Hyperventilation in Neurological Patients

Introduction

The small blood vessels in the brain change their diameter according to carbon dioxide and oxygen concentrations. The blood vessels in the brain relax and contract according to the concentration of carbon dioxide, called cerebral vasomotor tone. Carbon dioxide controls the relaxation and contraction of blood vessels based on its concentration in the blood. Lower levels of carbon dioxide (hypocapnia) cause contraction of cerebral blood vessels, and higher levels of carbon dioxide (hypercapnia) cause relaxation of blood vessels in the brain. Hyperventilation reduces intracranial pressure and relaxes a tense brain to make it small and soft. The mechanism behind this phenomenon is that hyperventilation causes a drop in levels of carbon dioxide, which causes vasoconstriction, resulting in reduced cerebral blood flow and blood volume. As a result, intracranial pressure is reduced. But prolonged and forced hyperventilation is dangerous. So, treating neurological patients with hyperventilation comes with both advantages and disadvantages.

What Is the Effect of Hyperventilation on the Normal Brain?

The brain is relaxed, and there is a reduction in intracranial pressure when the patient is under hyperventilation. Intracranial pressure is measured within a closed cranium and is exerted by all of the contents inside the skull. Relaxation of the brain is related to the firmness and size of the brain according to the volume of the cranium, which is examined by the surgeon when a craniotomy is performed. Hyperventilation brings about hypocapnia, which produces cerebral vasoconstriction and reduced cerebral blood volume, decreasing the pressure within the cranium.

What Are Cerebral Blood Flow and Cerebral Blood Volume?

Two parameters to know that are used in hyperventilation: Cerebral blood volume (CBV) and cerebral blood flow (CBF). Cerebral blood volume is the volume of blood in the brain, which is a static concept. But cerebral blood flow is a dynamic concept defined as the blood volume penetrating the brain per unit of time. In healthy people, during hyperventilation, there is approximately a 30 percent decrease in cerebral blood flow and a seven percent decrease in blood volume when carbon dioxide levels are decreased from a normal concentration to a lesser concentration. And in neurological patients like those with traumatic brain injury, there is approximately a 34 percent decrease in cerebral blood flow and a nine percent decrease in cerebral blood volume when the concentration of carbon dioxide is reduced from normal during hyperventilation.

What Are the Effects of Hyperventilation on Metabolic Activity in the Brain?

Hyperventilation causes hypocapnia, which in turn increases cerebral metabolic activity. There is an increase in neurons' excitability and seizure activity duration during hypocapnia. This results in an increase in the consumption of oxygen and glucose and an increase in the production of amino acids. Because of all these changes, the metabolism changes to an anaerobic type. Also, there is a shift in the oxygen-hemoglobin dissociation curve to the left, resulting in a decreased concentration of oxygen in tissue and a decrease in oxygen due to reduced cerebral blood flow. These changes disrupt the balance between demand and supply, and thus there is increased demand than supply of oxygen. Hyperventilation has different effects on a healthy brain and a brain with an acute injury. Although hyperventilation has the same effects on the normal and injured brains as a reduction in cerebral blood flow, the reduction is stronger in the normal brain. This happens because cerebral blood flow is redistributed from normal to injured brain cells. The fact that an acutely injured brain has increased blood flow supports the above statement. Hyperventilation reduces cerebral blood flow, which leads to decreased oxygen supply to the brain, and reduces cerebral blood volume, which relaxes the brain and decreases intracranial pressure. The effects of hyperventilation on the metabolic activity of the brain need to be studied further.

What Is the Effect of Hyperventilation on Traumatic Brain Injury?

• According to research, prolonged hyperventilation has a negative impact on patients with traumatic brain injury.

• It is not recommended to use prophylactic hyperventilation to create a partial pressure of fewer than 25 millimeters of mercury or carbon dioxide during the first 24 hours after the brain injury when there is a very low level of cerebral blood flow.

• Thus, hyperventilation is only a temporary remedy for reducing intracranial pressure. When used, there should be continuous monitoring of jugular venous oxygen saturation (SjvO2) and partial pressure of oxygen in the brain to determine the amount of oxygen saturation and the blood perfusion in brain tissue.

What Is the Effect of Hyperventilation During Intracranial Hemorrhage?

• Patients with intracranial hemorrhage will often result in mortality or permanent disabilities like motor and sensory paralysis.

• Hyperventilation is used as a temporary treatment to reduce the raised intracranial pressure and stop further damage to the brain.

But studies show that hyperventilation has both good and bad effects, so it is unclear whether it should be used during an intracranial hemorrhage.

What Is the Effect of Hyperventilation During a Craniotomy?

• A craniotomy is a surgical procedure that involves making a hole in the skull to visualize the brain, either for tumor removal or to take a sample of brain tissue for a biopsy.

• Hyperventilation is used for a craniotomy to make the brain small and soft. This will make the brain more relaxed and thus improve the surgical condition. But the question is whether normal brain tissue will experience less tissue perfusion simultaneously.

• To conclude, a balance between the operating condition and tissue perfusion in the brain should be maintained while hyperventilating.

• Hyperventilation is commonly used to relax the brain while performing craniotomies, but supporting evidence on their beneficial effects on the prognosis after craniotomies is still lacking.

Conclusion:

Hyperventilation is beneficial to neurological patients in that it helps relax a tense brain and elevated intracranial pressure. But it is crucial to make a precise decision to balance hyperventilation's good and bad outcomes. Hyperventilation, in particular, has been helpful during craniotomies, but its consequences during traumatic brain injury and intracranial bleeding are still unknown. Further research and investigation are needed to make hyperventilation the primary choice of management for all neurological patients, irrespective of how long and at what potency it is used.