73: Anesthesia for Craniotomy

Published on 06/02/2015 by admin

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CHAPTER 73 Anesthesia for Craniotomy

3 Should monitoring be different during a craniotomy?

The usual noninvasive monitors are used for every patient, including pulse oximetry, stethoscope, noninvasive blood pressure cuff, temperature, electrocardiogram, end-tidal and inspired gas monitors, and peripheral nerve stimulator. End-tidal anesthetic agent monitoring has some theoretic value, particularly in managing emergence. Continuous arterial pressure monitoring is often used to assess hemodynamic changes, which may develop acutely with cranial nerve root stimulation or slowly because of minimal intravascular volume repletion. Some forego the radial artery catheter for very superficial craniotomies such as mapping of the seizure focus directly with cortical electrodes; few anesthesiologists would use a central venous catheter unless there were a high risk of air entrainment in the venous system or a likelihood of using vasoactive infusions perioperatively. Occasionally continuous electroencephalography is used, not so much as an intraoperative monitor but rather as a means for the surgeon to localize diseased tissue. The various forms of processed electroencephalogram (EEG) monitors may facilitate the use of total intravenous anesthesia when indicated. Comparison of ipsilateral and contralateral evoked potentials has been reported during aneurysm surgery. Jugular bulb venous oxygen saturation and transcranial oximetry have been described as monitors of oxygen delivery and metabolic integrity of the brain globally but are not used regularly in intraoperative settings. Some patients, especially after trauma, have subdural, intraventricular, or cerebrospinal fluid pressure monitors in use intraoperatively.

4 Discuss the considerations for fluid administration during craniotomy

Volume depletion from overnight fasting and volume redistribution from vasodilating anesthetic agents result in relative hypovolemia. Each patient should be evaluated individually to ensure adequate myocardial, central nervous system, and renal perfusion. Special attention must be directed toward stability of intracranial volume. Before opening of the dura, sudden increases in intravascular volume may cause deleterious increases in ICP, especially in situations involving intracranial masses or contusions or intraparenchymal, subdural, or epidural hematomas. Therefore, although fluids must be given to avoid hypovolemia and hypotension, exuberant bolus administration is to be avoided.

The content of the fluids used during a craniotomy is also important. An isosmolar intravenous fluid should be chosen. Unless hypoglycemia is documented, glucose-containing solutions should be avoided. In both clinical and experimental settings in which glucose is used in the resuscitation fluids after head injury, outcome is worse. Saline is the appropriate fluid for use during craniotomy. Balanced salt solutions may be used if their osmolarity approximates or exceeds that of the serum. Ringer’s lactate has a slight theoretic disadvantage because lactate is metabolized and the solution becomes hypotonic. Colloid solutions such as 5% albumin or 3% NaCl are equivalent solutions for acute volume replacement before packed red cell administration. Often 25% albumin is used for pressure support when blood replacement is not needed. Hetastarch solutions should be limited to 15 to 20 ml/kg body weight during craniotomies because of concerns that larger quantities are associated with impaired coagulation in vitro.

6 How can the brain be protected?

Historically long-acting barbiturates have been used for metabolic suppression for refractory intracranial hypertension. The goal is suppression of brain activity with resultant reduction of metabolism, which is reflected by a flat EEG.

In the intraoperative setting metabolic suppression is needed when a major artery is temporarily clipped to facilitate access to an aneurysm. The EEG correlate is “burst suppression” in which the typical anesthetic slow-wave activity slows to random bursts of electrical activity. Burst suppression can be achieved by rapid infusion of thiopental, propofol, or etomidate. Hypothermia has long been known to reduce brain metabolism (and to slow the EEG). Mild to moderate hypothermia (32.5° to 34° C) has not been found to be useful for intraoperative brain protection. The global metabolic suppression secondary to hypothermia decreases not only neuronal electrical activity but also housekeeping functions, including cellular homeostasis and membrane integrity. Production of excitatory neurotransmitters during reperfusion of ischemic tissue may also be suppressed by modest hypothermia.

Much attention has been directed to suppression of the neuroexcitation that occurs with reperfusion after regional or global brain ischemia. Calcium influx into glial cells and vascular smooth muscle may be suppressed by calcium channel blockade; free radicals that are generated may be scavenged by mannitol, and increased intracellular hyperglycemia may be prevented by avoiding systemic hyperglycemia. Cerebral protection remains a fruitful area of investigation.

7 How is the choice of anesthetic agent made?

Choice of anesthesia for craniotomy is based on an understanding of the pharmacologic properties of hypnotic agents, inhalation agents, opioids, and muscle relaxants and on a balancing of beneficial and potentially adverse effects. Whichever agents are chosen, the goals are postoperative hemodynamic stability associated with an awake, neurologically assessable patient.

9 Why do some patients awaken slowly after a craniotomy?

Continuous infusion of opioid as part of balanced anesthesia leads to prolonged redistribution and persistent sedation. Residual volatile anesthetic or barbiturate may contribute to slow awakening. However, all these residual anesthetic effects are overcome simply by waiting and providing respiratory support. Use of agents of short duration is beneficial. Slow awakening that persists for more than 2 hours is rarely an effect of residual anesthesia. The patient who is unresponsive for several hours after a craniotomy should be evaluated for increased ICP, embolic phenomenon, brainstem ischemia, or intracranial masses. Evaluation should be a joint effort of the neurosurgeon and anesthesiologist. The anesthetic technique should be tailored to facilitate a rapid emergence for early testing of neurologic function.

10 What anesthesia problems are unique to surgery on the intracranial blood vessels?

11 Are there special anesthetic problems associated with brain tumors?

Mass lesions of the brain cause problems for the anesthesiologist because of their size and location. Frontal tumors grow to large size without producing neurologic symptoms or increased ICP. Supratentorial tumors of the motor and sensory cortical regions present with seizures, localizing neurologic signs, and increased ICP. Posterior fossa masses in adults cause disturbances in gait, balance, proprioception, or cranial nerve impingement. There is a penumbra around all intracranial tumors where the adjacent brain loses autoregulatory function. Thus on induction regional blood flow in these areas may increase in response to aggressive fluid replacement or increased systolic blood pressure. After the resection is completed, this penumbra may respond to reperfusion with swelling. The end result may be either preincisional or postoperative increases in ICP. Infratentorial posterior fossa tumors cause particular problems for the anesthesiologist. Tumors are generally small but may surround complex vascular channels of the basilar, posterior communicating, and cerebellar arteries. Tumors may arise from the glia surrounding the cranial nerve roots or impinge on them. Simple dissection of a brainstem tumor can cause disturbance of heart rate and rhythm or blood pressure when nerve roots are retracted. The surgical approach to the posterior fossa involves awkward positioning, from sitting to lateral to prone to park bench. At the least, any of these positions requires careful attention to the position of the endotracheal tube to avoid migration to an endobronchial position or out of the glottis. Venous air embolism must be anticipated. The plan for anesthesia must also allow for intraoperative monitoring of auditory-evoked potentials, somatosensory-evoked potentials, or motor-evoked potentials if indicated. Any of these evoked potentials can be suppressed by hypnotic and inhaled anesthetic agents.