3. Prone Position

Turning the patient is often problematic, particularly in those with raised BMI. Several handlers will be needed to ensure that the patient is logrolled into position without any twisting of the lumbar and cervical spines. The head must move as one with the shoulders with obvious care not to dislodge whichever airway device is in place.

It is accepted wisdom that restricted access to the airway once the patient is inverted mandates endotracheal intubation, probably with an armoured tube, and this would be the ‘safe’ exam answer. In clinical practice, however, by no means all anaesthetists are quite as dogmatic. The prone position confers some respiratory advantages in a patient who is breathing spontaneously and a standard laryngeal mask airway is relatively easy to resite should it move, unlike a tracheal tube (or a reinforced laryngeal mask airway). It would probably be wise not to volunteer this option as it remains contentious, but be prepared to discuss it should an examiner raise the issue.

Ophthalmic problems
Peripheral nerve injuries.

. These range from relatively benign complications such as chemosis and subconjunctival haemorrhage through retinal detachment to complete visual loss (which is quoted as occurring in 0.02–0.2% following spinal surgery). Decreased ophthalmic perfusion pressure secondary to hypotension, hypovolaemia and an increase in direct orbital pressure can contribute to ischaemic optic neuropathy or central retinal artery occlusion, both of which may result in blindness. The central retinal artery can also thrombose secondary to a reduction in flow, but in some cases this may be unilateral. Intraocular pressure can rise in patients with narrow-angle glaucoma who are positioned prone.

Nerves of the brachial plexus are at particular risk of traction injury because they are effectively fixed at the cervical vertebrae, and they may also be compressed at the level of the first rib, clavicle and head of the humerus as they pass down into the upper limb across these potentially mobile structures. The ulnar nerve is vulnerable both to ischaemia and to direct compression, as is the lateral cutaneous nerve of the thigh, which may be in prolonged contact with pelvic supports.

The prone position is no more likely to cause pressure sores than the supine, except that there are probably more bony prominences to put the overlying tissues at risk. These include the anterior ankle joints, knees, anterior superior iliac spines, thorax, chin and forehead. Ischaemia develops after around 2 hours of unrelieved pressure and tissue necrosis after around 6 hours. Duration of surgery is therefore the major risk factor along with increased skin fragility associated with advanced age or corticosteroid therapy. Obesity is also a contributing

. The venous drainage from the spine is via the vertebral valveless venous plexus of Batson. The absence of valves means that any increase in intraabdominal pressure secondary to external compression is likely to cause a significant increase in operative bleeding and potential compromise of any planned spinal surgery.

consistent finding across various studies has been a decrease in cardiac index. This fall in cardiac output is attributed primarily to a reduction in stroke volume secondary to reduced venous return, which is due largely to venous pooling but to which inferior caval obstruction can make a contribution if positioning is poor and the abdomen is compressed The compensatory sympathetic response to this effective hypovolaemia is reflected by a tachycardia and a rise in systemic vascular resistance. Patients who are anaesthetized may also tolerate poorly rapid changes in position.

Functional residual capacity decreases by around 45% from the conscious, upright position to the anaesthetized supine position, but only by 12% from upright to prone. The change from supine to prone therefore, with all other factors unchanged, is associated with an improvement in PaO2 which is due to better matching of ventilation-perfusion However, more sophisticated investigation has demonstrated that blood flow is much less altered with changes of position (and in weightlessness) than previously believed, and a different model based on the structural features of the airway and associated vasculature has been proposed A factor that is simpler to understand is the fact that the dorsal lung areas receive preferential perfusion independently of position, and this may be related to an intrinsically lower pulmonary vascular resistance in those regions. In the injured lung, the proximal architecture of the bronchioles and vessels may be distorted by oedema and inflammation, and this mismatch can be amplified by the distal branching with substantial V/Q mismatch in the alveoli.