Reversible Causes of Cardiac Arrest: The H’s and T’s

During any cardiac arrest, part of the resuscitation effort must focus on identifying and treating potentially reversible causes — the conditions that led to the arrest in the first place. ACLS providers use the mnemonic “H’s and T’s” to quickly recall these critical culprits. If these conditions are not addressed, resuscitation efforts are unlikely to succeed. This lesson explains each reversible cause in practical, real-world terms.

The H’s

• Hypovolemia: Low blood volume is a leading cause of pulseless electrical activity (PEA). It can result from severe bleeding, trauma, dehydration, or massive fluid loss (e.g., from vomiting or diarrhea). Clues may include flat neck veins, pale/cool skin, and a history of bleeding or fluid loss. Treatment involves rapid volume replacement with IV fluids or blood products.
• Hypoxia: Inadequate oxygen delivery to tissues can lead to bradycardia, PEA, and asystole. Common causes include respiratory failure, airway obstruction, or drowning. Suspect hypoxia if the patient has poor color, low oxygen saturation, or a known respiratory issue. Ensure airway patency, provide high-flow oxygen, and ventilate the patient.
• Hydrogen Ion (Acidosis): Metabolic acidosis — especially due to prolonged arrest or shock — interferes with cardiac contractility and drug effectiveness. Suspect in cases of renal failure, sepsis, or prolonged downtime. Confirm with blood gas if available. Treatment may include effective ventilation and, in some cases, sodium bicarbonate.
• Hypo-/Hyperkalemia: Potassium imbalances can lead to life-threatening arrhythmias. Hyperkalemia (common in renal failure) may cause peaked T waves, widened QRS, and sine-wave ECGs. Hypokalemia may show flattened T waves and U waves. Treat hyperkalemia with calcium, insulin/glucose, or sodium bicarb. Treat hypokalemia with potassium replacement.
• Hypothermia: Severely low core body temperature (usually <30°C / 86°F) can reduce metabolism, depress cardiac function, and render defibrillation less effective. Look for cold skin, bradycardia, or a known exposure history. Warm the patient gradually with external or internal warming techniques.

The T’s

• Tension Pneumothorax: Air trapped in the pleural space compresses the lung and shifts mediastinal structures, preventing venous return. Look for unilateral breath sounds, distended neck veins, and tracheal deviation (if advanced). Treat immediately with needle decompression followed by chest tube insertion.
• Tamponade (Cardiac): Fluid accumulation in the pericardial sac prevents the heart from filling properly. Signs include muffled heart sounds, distended neck veins, and hypotension (Beck’s triad). May occur after trauma or in cancer patients. Treat with pericardiocentesis to relieve pressure.
• Toxins: Drug overdoses (e.g., tricyclics, opioids, calcium channel blockers) or toxic exposures can disrupt cardiac function. History, pill bottles, or pupils may offer clues. Administer antidotes when indicated (e.g., naloxone, sodium bicarb, calcium, lipid emulsion) and provide supportive care.
• Thrombosis (Pulmonary): Massive pulmonary embolism blocks blood flow from the right heart. Consider in patients with recent surgery, DVT, or sudden collapse. May present with PEA, hypoxia, and clear lungs on auscultation. Treat with fibrinolytics or embolectomy if diagnosed.
• Thrombosis (Coronary): Acute myocardial infarction can trigger VF, VT, or asystole. Suspect if the patient had chest pain or known coronary disease. Treat with early CPR, defibrillation, and post-ROSC transfer to a PCI-capable facility for coronary intervention.

Putting It Into Practice

Reversible causes should always be on your mental checklist during a code. If a patient is in PEA or asystole and not responding to standard ACLS treatment, assume one of the H’s or T’s is to blame until proven otherwise. Use clues from history, physical exam, and point-of-care tools (ultrasound, capnography, blood gas) to guide your diagnosis and treatment.

Resuscitation isn’t just about pushing drugs or shocking — it’s about thinking. Find the fixable cause, and you may just reverse the arrest.