By Holly Goodrick
Hypothermia is a condition where the body’s temperature is lower than 37 degrees Celsius because the body loses more heat than it can produce. It can sometimes be fatal and around 20,000 people in the United Kingdom die of hypothermia every year. However, hypothermia can be deliberately induced in hospitals to help patients whose brain doesn’t receive sufficient oxygen to function.
Therapeutic hypothermia, also known as ‘targeted temperature management’, is a procedure where surgeons reduce the core temperature of cardiac arrest patients to around 32-34 degrees Celsius to prevent brain damage and even death due to lack of oxygen. The procedure can also be used when a surgeon operates on a new-born baby with heart defects because during the operation the surgeon must ‘turn off’ the circulatory system temporarily and this could result in brain damage due to lack of oxygen. In addition, therapeutic hypothermia protects the brain so allows the doctors to operate for longer during open-heart surgery.
This method is extremely effective because every 1 degree Celsius drop in body temperature leads to 6-10% less cerebral metabolism and therefore 6-10% less demand for oxygen. This means that cellular damage is less likely.
On the other hand, this method is dangerous because if core temperature drops by only a few degrees, it could cause organ damage and even death.
The History of Therapeutic Hypothermia
In the 1950s, scientists discovered that therapeutic hypothermia could protect the grey matter in the CNS from ischemia (the lack of blood flow) and anoxia (the lack of oxygen). Therefore, therapeutic hypothermia became a common procedure in hospitals in the 1950s during open-heart surgery. Recently, scientists have found methods that allow most cardiac surgeries to take place on beating hearts, but before this, surgeons had to stop the heart to perform the operation. This meant that, without therapeutic hypothermia, doctors had around 5 minutes before they had to get the heart pumping again because brain damage would start to occur.
However, scientists believed that the method led to severe side effects like pneumonia, so it wasn’t used very much until the 1990s.
What Happens without Therapeutic Hypothermia
If someone suffers from cardiac arrest out of hospital, it will kill 94% if people. If a person survives, their organs could suffer if the blood flow providing oxygen is cut off or if it returns suddenly. This is known as ischemia-reperfusion injury.
Without oxygen, mitochondria can’t perform aerobic respiration so cells use anaerobic respiration, which causes lactic acid, a toxic substance, to form and causes calcium to build up in the cell. The increased levels of calcium leads to the neurotransmitter glutamate being secreted, which excites the brain, leading to an increased need for oxygen and triggering harmful chemicals to be released, like free radicals, which are very reactive and cause inflammation and swelling, causing more brain damage.
How Therapeutic Hypothermia Helps
Its main function is to slow the metabolic rate of the brain, which reduces oxygen demand by the brain, which prevents the mitochondria producing lactic acid and prevents glutamate being secreted to prevent cell excitation.
Inducing Therapeutic Hypothermia
Many methods are used by hospitals to induce the hypothermic state:
Trans-nasal evaporative cooling- this involves a tube spaying a coolant mist into the patient’s nasal cavity, cooling the brain and bloodstream
Water blankets and caps- a relatively affordable method that involves cold water being circulated over the patient. However, attention is needed to prevent any fluctuations in temperature
Cooling catheter- a cold saline solution is injected into the bloodstream through a catheter put into the femoral vein
The surgeon cools the patient to the target temperature very quickly using one of the methods above. During the cooling, the doctors sedate the patient and prevent any shivering by using a paralytic, because shivering increases metabolic activity, raising body temperature. However, if the target temperature is too low, it could lead to problems with heart rhythms and if it’s too high, it will decrease the positive effects.
The patient is then kept in the target range (usually between 32 and 34 degrees Celsius) during the procedure and all fluctuations of temperature are avoided.
The patient is then slowly rewarmed at about 0.3 degrees per hour.
It’s a risky method because it could cause a myriad of side effects like blood glucose problems, pneumonia, and if the body is warmed up too fast, even hyperthermia (an increase in body temperature). Therefore, vital signs need to be carefully monitored during the procedure.
The future of Therapeutic Hypothermia
There are numerous problems with the technique that need to be solved:
The procedure is difficult and expensive and patients must remain in hospital for almost a week afterwards.
Scientists don’t know the precise temperature a patient should be cooled to. Some say 33 degrees gives the best results but others say that 36 degrees gives the same results and reduces complications and side effects.
In conclusion, therapeutic hypothermia has helped many people who have suffered from cardiac arrest; however, a drug that stops the brain needing so much oxygen with fewer severe side effects would be a good replacement.