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Craniotomy
1
How do anesthetic requirements differ during various time points in a craniotomy?
A craniotomy is unique in that the level of nociceptive stimulus varies greatly and the portions of the procedure that require deep anesthesia are mostly at the beginning. Deep anesthesia is essential during laryngoscopy (and intubation) to block any harmful increases in heart rate, blood pressure, and brain metabolic activity, which may increase intracranial pressure (ICP). Soon after intubation, placement of pins in the skull for head positioning is common and often necessitates deep anesthesia. Once these events conclude, considerable time may pass with little to no noxious stimuli. Patient positioning and operative preparation often takes considerable time, and maintenance of deep anesthesia may require vasoactive agents for hemodynamic support. If the plane of anesthesia is “light” during this period, careful anticipation that the depth of anesthesia will need to be increased immediately before incision of the scalp, opening of the skull, and reflection of the dura because these events provide increased surgical stimuli. Once the surgeon begins dissection of the brain or pathological tissue, noxious stimuli are minimal because these structures are essentially void of nociceptive nerve fibers.
2
Discuss the various monitors used for a craniotomy.
Aside from the standard American Society of Anesthesiology monitors, patients may require invasive monitoring, central venous access, and neuromonitoring. Invasive monitoring typically includes an arterial line to assess hemodynamic changes and intravascular volume status. A central venous catheter should be considered if there is an elevated risk of venous air embolism, a high likelihood of using vasoactive infusions perioperatively, or to administer hypertonic saline to treat intracranial hypertension. Neuromonitoring, such as continuous electroencephalogram (EEG); somatosensory, motor, and brainstem auditory evoked potentials; and ICP monitoring may be helpful, depending upon the nature of the surgery and surgeon preference. 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 the intraoperative setting.
3
What are the implications with fluid administration during a craniotomy?
The patient’s intravascular volume status can significantly affect the surgeon’s ability to visualize, dissect, and/or resect tissue. Sudden increases in intravascular volume, before opening the dura, may cause an exponential increase in ICP, especially in the setting of intracranial hypertension or when the ICP is already borderline elevated. However, hypotension because of hypovolemia may require volume resuscitation to restore normal cerebral perfusion. Therefore fluids should be administered judiciously to avoid both hypo- and hypervolemia.
4
Which fluids are safe to administer? Which should be avoided?
Only isotonic or hypertonic intravenous fluids should be administered. Hypotonic fluids should be avoided, which can exacerbate cerebral edema. Recall, the tonicity of fluids refers to normal patient serum osmolarity where hypertonic (hyperosmolar) fluids will have a higher osmolarity compared with normal serum osmolarity (275–295 mOsm/L). Unless hypoglycemia is documented, glucose-containing solutions should also be avoided, because hyperglycemia can negatively affect neurological outcomes. Normal saline (0.9%) and balanced salt solutions are categorized as isotonic fluids and are safe to give; although, technically, normal saline (0.9%) has a slightly higher osmolarity compared with balanced salt solutions (i.e., PlasmaLyte, Ringer’s lactate). Colloid solutions, such as isotonic 5% albumin or hypertonic 3% saline, are equivalent solutions for acute volume replacement. Hypertonic 25% albumin may be considered in situations where the patient is overall hypervolemic but intravascularly “dry,” which often occurs with hypoalbuminemia (e.g., cirrhosis, malnutrition, nephrotic syndrome). There is a concern that albumin administration is associated with worse clinical outcomes in the setting of traumatic brain injury (TBI). However, the oft-cited study (Saline vs. Albumin Fluid Evaluation trial) found this association with hypotonic 4% albumin, so it is uncertain if this finding can also be extrapolated for isotonic (5%) or hypertonic (25%) albumin.
5
What are the goals to “protect” the brain?
Brain protection refers to strategies to support the balance between brain metabolism and substrate delivery, while also preventing secondary injury to regions of the brain, following an episode of ischemia. The need for brain protection should be anticipated in the setting of TBI, stroke, and in various neurosurgical operations. Of primary importance is the adequate delivery of oxygen and energy substrates to brain tissue by maintaining optimal blood oxygen content and cerebral blood flow (CBF).
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