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Postoperative Cognitive Dysfunction and the Change of Regional Cerebral Oxygen Saturation in Elderly Patients Undergoing Spinal Surgery

Kim, Jeongmin MD, PhD; Shim, Jae-Kwang MD, PhD; Song, Jong Wook MD, PhD; Kim, Eui-kyung MD; Kwak, Young Lan MD, PhD

doi: 10.1213/ANE.0000000000001352
Neuroscience and Neuroanesthesiology: Original Clinical Research Report

BACKGROUND: In this study, we examined the relationship between postoperative cognitive dysfunction (POCD) and intraoperative regional cerebral oxygen saturation (rSO2) in elderly patients undergoing spinal surgery.

METHODS: We enrolled 87 patients older than 65 years. All patients were tested using a battery of cognitive function tests (Korean Mini-Mental State Examination and visuomotor test of Dynamic Lowenstein Occupational Therapy Cognitive Assessment–Geriatric Version) the day before their surgical operation and on the seventh postoperative day. Our threshold for defining POCD for a given patient was a Reliable Change Index score of <−1.96 occurring on 2 tests.

RESULTS: POCD was detected in 20 patients (23%) at the seventh postoperative day. Between-patient baseline characteristics, surgical data, and baseline cognitive function were similar for both those who developed POCD and those who did not. A univariate analysis that included age, female sex, education level, presence of diabetes, and duration of intraoperative decline in rSO2 to a level of <60% of baseline revealed that only diabetes and duration of rSO2 <60% (odds ratio, 1.01; 95% confidence interval [CI], 1.005–1.010) were found to be risk factors for POCD. After multivariate logistic regression analysis of these 2 variables, only the duration of rSO2 <60% (odds ratio, 1.006; 95% CI, 1.00–1.01, P = 0.014) remained as an independent risk factor for POCD. The area under the receiver operation characteristic of the duration of rSO2 <60% was 0.70 (95% CI, 0.57–0.82; P = 0.008). The optimal cutoff value was 157 minutes with a sensitivity of 75% and specificity of 72%.

CONCLUSIONS: This study showed that the duration of decline in rSO2 <60% during lumbar spinal surgery was correlated with the development of POCD at the seventh postoperative day in elderly patients.

Published ahead of print June 9, 2016

From the *Department of Anesthesiology and Pain Medicine, Severance Hospital, Anesthesia and Pain Research Institute, Yonsei University College of Medicine, Seoul, Republic of Korea; and Department of Anesthesiology and Pain Medicine, Severance Cardiovascular Hospital, Anesthesia and Pain Research Institute, Yonsei University College of Medicine, Seoul, Republic of Korea.

Accepted for publication March 21, 2016.

Published ahead of print June 9, 2016

Funding: None.

The authors declare no conflicts of interest.

Reprints will not be available from the authors.

Address correspondence to Young Lan Kwak, MD, PhD, Department of Anesthesiology and Pain Medicine, Severance Cardiovascular Hospital, Anesthesia and Pain Research Institute, Yonsei University College of Medicine, Yonsei-ro, Seodaemun-gu, Seoul 120–752, Republic of Korea. Address e-mail to

Alteration of cerebral function is frequently observed after surgery and includes a depressed level of consciousness and impairments in attention, memory, and reaction time. Surgery exerts relatively greater adverse effects on the brains of elderly patients than on those of younger patients, manifested by the higher prevalence of postoperative delirium and cognitive dysfunction.1 The reported incidence of postoperative cognitive dysfunction (POCD) after major noncardiac surgery in patients older than 60 years is 26% at 1 week and 10% at 3 months.2 Of note, POCD may persist for months after surgery, hindering patients’ return to their everyday lives.2 More importantly, persistent POCD may actually herald an increase in both morbidity and mortality in elderly patients.3 Thus, attention to appropriate monitoring and early recognition of POCD in elderly patients may allow appropriate risk stratification and assessment of the efficacy of potentially preventive measures.

Near-infrared spectroscopy (NIRS) allows continuous and noninvasive monitoring of regional cerebral oxygen saturation (rSO2). It provides real-time information assessing the adequacy of cerebral perfusion, although not cerebral perfusion per se.4–6 Age and the maximum percentage decrease in intraoperative rSO2 were shown to be risk factors for POCD at 7 days after total hip arthroplasty.7 In contrast, another study involving patients undergoing shoulder surgery in the beach-chair position failed to demonstrate this relationship and thus argued that the degree and duration of rSO2 were not correlated with POCD.8 A recent systematic review of cerebral oxygenation, as assessed by NIRS during noncardiac surgery, depicted only a weak association between severe intraoperative cerebral desaturation and POCD. In addition, the association was limited to certain types of surgeries.9 The available literature regarding the relationship between POCD and rSO2 in noncardiac surgery is minimal and is even more limited in elderly patients undergoing extensive surgeries. Given these limitations, the evidence for such a putative relationship should be considered inconclusive at best.

We hypothesized that intraoperative cerebral desaturation is associated with POCD in elderly patients undergoing major noncardiac surgeries. The major objective of this study was to investigate the relationship between intraoperative changes in rSO2 and development of POCD in elderly patients undergoing lumbar spinal surgery. In addition, we evaluated the putative risk factors associated with POCD.

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After institutional review board approval and registration at (ID NCT01839227), 132 patients 65 years or older and scheduled for lumbar spinal surgery were enrolled between November 2012 and December 2013. Enrolled patients had either degenerative lumbar spinal stenosis or spinal cord tumor. The planned surgical procedures were all performed in the prone position using the Wilson frame with the head and neck in the neutral position and included laminectomy and discectomy, spinal fusion, and (when present) removal of the spinal cord tumor. Written informed consent for participation in this study was obtained from all patients. Patients were screened within 1 week before the scheduled surgery. Details and medical comorbidities, surgical history, and the number of years of education received were recorded. Patients were excluded from participation in the study if they had literacy problems, language difficulties, hearing or visual impairment, history of taking any medication for mental disorders, previous neurologic disease (eg, stroke, seizures, or dementia), and memory impairment with a score on the Korean Mini-Mental State Examination (K-MMSE) of 23 or less.

Cognitive function testing was performed on the evening before the surgery, on the seventh postoperative day (median [5–95 percentiles], 6 [4–7] days), and 1 month (median [5–95 percentiles], 36 [34–41] days) after surgery. All cognitive function tests were conducted by a trained research staff member who was not involved in the intraoperative management of the patients. All tests were conducted in a separate quiet room at the same time of day to avoid any confounding influence of circadian rhythm. The neuropsychologic tests given included the K-MMSE and visuomotor test of the Dynamic Lowenstein Occupational Therapy Cognitive Assessment–Geriatric Version (DLOTCA–G).10 The K-MMSE consists of 11 simple questions or tasks grouped into 7 cognitive domains: orientation to time, orientation to place, registration of 3 words (ie, immediate memory), attention and calculation, recall of 3 words, language, and visual construction. The DLOTCA-G visuomotor subscale is composed of 6 individual tests: copying a geometric form, reproducing a 2-dimensional model, pegboard construction, color block design, reproduction of a puzzle, and drawing a clock. The visuomotor organization subtest combines perceptual activity, motor responses, and a spatial component and includes 3 large classes of activities: copying, drawing, and building or assembling. To minimize the learning effect, we used the parallel forms of each test and randomized the sequence of test administration. The first postoperative evaluation was done at the seventh postoperative day to avoid the effects of postsurgical narcotic analgesics used for immediate pain control.

To define the POCD, we used the Reliable Change Index (RCI) proposed by Jacobson and Truax.11 In brief, change was calculated for each test in the individual by subtracting the preoperative performance from the postoperative performance as a function of the SE of the difference without the adjustment from an age-matched nonsurgical group. POCD was defined in an individual when, at the seventh postoperative day, the RCI scores were <−1.96 on the K-MMSE and the DLOTCA-G visuomotor test. The presence of delirium was assessed twice daily (morning and night) on the ward for 7 days after surgery according to the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition.

No premedication was given. In the operating room, patients were laid in the supine position, and a Bispectral Index (BIS) sensor (BIS vista monitor Revision 3.0; Aspect Medical Systems, Norwood, MA) and 2 NIRS sensors (INVOS 4100; Covidien, Mansfield, MI) were applied to the patient’s forehead before the induction of anesthesia. The patient’s baseline rSO2 data were acquired before anesthetic induction while the patient breathed room air. The anesthesiologists and surgeons were blinded to the rSO2 data, which were recorded at 30-second intervals on the device’s accessory disk drive for later analysis. No interventions were attempted based on the device. Both the right and the left frontal rSO2 values were recorded simultaneously, and the lower value of either side was collected for analysis. Recorded data were analyzed by using Excel (Microsoft, Seattle, WA) to calculate the duration of rSO2 below the threshold and the area of the rSO2 tracing under the threshold. The rSO2 data were analyzed at multiple thresholds (<60%, <55%, and <50%) to determine 1 suitable for comparison. Both mean duration below the threshold value of rSO2 and the area under the threshold value of rSO2 were measured.

Anesthesia was induced with propofol (1–1.5 mg/kg IV), remifentanil (0.05–0.2 μg/kg/min), and rocuronium (0.6 mg/kg). After anesthesia induction, rSO2 measurements were made with the patients in both supine and prone positions. In each position, data collection began 5 minutes after assuming the position. During the surgery, the concentration of sevoflurane was adjusted to achieve a BIS value between 40 and 60. If mean arterial blood pressure (MAP) decreased to 80% of baseline during the surgery while within the BIS range 40 to 60, infusion of remifentanil was decreased at 0.01-μg/kg/min decrements, and either ephedrine (4–8 mg) or norepinephrine (starting at 0.02 μg/kg/min, with further adjustments in 0.01-μg/kg/min increments) was administered. Conversely, when MAP increased to >120% of baseline, the infusion rate of the drugs was adjusted in the opposite direction and in a similar stepwise fashion. The patients’ lungs were ventilated with a 50% oxygen/air mixture. Respiratory rate was set to meet the end-tidal CO2 35 to 40 mm Hg. The intraoperative threshold for transfusion of packed erythrocytes was hematocrit <27%. For monitoring purposes, we registered electrocardiogram, invasive arterial blood pressure, peripheral oxygen saturation, and end-expiratory CO2 every 5 minutes. Arterial blood gas analysis was performed hourly during the surgery. All patients used IV patient-controlled analgesia (opioid) postoperatively for pain control. The IV patient-controlled analgesia opioid consisted of 15 to 20 μg/kg fentanyl and 0.3 mg ramosetron (total volume of 100 mL including 0.9% normal saline, basal rate 2 mL/h, bolus 0.5 mL, and lockout time 15 minutes). Postoperative pain assessments were performed at 6 hours and the first day, second day, and seventh day after surgery by using a verbal numerical rating scale (“0”  =  no pain; “10”  =  worst possible pain). A rescue analgesic was administered at the discretion of the clinician in charge of postoperative care in the wards or on a patient’s request or verbal numerical rating scale >5.

As a prior assumption based on a previous study performed in patients undergoing thoracic surgery,12 we hypothesized that the duration of rSO2 <60% would be associated with the incidence of POCD in spine surgery, and similar to the previous study, Spearman correlation coefficient would be 0.3. Thus, we determined that a sample size of 84 patients would provide 80% statistical power to a correlation between the duration of rSO2 <60% and the incidence of POCD (2-sided α = 0.05).

Statistical analyses were performed using IBM SPSS Statistic 20 (SPSS Inc, Chicago, IL). Statistical comparisons were performed between patients developing POCD (POCD group) or not (non-POCD group) on the seventh postoperative day. Categorical values are presented as a number (percentage) and were analyzed using a contingency table or Fisher exact test. The normality of the continuous variables was tested with the Shapiro–Wilk test. The independent t test or Mann-Whitney U test was then used to determine differences in the means of continuous variables, as indicated. For intergroup comparisons of repeatedly measured variables such as heart rate, MAP, BIS, PaCO2, and hematocrit level, we used a linear mixed model with compound symmetry. Two fixed effects were included, 1 addressing the between-participant POCD (level: POCD and non-POCD) and 1 addressing the within-participant direction effect (direction: time). The correlation between 2 continuous variables was analyzed by Pearson or Spearman correlation.

The following variables were first introduced to the univariate analysis: age, female sex, education level, and variables with a statistically significant difference (P < 0.2) between patients who developed POCD and those who did not. Only the variables with P < 0.2 were then used in the multivariate logistic regression analysis to identify risk factors for POCD.

The ability of the duration of rSO2 <60% to discriminate patients who developed POCD or not was assessed with a receiver operation characteristic curve analysis. The optimal cutoff value was defined as the point of the duration of rSO2 <60% exhibiting the greatest sum of sensitivity and specificity. All reported P values are 2-tailed. P < 0.05 was considered statistically significant.

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One hundred thirty-two patients were screened for eligibility. Of them, 87 were enrolled over the course of a 1-month follow-up period (Figure 1). Among these 87 patients, POCD was detected in 20 (23.0%) at the seventh postoperative day. The 95% confidence interval of the POCD proportion was 0.40 to 0.60. Among these patients, 8 (9.2%) showed persistent POCD at 1 month. The remaining 12 patients’ cognitive function had recovered to their baseline values at 1 month after surgery, whereas no patient exhibited newly developed POCD at 1 month after surgery (data not shown).

Figure 1

Figure 1

Patients’ demographic characteristics were similar between the groups (Table 1). Operative data including the duration of anesthesia, surgical procedures, and transfusion requirements were also similar between groups. The incidence of postoperative delirium was also similar between groups and in all cases had resolved completely by 3 days after surgery. Values of rSO2 were not different between patients with or without delirium (data not shown). Reported pain intensities were similar between the 2 groups up to the seventh postoperative day (Table 1).

Table 1

Table 1

Intraoperative changes in heart rate, MAP, BIS, PaCO2, and hematocrit levels were similar between groups according to the mixed model (Figure 2). The incidence and total amount of intraoperative vasopressor use were not different between groups (non-POCD versus POCD; incidence of vasopressor use, 58 [86.8%] vs 16 [80%], P = 0.485; total amount of intraoperative ephedrine, 10 (4–20) vs 6 (0–12) mg, P = 0.224).

Figure 2

Figure 2

Test scores on the K-MMSE and visuomotor tests of the DLOTCA-G before surgery and on the seventh postoperative day are summarized in Table 2. Baseline cognitive function results were not different between groups (Table 2). At the seventh postoperative day, values for both orientation to time and place and complex command tests in the K-MMSE were significantly lower in the POCD group than in the non-POCD group. Each score of the DLOTCA-G visuomotor tests, except those of copying geometric forms and performance on the pegboard task, was significantly lower in the POCD group compared with the non-POCD group at the seventh postoperative day. RCIs of the K-MMSE (0 [−3.3 to 6.7] vs −8.3 [−15.8 to −8.3], P < 0.001) and visuomotor test (1.9 [−2.0 to 3.9] vs −6.8 [−9.8 to −2.4], P < 0.001) were significantly lower in the POCD group compared with the non-POCD group (Table 2). Postoperative cognitive function at 1 month was compared between groups scoring at different levels on the K-MMSE and the visuomotor test: K-MMSE, 27.4 ± 2.2 vs 24.4 ± 5.4, P < 0.001; visuomotor test, 25.8 ± 4.2 vs 22.8 ± 6.3, P = 0.013. However, the scores of each subject on the K-MMSE and visuomotor test were not different between groups (data not shown).

Table 2

Table 2

The changes in intraoperative rSO2 values are presented in Figure 3. Baseline rSO2 values were similar between groups (Table 3). Right- and left-side rSO2 values were highly correlated (r = 0.770, P < 0.001) with no differences between the rSO2 values of the 2 sides identified for the duration of the study period. Because there were no differences, and because from a clinical standpoint it is more practical, we used the lower value of the 2 sides for further analysis. In both groups, the degree of changes in rSO2 values between supine and prone positions was not significantly different. These rSO2 values were also not significantly different between groups (non-POCD group versus POCD group, −3 ± 7.5 vs −4 ± 4.8, P = 0.783).

Table 3

Table 3

Figure 3

Figure 3

There were significant differences in intraoperative rSO2 values between groups. The lowest intraoperative rSO2 value of the POCD group was significantly lower than the non-POCD group (51% ± 10% vs 56% ± 8%, P = 0.025). Furthermore, total duration of rSO2 <60% was significantly longer in the POCD group (P = 0.019). The duration of intraoperative rSO2 <60% and the area of the curve under rSO2 <60% were highly correlated according to Spearman regression (r = 0.973, P < 0.001). Notably, the logit function of POCD and the duration of rSO2 <60% were significantly linear, whereas the interaction of the continuous predictor with its log transform was not significant (P = 0.920). Thus, the assumption of linearity was not violated (Figure 4). In univariate analysis, the duration of intraoperative rSO2 <60% was positively correlated with the development of POCD (Table 4). Only diabetes exhibited a P value of <0.2 in the univariate analysis. When diabetes and the duration of rSO2 <60% were then included in the multivariate analysis, only the duration of rSO2 <60% remained an independent risk factor for POCD. When the area of the curve under rSO2 <60% was introduced instead of the duration of rSO2 <60%, similar results were observed (Table 4). The duration of intraoperative rSO2 <60% and the area under rSO2 <60% showed significant correlations with the RCI of the K-MMSE (r = −0.25, P = 0.019), but not that of the DLOTCA-G visuomotor test (r = −0.14, P = 0.202), using Spearman correlation.

Table 4

Table 4

Figure 4

Figure 4

The area under the receiver operation characteristic curve of the duration of rSO2 <60% for POCD was 0.70 (95% confidence interval, 0.57–0.82; P = 0.008). The optimal cutoff value was 157 minutes with sensitivity of 75% and specificity of 72%.

Repeatedly measured variables that may influence rSO2 values such as MAP (P = 0.475), BIS (P = 0.794), PaCO2 (P = 0.274), and hematocrit (P = 0.609) were not found to be correlated with intraoperative changes in rSO2 according to the mixed model. Nonrepeatedly measured variables such as lowest hematocrit level (r = 0.707, P < 0.001) and amount of intraoperative blood loss (r = 0.441, P < 0.001) and transfusion (r = 0.611, P < 0.001) were significantly correlated with the duration of rSO2 <60% according to either Spearman or Pearson correlation, as appropriate.

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In this prospective observational study of the association between intraoperative rSO2 changes and POCD in elderly patients undergoing lumbar spinal surgery, the duration of rSO2 <60% was found to be the only independent risk factor for POCD at 1 week after surgery.

Cognitive deterioration after surgery, including a depressed level of consciousness and impairments of attention, memory, and reaction time, is an important issue in perioperative medicine. In addition, the number of geriatric patients undergoing major surgery is increasing annually such that in Korea, approximately 36% of all surgical procedures were performed in adults older than 65 years in 2013.13 The adverse effects of surgery and general anesthesia on cognitive function seem to be more pronounced in elderly patients than in their younger counterparts, resulting in a higher prevalence of postoperative delirium and POCD in the former.14,15 Indeed, POCD was reported to occur in approximately one-fourth of elderly (>60 years) patients 1 week after surgery.2 Early POCD is believed to be a reversible condition in the majority of cases. However, persistent cognitive dysfunction can affect quality of life and may even be an indicator of an adverse outcome, potentially including mortality.3,16 Considering the clinical importance of POCD in elderly patients, the ability to predict its occurrence would be of value in terms of improving risk stratification. Notably, intraoperative cerebral hypoperfusion may be linked to POCD,17 suggesting that monitoring intraoperative cerebral perfusion could be the aforementioned valuable predictive indicator.

The rSO2 measured with NIRS represents oxygen saturation of arterial, capillary, and venous blood and thus could be a relative gauge of actual regional oxygen supply. In fact, several studies have suggested that cerebral perfusion and resulting oxygenation can be assessed using rSO2.6,18 The relationship between the decline in rSO2 and postoperative cognitive function assessed by the MMSE has been studied in various surgical patients.12,19,20

Several studies performed with the INVOS 4100 reported that the baseline rSO2 of elderly patients (>65 years) was significantly lower than of healthy young volunteers aged 20 to 36 years (63% ± 8% vs 71% ± 6%, respectively).20,21 Also, preoperative baseline rSO2 values <60% (INVOS 5100; Covidien) are common in elderly groups and may themselves be associated with a higher incidence of perioperative cognitive dysfunction.19 In this study, approximately one-third of the patients had baseline rSO2 <60% (Figure 3). Lower baseline rSO2 might be a narrower safety margin of cerebral oxygenation in elderly versus younger patients. In addition, a long duration of being bedridden before surgery adds to the risk of POCD, because a depressed mood and anxiety, which are sometimes characteristics of these patients, can aggravate the development of postoperative delirium or POCD.22–24 Recently, anesthetic management based on intraoperative cerebral oximetry monitoring has shown promising results in preventing POCD, although these results came from a small number of patients undergoing lumbar spinal surgery in the prone position.25 Still, current evidence indicates that if there is a link between the intraoperative rSO2 values and POCD at all, it is a weak link. Furthermore, there are no comprehensive data regarding the critical value of rSO2 needed to predict POCD in elderly patients undergoing extensive surgery such as lumbar spinal fusion.

In this study, the incidence of POCD at the postoperative seventh day was 23%. The mean age of patients was 72 ± 5 years, and the baseline rSO2 was 63% ± 7%, which was in accordance with the results of earlier studies.20 In univariate analysis, the duration of decline in rSO2 was proportionally associated with the occurrence of POCD. In multivariate analysis, the duration of desaturation of rSO2 <60% was revealed as an independent risk factor for POCD.

When tested at 1 month after surgery, only 8 patients (9.2%) showed persistent cognitive dysfunction and reported subjective decreases in learning and memory abilities. The intraoperative decline of rSO2 in patients with POCD at 1 month was similar to patients with POCD at the seventh postoperative day. However, no significant statistical correlation was found between the intraoperative decline of rSO2 and POCD at 1 month after surgery. These data must be interpreted with caution, because the sample size was relatively small, and the patients’ postoperative pain and emotional stress were not evaluated simultaneously on the seventh postoperative day. Likewise, the finding that none of the patients developed new POCD at 1 month may be attributable to the relatively small sample size or variability in performance.

Various perioperative factors related to cerebral blood flow and oxygen saturation could affect the rSO2 value. In this study, significant correlations were observed between rSO2 values and the following factors: lowest intraoperative hematocrit level, intraoperative blood loss, and intraoperative blood transfusion. These results are in agreement with the findings of other studies, in which rSO2 was affected by cerebral blood flow, cerebral vasodilation, hematocrit, and intraoperative blood loss.26–28 We did not introduce these variables into further analysis, because the purpose of multivariate logistic regression analysis in this study was to find risk factors for POCD and not to find factors that influence rSO2. The interdependence of blood loss, hematocrit level, and transfusion requirements would also prove problematic when introduced into the multivariate model owing to their multicollinearity. Nonetheless, when introducing these variables into the univariate logistic regression analysis for POCD, they did not yield the statistically significant differences necessary to be introduced into the multivariate analysis (blood loss, P = 0.452; hematocrit level, P = 0.061; transfusion requirements, P = 0.207). Whether application of blood conservation strategies and efforts to minimize blood loss would actually result in decreased POCD remain to be proven.

As a prior assumption based on a previous study, we used an arbitrary cutoff value of 60% but also tested further with other thresholds (55% and 50%). However, baseline rSO2 values <60% appear to be common in elderly patients and, thus, the chosen threshold of 60% is debatable. Indeed, we observed a significant difference in the lowest rSO2 values between patients who developed POCD and those who did not. The lowest value in the POCD group was 51%, which corresponded to approximately a 20% decrease from baseline. However, no difference was observed regarding the durations of rSO2 <55% or 50% thresholds. In addition, baseline rSO2 values were similar between groups and followed a normal distribution in each group, which indicated that patients with lower baseline rSO2 values were not at greater risk of developing POCD than their counterparts. Our results clearly show that a prolonged period during which the rSO2 value is decreased was more closely associated with POCD in elderly patients undergoing extensive surgery than was the degree of the decrease in rSO2 value from baseline. Although in this study we did not manage the patients according to their rSO2 values, our standardized hemodynamic management, based on continuous arterial blood pressure monitoring and avoiding anemia, may have limited the occurrence of dangerously low cerebral desaturation for prolonged periods. Given the ethics of withdrawing such hemodynamic management, it may be difficult to depict any relationship between a considerable decrease in rSO2 value and POCD in modern anesthetic practice. Considering the relatively low odds ratio of the duration of rSO2 <60% (1.181 for every 30-minute increase in duration), it is also possible that our results may merely indicate a weak correlation between rSO2 and POCD.

The findings of this study are subject to the following limitations. First, there are fundamental limitations related to rSO2 values such as wide intraindividual and interindividual baseline variability and the potential for “contamination” of the signal by extracranial tissue.29 Second, we used parallel forms of each test to minimize the potential of a learning effect from repeated cognitive testing.30 However, because we did not include a nonsurgical control group, we used the original RCI method proposed by Jacobson and Truax,11 which was designed to control for test–retest variability but not for learning effects. Third, this study is subject to possible selection bias, owing to the high dropout rate of patients with postoperative morbidities who were unable to perform the cognitive tests.

In conclusion, this study demonstrated that during spinal surgery (in the prone position), the duration of cerebral rSO2 <60% was correlated with the development of POCD at 1 week after surgery in elderly patients.

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Name: Jeongmin Kim, MD, PhD.

Contribution: This author helped conduct the study, analyze the data, and write the manuscript.

Name: Jae-Kwang Shim, MD, PhD.

Contribution: This author helped the analyze the data, design the study, review the analysis of the data, and write the manuscript.

Name: Jong Wook Song, MD, PhD.

Contribution: This author helped design the study and description of the data.

Name: Eui-kyung Kim, MD.

Contribution: This author helped recruitment and collect the data.

Name: Young Lan Kwak, MD, PhD.

Contribution: Young Lan Kwak helped design the study, conduct the study, and write the manuscript.

This manuscript was handled by: Gregory Crosby, MD.

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We thank our clinical assistant, Ms. Lee Min-Jeong, for her help with the data collection.

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