Manipulation of a patient into the desired position for surgery carries its own set of problems. The main hazards are related to the effects of pressure and the physiological changes associated with a change in posture.
The anaesthetised patient is at risk of developing pressure sores in those areas where perfusion may be compromised. Likely sites are the occiput, the sacrum and the heel, all of which must be padded. External pneumatic compression devices applied to the lower limbs both confer a degree of protection from pressure effects and also improve circulation which helps to prevent deep vein thrombosis. No patient should ever be allowed to lie with the legs crossed and where possible an evacuatable mattress should be used. Compartment syndrome, usually related to trauma or arterial surgery, and for which immediate fasciotomy is essential to save life or limb, can be a rare complication of prolonged (four hours or more) lower limb compression in the lithotomy position.
The physiological effects which result from positioning are posture related. In the respiratory system a reduction in functional residual capacity (FRC) and total lung volume (TLV) is usually seen, accompanied by ventilation and perfusion imbalance and a degree of mechanical embarrassment to the respiratory process.
Cardiovascular effects are generally the result of gravitationally dictated venous pooling which leads to a fall in pre load and reduced cardiac output. Embarrassment of venous return due to abdominal compression is a specific complication of the prone position, but one that may be overcome by correct support so that the abdomen is unrestricted.
Steep head down (Trendelenburg) positioning may increase intra-ocular pressure. Air embolism is a risk if the operative site lies above the right atrium.
The potential damage to nerves from the common positions is detailed in Figure IN.1. Maintenance of Anaesthesia
Techniques for Maintenance of Anaesthesia and Analgesia
The maintenance of anaesthesia may be conveniently divided into those techniques applied to patients who are self ventilating and those techniques applied to patients who are receiving artificial ventilation of their lungs. Techniques suitable for self ventilating patients are summarised in Figure IN.2.
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