Deep Hypothermic Circulatory Arrest

Introduction

The hypothermic circulatory arrest is implanted as the primary neuroprotection mechanism, as it limits brain metabolism and slows injury-inducing pathways. Deep hypothermic circulatory arrest is used in certain surgeries where the underlying condition or the nature of the surgery necessitates complete cessation of blood circulation to provide benefits such as improved surgical field exposure due to its capacity to give less field and reduced postoperative edema.

What Is a Deep Hypothermic Circulatory Arrest?

The technique of body cooling combined with the stoppage of blood flow is called deep hypothermic circulatory arrest. This technique maintains perfusion to other body organs. Preservation of this organ functions during a circulatory arrest is done by decreasing the body's core temperature. A deep hypothermic circulatory arrest is a simple and effective technique for brain protection during lengthy and complex surgeries. In addition, it also drastically reduces the risk of morbidity and mortality following such surgeries.

What Are the Indications for the Use of Deep Hypothermic Circulatory Arrest?

1. Cardiac Surgery:

• Aneurysm, rupture, or dissection of the aortic arch.

• Pulmonary thromboendarterectomy (surgery to remove large blood clots from blood vessels of the lung).

• Complex congenital heart defects repair surgeries such as transposition of the great arteries, hypoplastic left heart syndrome, and total anomalous pulmonary venous return.

• Vascular reconstruction during a heart transplant.

2. Non-Cardiac Surgery:

• Thoracoabdominal aorta procedures.

• Giant cerebral aneurysm repair surgery.

• Cerebral arteriovenous malformations rejection surgery

• Renal cell carcinoma with caval invasion resection surgery.

• Other tumors with caval invasion resection surgery.

What Is the Mechanism by Which Deep Hypothermic Circulatory Arrest Protects the Brain?

The brain utilizes 20 % of the total body oxygen consumption, and the heart provides 15 to 20 % of the total circulating blood volume to the brain. This is because the brain’s metabolic rate of glucose and oxygen consumption is multiple times quicker than other body organs. Unlike the liver or other tissues and muscles, the brain does not store glucose. Thus, a shortage of glucose delivery immediately impairs neuronal function. Relevant changes in the blood flow compensate for changes in oxygen and glucose delivery levels. This phenomenon is known as autoregulation of cerebral flow.

So, in the case of surgical procedures following events can lead to neural injury,

1. When there is a lack of oxygen, anaerobic glycolysis takes place to maintain normal neuronal function, which is also insufficient for the brain. Concurrently, lactate also gets collected in the neurons, which lowers the intracellular pH. Such depletion of brain energy and waste product accumulation in the brain cells rapidly leads to permanent damage and necrosis of brain tissues.

2. Lack of oxygen in brain cells leads to the release of excitatory neurotransmitters like glutamate, which activates specific brain channels. As a result, calcium ions can quickly enter the cells and accumulate from these activated channels. Such calcium level imbalance leads to the activation of intracellular dysfunctions, which in turn results in brain cell death.

Hypothermia significantly decreases glucose and oxygen's global cerebral metabolic rate. The cellular metabolism slows down by five to seven percent for every one-degree celsius decline in body temperature. Thus hypothermia decreases the demand for oxygen in brain cells. Experimental studies established that at 18 degrees Celsius, the body's metabolic rate is 12 % to 25 % of the average temperature metabolic rate.

What Is the Technique of Deep Hypothermic Circulatory Arrest?

The essential components of attaining deep hypothermic circulatory arrest as given below,

• Adequate anticoagulation before beginning deep hypothermic circulatory arrest is essential.

• Elimination of glucose from all intravenous solutions to decrease the risk of hyperglycemia (increased blood glucose levels).

• Administration of anesthetic agents and neuromuscular blocking drugs to reduce oxygen consumption and assure paralysis.

• Deep anesthesia levels may reduce the harmful physiologic stress reactions to this procedure.

• Reduction of temperature to 15 to 22 degrees Celsius. The cooling is induced slowly (over 30 to 60 minutes) to provide homogenous hypothermia. Cooling takes 30 to 40 minutes, depending on the patient's size.

• Temperature monitoring is performed solely by a probe in the urinary bladder.

• The head is sealed in ice to achieve topical cooling.

• Steroids are routinely administered before all surgeries.

• Alpha stat strategies manage the acid-base balance in the blood.

• Verification of cerebral electrical silence is done via electroencephalography.

• After the termination of deep hypothermic circulatory arrest, the rewarming usually takes about an hour. Gentle rewarming is preferred to stop potential protein denaturation.

What Are the Advantages of the Deep Hypothermic Circulatory Arrest Technique?

• Provides safety for short periods of circulatory arrest.

• Avoids cross-clamping of a diseased aorta.

• Generally can be used in straightforward cases.

What Are the Limitations of the Deep Hypothermic Circulatory Arrest Technique?

• Questions of safety in case of more extended periods of circulatory arrest.

• More incidence of permanent brain injury with more than 45 minutes of circulatory arrest.

• They contradict data on brief effects with shorter (around 25 minutes) ischemia.

• Modest, long-term cognitive dysfunction, particularly problems with short- and long-term information processing and memory, are common.

What Are the Complications of Deep Hypothermic Circulatory Arrest?

Rewarming the patient from deep hypothermic circulatory arrest is not without risk. Rewarming reperfusion with cold blood before at least ten minutes facilitates the removal of free radicals and metabolic waste.

• However, due to rising cerebral blood flow, excessively fast rewarming upsurges the risk of cerebral embolization, edema, and hyperthermic cerebral injury.

• The long duration of surgical procedures with a deep hypothermic circulatory arrest can cause pressure sores and accidental damage to the eyes, peripheral nerves, nerve plexuses, and pressure points.

• Coagulopathic hemorrhage is a significant cause of early death and morbidity after deep hypothermic circulatory cardiac arrest.

Conclusion

Deep hypothermic circulatory hypothermic arrest provides effective and safe neural protection for most emergency and elective cases. This technique surpasses other cerebral protection methods to minimize stroke rates and mortality and preserve postoperative cognitive function. It is convenient, simple, effective, and can justifiably be a preferred technique for cerebral protection. However, 8 to 15 % mortality rates are reported following this procedure, with stroke rates of 7 to 11 %. In addition, most can tolerate 30 minutes of deep hypothermic circulatory arrest without significant neurological dysfunction. However, when this extends longer than 40 minutes, there is a substantial increase in the occurrence of brain injury. Above 60 minutes, most individuals will suffer irreversible brain damage.