Cardiac arrest is a sudden, catastrophic event where the heart abruptly stops pumping blood effectively, leading to the rapid cessation of circulation. While often fatal, many cardiac arrests are triggered by underlying causes that are treatable and reversible if identified and managed quickly. Recognizing these specific causes during resuscitation offers a clear path to restoring heart function and improving survival chances. Medical teams use a standardized, systematic approach to rapidly uncover these issues while simultaneously performing life support measures.
Understanding the H and T Framework
Medical professionals, particularly those trained in Advanced Cardiac Life Support (ACLS), use the “H’s and T’s” mnemonic to categorize and check for the most common reversible causes of cardiac arrest. This framework serves as a checklist for clinicians to systematically investigate potential non-cardiac reasons for the heart stopping. Addressing these causes must happen immediately and concurrently with cardiopulmonary resuscitation (CPR) to maximize the patient’s chance of a positive outcome. The H’s represent internal metabolic imbalances, while the T’s signify physical obstructions or external agents.
Causes Related to Internal Imbalances (The H’s)
The first group of reversible causes, the H’s, involves five distinct internal imbalances. These imbalances destabilize the body’s systems enough to cause cardiac arrest.
Hypovolemia
Hypovolemia, or low fluid volume, is a common and treatable cause, resulting from significant fluid loss (e.g., severe bleeding or dehydration). When blood volume drops, the heart cannot pump enough blood to maintain circulation, leading to shock and eventual arrest. Rapid replacement of fluids or blood products treats this circulatory failure.
Hypoxia
Hypoxia, or insufficient oxygen supply to the body’s tissues, is a frequent cause. The lack of oxygen forces cells to switch to anaerobic metabolism, resulting in cellular energy failure and increased acid production. This quickly impairs the heart’s electrical and mechanical function, often leading to pulseless electrical activity (PEA) or asystole. Causes range from airway obstruction to severe lung conditions like pneumonia.
Acidosis
Excessive Hydrogen ions, or acidosis, severely compromise heart function by interfering with muscle contractility and reducing responsiveness to adrenaline. Acidosis develops rapidly during cardiac arrest due to the buildup of metabolic lactic acid and carbon dioxide from poor circulation. This acidic environment further impairs the heart’s ability to contract.
Hypo-/Hyperkalemia
Disruptions in the body’s electrolyte balance, particularly with potassium, can lead to Hypo-/Hyperkalemia. Potassium is necessary for maintaining the normal electrical rhythm of the heart, and levels that are too high or too low can cause life-threatening arrhythmias or a complete standstill. Elevated potassium levels (hyperkalemia) decrease the excitability of heart muscle cells, often leading to a slow heart rate, conduction blocks, and ultimately cardiac arrest.
Hypothermia
Hypothermia, defined as a core body temperature below 95°F (35°C), causes cardiac arrest by slowing the heart’s electrical activity and making the heart muscle irritable. As body temperature drops further, the heart becomes prone to dangerous rhythms like ventricular fibrillation or asystole. Severe hypothermia can be protective, and patients are often not considered dead until they have been actively rewarmed to a near-normal temperature.
Causes Related to Physical Obstruction or External Agents (The T’s)
The second group of reversible causes, the T’s, involves four physical or external factors. These factors abruptly stop the heart by creating an obstruction or toxicity.
Tension Pneumothorax
Tension Pneumothorax occurs when air becomes trapped in the chest cavity, typically from a lung injury, creating immense pressure that cannot escape. This pressure collapses the lung and pushes vital structures to the opposite side of the chest, severely impeding the return of blood to the heart and causing circulatory collapse.
Cardiac Tamponade
Cardiac Tamponade is a mechanical obstruction where fluid or blood rapidly accumulates in the pericardial sac, the thin membrane surrounding the heart. Because the sac is relatively inelastic, the fluid compresses the heart chambers, preventing them from filling properly with blood. This physical compression drastically reduces the heart’s output, leading to obstructive shock and eventual cardiac arrest.
Toxins
Toxins encompass a wide range of external agents, including drug overdoses and accidental poisonings, that can precipitate cardiac arrest through various mechanisms. These substances may directly poison the heart muscle, depress the central nervous system to stop breathing, or disrupt the heart’s delicate electrical pathways, leading to fatal arrhythmias. Prompt identification of the substance is essential for administering specific antidotes when available.
Thrombosis
Thrombosis, or the formation of a blood clot, represents two distinct causes: a massive Pulmonary Embolism (PE) or a large Coronary Thrombosis. A pulmonary embolism is a clot that lodges in the main artery of the lungs, causing a backup of blood into the right side of the heart and leading to sudden heart failure. A coronary thrombosis is the clot responsible for a major heart attack, blocking blood flow to the heart muscle and triggering a lethal electrical rhythm.
Why Rapid Identification is Vital
The ability to reverse a cardiac arrest depends on the speed at which the underlying cause is identified and treated. Correcting an H or T provides a pathway for the heart to resume a normal, functional rhythm, unlike non-reversible causes (e.g., end-stage heart disease). Prompt diagnosis during resuscitation significantly increases the patient’s survival rate and improves neurological outcome.
Diagnosis often relies on quickly gathering information from bystanders or family members about the events leading up to the collapse, which is crucial for pointing toward an obvious H or T. Medical teams also use diagnostic tools, such as bedside ultrasound or blood gas analysis, performed while chest compressions continue. For instance, an ultrasound can reveal a cardiac tamponade or pulmonary embolism, allowing for a life-saving procedure to be performed.
Correcting the reversible cause must happen simultaneously with high-quality CPR. Chest compressions buy time, but only treating the underlying problem will achieve a sustained return of circulation. For instance, simply providing oxygen for a patient with hypoxia is not enough if the cause is an obstructed airway that requires immediate clearance. The coordinated effort to diagnose and treat these specific conditions transforms an otherwise fatal event into a successful resuscitation.