Anesthesiology

Patient Positioning During Anesthesia: Prone Position

Patient transfer

Prior to achieving any surgical position, the patient must be transferred onto the operating room table. The final position of the patient is of the utmost importance, but achieving these positions requires careful planning and coordination by the operating room team. The overall plan for each patient transfer should be discussed prior to any movement.

Frequently, the patient can assist in positioning prior to induction of anesthesia. However, under general anesthesia, the operating room team must carefully move and position each patient. Pertinent patient comorbidities should be reviewed. For example, patients with morbid obesity or unstable spine fractures will require additional staff for transfer and positioning. When the patient is moved after the induction of general anesthesia, the anesthesiologist must be aware of any blood pressure alterations and ensure a safe systemic blood pressure prior to any patient movement.

All monitors, intravenous lines, and the endotracheal tube need to be carefully managed when moving a patient. The eyes should be taped to avoid corneal abrasion. With excellent communication, patients can be safely and successfully transferred within the operating room.

Prone position

The prone or ventral decubitus position allows access to the posterior cranial fossa, spine, buttocks, rectum, and posterior lower extremities. The evolution of the prone position has provided improvements to allow for excellent surgical exposure and mitigate risk to the patient. Understanding of the physiologic changes associated with prone positioning as well as common pitfalls will allow for the safe maintenance of anesthesia for various operations.

Typically, anesthesia is induced on a stretcher next to the operating room table within reach of all anesthesia equipment. After the trachea is intubated and any invasive monitors are placed, the patient is placed in the prone position. This is safely performed as a coordinated effort of the anesthesia, surgical, and nursing personnel in the operating room.

Once in the prone position, all monitors must be re-established as well as confirming continued adequate ventilation. The head is rested on a foam pillow with cutouts for the eyes, nose and mouth, or held using the Mayfield headrest and pinning the head.

Various positioning devices or tables, described in detail in the sections to follow, are used to balance the weight of the patient on their thoracic cage and bony pelvis. The arms are either positioned and secured at the patient’s side or placed on arm boards with the arms abducted to less than 90 degrees at the shoulder and flexed at the elbow, often called the "prone superman" position.

However, some reports indicate that abduction of the arm to greater than 90 degrees might be better tolerated in the prone position with less resultant injuries to the brachial plexus. This should only be reserved for cases that demand extreme abduction rather than for routine practice.

All bony prominences must be examined and padded.

The patient’s genitalia must not be compressed and the breasts should be placed medial to the thoracic bolsters.

A thorough inspection must be performed prior to applying the surgical drapes. Proper positioning of the head and neck must be frequently checked while in the operating room, with careful attention to the eyes.

At the conclusion of the operation, the patient must be safely returned to a stretcher in the supine position prior to extubation.

What common procedures are performed in this position?

The prone position allows access to the dorsal surface of the body. Common procedures include posterior cranial fossa surgery, posterior spine surgery at all levels, buttock and perirectal procedures, and operations on the posterior components of the lower extremity. Additionally, endoscopic retrograde cholangiopancreatography (ERCP) procedures are often performed in a modified prone position.

What are the common variations of this position?

Various tables and modified positions have been established to hold the patient in the prone position. The common theme is to avoid abdominal compression. Increased abdominal pressure is transmitted to the venous system causing IVC obstruction. Venous blood is forced to return to the heart via alternative routes including the vertebral column venous plexus of Batson. Distention of the valveless epidural veins increases intraoperative blood loss in spinal surgery, limiting visualization in the operative field.

Common operating room tables and positioning devices include the Wilson frame, Jackson table, Relton frame, Mouradian/Simmons modification of the Relton frame, and gel bolsters. Variations in prone positioning include the prone jackknife position for rectal procedures.

What are the physiologic changes when placing a patient in this position?

Cardiovascular physiology

The prone position is associated with several alterations in cardiovascular and respiratory physiology. When positioning a patient prone, there is a predictable decrease in cardiac index of up to 20%. This decrease is caused by a reduction in stroke volume secondary to decreased venous return. Heart rate is minimally changed.

However, mean arterial pressure usually remains unchanged secondary to an increase in systemic vascular resistance (SVR). Thus, the decrease in cardiac index would only be noted if cardiac output were specifically being monitored. Arterial pressure, whether invasive or noninvasive, should not change significantly when placing a patient in the prone position.

Respiratory physiology

The respiratory system has a more pronounced and clinically significant change in the prone position. Overall, functional residual capacity (FRC) decreases compared to the erect position. However, compared to the supine patient, FRC increases in the prone position. Forced vital capacity (FVC) and forced expiratory volume in 1 second (FEV1) change minimally. In addition, pulmonary blood flow changes in the prone position.

Classic teaching described pulmonary blood flow to be gravity dependent. In the prone position, perfusion of the dependent lung would be increased compared to the nondependent lung. However, recent work has found that blood flow is distributed more uniformly throughout the lung in the prone position compared to the supine position. As with pulmonary perfusion, lung ventilation is probably less dependent on gravitational forces than was once thought.

Recent work emphasizes that the architecture of the airway has a greater impact than gravity on the distribution of ventilation. Overall, this leads to improved matching of ventilation and perfusion, allowing for better oxygenation when properly placed in the prone position.

If not positioned correctly, excess abdominal compression could cause cephalad displacement of the diaphragm and encroach the lung. This could lead to a decrease in FRC and lung compliance leading to V/Q mismatch.

What are the options for anesthetic management?

Procedures in the prone position are often performed under general anesthesia with intubation of the trachea. Certain procedures, in acceptable patients, can be accomplished using MAC anesthesia. Examples include ERCP and perirectal procedures. There are reports of the successful use of LMAs in the prone position.

What are the complications associated with this position?

Perioperative visual loss (POVL)

POVL is a devastating, but extremely rare event occurring in 0.0008% for all anesthetics. Spine and cardiac surgery have the highest reported frequency of POVL. In fact, the American Society of Anesthesiologists (ASA) POVL registry implicated spine surgery in the prone position in 67% of cases of documented POVL. In patients undergoing cardiac surgery with cardiopulmonary bypass, the incidence appears to be 0.06% to 0.113% and for patients undergoing spine surgery in the prone position, the incidence is 0.09%. Two main patterns of injury are seen in the prone position: ischemic optic neuropathy (ION) and central retinal artery occlusion (CRAO) .A detailed discussion of POVL is beyond the scope of this chapter. The following will detail patient positioning related effects of visual loss.

CRAO occurs due to improper positioning and direct external pressure on the eye, increasing intraocular pressure and leading to retinal ischemia. More often than not, CRAO is a preventable injury that takes diligence and frequent eye checks throughout the procedure. Following CRAO, overall prognosis is poor with limited treatment options available. For this reason, prevention must be the focus of the anesthesiologist with frequent eye checks and careful positioning of the head.

ION has occurred following instrumented spine surgery in the prone position. The specific etiology and risk factors of ION remains unclear. Certain factors occur more frequently, including long surgery times, extensive blood loss, large crystalloid resuscitation, and hypotension. However, ION has occurred in the absence of each of these findings or in combination. The prone position specifically does not appear to be an independent risk factor for ION.

In high-risk patients, those undergoing extensive or revision spinal fusions, exact preventative measures have not been firmly established. Maintaining elevated mean arterial pressure, lowering the transfusion trigger, or altering intraocular pressure has not been proven in the literature to affect outcomes. Current recommendations for intraoperative management include consideration of colloids when needed for large volume resuscitation, avoiding the head down position to maximize venous drainage, and consideration of staging for complex spine procedures.

Nerve injury

Peripheral nerve injury can be seen after any position under anesthesia, but the pattern of injury varies when the patient is in the prone position.

Using somatosensory evoked potential (SSEP) changes as a surrogate for peripheral nerve injuries, prone “superman” position with arms abducted, elbows flexed with hands by the head had a significantly greater reversible change compared to when the arms were tucked by the patient’s side. Some authors recommend placing the patient’s arms at their side whenever possible.

Every attempt should be made to maintain the arms abducted to no greater than 90 degrees to decrease risk of nerve injury, specifically to the brachial plexus. Ulnar neuropathy can occur, but prone positioning has not been found to be an independent risk factor. Prone position does increase the risk of injury to the lateral cutaneous nerve of the thigh.

Other injuries

Although other extremely rare injuries occur while in the prone position, it is worth noting several of these to raise awareness to those providing anesthesia in the prone position. It has been previously mentioned that maintaining the head and neck in a neutral position is essential to limit both spinal cord and brachial plexus injuries.

Additionally, there have been a limited number of reports of carotid and vertebral artery injuries due to excess rotation, flexion, or extension of the neck. This malposition of the neck could also impede venous outflow from the head, not only increasing the risk of POVL as previously described, but also causing macroglossia via impingement of venous drainage from the tongue possibly complicating extubation.

Additionally, direct pressure on external parts of the patient including the malar regions, chin, breast, iliac crest, and genitalia could lead to localized skin ischemia and necrosis.

Less recognized is the potential for abdominal compression on specific solid organs. Although rare, there have been several case reports of hepatic ischemia attributed to direct compression.

As stated previously, one should limit abdominal compression to improve hemodynamics, but also to avoid potentially devastating and rare postoperative complications.

What strategies can be used to decrease the risk of injury in this position?

As with other procedures, proper positioning of the head and neck in the prone position must be performed and continuously checked throughout the case to prevent spinal cord and brachial plexus nerve injuries.

Every attempt should be made to avoid excessive flexion, extension, or rotation of the neck that is not deemed necessary for adequate surgical exposure.

CRAO related to patient positioning in the prone position appears to be a preventable injury through careful and repeated eye checks. Although the exact time between eye checks to prevent CRAO has not been established, studies in animals suggest the eyes should be checked at least every 20 minutes. Specific devices designed to support the head during prone position are fashioned to leave a window open for the eyes, distributing the weight of the head on the bony structures.

Mirror systems placed on the operating room table can assist with eye checks.

When the patient is placed in the Mayfield headrest with their head pinned, there is no direct pressure on the eyes, but one must be cognizant of any surgical or anesthesia equipment that might compromise the patient’s eyes.

Simply taping the eyes prior to placing the patient prone appears to be better than using eye protecting devices such as goggles. These devices might shift during the procedure and inadvertently compress the eyes.

Using the horseshoe headrest in the prone position is not advised due to inadvertent movement of the head causing compression of either eye.

Although the incidence of ION is increased in patients undergoing spine surgery in the prone position, it appears that the prone position alone does not increase the risk of this form of POVL. Rather, ION is a combination of factors leading to a devastating complication. In an effort to decrease the risk of ION when placing a patient prone, the head down position should be avoided. In this position, venous drainage from the head may be compromised, leading to venous engorgement and possibly decreased perfusion of the optic nerve. A neutral or slightly head up position will allow for better venous drainage of the head.

To avoid other pressure related injuries, all potential locations of injury must be checked during positioning. This includes the eyes, but should also include all bony prominences, breasts, genitalia, arms, and abdomen. Avoidance of abdominal compression limits IVC obstruction and also the rare complication of intra-abdominal solid organ ischemia.

What's the Evidence?

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(Book chapter on patient positioning in the operating room.)

Cheney, FW, Domino, KB, Caplan, RA, Posner, KL. "Nerve Injury Associated with Anesthesia". Anesthesiology. vol. 90. 1999. pp. 1062-69.

(Closed claims database evaluation of anesthesia-related nerve injury.)

Coonan, TJ, Hope, CE. "Cardio-respiratory effects of change of body position". Can Anaesth Soc J. vol. 30. 1983. pp. 424-37.

(Basic physiology of many common surgical positions.)

Dunn, PF. "Physiology of the lateral decubitus position and one-lung ventilation". Int Anesthesiol Clin. vol. 38. 2000. pp. 25-53.

(A detailed description of ventilation and perfusion mismatch in the lateral decubitus position.)

Edgecombe, H, Carter, K, Yarrow, S. "Anaesthesia in the prone position". Br J Anaesth. vol. 100. 2008. pp. 165-83.

(Comprehensive and in depth review of the prone position.)

Gale, T, Leslie, K. "Anaesthesia for neurosurgery in the sitting position". J Clin Neurosci. vol. 11. 2004. pp. 693-6.

(Review of the sitting position and discussion of venous air embolism.)

Higuchi, H, Takagi, S, Zhang, K, Furui, I, Ozaki, M. "Effect of lateral tilt angle on the volume of the abdominal aorta and inferior vena cava in pregnant and nonpregnant women determined by magnetic resonance imaging". Anesthesiology. vol. 122. 2015. pp. 286-93.

(Discussion of aortocaval compression in the parturient and the effects of various degrees of left lateral tilt in the supine position.)

Knight, DJW, Mahajan, RP. "Patient position in anaesthesia. Continuing Education in Anaesthesia". Critical Care & Pain. vol. 4. 2004. pp. 160-3.

(Brief overview of patient position during anesthesia.)

Koh, JL, Levin, SD, Chehab, EL, Murphy, GS. "Neer Award 2012: Cerebral oxygenation in the beach chair position: a prospective study on the effect of general anesthesia compared with regional anesthesia and sedation". J Shoulder Elbow Surg. vol. 22. 2013. pp. 1325-31.

(A prospective study suggesting the possible benefits of avoidance of general anesthesia in the BCP.)

Lee, JR. "Anesthetic considerations for robotic surgery". Korean J Anesthesiol. vol. 66. 2014. pp. 3-11.

(An update and review of robotic surgery including a detailed discussion on the anesthetic implications of many common robotic surgeries.)

Lohser, J. "Evidence-based management of one-lung ventilation". Anesthesiol Clin. vol. 26. 2008. pp. 241-72.

(Ventilation and perfusion in the lateral decubitus position with further discussion of one-lung ventilation management.)

Murphy, GS, Szokol, JW. "Blood pressure management during beach chair position shoulder surgery: what do we know?". Can J Anesth. vol. 58. 2011. pp. 977-82.

(Brief discussion of the beach chair position and intraoperative blood pressure management.)

Picton, P, Dering, A, Alexander, A, Neff, M, Miller, BS, Shanks, A, Housey, M, Mashour, GA. "Influence of ventilation strategies and anesthetic techniques on regional cerebral oximetry in the beach chair position". Anesthesiology. vol. 123. 2015. pp. 765-74.

(Prospective study showing that increasing the inspired oxygen fraction and end-tidal carbon dioxide during general anesthesia increases regional cerebral oxygenation in the BCP.)

Prielipp, RC, Morell, RC, Butterworth, J. "Ulnar nerve injury and perioperative arm positioning". Anesthesiol Clin NA. vol. 20. 2002. pp. 589-603.

(Review of perioperative ulnar neuropathy including anatomy, risk factors, and legal implications.)

Rains, DD, Rooke, GA, Wahl, CJ. "Pathomechanisms and complications related to patient positioning and anesthesia during shoulder arthroscopy". Arthroscopy. vol. 27. 2011. pp. 532-41.

(Discussion of anesthetic options and positioning during shoulder arthroscopy.)

Roth, S. "Perioperative visual loss: what do we know, what can we do?". Br J Anaesth. vol. 103. 2009. pp. i31-i40.

(Review of perioperative visual loss including updates on risks factors and preventative recommendations.)

Washington, SJ, Smurthwaite, GJ. "Positioning the surgical patient". Anaesth Intens Care. vol. 10. 2009. pp. 476-9.

(Brief overview of patient position during anesthesia.)

Winfree, CJ, Kline, DG. "Intraoperative positioning nerve injuries". Surg Neurol. vol. 63. 2005. pp. 5-18.

(Comprehensive review of position related nerve injuries.)
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