The “Hs and Ts” are not just a list to recite during ACLS. They are a structured way to ask a more important question: What potentially reversible problem could be driving this cardiac arrest?
The 2025 American Heart Association adult cardiac-arrest algorithm continues to list reversible causes that can be organized into the familiar H-and-T framework. The value of the mnemonic is not memorization by itself. Its value is helping the resuscitation team connect history, physical findings, monitor data, laboratory information, point-of-care testing, and the circumstances of the arrest to causes that may be treatable.
The 2025 AHA Reversible Causes
The current adult cardiac-arrest algorithm identifies the following reversible causes:
- Hypovolemia
- Hypoxia
- Hydrogen ion (acidosis)
- Hypo-/hyperkalemia
- Hypothermia
- Tension pneumothorax
- Cardiac tamponade
- Toxins
- Pulmonary thrombosis
- Coronary thrombosis
These causes should be considered during the resuscitation, not saved for a final checklist after several cycles have already passed.
Hypovolemia
Think about inadequate circulating volume when the history or scene suggests hemorrhage, severe fluid loss, trauma, gastrointestinal bleeding, ruptured vascular disease, dehydration, or another cause of major volume depletion.
During a megacode, the important skill is linking clues to the possibility. A patient who deteriorates after major bleeding presents a very different reversible-cause problem than a patient with sudden collapse and no evidence of volume loss.
Hypoxia
Hypoxia is a major reversible cause and should prompt attention to ventilation, oxygenation, airway patency, equipment function, and the events leading to arrest. Think about respiratory failure, airway obstruction, drowning, severe pulmonary disease, overdose, or another condition capable of causing critical oxygen failure.
Do not allow advanced airway procedures to create long interruptions in chest compressions. In ACLS, airway management exists within the larger resuscitation, not apart from it.
Hydrogen Ion (Acidosis)
Severe acidosis can accompany prolonged arrest and can also reflect an underlying metabolic or respiratory problem. The key learner point is that acidosis is not a reason to give sodium bicarbonate routinely to every adult in cardiac arrest. Current AHA guidance does not recommend routine sodium bicarbonate administration in undifferentiated adult cardiac arrest.
Instead, look for the clinical context and special circumstances in which targeted treatment may be appropriate.
Hypokalemia and Hyperkalemia
Potassium abnormalities can produce dangerous dysrhythmias and cardiac arrest. Consider renal failure, dialysis, medication effects, endocrine disorders, gastrointestinal losses, major tissue injury, and other clues that could make a potassium disturbance plausible.
The treatment of a specific electrolyte emergency is different from routine ACLS drug administration. This is why the reversible-cause framework should be integrated with history and available diagnostic information.
Hypothermia
Environmental exposure, cold-water immersion, prolonged exposure, or another cause of severe core cooling can alter the resuscitation strategy. Hypothermic arrest is a special circumstance, so the team should recognize the possibility early and follow current specialized guidance rather than assuming that every arrest is managed identically.
Tension Pneumothorax
Tension pneumothorax can obstruct circulation and produce rapidly worsening hemodynamics or cardiac arrest. Clues may include trauma, positive-pressure ventilation, sudden deterioration, severe respiratory distress before arrest, or other findings consistent with obstructive physiology.
In a training scenario, focus on recognizing when the clinical story makes tension pneumothorax plausible and understanding that a reversible mechanical problem requires correction of that problem, not simply another medication cycle.
Cardiac Tamponade
Cardiac tamponade can prevent adequate ventricular filling and lead to obstructive shock or arrest. Penetrating trauma, pericardial disease, recent cardiac procedures, and other relevant history may increase suspicion.
Current AHA advanced-life-support guidance also discusses point-of-care ultrasound as a possible aid in identifying selected reversible causes such as tamponade, pulmonary embolism, myocardial infarction, and hypovolemia when it can be used without disrupting high-quality resuscitation.
Toxins
Toxicologic cardiac arrest is a broad category. Medication overdose, recreational substances, industrial exposures, and poisoning can produce very different physiologic effects and may require specific antidotes or specialized resuscitation strategies.
The 2025 AHA special-circumstances guidance includes multiple poisoning syndromes. For ACLS preparation, the main lesson is to ask whether the history, scene, medications, or exposure pattern suggests a toxicologic cause and to seek appropriate poison-control, toxicology, medical-direction, or protocol support.
Pulmonary Thrombosis
A massive pulmonary embolism can obstruct pulmonary blood flow and cause sudden cardiovascular collapse. Consider the history and risk factors, including recent immobility, surgery, thromboembolic disease, pregnancy or postpartum status, malignancy, or other relevant clinical context.
The reversible-cause framework helps the team recognize that PEA is not a diagnosis of why the patient arrested. It is a rhythm state that should trigger active investigation for causes such as massive pulmonary embolism.
Coronary Thrombosis
Acute coronary occlusion can precipitate malignant ventricular dysrhythmias and cardiac arrest. The patient’s preceding symptoms, ECG findings when available, known coronary disease, and the circumstances of collapse can influence suspicion and post-resuscitation planning.
How to Use the Hs and Ts During a Megacode
Do not simply announce all ten causes. Use a focused process:
- Look at the story. What happened before the arrest?
- Look at the rhythm. Does it suggest a particular group of causes?
- Look at the patient and equipment. Are there findings suggesting airway, ventilation, trauma, bleeding, or obstructive problems?
- Use available data. ECG, capnography, laboratory information, ultrasound, medication history, and scene evidence can refine the differential.
- Prioritize plausible causes. Act on the problems that fit rather than reciting every mnemonic item with equal weight.
- Reassess. New information can change which reversible cause is most likely.
Pair Reversible Causes With the Rest of ACLS
The Hs and Ts make more sense when you study them alongside ACLS rhythm recognition, ACLS medications, and megacode preparation. Together, those resources help turn a memorized list into clinical reasoning.
For a complete preparation sequence, use the ACLS Precourse Preparation Checklist. To compare training formats, visit the LSE ACLS Training Hub.
Educational note: This article is intended for ACLS course preparation. It does not replace current AHA algorithms, special-circumstances guidance, medical direction, institutional policy, toxicology resources, or local clinical protocols.
