Advanced Cardiac Life Support (ACLS) protocols manage life-threatening emergencies, primarily cardiac arrest. While immediate focus is on high-quality cardiopulmonary resuscitation (CPR) and defibrillation, effective management requires identifying the root cause. Cardiac arrest is often the result of an underlying, reversible condition, not a primary electrical problem. The “H’s and T’s” is a mnemonic device used in resuscitation to quickly categorize and recall these potentially correctable causes, guiding the medical team’s diagnostic and therapeutic efforts.
The H’s: Hypoxia, Volume, and Metabolic Causes
The five “H” causes primarily involve systemic, metabolic, or volume issues that deprive the heart of the resources it needs to function.
Hypoxia
Hypoxia, an inadequate oxygen supply to the tissues, is a common and treatable cause of cardiac arrest. Lack of oxygen starves the heart muscle, slowing electrical activity, and is immediately addressed by ensuring a clear airway and providing supplemental oxygen or ventilation.
Hypovolemia
Hypovolemia is a significant decrease in circulating blood or fluid volume, often resulting from severe bleeding or dehydration. This prevents the heart from pumping enough blood to maintain pressure. Intervention is the rapid infusion of intravenous (IV) fluids, such as saline or Ringer’s lactate, to restore volume and pressure.
Hydrogen Ion (Acidosis)
Hydrogen Ion (Acidosis) is an excessive buildup of acid in the blood, impairing heart function and reducing the effectiveness of resuscitation medications. This imbalance can be metabolic (e.g., diabetic ketoacidosis) or respiratory (from inadequate breathing and carbon dioxide buildup). Treatment involves providing adequate ventilation or administering sodium bicarbonate for severe metabolic cases.
Hypo/Hyperkalemia
Hypo/Hyperkalemia refers to dangerously low or high levels of potassium, an electrolyte fundamental to the heart’s electrical signaling. High potassium (hyperkalemia) causes distinctive ECG changes, requiring immediate drug therapy to shift potassium out of the bloodstream. Conversely, low potassium (hypokalemia) can lead to dangerous heart rhythms, necessitating a controlled infusion of potassium to correct the deficit.
Hypothermia
Hypothermia, a core body temperature below 30°C (86°F), dramatically slows all metabolic processes, including electrical conduction. Severe hypothermia makes the heart resistant to defibrillation and resuscitation medications. Correction involves active and passive rewarming techniques to gradually increase the patient’s core temperature, and resuscitation efforts must continue until the patient is warm.
The T’s: Mechanical, Vascular, and Toxic Causes
The five “T” causes generally involve physical obstructions, mechanical failures, or the presence of poisons that directly interfere with the heart’s ability to pump blood.
Tension Pneumothorax
Tension Pneumothorax is a mechanical collapse of a lung where air builds up in the chest cavity, creating intense pressure. This pressure pushes the heart and major blood vessels aside, preventing the heart from filling properly and leading to failure. It requires immediate needle decompression or chest tube insertion to relieve the pressure.
Tamponade (Cardiac)
Tamponade (Cardiac) is an obstructive problem where fluid or blood accumulates in the pericardial sac surrounding the heart. This fluid physically squeezes the heart chambers, preventing them from expanding to receive blood. The condition is relieved through pericardiocentesis, a procedure where a needle drains the fluid from the sac.
Toxins
Toxins, including drug overdoses or accidental poisonings, cause cardiac arrest by suppressing the heart muscle or disrupting its electrical system. Identifying the specific substance, often via patient history or toxicology screening, is paramount. Treatment involves administering specific antidotes or using enhanced elimination techniques like dialysis.
Thrombosis (Pulmonary and Coronary)
The final two T’s relate to circulatory blockages: Thrombosis (Pulmonary) and Thrombosis (Coronary). Pulmonary Thrombosis (massive pulmonary embolism or PE) occurs when a large blood clot lodges in the lung arteries, creating a mechanical barrier to blood flow that causes heart failure. Coronary Thrombosis (heart attack or acute myocardial infarction) is the most common cause of cardiac arrest, resulting from a blood clot blocking a coronary artery and cutting off blood supply. Both types are often treated with fibrinolytics (clot-busting drugs) or require immediate catheterization or surgery to remove the obstruction.
Rapid Identification and Diagnostic Clues
Identifying the responsible H or T must happen concurrently with CPR, as the window for successful reversal is narrow. The resuscitation team gathers rapid information from bystanders or EMS regarding the patient’s immediate past. A focused physical examination during pulse checks can provide immediate clues, such as checking for distended neck veins, which may indicate cardiac tamponade or tension pneumothorax.
Diagnostic Tools
Immediate diagnostic tools secure objective data points within seconds. Point-of-Care Ultrasound (POCUS) allows the team to visualize the heart and surrounding structures without prolonged interruption of chest compressions. POCUS can quickly confirm fluid around the heart, a non-sliding lung, or an empty heart suggesting hypovolemia. Rapid blood gas analysis confirms severe metabolic disturbances, such as acidosis or extreme potassium imbalances, guiding targeted medication administration.
Core Principles of Reversible Cause Management
The overarching philosophy in managing the H’s and T’s is concurrent treatment: the search for and treatment of the underlying cause occurs simultaneously with basic life support. High-quality chest compressions and ventilation are maintained while the team investigates reversible factors. This dual-focus approach ensures immediate circulatory support while addressing the cause of the arrest.
Targeted Reversal
A core principle is the need for immediate, targeted reversal, meaning treatment is specific to the identified cause. If a toxin is identified, a specific antidote is administered rather than generalized cardiac medications. For suspected mechanical causes, such as tension pneumothorax, procedural intervention (e.g., needle decompression) is performed empirically without waiting for definitive imaging. Effective management also relies on resource mobilization, involving the rapid preparation of specialized equipment and personnel for time-sensitive procedures.