Abnormal responses of the human cerebral microcirculation to papaverin during aneurysm surgery.

BACKGROUND AND PURPOSE: The role of the cerebral microcirculation in delayed ischemia after subarachnoid hemorrhage remains obscure. To test the hypothesis that cerebral arterioles have a reduced capacity to dilate after subarachnoid hemorrhage, we studied the microvascular responses to papaverine (PPV) in patients undergoing aneurysm surgery. Method- In 14 patients undergoing aneurysm surgery, the diameter changes of cortical microvessels after topical application of PPV were observed using orthogonal polarizing spectral imaging. RESULTS: In control subjects, neither arterioles nor venules showed diameter changes in response to topical PPV. In patients operated <48 hours after subarachnoid hemorrhage, PPV resulted in vasodilatation of arterioles with 45+/-41% increase in arteriolar diameter (P=0.012). In 2 of these patients, arteriolar diameter returned below baseline value. In patients undergoing late aneurysm clipping, the diameter increase of the arterioles after PPV was 25+/-24% (not significant). In 2 patients of this group, no vasodilatation but focal arteriolar narrowing occurred. CONCLUSIONS: In patients with subarachnoid hemorrhage, unpredictable response patterns to PPV were observed with "rebound" vasoconstriction suggesting increased contractility of the microcirculation. Yet, diminished vasodilatory capacity of the cerebral microcirculation after subarachnoid hemorrhage was not confirmed by this study.

Brain tissue oxygen pressure monitoring in awake patients during functional neurosurgery: the assessment of normal values.

Local brain tissue oxygen (ptiO2) monitoring is frequently applied in patients at risk for cerebral ischemia. To identify ischemic thresholds, the normal range of local brain tissue oxygen pressure (ptiO2) values needs to be established. Ideally, such normal values are determined in healthy and awake subjects, so as to eliminate the possible influences of anesthetics on cerebral physiology or ptiO2. Thus far, however, such measurements have not been conducted, and to fill this void, we determined the ptiO2 values in normal white matter of awake patients undergoing functional stereotactic brain surgery. In 25 otherwise healthy patients, who underwent functional neurosurgery for treatment of a refractory movement disorder under local anesthesia, the ptiO2 of white matter was recorded continuously using a polarographic Clark type electrode monitoring system. Preoperative screening ruled out cognitive dysfunction or structural cerebral lesions. Reliable intraoperative ptiO2 values were obtained in 22 patients. After an adaptation period of 118+/-35 min (range, 47-171 min), we found an average normal ptiO2 of 22.6+/-7.2 mm Hg in the frontal white matter. In 11 patients, ptiO2 measurements were continued postoperatively for 24 h. During this period, a similar normal ptiO2 value of 23.1+/-6.6 mm Hg was found. No iatrogenic complications occurred. In conclusion, the normal ptiO2 of cerebral white matter is most likely lower than previously assumed. Further, the long adaptation time renders this widely applied monitoring instrument unreliable in detecting ischemia early after insertion and limits its usefulness for intraoperative monitoring.

Continuous real-time visualization of the human cerebral microcirculation during arteriovenous malformation surgery using orthogonal polarization spectral imaging.

OBJECTIVE: After excision of an arteriovenous malformation (AVM), intracerebral hemorrhage or edema can develop, most probably resulting from hyperperfusion. Changes in the perinidal cerebral microvessels probably play a role in the development of this complication but have not been well studied so far. In this study, microvascular changes associated with resection of an AVM were observed and quantified intraoperatively using orthogonal polarization spectral imaging. METHODS: In two patients undergoing craniotomy for excision of an AVM, microvessel diameter, functional capillary index, and microvascular flow index were assessed during surgery using orthogonal polarization spectral imaging and compared with controls (n = 2). RESULTS: Before excision of the AVM, arterioles were characterized by the observation of individual erythrocytes caused by slowing of flow. In venules, microvascular flow index was 2.0 per image field (sludging flow), and functional capillary index was 1.4 +/- 1.3 cm/mm. After resection, flow velocity increased to a level that individual erythrocytes could not be traced any more in arterioles. Furthermore, both microvascular flow index and functional capillary index increased to 3.7 (high flow) and 2.1 +/- 0.8 cm/mm, respectively. CONCLUSION: With intraoperative orthogonal polarization spectral imaging, microcirculatory hemodynamic changes in the human brain can be readily observed and quantified. In AVM surgery, a dramatic increase in microvascular flow was observed in the perinidal brain tissue, which seems consistent with current hypotheses regarding normal perfusion pressure breakthrough.

Direct observation of the human cerebral microcirculation during aneurysm surgery reveals increased arteriolar contractility.

BACKGROUND AND PURPOSE: The effects of aneurysmal subarachnoid hemorrhage on morphology and function of the cerebral microcirculation are poorly defined, partly due to the lack of suitable techniques to visualize the microvessels in vivo. We used orthogonal polarization spectral (OPS) imaging on the brain cortex during aneurysm surgery to directly observe the small cortical blood vessels and quantify their responses to hypocapnia. METHODS: In 16 patients undergoing aneurysm surgery, the diameter changes of small cortical vessels (15 to 180 microm) were observed using OPS imaging. Ten patients were operated on early (within 48 hours after bleeding) and 6 underwent late surgery. Immediately after dura opening, the response to hyperventilation of arterioles and venules was observed with OPS imaging under sevoflurane anesthesia. RESULTS: In patients operated on early, layers of subarachnoid blood were clearly visible. In this group, hyperventilation resulted in a 39+/-15% decrease in arteriolar diameter with a "bead-string" constriction pattern occurring in 60% of patients. In late surgery and in controls, no subarachnoid blood was seen. The arteriolar diameter decrease with hyperventilation was 17+/-20% in patients undergoing late surgery and 7+/-7% in controls. Venules were not affected by hyperventilation in any of the groups studied. CONCLUSIONS: OPS imaging allows direct in vivo observation of the cerebral microcirculation enabling us, for the first time, to visually observe and quantify microvascular reactivity in the human brain. The present study demonstrates increased contractile responses of the cerebral arterioles in the presence of subarachnoid blood, suggesting increased microvascular tonus with possibly greater susceptibility to ischemia.

Intraoperative monitoring of brain tissue oxygen and carbon dioxide pressures reveals low oxygenation in peritumoral brain edema.

Brain edema and swelling often complicate surgery for brain tumors. Its pathophysiology is unclear, as is the relationship with brain tissue oxygenation. Our hypothesis was that brain edema around tumor is cytotoxic type caused by impaired local tissue oxygenation due to increased local tissue pressure. Therefore, we monitored brain tissue oxygen pressure (p(ti)O2) and carbon dioxide pressure (p(ti)CO2) in 19 patients undergoing craniotomy for removal of a brain tumor and specifically studied the effect of decompression by dura opening and by tumor removal with respect to the presence of brain swelling. Before craniotomy, multiparameter sensors were inserted into the peritumoral brain tissue guided by MRI-based stereotaxy. In eight patients who had severe brain swelling upon opening of the dura mater, p(ti)O2 immediately rose from 7 +/- 8 mm Hg to 24 +/- 15 mm Hg ( < 0.05), whereas in patients who did not have swelling, p(ti)O2 went from 16 +/- 9 to 18 +/- 10 mm Hg after opening of the dura. The mean p(ti)O2 of all patients at the start of resection of the tumor was 18 +/- 11 mm Hg, and increased to 30 +/- 15 mm Hg after resection was completed ( < 0.05). The effect on p(ti)O2 of raising the FiO2 to 1.0 was limited in this group of patients, as an increase greater than 50% was found in only six of twelve patients. Notably, in six patients, sensor malfunctions or associated hardware problems occurred, prohibiting useful data acquisition. We conclude that brain tissue oxygenation is reduced in the peritumoral area and improves after local tissue pressure relief, especially in patients with brain swelling. Thus, ischemic processes may contribute to brain edema around tumors. Intraoperative p(ti)O2 monitoring may enhance the safety of neuroanesthesia, but the high incidence of failures with this type of sensor remains a matter of concern.