Changes in cerebral blood flow during anaesthesia and surgery in the sitting position

Differences between internal jugular vein and vertebral vein flow examined in real time with the use of multigate ultrasound color Doppler

BACKGROUND AND PURPOSE

The hypothesis that MS could be provoked by a derangement of the blood outflow from the brain has been largely discredited. In part, it was because data on the normal pattern of outflow are scarce and obtained with different methods. The aim of this study was to evaluate the normal pattern of outflow for the vertebral and internal jugular veins in healthy subjects with multigate color Doppler.

MATERIALS AND METHODS

Twenty-five volunteers were studied to assess vessel area, mean velocity, and flow for the vertebral and internal jugular veins in the supine and sitting positions.

RESULTS

In the sitting position, flow decreases, both in vertebral veins and internal jugular veins, as the total vessel area decreases (from 0.46 ± 0.57 to 0.09 ± 0.08 cm(2)), even if the mean velocity increases (from 12.58 ± 10.19 to 24.14 ± 17.60 cm/s). Contrary to what happens to the blood inflow, outflow in the supine position, through vertebral and internal jugular veins, is more than twice the outflow in the sitting position (739.80 ± 326.32 versus 278.24 ± 207.94 mL/min). In the sitting position, on application of very low pressure to the skin with the sonography probe, internal jugular veins rarely appear to occlude. A pronounced difference of diameter between internal jugular veins was present in approximately one-third of subjects.

CONCLUSIONS

Our results support the view that other outflow pathways, like the vertebral plexus, play a major role in the normal physiology of brain circulation and must be assessed to obtain a complete picture of blood outflow.

The Effect of the Sitting Upright or ‘Beachchair’ Position on Cerebral Blood Flow during Anaesthesia for Shoulder Surgery

The sitting upright or ‘beachchair’ position is commonly used for shoulder arthroscopic surgery. There is a theoretical concern that anaesthetised patients placed in this posture are at risk of reduced cerebral blood flow (CBF), especially if there is associated hypotension. This study investigated the effect of anaesthetic-induced hypotension on estimated cerebral blood flow in patients placed in the beachchair position for shoulder surgery.

Forty patients were randomised to either sedation (propofol infusion 10 to 20 mg.hour-1, n=20) or general anaesthesia using sub minimum alveolar concentration of sevoflurane (n=20). All patients received an interscalene brachial plexus regional block. Internal carotid artery blood flow was measured using the time averaged velocity of the spectral Doppler waveform, and was then used as an estimate of global CBF. Following a pre-anaesthesia study, measurement of internal carotid artery blood flow was made before and after beachchair positioning, and at five-minute intervals during surgery.

Beachchair positioning during general anaesthesia significantly decreased the mean arterial pressure (34±10 mmHg) compared to sedation (4±2 mmHg, P <0.01), and vasopressor therapy was required more often. However, CBF remained constant in both anaesthetised (P=0.83) and sedated patients (P=0.68) despite beachchair positioning, and the fall in mean arterial pressure in the anaesthetised patients. There was no significant difference in CBF between groups (P=0.91). These findings indicate that in patients in the beachchair position receiving sevoflurane anaesthesia, CBF is maintained when mean arterial pressure is above 70 mmHg, consistent with intact autoregulation.

Shy-Drager syndrome. Effect of fludrocortisone and L-threo-3,4-dihydroxyphenylserine on the blood pressure and regional cerebral blood flow

In nine cases of Shy-Drager syndrome, the changes in blood pressure and cerebral blood flow on sitting up from a supine position were studied. The influence of fludrocortisone, a synthetic mineralocorticoid, and L-threo-3,4-dihydroxyphenylserine (DOPS), a precursor of norepinephrine, on these changes was examined. On sitting up, the regional cerebral blood flow (rCBF) measured by Xe133 inhalation showed a tendency to decrease. Fludrocortisone reduced the fall of the mean blood pressure significantly. DOPS reduced the fall of both the diastolic blood pressure and rCBF significantly.

Effect of the knee-chest position on cerebral blood flow in patients undergoing lumbar spinal surgery

The cerebral haemodynamic effect of the knee-chest position was evaluated in 15 anaesthetised patients undergoing elective lumbar disc surgery and divided into a control group (n = 8) where cerebral blood flow (CBF) was measured twice in the supine position and an experimental group (n = 7) where the first CBF was measured in the supine position and the second in the knee-chest position. CBF was measured by a modified intravenous 133xenon washout technique. Mean global CBF did not change in control group (56.1, SD 9.2 versus 52.8, SD 10.8 units) and was not significantly modified by the knee-chest posture, 51.8, SD 8.8 units versus 53.9, SD 7.4 units in the supine position. The results indicate that mean global CBF in the knee-chest position is not different from CBF in the supine position in healthy patients.