Cardiac arrest occurs when the heart stops beating effectively, resulting in the cessation of blood flow to the brain and other organs. This collapse of the circulatory system is often caused by an electrical malfunction in the heart, leading to a life-threatening arrhythmia. While cardiac arrest is frequently fatal, many cases are attributed to underlying conditions that are immediately treatable, known as reversible causes. Identifying and correcting one of these conditions is the primary strategy for achieving a return of spontaneous circulation and improving survival.
Causes Stemming from Internal Fluid and Chemical Imbalances
The most common group of reversible causes involves profound disturbances in the body’s internal environment, often summarized by several conditions beginning with the letter H. A severe loss of fluid volume, known as hypovolemia, is one such cause, typically resulting from major hemorrhage or extreme dehydration. When the total volume of blood drops significantly, the heart cannot generate enough pressure to circulate oxygen, causing the heart muscle to fail from lack of fuel and mechanical strain.
Hypoxia, or an inadequate supply of oxygen to the tissues, is another fundamental cause that leads to cardiac arrest through a cascade of cellular failure. Without oxygen, the heart muscle cells shift from efficient aerobic metabolism to anaerobic metabolism, which rapidly depletes their energy stores of adenosine triphosphate (ATP). This energy depletion prevents the proper function of ion pumps within the cell membranes, leading to cellular dysfunction and the inability of the heart to contract effectively.
Profound acidosis, a state of excessive acidity in the blood, often develops as a consequence of this anaerobic metabolism due to the buildup of lactic acid and carbon dioxide. This chemical imbalance directly interferes with myocardial contractility, reducing the force of the heart’s squeeze regardless of the electrical activity present. Furthermore, acidosis reduces the effectiveness of life-saving medications like adrenaline, as the highly acidic environment prevents these drugs from binding properly to their target receptors.
Abnormal levels of the electrolyte potassium, termed hypo- or hyperkalemia, represent a direct threat to the heart’s electrical stability. Potassium is necessary for regulating the resting electrical potential of heart muscle cells. Too little potassium (hypokalemia) can make the cells overly excitable, leading to erratic, rapid rhythms like ventricular fibrillation. Conversely, an excess of potassium (hyperkalemia) makes the heart muscle sluggish and less able to conduct impulses, resulting in a slow, weak rhythm or complete asystole.
The final chemical imbalance is hypothermia, where a core body temperature below 89.6°F (32°C) slows all metabolic processes. While cold can be protective by reducing the heart’s oxygen demand, severe hypothermia causes the heart to become irritable and highly prone to developing lethal arrhythmias.
Causes Related to Physical Compression or Pressure
A different class of reversible causes involves mechanical obstructions that prevent the heart from performing its pumping function. One such obstruction is Cardiac Tamponade, which occurs when fluid collects within the pericardial sac surrounding the heart. This fluid accumulation places external pressure on the heart chambers, restricting them from fully expanding to receive blood. This restriction significantly reduces the volume of blood the heart can pump, leading to a drop in blood pressure and circulatory collapse. Prompt drainage of this fluid is necessary to relieve the pressure and restore proper heart function.
Another mechanical cause is Tension Pneumothorax, which involves a buildup of air pressure in the chest cavity, typically due to a lung injury. The trapped air causes the lung to collapse and pushes the central chest structures (mediastinum) to the opposite side. This shift compresses the large veins that return blood to the heart, severely restricting the heart’s ability to fill. This lack of blood return (decreased preload) starves the heart of the volume needed to maintain circulation, quickly leading to cardiac arrest. Rapid decompression is required to release the trapped air and restore normal circulatory mechanics.
Causes Involving Acute Vascular Blockages
Blockages within the vascular network can cause the circulatory system to fail, most notably through the sudden formation of a thrombus, or blood clot. Coronary Thrombosis involves a clot forming in an artery supplying the heart muscle, leading to a heart attack. This blockage starves the heart muscle of oxygen, making it electrically unstable. The lack of oxygen often triggers a fatal electrical short circuit, resulting in ventricular fibrillation, where the heart quivers uselessly instead of beating.
In contrast, Pulmonary Thrombosis, commonly referred to as a massive Pulmonary Embolism (PE), involves a large clot traveling to the arteries of the lungs. This clot obstructs the pathway for blood leaving the right side of the heart, preventing it from reaching the lungs for oxygenation. The right ventricle faces an insurmountable pressure barrier, causing it to fail rapidly. The resulting circulatory collapse is due to a sudden obstruction of blood flow leaving the heart, leading to profound shock.
Causes Resulting from External Agents
A final category of reversible causes involves external agents, such as pharmaceutical overdoses or exposure to poisons, that interfere with the heart or the body’s ability to oxygenate blood. Certain drugs, like cocaine or amphetamines, are sympathomimetics that act like an excessive rush of adrenaline. They can push the heart into an electrical overdrive, causing the heart rate to become fast and disorganized, leading to cardiac arrest. Conversely, depressant agents, such as opioids or beta-blockers, can severely depress the heart’s electrical system or cause the respiratory drive to fail, leading to severe hypoxia.
The direct cardiotoxic effect of some agents can cause the heart to slow down until it stops beating. Other poisons, such as carbon monoxide or cyanide, cause arrest by preventing the body’s cells from using the oxygen that is present. These agents are considered reversible because identifying and neutralizing the toxin can allow the heart and body systems to recover.