PROBLEMS RELATED TO PATIENT POSITIONING

Patient positioning (see Chapter 28 ) depends on the site of surgery. Whereas head-down tilt is used for pelvic and inframesocolic surgery, the head-up position is preferred for supramesocolic surgery. The patient is often placed in the lithotomy position. These positions may be responsible for or contribute to the development of pathophysiologic changes or injury during laparoscopy. The steepness of the tilt requested directly affects the magnitude of these changes.

Cardiovascular Effects

In normotensive subjects, the head-down position results in an increase in central venous pressure and cardiac output. The baroreceptor reflex response to increased hydrostatic pressure consists of systemic vasodilation and bradycardia, which stabilizes the cardiovascular status. Although these different reflexes may be impaired during general anesthesia, the hemodynamic changes induced by this position during laparoscopy remain insignificant.^{[79] [84] [85]} However, central blood volume and pressure changes are greater in patients with coronary artery disease, particularly with poor ventricular function, leading to potentially deleterious increased myocardial oxygen demand.^[39] The Trendelenburg position may also affect the cerebral circulation, particularly in case of low intracranial compliance,^[140] and result in elevation of the intraocular venous pressure (which can worsen acute glaucoma).^[143] Although the intravascular pressure increases in the upper torso, the head-down position decreases transmural pressures in the pelvic viscera, reducing blood loss but increasing the risk of gas embolism.^[39]

With the head-up position, a decrease in cardiac output and mean arterial pressure results from the reduction in venous return.^{[79] [85] [86]} This decrease in cardiac output compounds the hemodynamic changes induced by pneumoperitoneum. The steeper the tilt, the greater is the fall in cardiac output.

Venous stasis in the legs occurs during the head-up position and may be aggravated by the lithotomy position with the knees flexed.^[39] Because pneumoperitoneum further increases blood pooling in the legs,^{[96] [114]} any additional factor contributing to circulatory dysfunction should be avoided. The legs must be freely supported and not tightly strapped, and pressure on the popliteal space must be prevented.