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Anaesthesia in the prone position

2008; Elsevier BV; Volume: 100; Issue: 2 Linguagem: Inglês

10.1093/bja/aem380

1471-6771

H. Edgcombe, K. Carter, S. Yarrow,

Spine and Intervertebral Disc Pathology

Prone positioning of patients during anaesthesia is required to provide operative access for a wide variety of surgical procedures. It is associated with predictable changes in physiology but also with a number of complications, and safe use of the prone position requires an understanding of both issues. We have reviewed the development of the prone position and its variants and the physiological changes which occur on prone positioning. The complications associated with this position and the published techniques for various practical procedures in this position will be discussed. The aim of this review is to identify the risks associated with prone positioning and how these risks may be anticipated and minimized. Prone positioning of patients during anaesthesia is required to provide operative access for a wide variety of surgical procedures. It is associated with predictable changes in physiology but also with a number of complications, and safe use of the prone position requires an understanding of both issues. We have reviewed the development of the prone position and its variants and the physiological changes which occur on prone positioning. The complications associated with this position and the published techniques for various practical procedures in this position will be discussed. The aim of this review is to identify the risks associated with prone positioning and how these risks may be anticipated and minimized. The prone position has been described, used, and developed as a result of the requirement for surgical access. However, pioneers of spinal surgery in the 1930s and 1940s were hampered because no effort was made to avoid abdominal compression when positioning the patient, somewhat surprisingly given that the valveless nature of the venous system was well understood at the time. Increased intra-abdominal pressure forced blood from the inferior vena cava (IVC) into the extradural venous plexus, resulting in increased bleeding and a poor surgical field. The position adopted enhanced the natural anterior curvature of the lumbar spine, making surgical access even more difficult. In addition, the aorta, vena cava, and small bowel were forced against the lumbar spine where they were at risk of injury during surgery. Surgical access was also hindered by the limitations of contemporary anaesthetic techniques—most operations were performed with the patient breathing spontaneously, and increased muscle tone served to increase bleeding and impair the surgical field even more. Local anaesthesia was only partially successful, having a limited effect on inflamed spinal nerve roots. In 1949, Ecker47Ecker A Kneeling position for operations on the lumbar spine.Surgery. 1949; 25: 112PubMed Google Scholar provided the first description of a new position which attempted to overcome some of the adverse effects of increased intra-abdominal pressure in the prone position. Since then, many positions and devices have been described to refine this, all under the blanket term ‘prone position’, but with subtle differences (Table 1) and varying advantages and disadvantages.Table 1Historical variations on the prone position. *Modern versions include the Wilson Frame, whose two curved full-length pads are adjustable laterally to optimize positioning, and the Cloward Surgical Saddle (US Patent No. 4398707). †The ‘Buie’ position175Smith R The prone position.in: Martin JT Positioning in Anesthesia and Surgery. WB Saunders, Philadelphia1978Google Scholar is similar to the Georgia prone, but involves a head-down tilt and is useful for ano-rectal surgery. A hypobaric spinal block is possible. ‡The Ray Frame155Ray CD New kneeling attachment and cushioned face rest for spinal surgery.Neurosurgery. 1987; 20: 266-269Crossref PubMed Google Scholar assumes a similar position but with more even weight distribution on the knees, and the arms adducted across the chest. ¶The Seated Prone position as described by Tarlov190Tarlov IM The knee–chest position for lower spinal operations.J Bone Joint Surg Am. 1967; 49: 1193-1194Crossref PubMed Google Scholar is also referred to as the ‘Knee–Chest’ position. A similar position is adopted using the Andrews Table and the Codman Frame. §Other authors describing similar positions to Tarlov include Hastings,69Hastings DE A simple frame for operations on the lumbar spine.Can J Surg. 1969; 12: 251-253PubMed Google Scholar Laurin and colleagues,101Laurin CA Migneault G Brunet JL Roy P Knee–chest support for lumbosacral operations.Can J Surg. 1969; 12: 245-250PubMed Google Scholar and Dinmore.42Dinmore P A new operating position for posterior spinal surgery.Anaesthesia. 1977; 32: 377-380Crossref PubMed Scopus (4) Google Scholar ||The Hastings Frame is also known as the ‘Toronto’ or ‘Canadian’ Frame in some publications. Similar devices include the Heffington Frame.186Sutterlin C Rechtine GR Using the Heffington frame in elective lumbar spinal surgery.Orthop Rev. 1988; 17: 597-600PubMed Google Scholar #The ‘Concorde’ position94Kobayashi S Sugita K Tanaka Y Kyoshima K Infratentorial approach to the pineal region in the prone position: concorde position. Technical note.J Neurosurg. 1983; 58: 141-143Crossref PubMed Google Scholar is identical except that the head is flexed on the neck and tilted to the right. This allows good access to the pineal region without the disadvantages of the sitting positionVariationDescriptionAdvantagesDisadvantages‘Kneeling’ prone47Ecker A Kneeling position for operations on the lumbar spine.Surgery. 1949; 25: 112PubMed Google ScholarThighs and knees flexed and splayedReduced intra-abdominal pressureRisk of crush injury to thigh and calf musclesChest supported in slingFlexion of spineArms crossed in front of faceReduced tension on nerve roots‘Mohammedan Prayer’111Lipton S Anaesthesia in the surgery of retropulsed vertebral discs.Anaesthesia. 1950; 5: 208-212Crossref PubMed Google ScholarAs for ‘Kneeling’, but: chest supported with pillowsReduced intra-abdominal pressureRisk of crush injury as aboveArms abducted at shoulder above headFlexion of spineIncreased tension in paraspinal musclesHead-up tiltManual handling issuesMoore and Edmunds Frame129Moore DC Edmunds LH Prone position frame.Surgery. 1950; 27: 276-279PubMed Google ScholarPatient suspended between longitudinal bars of curved frameReduced intra-abdominal pressurePressure injuryPortableCheapOverholt position49Etsten BE Respiratory acidosis during intrathoracic surgery; the Overholt prone position.J Thorac Surg. 1953; 25: 286-299PubMed Google ScholarCrest of ilium supported by edge of table. Adjustable pad under upper third of sternum. Head in headrestFree chest expansion allows spontaneous respiration during thoracic surgeryMackay Frame117Mackay I A new frame for the positioning of patients for surgery of the back.Can Anaesth Soc J. 1956; 3: 279-282Crossref Google ScholarTwo longitudinal curved bolstersAdjustable curvature for any degree of flexion/extensionPatient supported on iliac crests and mid-claviclesCheap and robustArms extended above head*Reduced intra-abdominal pressure‘Prone Jack Knife’191Taylor AR Gleadhill CA Bilsland WL Murray PF Posture and anaesthesia for spinal operations with special reference to intervertebral disc surgery.Br J Anaesth. 1956; 28: 213-219Abstract Full Text PDF PubMed Scopus (0) Google ScholarPelvis supported (weight borne on anterior pelvis and sternum)Much reduced intra-abdominal pressureDoes not undo lumbar lordosis (poor surgical access to spine)Knees semi-flexedRisk of femoral vessel injuryThighs flexed to 45°Arms abducted at shoulders‘Georgia Prone’†,‡ 177Smith GZ RH Volpitto PP Problems related to the prone position for surgical operations.Anesthesiology. 1961; 22: 189-193Crossref Google ScholarKneeling on shelf, hips flexed at 90°; weight borne on kneesPatient comfort (derived from experiments on awake subjects)Manual handling issuesPelvis supported under iliac crestsGood surgical access (thigh flexion flattens lumbar spine)All weight borne by kneesPillows under chestIncreased CVPArms abducted above headTight paraspinal muscles can limit lateral surgical accessHead turned to sideRelton and Hall Frame157Relton JE Hall JE An operation frame for spinal fusion. A new apparatus designed to reduce haemorrhage during operation.J Bone Joint Surg Br. 1967; 49: 327-332Crossref PubMed Google ScholarFour individually adjustable supports in two V-shaped pairs tilting inwards at 45°Reduced intra-abdominal pressureIncreases lumbar lordosis (unsuitable for disc surgery)Supports lateral thoracic cage and antero-lateral pelvisTends to correct scoliosisAdjustable for any body habitus and degree of scoliosisVery stableCan be modified to allow skeletal traction131Mouradian WH Simmons EH A frame for spinal surgery to reduce intra-abdominal pressure while continuous traction is applied.J Bone Joint Surg Am. 1977; 59: 1098-1099Crossref PubMed Google Scholar‘Seated Prone’¶,§ 190Tarlov IM The knee–chest position for lower spinal operations.J Bone Joint Surg Am. 1967; 49: 1193-1194Crossref PubMed Google ScholarWeight supported on ischial tuberositiesVery low venous pressures32Cook AW Siddiqi TS Nidzgorski F Clarke HA Sitting prone position for the posterior surgical approach to the spine and posterior fossa.Neurosurgery. 1982; 10: 232-235PubMed Google ScholarTheoretical risk of venous air embolus (because of low venous pressures)45° head-up tilt making back horizontalWeight borne on ischii, not kneesUnstable positionChest paddedReduced risk of crush injury and deep venous thrombosisTight paraspinal muscles can limit lateral surgical accessHead rotatedArms abducted above head‘Tuck’ position214Wayne SJ The tuck position for lumbar-disc surgery.J Bone Joint Surg Am. 1967; 49: 1195-1198Crossref PubMed Google ScholarVery similar to ‘Prayer’ positionLow venous pressuresRisk of crush injury and deep venous thrombosisHips flexed >90°Spinal flexion improves surgical accessTight paraspinal muscles can limit lateral surgical accessHead-down tiltFewer manual handling problemsHastings Frame||69Hastings DE A simple frame for operations on the lumbar spine.Can J Surg. 1969; 12: 251-253PubMed Google ScholarAs for ‘Seated prone’More stable than ‘Seated prone’Venous pooling in legsWooden frame with adjustable seatDegree of spinal flexion variableSmith Frame176Smith RH One solution to the problem of the prone position for surgical procedures.Anesth Analg. 1974; 53: 221-224Crossref PubMed Google ScholarTwo curved supports under iliac crestsFree abdomen, adjustable for any degree of obesityRisk of pressure necrosis over iliac crestsPile of folded sheets under chestRisk of lateral femoral cutaneous nerve palsyNeck flexed and rotated‘Sea lion’ position#78Iwabuchi T Ishii M Julow J Biparieto-occipital craniotomy with hyperextended neck—‘sea lion’ position.Acta Neurochir (Wien). 1979; 51: 113-117Crossref PubMed Scopus (5) Google ScholarBack hyperextended on the pelvis; head up and extended on the neckGood access to posterior cerebral structures with low venous pressuresRisk of venous air embolism‘Tuck Seat’213Wayne SJ A modification of the tuck position for lumbar spine surgery. A 15-year follow-up study.Clin Orthop. 1984; 184: 212-216Google ScholarAs for ‘Tuck’ position, but padded seat below ischial tuberositiesAs for ‘Tuck’ positionRisk of crush injury and deep venous thrombosis lowEvacuatable Mattress183Sunden G Walloe A Wingstrand H A new device to reduce intra-abdominal pressure during lumbar surgery.Spine. 1986; 11: 635-636Crossref PubMed Google ScholarAirtight flexible mattressFewer pressure effects (spreads load over whole body)Becomes rigid on evacuation.Adjustable for any body habitus.Moulded around patient to support iliac crests and thorax but with abdomen freeHeat-retainingProne lithotomy180Srivastava S Pandey CK Anesthesia in the prone lithotomy position.Can J Anaesth. 2001; 48: 827Crossref PubMed Google ScholarLithotomy position of legs superimposed on prone positionSome compensation for impaired venous returnRarely used, hence limited data availableGood access for ano-rectal surgery Open table in a new tab When moving a patient into the prone position, an almost universal finding is a decrease in cardiac index (CI). In 16 patients16Backofen JE SJ Hemodynamic changes with prone positioning during general anethesia.Anesth Analg. 1985; 64: 194Google Scholar with cardiopulmonary disease during surgery in the prone position, the most marked finding was an average decrease in CI of 24% 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 the majority of patients. No changes were noted in mean right atrial or pulmonary artery pressures (PAP). Interestingly, these alterations in cardiac function were only noted because cardiac output was measured and central venous and intra-arterial pressure measurements would not have identified this. This decrease in CI in the prone position has been confirmed elsewhere,70Hatada T Kusunoki M Sakiyama T et al.Hemodynamics in the prone jackknife position during surgery.Am J Surg. 1991; 162: 55-58Abstract Full Text PDF PubMed Scopus (18) Google Scholar although in contrast, one study using transoesophageal echocardiography in patients undergoing lumbar laminectomy199Toyota S Amaki Y Hemodynamic evaluation of the prone position by transesophageal echocardiography.J Clin Anesth. 1998; 10: 32-35Abstract Full Text PDF PubMed Scopus (0) Google Scholar showed that although central venous pressure (CVP) increased slightly when patients were moved from supine to prone, CI did not change. However, it appears that the specific prone position used may influence these findings. A study of 21 patients undergoing lumbar surgery with direct PAP or IVC pressure monitoring226Yokoyama M Ueda W Hirakawa M Yamamoto H Hemodynamic effect of the prone position during anesthesia.Acta Anaesthesiol Scand. 1991; 35: 741-744Crossref PubMed Google Scholar demonstrated that the flat prone position did not interfere with circulatory function but that positioning in a convex saddle frame caused a decrease in CI and stroke volume index with no significant increase in IVC pressure. It was suggested that in these situations, the position of the heart at a hydrostatic level above the head and limbs may have caused reduced venous return to the heart and consequently a decreased CI. A study207Wadsworth R Anderton JM Vohra A The effect of four different surgical prone positions on cardiovascular parameters in healthy volunteers.Anaesthesia. 1996; 51: 819-822Crossref PubMed Google Scholar of four different surgical prone positions in 20 healthy non-anaesthetized volunteers (support on pillows under the thorax and pelvis with abdomen free, on an evacuatable mattress, on a modified Relton–Hall frame and the knee–chest position) found no substantial changes in heart rate or MAP in any position, but CI decreased by 20% on assuming the knee–chest position and by 17% on assuming the modified Relton–Hall position. In the prone jack-knife position,70Hatada T Kusunoki M Sakiyama T et al.Hemodynamics in the prone jackknife position during surgery.Am J Surg. 1991; 162: 55-58Abstract Full Text PDF PubMed Scopus (18) Google Scholar head-down tilt caused CI to return to supine values, attributed to decompression of the IVC allowing an increase in venous return to the heart. It has been suggested that the decrease in CI could be attributed to increased intra-thoracic pressures causing a decrease in arterial filling, leading to an increase in sympathetic activity via the baroceptor reflex. Consistent with this theory is the work which demonstrated decreased stroke volume accompanied by an increased sympathetic activity (increased heart rate, total peripheral vascular resistance, and plasma noradrenaline) in prone patients.153Pump B Talleruphuus U Christensen NJ Warberg J Norsk P Effects of supine, prone, and lateral positions on cardiovascular and renal variables in humans.Am J Physiol Regul Integr Comp Physiol. 2002; 283: R174-R180Crossref PubMed Google Scholar Another study has suggested that in addition to reduced venous return, left ventricular compliance may also decrease secondary to increased intra-thoracic pressure which could contribute to the observed decrease in cardiac output.182Sudheer PS Logan SW Ateleanu B Hall JE Haemodynamic effects of the prone position: a comparison of propofol total intravenous and inhalation anaesthesia.Anaesthesia. 2006; 61: 138-141Crossref PubMed Scopus (35) Google Scholar Recent work suggests that the anaesthetic technique could affect haemodynamic variables in the prone position. One study142Ozkose Z Ercan B Unal Y et al.Inhalation versus total intravenous anesthesia for lumbar disc herniation: comparison of hemodynamic effects, recovery characteristics, and cost.J Neurosurg Anesthesiol. 2001; 13: 296-302Crossref PubMed Scopus (55) Google Scholar compared total i.v. 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. A study182Sudheer PS Logan SW Ateleanu B Hall JE Haemodynamic effects of the prone position: a comparison of propofol total intravenous and inhalation anaesthesia.Anaesthesia. 2006; 61: 138-141Crossref PubMed Scopus (35) Google Scholar comparing inhalation with i.v. maintenance anaesthesia used non-invasive cardiac output measures with the patients supine and then prone on a Montreal mattress. The authors found a decrease in CI and increase in SVR on turning the patient prone. The changes were greater during TIVA (decrease in CI of 25.9%) than during inhalation anaesthesia (12.9%). However, a contributor to these observations could be a change in propofol pharmacokinetics in the prone position. Measured propofol concentrations have been observed to increase during target-controlled infusions when patients are transferred from supine to prone, probably as a result of the decrease in cardiac output.189Takizawa D Hiraoka H Nakamura K Yamamoto K Horiuchi R Influence of the prone position on propofol pharmacokinetics.Anaesthesia. 2004; 59: 1250-1251Crossref PubMed Scopus (7) Google Scholar 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. Obstruction to venous drainage forces blood to return to the heart by an alternative route (usually the vertebral column venous plexus of Batson). As these veins are thin walled, containing little or no muscle tissue and few valves, any increase in pressure is transmitted and causes distension. This is likely (especially during lumbar spinal surgery) to cause increased blood loss and difficulty with the surgical field. The problem of IVC obstruction is well recognized and various methods have been attempted to reduce blood loss, including the use of local anaesthetic infiltration, spinal and epidural anaesthesia, and deliberate hypotension. In one study,144Pearce DJ The role of posture in laminectomy.Proc R Soc Med. 1957; 50: 109-112PubMed Google Scholar IVC pressure was measured in six patients with the abdomen hanging free or compressed. In all patients, abdominal compression resulted in a large increase in venous pressure, increasing to more than 30 cm H2O in one patient. The position resulting in 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. A comparison of IVC pressures found that patients in the flat prone position had pressures 1.5 times greater than in patients on the Relton–Hall frame,105Lee TC Yang LC Chen HJ Effect of patient position and hypotensive anesthesia on inferior vena caval pressure.Spine. 1998; 23 (discussion 7–8): 941-947Crossref PubMed Scopus (0) Google Scholar demonstrating the benefit of a support system allowing a free abdomen. This study also found that induced hypotension had no significant effect on IVC pressure. In summary, turning a patient into the prone position has measurable effects on cardiovascular physiology, the most consistent of which is a reduction in CI. This has variously been attributed to reduced venous return, direct effects on arterial filling, and reduced left ventricular compliance secondary to increased thoracic pressure. Other haemodynamic variables change less predictably, although at least some patients demonstrate an increased sympathetic response to the change in position, and choice of anaesthetic technique may influence the degree to which such changes occur. Obstruction of the IVC is a well-recognized complication of prone positioning and is exacerbated by any degree of abdominal compression, leading to decreased cardiac output and increased bleeding, venous stasis, and consequent thrombotic complications. Careful positioning is therefore essential to minimize these risks. Lung mechanics have been studied in different positions, and interest has grown in the use of the prone position for improving oxygenation in patients with acute lung injury. This review does not address the changes occurring in the intensive care setting. It should be noted that studies carried out on awake spontaneously breathing subjects cannot necessarily be extrapolated to those who are anaesthetized and ventilated. In addition, the type of frame or support used and the body habitus of the patient may influence results. 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.115Lumb AB Nunn JF Respiratory function and ribcage contribution to ventilation in body positions commonly used during anesthesia.Anesth Analg. 1991; 73: 422-426Crossref PubMed Google Scholar Coonan and Hope33Coonan TJ Hope CE Cardio-respiratory effects of change of body position.Can Anaesth Soc. 1983; 30: 424-437Crossref PubMed Google Scholar have discussed in detail the cardio-respiratory effects of change in body position. The change in FRC in a patient going from upright and conscious to supine, anaesthetized, and paralysed is a decrease of 44%, but from upright to prone is considerably less at 12%. These findings were confirmed in a clinical context in patients undergoing intervertebral disc surgery.145Pelosi P Croci M Calappi E et al.The prone positioning during general anesthesia minimally affects respiratory mechanics while improving functional residual capacity and increasing oxygen tension.Anesth Analg. 1995; 80: 955-960PubMed Google Scholar Measurements of FRC and arterial oxygen tension (Pao2) were made with patients supine and again after 20 min prone. On changing from supine to prone there was a significant increase in the FRC and Pao2 [1.9 (sd 0.6) vs 2.9 (0.7) litre and 160 (37) vs 199 (16) mm Hg]. 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 with the change in position. The authors related the increase in FRC to the reduction of cephalad pressure on the diaphragm and the reopening of atelectatic segments. The study was repeated in obese patients (BMI>30 kg m−2),146Pelosi P Croci M Calappi E et al.Prone positioning improves pulmonary function in obese patients during general anesthesia.Anesth Analg. 1996; 83: 578-583Crossref PubMed Google Scholar using similar methodology and positioning, and found an increase in lung volumes, lung compliance, and oxygenation when patients were turned into the prone position, although the average FRC in obese subjects when supine was significantly smaller than in the non-obese group [1.9 (0.6) litre compared with 0.894 (0.327) litre]. It should be noted that some older work came to different conclusions, based on findings of marked (30–35%) decreases in respiratory compliance and increased peak airway pressure.116Lynch S Brand L Levy A Changes in lung–thorax compliance during orthopedic surgery.Anaesthesiology. 1959; 20: 278-282Crossref PubMed Google Scholar 163Safar P Aguto-Escarraga L Compliance in apneic anesthetised adults.Anesthesiology. 1959; 20: 283-289Crossref PubMed Google Scholar However, the position used by these authors was either inferior in terms of allowing free abdominal and chest wall movement116Lynch S Brand L Levy A Changes in lung–thorax compliance during orthopedic surgery.Anaesthesiology. 1959; 20: 278-282Crossref PubMed Google Scholar or not described.163Safar P Aguto-Escarraga L Compliance in apneic anesthetised adults.Anesthesiology. 1959; 20: 283-289Crossref PubMed Google Scholar It is clear that observed changes in lung volumes will depend on the exact prone position used. This has been demonstrated in one study118Mahajan RP Hennessy N Aitkinhead AR Jellinek D Effect of three different surgical prone positions on lung volumes in healthy volunteers.Anaesthesia. 1994; 49: 583-586Crossref PubMed Scopus (14) Google Scholar which compared lung volumes in three different prone positions (knee–chest, Eschmann frame, and chest/pelvic supports) with those in a ‘control’ prone position in 10 healthy non-obese subjects who were awake, breathing spontaneously. In all positions, the FRC tended to increase compared with control (significantly in the knee–chest and frame positions). Overall, the knee–chest position allowed the largest lung volumes with the exception of the inspiratory capacity which decreased significantly compared with the control. The cause of the improvement in lung volumes is thought to be the weight being supported by the knees, allowing the lower chest and abdomen to be suspended. However, these findings cannot necessarily be extrapolated to the anaesthetized ventilated patient. Early studies described redistribution of pulmonary blood flow to dependent lung areas when patients were moved from supine to prone.87Kaneko K Milic-Emili J Dolovich MB Dawson A Bates DV Regional distribution of ventilation and perfusion as a function of body position.J Appl Physiol. 1966; 21: 767-777Crossref PubMed Google Scholar More recently, it has been observed in animal studies that regional perfusion is directed preferentially towards the dorsal lung areas regardless of position.55Glenny RW LW Albert RK Robertson HT Gravity is a minor determinant of pulmonary blood flow distribution.J Appl Physiol. 1991; 71: 620-629PubMed Google Scholar Work in humans has similarly shown attenuation of the preferential perfusion of dependent lung areas in prone compared with supine positioning; one study found that lung perfusion was more uniformly distributed in the prone compared with the supine position.136Nyren S Mure M Jacohbsson H Larrson SA Lindahl SGE Pulmonary perfusion is more uniform in the prone than in the supine position: scintigraphy in healthy humans.J Appl Physiol. 1999; 86: 1135-1141PubMed Google Scholar These findings are consistent with the theory that gravity has only a minor role in determining regional lung perfusion; an intrinsically lower pulmonary vascular resistance in dorsal regions of lung could be the explanation. In the prone position, blood flow may be relatively uniform as gravitational forces are opposing rather than augmenting the regional differences in pulmonary vascular resistance. The role of gravity in the distribution of pulmonary blood flow has recently been reviewed and lung architectural changes may be more important.52Galvin I Drummond GB Nirmalan M Distribution of blood flow and ventilation in the lung: gravity is not the only factor.Br J Anaesth. 2007; 98: 420-428Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar Redistribution of lung ventilation is another proposed mechanism by which gas exchange is thought to improve in the prone position. Work carried out in the 1960s87Kaneko K Milic-Emili J Dolovich MB Dawson A Bates DV Regional distribution of ventilation and perfusion as a function of body position.J Appl Physiol. 1966; 21: 767-777Crossref PubMed Google Scholar demonstrated the apparent dependence of both ventilation and perfusion on gravity. However, it is now suggested that variation in regional lung ventilation may be related primarily to the structural features of the airways and blood vessels and that gravity has a less important role. Early studies87Kaneko K Milic-Emili J Dolovich MB Dawson A Bates DV Regional distribution of ventilation and perfusion as a function of body position.J Appl Physiol. 1966; 21: 767-777Crossref PubMed Google Scholar suggested that the greater ventilation observed in the dependent lung was secondary to gravitational differences in interpleural pressure (IPP), IPP becoming less sub-atmospheric with gravity. Both animal and human studies have subsequently shown that the pleural pressure gradient when prone is considerably reduced compared with supine.121Mayo JR Mackay AL Whittall KP Kaile EM Pare PD Measurement of lung water content and pleural pressure gradient with magnetic resonance imaging.J Thorac Imaging. 1995; 10: 73-81Crossref PubMed Google Scholar 132Mutoh T Guest RJ Lamm WJ Albert RK Prone position alters the effect of volume overload on regional pleural pressures and improves hypoxemia in pigs in vivo.Am