The transmural pressure of an aneurysm is related to the difference between the Mean Arterial Pressure (MAP) and Intracranial Pressure (ICP):
Transmural Pressure= CPP= MAP – ICP
Therefore, an increase in the MAP or an abrupt decrease in ICP leads to an increase in the transmural pressure. Thus, increases in MAP and decreases ICP can increase the risk of aneurysm rupture. If an aneurysm ruptures during induction of anesthesia, the mortality is high (approximately 75%), so diligent control of transmural pressure is very important during induction of anesthesia. During the induction of anesthesia and tracheal intubation, it is critical to avoid hypertension and increases in MAP. Ketamine should be avoided due to its increased in CBF and ICP. Also, lidocaine, esmolol, and/or fentanyl should be used prior to intubation to blunt hemodynamic response to intubation. After the loss of consciousness, a normal PaCO2 should to be maintained and extreme hyperventilation should be avoided to prevent increases in transmural pressure (hyperventilation lowers PaCO2, decreasing CBF, which may lower ICP to a point that transmural pressure is increased). Conversely, care should be taken to preserve cerebral perfusion pressure (CPP). CPP should not get low enough that ischemia can develop. Extreme reductions in MAP (> 35%) may compromise CPP in patients with increased ICP.