Our preliminary Phase I studies using awake volunteers confirmed the increase in IOP associated with the prone position which is attenuated, although not eliminated, with a 15 cm head up table elevation. In addition we found evidence of retinal vascular changes consistent with venous congestion over a one hour period in the prone position in five awake volunteers using digital retinal imaging.

It is known that general anesthesia decreases IOP. A critical element, however, which has not been definitively studied is the effect of individual anesthetics, specifically nitrous oxide, on IOP. It is known that nitrous oxide does increase IOP when inert gases are injected into the vitreous for treatment of retinal detachment and this phenomenon can last up to six weeks, resulting in permanent vision loss with its use. It has also been shown that nitrous oxide increases intracranial pressure (ICP,9) although this effect may be attenuated by narcotics and additional intravenous anesthetic agents. It also increases cerebral metabolic rate, potentially aggravating any compromise in blood flow. Several studies showed a significant association between elevated IOP and ICP. Therefore, it is critical to attempt to isolate the effect of nitrous oxide on IOP. Because nitrous oxide is used as an adjunct to other anesthetics, it is the most frequently used of all the inhalation anesthetics and delineating its effect on this variable will help clinicians decide whether the advantage of its use outweighs the risk.

Because general anesthesia decreases IOP and there is significant variation between subjects this study is planned using a within subject design. Patients undergoing open gynecological procedures such as hysterectomies and patients undergoing orthopedic surgery are ideal candidates for the proposed study for the following reasons. The majority of patients undergoing general anesthesia in University Hospital operating rooms require frequent table position changes which might affect the results. Therefore, we choose to study patients who will remain in one position for the majority of the operation. The hysterectomy patients offer a unique benefit because It would also be advantageous to study patients in a position known to increase IOP in order to better detect differences in anesthetic regimen and isolate the effects of a specific agent. In addition, patients undergoing orthopedic surgery such as hand and ankle procedures are commonly in a fixed supine position for the predominance of the operation. University Hospital conducts a considerably greater number of orthopedic cases than gynecological cases daily which allows for greater ease in patient enrollment. Measuring IOP in the supine Trendelenburg position offers significant technical and logistical advantages compared to the prone position and therefore taking our measurements in this position is desirable.