The “H” causes of reversible cardiac arrest are a set of conditions that can stop the heart but can be corrected if identified quickly. They come from a mnemonic used in advanced cardiac life support (ACLS) that pairs “H” causes with “T” causes to help medical teams rapidly troubleshoot why a heart has stopped beating. The current guidelines list five H causes: hypovolemia, hypoxia, hydrogen ion excess (acidosis), hypo- or hyperkalemia, and hypothermia. Some training programs still list these as seven separate items by counting hypokalemia and hyperkalemia individually.
Hypovolemia: Not Enough Blood Volume
Hypovolemia means the body has lost so much fluid or blood that the heart can no longer pump effectively. When total blood volume drops by 20% to 25%, the body’s ability to compensate is overwhelmed and shock sets in. A loss greater than 40% is immediately life-threatening and can lead to cardiac arrest if not reversed.
The most common cause is hemorrhage from trauma, but gastrointestinal bleeding, ruptured aneurysms, and postpartum bleeding can all drain enough volume to stop the heart. Non-bleeding causes matter too. Severe vomiting, diarrhea, burns, and heavy sweating (people exercising in heat can lose 1 to 2 liters per hour) all reduce circulating volume. Fluid can also shift out of blood vessels into surrounding tissues, a process called third-spacing, which effectively shrinks blood volume even without visible fluid loss.
Reduced blood volume starves tissues of oxygen, triggers acidosis, and impairs blood vessel function. The heart, already working harder to compensate, eventually fails.
Hypoxia: Oxygen Deprivation
Hypoxia occurs when the body’s tissues don’t receive enough oxygen. It is one of the most common reversible triggers of cardiac arrest, particularly in drowning, airway obstruction, severe asthma, and respiratory failure.
The timeline from falling oxygen levels to cardiac arrest is surprisingly short. Research tracking patients in controlled settings found that a terminal drop in oxygen saturation typically begins about 14 minutes before the heart stops. Blood pressure starts falling around 8 minutes before arrest, and heart rate plummets in the final 4 minutes. Once oxygen saturation drops below 35%, blood pressure begins its collapse. At saturations below 40%, the brain enters critical hypoxia, with rising lactate levels signaling cellular damage. In that study, 85% of patients had saturations below 40% for more than 3 minutes before their hearts stopped.
Because the window is narrow, restoring oxygen delivery through clearing the airway, ventilation, or supplemental oxygen is among the highest priorities during resuscitation.
Hydrogen Ion Excess (Acidosis)
When blood becomes too acidic, the heart’s electrical system malfunctions. Normal blood pH hovers around 7.35 to 7.45. Acidosis severe enough to contribute to cardiac arrest generally involves a pH below 7.20, and survival becomes increasingly unlikely as pH drops further. One large study found that in-hospital mortality increased by 45% for every 0.1-point drop in pH below 7.25. Survival below a pH of 6.70 was less than 1%.
Acidosis during cardiac arrest is usually multifactorial. On the metabolic side, poor blood flow causes tissues to produce lactic acid. Kidney failure, diabetic emergencies, poisoning, and sepsis can all flood the blood with acid. On the respiratory side, if the lungs can’t expel carbon dioxide (from airway obstruction, for example), CO2 builds up and drives pH down. In many cardiac arrests, both mechanisms operate simultaneously, which is why acidosis often accompanies other H and T causes rather than acting alone.
Hypokalemia and Hyperkalemia
Potassium is the mineral most responsible for keeping heart cells electrically stable. Normal blood potassium sits between 3.5 and 5.3 mmol/L. Deviations in either direction can destabilize the heart’s rhythm and lead to arrest.
Low Potassium (Hypokalemia)
When potassium drops below 3.5 mmol/L, the risk of dangerous heart rhythms rises. Below 2.5 mmol/L, hypokalemia is considered severe and potentially fatal. Low potassium changes the electrical gradient across heart cell membranes, making them harder to activate normally. It also disrupts calcium handling inside cells, which can trigger abnormal heartbeats ranging from atrial fibrillation to ventricular fibrillation. Common culprits include diuretic medications, prolonged vomiting or diarrhea, and eating disorders.
High Potassium (Hyperkalemia)
Potassium levels above 5 mmol/L start to cause problems. The classic warning sign on a heart tracing is a tall, peaked T wave, followed by a widening of the electrical complex that represents each heartbeat. At levels of 8 mmol/L or higher, every patient in one review showed ECG abnormalities, and 56% experienced dangerous cardiac events. If untreated, the heart’s electrical signals merge into a chaotic sine-wave pattern before stopping entirely. Kidney failure is the most frequent cause, but tissue destruction from crush injuries, burns, or massive blood transfusions can release enough potassium to be lethal. The 2025 AHA guidelines note that the role of intravenous calcium for hyperkalemia-related arrest is still not well established, highlighting how challenging this cause can be to treat in the moment.
Hypothermia: Dangerously Low Body Temperature
Hypothermia is classified in four stages. Mild hypothermia (35 to 32°C, or about 95 to 90°F) causes shivering and confusion. Moderate hypothermia (32 to 28°C) impairs consciousness and may stop shivering. Severe hypothermia (below 28°C, or about 82°F) renders a person unconscious, and below roughly 24°C (75°F), vital signs may be absent entirely.
In healthy younger adults, hypothermia-induced cardiac arrest generally occurs below 30°C (86°F). In older adults or those with existing heart conditions, the threshold is higher, around 32°C. Cold temperatures make the heart muscle irritable and prone to ventricular fibrillation. What makes hypothermia unique among these causes is that it also protects the brain by slowing metabolism. People have survived prolonged cardiac arrest from hypothermia with good neurological outcomes, which is why the resuscitation principle “no one is dead until they are warm and dead” persists in emergency medicine. The 2025 AHA guidelines recommend using advanced rewarming techniques for patients in hypothermic arrest and suggest that even patients with severe hypothermia who haven’t yet arrested may benefit from aggressive rewarming.
Why These Causes Are Grouped Together
The H’s and T’s mnemonic exists because certain cardiac arrests don’t respond to standard resuscitation alone. You can perform perfect chest compressions and deliver medications on schedule, but if the underlying trigger isn’t fixed, the heart won’t restart. These causes are called “reversible” because correcting them, whether that means replacing lost blood, warming the body, or restoring normal potassium, can allow the heart to resume a functional rhythm.
During resuscitation, teams are trained to run through the H’s and T’s as a mental checklist while CPR continues. Hypoxia and hypovolemia are typically considered first because they are the most common and the most rapidly treatable. The others are evaluated based on the patient’s history and clinical clues. A dialysis patient who arrests, for example, prompts immediate suspicion of hyperkalemia. Someone pulled from cold water points to hypothermia.
A Note on Hypoglycemia
You may encounter older materials listing hypoglycemia (low blood sugar) as a sixth or seventh “H” cause. It appeared in the 2005 ACLS guidelines but was removed in 2010 and has not been reinstated in the 2015, 2020, or 2025 editions. That said, dangerously low blood sugar still matters during resuscitation. Research has shown that blood glucose below 100 mg/dL during cardiac arrest is associated with significantly lower odds of getting the heart restarted. The practical takeaway is that while hypoglycemia is no longer officially on the list, blood sugar extremes still influence outcomes.