Until recently, postoperative cognitive deficit (POCD) was considered predominantly a complication of coronary bypass surgery, but studies following a wide range of surgical and interventional procedures without cardiopulmonary bypass have revealed a considerable incidence of POCD.1 A substantial cognitive decline has been described after carotid endarterectomy,2 but with no important difference between patients receiving general and other forms of anaesthesia. Consequently, the contribution of factors that might enhance the risk of POCD remains unclear.3 Intraoperative positioning with cervical spine rotation during carotid endarterectomy may contribute to POCD, as cerebral blood flow (CBF) may become compromised by distortion of major blood vessels such as the vertebral and carotid arteries.4,5 However, rotation or flexion of the cervical spine is unavoidable during positioning for surgical procedures such as carotid endarterectomy, thyroidectomy and shoulder surgery. Generally, sufficient cerebral blood supply is maintained by intracerebral collateral flow even if flow in one or more arteries is compromised and CBF is kept almost constant by autoregulation. However, extensive head rotation or flexion may induce intraoperative cerebral ischaemia if collateral flow is insufficient due to congenital abnormalities or atherosclerosis. This may be aggravated by intraoperative hypotension. The assessment of blood flow in intracranial and extracranial arteries during head rotation remains controversial. A significant decrease of blood flow in the intracranial vertebral artery that was more pronounced on the side opposite to the direction of rotation has been observed in normal individuals following cervical spine rotation.6,7 Neck rotation also markedly reduced flow in six of 31 patients in another study,8,9 and again in a further investigation, the extracranial vertebral artery was compressed in 5% of 1108 patients.10 However, other investigators failed to find any significant impairment of blood flow in the carotid and vertebral arteries following extension and rotation of the neck.11,12
The middle cerebral artery (MCA) is an important vessel for the distribution of CBF. Hence, a largely decreased flow in the MCA may serve as a surrogate for a comparable decrease of global CBF caused by carotid and vertebral artery stretching or narrowing. Transcranial Doppler ultrasonography (TCD) is an established diagnostic tool for monitoring vasospasm after subarachnoid haemorrhage and for stroke prevention in sickle cell disease.13 It is an accepted tool in the measurement of mean blood flow (MBF) velocity in the MCA and should be capable of detecting the effects of changes to positions of the head and neck. A positive result on MBF in the MCA would identify TCD as a potential screening tool for the prediction of critical reductions in CBF following head positioning during surgery.
The aim of this study was to investigate the effect of five different extended and/or rotated cervical spine positions on MBF in the MCA and to measure the incidence of decreased flow greater than 20% from baseline MBF.
Ethical approval for this study (Protocol number A 132/09) was provided by the Ethics Committee of the University Hospital Schleswig-Holstein, Campus Kiel, Kiel, Germany (Chairperson Professor Dr medicine H. M. Mehdorn) on 19 October 2009 and written informed consent was obtained from each patient.
An arbitrary 100 patients (age >17 years) presenting in the preanaesthesia outpatient clinic of the University Hospital Schleswig-Holstein, Campus Kiel were approached for the study. Exclusion criteria were cranial spine pathology and cerebral vascular disease.
Mean blood flow velocity measurement
MBF in the MCA was measured bilaterally using a bidirectional multigate 2 MHz pulsed wave transcranial Doppler (Multidop T1, Software Multiflow MF version 8.27j, DWL Corp., Sipplingen, Germany). After establishing a transtemporal approach to the proximal segment (M1) of the MCA, the Doppler probes were fixed bilaterally with an adjustable mounting (SPENCER helmet) to keep the angle and depth of insonation constant over time. With the upper cervical spine in a neutral position, the insonation depth was chosen between 45 and 55 mm and the position giving the best signal was fixed. A sample volume of 5 mm, an ultrasound frequency of 128 Hz and a fast sweep for temporal overlap were chosen. All measurements were performed between 10 : 00 a.m. and 2 : 00 p.m. by the same experienced examiner (J.L.) in a quiet, warm, well lit room. Each participant was asked to take a relaxed, supine position. Patients were asked to keep their eyes closed to avoid stimulation of the occipital cortex. Three minutes were allowed for acclimatisation and stabilisation. Noninvasive blood pressure was measured in both arms before and after the procedure to ensure that any possible change in blood flow was not related to blood pressure changes. The time-averaged MBF on both sides was measured with the cervical spine in a neutral position, rotated maximally to the left, rotated maximally to the right, hyperextended in the neutral position and then rotated to the left and the right in hyperextension. Measurements were performed after 1 min of constant blood flow. The neutral position was re-established after each position and the next positioning manoeuvre started only after the blood velocity in this position had returned to the value obtained in the neutral position during the first measurement.
Statistical analysis was performed with the statistics software SPSS (Version 17.0, SPSS Inc., Chicago, Illinois, USA). Normal distribution of blood velocity data was tested with the Kolmogorov–Smirnov goodness-of-fit test and the Shapiro–Wilk test. Subgroups were defined a priori for female and male participants and for three age groups (18 to 39 years, 40 to 59 years and ≥60 years). The effect of the factors ‘age’ and ‘sex’ on MBF in the MCA in the neutral position was analysed by two-factorial ANOVA. Significance of differences between subgroups was analysed by multiple comparisons (Scheffé's test). The effect of the factors ‘head position’ and the interaction of the factors ‘age’ and ‘sex’ on MBF in the MCA were analysed by two-factorial repeated measurements ANOVA.
The significance of a decreased mean MBF from baseline for a subgroup of patients with a decrease of MBF of more than 20% from baseline in at least one rotated and/or extended head position was analysed exploratively for all positions without correction for multiple testing by one sample Student's t-test and Wilcoxon signed rank test (Graph Pad Prism, version 5.03, Graph Pad Software, San Francisco, California, USA). Significance was recognised when P was less than 0.05. Results are shown as number and mean (SD) as appropriate.
One hundred consecutive patients presenting in the preanaesthesia clinic of the University Hospital Schleswig-Holstein, Campus Kiel were approached and 80 were enrolled in this study during March and April 2010. Twenty were unsuitable due to an insufficient bone window for TCD. The personal characteristics of the enrolled patients are shown in Table 1. The scheduled procedures of these patients are shown in Table 2. Allocation according to age and sex resulted in subgroups for female (N = 24) and male individuals (N = 56) and three age groups, 18 to 39 years (N = 17), 40 to 59 years (N = 28) and ≥60 years (N = 35). There were no important changes in blood pressure during TCD measurements nor any reported discomfort or neurological symptoms during and after the examination.
Transcranial Doppler ultrasonography measurements in the neutral position
Increasing age was associated with significant reduction in MBF in the left but not in the right MCA in the neutral position (P = 0.047 left, P = 0.248 right). Patients older than 59 years had significantly less MBF in the left MCA when compared to those younger than 40 years [47.7 (16.2) cm s−1 vs. 61.2 (16.6) cm s−1, P = 0.015; Table 3].
On the right side but not on the left side a significant effect of sex on MBF in the MCA was observed. Male patients had significantly lower MBF in the right MCA compared with women [48.6 (13.8) cm s−1 vs. 58.0 (9.7) cm s−1, P = 0.014]. Table 4 shows the results of two-factorial repeated measurements ANOVA for the neutral position.
Transcranial Doppler ultrasonography measurements during rotation and extension
The two-factorial repeated measurements ANOVA revealed a significant effect of head position on MBF in the left and right MCA (P = 0.039 left, P = 0.025 right). The combination of ‘head position’, ‘age’ and ‘sex’ on MBF in the right and left MCA failed to show a significant effect (Table 5). Figure 1 shows the MBF in the right and left MCA during rotation and extension relative to baseline in the neutral position.
Patients with a decrease of mean blood flow of more than 20% from baseline
Twenty patients had a decrease of MBF of more than 20% from baseline in at least one head position. Four of these patients had a decrease of MBF of more than 30% from baseline in at least one head position but none had a decrease of more than 40%. Median MBF (relative to baseline) in all rotated and/or extended head positions of this subgroup was significantly different from baseline MBF in the neutral position (P < 0.05, Wilcoxon signed rank test and one sample Student's t-test, explorative testing without correction for multiple comparisons). Figure 2 shows the MBF in the right and left MCA during rotation and extension as a percentage of baseline MBF in the neutral position.
We chose TCD as a potential screening tool for the detection of reduced MBF in the MCA during different positions of the cervical spine because it has been used successfully to measure decreased blood flow in the intracranial vertebral artery following cervical spine rotation, and we expected a similar effect on the MCA.6–9 Measurement of MBF in the MCA by transcranial Doppler ultrasound is regarded as reliable because the blood flow direction nearly parallels the direction of the Doppler beam. However, a validation study comparing transcranial Doppler measurement of MBF in the MCA with the xenon technique showed only a weak correlation of the absolute velocity between methods, while the sensitivity to velocity changes was similar for both. Thus, changes in comparison to baseline are probably more reliable than absolute values when using TCD to measure CBF.14 A change obtained by TCD compared to baseline can only be regarded as important if it exceeds intraoperator variability of serial measurements. Intraoperator 95% limits of agreement were 10.9 cm s−1 for experienced operators in a former study, averaging 20% of mean MBF at the neutral position (51 cm s−1 at the left side and 52 cm s−1 at the right side) in our study.15 Hence, a decrease exceeding more than 20% from baseline was regarded as significant. Of our 80 patients, 20 met this requirement in at least one of the cervical positions.
Age and sex
Our results are consistent with previous investigations showing that MBF in the internal carotid and vertebral arteries decreases with age. This is explained by an increasing diameter of the vertebral artery, lower blood flow in the internal carotid artery and decreasing resistance in the internal carotid artery and vertebral artery with age.16–20 Age has also been associated with compression of the extracranial vertebral artery by neck rotation.10
The higher MBF measured on the right side in women is consistent with higher CBF predominantly in anterior brain regions.21This may partially be explained by the lower haematocrit and smaller vascular diameters of the carotid arteries in females.18,22,23 MBF increase during mental activities was more pronounced in women and the increase of MBF during attention tasks is generally larger on the right side than on the left side.24–26
In keeping with earlier studies, we failed to see any interaction between age and sex regarding cerebral MBF.27
Head position caused a significant change in blood flow in the MCA and 20 patients showed a decrease of more than 20% from baseline MBF in at least one position of the head. However, only four patients showed a decrease of more than 30% from baseline MBF, and none experienced a decrease of more than 40%. These are modest changes, but we do not know what the safety margin for MBF might be. The decrease we have measured may have been limited by the normal blood pressure maintained during the examination in the supine position. This contrasts with clinical practice when intraoperative blood pressure might fall below baseline levels, leading to a reduction in CBF. Moderate falls in blood pressure are usually compensated for by autoregulation, but if CBF is additionally reduced by the narrowed diameter of distorted arteries, hypoperfusion may result. Moreover, some surgical procedures such as surgery of the shoulder are performed with the upper part of the body elevated and the head turned to the opposite side from the site of surgery, thus combining the risk of distorted arteries with the increased chance of an accidental decrease of blood pressure.
Besides positioning, preexisting diseases with compromised cerebral oxygen delivery may make an additional contribution to the effect of decreasing CBF, even if it is only small. Patients with cerebral vascular disease, chronic arterial hypertension and low oxygen transport capacity due to anaemia may need higher mean arterial blood pressure for adequate cerebral perfusion. Adverse intraoperative events such as arrhythmia, blood loss, thromboembolism, low haematocrit, low cardiac output or other conditions leading to a DO2/VO2 imbalance may contribute to cerebral hypoxaemia. In susceptible patients it is possible that, if additional risk factors are present, head positioning might lead to a critical reduction in cerebral perfusion.
It is not possible to draw immediate conclusions or make recommendations for preoperative screening from our results. The significant influence of head rotation on MBF indicates that TCD can identify patients at risk of a clinically important decrease of MBF during surgery, but the effect on postoperative outcome of preoperative screening was not examined in our study. However, obtaining a predictive value with respect to clinical endpoints may be possible, as the significant changes in MBF in the left and in the right MCA following head positioning may become clinically important in patients who already have decreased MBF, particularly in the presence of additional adverse factors.
Possible confounders leading to bias were considered in the study design. Variation of blood pressure and heart rate could have increased MBF by increasing cardiac output and cerebral perfusion pressure, but no change in these variables was observed during measurements. Interobserver variability was excluded by performing TCD using one single experienced physician (J.L.). Displacement of transducers by head rotation was carefully avoided. A further limitation of the study is that it was not possible to blind the observer to the study design.
As an immediate conclusion for clinical practice, understanding that head position can influence MBF may improve the awareness of anaesthetists to the potentially harmful effect of head position on CBF, and encourage the optimisation of compounding factors. A further study should address whether routine TCD before surgery with head rotation, combined with standardised intraoperative precautions, such as modified positioning and improved perioperative management of additional factors, can reduce the incidence of adverse events such as POCD.
Assistance with the study: We would like to thank the nursing staff, Department of Anaesthesiology and Intensive Care Medicine, University Hospital Schleswig-Holstein, Campus Kiel, for their support.
Sources of funding: Source of financial support is Department of Anaesthesiology and Intensive Care Medicine, University Hospital Schleswig-Holstein, Campus Kiel, Schwanenweg.
Conflicts of interest: None declared.
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