Reducing Risk of Rebleeding During Microscopic Dissection and Aneurysmal Clipping

Two techniques—controlled systemic hypotension and temporary occlusion of the major feeding arteries—have been applied to decrease TMP, during the critical part of aneurysmal surgery, theoretically reducing the risk of intraoperative aneurysmal rupture and improving conditions for the final clip placement. Anesthetic considerations for these two techniques differ significantly.

Controlled systemic hypotension has been the traditional approach for several reasons: (i) the aneurysm wall tension increases almost linearly with the MABP (49), and control of the systemic blood pressure is a high priority until final clipping of the aneurysm; (ii) all necessary drugs and techniques are familiar to the anesthesiologist and accessible during aneurysmal surgery; (iii) controlled hypotension might also reduce surgical bleeding, thereby improving visualization during microscopic dissection. However, the risks of perioperative arterial hypotension are significant and include global cerebral ischemia, as well as focal ischemic damage in areas affected by brain retraction (50). In addition, coronary ischemia, reduced hepatic and renal blood flow, hyperglycemia, and, depending on the agent used, inhibition of hypoxic vasoconstriction, have been described (51). After aSAH, the risk for ischemia is especially high due to cerebral vasospasm and impaired autoregulation (52,53). A safe limit of intraoperative hypotension has never been established. Patients with preexisting arterial hypertension, as well as those at risk for cerebral vasospasm, might require a significantly higher MABP to maintain adequate CPP during surgery. Intraoperative neuromonitoring (e.g., EEG and somatosensory-evoked potentials) might help to detect ischemia in areas at risk during deliberate hypotension, but the reliability of these methods is questionable. In addition, the duration of systemic hypotension could be an important confounding factor for adverse outcome (54,55).

Frequently, isoflurane and sodium nitroprusside are administered to reduce blood pressure during aneurysmal surgery (56). Others report the successful use of nitroglycerin, adenosine, prostaglandin E, and P-receptor blockers (labetalol and esmolol) (57). However, the effectiveness of this method to reduce the risk of perioperative rebleeding has never been precisely determined, and its use has declined over the last years, especially with the availability of improved techniques for temporary occlusion of upstream arteries (58) . Today, deliberate hypotension should be limited in degree and duration and applied only during short periods immediately prior to aneurysmal clipping; the risk-benefit ratio must be assessed for each patient (59).

Temporary occlusion of the parent artery immediately reduces blood flow, resulting in reduced TMP and attenuated risk of excessive bleeding if the aneurysmal dome is violated during dissection (60). At the same time, it results in various degrees of transient focal ischemia in dependent brain areas. The risk of such a maneuver to trigger permanent neurologic damage is apparent, and controversy exists regarding duration and technique for safe "temporary clipping." Short periods of artery occlusion (5-7 min) are usually well tolerated, although this is not sufficient for every surgical procedure. Currently, it is believed that 15 to 20 min of temporary artery occlusion might represent the critical threshold for the development of cerebral infarction. Longer durations of up to 120 min have been reported without neurologic sequelae, but others report a significant lower ischemia tolerance (maximum, 10 min), e.g., for brain stem and certain nuclei (61). The effects could be strongly related to the area involved and to certain risk factors, such as age > 61, unfavorable clinical grades (H&H III-IV), and the state of collateral circulation (62). In addition, hypothermia, the application of arterial hypertension to improve collateral blood flow, or putative neuroprotective pharmacologic agents might increase the tolerance period for temporary vessel occlusion (63). Alternatively, temporary clipping can be interrupted after approximately 7 min and reinstituted after sufficient and profound reperfusion has been accomplished.

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