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Sudden cardiac arrest (SCA) is a medical emergency that can cause irreversible brain damage or even death, if not treated within minutes. The heart pumps blood to the brain at a particular rate and in a regular rhythm so that there is a constant supply of oxygen to the brain. This is very essential for the proper functioning of the brain. Any disturbance in the electrical system of the heart disrupts the normal pumping mechanism and can cause severe and irreparable damage to the brain.
This technique of lowering the body temperature to prevent or reduce damage to the tissues is called therapeutic hypothermia.
Sudden cardiac death – unexpected death due to cardiac cause
An unexpected loss of heart function (sudden cardiac arrest) represents danger to every system of the human body. Blood flow to the vital organs stops abruptly and the main sufferer is the brain. Unexpected death that occurs due to a sudden cardiac arrest is referred to as Sudden Cardiac Death (SCD). It stands out as a leading cause of death worldwide, and ventricular fibrillation is reported to be the underlying cause in most of the cases. Myocardial infarction (heart attack), coronary artery disease, other rhythmic and structural abnormalities of the heart, stroke and pulmonary embolism constitute some of the other causes of SCD.
A few minutes of disrupted blood supply to the brain can cost lives
A constant supply of oxygen and glucose is essential for the survival of the brain tissues. When blood flow to the brain ceases following a cardiac arrest, the supply of oxygen and glucose is cut off.
The brain tissues switch on to anaerobic metabolism and the resulting cascade of metabolic events cause anoxic brain injury to set in.
Even when circulation is restored after some time, the reperfusion (return of blood to the brain tissues) can itself cause secondary brain injury by various inflammatory responses and chemical reactions that occur over a period of time (minutes, hours or days). Many patients who survived a cardiac arrest after return of spontaneous circulation (ROSC) die after some days due to the brain injury. One study suggests that two-thirds of the patients who died after out-of-hospital cardiac arrest and one-fourth of the patients who died after in-hospital cardiac arrest died due to such brain injuries. Hence it is crucial that the brain’s metabolic rate be reduced during the period after reperfusion so that the brain damage is minimized.
Therapeutic hypothermia can help reduce brain injury
It has been suggested that efforts to cool the brain by inducing hypothermia can limit the brain damage and thus improve the survival rate and the neurological outcome. The several possible mechanisms by which hypothermia limits brain damage are:
- Hypothermia reduces the metabolic rate of brain.
- Hypothermia suppresses certain chemical reactions like the release of excitatory amino acids and calcium shifts that are associated with the reperfusion injury.
- Hypothermia suppresses the release of free radicals and attenuates the inflammatory responses that follow reperfusion injury.
- It reduces blood brain barrier disruptions and decreases the permeability of blood vessels, thus preventing the development of cerebral edema.
- Hypothermia helps stabilize the cell membranes.
- It improves brain glucose metabolism and hence the ability of brain to utilize glucose is increased.