Frederik A. Pennings

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 μ m) 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 (ptiO2) and carbon dioxide pressure (ptiCO2) 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, ptiO2 immediately rose from 7 ± 8 mm Hg to 24 ± 15 mm Hg (P < 0.05), whereas in patients who did not have swelling, ptiO2 went from 16 ± 9 to 18 ± 10 mm Hg after opening of the dura. The mean ptiO2 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 (P < 0.05). The effect on ptiO2 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 ptiO2 monitoring may enhance the safety of neuroanesthesia, but the high incidence of failures with this type of sensor remains a matter of concern.

Intraoperative Monitoring of Brain Tissue Oxygen and Carbon Dioxide Pressure in Peritumoural Oedema by Stereotactic Placement of Multiparameter Microsensors

Ischaemia may play an important role in peritumoural brain oedema and swelling, but little data exist so far on brain tissue oxygenation adjacent to a tumour mass. We have monitored brain tissue oxygen tension (ptiO2) and brain tissue CO2 tension (ptiCO2) in 19 patients undergoing craniotomy for resection of a brain tumour using a multiparameter sensor placed in the brain parenchyma. Accurate placement of this probe in the peritumoural area was accomplished with the aid of a 3-D neuronavigation system. Due to various problems we obtained useful data in only 13/19 patients. The presence of brain swelling was associated with a significant rise in ptiO2 upon opening of the dura from 7.1 +/- 7.8 to 23.6 +/- 14.7 mm Hg. The average ptiO2 before tumour resection was 18.1 +/- 10.8 mm Hg. A significant improvement in ptiO2 occurred after tumour resection to an average ptiO2 of 29.7 +/- 15.2 mm Hg. From these preliminary data, we conclude that ptiO2 is depressed in the peritumoural area, and improves following tumour resection. Stereotactic placement of sensors for intraoperative ptiO2 monitoring is feasible and may enhance data quality. Nevertheless, the high incidence of failures with this type of sensor remains a matter of concern.

Continuous Real-Time Visualizationof the Human Cerebral Microcirculation During Avm 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.