhythm disturbance. However, their purpose, application, and intended effect on the heart’s electrical system are fundamentally different. Both are emergency interventions, but they address opposite problems within the heart’s conduction system.
Transcutaneous Pacing: A Temporary Electrical Support
Transcutaneous pacing (TCP) is a temporary, non-invasive method used to artificially stimulate the heart when its natural pacemaker is failing to generate an adequate rate. The primary indication for this procedure is symptomatic bradycardia, which is an abnormally slow heart rate that is causing the patient to become unstable. This slow rate, typically below 60 beats per minute, might be due to conditions like complete heart block or sinus node dysfunction.
The procedure involves placing two large electrode pads on the patient’s chest, usually in an anterior-posterior configuration to effectively “sandwich” the heart. These pads are connected to an external generator that delivers regular, low-energy electrical impulses. The energy is measured in milliamperes (mA) and is set to a rate, often between 60 and 80 beats per minute, to maintain an acceptable level of blood circulation.
The electrical impulses travel from the pads through the chest wall, stimulating the underlying heart muscle to contract. Clinicians slowly increase the output current until electrical and mechanical capture is achieved, meaning the pacing spike on the electrocardiogram (ECG) is consistently followed by a heartbeat and a corresponding pulse. TCP maintains cardiac output until the underlying cause of the slow heart rate can be reversed or a permanent pacemaker can be implanted. Since the electrical energy stimulates surrounding skeletal muscle, the procedure can be uncomfortable for the conscious patient, often requiring sedation.
Cardioversion: Resetting the Heart’s Rhythm
Cardioversion is an electrical procedure used to terminate tachyarrhythmias, which are heart rhythms that are too fast or chaotic. The goal of cardioversion is to deliver a precisely timed, therapeutic electrical shock that momentarily stuns the entire heart muscle. This allows the heart’s natural pacemaker, the sinoatrial (SA) node, to regain control and restore a normal, coordinated rhythm.
This procedure is most commonly used for rhythms like atrial fibrillation, atrial flutter, or ventricular tachycardia when the patient still has a pulse but is hemodynamically unstable. The shock is a high-energy discharge, measured in Joules (J), and is delivered using a defibrillator device through pads or paddles placed on the chest. Unlike pacing, which involves continuous, low-level stimulation, cardioversion is a single, powerful event designed to completely reset the electrical cycle.
Synchronized Cardioversion
Synchronized cardioversion is used for patients who still have a pulse. The device is timed to deliver the shock precisely with the peak of the heart’s R-wave on the ECG. Timing the shock this way avoids the vulnerable period of the heart’s electrical cycle, specifically the T-wave, which could otherwise induce a lethal rhythm like ventricular fibrillation.
Unsynchronized Cardioversion (Defibrillation)
Unsynchronized cardioversion, often referred to as defibrillation, delivers the high-energy shock immediately without timing it to the heart’s electrical activity. This is reserved for critical situations where there is no organized electrical activity or the patient is pulseless, such as in ventricular fibrillation or pulseless ventricular tachycardia.
Key Differences in Purpose and Application
The most fundamental difference lies in the problem each procedure is designed to solve; transcutaneous pacing treats rhythms that are too slow, while cardioversion treats rhythms that are too fast or disorganized. Pacing aims to support the heart by generating an electrical signal when the intrinsic rate is insufficient, acting as a temporary substitute for the SA node. Cardioversion, by contrast, seeks to terminate the abnormal electrical activity entirely, allowing the SA node to restart the heart’s rhythm from a blank slate.
The energy and delivery mechanism also vary significantly. Pacing uses repetitive, low-energy electrical pulses measured in milliamperes (mA) to capture the rhythm and maintain a minimum heart rate. In contrast, cardioversion uses a high-energy electrical shock measured in Joules (J), which is delivered as a singular event. Synchronized cardioversion requires precise timing to align the shock with the R-wave, a feature unnecessary for continuous pacing.
Application and patient condition further distinguish the two procedures. Transcutaneous pacing is used as a temporary measure for patients symptomatic from a slow rate, maintaining perfusion until a more definitive treatment can be arranged. Cardioversion, especially defibrillation, is a higher-stakes intervention used to treat unstable or life-threatening tachyarrhythmias that pose an immediate risk of cardiac arrest.
While both treatments utilize electricity, their indications are mutually exclusive. Pacing adds electrical input when it is lacking to increase the heart rate, whereas cardioversion employs an electrical discharge to interrupt and reset a dangerously fast or chaotic rhythm.