Introduction
Patients with diseases involving the central nervous system (CNS) may require surgery to treat the underlying cause. The patient’s positioning, need for vigilant monitoring during surgery, and risk of acute and long-term neurologic sequelae make neurosurgical procedures unique. The knowledge of cerebral physiology and the effect of anesthetic drugs on cerebral homeostasis are a must to ensure optimal patient care with improved outcomes. This chapter will overview the important neurosurgical conditions along with the anesthetic considerations.
Cerebral Physiology and Pharmacology
Almost two-third of total oxygen consumption by the brain is used for adenosine triphosphate (ATP) production to support neuronal electrical activity. Interruption of cerebral perfusion for 10 seconds or more results in complete unconsciousness. Failure of restoration of blood flow within 3 to 8 minutes results in irreversible cellular injury. Important cerebral physiological parameters are discussed below.
Cerebral Blood Flow
Cerebral blood flow (CBF) depends on cerebral metabolic activity, cerebral perfusion pressure, arterial carbon dioxide, and oxygen tension. CBF averages 50 mL/100 g/min at a PaCO2 of 40 mm Hg in an awake healthy individual with intact autoregulation. The flow in gray matter is around 80 mL/100 g/min and white matter is 20 mL/
100 g/min. The total flow in adults averages is 750 mL/min (15 to 20% of cardiac output). CBF can be measured by various methods like CT perfusion scans directly, but bedside monitoring of CBF is not possible by these methods. Indirect measures to estimate the adequacy of CBF and brain tissue oxygen delivery are transcranial Doppler (TCD), infrared spectroscopy, and brain tissue oximetry. The CBF is autoregulated and depends on multiple factors, as discussed below:
1. Cerebral perfusion pressure (CPP): It is the difference between mean arterial pressure (MAP) and intracranial pressure (ICP) or central venous pressure (CVP) if greater than ICP:
CPP is 80 to 100 mm Hg. Moreover, ICP is normally less than 10 mm Hg. Hence, CPP is proportional to MAP. CPP values less than 50 mm Hg often show slowing on the EEG, whereas those with a CPP between 25 and 40 mm Hg show flat EEG; less than 25 mm Hg can lead to irreversible brain damage.
Cerebral vasculature rapidly adapts to change in CPP. A decrease in CPP results in cerebral vasodilation, whereas elevations induce vasoconstriction. It remains constant between MAP of 60 and 160 mm Hg; outside these limits, blood flow will become pressure dependent.
Arterial tension of carbon dioxide: CBF changes approximately 1 to 2 mL/100 g/min per mm Hg change in PaCO2. Marked hyperventilation (PaCO2 < 20 mm Hg causes cerebral vasoconstriction with consequent fall in CBF).
Temperature: CBF changes 5 to 7% per 1°C change in temperature.
Viscosity: A decrease in hematocrit decreases viscosity and can improve CBF and vice versa with elevated viscosity.
3. Autonomic factors: The sympathetic and parasympathetic systems innervate intracranial vessels. Sympathetic stimulation induces vasoconstriction and limits CBF.
Effect of Anesthetic Agents on Cerebral Physiology
The anesthetic agents can affect the various aspects of cerebral physiology, as shown in Table 23.1.
Abbreviations: CBF, cerebral blood flow; CBV, cerebral blood volume; CMR, cerebral metabolic rate; CSF, cerebrospinal fluid; ICP, intracranial pressure.
It is important to understand the concept of flow-coupling metabolism to realize the effect of anesthetic agents on cerebral physiology. According to the flow-coupling metabolism theory, the CBF correlates with cerebral metabolism. The inhaled anesthetic decreases the cerebral metabolic rate, leading to a decrease in CBF. However, these agents are intrinsic vasodilators, causing a rise in CBF. Therefore, metabolism suppression-related decline in CBF is compensated by increased blood flow due to intrinsic vasodilation and matches at 1 MAC. All the inhaled anesthetics do not increase CBF and ICP and are safe even in patients with raised ICP.
General Anesthetic Considerations
The important anesthetic considerations for patients undergoing neurosurgical procedures are:
Provision of optimal operating conditions.
Use of physical/pharmacological means to maintain a stable ICP.
Maintenance of stable hemodynamics, oxygenation, and ventilation.
Maintaining appropriate CPP and oxygenation.
Early detection and prompt management of intraoperative complications:
Controlled but rapid emergence from anesthesia to enable early assessment and monitoring of neurological status.
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