Despite the significant decline in its use over the past 20 years, the semisitting position (SSP) for patients undergoing surgery still has a unique and irreplaceable role in neurosurgery, especially in posterior fossa surgery.^{20,27,28,38,41,42,44,47,48} This position enables better gravitational drainage of cerebrospinal fluid and blood from the surgical field, which provides the neurosurgeon a clean and unobstructed exposure of the lesion. Other benefits conferred by the SSP include opening the possibility for the surgeon to use both hands for micromanipulation rather than relegating 1 hand to aspiration; providing better anatomical orientation; decreasing intracranial pressure; necessitating less retraction; providing an unobstructed view of the patient’s face for the anesthesiologist; enabling easier manipulation of the chest wall, in case of emergency; and enabling observation of the patient’s motor responses to cranial nerve stimulation.^{2,20,27,28,38,43,44,47,48} Despite these advantages, however, use of the SSP declined primarily because of the risk for venous air embolism (VAE), paradoxical air embolism, and tension pneumocephalus.¹³

Intraoperative VAE can be defined as an obstruction of the vascular circulation by air introduced into the venous system from the surgical field or from other interventions to the venous system during surgery. Changing the position of the patient from supine to semisitting brings the venous pressure at heart level below atmospheric pressure, which places the patient at particular risk for rapid air inflow into the venous circulation.²² Common sites of venous air entrainment with a patient in the SSP include the puncture sites of the 3-pin head holder, veins within the muscles, emissary veins, diploic veins, the intracranial venous sinuses, and cerebral veins.^8,45,47,48

Despite minor controversies, the pathophysiology and treatment modalities for VAE have been reported with scientific consensus in numerous publications. Still, the reason for the surprisingly large variation in the reported rates of VAE, from 1.6% to 76%, is still unknown.^18,20,21 This range is too wide to be explained by only different detection methods and is probably the result of different patient-positioning techniques, because there is no standardized angle of head elevation.^{15,18,20,21,27,28,38,43,44,47} Moreover, none of the studies was prospective. To the best of our knowledge, there are currently no data to define the relationship between the degree of head elevation and the rate and severity of VAE.

In this study, we compared data regarding VAEs in patients undergoing an elective infratentorial neurosurgical procedure in the SSP. We compared data from 2 groups, one of which included patients who underwent surgery with a head elevation of 30° and the other of which included patients who underwent surgery with a head elevation of 45°. We also defined a standard protocol for the perioperative management of patients undergoing surgery in the SSP.

Methods

During the study, the senior author (U.T.) used a 30° or 45° head elevation for patients in the SSP depending on the location of the patient’s lesion. Patients with a moderate- to large-size vestibular schwannoma, lower fourth ventricle lesion, or midbrain lesion had a head elevation of 30°. Those with a lesion in the pineal region, thalamus, or upper part of the fourth ventricle had a head elevation of 45°. These technical distinctions were applied to 2 groups of patients who underwent surgery in the SSP.

With approval of the Yeditepe University School of Medicine ethical committee and written informed consent from each patient, 50 patients were enrolled in each group according to the inclusion criteria. Patients were eligible for inclusion if they were 18 to 60 years of age, had an American Society of Anesthesiologists (ASA)³ physical status score of I or II, and elected to have an infratentorial craniotomy in the SSP to remove a tumor or cavernous malformation.

In this study, because the vascular hemodynamics of arteriovenous malformations and aneurysms differ from those of tumors and cavernous malformations, we excluded patients with an arterial lesion. Patients were also excluded if they had clinically severe cardiovascular disease (especially hypertension, atherosclerotic myocardial disease, cardiac failure, aortic stenosis, carotid artery stenosis, or a cardiac septal defect), a poor preoperative condition (such as a severe neurological deficit or disability with a Karnofsky Performance Scale score of less than 80), or severe hydrocephalus. Patients who had undergone a craniotomy in the SSP previously were excluded from the study because of various degrees of meningocerebral adhesions. Patients who had undergone radiosurgery, radiotherapy, or chemotherapy were excluded also. Furthermore, we excluded patients who were more than 130% of their ideal body weight, taking a vasodilator agent or multiple antiepileptic drugs, had a known allergy to any of the study medications, or had a known severe degenerative disease of the cervical spine.

Patients were assessed for contraindications to transesophageal access, including a history of esophageal obstruction such as stricture or a mass, previous esophageal surgery, or a history of dysphagia for reasons other than the intracranial pathology. Patients with any of these contraindications were excluded from the study.

Standard Technique for Surgery With Patients in the SSP

Preoperative Evaluation

During the preoperative visit, after routine physical examination and laboratory testing, patients were evaluated by the same anesthesiologist (H.T.). In addition to answering preoperative and preanesthetic questionnaires, the patients were evaluated also with a standard form to assess their suitability for undergoing surgery in the SSP (Fig. 1).

Preanesthetic questionnaire for SSP. GERD = gastroesophageal reflux disease.

During the preanesthetic examination, patients were tested for range of motion through a simulation of the SSP in the patient’s bed to ensure that their movements and position corresponded to their physiological range. The simulation consisted of flexing the patient’s legs 90° at the hip and approximately 90° at the popliteal fossa. Cervical flexion-extension and cervical abduction-adduction were tested also. If any restriction in range of motion was detected, further examinations were performed. Moreover, in each case, sagittal MR images provided adequate information on the cervical spine. If severe cervical degenerative disease was diagnosed, then the patient was not enrolled into the study.

Before surgery, each patient was assessed by a cardiologist for a right-to-left cardiac shunt by using transthoracic echocardiography during the Valsalva maneuver with an intravenous echo-contrast medium. Any sign of a right-to-left cardiac shunt is a contraindication for undergoing surgery in the SSP.

Anesthesia

On the day of surgery, after being dressed with elastic antiembolic stockings, each patient was given intravenous midazolam (maximum dose 0.02 mg/kg⁻¹) while being monitored with a pulse oximeter and transferred to the operating room. In the operating room, 5-lead electrocardiography, noninvasive blood pressure measurements, pulse oximetry, and the bi-spectral index scale were used for patient monitoring. The patient’s head was kept at a 15°–20° elevation during the induction of anesthesia to maintain venous drainage and prevent gastric aspiration. Anesthesia was induced with 4–6 mg/kg intravenous thiopental, 1–2 μg/kg fentanyl, 0.15–0.2 mg/kg cisatracurium, and 100% oxygen. Anesthesia was maintained with intravenous infusions of remifentanil (0.1–0.5 μg/kg per min) and propofol (75–200 μg/kg per min). The patient’s trachea was intubated, and mechanical ventilation was started (oxygen/air; oxygen 40%–50%). Additional cisatracurium was titrated to the patient’s train-of-4 ratio if required.

A urinary catheter was placed, and urinary output was monitored during the surgery. A 20-gauge catheter was placed in the radial artery, and the subclavian vein was cannulated with a multilumen central venous catheter. We confirmed with radiography that the central catheter was placed at the entrance of the right atrium. The invasive arterial blood pressure, right atrial pressure, end-tidal carbon dioxide levels, and temperature from the urinary catheter were monitored and recorded during surgery. Pneumatic sequential compression devices were applied to the patient’s legs to minimize venous pooling. At our institution, diuretics are not routinely used to prevent intraoperative imbalance of blood pressure in patients who are placed in the SSP.

Electrodes from the intraoperative electrophysiological monitor were placed by the neurophysiology team after the induction of anesthesia. Transesophageal echocardiography (TEE) was performed using a multiplane 3.5- to 7-MHz probe (Philips T6H). The probe was advanced into the esophagus and rotated until the right heart was clearly visible, and the probe was angled to enhance right ventricular imaging at the midesophageal level.

Next, the oral cavity was prepared carefully, and the tongue was secured through the pressure of the TEE probe and endotracheal tube. When both devices are crossed at the back of the mouth, they can compress the base of the tongue after the neck is flexed. A mouth guard was used to secure the oral opening and prevent the patient from unintentionally biting the endotracheal tube, TEE probe, or his or her tongue. However, this maneuver was not comfortable for some patients. In such cases, we inserted a small oral airway to secure the endotracheal tube and TEE probe.

In each case, the patient’s lungs were ventilated manually, and the position of the TEE probe and the absence of a right-to-left cardiac shunt were confirmed with the injection of 10 ml of agitated isotonic saline solution through the subclavian catheter. During this examination, the airway pressure was kept at 25–30 cm H₂O. In addition, 5 cm H₂O positive end-expiratory pressure (PEEP) was applied to further exclude a right-to-left cardiac shunt. If a shunt was detected at this step, we kept the patient in the SSP unless there was a feasible alternative position. To maintain the homogeneity of the observed groups, however, these patients were excluded from the study.

We emphasize that these patients were assessed for a right-to-left cardiac shunt twice. The first time was by a cardiologist with transthoracic echocardiography, and the second time was with TEE examination with agitated isotonic saline solution and PEEP just before surgical positioning.

Positioning

After the induction of anesthesia and preparation and titration of the bolus dose of remifentanil (1–1.5 mg/kg), the Mayfield-Kees 3-pin head holder (Mayfield Clinic) was applied. While the surgeon held the patient’s head securely, the patient’s legs were flexed approximately 90° toward the torso with 90° knee flexion. The seat section of the operating table was kept parallel to the ground, and the back section was tilted upward, 30° for patients in Group 1 and 45° for those in Group 2 (Fig. 2). This degree of elevation was measured by a simple protractor. The head holder was fixed to the Mayfield crossbar adaptor while the patient was faced straight forward with his or her head flexed.

Lateral view of patients in the SSP. A: Group 1, 30° head elevation. The seat section (ss) of the operating table is parallel to the ground, and the back section (bs) is tilted upward. The accessory rail of the back section is used for the Mayfield crossbar adaptor (cba), which enables easy changes in the degree of head elevation without having to go under the drapes to disconnect the Mayfield system from the operating table during surgery. The patient is monitored with TEE. A custom-made leg holder (lh) is used for all patients. B: Group 2, 45° head elevation. The degree of head elevation is the only difference in positioning from that of patients in Group 1. bu = base unit; scl = skull clamp; swa = swivel adaptor; tee-m = TEE monitor; tee-p = TEE probe.

A 2-finger space is classically maintained between the sternal notch and the patient’s chin to prevent obstruction of venous outflow. At our institution, we prefer more flexion of the head, because it increases venous pressure slightly, which permits fewer VAEs. According to the patient’s physiological range of motion and airway pressure, his or her head was flexed as much as possible in a slow and cautious manner, especially if any restriction in range of motion was detected preoperatively. While positioning the patient’s neck, we monitored the airway pressure and changes from the baseline level, which indicated the change of degree in head flexion. This was done to prevent airway obstruction and potential kinking of the endotracheal tube. Furthermore, the anesthesiologist tested the patient’s neck position to see if it was suitable for jugular compression. Depending on the patient’s neck length, range of motion, age, and weight, the degree of the neck flexion varies. For this reason, we did not enroll obese or elderly patients or patients with degenerative disease of the cervical spine. Flexing the neck without causing any increase in the airway pressure was the best standardization possible for this study. Last, the operating table was elevated to the desired position.

An underestimated detail of the SSP is the fixing of the crossbar adaptor and the armrests to the back section of the operating table rather than the seat section. This step is crucial, because it easily allows the necessary changes in position from semisitting to supine without having to go under the drapes to disconnect the Mayfield system from the operating table adaptor in case of emergency.

A full-length viscoelastic foam mattress (Tempur-Pedic International, Inc.) was overlaid on the operating table to reduce the possibility of pressure sores during the lengthy surgery. Any direct contact between the patient and the surface of a surgical table or positioning device was avoided to prevent injury. All extremities were supported with gel pads. We did not use a warming blanket under the patient, because it might be a risk factor for pressure sores during such a lengthy operation.

Evaluation

There are several surgical stages during which the risk of venous air entrainment arises, including placement of the 3-pin head holder; incisions in the skin, subcutaneous tissue, or muscle; craniotomy; opening of the dura mater; resection of the lesion; and closure. All signs of air bubbles observed on the TEE screen during these stages were recorded. In addition, the density of bubbles detected on the TEE screen was classified as Grade 0, 1, 2, 3, or 4 (Table 1). If multiple VAE instances occurred, the highest graded attack for each patient was used for statistical analysis. Fluid and vasopressor requirements were recorded after each VAE.

TABLE 1.

Classification of VAE severity detected with TEE

Grade	Level	Characteristic(s)
0	None	No air bubbles
1	Mild	Air bubbles rarely seen on TEE screen; no hemodynamic or end-tidal carbon dioxide changes
2	Moderate	Air bubbles on TEE screen; decreasing level of end-tidal carbon dioxide
3	Severe	Air bubbles on TEE screen; hemodynamic changes such as increased heart rate & decreased arterial pressure
4	Life-threatening	Air bubbles on TEE screen; hemodynamic changes that necessitate cardiopulmonary resuscitation

Hemodynamics

With the patient supine, just before the 3-pin head holder was placed, several measurements were recorded as basal hemodynamics. If the heart rate and mean arterial blood pressure decreased more than 30% after positioning from supine to the SSP, the patient was given a bolus of fluid and a bolus of ephedrine HCL and/or intravenous glycopyrrolate, if required. For each group, we recorded the number of patients with hemodynamic changes that required treatment after moving them from supine to the SSP. Surgical and anesthetic complications during the perioperative period were recorded also.

Management of VAE

When air bubbles were detected on the TEE screen, the senior author (U.T.) controlled the site of venous air entrainment. Each VAE was managed successfully through efforts of the team. The anesthesiologist compressed the jugular vein while the neurosurgeon covered bony edges with bone wax, applied fibrin glue, and covered the surgical field with saline-soaked pads. The central venous catheter was prepared to remove air bubbles from the right atrium. If venous air entrainment continued even after these manipulations, we temporarily decreased the degree of head elevation.

Results

Two patients were excluded from the study because a patent foramen ovale was visible on the TEE screen. The cardiologist’s preoperative assessment with transthoracic echocardiography, including the Valsalva maneuver with an intravenous echo-contrast medium, did not identify the problem in these 2 patients. Thus, only 48 patients in Group 2 completed the study.

Demographic data such as sex, age, weight, ASA status score, and the type of tumor were similar between the groups (Table 2). We found no statistically significant differences between the groups but did find a trend in Group 2 toward more patients who required treatment for hemodynamic changes after positioning from supine to the SSP (0 [0%] of 50 vs 4 [8%] of 48, respectively; OR 0.0 [95% CI 0.0–1.8]; p = 0.05) (Table 2).

The rates and severity of VAE between groups are shown in Table 3. We found statistically significant differences between the groups for the total rate of VAEs detected on the TEE screen. The total numbers of VAEs were significantly lower in Group 1 than in Group 2 (11 [22%] of 50 vs 30 [62.5%] of 48, respectively; OR 0.16 [95% CI 0.06–0.41]; p = 0.0002). The rate of Grade 2 or higher (clinically important) VAE was significantly lower in Group 1 than in Group 2 also (4 [8%] of 50 vs 24 [50%] of 48, respectively; OR, 0.08 [95% CI 0.02–0.27]; p < 0.0001).

When we analyzed the associations between VAE and patient demographics and surgical pathology, we found no associations between the rate and severity of VAE with demographics (p > 0.05). However, we did find an association between the rate of VAE and the type of surgical pathology (p < 0.001). In Groups 1 and 2, VAE occurred more frequently in patients with a meningioma (p < 0.001 and p = 0.035, respectively), and their rates were 78% (7 of 9) and 91% (10 of 11), respectively. Therefore, the Mantel-Haenszel ln(estimate) common OR was found to be 2.7 (ln[estimate] 95% CI 1.2–4.2); p = 0.001). These high rates are possibly a result of tumor involvement and surgical manipulation of the venous sinuses and of tentorial and dural venous lakes.

In most patients, air entrainment was managed successfully as described. For 1 patient in Group 2, however, we could not stop the entrance of air until we discovered that the 3-pin head-holder fixation points were the source. Air entrainment stopped after these points were covered with fibrin glue and bone wax. The severity of air entrainment in this patient was Grade 2.

None of the patients required a blood transfusion during surgery. Two patients in Group 2, who experienced Grade 3 VAE during surgery, required a bolus of fluid and ephedrine HCL to treat VAE-related hypotension. All patients were extubated safely after surgery.

No tension pneumocephalus, hemorrhage, or major surgical or anesthetic complication was experienced in this series of patients. No additional postoperative cardiovascular complications were observed in the patients who experienced air entrainment, and none of the patients had position-related adverse effects such as peripheral nerve injury, decubitus ulcer, or skin lesions.

Discussion

Our study determined that the degree of head elevation of patients in the SSP is related directly to air entrance into the bloodstream. When a patient’s head was elevated 30°, the rate of air entrainment was lower than in one whose head was elevated 45°.

Neurosurgeons became aware of the advantages of the sitting position in 1917 after De Martel stated, “I believe in the necessity of operating on the skull with the patient in the sitting position.”^2,6,12 In the 1970s, the sitting position was resurrected and refined by pioneer neurosurgeons.^{23,28,36,37,40,41,44,47,48} With the modification of the position to semisitting, technical improvements, and the interest of neuroanesthesiologists to perfect the technique, many of the associated complications were eradicated, and others were decreased.^1,11,25,27 In addition, improvements in monitoring techniques for VAE and their use in routine clinical practice contributed to better outcomes. Despite numerous publications describing how to prevent complications and the vast experience accumulated during the golden years of the SSP, the newer generation of neurosurgeons and anesthesiologists could not be persuaded that its risks were comparable with those of other positions and, more importantly, that this position is necessary in certain cases.

The usual suspect in the decline of the SSP is VAE, and this frightening complication is addressed widely in the literature.^{1,2,10,11,16,20,24,27,32,34,35} Other possible complications are hemodynamic instability, pneumocephalus, airway edema, and compressive peripheral neuropathy.^33,40

Although the risk of VAE is lower with other positions than with SSP, there is still a risk of VAE with every surgical position. Abandoning use of the SSP because of the potential risk of VAE does an injustice to our patients. It is also a disservice to new generations of neurosurgeons who are less and less exposed to clean surgeries with patients in this position. In our practice, and perhaps in many others, the SSP is not the first choice, but according to several authors, use of the SSP is required for a successful outcome in certain patients.^{27,28,38,41,43,44,47,48} The pioneers of neurosurgery enjoyed the clean and relaxed surgical fields supplied by the SSP, and they dignified the role of this position through their impressive success with selected lesions,^{28,38,44,47,48} especially in patients with a large vestibular schwannoma.

The story of the SSP resembles that of the transcallosal and transsphenoidal approaches, which were abandoned because of high complication rates but were revitalized with the advent of new technologies and techniques. When Cushing first performed the transcallosal and transsphenoidal approaches, the surgical equipment and technology were not sufficient.⁹ Therefore, his results were disappointing and led to abandonment of the procedures. With the development of microneurosurgical techniques and equipment, and the use of bipolar coagulation, the transsphenoidal and transcallosal approaches have become indispensable.^17,47 For neurosurgeons who have unpleasant memories of the SSP, the choice now is to look for ways to make the SSP better instead of abandoning the approach and settling for less than patients deserve.

In most cases, the alternative to the SSP is the prone position, which has its own potential risks. Therefore, it is not accurate to say that neurosurgeons can substitute the SSP with a safer position. In their publications, neurosurgeons should also discuss the risks of the prone position as critically as they discuss those associated with the SSP. Nonetheless, a reasonable argument is the difficulty of defending in malpractice claims use of the SSP over the prone position.

The association of VAE with only the SSP leaves young neurosurgeons with a false impression of the safety of other surgical positions. Venous pressure in any surgical site that is lower than that in the right atrium is enough to cause a VAE through an open vein. Albin et al.² showed that a VAE can occur with as little as 5 cm of gravitational gradient between the venous entrance point and the right atrium. Using precordial Doppler ultrasonography, they detected VAE in 100 (25%) of 400 patients who underwent surgery in the sitting position, 5 (8.3%) of 60 patients in the lateral position, 7 (14.5%) of 48 patients in the supine position, and 1 (10%) of 10 patients in the monitored prone position.² In his letter to the editor in 1984, Cucchiara¹⁰ confirmed the safety of the SSP with a report of 3827 cases from the years 1966–1983 with no deaths in the operating room, although no TEE assessments for patent foramen ovale were performed. The author reported only 1 postoperative death related to VAE and 1 death associated with paradoxical air embolism. The only intraoperative death directly caused by VAE was that of a patient who had been placed in the prone position.¹⁰

There have been frequent reports of the rate of VAE during spinal procedures performed on patients in the prone position and in orthopedic surgeries.^30,39,42,46 Even perioperative blood-product transfusion has been shown to cause VAE.^4,5,26 Other predisposing factors include a patient’s high body mass index, poor preoperative condition, existing hypertension, atherosclerotic myocardial disease, cervical stenosis, and low central venous pressure. Risk factors related to surgery include the preoperative use of potent vasodilator agents, spontaneous ventilation during surgery, and inadequate hemostasis during surgery.^2,28,43 Factors related to the ingress of air from opened veins are patient age, ASA status score, body mass index, cervical stenosis, preoperative cardiac disease and shunts, perioperative anesthetic agents such as nitrous oxide, the use of muscle relaxants, the depth of ventilation, low intravascular volume, and poor surgical technique.^1,29,31,35

In our study, we used a custom-made leg holder and 1 pillow, and the level of leg elevation was almost the same for each patient within his or her physiological range (Fig. 2). However, leg length varies according to the length of the femur. In Group 1, the patients’ knees were almost level with their ears. With the head higher up for those in Group 2, however, the ears were higher than the knees. As a result, the rate and severity of VAE increased with no changes in hemodynamics. The degree of head elevation is one of the main factors for VAE from the surgical site when the patient’s legs are higher than his or her heart. Lower head elevation is associated with a lower rate and severity of VAE.

The main difference between our study and those described in previously published reports on the degree of head elevation and VAE is the higher sensitivity and objectivity of VAE-monitoring methods. We used TEE, which is considered the gold standard for detecting VAE. TEE can detect VAE that does not cause any hemodynamic changes. Using TEE, we recorded every incidence of VAE and graded the severity of every occurrence. VAE can be detected by using precordial Doppler ultrasonography and transesophageal echocardiography and by measuring end-tidal carbon dioxide levels. With such technological and pharmacological improvements, many institutions have started to report positive experiences with patients undergoing surgery in the SSP and a decreasing number of VAEs.^{14,18,20,27,34} However, these studies were not prospective and relied only on retrospective anesthetic charts. We designed this prospective study using TEE to compare the rates and severity of VAE and found a significant difference related to the degree of head elevation in the SSP. Because TEE is the most sensitive tool for detecting air bubbles within the circulation, we prefer this method for every patient placed in the SSP. Without the use of TEE, we would not be able to detect mild VAE, which cannot be detected through end-tidal carbon dioxide levels or hemodynamic changes. Nonetheless, the shadow of any infusion can be wrongly assumed to be VAE. Especially with cardiovascular anesthesia, the use of TEE is almost routine. Thus, TEE is becoming widely accepted as a standard monitoring tool for detecting air in the blood circulation of patients in the SSP.

TEE is also the gold standard for the preoperative assessment of patent foramen ovale. With TEE, we detected a right-to-left shunt in 2 patients in Group 2 when transthoracic echocardiography missed this pathology. As a consequence, these patients were excluded from our study.

The gravitational effect of low central venous and negative intravenous pressures relative to atmospheric pressure can trigger VAE.³⁵ PEEP can also be used for this purpose. As early as 1962, however, an author claimed that intermittent positive-pressure ventilation increased the risk of VAE,¹⁹ but subsequent investigations have not substantiated this claim.⁷ Some clinicians recently described their retrospective experience with a low VAE rate by using positive-pressure ventilation for patients in the SSP, but they did not mention hemodynamic changes.²⁰ Because numerous publications have stated that PEEP increases the paradoxical embolism rate, TEE should be used to monitor patients for VAE, especially if PEEP is used to prevent it. The patient’s leg elevation combined with more head flexion could be the main reason for the lower rate of clinically important air emboli, probably because of the decrease in the venous return to the right heart for patients in the SSP.

We found that, when a patient’s head was elevated 30°, we did not find a clinically important VAE. Another question is, which degree of head elevation is blameless? In this study, we tried to discern whether the rate and severity of VAE decrease when the head elevation is decreased to 30°. The degree of elevation, however, was determined by the surgeon (U.T.) according to the location of the patient’s lesion, the surgical route, and the comfort of the patient and surgeon. We found that a 45° elevation leads to a higher rate of VAE than does a 30° elevation. Furthermore, although none of the VAEs in either group was clinically life-threatening, only 2 patients in Group 2 had a VAE of Grade 3 (severe venous air entrainment that caused hemodynamic changes). These patients had hypotension and were treated with a bolus dose of fluid and ephedrine HCL until the VAE dissolved.

Hypotension is a critical issue for patients undergoing neurosurgery, because it can lead to cerebral ischemia and stroke. As some authors have noted, hypotension is more life-threatening than VAE during cranial surgery and, unfortunately, has been underestimated in studies.^1,2,35 Thus, the anesthesiologist should routinely maintain blood pressure during surgery, and hypotension should be treated by using fluids or vasoactive agents after positioning.²⁷ The definition of normotension during the craniotomy is crucial, but in our study, we defined the basal blood pressure as that recorded when the patient was supine after anesthesia induction and just before the head holder was placed. We treated patients for hypotension if their blood pressure decreased more than 30% from their basal blood pressure. Despite the statistical insignificance between the groups (p = 0.05) for the number of patients who required treatment for hemodynamic changes after head elevation from the supine position to the SSP, it is noteworthy that no patients in Group 1 needed treatment, whereas 4 (8.3%) patients in Group 2 required treatment for hypotension.

During surgery, a clinically important VAE can compromise hemodynamics and respiratory function. This severe complication was reported to occur in 3.3% of patients in the series of Ganslandt et al.¹⁴ and in 1.45% of patients in the series of Himes et al.¹⁸ However, these anesthetic chart analyses were retrospective.¹⁴ If investigators do not plan to record every detail ahead of time, it is too difficult to take notes during an emergency, and there is no section on the regular records for a detailed description, especially for when VAE occurs. In prospective studies, however, we can plan to describe every detail of changes in carbon dioxide levels, hemodynamics, or VAEs. In our prospective series, although the rates of Grade 1 VAE were similar in both groups, both groups were comparable in rates of clinically important VAE. The total rates of VAE and clinically important VAE were higher in patients with a head elevation of 45° (22.0% vs 62.5% and 8.0% vs 50%, respectively). The appropriate choice for elevation, of course, is determined by the physiological range of the patient, the surgeon, and the specifications of the surgical microscope. In all of our most recent cases, the details of which have been previously described,⁴³ we elevated each patient’s head 25°–30°.

The use of the SSP in neurosurgery involves teamwork among the neurosurgeon, neuroanesthesiologist, nurse, and surgical technician. All aspects of positioning should be planned in advance to assign specific tasks to the appropriate person, to check the accessories used during surgery beforehand, and to coordinate and cooperate with the leader of the group. Positions associated with major physiological changes should be accomplished in stepwise fashion to allow for a review of the hemodynamics and other parameters and to adjust the depth of anesthesia appropriately. We recommend that at least 1 neuroanesthesiologist and 1 anesthesia technician always be present in the operating theater during the entire surgery.

Use of the SSP decreased in neurosurgery in the late 20th century as complications were reportedly attributed to VAE, even though some authors emphasized that hemodynamic complications were the main problem and that the results of these complications are worse than those that stem from VAE. If the most feared problem of the SSP, in opposition to all the benefits, is VAE, then the following points must be taken into consideration: 1) With the current means of full monitoring and instant intervention, the undesired effects of VAE can be minimized and controlled. 2) VAE can be detected through continuous close follow-up and full monitoring of the patient, which includes TEE with end-tidal carbon dioxide monitoring. 3) Defining and closing fields of venous air entrainment in the surgical area with effective jugular compression can prevent the further admission of air. 4) A multiorifice central venous pressure catheter placed at the entrance to the right atrium can be used to aspirate air if necessary. 5) Morbidity can be prevented through instant intervention for complications, such as hemodynamic changes, for which the necessary preparations have been made already.

A surgical environment that has been prepared according to these principles will minimize morbidity and mortality rates, and the patient can benefit from the surgical procedure to the fullest extent possible.

Conclusions

The SSP remains a special concern for neurosurgeons and anesthesiologists. The use of new anesthetic agents and techniques and an increased understanding of neurophysiology are the basis for constant change in our specialties. Given the advantages of the SSP, instead of discarding this method of positioning patients, we can focus on the reasons that VAE occurs and work to prevent or minimize this phenomenon while developing advanced techniques to deter venous air entrainment. The results of our study show that the degree of head elevation is one of the most important factors in venous air entrainment from the surgical site. With these improvements in detecting and preventing VAE and other feared complications, it is time to revitalize the SSP. With a 30° head elevation and our standardized technique for patient positioning, the SSP can be used safely in neurosurgical practice.

Acknowledgments

We thank Julie Yamamoto for editing the text.

Disclosures

The authors report no conflict of interest concerning the materials or methods used in this study or the findings specified in this paper.

Author Contributions

Conception and design: U Türe, H Türe, Harput. Acquisition of data: U Türe, H Türe, Keskin, Köner. Analysis and interpretation of data: all authors. Drafting the article: U Türe, H Türe, Harput. Critically revising the article: U Türe, H Türe, Harput. Reviewed submitted version of manuscript: all authors. Approved the final version of the manuscript on behalf of all authors: U Türe. Statistical analysis: H Türe, Bekiroğlu. Administrative/technical/material support: U Türe, H Türe, Harput. Study supervision: U Türe, H Türe. Senior neurosurgeon: U Türe.