Abstract

Objective

The beach chair position (BCP) during shoulder arthroscopy is a known risk factor for cerebral ischemia. We retrospectively investigated whether maintaining the arterial blood pressure (ABP) prevented the decrease in the regional cerebral tissue oxygen saturation (rSO₂).

Methods

We analyzed 20 consecutive patients who underwent elective shoulder surgery in the BCP under general anesthesia. The bilateral rSO₂ was monitored continuously throughout the procedure with the help of near-infrared spectroscopy (INVOS 5100 Cerebral Oximeter, Somanetics Corporation, Troy, MI, USA). Anesthesiologists maintained patient blood pressure while they were in the BCP, which was measured using an ABP transducer placed at the level of the external auditory meatus. We compared rSO₂ measured in the supine position and in the BCP.

Results

Measurement of cortex level mean ABP (mABP) values in the BCP were >50 mmHg and over 60% higher than those noted for the supine position in most patients. Although all bilateral rSO₂ values in the BCP were significantly lower than those in the supine position, the reductions was <20%. Further, 35% (7 of 20) patients that were part of the study experienced cerebral desaturation events at any given point during the procedure. None of the patients experienced clinical postoperative neurological complications.

Conclusions

Although cortex level mABP in the BCP was >50 mmHg, a decrease was recorded in the rSO₂ values. This rSO₂ decrease was proportional to the reduction in the cortex level mABP induced by a postural change to the BCP. Therefore, despite appropriate blood pressure management, shoulder surgery in the BCP might involve certain risks for patients with cerebrovascular diseases.

Keywords

Beach chair position; Blood pressure; Cerebral desaturation events;

1. Introduction

The beach chair position (BCP) is commonly used during arthroscopic shoulder surgery.¹ The BCP facilitates the approach to the shoulder joint and improves the arthroscopic visualization.¹ However, this position increases the risk of neurologic complications such as vision loss, spinal cord injury, cerebral infarction, and death.^2,3It has been reported that a change from the supine position to the BCP under general anesthesia causes a significant decrease in the cardiac output and the mean arterial blood pressure (mABP).⁴ Pohl and Cullen reported a case of brain death and three of severe cerebral dysfunction after arthroscopic shoulder surgery in the BCP under general anesthesia.² Near-infrared spectroscopy (NIRS) is typically used in the measurement of regional cerebral tissue oxygen saturation (rSO₂).⁵ A previous report indicates that despite similar intraoperative hemodynamics, patients having undergone surgery in the BCP are at a higher risk of cerebral desaturation events (CDEs) than patients in the lateral decubitus position.⁶ According to the report, blood pressure in BCP patients is overestimated when compared with the actual blood pressure at the level of the brain because it was measured at the brachial artery in both groups. In order to prevent cerebral hypoperfusion and CDEs, previous reports recommend that the blood pressure level measured by a cuff at the upper extremity be maintained at over 80% of its preoperative value. This is because the rSO₂ value depends on cerebral blood flow and oxygen transport.^2,6 The ABP transducer should be placed at the level of the external auditory meatus and not at the level of the heart upon insertion of an invasive percutaneous arterial catheter. Further, anesthesiologists should maintain blood pressure based on the values obtained from this level, which reflects the cerebral cortex blood pressure.² Our hospital typically aims to maintain the cortex level mABP at above 60 mmHg. The purpose of this study is to investigate whether our targeted mABP would decrease the occurrence of CDEs during the BCP.

2. Methods

The retrospective study protocol was approved by the Institutional Review Board of The Gifu University Hospital. We examined 20 consecutive patients aged 21–82 years who underwent elective shoulder surgery in the BCP under general anesthesia at the Gifu University Hospital, between January 2011 and December 2012. Anesthesia was induced with thiopental or propofol and remifentanil. The administration of rocuronium produced a neuromuscular blockade. The effect of anesthesia was maintained with sevoflurane and remifentanil titrated to bispectral index values between 40 and 60. All patients received orotracheal intubation with an endotracheal tube and mechanical ventilation. The ventilator settings were maintained at 35–45 mmHg PaCO₂ and the inhalation oxygen concentration was maintained at 50%. Bilateral forehead rSO₂ was monitored continuously for the length of the surgery using NIRS (INVOS 5100 Cerebral Oximeter, Somanetics Corporation, Troy, MI, USA). A percutaneous arterial catheter was inserted into the non-operative radial artery after anesthesia induction. Patients were positioned in the BCP and the ABP transducer was placed at the level of the external auditory meatus. The target cortex level mABP was set at above 60 mmHg. Both values of the rSO₂ and cortical mABP were extracted at the following time points: (a) the first value was measured in the supine position just before the posture change (baseline indicated as “Pre” in Figure); (b) subsequent measurements were taken after switching to the BCP at 5 min intervals for 90 min; (c) the final measurement was after the return to the supine position at the end of the surgery (indicated as “Supine” in Figure). When the blood pressure decreased, ephedrine, phenylephrine, or fluid infusion was used to maintain mABP. The total amounts of ephedrine or phenylephrine administered during 90 min of the surgery after the posture change to the BCP are shown in Table 1. Based upon a previous report,⁷CDEs are defined as when the rSO₂ values decreased ≥20% of the baseline value or when the absolute rSO₂ values were <50%.

We used SPSS 11.0.1 (SPSS Japan, Tokyo, Japan) for the statistical analysis of the data. Each rSO₂ value was compared to the control (Pre) using the paired Student's t-test. Data are expressed as the mean ± standard deviation. P-values < 0.05 were considered statistically significant.

Table 1. Frequency of cerebral desaturation events and the amount of ephedrine or phenylephrine administration in 90 min in the beach chair position.

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3. Results

No subjects were excluded from this study. Table 1 outlines patient characteristics. The non-invasive mABP was recorded as 91.2 ± 10.8 mmHg in the supine position before anesthesia induction. The mABP immediately prior to the posture change was noted as 73.5 ± 12.5 mmHg (Pre). Post patient positioning in the BCP, the mABP was recorded as 66.8 ± 17.7 mmHg (1 min), 66.9 ± 21.2 mmHg (5 min), 62.2 ± 11.8 mmHg (10 min) and 61.2 ± 9.6 mmHg (90 min). The mABP was found to be 78.7 ± 11.1 mmHg (Supine in Fig. 1A) upon return to the supine position. Regardless of the method used for mABP elevation, all values of cerebral level mABP in the BCP were lower than those noted for the supine position (Pre). Although the target mABP in the BCP was over 60 mmHg, cerebral level mABP recorded for 98.5% of the time points was over 60% of its value in the supine position and was above 50 mmHg in 96.5% of the time points.

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Fig. 1. Changes in the mean arterial blood pressure and the regional cerebral tissue oxygen saturation value. (A) A percutaneous arterial catheter is inserted into the non-operative radial artery upon induction of anesthesia. The arterial blood pressure transducer is placed at the level of the external auditory meatus upon bringing the patients into the beach chair position (BCP). (B) Bilateral regional cerebral tissue oxygen saturation (rSO2) is monitored on the forehead continuously with near-infrared spectroscopy. The right and the left rSO2 values are expressed as open and filled circles, respectively. The difference between rSO2 values is recorded in the patients in the BCP and before the posture change (Pre) was statistically significant (*P < 0.05). mABP: mean arterial blood pressure. Pre: immediately prior to posture change. Supine: post-surgery when the patients are assisted to the supine position. Time: minutes after changing the patient position to BCP. Data are expressed

Immediately prior to the posture change, the right rSO₂ was recorded as 68.1 ± 10.5% (Pre). Upon positioning in the BCP, the right rSO₂ was measured as 63.7 ± 10.3% (1 min), 62.4 ± 11.1% (5 min), 62.5 ± 11.6% (10 min) and 65.0 ± 8.0% (90 min). Upon return to the supine position, the right rSO₂ reading was recorded as 68.2 ± 7.7% (Supine) (Fig. 1B, open circles). Similarly, the left rSO₂ was measured as 70.7 ± 12.1% (Pre). Upon positioning in the BCP, the left rSO₂ reading was recorded as 65.7 ± 13.4% (1 min), 65.3 ± 12.4% (5 min), 64.5 ± 13.4% (10 min) and 66.0 ± 9.9% (90 min). Upon retune to the supine position, the left rSO₂ was noted as 69.5 ± 8.9% (Supine) (Fig. 1B, filled circles). All bilateral rSO₂ values in the BCP (measured 1–90 min) were significantly lower than those recorded in the supine position (Pre and Supine) (Fig. 1B). The rSO₂ values recorded at the majority of time points (95.8%) in the BCP was over 80% of the values in the supine position and the absolute values were above 50% at 95.5% of the time points. The number of patients who experienced CDEs at any point was 35% (7 of 20 patients). None of the patients experienced clinical postoperative neurologic complications.

4. Discussion

In the present study, rSO₂ in anesthetized patients undergoing shoulder surgery decreased with a change in their positions from supine to BCP. It has been reported that following the stimulation of the sympathetic nervous system caused by a change from the supine to the upright position in awake healthy humans, the heart rate increases by 22%, the cardiac output decreases by 20%, the stroke volume decreases by 30%, the systemic blood pressure increases by 10–15%, and the systemic vascular resistance increases by 30–40%.^8,9 Previous studies shown that autoregulation of cerebral blood flow in awake patients is normal when cortex level mABP is maintained between 50 and 150 mmHg.¹⁰ In our study, while most values of cortex level mABP in the BCP were above 50 mmHg, some patients experienced CDEs. These patients tended to have cardiovascular comorbidities (Table 1). It has recently been suggested that the lowest cortex level mABP required to maintain brain autoregulation is 70 mmHg in awake and supine humans.⁸ Therefore, it is estimated that a cortex level mABP elevation higher than 50 mmHg may be required to maintain cerebral perfusion, especially in patients with cardiovascular diseases.

Cerebral perfusion pressure decreases under general anesthesia after a change from the supine to the sitting position.^8,11–14 A common hypothesis is that neurological dysfunctions in the BCP are caused by cerebral hypoperfusion.^2,8,11–13 Notably, the head-up position and the hydrostatic pressure gradients reduce the cortex level mABP due to the weight of the heart-cortex blood column. This reduction is approximately 2 mmHg for each inch of vertical displacement. Therefore, it is recommended that anesthesiologists manage patients who are in the BCP and under general anesthesia by taking the blood pressure at the level of cerebral cortex into consideration or adjust the blood pressure according to the distance between the cerebral cortex and the heart.^8,11,12 The upper extremity level mABP in the BCP under general anesthesia should be maintained at more than 80% of the baseline upper extremity blood pressure value. This action might prevent the decrease of the rSO₂.^6,11 Upon insertion of an invasive percutaneous arterial catheter, it is recommended to place the ABP transducer at the level of the external auditory meatus and not at the level of the heart; cortex level mABP should be maintained according to the values measured at this level.^2,8,11,12 In our study, most cortex level mABP measurements were found to be above 50 mmHg and were calculated as 66 mmHg at the heart or the upper extremity level. This value was found to be 90% of the mABP before the posture change (73.5 mmHg). However, this value was also 72% of mABP in the supine and awake conditions (91.2 mmHg). We suggest that the cortex level mABP in the BCP should be increased further.

Murphy et al reported that CDEs, defined as 20% decrease in the baseline rSO₂, occur in 80% of subjects undergoing shoulder arthroscopic surgery in the BCP under general anesthesia.⁶ In their study, the measurement of blood pressure using a cuff placed on the upper extremity and the maintenance of mABP at over 70 mmHg,⁶which is equivalent to 55 mmHg at the cerebral cortex level. On the other hand, it has been reported that even if the mABP is maintained at above 50 mmHg, rSO₂ values decrease under sevoflurane-nitrous oxide or propofol-remifetanil anesthesia and CDEs occur in about 30% of patients.¹³ Our results also showed that rSO₂ values decreased under sevoflurane-remifentanil anesthesia and that the incidence of CDEs is similar to that in the latter report. Contrary to this report, another study showed that all patients, including those with cardiovascular diseases, recorded the absolute rSO₂to be above 70% at any point (within the normal range) if the heart level mABP was maintained at above 60 mmHg.¹⁴ However, it is a fact that cortex level mABP decreases with a change in posture to the BCP. Maintenance of the normal cerebral perfusion in the BCP is required. Healthy patients can tolerate decreased levels of rSO₂ but it is dangerous for elderly patients or those with cardiovascular diseases. Anesthesiologists must monitor blood pressure directly from an artery and maintain normal blood pressure at the cerebral cortex level.

However, we admit that there are certain limitations of this study. First, NIRS devices measure hemoglobin saturation not only in the arterial, venous, and capillary networks but also in the avascular tissues.⁵ Thus, a decrease in rSO₂ may not always indicate cerebral desaturation induced by a change in position from the supine to the BCP because position change can affect the arterial and the venous blood distribution with respect to the cerebral circulation.⁵ Second, no commonly accepted definition of CDE exists so far. Finally, our results included statistical variation due to the small sample size of our study. Further studies with a larger sample size must be conducted to explore CDEs in the BCP.

5. Conclusion

Although the cortex level mABP in the BCP were found to be >50 mmHg, the rSO₂values recorded a decrease. The reduced rSO₂ values correlated with the decrease in mABP induced by a posture change from supine to BCP. Therefore, despite appropriate blood pressure management, shoulder surgery in the BCP might involve risks for the patients with cerebrovascular diseases.

Conflicts of interest

There are non-financial conflicts of interest.

Declaration

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Acknowledgement

We would like to thank Editage (www.editage.jp) for their support in the English language editing of this manuscript.