This increased intra-abdominal pressure also pushes on the vena cava—the
large vein that returns blood to the heart—
forcing blood to make its way back to the
heart through collateral vessels, including veins near the spine. This results in
increased blood loss during spinal surgery
and can lead to hemodilution when the
lost blood is replaced by nonblood fluids.
The increased intra-abdominal pressure
also decreases blood flow into the heart
from the venous system, which in turn
causes lower blood pressure.
Some surgeons prefer that the anesthesiologist maintain the patient’s blood
pressure lower than normal during surgery to reduce bleeding and keep the
surgical field clear. And often a surgeon
will keep a patient’s head lower than the
heart to more comfortably access the
surgical field. This combination—
especially when a patient’s blood has been
diluted—can cause permanent damage
to the optic nerve.
When low arterial blood pressure,
diluted blood, and increased venous
and tissue pressures are maintained for
a long time, inadequate oxygenation of
the optic nerves may result in permanent visual loss. The American Society of
Anesthesiologists (ASA) has associated
POVL with “prolonged surgical duration
and substantial blood loss.”10
One important question has confounded researchers: If optic nerve
injury is caused by prolonged low blood
flow and low oxygenation, why don’t
patients suffer hypoxic brain injury as
well? This question was answered in
2008 when the journal Anesthesiology
published a study involving pigs.
The researchers examined the effects
of blood flow and hemodilution on
oxygen delivery to the brain and optic
nerves.11 They discovered that the brains
of pigs with diluted blood and low blood
pressure maintained adequate oxygenation by increasing cerebral blood flow.
But the optic nerves did not do this.
This study is the best and most up-to-date science on the subject. Because it
explains why blindness occurs without
brain injury, it can help to refute causation defenses.
In the most recent published study
of POVL, researchers found that five
hours in the prone position resulted in
increased intraocular pressure, choroid
layer thickness in the eye, and an increase
in optic nerve diameter. This study
shows that these physiologic changes are
independent of the anesthetic used and
demonstrates the mechanism of injury
in human subjects.12
Investigation and Experts
When investigating a POVL case, look
for evidence of factors associated with
POVL: a surgery in the prone posi-
surgery. Be sure to look past the laboratory reports, because hematocrit often is
measured with a bedside arterial blood
gas test that is recorded in cryptic form
in the anesthesia record rather than in
a lab report.
Check the hospital billing record for
all arterial blood gas tests that were billed
for, and make sure they are recorded
somewhere in the surgical record. If
they are not, you can be sure valuable
and potentially incriminating information is being withheld.
Often, there is scant documentation
of the hematocrit during surgery, so look
for evidence of multiple units of fluids
administered during surgery or blood
products administered in the post-op
room. Their use can shed light on how
hemodiluted the patient became.
tion lasting six hours or more with at
least one liter of blood loss. Gather the
patient’s preoperative medical records
to determine his or her baseline blood
pressure and to look for evidence of
preexisting vascular disease, hypertension, diabetes, and physical factors such
as a large protuberant belly that would
contribute to excessive intra-abdominal
pressures during surgery.
Comb the surgical record for evidence
of insufficient blood flow, including excessive acid in arterial blood gases and abnormally low urine output during surgery.
Also find out the type of surgical bed or
frame used, and look for evidence of a
head-down position during the surgery.
Look for evidence of a low hematocrit
before, during, or immediately following
The defense will try to minimize
evidence of blood loss, so do not simply
rely on the estimated blood loss in the
anesthesia record. Comb the hospital’s
itemized billing for the number of units
of blood that were billed for, and compare that with the number the anesthesia record says were administered.
Search the hospital billing record to
determine how many lap pads, sponges,
and towels were billed for, as the true
amount of blood loss is not properly
estimated until these are considered.
Also examine the anesthesia record for
evidence of long periods of low blood
pressure—25 percent or more below
the baseline. Prolonged periods of mean
arterial pressure in this range, along with
the factors described above, solidify the
