Reduction of Mean Arterial Pressure at Incision Reduces Operative Blood Loss in Adolescents

          Surgical management of spinal deformity is associated with significant blood loss and transfusion requirements. Patient factors affecting operative blood loss include the severity and type of spinal deformity, and patient height [1-3]. Surgery-dependent factors include operative time, duration of exposure, procedure performed, surgical approach, number of vertebrae fused, number of screws placed, mean arterial pressure (MAP) during surgery, blood salvage techniques, and the use of antifibrinolytic medications [4]. Large quantities of lost intraoperative and postoperative blood require blood transfusion to maintain tissue perfusion and prevent organ damage. The use of allogenic blood, however, confers an additional risk for blood-borne pathogens, transfusion-related reactions, immune suppression, and a decrease in coagulation factors and increase in surgical site infection [2,5]. To manage blood loss and prevent allogenic transfusion, much attention has been given to blood salvage techniques, Author disclosures: KV (none); BL (board member for Spine Search; consultancy for DePuy Spine; grants from DePuy Spine and AOSpine; payment for lectures for DePuy Spine and K2M; royalties from DePuy Spine; stock/stock options from Paradigm Spine and Spine Search); LD (none); DV (none); VL (consultancy for Medtronic; grants to author’s institution from NIH, DePuy Spine, Medtronic, International Spine Study Group; payment for lectures from DePuy Spine; stock/stock options from Nemaris, no money received). *Corresponding author. New York University Hospital for Joint Diseases, 820 Second Avenue, New York, NY 10017, USA. Tel.: (212) 986-0140; fax: (212) 986-0160. E-mail address: (B. Lonner). 2212-134X/$ - see front matter 2013 Scoliosis Research Society. Spine Deformity 1 (2013) 115e122 including preoperative autologous donation, hemodilution, use of cell saver, and induction of hypotension during surgery.

         Although the ideal MAP is not known, targeted hypotension has been reported with the use of sedative anesthetics, narcotics, vasoactive agents, inhalation anesthetics, and ß-receptor blockade. Each of these medications reduces blood pressure through variable mechanisms. Complications from hypotensive anesthesia have been described, but they remain exceedingly rare in the young healthy population undergoing major surgery spinal surgery [4,6-14]. There has been ongoing discussion regarding the use of hypotension as a blood salvage technique for spinal deformity, and particularly for patients with adolescent idiopathic scoliosis (AIS), a population with relatively few medical comorbidities [4,6,7,12-14]. In this study we assessed the degree and timing of hypotension, operative blood loss, and complications from a large cohort of AIS patients treated by a single surgeon. We hypothesized that using hypotensive anesthesia at incision would be associated with a reduction in operative blood loss and transfusion rates. Methods Study design This study was a retrospective review of 340 consecutive medical records from patients diagnosed with AIS, treated by the senior author from 2000 to 2009.

         The study received approval by the investigational review board at 2 institutions. Inclusion criteria were age between 10 and 21 years; greater than 4 vertebral levels fused; and anterior, posterior, or combined anteroposterior surgical approach. On average, anterior fusions required shorter fusions (4.5 1.2 levels), whereas posterior fusions were longer (9.5 2.3 levels). Exclusion criteria were significant medical comorbidities including history of blood dyscrasia or abnormalities in prothrombin time, partial thromboplastin time, and international normalized ratio test. Data gathered Preoperative data gathered included patient demographics (height, weight, age, and gender) volume of autologous donation, systolic/diastolic blood pressure, and estimated blood volume (EBV). We estimated EBV to be 75 mL multiplied by the body weight (in kilograms). The anesthesiologist and senior surgeon agreed on estimated blood loss (EBL) at the conclusion of surgery, based on cell-saver collected volume, number of blood soaked pads, and volume of intravenous blood products and fluid administered. Estimated blood loss in this study is reported as an absolute volume (EBL) as well as a percentage of total blood volume (%EBV).

          Intraoperative data gathered from the anesthesia flow sheet and operative note included the number of levels fused, time from incision to closure, use of hypotensive agents, volume of autologous, allogenic or cell-saver transfusion, EBL, and intraoperative systolic/diastolic blood pressure at 15-minute intervals. Postoperative data gathered included systolic/diastolic blood pressure and any perioperative complications. Postoperative drain output and transfusion data were inconsistently recorded in the medical record. No patients were treated with antifibrinolytic agents. Method of hypotensive anesthesia The senior surgeon requested hypotensive anesthesia (MAP ! 65) before incision for every case. Hypotension was most commonly induced with inhalation anesthetic agents, sedative agents, and narcotic analgesics. Cardiac medications required to reduce blood pressure in only a few cases. At incision, numerous patients were noted to have a sharp increase in heart rate and blood pressure, which the anesthesiologist controlled with appropriate analgesics. During pedicle screw placement, the senior author requested that the blood pressure be permitted to increase gradually to an MAP O 80 for surgical correction.

          The anesthesiologist on each case varied among several individuals. Estimation of MAP Yoshimoto et al. [15] described a method to estimate the average MAP for the duration of an operation. In that study, the authors measured MAP during surgery at 5-minute intervals to calculate an average MAP for the entire surgery. In the same way, we recorded the intraoperative systolic/diastolic blood pressure from the anesthesia records at 15-minute intervals from incision to closure, to estimate the average MAP (Avg-MAP). We calculated MAP using the formula: MAP 5 1/3 systolic blood pressure þ 2/3 diastolic blood pressure. The MAP at the point of surgical incision (‘‘IMAP’’) was explicitly noted on the anesthesia flow sheet in almost all cases. When the exact incision point was not known, the I-MAP was defined as the MAP 60 minutes after induction of anesthesia. From a review of these anesthesia records, 45e60 minutes was the average time required for patient positioning and draping before incision. Statistics We assessed demographic data using descriptive statistics (means and standard deviation). Patients were first stratified according to surgical approach: posterior, anterior, or combined anteroposterior.

         We compared these surgical groups in terms of operative blood loss, transfusion requirements, I-MAP, and Avg-MAP. Alternatively, all patients were also stratified according to the I-MAP or the Avg-MAP. Groups were as follows: low (MAP!65), medium (MAP 65e75), or high (MAP O 75). The high group was the control group or the no-hypotension group. To analyze the effect of blood pressure elevation at incision, we calculated the difference between the I-MAP and the Avg-MAP.We used this to stratify patients as follows: reduced (Avg MAP I-MAPO5), stable (Avg MAP I-MAP ! 5), or elevated (I-MAP Avg MAP 116 K. Verma et al. / Spine Deformity 1 (2013) 115e122 O 5). In this analysis, the stable group was the control group. We compared groups using an analysis of variance with Tukey’s Multiple Comparison Test. This analysis was done on the entire set of patients (all approaches) and again according to surgical approach (anterior vs. posterior approach). p 5 .05 indicated statistical significance. Results Patient demographics Of the 340 consecutive medical records reviewed from patients with AIS, 327 adolescents (average age, 15 years; range, 10e21 years; 248 females and 79 males) had complete anesthesia records with blood pressure recordings every 15 minutes.

         Average major and secondary curve magnitudes were 51 12 and 47 13, respectively. Average T2e12 kyphosis was 32. The most common curve types were Lenke 1 (49%) and Lenke 5 (24%). Table 1 lists demographic and curve parameters by gender. Estimated blood loss, %EBV, and transfusion rate Table 2 lists the EBL for anterior fusions (322 171 mL), posterior fusions (920 720 mL), and combined anteroposterior fusions (1,321 876). The %EBV for anterior fusions (8.4% 4.8%), posterior fusions (21.2 17.0), and combined anteroposterior fusions (34.6 20.3) is also listed. The vast majority of patients received all pedicle screw constructs, whereas a few early in the series received hybrid constructs consisting of both pedicle screws and hooks. Seven patients in this series underwent iliac crest bone graft (ICBG) harvesting and the remainder had allograft only. The ICBG patients all underwent a posterior spinal fusion of greater than 10 levels from 2000 to 2001 (1 case from 2004). The average blood loss for patients undergoing ICBG harvesting was 1,386 1,135 mL (31.8% 19.5%). Of the 327 operative cases, 129 patients received blood transfusions. Of the patients given a transfusion, most received only autologous blood (71%; n 5 91), whereas a minority were given at least some component of donordirected or allogenic blood (29%; n 5 37). For 3 patients receiving a blood transfusion, the source of the blood product was not found in the hospital record. The EBL and transfusion rates differed significantly with surgical approach (p ! .01) (Table 2). There was no set transfusion trigger.

          The decision to transfuse blood during surgery or in the postoperative period was at the discretion of the senior surgeon. Degree and timing of hypotension Mean arterial flow at incision (mean, 68 8.4 mm Hg) and Avg-MAP (mean 68 5.6 mm Hg) showed no statistical difference when we compared all 327 patients together. The lowest and highest MAP recorded for any patient was 45 and 113, respectively. Similarly, there were no differences in the I-MAP or the Avg MAP with surgical approach (posterior 181, anterior 122, and combined 24) (Table 2). For patients treated with a combined anteroposterior approach, we calculated the I-MAP (or Avg MAP) for each stage and then averaged them. Comparing all surgical approaches together, there was a significant reduction in EBL and %EBV for low I-MAP (584 696 mL; 14.0% 16.7%; n 5 112) versus high I-MAP (871 730 mL; 20.3% 17.0%; n 5 60) (p 5 .03) (Table 3 and Fig. 1). However, there was also no significant difference between low and medium I-MAP (769 706 mL; 18.0% 16.5%; n 5 155) (p 5 .11). There was no significant difference in blood transfusion rate between Table 1 Demographics and curve parameters in AIS patients, by gender. Demographics Male (n579) Female (n5248) Age, years 15.8 (10.1e21.3) 15.2 (10.5e21.8) Height, cm 170.99.1 1597.7 Weight, kg 64.214.3 53.911.7 Body mass index, kg/m2 21.84.9 21.34.3 Radiographic parameters n573 n5234 T2eT12 kyphosis ( ) 29 (22 to 72) 33 (17 to 68) Major curve ( ) 5112 5310 Lenke classification type 1 41 (52%) 118 (48%) 2 11 (14%) 19 (8%) 3 4 (5%) 11 (4%) 4 1 (1%) 4 (2%) 5 13 (16%) 67 (27%) 6 3 (4%) 15 (6%) Categorical values are listed as absolute number (n) and percentage of total (%); numerical values are listed as mean (range) or mean standard deviation. Table 2 Intraoperative data, by surgical approach. Posterior (n5181) Anterior (n5122) Combined (n524) Rib resection 11 (6.1%) 2 (1.6%) 5 (20.8%) Thoracoplasty 11 (6.1%) 12 (9.85) 6 (25%) ICGB harvested 6 (3.3%) 0 0 Posterior levels fused 9.52.3 10.91.6 Anterior levels fused 4.51.2 5.51.2 Number of screws placed 13.13.9 7.92 14.13.8 Operative time, min 21260 23071 38476 Incision MAP 689 677 728 Average MAP 675 696 675 Units transfused 0.71.5 0.30.5 1.71.1 Cell-saver (mL) 326263 3578 436260 EBL (mL) 920780 322171 1,321876 EBL/EBV (%) 21.217.0 8.44.8 34.620.3 EBL, estimated blood loss; EBV, estimated blood volume; ICGB, iliac crest bone graft; MAP, mean arterial pressure. Categorical values are listed as absolute number (n) and percentage of total (%); numerical values are listed as mean standard deviation. K. Verma et al. / Spine Deformity 1 (2013) 115e122 117 I-MAP groups. For Avg-MAP, there was no difference in EBL or transfusion rate between low (n 5 97), medium (n 5 189), and high (n 5 41) Avg MAP groups. Of note, blood loss per minute also was reduced, from 3.8 mL/min (I-MAP O 75 mm Hg) to 2.6 mL/min (I-MAP ! 65 mm Hg) (Table 3). Mitigating factors possibly influencing blood were also similar between groups (Table 4).

          When we compared anterior and posterior approaches independently, there was no significant difference between low, medium, and high I-MAP groups. However for both approaches, there was a notable trend toward decreased blood loss in patients with a lower I-MAP (Table 6). Elevations in MAP during surgical exposure Finally, we considered the effect of elevations in I-MAP over Avg MAP possibly related to extent of analgesia. The groups were as follows: reduced (Avg MAP e I-MAP O 5), stable (5 ! Avg MAP I-MAP ! 5), and elevated (I-MAP Avg MAP O 5). Comparing all surgical approaches together, there was a significant reduction in EBL and %EBV for the reduced (510 648 mL; 11.5% 14.6%; n 5 82) versus the stable group (735 682 mL; 17.6% 16.3%; n 5 193) (p 5 .04) and versus the elevated group (1,033 813 mL; 24.9% 19.6%; n 5 52) (p ! .000) (Table 5 and Fig. 2). There was also a significant difference in blood loss for the stable versus the elevated group (p 5 .02). Operative time was decreased by 29 minutes for the stable versus the elevated group (p 5 .05) and by 48 minutes for the reduced versus the elevated group (p 5 .002) (Table 5). Although there was a trend toward increased curve magnitude in the elevated group, this was not statistically significant and probably not a strong confounding variable (50 vs. 54; p 5 .07). Within the group of patients treated by anterior approach, there was a significant reduction in blood loss for the reduced (271 124 mL; 6.9% 3.6%; n 5 42) versus the stable group (344 184 mL; 9.0% 5.3%; n 5 67) and the reduced versus the spike group (405 209 mL; 10.3% 5.3%; n 5 11) (p ! .029 and .021, respectively). For the posterior approach, the reduced group (774 884 mL; 16.8% 19.9%; n 5 37) had significantly less blood loss than the spike group (1,198 918 mL; 25.8% 18.1%; n 5 29) (p ! .028) (Table 7 and Fig. 2). Hemodynamic complications One patient had intraoperative hemodynamic instability (Avg-MAP 61 6.4 mm Hg; range, 45e70 mm Hg). The operation performed was a combined anteroposterior spinal fusion for a Lenke 6 curve with a major coronal curve magnitude of 85. During the surgery, the patient lost 2,725 mL of blood (76.9% EBV) and was transfused 4 units.

          In this case, hemodynamic instability was most likely related to excessive blood loss and not the degree of hypotension during surgical exposure (I-MAP 5 66 mm Hg). We noted no other operative or postoperative complications related to blood loss or hypotension. Discussion Gardner [16] first described hypotensive anesthesia to reduce intraoperative blood loss in 1946; it has been used in major orthopedic procedures since that time [12]. Although the potential benefit of hypotension to manage blood loss has been reported, it remains controversial. In a recent review of 17 randomized orthopedic trials, Paul et al. [12] reported a significant decrease in blood loss (287 mL) and transfusion volume (667 mL) when patients were Table 3 Operative blood loss and transfusion rate, grouped according to mean arterial pressure (MAP). Low (MAP !65) (N5112) Medium (MAP 65e75) (N5155) High (MAP O75) (N560) I-MAP EBL, mL 584696 769706 871730 EBL/EBV (%) 14.016.7 18.016.5 20.317.0 p[.11* ns p[.03y PRBC transfused (units) 0.711.76 0.460.74 0.770.89 ns ns ns N597 N5189 N541 Avg-Map EBL, mL 712774 758728 609449 EBL/EBV (%) 17.719.2 17.917.2 12.99.5 ns ns ns Paced red blood cells transfused (units) 0.761.83 0.560.83 0.370.60 ns ns ns Avg-MAP, Mean arterial pressure at incision; EBL, estimated blood loss; EBV, estimate blood volume; I-MAP, mean arterial pressure during entire surgery; ns, not significant.

         Numerical values are mean standard deviation. Values for MAP are compared: *low versus medium and y low versus high. Reduction of I-MAP below 65 mm Hg resulted in a significant decrease in operative blood loss compared with I-MAP above 75 mm Hg (p 5 .03). Reduction in MAP for the duration of surgery (Avg-MAP) did not reduce blood loss. We performed comparisons between groups using analysis of variance with Tukey’s Multiple Comparison Test. 118 K. Verma et al. / Spine Deformity 1 (2013) 115e122 treated with hypotension. Especially for patients undergoing spinal fusion, concerns exist about spinal cord perfusion and the potential for vision loss. Furthermore, patients with renal, cardiac, or pulmonary disease may be at greater risk for organ injury with induced hypotension [11,12,17]. For adolescents undergoing multilevel spinal fusion, hypotensive anesthesia has been shown to significantly reduce operative blood loss and transfusion rate [4,7,10,15,18]. In a prospective study of 24 patients, Grundy et al. [18] reported a 44% decrease in blood loss and a 19% reduction in transfusion volume. Similarly, Patel et al. [10] retrospectively reported a 28% reduction in blood loss and a 49% reduction in transfusion volume in 49 patients. However, neither the timing of hypotension nor the method used to estimate average MAP was well described in either study.

         For both of these studies, the sample size was small, as well. In comparison, Lawhon et al. [7] and Hur et al. [4] reported a larger cohort of adolescents and children (n 5 132 and 119, respectively), but the children’s heterogeneous group included idiopathic, congenital, neuromuscular, and miscellaneous spinal deformities. Lawhon et al. reported a 49% reduction in blood loss and a 42% reduction in transfusion rate, whereas Hur et al. reported an 82% reduction in homologous blood transfusion when patients were treated with a combination of hypotension and hemodilution. Neither of these studies measured MAP intraoperatively. Hypotension also reduced operative time for multilevel spinal fusion by 20e30 minutes in several studies [8,19,20]. In a small series of 12 Jehovah’s Witnesses undergoing spinal instrumentation, Brodsky et al. [20] reported a 280-mL reduction in blood loss compared with case-matched controls. Using a linear regression, however, the reduction in blood loss was independently attributed to a reduced operative time rather than blood pressure.

         In contrast, linear regression analyses from this cohort in a separate analysis identified both operative time (for posterior fusion) and the MAP at incision (for anterior fusion) as independent predictors of blood loss. Of note, blood loss per minute was reduced as well, from 3.8 mL/min (I-MAPO 75 mm Hg) to 2.6 mL/min (I-MAP ! 65 mm Hg). Similarly, Mandel et al. [8] reported a 44% reduction in operative blood loss compared with controls. Blood loss per minute was also reduced from 5.4 to 3.3 mL/min for the hypotensive group. Intuitively, reduced bleeding from paraspinal musculature should also reduce the time necessary for surgical exposure. Maintenance of adequate spinal cord perfusion is of paramount importance when using this technique. However, hypotension from excessive blood loss is physiologically different from induced hypotension. In the former, the patient is vasoconstricted with poor cardiac output and low blood flow. In the latter, the patient is hypotensive as a result of vasodilatation and maintains a high flow state with preservation of peripheral oxygen delivery [6]. Therefore, induction of hypotension at incision, before significant blood loss, should not adversely affect spinal cord perfusion in healthy young patients. Moreover, spinal cord perfusion is most critical during surgical correction when blood vessels supplying the spinal Fig. 1.

          Estimated blood loss is reduced with hypotension at incision. Reduction of the mean arterial pressure at incision (I-MAP) below 65 results in a significant reduction in operative blood loss. UMean arterial pressure less than 65 versus MAP greater than 75; p 5 .03. The number of patients in each group (N) is noted. Table 4 Mean arterial pressure at incision (I-MAP) groups compared for parameters associated with increased blood loss. Low (I-MAP !65) (N5112) Medium (I-MAP 65e75) (N5155) High (I-MAP O75) (N560) p Age, years 15.02.3 15.52.2 15.72.5 ns Body mass index 21.03.9 21.64.3 21.95.6 ns Major coronal curve ( ) 5111 5212 5011 ns T2eT12 kyphosis ( ) 3214 3315 3212 ns Rib resection 4 (3.6%) 9 (5.8%) 5 (8.3%) Thoracoplasty 4 (3.6%) 18 (11.6%) 7 (11.7%) ICBG harvest 1 (0.9%) 5 (3.2%) 0 Posterior levels fused 9.72.5 9.72.3 9.52.2 ns Anterior levels fused 4.81.2 4.61.3 4.41 ns Number of screws placed 10.84.4 11.03.8 12.14.5 .21 Operative time, minutes 22969 23380 23189 ns ICBG, iliac crest bone graft; ns, not significant. Categorical values are listed as absolute number (n) and percentage of total (%); numerical values are listed as mean standard deviation. We found no differences between low, medium, and high I-MAP groups comparing various patient specific, radiographic, and surgical parameters associated with increased blood loss.

         We performed comparisons between groups using analysis of variance with Tukey’s Multiple Comparison Test. K. Verma et al. / Spine Deformity 1 (2013) 115e122 119 cord are stretched [14,17]. Hypotension is most commonly induced with the anesthetic agent (ie, propofol), but can also be induced with inhalation agents, neuromuscular blockers, narcotics, and b-blockers [10,17]. As described, suboptimal pain control at incision can lead to a spike in heart rate and blood pressure. A conscientious anesthesiologist, however, may administer pain medications at the start of incision to manage this raise in blood pressure [6,15,17,21]. To our knowledge, we are the first group to study the rise in blood pressure at incision and its effect on blood loss. We report that a rise in heart rate and blood pressure at incision is associated with a 29% increase in blood loss. There is also a corresponding increase in operative time, by 29 minutes. In this series of 327 operative AIS patients, most (n 5 267; 82%) had an MAP ! 75 mm Hg at incision. Although we report no complications related to operative hypotension, we recognize that this study is underpowered to adequately detect neurologic complications related to hypotension.

          As mentioned, 1 patient experienced intraoperative hemodynamic instability that was presumably related to fluid shifts from excessive blood loss (2,725 mL; 76.9% EBV) rather than induced hypotension. Whereas complications from hypotension remain exceedingly rare in healthy patients, it is relatively contraindicated in those with a significant history of cardiac, renal, pulmonary, hepatic, or peripheral vascular disease. The most common complications reported after spinal surgery include delayed hemorrhage, persistent hypotension, anuria/oliguria, delayed awakening, cardiovascular, embolic phenomena, and visual disturbances [4, 6-14]. Visual loss remains 1 of the most worrisome complications of spine surgery. In a National Inpatient Sample analysis of 4.7 million spine surgery patients, Patil et al. [11] reported a 0.29% rate of visual loss for scoliosis patients. Intraoperative hypotension was strongly associated with ischemic optic neuropathy (ION) (odds ratio [OR] 5 10.1), but was not associated with non-ION visual loss. Ischemic optic neuropathy risk was also strongly associated with anemia (OR 5 5.9) and blood transfusion (OR 5 4.3), but has only been reported in patients older than 17 years. The authors concluded that despite the strong associations, the development of postoperative ION is multifactorial, and that neither hypotension nor anemia is sufficient for ION.

          Peripheral vascular disease (OR 5 6.4), diabetes (OR 5 2.3), and operative times greater than Table 6 Operative blood loss, grouped according to mean arterial pressure (MAP): Analysis by surgical approach. Low (I-MAP !65) Medium (I-MAP 65e75) High (I-MAP O75) p Anterior approach EBL 289136 336176 382228 ns %EBV 7.64.1 8.44.8 11.27.0 n547 n546 n517 Posterior approach EBL 782865 946688 1,056833 ns %EBV 18.419.0 21.819.0 25.016.1 n558 n588 n536 EBL, estimated blood loss; %EBV, percent estimated blood volume; I-MAP, MAP at incision; ns, not significant. Numerical values are listed as mean standard deviation. When anterior or posterior approaches were analyzed independently, a reduction of in MAP at incision below 65 mm Hg did not result in a significant decrease in operative blood loss compared with other groups. However, there was a noticeable trend toward decreased blood loss in this group for both approaches. We performed comparison between groups using analysis of variance with Tukey’s Multiple Comparison Test. Table 5 Comparison of reduced, stable, and elevated mean arterial pressure at incision (I-MAP) groups. Reduced (I-MAP ! Avg-MAP) (N582) Stable (I-MAP 5 Avg-MAP) (N5193) Spike (I-MAP O Avg-MAP) (N552) p Age, years 15.42.4 15.32.2 15.62.4 ns Body mass index 21.84.9 21.34.3 21.24.0 ns Major coronal curve ( ) 5012 5111 5413 .07* T2eT12 kyphosis ( ) 3413 3215 3013 ns Operative time, minutes 21270 23172 260100 .002,* .05y EBL, mL 510648 735682 1,033813 .04,z .000,* .02y EBL/EBV (%) 11.514.6 17.616.3 24.919.6 Avg-MAP, MAP at incision; EBL, estimated blood loss; EBV, estimated blood volume; ns, not significant. Numerical values are listed as mean standard deviation; categorical values are listed as absolute number (n) and percentage of total (%).

         Mean arterial blood pressure at incision was considered equal to Avg-MAP when within 5 mm HG. There was a significant difference in blood loss comparing reduced, stable, and elevated groups. Operative time also decreased by 29 minutes comparing stable versus elevated groups, and by 48 minutes comparing reduced versus elevated groups. We performed comparisons between groups performed using analysis of variance with Tukey’s Multiple Comparison Test. *Compares reduced versus spike. y compares stable versus spike. z compares reduced versus stable. 120 K. Verma et al. / Spine Deformity 1 (2013) 115e122 6 hours (96% of ION cases) are also likely to increase the risk [11,22]. In a review by Myers et al. [23] of 37 cases of postoperative ION, perioperative hematocrit and blood pressure were equivalent between patients with visual loss and unaffected surgical patients. The largest studies examining the complications from hypotensive anesthesia during spinal surgery have reported no visual complications [4,7,8]. Lawhon et al. [7] (n 5 210) and Mandel et al. [8] (n 5 77) both reported no mortalities or neurologic complications related to this technique. Hur et al. [4] reported only 3 cases of clinical thrombosis in over 500 pediatric and adult patients treated with hemodilution and hypotensive anesthesia.

          No cardiorespiratory, renal, hepatic, neurologic, visual, or metabolic complications were reported. Numerous studies with smaller sample sizes have similarly reported no complications [9,12-14,19]. There continues to be little to no evidence supporting an association between hypotension and visual loss or organ failure in multilevel spinal surgery for AIS. In this study, we estimated the I-MAP and Avg-MAP. For all patients, we find that a reduction of the MAP below 65 mm Hg at incisiondbut not the duration of surgerydreduces operative blood loss by 33% in AIS. This difference was not statistically significant when anterior or posterior procedures were analyzed independently, perhaps because of a small sample size. Moreover, we found that a spike in blood pressure at incision may increase operative blood losses by 29% and operative time by 29 minutes when considering all patients and procedure types. The spike also increased blood losses when either anterior or posterior approaches were analyzed independently.

         We did not detect differences in transfusion rate, likely because of a low incidence of transfusion in AIS and an inability to detect a subtle difference in transfusion rate between groups. Adolescents with idiopathic scoliosis are also a homogeneous subset with relatively little medical comorbidity. Patient factors that are known to raise or lower operative blood losses are therefore better controlled than with adult deformity patients. Patients with AIS often have flexible curves that generally require fewer osteotomies and less operative manipulation than patients with adult deformity and arthritic facet joints [2]. Because a single surgeon treated all patients, variability in the duration of surgical exposure, surgical technique, and instrumentation is significantly reduced. This offers a unique advantage over other large cohort studies, in which surgeon-to-surgeon bias may be of significant concern. The vast majority of patients received all pedicle screw constructs and did not require ICBG harvesting. The subgroup of patients undergoing ICBG or receiving hybrid represents less than 5% of cases, which are not likely to significantly impact blood loss.

         Our hope is that this study will be applicable to all pediatric spinal deformity patients, with the assumption that more complex deformities will require greater surgical manipulation and have increased EBL. Table 7 Operative blood loss for reduced, stable, and elevated mean arterial blood pressure at incision (I-MAP) groups: Analysis by surgical approach. Reduced (I-MAP ! Avg-MAP) Stable (I-MAP 5 Avg-MAP) Spike (I-MAP O Avg-MAP) p Anterior approach EBL 271124 344184 405209 .029,* .021y %EBV 6.93.6 9.05.3 10.35.3 n542 n567 n511 Posterior approach EBL 774884 890692 1,198918 .028y %EBV 16.819.9 21.315.4 25.818.1 n537 n5116 n529 AVG-MAP, average MAP; EBL, estimated blood loss; %EBV, percent blood volume; ns, no significant. Numerical values are listed as mean standard deviation. When anterior or posterior approaches are analyzed independently, there continues to be a statistically significant decrease in operative blood loss when comparing the reduced, stable, and elevated groups. We performed comparisons between groups using analysis of variance with Tukey’s Multiple Comparison Test. *Compares reduced versus stable. y compares reduced versus spike. Fig. 2. Blood loss is increased when the mean arterial pressure (MAP) is elevated at incision.

          Elevations in the MAP at incision (I-MAP) above the MAP during the entire surgery (Avg-MAP) are associated with significant increases in operative blood loss. ({ Reduced versus stable; p 5 .04. UReduced versus elevated; p ! .000. VStable versus elevated; p 5 .02). The number of patients in each group (N) is noted. K. Verma et al. / Spine Deformity 1 (2013) 115e122 121 To our knowledge, this is the largest study to examine the efficacy of operative hypotension in the AIS population. This seems to also be the first study to specifically focus on hypotensive anesthesia at the start of incision. Concerns about spinal cord perfusion during surgical correction or the effect of extended periods of hypotension on renal, cardiac, pulmonary, or visual function can be mitigated by confining controlled hypotension to surgical exposure when it is most effective. During the screw placement and correction stages of surgery, blood pressure may be elevated without significant increases in blood loss.

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