Cardiac arrest is the sudden and complete cessation of effective blood circulation, leading to immediate loss of consciousness and collapse. It is often confused with a heart attack, which is a circulation problem caused by a blocked artery. Unlike a heart attack, where the heart usually continues to beat, cardiac arrest is an electrical problem where the heart malfunctions and stops pumping blood entirely. Many causes of cardiac arrest are reversible, often stemming from non-cardiac factors that affect the body’s environment or mechanical function.
Systemic and Chemical Imbalances (The H Causes)
Systemic and chemical factors, known as the “H” causes, can disrupt the body’s internal environment and halt the heart’s function. Hypoxia, a lack of sufficient oxygen delivery to tissues, is a common reversible cause. Without adequate oxygen, cells produce acidic byproducts, leading to cellular dysfunction and the heart’s failure to beat effectively.
Hypovolemia, or severe loss of blood or bodily fluids, is another frequent cause, often stemming from trauma or dehydration. A reduced circulating volume decreases the blood returning to the heart, drastically lowering output and preventing vital organs from receiving necessary blood flow. This can result in pulseless electrical activity (PEA), where the heart shows electrical activity but cannot generate a measurable pulse.
An imbalance in hydrogen ions, known as acidosis, represents a state where the blood becomes too acidic, which significantly impairs heart function. This acidity often results from prolonged hypoxia or poor circulation, as the body struggles to eliminate metabolic waste. Critical imbalances of the electrolyte potassium (hypo- or hyperkalemia) can also cause immediate cardiac arrest. Potassium is essential for electrical signaling in heart cells, and levels that are too high or too low destabilize the heart’s rhythm.
Hypothermia, defined as a core body temperature below 36.0°C, slows all metabolic processes, including the heart’s electrical activity. As temperature drops, the heart becomes irritable and prone to dangerous rhythms, or it may slow to a halt. Rewarming is a necessary first step, as defibrillation and other resuscitation efforts are often ineffective in a severely cold state.
Physical and Pressure-Related Causes
Physical and pressure-related issues can mechanically obstruct the heart’s ability to fill or pump blood, leading to obstructive shock. Tension pneumothorax occurs when air becomes trapped in the chest cavity, often due to trauma. This trapped air builds pressure, collapsing the lung and pushing central chest structures, including the heart, to the opposite side. This compression prevents blood from returning to the heart, causing circulatory collapse.
A second mechanical cause is cardiac tamponade, which occurs when fluid, typically blood, rapidly accumulates in the pericardial sac surrounding the heart. Since the pericardial sac is not highly elastic, this fluid buildup squeezes the heart, preventing its chambers from fully relaxing and filling with blood. This effect drastically reduces the amount of blood the heart can pump out, causing circulation to fail. Both conditions require immediate physical intervention, such as draining the fluid or releasing the trapped air, to relieve the pressure.
Vascular Blockages and Toxic Exposure
Failures of blood flow and external chemical interference are grouped under the “T” causes. Thrombosis, the formation of a blood clot, is a major cause, especially in the coronary arteries or the pulmonary circulation. Coronary thrombosis blocks blood flow to the heart muscle, causing a heart attack that can trigger a fatal electrical disturbance.
Pulmonary thrombosis, or pulmonary embolism (PE), involves a clot that blocks a major artery in the lungs. This blockage creates severe resistance to blood flow from the right side of the heart, preventing oxygenated blood from returning to the left side. The heart fails because it cannot overcome this resistance, leading to rapid circulatory shock.
Toxins, including drug overdoses or exposure to poisons, represent a broad category of chemical interference that can lead to cardiac arrest. Many substances disrupt the heart’s electrical system, causing chaotic and ineffective rhythms. Others cause a profound drop in blood pressure or depress the heart muscle’s ability to contract. Identifying the specific toxin is the first step in treatment, as targeted therapies or antidotes may be available to reverse the drug’s effects.
Why Quick Recognition Saves Lives
The array of reversible causes highlights why resuscitation extends beyond chest compressions and delivering a shock. Medical professionals use a systematic approach, known as a differential diagnosis, to simultaneously look for and treat these underlying issues. The initial heart rhythm provides a rapid clue; for example, a non-shockable rhythm like PEA is frequently associated with the “H” and “T” causes.
The urgency of treatment is directly tied to the specific cause, as each requires a different, immediate action. Hypovolemia is treated with rapid fluid administration, while a tension pneumothorax demands a needle decompression to release trapped air. This quick diagnostic process allows clinicians to deliver a targeted intervention that resolves the root cause. The time-critical nature of these treatments means that failure to address the underlying issue significantly lowers the chance of survival.