The editors and publisher would like to thank Drs. Jae-Woo Lee and Lydia Cassorla for contributing to this chapter in the previous edition of this work. It has served as the foundation for the current chapter.
Patient positioning in the operating room facilitates surgical procedures; however, positioning can be a source of patient injury and can alter intraoperative physiology. Positioning injuries during surgery remain a significant source of perioperative morbidity. Anesthesia providers share a critical responsibility for the proper positioning of patients in the operating room. This chapter will review general physiologic changes during positioning, general intraoperative positions, some specific positioning concerns, and intraoperative positioning related injuries.
Physiologic Aspects of Positioning
Physiologic responses play an essential role in blunting hemodynamic changes that would otherwise occur from positional changes in our day-to-day lives. Central, regional, and local mechanisms are involved. When a person reclines from an upright to a supine position, venous return to the heart increases and this increases preload, stroke volume, and cardiac output. These changes cause a brief increase in arterial blood pressure, which in turn activates afferent baroreceptors from the aorta (via the vagus nerve) and within the walls of the carotid sinuses (via the glossopharyngeal nerve) to decrease sympathetic outflow and to increase parasympathetic impulses to the sinoatrial node and myocardium. This parasympathetic outflow counters the increase in arterial blood pressure from increased preload and as a result systemic arterial blood pressure is maintained within a narrow range during postural changes in the nonanesthetized setting.
Central, regional, and local physiologic responses are important in maintaining hemodynamics when changing positions during a normal day-to-day life. During various types of anesthesia some of these responses can be blunted, which can change a patient’s hemodynamic responses to positional changes.
Pulmonary physiology is also altered by positional changes, which are further exaggerated during anesthesia. For example, when nonanesthetized people lie down, their functional residual capacity (FRC) decreases as a result of the diaphragm shifting upward. In anesthetized patients, the decrease in FRC is more dramatic, and often closing capacity exceeds FRC, leading to increases in ventilation-perfusion ( <SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='V˙/Q˙’>V˙/Q˙V˙/Q˙
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) mismatching and hypoxemia. Furthermore, positioning that limits diaphragmatic movement pushes on the chest wall or abdomen, causing intrapulmonary shunting from atelectasis.
Proper positioning requires the cooperation of anesthesia providers, surgeons, and nurses to ensure patient well-being and safety while permitting surgical exposure. Positioning also involves maintaining spine and extremity neutrality, proper padding, and securing the patient in order to prevent inadvertent changes in position. Patients often remain in the same position for long periods; therefore, prevention of positioning-related complications often requires compromise and judgment. During normal sleep we change positions, which prevents prolonged compression and excessive stretch. During anesthesia patients lose the ability to both sense injury and change position, increasing their risk for injury. Ideally, patients are placed in a surgical position that they can tolerate when awake. The duration of more extreme positions, when necessary, should be limited as much as possible. Tissues overlying all bony prominences, such as the heels and sacrum, must be padded to prevent soft tissue ischemia due to pressure. Maintaining neutrality of the patient’s spine and extremities prevents undue stretch.
The supine position, also called the dorsal decubitus position, is the most common position for surgery ( Fig. 19.1A ). In the classic supine position the head, neck, and spine all retain neutrality. One or both arms can be abducted or adducted alongside the patient. Arm abduction should be limited to less than 90 degrees in order to prevent brachial plexus injury from the head of the humerus pushing into the axilla.
Hands and forearms are either supinated or kept in a neutral position with the palm toward the body to reduce external pressure on the ulnar nerve ( Fig. 19.1B ). When the arms are adducted, they are usually held alongside the body with a “draw sheet” that passes under the body and over the arm and is then tucked directly under the torso (not the mattress) to ensure that the arm remains properly placed next to the body. The anesthesia provider should pad all bony prominences as well as stopcocks or intravenous lines that may exert pressure on the skin during the operation ( Fig. 19.1C ).
Variations of the Supine Position
Variations of the supine position are also frequently used such as the lawn-chair position, frog-leg position, and Trendelenburg positions. The lawn-chair position ( Fig. 19.1D ) flexes the hips and knees slightly, which reduces stress on the back, hips, and knees. This modified supine position is often better tolerated by patients who are awake or undergoing monitored anesthesia care. The legs are placed slightly above the level of the heart, which facilitates venous drainage from the lower extremities. Furthermore, the xiphoid to pubic distance is decreased, reducing tension on the abdominal musculature. Typically the back of the bed is raised, the legs below the knees are lowered to an equivalent angle, and a slight Trendelenburg tilt is used to level the hips with the shoulders.
The frog-leg position, in which the hips and knees are flexed and the hips are externally rotated with the soles of the feet facing each other, facilitates procedures to the perineum, medial thighs, genitalia, and rectum. The knees must be supported in order to minimize stress or dislocation of the hips.
Tilting a supine patient head-down with the pubic symphysis as the highest part of the trunk is called the Trendelenburg position ( Fig. 19.1E ). It is named after a 19th century German surgeon who first described its use for abdominal surgery. Walter Cannon, a Harvard physiologist, is credited with popularizing the use of Trendelenburg positioning to improve hemodynamics for patients in hypovolemic shock during World War I. Trendelenburg positioning is commonly used today to increase venous return during hypotension, improve exposure during abdominal and laparoscopic surgery, and prevent air emboli during central line placement.
The Trendelenburg position does produce hemodynamic and respiratory changes. Initially, placement of the patient head-down causes an autotransfusion from the legs with about a 9% from baseline increase in cardiac output in 1 minute. However, these changes are not sustained and within 10 minutes many hemodynamic variables, including cardiac output, return to baseline values. Nevertheless, Trendelenburg positioning is still part of the initial resuscitative efforts to treat hypovolemia. The abdominal contents are displaced toward the diaphragm, which decreases FRC and can also decrease pulmonary compliance necessitating higher airway pressures during mechanical ventilation. Intraocular pressure and intracranial pressure (ICP) can also increase. In patients with increased ICP and impaired cerebral autoregulation, Trendelenburg positioning should be avoided. For patients receiving general anesthesia who will be placed in the Trendelenburg position, endotracheal intubation is strongly recommended over supraglottic airways because of the risk of pulmonary aspiration of gastric contents. Prolonged head-down position can lead to swelling of the face, conjunctivae, larynx, and tongue with an increased potential for postoperative upper airway obstruction. An air leak should be verified around the endotracheal tube or the larynx visualized prior to extubation.
When placing a patient in the Trendelenburg position, measures should be taken to ensure the patient does not slide or shift. Nonsliding mattresses are recommended to prevent the patient from sliding cephalad. Caution should be exerted when shoulder braces are used because of considerable risk of compression or stretch injury to the brachial plexus.
Conversely, the reverse Trendelenburg position ( Fig. 19.1E ) tilts the supine patient upward so that the head is higher than any other part of the body. This position is most often used to facilitate upper abdominal surgery. Again, patients must be prevented from slipping on the table. Patients in reverse Trendelenburg, particularly those patients who are hypovolemic, are at risk for hypotension due to decreased venous return. Invasive arterial blood pressure monitoring should be calibrated (i.e., zeroed) at the level of the external auditory meatus in order to optimize cerebral perfusion.
Backache may occur in the supine position as the normal lumbar lordotic curvature is lost during general anesthesia with muscle relaxation or a neuraxial blockade. Consequently, patients with extensive kyphosis, scoliosis, or a previous history of back pain may require extra padding of the spine or slight flexion at the hip and knee.
With obese patients, caution is advised when placing them in reverse axis on the operating room table (also see Chapter 29 ). The base of the operating room table is asymmetric, with the torso usually over the foot of the table. However, patients are often positioned with the torso over the open side of the table to improve surgical access or to permit use of equipment such as C-arm x-ray devices. This places the heaviest portion of the body, and therefore the patient’s center of gravity, opposite the weighted foot of the table, with substantial leverage. The operating room table can tilt and tip over if sufficient weight is placed away from the base, particularly if extensions are used or the bed is tilted in the Trendelenburg position. Operating room table weight limits should be strictly observed; they differ substantially with regard to normal and reverse positioning.
Lithotomy position ( Fig. 19.2A to C ) is frequently used during gynecologic, rectal, and urologic surgeries. The legs are abducted 30 to 45 degrees from the midline, the knees are flexed, and legs are held by supports. The patient’s hips are flexed to varying degrees depending upon the type of lithotomy required for the procedure; standard, low, or high lithotomy. Legs should be raised and lowered simultaneously in order to prevent spine torsion. The lower extremities should be padded to prevent compression against the leg rests. The common peroneal nerve wraps around the head of the fibula on the lateral leg and is at significant risk of injury if insufficiently padded.
The foot section of the operating room table is lowered or taken away in order to facilitate the procedure. If the arms are on the operating table alongside the patient, the hands and fingers may lie near the open edge of the lowered section of the table. When the foot of the bed is raised again at the end of the procedure strict attention to the position of the hand must be paid to avoid a potentially disastrous crush injury to the fingers ( Fig. 19.2D ). For this reason, positioning the arms on armrests far from the table hinge point is recommended at all times when patients are in the lithotomy position.
The lithotomy position causes some physiologic changes. When the legs are elevated, preload increases, causing a transient increase in cardiac output. In addition, the lithotomy position causes the abdominal viscera to displace the diaphragm cephalad, reducing lung compliance and potentially resulting in a decreased tidal volume. Again, the normal lordotic curvature of the lumbar spine is lost in this position, potentially aggravating any previous lower back pain.
Lower extremity compartment syndrome is a rare but devastating complication associated with the lithotomy position. It occurs when perfusion to an extremity is inadequate because of either restricted arterial flow (from leg elevation) or obstructed venous outflow (direct limb compression or excessive hip flexion). This results in ischemia, edema, and rhabdomyolysis from increased tissue pressure within a fascial compartment. In a large retrospective review of 572,498 surgeries, the incidence of compartment syndromes was higher in the lithotomy (1 in 8720) and lateral decubitus (1 in 9711) positions as compared to the supine (1 in 92,441) position. Long surgical procedure time was the only distinguishing characteristic of the surgeries in which patients developed lower extremity compartment syndromes. In a retrospective multicenter review of 185 urologic patients who were placed in high lithotomy position, the overall complication rate due to positioning was 10%. Neurapraxia was the most common positioning-related complication (12 of 18 patients). Two patients from this cohort had compartment syndrome, and for both of these patients the time in high lithotomy exceeded 5 hours. Therefore, it is recommended to periodically lower the legs to the level of the body if surgery extends beyond several hours.
In the lateral decubitus position the patient lies on the nonoperative side in order to facilitate surgery in the thorax, retroperitoneum, or hip ( Fig. 19.3A ). The patient must be well secured to avoid falling or tilting forward to backward. Often a beanbag or bedding rolls are used. A kidney rest is sometimes also used to help secure the patient.
The extremities must be carefully positioned in order to prevent injury. The dependent leg should be somewhat flexed. A pillow or other padding is generally placed between the knees with the dependent leg flexed to minimize excessive pressure on bony prominences and stretch of lower extremity nerves. The dependent arm is placed in front of the patient on a padded arm board. The nondependent arm is often supported over folded bedding or suspended with an armrest or foam cradle ( Fig. 19.3B ). Neither arm should be abducted more than 90 degrees in order to prevent injury to the brachial plexus from the humeral head. Additionally, an axillary roll should be placed underneath the patient just caudal to the axilla, not placed in the axilla itself. The axillary roll prevents compression injury to the dependent brachial plexus and dependent axillary vascular structures ( Fig. 19.3C ). Sometimes an axillary roll is not used if an inflatable beanbag is being used for positioning; however, the team must ensure that there is no compression in the axilla. With invasive arterial monitoring consider placing the catheter in the dependent arm in order to detect positioning compression of the axillary neurovascular structures.
The patient’s head must be kept in a neutral position to prevent excessive lateral rotation of the neck and stretch injuries to the brachial plexus. This positioning may require additional head support ( Fig. 19.3B ). The dependent ear should be checked to avoid folding and undue pressure. The eyes should be securely taped before repositioning if the patient is asleep. The dependent eye must be checked frequently for external compression.
Lastly, the lateral decubitus position changes pulmonary function. In a patient who is mechanically ventilated, the combination of the lateral weight of the mediastinum and disproportionate cephalad pressure of abdominal contents on the dependent diaphragm decreases compliance of the dependent lung and favors ventilation of the nondependent lung. Simultaneously pulmonary blood flow to the dependent lung increases because of the effect of gravity. This causes ventilation-perfusion mismatching and can affect alveolar ventilation and gas exchange.
The prone or ventral decubitus position ( Fig. 19.4A ) is used primarily for surgical access to the posterior fossa of the skull, the posterior spine, the buttocks and perirectal area, and the lower extremities. When general anesthesia is required in the prone position, endotracheal intubation, intravenous access, Foley catheter, and invasive hemodynamic access should all be obtained in the supine position first while the patient is still on a gurney. Make sure all lines and tubes are very well secured to prevent dislodgement during turning and to prevent tube migration during the case.
Turning the patient from supine to prone requires coordination of all operating room providers. The anesthesia provider is primarily responsible for coordinating the move and for the repositioning of the head. An exception is in cases in which the head is placed in rigid pin fixation and the surgeon holds the pin frame. During the turn to prone, the head, neck, and spine are maintained in a neutral position. Some patients requiring prone positioning have unstable spines necessitating surgical operation. Also, strokes apparently can occur from presumed carotid and vertebral artery injury during turning. For some cases when neuromonitoring will be used for the surgical procedure “pre-flip,” baseline recordings are obtained prior to turning the patient prone for safety documentation.
In order to minimize risk of dislodgement, disconnect as many monitors and lines as is safe and possible before turning the patient from supine to the prone position. This is particularly helpful for lines and monitors on the side that rotates the furthest (the outside arm). Our practice is to disconnect the endotracheal tube during movement and to reconnect immediately upon prone positioning.
The position of the head is very important. In most cases, the head is maintained in a neutral position using a surgical pillow, horseshoe headrest, or Mayfield rigid head pins. Several commercially available pillows are specially designed for the prone position. Most, including disposable foam versions, support the forehead, malar regions, and the chin with a cutout for the eyes, nose, and mouth. The prone position is a risk factor for perioperative visual loss, which is discussed in a separate section later in this chapter. Mirror systems are available to facilitate checking face positioning ( Fig. 19.4B ). The anesthesia provider must ensure that the eyes and nose are free from pressure and document these findings at regular intervals throughout the case. Facial pressure wounds are a complication of prone positioning. The horseshoe headrest supports only the forehead and malar regions and allows excellent access to the airway ( Fig. 19.4C and D ). Rigid fixation pins support the head without any direct pressure on the face, allow access to the airway, and hold the head firmly in one position that can be finely adjusted for optimal neurosurgical exposure ( Fig. 19.5A ). Patient movement must be prevented when the head is held in rigid pins; slipping out of pins can result in scalp lacerations, skull fractures, and even cervical spine injury.
The legs should be padded and flexed slightly at the knees and hips. Both arms may be positioned to the patient’s sides, tucked in the neutral position as described for the supine patient, or placed next to the patient’s head on arm boards. Again, the arms should not be abducted greater than 90 degrees to prevent excessive stretching of the brachial plexus. Extra padding under the elbow will be needed to prevent compression of the ulnar nerve.
The abdomen should hang relatively freely for patients in the prone position. This alleviates external pressure on the abdomen, which can otherwise cause problems with ventilation and hypotension by compressing the inferior vena cava and reducing venous return. The thorax should be supported by firm rolls or bolsters placed along each side from the clavicle to the iliac crest. Multiple commercial rolls and bolsters are available including the Wilson frame ( Fig. 19.4A ), Jackson table, Relton frame, and the Mouradian/Simmons modification of the Relton frame. All devices and special operating room tables for the prone position serve to minimize abdominal compression. To prevent tissue injury, pendulous structures (e.g., male genitalia and female breasts) should be clear of compression; the breasts should be placed medial to the bolsters. The lower portion of each roll or bolster must be placed under its respective iliac crest to prevent pressure injury to the genitalia and the femoral vasculature.
Similar to the supine position, hemodynamics are well maintained, and pulmonary function is actually superior to the supine position. The FRC is actually improved compared to the supine positioning, leading to improved oxygenation. For obese patients, pulmonary compliance is improved in the prone position with the abdomen hanging freely (also see Chapter 29 ). The prone position has been utilized to improve respiratory function and mortality rate in patients with adult respiratory distress syndrome.
Proper prone positioning of patients relies on the table and headrest equipment. Horseshoe and rigid fixation pin headrests attach to adjustable articulating supports; any slippage or failure of this bracketing device may lead to complications if the head suddenly drops. Jackson tables can actually tilt or flip 180 degrees as a result of disengagement of the turning locking mechanisms.
In the sitting position ( Fig. 19.5B ) the patient’s head and also the operative field are located above the level of the heart. Sitting position can provide excellent surgical exposure for some cervical spine and neurosurgical procedures, particularly of the posterior fossa and superior cervical spine. Blood loss may also be reduced owing to decreased venous pressure in the operative field. A variation of the sitting position, the “beach chair” position, has been increasingly used for shoulder surgeries including arthroscopic procedures. This position offers access to the shoulder from both the anterior and posterior aspect, and potential for great mobility of the arm at the shoulder joint.
In the sitting position, the patient’s head must be adequately fixed. This can be done either with a head strap, tape, or rigid fixation. The arms should be supported and padded. The anesthesia provider should ensure that the shoulders are even or very mildly elevated in order to avoid stretch injury between the neck and shoulders. The knees are usually slightly flexed for balance and to reduce stretching of the sciatic nerve, and the feet are also supported and padded.
The most significant complication from the sitting position is risk of venous air embolism (VAE). During intracranial procedures, a significant amount of air can be entrained through the open dural venous sinuses. Low venous pressure in the operative field creates a gradient for air entry into the venous system, similar to the risk of venous air entry during central line placement. The important fear is the occurrence of a paradoxical air embolism. Patients undergoing planned surgery in the sitting position should be first evaluated to rule-out anatomic intracardiac shunts. If an intracardiac shunt is present, even small amounts of entrained venous air may result in a stroke or myocardial infarction. Transesophageal echocardiography (TEE) has shown some degree of venous air in most patients undergoing neurosurgery in the sitting position even as high as 100%. Clinically significant VAE has a much smaller incidence of 0.5% to 3%. Currently TEE is the gold standard for detection of intracardiac shunts. Even with screening, contrast echocardiography septal patency may not always be detected. A recent meta-analysis assessing accuracy of TEE for detection of intracardiac shunts compared to autopsy, cardiac catheterization, or surgery found a sensitivity of 89% and specificity of 91% for TEE. Other means of evaluating for intracardiac shunts include transthoracic echocardiography (TTE) and transcranial Doppler (TCD). Recent studies comparing TTE or TCD to TEE reveal sensitivities and specificities of 46% and 99% for TTE and 97% and 93% for TCD, respectively. Other complications of VAE include arrhythmias, acute pulmonary hypertension, and circulatory collapse. Preoperative diagnosis of an intracardiac shunt is a contraindication to surgery in the sitting position. With adequate intravascular volume, the use of intraoperative TEE or precordial Doppler ultrasound may aid in early detection of entrained air.
Patients are at risk for hypotension from pooling of blood in the lower body. The lower extremities are often wrapped in Ace bandages or compression stockings. Intravenous fluids and vasopressors are usually required in order to raise mean arterial pressure. Invasive arterial blood pressure monitoring is recommended for these cases and should be measured at the level of the external auditory meatus in order to optimize cerebral perfusion pressure. Central venous catheter (CVC) access is also recommended for these cases. Long-arm CVCs provide intravenous access without being near the surgical field. Multiorifice CVCs offer an advantage over conventional CVCs for improved aspiration of air should a VAE occur.
Pneumocephalus occurs in almost all patients undergoing cervical spine or posterior fossa surgery in the sitting position if diagnosed on postoperative imaging. Clinically significant pneumocephalus is more rare and occurs because of the lower pressure of cerebrospinal fluid in the sitting position. Symptomatic patients may experience headache, confusion, seizures, or even temporary hemiparesis. Patients experiencing any of these symptoms need to also be evaluated to rule out other postoperative complications, such as intravascular bleeding or stroke. Complications from head and neck positioning are also a risk of the sitting position. Excessive flexion of the cervical spine can impede cerebral venous outflow contributing to swelling and can also impede cerebral arterial inflow causing hypoperfusion of the brain. Macroglossia can also occur with excessive neck flexion. TEE monitoring combined with neck flexion can cause compression of laryngeal structures and the tongue. A minimum distance of two fingerbreadths between the mandible and the sternum is recommended for a normal-sized adult in order to prevent these complications. If preoperative examination reveals that the patient has a further decreased range of motion, then intraoperative positioning should not extend beyond the patient’s normal limitations.