Reversible Causes of Cardiac Arrest: Hs and Ts
This guide provides a comprehensive overview of the Hs and Ts of cardiac arrest for healthcare providers, ACLS learners, and emergency responders. By the end of this article, you will be able to rapidly recall these potentially reversible causes, identify their clinical presentations, and initiate the correct targeted treatments during a resuscitation event.
ACLS Certification Association videos have been peer-reviewed for medical accuracy by the ACA medical review board.
Article at a Glance
- Although cardiac arrest is sometimes sudden and deadly, in many cases, there are reversible causes.
- The Hs and Ts mnemonic helps healthcare providers remember the reversible causes of cardiac arrest.
- The Hs: Hypovolemia, Hypoxia, Hydrogen ion (acidosis), Hypo-/Hyperkalemia, and Hypothermia.
- The Ts: Tension pneumothorax, Tamponade (cardiac), Toxins, and Thrombosis (pulmonary or coronary).
- Clinical Context: While hypoxia and hypovolemia are frequently encountered in cardiac arrest scenarios, providers must systematically evaluate all Hs and Ts to ensure no reversible cause is missed.
What Are the Hs and Ts?
The Hs and Ts mnemonic is a valuable tool to help healthcare providers remember the reversible causes of cardiac arrest, which are integrated into the adult cardiac arrest algorithm:
- Hs: Hypovolemia, hypoxia, hydrogen ion (acidosis), hypo- or hyperkalemia, and hypothermia.
- Ts: Tension pneumothorax, tamponade (cardiac), toxins, and thrombosis (pulmonary or cardiac).
You may encounter variations like the “4Hs and 4Ts” depending on the region or specific guideline. Additionally, while not officially listed in the adult ACLS algorithm, hypoglycemia is often considered alongside the Hs, especially in pediatric resuscitation.
Sudden cardiac death can occur extremely rapidly after the onset of symptoms, often within minutes.1 It can even happen to a healthy, asymptomatic person. That phenomenon makes it difficult to classify the primary cause of mortality in sudden cardiac arrest. However, there are transient causes of cardiac arrest that are reversible. These underlying mechanisms include myocardial ischemia and reperfusion, hemodynamic dysfunction, electrolyte imbalance, changes in physiologic pH or partial-pressure oxygen, effects of central and peripheral neurophysical action, and the transient effects of toxins, drugs, and alcohol.
For a brief overview of this concept, watch One Quick Question: What are Hs and Ts? to understand how these reversible causes fit into your rapid assessment.
For a more detailed breakdown of these conditions, our Introduction to the Hs and Ts video provides an excellent foundation.
Hypovolemia
How to identify: Look for signs of severe dehydration, history of bleeding, flat neck veins, or a rapid, narrow-complex tachycardia before the arrest. Excessive fluid loss due to severe diarrhea is a common cause of hypovolemia. Viral or bacterial pathogens can cause diarrhea from acute gastroenteritis. It can also be due to an autoimmune process, such as celiac disease, where gluten in the diet damages the intestinal villi and its lining. Extensive intestine dysfunction causes irritable bowel syndrome, which is a chronic disease. Some medications also cause diarrhea. Excessive sweating leads to excessive fluid loss as well. Common causes include heatstroke in the older population where the risk of mortality is high. Prolonged exercise or exposure to a hot climate is also a risk factor. Traumatic blood loss is also a common cause of hypovolemia.2
Fluid loss from diarrhea is a common cause of hypovolemia.
Treat now: To correct hypovolemia, intravenous fluid resuscitation is recommended. If bleeding is suspected, immediate blood transfusion and hemorrhage control are required.
To learn more about fluid loss and restoration during an arrest, watch our video on Hs and Ts – Hypovolemia.
Hypoxia
How to identify: Signs before arrest may include cyanosis, low SpO2, and severe respiratory distress. Hypoxia occurs when there is a lack of oxygen perfusion to tissues. Ventilation problems are common causes of hypoxia, and unlike shockable ventricular rhythms, hypoxia-related arrests are managed by correcting oxygen delivery rather than defibrillation.3 Examples are acute exacerbations of chronic obstructive pulmonary disease (COPD) and asthma. Foreign body obstruction or other causes of significant upper airway obstruction, such as croup or epiglottitis, cause hypoxia as well.
Treat now: Provide high-quality CPR and ensure a patent airway with 100% oxygen during arrest. Hypoxia is treated by clearing the airway obstruction, such as by using bronchodilators and corticosteroids for asthma, COPD, croup, and epiglottitis, or extracting the foreign body obstruction. Treating hypoxia also requires oxygen supplementation via a nonrebreather oxygen mask or nasal cannula if the patient is breathing spontaneously. Treatment may necessitate the use of positive-airway pressure or mechanical ventilation if the patient has abnormal breathing.
Foreign body obstruction, or choking, can lead to hypoxia.
Review the primary interventions for a compromised airway in our Hs and Ts – Hypoxia video lesson.
Hydrogen Ion (Acidosis)
How to identify: An Arterial Blood Gas (ABG) is the primary evaluation tool to detect acidosis. Look for a low pH and consider the patient’s history (e.g., renal failure, diabetic ketoacidosis, or respiratory failure). Carbon dioxide is acidic.
- Respiratory Acidosis: Due to the increased serum concentration of carbon dioxide resulting from a decreased respiratory drive from the patient.
- Metabolic Acidosis: Can be due to intestinal losses from diarrhea or kidney problems.
Treat now: Correcting respiratory acidosis can be achieved by providing positive-pressure ventilation.4 For metabolic acidosis, sodium bicarbonate can temporarily correct the pH while the provider diagnoses and treats the underlying cause.
Acidosis is broadly categorized as respiratory acidosis and metabolic acidosis.
For a closer look at managing pH imbalances during a code, check out the Hs and Ts – Hydrogen Ions video.
Hypokalemia or Hyperkalemia
Hypokalemia and hyperkalemia are electrolyte imbalances that pertain to serum potassium concentration.5 Potassium is an electrolyte that myocytes (cardiac cells) need to contract. Sudden increases or losses of potassium can cause sudden cardiac death. Diarrhea, vomiting, and the use of diuretic medications can cause hypokalemia.
The kidneys excrete potassium. Patients with severe kidney disease can have an abnormal increase in potassium excretion.
Cells store potassium. When there is massive cellular destruction, potassium will leak into the systemic circulation. Also, low levels of insulin depress the activity of the sodium-potassium adenosine triphosphate (ATPase) pump in the cell walls. In this instance, potassium does not enter the cell and eventually increases serum potassium concentration or hyperkalemia.
Metabolic acidosis causes a shift of potassium from the cells into the systemic circulation in exchange for extracellular hydrogen ions. That also causes hyperkalemia.
To correct hyperkalemia, providers identify and treat the underlying cause. If there is kidney failure, hyperkalemia may be treated with hemodialysis. If there are low levels of insulin, administering exogenous insulin corrects hyperkalemia. If there is metabolic acidosis, sodium bicarbonate is a solution.
Understand the mechanics of potassium shifts and their treatments by watching Hs and Ts – Hypo-Hyper Kalemia.
Hypothermia
How to identify: Check a core body temperature using an esophageal, rectal, or bladder probe. Hypothermia is generally defined as a core body temperature of less than 35.0°C (95°F), but cardiac arrest risk becomes critically high as the temperature drops lower. Patients at risk are those exposed to an extremely cold environment. As soon as the core temperature drops below 30°C, the body experiences severe bradycardia, which compromises cardiac output and eventually causes cardiac arrest.
Severity is often classified using staging frameworks like the Swiss Staging System:
- Stage I (Mild): 32-35°C; conscious, shivering.
- Stage II (Moderate): 28-32°C; impaired consciousness, not shivering.
- Stage III (Severe): 24-28°C; unconscious, vital signs present.
- Stage IV (Cardiac Arrest): <24°C; no vital signs.
Treat now: Cardiopulmonary resuscitation and defibrillation may have reduced efficacy if the core temperature is profoundly low (e.g., below 30°C); hence, active internal rewarming is a major priority while continuing high-quality CPR. You cannot pronounce a patient dead until they are “warm and dead.”
Read: Asystole and Pulseless Electrical Activity Algorithm
Tension Pneumothorax
When the integrity of the lungs is compromised, air will leak into the pleural space.6 The increase in air pressure compresses the mediastinal structure and the contralateral lung. When mediastinal structures are compressed, venous return to the heart is decreased.
Treatment involves decreasing the air pressure in the mediastinum. Needle decompression in minor cases or closed tube thoracostomy in severe cases is the treatment of choice.
Watch our video on Hs and Ts: Tension Pneumothorax to see the pathophysiology and treatment of this obstructive shock condition.
Tamponade, Cardiac
How to identify: Check for Beck’s Triad (hypotension, JVD, and muffled heart sounds) or perform a rapid bedside ultrasound (POCUS) to detect pericardial fluid. Cardiac tamponade is the accumulation of fluid or blood within the pericardium, which causes an increase in pressure between the pericardial space and myocardium, restricting the heart’s motion. That accumulation of fluid can be exudative or transudative secondary to an infection or inflammation.
Treat now: Rapid fluid evacuation is essential. Pericardiocentesis (preferably under ultrasound guidance if available) or emergency thoracotomy in severe cases is used to relieve fluid accumulation.
Cardiac tamponade is the accumulation of fluid within the pericardium.
See how fluid compresses the heart and why it causes PEA in our Hs and Ts: Cardiac Tamponade video.
A common cause of reversible sudden cardiac arrest is the ingestion of toxins, such as recreational or prohibited drugs, or an overdose of certain prescribed or ethical drugs. Prolonged QT interval is a common finding. Supportive care reverses this presentation. Likewise, an antidote or reversing agent must be given to prevent the development of sudden cardiac arrest due to toxins. For example, the effects of a narcotic overdose with opiates can be treated with naloxone.7Toxins
Thrombosis, Pulmonary
Pulmonary embolism is caused when a thrombus travels from the body and lodges in the pulmonary arteries or its branches. Patients at risk for thrombus formation, such as patients with chronic venous insufficiency, nonambulant patients, or those with cardiac dysrhythmia, are susceptible to pulmonary embolism.
Patients experiencing pulmonary embolism will report chest pain and sudden shortness of breath. It is treated by fibrinolytic therapy, anticoagulant therapy, or embolectomy.
In pulmonary embolism, a clot travels from the body to the right chambers of the heart, then lodges in the lungs.
Learn how large clots disrupt pulmonary circulation and lead to arrest in our Hs and Ts – Pulmonary Embolism lesson.
Thrombosis, Coronary
Coronary thrombosis occurs when a thrombus restricts blood flow inside a coronary artery.8 If the perfusion is large enough, cardiac arrest will likely occur. Treatment involves angioplasty or coronary artery bypass graft surgery, guided by the acute coronary syndrome algorithm.
A coronary thrombus is a blood clot in the coronary artery that can lead to restricted blood flow.
This article summarizes the conditions that make up the Hs and Ts mnemonic. The Hs and Ts mnemonic is useful for remembering the reversible causes of cardiac arrest.
Summary
The Hs and Ts mnemonic is a vital cognitive aid for remembering the potentially reversible causes of cardiac arrest[cite: 753]. To maximize a patient’s chance of survival, healthcare providers must memorize the quick list: Hypovolemia, Hypoxia, Hydrogen ion (acidosis), Hypo-/Hyperkalemia, Hypothermia, Tension pneumothorax, Tamponade (cardiac), Toxins, and Thrombosis (pulmonary and coronary).
At the bedside, the most effective approach is a rapid “Identify and Treat” sequence. While performing high-quality CPR and following the standard adult cardiac arrest algorithm, the team leader should continuously run through the Hs and Ts to evaluate clinical clues and initiate targeted interventions.
- Delaying Evaluation: Do not wait until the end of the algorithm to consider the Hs and Ts. Begin evaluating reversible causes immediately during rhythm checks.
- Assuming Single Causes: Avoid absolute thinking; a patient can have multiple concurrent issues (e.g., hypoxia leading to acidosis). Treat all suspected causes aggressively.
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