What Are the 5 H’s and 5 T’s of Cardiac Arrest?

The 5 H’s and 5 T’s are a mnemonic that emergency medical teams use to quickly recall the ten most common reversible causes of cardiac arrest. The five H’s are hypovolemia, hypoxia, hydrogen ion excess (acidosis), hypo- or hyperkalemia, and hypothermia. The five T’s are tension pneumothorax, cardiac tamponade, toxins, thrombosis of the lungs (pulmonary embolism), and thrombosis of the coronary arteries (heart attack). These aren’t just textbook categories; they form the mental checklist that rescuers run through in real time while performing CPR, because standard chest compressions and defibrillation can only do so much if an underlying cause keeps the heart from restarting.

Why a Checklist Matters During Cardiac Arrest

When someone’s heart stops, the default response follows a structured protocol: chest compressions, airway management, defibrillation if the rhythm is shockable, and medications like epinephrine. But roughly half the time, there is a specific, treatable problem driving the arrest, and until that problem is fixed, the heart may refuse to restart no matter how good the CPR is. A study examining both out-of-hospital and in-hospital cardiac arrests found that identifying a presumptive cause was significantly associated with return of spontaneous circulation in out-of-hospital cases.¹ In other words, the team that figures out why the heart stopped has a meaningfully better shot at getting it going again.

The H’s and T’s exist because cardiac arrest is a terrible time to brainstorm. The mnemonic gives clinicians a rapid mental framework so they can systematically consider each treatable cause even under extreme time pressure. Not every cause applies to every patient, but skipping through the list ensures nothing obvious gets missed.

The Five H’s

Hypovolemia

Hypovolemia simply means the body has lost too much fluid, usually blood. Massive bleeding from trauma, a ruptured blood vessel, or severe dehydration from any cause can drop blood volume so low that the heart has nothing left to pump. In trauma settings, this is one of the first causes considered. Treatment is aggressive fluid replacement and, in bleeding cases, surgical control of the source. Without restoring volume, compressions push an almost-empty system around in circles.

Hypoxia

When oxygen levels in the blood drop critically low, the heart muscle itself becomes starved of the fuel it needs to generate electrical signals and contract. Choking, drowning, severe asthma attacks, and airway obstruction are classic triggers. Research on asphyxia-induced cardiac arrest shows that progressive oxygen depletion over just a few minutes is enough to cause the heart to stop.² The fix is straightforward in principle: secure the airway, ventilate the patient, and deliver high-concentration oxygen. In practice, the challenge is doing it fast enough.

Hydrogen Ion Excess (Acidosis)

When the blood becomes too acidic, the heart’s ability to contract weakens. Severe acidosis can result from prolonged cardiac arrest itself (a vicious cycle), diabetic emergencies, kidney failure, or poisoning. The damage is not subtle: acidosis impairs the heart’s pumping strength, drops blood pressure, and can injure the kidneys, liver, and brain, potentially progressing to multi-organ failure.³ Sodium bicarbonate is sometimes given to buffer the acid, though its use remains debated in routine cardiac arrest. The best treatment is correcting whatever is generating the acid in the first place.

Hypokalemia and Hyperkalemia

Potassium is one of the key electrolytes that control the heart’s electrical rhythm. Too little (hypokalemia) or too much (hyperkalemia) can throw the heart into fatal rhythm disturbances. Hyperkalemia is particularly dangerous and common in people with kidney disease or those taking certain medications. It can cause the heart to slow down and eventually stop. Treatment includes calcium to stabilize the heart, insulin with glucose to push potassium back into cells, and sometimes emergency dialysis. Hypokalemia, often caused by severe vomiting, diarrhea, or diuretic use, is treated with intravenous potassium replacement. A quick blood test during resuscitation can reveal either problem.

Hypothermia

When core body temperature drops severely, the heart becomes increasingly irritable and prone to dangerous rhythms before eventually stopping altogether. Exposure to cold water, prolonged outdoor exposure in winter, and certain medical conditions can all trigger it. Hypothermia is unique among the H’s and T’s because a cold heart often will not respond to standard resuscitation drugs or defibrillation until it has been warmed. Most cases are managed with external warming techniques, but patients in cardiac arrest from severe hypothermia may require internal rewarming methods.⁴ There is a saying in emergency medicine: “You’re not dead until you’re warm and dead,” reflecting the fact that some deeply hypothermic patients have been successfully resuscitated after extraordinarily long periods of arrest.

The Five T’s

Tension Pneumothorax

A tension pneumothorax occurs when air leaks into the space between the lung and the chest wall and gets trapped there, building pressure. That pressure collapses the lung on the affected side and eventually pushes on the heart and the opposite lung, choking off blood flow. It can happen after chest trauma, certain medical procedures, or sometimes spontaneously. The traditional first-line treatment is needle decompression, inserting a large needle through the chest wall to release the trapped air. However, needle decompression has surprisingly high failure rates and the evidence behind it is not as strong as many people assume.⁵ Because of this, many trauma teams now prefer a surgical approach called thoracostomy, which involves making a small incision in the chest wall. In one study of prehospital traumatic cardiac arrest, patients who underwent chest decompression for tension or bilateral pneumothorax were able to regain a heartbeat.⁶

Cardiac Tamponade

Tamponade happens when fluid, often blood, collects in the sac surrounding the heart (the pericardium). The fluid compresses the heart so tightly that it cannot fill between beats, strangling its output. Trauma, recent heart surgery, cancer, and certain infections are common causes. The emergency treatment is pericardiocentesis, using a needle to drain the fluid from around the heart. In a case report of a child who went into cardiac arrest during surgery from traumatic tamponade, ultrasound-guided drainage of blood from the pericardium led to immediate return of a heartbeat.⁷ Without drainage, no amount of chest compressions can force a squeezed heart to pump effectively.

Toxins

A wide range of drugs and poisons can trigger cardiac arrest, and the treatment depends entirely on what was ingested. Opioid overdoses respond to naloxone. Certain antidepressant overdoses, like tricyclics, are treated with sodium bicarbonate to counteract the drug’s effect on the heart’s electrical system. A newer rescue treatment, intravenous lipid emulsion therapy, has shown promise for poisonings involving fat-soluble drugs. Case reports have documented its successful use in cardiac arrest from amitriptyline overdose⁸ and cocaine toxicity.⁹ The toxins category is arguably the broadest of the ten causes, since the list of substances capable of stopping the heart is long, and each may call for a different antidote.

Thrombosis, Pulmonary

A massive blood clot that blocks the arteries feeding the lungs, known as a pulmonary embolism, can cause sudden cardiac arrest by preventing blood from reaching the left side of the heart. It is estimated that acute heart attacks and large pulmonary emboli account for more than 70% of the primary health deterioration leading to CPR.¹⁰ The treatment is thrombolytic medication, which dissolves the clot. Administering clot-busting drugs during CPR is a significant decision because they increase bleeding risk, but when a massive pulmonary embolism is suspected, most clinicians will use them. A survey of international practice found that most respondents used thrombolytics during cardiac arrest from known or suspected pulmonary embolism.¹¹

Thrombosis, Coronary

A heart attack, caused by a blood clot blocking one of the coronary arteries, is probably the single most common cause of cardiac arrest in adults. The American Heart Association has emphasized that the cardiac catheterization laboratory has unique importance in treating arrest caused by underlying coronary artery disease, since the blockage can potentially be opened with a catheter-based procedure.¹² In a study of patients who had out-of-hospital cardiac arrest, roughly 42% were found to have an acute coronary occlusion, and those who underwent successful catheter-based intervention had about half the risk of dying within 30 days compared to those who did not.¹³ The challenge is getting the patient to the cath lab quickly enough, especially when the arrest happens outside a hospital.

How Clinicians Identify the Cause in Real Time

Running through a mental list is only useful if you can actually figure out which cause is responsible while CPR is ongoing. History helps when it is available: a known heroin user in respiratory arrest suggests hypoxia and toxins; a dialysis patient suggests hyperkalemia; a post-surgical patient with sudden collapse suggests pulmonary embolism. But often there is little history to go on, especially in out-of-hospital arrests.

Point-of-care ultrasound, sometimes called POCUS, has become one of the most valuable bedside tools during cardiac arrest. An ultrasound probe placed on the chest can quickly reveal fluid around the heart (tamponade), a collapsed lung (pneumothorax), a dilated right ventricle (suggesting pulmonary embolism), or an empty, poorly filling heart (suggesting hypovolemia).¹⁴ A comparative study found that ultrasound identified reversible causes in about 73% of cardiac arrest cases, compared to roughly 43% using standard assessment alone. Ultrasound also cut the average time to identify a cause by nearly three minutes and led to faster interventions.¹⁵ Three minutes sounds modest until you consider that every minute of cardiac arrest reduces the chance of survival.

Blood gas analysis and basic blood chemistry drawn during resuscitation can flag acidosis and potassium abnormalities within minutes. Temperature probes reveal hypothermia. An electrocardiogram, when a rhythm is present, may point toward a heart attack or electrolyte disturbance. The H’s and T’s framework gives these diagnostic findings a home: each test result maps to one or more items on the list, guiding treatment decisions even when the clinical picture is chaotic.

Does Finding the Cause Actually Improve Survival?

The evidence strongly suggests it does. Among in-hospital cardiac arrests, a study found that 55% had a cardiac cause such as heart attack or arrhythmia, and those patients had significantly higher survival rates than patients with non-cardiac causes: about 44% versus 19%. Survivors with cardiac causes also had better neurological outcomes.¹⁶ For out-of-hospital arrests, identifying any presumptive cause was statistically linked to achieving return of spontaneous circulation.¹

That said, survival from cardiac arrest remains low overall, especially outside the hospital. The H’s and T’s framework improves the odds, but it is not a guarantee. Some causes, like massive hemorrhage or prolonged hypoxia with brain injury, carry grim prognoses even when identified promptly. The framework’s value is that it gives rescuers a structured way to find and treat the causes that can be fixed, rather than continuing futile compressions on a problem that has a specific solution.

How the Framework Has Changed Over Time

The H’s and T’s list is not set in stone. Earlier versions of the American Heart Association’s guidelines included hypoglycemia, dangerously low blood sugar, as one of the H’s. It was part of the 2005 ACLS guidelines but was removed in later editions.¹⁷ That decision is somewhat controversial, since case reports exist of patients who went into cardiac arrest from severe hypoglycemia and were resuscitated after receiving intravenous glucose. The removal likely reflects the rarity of hypoglycemia as a sole cause of arrest rather than any judgment that it is unimportant. Many experienced clinicians still check blood sugar during any resuscitation.

Different countries and training programs sometimes organize the list slightly differently. Some group the two thrombosis categories (pulmonary and coronary) together; some add trauma as a separate T. The European Resuscitation Council uses a similar framework called the “4 H’s and 4 T’s,” which combines certain categories. The core idea is identical across all versions: during CPR, systematically consider every treatable cause.

When Standard CPR Fails and the Cause Is Treatable

Sometimes a reversible cause is identified but the heart still will not restart despite ongoing CPR and targeted treatment. In these cases, a technology called extracorporeal CPR, or ECPR, can buy time. ECPR uses a machine similar to a heart-lung bypass to take over the work of pumping blood and oxygenating it, allowing the medical team to treat the underlying cause while the body’s organs continue receiving blood flow.¹⁸ Candidates typically include patients with witnessed arrest, good-quality CPR, at least 15 minutes of failed conventional resuscitation, and a potentially reversible cause.

A patient with a massive coronary blockage, for example, might be placed on ECPR and then taken to the catheterization lab to have the artery opened. A patient with severe hypothermia might be placed on ECPR to warm their blood directly. The evidence on whether ECPR improves survival with good neurological outcomes remains inconclusive for out-of-hospital arrests, though it is a rapidly evolving area of research.¹⁹ The technology is resource-intensive, requiring specialized teams and equipment, so it is available only at select centers. But for the right patient with the right reversible cause, it represents a final option when everything else has been exhausted.

What Differs Between Cardiac and Non-Cardiac Causes

Not all items on the H’s and T’s list behave the same way during resuscitation. Cardiac causes, particularly coronary thrombosis and certain arrhythmias, tend to present with shockable heart rhythms like ventricular fibrillation, which respond relatively well to defibrillation. Non-cardiac causes like hypoxia, tension pneumothorax, and tamponade more often present with non-shockable rhythms, where defibrillation is useless and the only path to survival is treating the root problem. Research comparing asphyxia-caused arrest to arrest from ventricular fibrillation found that the heart actually recovered better after asphyxial arrest, suggesting that the mechanism of injury matters for how the heart responds to resuscitation.²⁰

This distinction has practical implications. A patient found in cardiac arrest with a shockable rhythm gets immediate defibrillation and may not need the H’s and T’s at all if the shock restores a normal heartbeat. But a patient with a non-shockable rhythm, or whose shockable rhythm keeps returning after defibrillation, almost certainly has one of the ten reversible causes in play. That is when systematic use of the checklist becomes critical, and when bedside ultrasound, blood tests, and a careful clinical assessment offer the best chance of finding a fixable problem before time runs out.