The concept of ocular perfusion pressure (OPP), is defined as the difference between the
mean arterial pressure (MAP) and the intraocular pressure (IOP.)It has been hypothesized
that unopposed decreases in MAP, increases in IOP or a combination of the two may result in
hypo perfusion of the eye and can cause an ocular infarction at the level of the retina or
optic nerve, leading to varying degrees of visual loss which is frequently bilateral and
Both the prone position and downward head tilting (Trendelenburg position) have been
associated with increases in IOP compared to the supine horizontal position (2,4,5,6), but
it is not known whether the Trendelenburg position during surgery and anesthesia is
associated with POVL.
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.
- If I am between the ages of 18 and 65 years of age and mentally capable of giving
- If I am scheduled for gynecological surgery and will be placed in the Trendelenburg
position (head lower than my waist) on the operating room table.
- If I fail/refuse to provide an informed consent
- If I have a history of stroke or trans-ischemic attack (TIA).
- If I have a history of corneal disease.
- If I a have a sensitivity to eye drops called proparacaine
- If I have had a previous diagnosis of carotid disease.
- If I have a history of glaucoma
- If I am taking medications which may affect the pressure in my eyes