During posterior fossa aneurysm surgery, the perfusion of the brainstem can be compromised by temporary occlusion of feeding arteries or by retractor compression. Regions affected may include those involved in the regulation of respiration and cardiovascular activity. Injury to these centres can result in serious morbidity and even mortality. Monitoring of cardiovascular, respiratory and evoked potentials1 has been advocated during this type of surgery to detect brainstem ischaemia. Although evoked potential monitoring is advocated by many authors,2,3 baseline and subsequent recordings may be difficult to obtain owing to preoperative compression of the brainstem.1 It may not be available in all hospitals, as well. Cardiovascular monitoring is a common method for detection of brainstem ischaemia.4,5 However, it has been observed that the respiratory changes occur much earlier in the event of brainstem ischaemia; hence, monitoring of the respiratory pattern may be a better or additional option in such a scenario.6 In the present article, we describe a patient who underwent posterior fossa aneurysmal surgery in the lateral position, in which the spontaneous breathing pattern was used to monitor brainstem perfusion.
A 36-year-old woman presented with severe headache associated with transient loss of consciousness and weakness of both lower limbs. Computed tomographic (CT) scan revealed a subarachnoid haemorrhage (SAH) in the prepontine, premedullary cistern and 3rd ventricle with mild hydrocephalus. Cerebral angiography showed an aneurysm at the junction of the vertebral artery and basilar artery; hence, retromastoid suboccipital craniotomy in the left lateral position and clipping of the aneurysm was planned. All preoperative routine investigations were within the normal limits. A decision was made to temporarily clip the basilar artery and vertebral arteries before application of the permanent clip. The great risk of brainstem ischaemia made the anaesthesia team monitor brainstem perfusion by observation of cardiovascular and respiratory patterns as evoked potential monitoring was not available. In the operating room, routine monitors including ECG, arterial oxygen saturation (SpO2), end-tidal CO2 and arterial blood pressure (BP) were connected. A bispectral index (BIS) monitor was attached to assess the depth of anaesthesia. Anaesthesia was induced with fentanyl 100 μg, thiopental 300 mg and rocuronium 50 mg. Maintenance of anaesthesia was with O2, nitrous oxide, isoflurane and fentanyl. The neuromuscular blockade and the fentanyl infusion were maintained until opening of the dura, after which they were stopped. When the surgeon started dissecting in the vicinity of brainstem, spontaneous ventilation was allowed to resume, which took 5 min. Isoflurane was titrated to keep the BIS value between 40 and 60. During this period the partial pressure of carbon dioxide in arterial blood (PaCO2) rose to 48 mmHg. However, there was no brain bulge; hence, spontaneous respiration was continued. The duration of temporary occlusion of the basilar artery was 4 min and 23 s whereas that of the vertebral arteries was 6 min and 12 s. On application of a permanent clip to the aneurysm, an irregular respiratory pattern was observed which gradually progressed to apnoea. A few cardiac ectopics were also noted during this period. The neurosurgeon was informed, and the permanent clip was repositioned. Blood clot around the brainstem was also removed. Within 10 s, spontaneous breathing resumed. After the respiratory rate became regular with normal haemodynamics, the patient was given rocuronium and controlled ventilation was begun again. The total duration of spontaneous ventilation was 85 min. At the end of surgery the residual neuromuscular blockade was reversed and the trachea was extubated. The postoperative period was uneventful.
In the present article, we reported a case of successful anaesthetic management of vertebrobasilar junction aneurysm clipping using spontaneous ventilation as a monitoring tool. The use of the respiratory pattern as a clinical monitoring modality is well documented.1 The manifestations due to ischaemia include a changing pattern of breathing, decreased depth of respiration and, even, apnoea, as occurred in our patient. Common cardiovascular changes due to brainstem ischaemia are bradycardia, hypertension and dysrhythmia (commonly ventricular ectopics); none of which were seen in our patient.
There are concerns about spontaneous respiration during neurosurgery. First, hypoventilation may occur due to respiratory depression caused by anaesthetic agents, as in our patient, in whom PaCO2 increased up to 48 mmHg. We used mannitol for brain relaxation before opening of the dura. The infusion of fentanyl was stopped once the dura was opened to reduce opioid-induced respiratory depression. Fortunately, in this case there was no intraoperative brain swelling. Patients can move during surgery in the absence of muscle paralysis, resulting in hazardous complications. To ensure maintenance of an adequate depth of anaesthesia, BIS monitoring was used. The intraoperative apnoea could be explained by either the clots around the brainstem or the permanent clip, both of which may have kinked or compressed large arteries or small perforators. The presence of regular breathing and stable haemodynamics assured us that the brainstem was functioning normally so that we could decide on tracheal extubation at the end of the procedure.
Brainstem areas involved in respiratory and cardiovascular control and the sensory and motor conduction pathways are adjacent to each other but are anatomically discrete. Highly focal or early ischaemia may cause injury to one area only initially but will ultimately damage other areas due to ongoing ischaemia or oedema. Manninen et al.1 observed that respiratory changes occur earliest after occlusion of the vertebral artery system. It was opined that the cardiovascular changes did not occur as frequently or as early as the respiratory changes were observed. We feel that the cardiovascular changes at times may not occur or may be mistaken for the changes due to the use of vasoactive drugs or blood loss. Hence, despite the fact that spontaneous breathing during neurosurgical procedures is no longer advocated, the role of spontaneous ventilation monitoring as an adjunct to cardiovascular monitoring to assess the adequacy of brainstem perfusion during posterior fossa vascular surgery should not be forgotten.
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