The Importance of the Temporary Clip Removal Phase on Exposure to Hypoxia: On-Line Measurement of Temporal Lobe Oxygen Levels During Surgery for Middle Cerebral Artery Aneurysms

Abstract

BACKGROUND: Most studies concerning intraoperative temporary arterial occlusion overlook the period between and after clip placement. OBJECTIVE: To analyze the brain tissue oxygen tension through the process by which anterograde arterial blood flow is re-established after temporary clipping (TR). METHODS: In this prospective observational study, patients who presented to surgery for middle cerebral artery aneurysms were continuously monitored with ICM+, to obtain temporal (downstream) P bt O 2 levels while M1 segment temporary clips were applied and removed. P bt O 2 changes were analyzed and compared with the clipping phase, and measures of exposure to hypoxia were defined and assessed during both phases and used in a model to test the impact of extending them. RESULTS: Eighty-six TRs (20 patients) were recorded. The mean acquired amount of time per clip release (CR) event was 336.7 seconds. Temporary clip removal produced specifically shaped, highly individual P bt O 2 curves that correlated with their corresponding clipping phase events but developing slower and less consistently. The CR phase was responsible for greater cumulative exposure to hypoxia than the clip application phase through the first and second minutes of each. In our model, the duration of the TR phase was mostly responsible for the total exposure to hypoxia, and longer CR phases reduced the mean exposure to hypoxia. CONCLUSION: During the clip removal phase, the brain tissue is still exposed to oxygen levels that are significantly below the baseline, reverting through a singular, dynamic process. Therefore, it must be regarded by surgeons with the same degree of attention as its counterpart.

Letter: Commentary: The Importance of the Temporary Clip Removal Phase on Exposure to Hypoxia: On-Line Measurement of Temporal Lobe Oxygen Levels During Surgery for Middle Cerebral Artery Aneurysms

“…Human studies deliberately testing "cutoffs" for cerebral ischemia are difficult to conceptualize, making it challenging to act practically on evidence. Both in reports by other authors 9,13,14 and our previous series documenting TAO and its effect on brain tissue oxygenation, 1,4,15 we could not identify new-onset ischemic lesions that could be attributed to TAO when its use was limited to the range of 120 to 180 seconds. If the surgical conditions permit it, that is our favored point to restore perfusion.…”

“…In their commentary, Wipplinger et al 3 derived from the findings of our study and previous literature on the subject to propose specific recommendations regarding the use of TAO. In a true spirit of contribution to the field based on a personal interpretation of our results concerning brain tissue oxygenation during the 2 phases of the temporary clip (application and removal), 1,4 we were motivated to write the present text in an attempt to both discuss and expand on those. We propose the following principles, around which our current clinical practice is established: The use of TAO in Neurosurgery is broadly justified by its unique advantages, but those must always be weighed against its potential for vascular injury and ischemia, considering the specific necessities of the patient and the intraoperative occurrences.…”

“…In the exceptions, such as the original studies by Griessenauer et al 7,8 that did not identify a correlation between TAO and patient outcome, relatively prolonged occlusion intervals were employed, but the authors underlined that ample reperfusion times (over 15 minutes between clipping events) were also a rule. The temporary clip release phase is paramount to limit the exposure to hypoxia throughout the TAO phase of surgery. Given its impact on the global exposure to hypoxia during TAO, 1 the clip removal phase must be taken in consideration, valued equally as an influencing factor, and again recognized as patient-specific. We agree 3 that at least 200/300 seconds (the approximate reference mark at which the P bt O 2 levels reached the baseline on average in our series 1 ) are the minimum, but we also suggest that the reperfusion interval be extended in certain circumstances, such as a previous prolonged TAO event, a perceived risk of rupture at dissection during the following TAO event (to have more “room” in case it occurs) or a ruptured aneurysm status.…”

In Reply: Commentary: The Importance of the Temporary Clip Removal Phase on Exposure to Hypoxia: On-Line Measurement of Temporal Lobe Oxygen Levels During Surgery for Middle Cerebral Artery Aneurysms

“…We thank the authors for their response to our commentary. 1-3 We generally agree with the authors' statements and would like to comment on their expansion of our recommendations. The authors modified our recommendation to avoid temporary artery occlusion (TAO) whenever possible and stated that it should be applied more broadly weighed against the potential risks.…”

“…Overall, the most valuable insight of the authors' study was the conclusion that sufficient clip release phases are equally, if not more, important than the actual occlusion time. Studies such this one 1 are critical to improve understanding of the impact of TAO during cerebral aneurysm surgery.…”