Keywords

1 Physiological Changes in the Prone Position

1.1 Cardiovascular

1.1.1 Decreased Cardiac Index

When a patient is put into the prone position, an almost universal finding is a decrease in cardiac index (CI). In 16 patients [1] with cardiopulmonary disease, the most remarkable finding during surgery in the prone position was an average decrease of 24% in CI, which reflected a decrease in stroke volume with little change in heart rate.

Mean arterial pressure (MAP) was maintained by increased systemic vascular resistance (SVR), and pulmonary vascular resistance (PVR) also increased in most patients.

No changes were noted in mean right atrial or pulmonary artery pressures (PAP). Interestingly, these cardiac function alterations were only noted because cardiac output was measured. Normally, central venous and intra-arterial pressure measurements would not have identified this. The decrease in CI during prone position has also been confirmed elsewhere [2]. On the other hand, one study that analysed transoesophageal echocardiography in patients undergoing lumbar laminectomy [3] showed that although central venous pressure (CVP) increased slightly when patients were moved from supine to prone, CI did not change.

Nevertheless, it appears that the specific prone position used may influence these findings. A study performed in 21 patients undergoing lumbar surgery with direct PAP or IVC pressure monitoring [4] demonstrated that the flat prone position did not interfere with circulatory function but positioning with a convex saddle frame caused a decrease in CI and stroke volume index without significant increase in IVC pressure. It suggests that heart position at a hydrostatic level above the head and limbs may result in reduced venous return and consequently a reduced CI. A study [5] of four different surgical prone positions in 20 healthy non-anaesthetized volunteers (pillows under the thorax and pelvis with a free abdomen, position on an evacuatable mattress, position on a modified Relton–Hall frame or the knee–chest position) found no substantial changes in heart rate or MAP in any of the different positions, but CI decreased by 20% on the knee–chest position and by 17% on the modified Relton–Hall position. In the prone jack-knife position [2], head-down tilt caused CI to return to supine values and this was attributed to decompression of the IVC allowing an increased venous return to the heart.

It has been suggested that the decrease in CI may be due to elevated intra-thoracic pressures causing reduced arterial filling which leads to sympathetic activity increase via the baroceptor reflex. Consistent with this theory is the work which demonstrated that in prone patients decreased stroke volume is accompanied by an increased sympathetic activity (increased heart rate, total peripheral vascular resistance and plasma noradrenaline).

Recent studies suggest that the anaesthetic technique could affect haemodynamic variables in the prone position. One study [6] showed that when comparing total intra venous anaesthesia (TIVA) with inhalation anaesthesia by measuring MAP and heart rate in patients undergoing spinal surgery, a greater decrease in arterial pressure in the TIVA group was observed.

Another study [7] compared inhalation with intra venous maintenance anaesthesia by using non-invasive cardiac output measures in supine patients which were then pronated on a Montreal mattress. The authors found a decrease in CI and increase in SVR when the patients where pronated. The changes were greater during TIVA (decrease in CI of 25.9%) compared with inhalation anaesthesia (12.9%). Notwithstanding, such results may be explained by the change in propofol pharmacokinetics during prone position. Measured propofol concentrations have been observed to increase during target-controlled infusions when patients are transferred from supine position to prone, probably as a result of the decrease in cardiac output [8].

1.1.2 Inferior Vena Cava Obstruction

Obstruction of the IVC is likely to play a role in reducing cardiac output in at least some patients positioned prone. It is also clear that such obstruction contributes to increased blood loss during spinal surgery. Venous drainage obstruction forces blood to return to the heart by an alternative route (usually the vertebral column venous plexus of Batson). Given that these veins are thin-walled and contain little or no muscle tissue with few valves, any increase in pressure is transmitted and causes distension. This probably causes increased blood loss which adds to the difficulty in the surgical field, especially during lumbar spinal surgery.

The IVC obstruction issue is well recognized and various methods have been attempted to reduce blood loss such as the use of local anaesthetic infiltration, spinal and epidural anaesthesia and deliberate hypotension. In one study, IVC pressure was measured in six patients by comparing positions with and without a compressed abdomen. In all patients [9] abdominal compression resulted in a large increase in venous pressure by reaching more than 30 cm H2O in a particular patient. The position that appeared to cause the least compression (changes of up to 4 cm H2O) involved placing a large block under the chest and small sandbags under each anterior superior iliac crest. It was also noted that hypercarbia and any increase in pressure during expiration caused an increase in venous pressure.

When comparing IVC pressures in patients in the flat prone position, it was found that pressures were 1.5 times greater with respect to patients on the Relton–Hall frame, demonstrating the benefit of a support system that allows a free abdomen. This study also showed that induced hypotension had no significant effect on IVC pressure.

In summary, turning a patient into the prone position has significant effects on cardiovascular physiology, the most consistent of which being the reduction in CI. This has been variably attributed to a reduced venous return, to direct effects on arterial filling and reduced left ventricular compliance secondary to increased thoracic pressure. Other haemodynamic measures vary in a less predictable manner, although at least some patients demonstrate an increased sympathetic response to the change in position and the different anaesthetic techniques may influence the degree to which such changes occur. IVC obstruction is a well-recognized complication of prone positioning, and it is exacerbated by any degree of abdominal compression leading to decreased cardiac output, increased bleeding, venous stasis and consequent thrombotic complications. Careful positioning is therefore essential to minimize these risks.

1.2 Respiratory

1.2.1 Changes in Respiratory Physiology

The respiratory system is affected by an even more pronounced and clinically significant change in the prone position. Overall, functional residual capacity (FRC) decreases in comparison to the erect position. On the other hand, when compared to the supine position, FRC is seen to increase in the prone patient. Forced vital capacity (FVC) and forced expiratory volume in 1 s (FEV1) change minimally. In addition, pulmonary blood flow changes in the prone position.

It is common knowledge that pulmonary blood flow is 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 with respect 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. 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 may result in a decrease in FRC and lung compliance potentiating V/Q mismatch.

1.2.2 Lung Volumes

The most consistent finding is a relative increase in functional residual capacity (FRC) when a patient is moved from a supine to a prone position; forced vital capacity and forced expiratory volume in 1 s (FEV1) change very little [10].

Coonan and Hope [11] discussed, in detail, the cardio-respiratory effects of different body positions. The FRC of a patient going from upright and conscious to supine, anaesthetized and paralysed may decrease up to 44%, but considerably less (12%) when going from upright to prone. These findings were confirmed in a clinical context involving patients undergoing intervertebral disc surgery [12].

Measurements of FRC and arterial oxygen tension (PaO2) were taken in supine patients and after 20 min in prone position. A significant increase was found in the FRC and PaO2 [1.9 (SD 0.6) vs. 2.9 (0.7) L and 160 (37) vs. 199 [13] mmHg] when changing from supine to prone. The delivered tidal volumes and inspiratory flow rates were unchanged by the position as were the static compliances of the respiratory system (chest wall and lung). Although the resistance of the respiratory system was found to increase by 20% primarily as a result of changes in the viscoelastic properties of the chest wall, this did not seem to be of any clinical significance. Airway resistance was not altered by the change in position. The authors attributed the increase in FRC to the reduction of cephalad pressure on the diaphragm and the reopening of atelectatic segments.

The same study was repeated in obese patients (BMI >30 kg/m2) [13] using a similar methodology and positioning, whereby the authors observed an increase in lung volumes, lung compliance and oxygenation when patients were turned to the prone position, although, in obese subjects, the average FRC when supine was significantly lower than in the non-obese group [1.9 (0.6) L compared with 0.894 (0.327) L].

In summary, there are clear differences in respiratory physiology between supine and prone positions, including an increase in FRC and the distribution changes of both ventilation and perfusion throughout the lungs. It is thought that this leads to an improved ventilation/perfusion matching, which results in a better oxygenation in the surgical patient.

1.3 Complications Associated with the Prone Position

1.3.1 Injury to the Central Nervous System

Injury to the central nervous system represents a rare but potentially catastrophic complication of the prone position. These injuries can be classified according to the underlying mechanism—arterial occlusion, venous occlusion, air entrainment, cervical spine injury or the repercussion of undiagnosed space-occupying lesions.

1.3.2 Injury to the Peripheral Nervous System

Peripheral nerve injury may occur in patients under anaesthesia in any position and is thought to be the result of nerve ischaemia from undue stretching or direct pressure. In this context, prone positioning might lead to a different pattern or frequency of nerve injury when compared with supine positioning.

1.4 Pressure Injuries

A wide variety of injuries can occur in the prone position as a result of the pressure applied to dependent parts of the body. These injuries can be thought of as being the result of either direct pressure or indirect pressure (when injury occurs as a result of pressure on or occlusion of the vascular supply).

1.4.1 Direct Pressure Injuries

1.4.1.1 Pressure Necrosis of the Skin

Direct pressure is a common cause of anaesthesia-related injury that can occur in the prone position. Most authors advise to closely pay attention to the positioning of the face, ears, breasts, genitalia and other dependent areas to prevent pressure sores or skin necrosis. However, there are few reports on the subject, and such complications are usually mentioned as part of case series of other complications. The skin areas mainly affected include the malar regions, iliac crests, chin, eyelids, nose and tongue [14,15,16].

1.4.1.2 Contact Dermatitis

A patient developed contact dermatitis of the face [17] with periorbital and lip swelling after undergoing surgery with the head placed in a specific device (PronePositioner). A case of contact dermatitis in response to a Bispectral Indexw monitor placed on the forehead was thought to have been exacerbated by the prone position as a continuous pressure potentiated contact with the electrode conductive gel [18].

1.4.1.3 Tracheal Compression

There have been four reported cases of tracheal compression occurring during surgery in the prone position [19,20,21,22]. In all patients, this was associated with thoracic scoliosis and the proposed mechanism involved a reduced anterior–posterior diameter of the chest, which resulted in the compression of the trachea between the spine and the sternum. Tracheal compression appears to be a problem only in patients with underlying anatomical abnormalities and has not yet been reported in those with normal habitus.

1.4.1.4 Salivary Gland Swelling

Bilateral painful swelling of the submandibular glands after surgery in the prone position with lateral rotation of the head has also been reported. Although the aetiology is not clear, the authors concluded that it probably resulted from salivary ducts stretching, leading to stasis and acute swelling.

1.4.1.5 Shoulder Dislocation

The distribution of pressure in the prone position can also lead to anterior dislocation of the shoulder.

1.4.2 Indirect Pressure Injuries

1.4.2.1 Macroglossia and Oropharyngeal Swelling

Macroglossia is a well-documented complication of surgery in the sitting position and is thought to result from excessive flexion of the head and neck causing obstruction to venous drainage. Swelling of the tongue and oropharynx can constitute a real emergency for the patient undergoing surgery in the prone position.

1.4.2.2 Visceral Ischaemia

Avoiding compression on the abdominal organs is as important as avoiding abdominal compression to facilitate the surgical field. Hepatic ischaemia with progressive metabolic acidosis and elevated liver enzymes has been described after prolonged surgery in the prone position [23, 24] with subsequent resolution, and a case of hepatic infarction after 10 h of surgery in the prone position has also been reported.

1.4.2.3 Peripheral Vessel Occlusion

The prone position can cause compression and occlusion of several peripheral vessels.

1.4.2.4 Ophthalmic Injury

Postoperative visual loss (POVL) after non-ocular surgery in any position is relatively rare. There are a few mechanisms by which prone positioning may lead to ophthalmic injury. The most obvious is the result of direct external pressure by a headrest or other support on the orbital contents which causes an increased intraocular pressure leading to retinal ischaemia and visual loss. This has been named ‘Hollenhorst Syndrome’ and is usually linked with findings related to central retinal artery occlusion. Ironically, such injury has recently been described as a result of using a device designed to protect the eyes [25]. Specific devices designed to support the head during the prone position are fashioned to leave an open space for the eyes by distributing the weight of the head between the bony structures. Mirror systems placed on the operating table can be helpful with eye checks.

POVL can occur in the absence of external impingement on the eyeball, for example, when the head has been pinned and no headrest or other support has been placed near the eyes. This situation tends to be associated with findings of ischaemic optic neuropathy on examination [26] and may also be bilateral (over 40% of patients in one review) [27]. The final common pathway in ischaemic optic neuropathy is the inadequate oxygenation of the optic nerve causing ischaemic damage and failure of impulse transmission. Some individuals may be more susceptible to this as a result of anatomical variation in the arterial supply or abnormal vessel autoregulation [28]. In any patient, however, oxygenation of the optic nerve depends on adequate perfusion of its neurones. Perfusion pressure to the optic nerve can be defined as the difference between MAP and intraocular pressure or venous pressure, whichever is greater. Consequently, an increase in intraocular or venous pressure or a decrease in arterial pressure result in greater likelihood of developing optic nerve ischaemia.

2 … and The Most Correct Anaesthetic Approach?

2.1 Pre-assessment

Firstly, discuss with the surgeon the required position and the predicted duration of the procedure. Then fully pre-assess the patient, including physical examination and informed consent for anaesthesia. Evaluate the airway carefully—cervical spine surgery is an indication for prone positioning and limited head and neck movements are common in these patients, complicating airway management. Focus on peripheral neuropathy risk factors (diabetes, alcohol consumption, B12 deficiency) and document pre-existing nerve injuries and neuropathies. Check for signs of vertebrobasilar insufficiency. Consider the need for invasive monitoring and appropriate consent. Perform pre-operative investigations as needed.

2.2 Pre-induction

Standard monitoring should be set up when the patient is in the supine position and an appropriate venous access should be established. The anterior cubital fossae should be avoided given that flexion of the arms will occlude this route when the patient is positioned prone for surgery. Place ECG electrodes on the patient’s back where they will not interfere with surgery. Ensure that there is an adequate number of staff present to turn the patient after induction; they should be instructed on the technique using an awake volunteer for practice. The correct operating table should be in place and induction should take place on a separate moveable bed.

2.3 Induction

Induce anaesthesia appropriately and then secure the airway. A reinforced endotracheal tube (ETT) is often used. A laryngeal mask has been used in the prone position, but it is intuitively safer to fully secure the airway as intra-operative access is difficult. Secure the tube, preferably with tape and not a tube tie. The main reason for this is that when the patient is positioned prone the tie may become tighter and occlude venous drainage from the head and neck resulting in morbidity as discussed earlier.

Protect the eyes carefully. Initially, by covering with tape and then by placing protective extra padding over them and securing that in place with additional tape. Hard goggles have been designed to help protect the eyes in the prone position—if used, ensure that they are correctly placed, making sure there is no pressure on the ocular globes. Consider temperature monitoring—if continuous nasopharyngeal monitoring is needed, then insert the tube prior to taping the ETT as access to the nose and mouth may be difficult. Place arterial and central lines if required, but keep in mind that CVP interpretation may be difficult in the prone position. A urinary catheter is recommended in major procedures to aid in the assessment of circulation and fluid balance.

2.4 Positioning

As soon as the airway and all lines are secure, tell the theatre team members that you are ready for the prone position. Place the stretcher with the patient next to the operating table. Take control of the head and airway—as with all positioning, it is safest to disconnect the patient from the breathing circuit at this point. At least five other staff members (one of whom should be the surgeon) are required to safely turn the patient—two on each side and one controlling the legs and feet. The patient should be turned prone slowly and gently onto the operating table, while the anaesthetist coordinates the procedure. Care should be taken to avoid misplacement of intravenous lines and cannulas. Once positioning is complete, head and neck should be placed carefully preferably in a neutral position, on a soft head ring avoiding ocular pressure. Then perform a rapid but thorough assessment of the airway, breathing and circulation. It is not uncommon that the endo-tracheal tube gets misplaced into the right main bronchus as a result of increased neck flexion.

Arm positioning depends on surgery indication. In the Montreal mattress prone position, arms should be placed alongside the head on additional arm support. When moving the arms, do so one at a time and not simultaneously, such that greater ROM is allowed at the shoulder joint level (as per butterfly versus freestyle swimming strokes). To avoid brachial plexus stretching, ensure that the axillae are not under tension.

Perform a full head-to-toe assessment of the patient to verify that every potential pressure point is protected by padded material. When dealing with a Montreal mattress, assure that the abdomen is correctly placed. Next, perform a secondary assessment of airway, breathing and circulation prior to the commencement of surgery.

2.5 Intra-Operative Management

The same principles of intra-operative management of any anaesthetic technique also apply to prone positioning. The main difference lies in the fact that if a problem that requires returning to the supine position arises, there may be some delay before this can be done safely. As with all anaesthetic procedures, extremely careful preparation and a double-check prior to surgery initiation is crucial to prevent problems or potential adverse events related to position.

2.6 Emergence from Anaesthesia

Maintain adequate anaesthesia until the patient is repositioned in the supine position on the stretcher, given that it is harder to safely reposition a coughing or non-compliant patient. Anaesthetic emergence follows the same principles as any other anaesthetic process including post-operative examination.