External pacing, more precisely called transcutaneous cardiac pacing, is a way to electrically stimulate the heart through the skin when it beats dangerously slowly or stops producing an effective rhythm. Two large adhesive electrode pads are placed on the chest (and sometimes the back), and a device sends brief electrical pulses through the chest wall to trigger heartbeats at a set rate. It is a temporary, emergency-oriented technique rather than a long-term fix, and it has been a staple of critical care and emergency medicine since Paul Zoll pioneered external pacemakers and defibrillators in the 1950s.
How the Device Actually Stimulates the Heart
The concept is straightforward: electricity applied to the outside of the body can travel through skin, muscle, and lung tissue to reach the heart and force it to contract. A pulse generator, often built into a standard defibrillator or monitor, delivers timed electrical impulses through large gel electrode pads. The current passes between the two pads, and if enough of it reaches the heart muscle, it depolarizes the cardiac cells and triggers a beat. Most patients with some remaining heart function require a current somewhere between 40 and 80 milliamps (mA) to achieve this, though the threshold can be substantially higher in people with emphysema, fluid around the heart, or those on a mechanical ventilator.1PubMed Central. Using transcutaneous cardiac pacing to best advantage: How to ensure successful capture and avoid complications
The electrical pulses are typically very short, on the order of a few milliseconds. Research on pulse characteristics has shown that the relationship between pulse duration and the current needed to stimulate the heart follows a predictable curve: shorter pulses need more current, longer pulses need less, down to a minimum threshold. Biphasic waveforms, which reverse polarity partway through each pulse, tend to lower the amount of current needed compared to monophasic (one-direction) waveforms, though the difference has not always reached statistical significance in studies.2PubMed. Strength-Duration Characterisation of Subcutaneous Pacing: A Preclinical Study
Where the Pads Go and Why It Matters
There are two common pad positions. The anterolateral position places one pad on the upper right chest and the other on the left side, roughly along the mid-axillary line. The anteroposterior position places one pad on the front of the chest, over the heart, and the other directly behind it on the back. Research comparing the two found that the anteroposterior arrangement required significantly less current to pace the heart, with a mean threshold about 33 mA lower than the anterolateral setup. In concrete terms, the average threshold was roughly 93 mA with pads front-and-back versus about 126 mA with pads on the front and side.3PubMed. Anteroposterior pacer pad position is better than anterolateral for transcutaneous cardiac pacing
This difference is not trivial. Lower current means less pain for the patient and less risk of skin burns. The anteroposterior position sends the current on a more direct path through the heart, wasting less energy stimulating chest wall muscles. In practice, though, clinicians sometimes use the anterolateral position because it is faster to place when the patient is lying on their back and access to the posterior chest is difficult.
When External Pacing Is Used
The primary indication is a dangerously slow heart rate, known as bradycardia, that is causing symptoms like low blood pressure, altered consciousness, or signs of poor circulation. If medications like atropine fail to speed up the heart, external pacing becomes the next step. Research in emergency departments has confirmed that transcutaneous pacing is clinically effective in patients with unstable bradycardia that does not respond to atropine, producing measurable improvements in vital signs.4The American Journal of Emergency Medicine. The efficacy of transcutaneous cardiac pacing in ED Earlier work similarly found that in hemodynamically compromising slow rhythms unresponsive to drugs, early use of transcutaneous pacing could improve survival in patients who might otherwise die.5The Journal of Emergency Medicine. Emergency transcutaneous pacing in the management of patients with bradyasystolic rhythms
External pacing is also used as a bridge, keeping the heart going while clinicians arrange a more permanent solution like a transvenous pacemaker wire threaded through a vein into the heart, or a permanent implanted pacemaker. It is explicitly a temporary measure, not a destination.
Situations Where It Does Not Work Well
Not every slow or absent heartbeat responds to external pacing. In asystolic cardiac arrest, where the heart has essentially flatlined, the evidence is discouraging. A large New England Journal of Medicine trial had emergency medical technicians apply pacing to patients found in asystole in the field. Among 278 patients who received pacing, only about 4 percent survived to hospital discharge, which was not significantly better than the control group at 2 percent. The study concluded that transcutaneous pacing offers no benefit in asystolic cardiac arrest, even when applied as early as possible.6PubMed. Out-of-hospital transcutaneous pacing by emergency medical technicians in patients with asystolic cardiac arrest An earlier prehospital study painted an even bleaker picture: while about half of asystolic patients showed electrical capture on the monitor, none survived to hospital discharge, with an average time to device application of 29 minutes after pulse loss.7PubMed. Transcutaneous pacing for bradyasystolic cardiac arrests in prehospital care A systematic review of prehospital transcutaneous pacing similarly found no evidence to support its use in cardiac arrest with asystole.8Resuscitation. Prehospital transcutaneous cardiac pacing for symptomatic bradycardia or bradyasystolic cardiac arrest: A systematic review
The distinction matters: external pacing works best when the heart still has some intrinsic electrical activity but is beating too slowly. When the heart muscle has no remaining electrical activity at all, sending current through the chest can produce electrical complexes on the monitor without producing actual mechanical pumping. And even when electrical capture is achieved, the underlying cause of the arrest often means the heart muscle simply cannot contract effectively.
Another scenario where external pacing often falls short is bradycardia caused by severely elevated potassium levels. In hyperkalemia, the excess potassium disrupts the electrical properties of heart cells in a way that standard treatments for slow rhythms, including both atropine and electrical pacing, are frequently ineffective. The priority in those cases is to treat the potassium imbalance directly.9PubMed Central. Hyperkalemia-Induced Bradydysrhythmias
Confirming That Pacing Is Actually Working
One of the trickiest aspects of external pacing is verifying that the electrical stimulus is truly making the heart pump blood, not just creating electrical artifacts on the monitor. On an electrocardiogram, successful capture typically appears as a widened complex after each pacing spike, followed by a distinct repolarization wave.1PubMed Central. Using transcutaneous cardiac pacing to best advantage: How to ensure successful capture and avoid complications But the monitor pattern alone can be misleading. A case series examining prehospital pacing found a high proportion of patients at risk of false electrical capture, where the monitor showed what looked like successful pacing and the paramedic reported a palpable pulse, but the heart was not actually pumping effectively.10PubMed. False Electrical Capture in Prehospital Transcutaneous Pacing by Paramedics: A Case Series
Part of the problem is that the large electrical current causes the chest muscles to twitch with every pulse, which can mimic a pulse when someone palpates the wrist or neck. This muscle twitching makes it genuinely difficult to feel whether the heart itself is producing output. Bedside ultrasound offers a solution: a clinician can aim an ultrasound probe at the heart and directly visualize whether the ventricles are contracting with each pacing spike. Case reports have described using ultrasound as the primary method to confirm mechanical capture and to differentiate between causes of persistent low blood pressure even when the electrical pacing looks successful on the monitor.11PubMed. Ultrasound for primary confirmation of mechanical capture in emergency transcutaneous pacing
Pain Management During Pacing
External pacing hurts. Each electrical pulse contracts not only the heart but also the skeletal muscles of the chest wall, and patients who are conscious often find the sensation intolerable. Adequate sedation and pain relief are considered essential for maintaining patient comfort during transcutaneous pacing.1PubMed Central. Using transcutaneous cardiac pacing to best advantage: How to ensure successful capture and avoid complications In hospital settings, intravenous sedatives and analgesics are standard. In prehospital and field settings, options are more limited. One study evaluated inhaled nitrous oxide (a 50:50 mix with oxygen) during transcutaneous pacing and found that about 83 percent of subjects reported improved tolerance, with significantly lower pain scores compared to breathing room air.12Prehospital and Disaster Medicine. An Evaluation of Nitrous Oxide Analgesia During Transcutaneous Pacing
The pain issue is one of the main reasons external pacing is considered a bridge rather than a definitive treatment. Even with good analgesia, patients generally cannot tolerate it for long periods. Moving to a transvenous pacing wire, which sits inside the heart and stimulates it directly with far less current, eliminates the chest wall pain problem entirely.
Skin Burns and Other Complications
Beyond discomfort, the most serious local complication of external pacing is skin burns, which range from superficial redness to full-thickness injuries requiring surgery. The risk increases with higher current output, longer pacing duration, and smaller body surface area. Case reports have documented burn injuries spanning a wide spectrum of circumstances. In one case, 12 hours of pacing at 80 mA led to third-degree burns attributed to inadvertent displacement of the electrode pad. In another, just two hours of pacing produced full-thickness burns, possibly because the anterior pad was positioned under heavy breast tissue, reducing heat dissipation. Severe obesity, prior surgical scars with reduced blood flow, and reuse of electrode pads have all been linked to pacing-induced burns in individual reports.13PubMed Central. Third-degree burns caused by transcutaneous pacing for third-degree heart block
Children and newborns face especially high burn risk because of their thin skin and small body surface area. A five-year-old weighing under 15 kg developed full-thickness burns after only 55 minutes of transcutaneous pacing at 60 mA, and neonates have suffered major burns requiring surgical repair after prolonged pacing at high outputs.13PubMed Central. Third-degree burns caused by transcutaneous pacing for third-degree heart block Prolonged use of gel pads without replacement has also been identified as a risk factor for burns, and the prophylactic application of pacing pads “just in case” has been discouraged for this reason.14BMJ Case Reports CP. Third-degree burns associated with transcutaneous pacing
How It Compares to Transvenous Pacing
Transvenous pacing involves threading a thin wire through a vein into the right ventricle, where it stimulates the heart directly. This requires considerably less current since the electrode sits against the heart tissue, which means no chest wall pain and no skin burns. In a canine study comparing the two methods, transcutaneous pacing produced similar cardiac output and blood-flow parameters to transvenous pacing, with the only notable difference being slightly lower mean arterial blood pressure during transcutaneous pacing.15The American Journal of Emergency Medicine. Hemodynamics of transcutaneous cardiac pacing A study comparing the two approaches in early cardiac arrest found no difference in clinical outcomes between the techniques.16Annals of Emergency Medicine. Transcutaneous and transvenous cardiac pacing for early bradyasystolic cardiac arrest
Where the two methods differ most is in speed of deployment versus sustainability. External pacing can be started in seconds: slap on the pads, press a button. Transvenous pacing requires sterile technique, vein access, fluoroscopic guidance in many cases, and a clinician experienced in the procedure. It takes considerably longer to set up. But once in place, a transvenous wire is far more comfortable and reliable for hours or days. The practical reality in most emergency settings is that external pacing buys time until a transvenous wire or a permanent pacemaker can be placed.
From an electrical standpoint, transcutaneous pacing causes greater widening of the QRS complex on the electrocardiogram compared to transvenous pacing, particularly in patients without structural heart disease. However, certain markers of ventricular repolarization that are associated with arrhythmia risk were actually more favorable during transcutaneous pacing than during transvenous pacing in some patient subgroups.17PubMed. Effects of transcutaneous cardiac pacing on ventricular repolarization and comparison with transvenous pacing
Prehospital Use and Real-World Outcomes
Paramedics regularly carry defibrillator-monitors equipped with pacing capability, and transcutaneous pacing is a core skill in prehospital emergency care. A large U.S. epidemiological study of prehospital pacing found sobering overall numbers: the mortality rate following transcutaneous pacing was about 63 percent. Among patients who had not already progressed to cardiac arrest before pacing was attempted, about one in five still deteriorated into arrest despite the pacing. Body weight over 100 kg, a heart rate that was not particularly slow before pacing, and low oxygen levels before pacing were all associated with higher odds of failure.18PubMed Central. Prehospital Transcutaneous Cardiac Pacing in the United States: Treatment Epidemiology, Predictors of Treatment Failure, and Associated Outcomes
Those numbers reflect the severity of the patients who need pacing in the first place rather than a failure of the technique itself. When prehospital pacing was compared to dopamine (a drug that can raise heart rate and blood pressure) in a randomized feasibility trial of symptomatic bradycardia, survival to 30 days was about 69 percent in the pacing group and 70 percent in the dopamine group, essentially identical.19Resuscitation. A randomized controlled feasibility trial comparing safety and effectiveness of prehospital pacing versus conventional treatment: ‘PrePACE’ The take-home message is that prehospital pacing works well for symptomatic bradycardia in patients who still have a pulse and some cardiac activity, but it is not a rescue tool for cardiac arrest.
External Pacing in Children
External pacing can be used in children, though the approach requires some adaptation. A study of 22 children ranging from infants to teenagers found that transcutaneous pacing successfully achieved capture in 53 out of 56 trials. The current needed with standard adult-sized electrodes averaged about 63 mA, comparable to what adults typically require. Smaller electrode pads specifically manufactured for pediatric use required less total current but delivered more current per unit of skin area. Interestingly, the threshold current, current density, and energy requirements did not vary with the child’s age, weight, or chest size. No complications were observed during the study. The researchers recommended using smaller electrode pads in children weighing less than 15 kg.20PubMed. Noninvasive transcutaneous cardiac pacing in children
These findings are reassuring in terms of effectiveness, but they should be read alongside the burn risk data: pediatric patients, especially neonates and very small children, face elevated risk of serious skin injury during pacing. The tension between effective capture and burn prevention is sharper in this population, which is one reason clinicians try to transition pediatric patients to transvenous or permanent pacing as quickly as possible.
Ultrasound Pacing and the Future of Non-Invasive Cardiac Stimulation
One of the most intriguing developments in this space is the possibility of pacing the heart with focused ultrasound instead of electrical current. Researchers have demonstrated proof of concept in large-animal studies, showing that image-guided focused ultrasound can stimulate the heart non-invasively, with quantifiable stimulation thresholds determined as a function of ultrasound pulse duration and amplitude.21Scientific Reports. Non-invasive cardiac pacing with image-guided focused ultrasound Because the ultrasound energy can be focused on the heart tissue directly without stimulating chest wall muscles, this approach could theoretically eliminate both the pain and the burn risk of conventional transcutaneous pacing.
Early human studies have already begun. In a first-in-human trial of non-invasive ultrasound pacing of the left ventricle, ultrasound pacing significantly narrowed the QRS complex and improved markers of synchrony compared to baseline, and it performed comparably to conventional cardiac resynchronization therapy. In seven of nine patients, the maximum QRS narrowing achieved with ultrasound pacing matched or exceeded what standard CRT could achieve.22PubMed Central. First-in-human noninvasive left ventricular ultrasound pacing: A potential screening tool for cardiac resynchronization therapy While this particular application targets a different clinical scenario than emergency bradycardia, the underlying technology holds broader promise. If focused ultrasound pacing can be made portable and reliable enough for emergency use, it could eventually offer a painless, burn-free alternative to the electrode-pad approach that has been standard since the 1950s.23Heart, Vessels and Transplantation. Evolution of cardiac pacemakers: a journey from Galvanic experiments to leadless pacemakers That transition is likely years away, but the early results are genuinely encouraging.