Providing an anesthetic for neurosurgical procedures that include a craniotomy can involve unique anesthetic goals otherwise not encountered in routine practice. For example, tight hemodynamic control and cerebral protection techniques during aneurysm clipping or providing moderate sedation during intracranial tumor resection allow patients to cooperate during speech mapping. Anesthesiologists have suggested numerous dosing regimens to meet these goals. This chapter will briefly summarize a few of those designed for aneurysm clipping and awake procedures for tumor resection.
CRANIOTOMY FOR ANEURYSM CLIPPING
The incidence of unruptured intracranial aneurysms is thought to be near 2%. Approximately 10 in 100,000 people suffer a brain aneurysm rupture, making it frequently encountered problem in the operating room. The prognosis is poor; 40% of ruptured brain aneurysms are fatal and of those that survive, about two thirds have permanent neurologic deficits.1,2
Perioperative management of these patients focuses on 5 potentially conflicting goals:
Maintain adequate cerebral perfusion pressure (CPP) to prevent cerebral ischemia and cerebral vasospasm.
Maintain a low transmural pressure (TMP) gradient to prevent rupture of the aneurysm.3
Minimize brain swelling.
Minimize large swings in intracranial pressure (ICP).
Provide an anesthetic that allows for a rapid emergence.
To accomplish this goal, a basic understanding of the aneurysm's TMP is useful. TMP (the equivalent of the CPP) is defined by Equation 35–1.
(35–1)TMP = CPP = MAP – ICP or CVP
where MAP is the mean arterial pressure and CVP is the central venous pressure. Normal values for MAP, ICP, CVP, and CPP are presented in Table 35–1. The higher value between ICP and CVP is used. TMP describes the relationship between the pressure within the aneurysm (arterial blood pressure) and the pressure surrounding the aneurysm. Abrupt increases in the transmural pressure gradient may lead to aneurysmal rupture and poor outcomes.
Table 35–1Normal values for pressures that influence aneurysm transmural pressures. ||Download (.pdf) Table 35–1 Normal values for pressures that influence aneurysm transmural pressures.
|Hemodynamic Parameter ||Normal Values (mm Hg) |
|Mean arterial blood pressure (MAP) ||80–100 |
|Central venous pressure (CVP) ||3–8 |
|Intracranial pressure (ICP) ||7–15 |
|Cerebral perfusion pressure (CPP) ||70–90 |
Subarachnoid hemorrhage (SAH) associated with aneurysmal rupture can be linked with numerous physiologic derangements (Table 35–2) that should be considered when formulating an appropriate anesthetic as well as drugs that may be used in the perioperative period to manage these derangements.
Table 35–2Physiologic derangements associated with subarachnoid hemorrhage. ||Download (.pdf) Table 35–2 Physiologic derangements associated with subarachnoid hemorrhage.
|Adverse Physiologic Problem ||Management |
|Central nervous system |
|Cerebral vasospasm || |
Triple H therapy (contraindicated prior to aneurysm clipping)
Oral dosing: 60 mg orally every 4 hours, with a maximal daily dose of 360 ...