Freeze—You’re Under Deep Hypothermic Circulatory Arrest!
Stephen R. Barone MD
Michael G. Fitzsimons MD
A 50-year-old male who works as an aeronautical engineer presents sporting a bicuspid aortic valve and a “grande burrito” size (63 mm) ascending aorta. He is scheduled for both aortic valve replacement and ascending aortic replacement requiring deep hypothermic circulatory arrest (DHCA). Following initiation of cardiopulmonary bypass (CPB), the heater/cooler exchange jacket is dropped to 15°C, and after the patient reaches a nasopharyngeal temperature of 18°C, the rest of the operating room team is ready for DHCA. Are you? Hopefully, he’ll return to work for another 15 years after this operation, and his space shuttle will have a successful mission. To allow that, you’ll have to protect that great brain of his! Cover his head with ice bags, and prepare a brain cocktail with steroids, magnesium, and mannitol. Once circulation is arrested, ensure that the selective anterograde cerebral perfusion circuit flow is appropriate for his body size. Watch the clock; time is the enemy now, and the surgeon needs to be informed of the circulatory arrest time at regular intervals. Following complete rewarming of both core and shell temperatures, make sure that platelets, plasma, and red blood cells are available for transfusion if needed. With these maneuvers and a technically successful surgery, the patient has a great chance of going right back to being a rocket scientist!
Certain adult cardiac surgical procedures, primarily involving the ascending and descending aorta, aortic arch, and pulmonary vasculature require temporary interruption of cerebral blood flow to improve visualization and technical repair. This technique is referred to as DHCA (aka “circ arrest”) and is associated with high rates of neurologic compromise. Advanced age (older than 80 years) and DHCA times of >30 minutes are associated with higher rates of neurologic injury. The immediate energy-supply imbalance that occurs with “circ arrest” requires that the anesthesiologist consider the direct application of ice to the head (i.e., inducing an ice cream headache) and administration of a fancy drug cocktail in an attempt to preserve neurologic function. In addition, selective cerebral perfusion during DHCA has been observed to be of clinical benefit.
MECHANISM OF BENEFITS
The brain is exquisitely vulnerable to ischemic episodes due to its high cerebral metabolic rate of oxygen consumption (CMRO2) and yet relatively
limited reserve of high-energy phosphates. Hypothermia decreases the metabolic rate of neural tissue and increases the safe time that the brain can survive without oxygen and metabolic substrate. Silence of the electroencephalogram is only achieved in 50% of patients at 18°C but up to 99% at 12.5°C. Other suggested mechanisms of protection include inhibition of capsases, maintenance of microvascular integrity, prevention of glutamate-associated transient spikes in activity, suppression of inflammation, and decrease in edema formation. Most clinicians will cool systemically to between 15°C and 20°C and then maintain circulation at this temperature for approximately 5 minutes before bypass flow is ceased.
limited reserve of high-energy phosphates. Hypothermia decreases the metabolic rate of neural tissue and increases the safe time that the brain can survive without oxygen and metabolic substrate. Silence of the electroencephalogram is only achieved in 50% of patients at 18°C but up to 99% at 12.5°C. Other suggested mechanisms of protection include inhibition of capsases, maintenance of microvascular integrity, prevention of glutamate-associated transient spikes in activity, suppression of inflammation, and decrease in edema formation. Most clinicians will cool systemically to between 15°C and 20°C and then maintain circulation at this temperature for approximately 5 minutes before bypass flow is ceased.
The acceptable duration of circulatory arrest depends on the degree of hypothermia. According to the Kirkland normogram, at 15°C, 30 minutes is possible, and at 10°C, up to 40 minutes is possible. There is no consensus opinion about the acceptable duration, and the limit likely varies between individuals.
Known risks of hypothermia include injury to blood elements associated with cooling and warming, platelet dysfunction, and coagulopathy.