Intraocular Pressure Clinical Trial
Official title:
The Effect of Aortic Infrarenal Clamping and Unclamping On 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.
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Time Perspective: Prospective
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