Intraocular Pressure During Abdominal Aortic Aneurysm (AAA) Repair
Intraocular Pressure During Abdominal Aortic Aneurysm (AAA) Repair
The objective of this study will be to answer a clinical question that has not already been
 investigated; that is, what are the effects of aortic infra-renal clamping and unclamping on
 intraocular pressure during Abdominal Aortic Aneurysm (AAA) repair? Depending on the results,
 this study may raise or alleviate concern that vascular surgery for abdominal aortic aneurysm
 could contribute to early perioperative exacerbation of pre-existing eye disease and increase
 a patient's vulnerability to developing a type of blindness known as ischemic optic
 neuropathy. The purpose of this observational study is to evaluate whether intraocular
 pressure measurements with a handheld tonometer will detect changes in intraocular pressure
 related to intraoperative events during aortic cross clamping and unclamping that may provide
 information on causes of perioperative blindness.
Perioperative blindness in nonocular surgery has gained significant clinical interest as an
 overwhelming complication with an increasing incidence. Initial published studies suggested a
 rare occurrence with an estimated postoperative visual loss of 0.002% and 0.0008% (1,2).
 However, retrospective reviews in spinal and cardiac surgery demonstrated higher rates of
 perioperative blindness; that is, between 0.2% and 0.06% respectively (3,4). The difference
 in published reports suggests that the true incidence is likely underestimated because of
 fears regarding litigation, ineffective means of reporting and extra work involved.
 Fortunately, numerous case reports and series in the middle to late 1990s were published;
 prompting the American Society of Anesthesiologists (ASA) Committee on Professional Liability
 to establish the ASA Postoperative Visual Loss Registry (POVL) in 1999 as a medium to collect
 confidential, comprehensive perioperative data on patients developing postoperative
 blindness.

 The POVL registry, along with other case series, has demonstrated that the most common cause
 of visual loss is non-arteritic ischemic optic neuropathy (5,6,7). Perioperative
 non-arteritic ischemic optic neuropathy (ION) is a consequence of patient and surgery
 specific factors that results from hypoperfusion and infarction of the optic nerve head (8).
 Anatomical factors such as variation in the number of short posterior ciliary arteries (sPCA)
 supplying the optic nerve head, location of sPCA watershed zone, and small optic cup to disk
 ratio increase susceptibility to optic nerve ischemia (8,9). Surgery specific factors involve
 profound blood loss, anemia, hypotension, prone positioning and duration of surgery
 (5,6,7,10). The interaction of these surgical variables and patient specific anatomical
 factors can decrease ocular perfusion pressure (the difference between mean arterial pressure
 and intraocular pressure) and result in ION (11).

 Based on the determinants of ocular perfusion pressure (OPP), low mean arterial pressure
 (MAP) and/or high intraocular pressure (IOP) can decrease OPP and lead to ION. Therefore, it
 is necessary to investigate changes in IOP incurred from different types of surgery that may
 increase vulnerability to developing ION. Normal IOP is between 12 - 20 mm Hg. Studies have
 been published assessing intraocular changes with prone positioning, laparoscopic surgery and
 cardiopulmonary bypass (12,13,14). However, there has been no literature evaluating
 intraocular pressure during abdominal aortic aneurysm (AAA) repair. AAA repair is a high-risk
 surgery associated with blood loss, hypotension and has been reported in a case series
 associated with ION (6). The surgery involves clamping and unclamping of the aorta to
 facilitate excision of the aneurysm and graft repair. Aortic cross clamping and unclamping is
 an intense physiologic insult affecting venous return, systemic vascular resistance, cardiac
 output, and acid base status. These physiologic changes are further pronounced with more
 proximal cross-clamping and longer duration. Because infrarenal abdominal aortic aneurysms
 are the most common type of AAA repair, we will be assessing IOP with patients undergoing
 elective infrarenal abdominal aortic reconstruction.

 Infrarenal aortic cross clamping is associated with increases in venous return, central
 venous pressure and arterial blood pressure (15). The hemodynamic changes with infrarenal
 unclamping entail decreases in venous return, central venous pressure and arterial blood
 pressure (15). The determinants of intraocular pressure involve extraocular muscle tone,
 aqueous flow, choroidal blood volume and central venous pressure (16). The volume
 redistribution proximal to the aortic cross-clamp should cause a rise in venous pressure,
 increase resistance to aqueous drainage and increase choroidal blood volume thereby
 increasing intraocular pressure. However, after aortic unclamping, choroidal blood volume and
 IOP should decrease as venous return and central venous pressure decline. Therefore, our
 hypothesis is that IOP will be increased during aortic cross clamping and decreased following
 aortic unclamping.
Inclusion Criteria:

 -  All patients presenting to St. Boniface General Hospital for elective AAA repair
 requiring an arterial line, central line and general anesthesia will be eligible for
 inclusion in this study.

 Exclusion Criteria:

 -  . The exclusion criteria for this study are:

 1. Patients with a history of acute or chronic eye disease

 2. Allergy to topical ophthalmologic anesthesia

 3. Patients receiving nitrous oxide, ketamine or succinylcholine during anesthesia

 4. If the surgical procedure will require the aorta to be re-clamped following
 unclamping, those patients will also be excluded

 5. Patients that are VRE+ or MRSA+ will be excluded for infection control purposes