Pulseless electrical activity (PEA) happens when the heart’s electrical system keeps firing normally, but the heart muscle fails to contract and pump blood. It’s a form of cardiac arrest, and unlike the chaotic rhythms most people picture, PEA looks deceptively organized on a heart monitor while producing no pulse. The causes fall into a well-known set of reversible conditions, and identifying the right one quickly is the difference between life and death.
Why Electrical Signals Stop Producing a Heartbeat
In a healthy heart, an electrical impulse triggers a carefully timed wave of muscle contraction. That contraction depends on calcium flowing into heart muscle cells at precisely the right moment. In PEA, the electrical signal travels through the heart as usual, but the mechanical response fails. This disconnect, sometimes called electromechanical dissociation, can happen because the heart muscle itself is too damaged or overwhelmed to respond, or because something outside the heart is preventing it from filling or ejecting blood.
One pathway involves the body’s own stress response turning against it. Under extreme physiological stress, the immune system can flood the heart with inflammatory molecules that interfere with calcium signaling inside heart cells. Without proper calcium movement, the muscle fibers can’t contract even though the electrical signal tells them to. The other pathway is purely mechanical: something physically prevents the heart from doing its job, like a blood clot blocking the lungs or fluid compressing the heart from outside.
The Reversible Causes: Hs and Ts
Emergency teams use a mnemonic called the “Hs and Ts” to quickly run through the treatable causes of PEA. First described in 1995, this checklist remains the standard framework because PEA is almost always triggered by a specific, identifiable problem rather than by the heart simply giving out on its own.
The H causes are:
- Hypoxia: severely low oxygen levels
- Hypovolemia: not enough blood or fluid in circulation
- Hydrogen ion excess (acidosis): dangerously acidic blood
- Hypo- or hyperkalemia: potassium levels that are too low or too high
- Hypothermia: critically low body temperature
The T causes are:
- Tension pneumothorax: a collapsed lung trapping air under pressure
- Tamponade: fluid compressing the heart
- Toxins: drug overdoses or poisoning
- Thrombosis: a massive blood clot in the lungs or coronary arteries
These causes are split into two broad categories. “Primary” PEA originates from a problem within the heart itself, like a massive heart attack cutting off blood supply to the muscle. “Secondary” PEA comes from something outside the heart, such as blood loss or a drug overdose, and tends to be more treatable because fixing the external problem can restore normal heart function.
Blood Loss and Low Fluid Volume
Hypovolemia is one of the most common triggers. When the body loses enough blood or fluid, the heart simply has nothing left to pump. The electrical system keeps firing, but the chambers are too empty to generate any meaningful output. This can happen with major trauma, internal bleeding from a ruptured organ, severe dehydration, or massive fluid loss from burns. It is considered the most likely cause of secondary PEA, and rapid fluid replacement or blood transfusion is the critical intervention.
Blood Clots in the Lungs
Massive pulmonary embolism, a large blood clot that blocks blood flow through the lungs, is a surprisingly frequent cause. In one study of patients with unexplained cardiac arrest presenting as PEA, 36% had pulmonary emboli. That’s more than a third. The clot blocks blood from returning to the left side of the heart, so even though the heart keeps generating electrical signals, there’s no blood arriving to be pumped out. This cause is particularly dangerous because it can strike without warning in people who seem otherwise stable, especially those with recent surgery, prolonged immobility, or a history of blood clots.
Potassium Imbalances
Potassium is essential for both the electrical and mechanical functions of the heart. When levels swing too high (hyperkalemia) or too low (hypokalemia), the heart’s ability to translate electrical signals into contractions breaks down. Hyperkalemia is especially dangerous and leaves a clue: it tends to produce a wide pattern on the heart monitor, where each electrical complex stretches out beyond the normal duration. In one study, nearly 50% of PEA patients with this widened pattern had dangerously high potassium levels, compared to about 27% of those with a normal-width pattern. Kidney failure, certain medications, and severe tissue injury are common culprits behind potassium spikes.
Tension Pneumothorax and Cardiac Tamponade
Both of these causes work by the same basic principle: external pressure crushes the heart so it can’t fill with blood. In a tension pneumothorax, air leaks into the chest cavity and has no way to escape. As pressure builds, it pushes the entire center of the chest to the opposite side, compressing the heart, major blood vessels, and airways. Venous blood can no longer return to the heart, and cardiac output drops to zero. This can happen after chest trauma, a stabbing injury, or even spontaneously in people with certain lung conditions.
Cardiac tamponade works similarly but involves fluid rather than air. Blood, pus, or other fluid accumulates in the sac surrounding the heart, squeezing it from the outside. The heart still generates electrical impulses, but the external pressure prevents the chambers from expanding enough to fill. Both conditions are treatable if recognized in time: releasing the trapped air or draining the fluid immediately relieves the pressure.
Drug Overdoses and Toxins
Certain medications can cause PEA when taken in excess. Tricyclic antidepressants, an older class of depression medication, are well-known triggers because they block sodium channels in the heart and disrupt the connection between electrical activity and muscle contraction. Beta-blockers, which slow the heart rate and reduce its force, can cause PEA in overdose by suppressing the heart’s mechanical response even while electrical signals persist. Antipsychotic medications have also been linked to PEA arrests. The key with toxin-related PEA is that specific antidotes or treatments exist for many of these poisonings, making rapid identification critical.
Hypoxia and Acidosis
When oxygen levels drop severely, heart muscle cells can’t produce the energy they need to contract. Drowning, choking, severe asthma attacks, and airway obstruction are common scenarios. The heart’s electrical system is more resilient than its mechanical function, so the rhythm on the monitor can look relatively normal even as the muscle itself is starving for oxygen.
Acidosis, where the blood becomes too acidic, often develops alongside other causes on this list. Prolonged low blood flow, severe infections, diabetic emergencies, and kidney failure can all push blood pH to dangerous levels. Acidic conditions interfere with the proteins that allow heart muscle to contract, creating the same electrical-mechanical disconnect.
True PEA vs. Pseudo-PEA
Not all PEA is the same. Bedside ultrasound has revealed that some patients labeled as having PEA actually have faint cardiac contractions that are too weak to produce a detectable pulse. This is called pseudo-PEA. In true PEA, the heart shows no mechanical movement at all on ultrasound. The distinction matters because patients with pseudo-PEA, those with some remaining cardiac wall motion, generally have a better chance of being resuscitated. Ultrasound during cardiac arrest is increasingly used to make this distinction in real time and to look for treatable causes like fluid around the heart or a severely enlarged right ventricle suggesting a pulmonary embolism.
How PEA Is Treated
PEA is a non-shockable rhythm, meaning a defibrillator won’t help. Unlike ventricular fibrillation, where a shock can reset a chaotic rhythm, PEA’s electrical pattern is already organized. The problem isn’t the rhythm. It’s that the heart can’t respond to it.
Treatment centers on two things happening simultaneously: high-quality CPR to keep blood moving, and aggressive investigation to find and fix the underlying cause. CPR for PEA means chest compressions at a rate of 100 to 120 per minute, pressing at least two inches deep, with minimal interruptions. Epinephrine is given every three to five minutes to support blood pressure and stimulate the heart. But the real treatment is solving whichever H or T triggered the arrest: replacing lost blood, relieving a tension pneumothorax, correcting potassium, warming a hypothermic patient, or breaking up a pulmonary clot.
Survival Outlook
PEA cardiac arrest carries a sobering prognosis. For out-of-hospital arrests presenting with PEA, survival to 30 days is roughly 7%. That’s significantly lower than arrests caused by ventricular fibrillation, which is shockable and has considerably better outcomes. The survival rate improves when the underlying cause is identified and treated quickly, which is why the Hs and Ts checklist exists. Secondary PEA from a correctable cause like blood loss or a tension pneumothorax has a better prognosis than PEA caused by a massive, irreversible heart attack. Speed of recognition and targeted treatment are the strongest predictors of whether someone survives.