Seemingly normal red blood cell
(RBC) volumes can be a sign a patient's at risk for
post-op cardiac problems. That's the startling finding
of a large new analysis in the June 13 JAMA.
Pre-operative screening of hematocrit
levels is common procedure, but little was previously
known about the relationship between anemia/polycythemia
and post-operative adverse effects. "We never knew what
the cut-off really was," says lead author Dr Wen Chih
Wu, of the Providence Veterans Affairs Medical Center.
"Ranges considered normal vary between different studies
and populations."
CUT-OFF
ENCROACHMENT
The study looked at data from over 310, 000 veterans,
mostly men aged 65 and older, who had major non-cardiac
surgery between 1997 and 2004, and for whom pre-operative
hematocrit data was available.
Using conventional hematocrit definitions,
subjects were stratified into groups of anemics (hematocrit
value of below 39%), normal range (39 to 54%), and polycythemics
(above 54%). Researchers compared the 30-day post-operative
mortality and cardiac event rates between groups.
"The level of risk was high, even
with small deviations," notes Dr Wu. Adverse events
were more common as soon as hematocrit levels went outside
the normal range. For every percentage point deviation
from optimal hematocrit range, there was a 1.6% increased
mortality risk. Those who fared best were within the
normal range (hematocrit value of 45-47.9%), and were
therefore selected as the reference group. They had
the lowest post-operative mortality at 1.5%, and a cardiac
event rate of 0.9%.
The shocker came when researchers
discovered that even deviations within the accepted
normal range led to significant dangers. Polycythemia
between 51-54% — at the upper limit of the normal
range — conferred a 3.1% increase in the 30-day
mortality rate compared with the reference group. "This
should trigger the health science community into changing
its thinking on this hematocrit group," suggests Dr
Wu.
GENDER
BENDER
The hematocrit-post-surgical outcome link didn't hold
for women. This is possibly due to the smaller sample
of women and the hematocrit cut-offs, which are known
to differ between women and men.
Dr Wu explains why men may be more
susceptible to the post-surgical risks of hematocrit
deviations. "Elderly men have more cardiovascular problems
in general, and less of a cardiovascular reserve to
compensate for abnormal hematocrits, particularly with
the stress of surgery."
Additionally, the paper notes that
a steep increase in blood viscosity occurs around the
51% mark — right around where the mortality rates
begin to significantly increase. This might explain
how mild polycythemia could be associated with increased
mortality, and again suggests that the 51-54% bracket
does not deserve its "normal" distinction.
UNDER
PRESSURE
Although a cause-and-effect relationship wasn't determined
by the retrospective methodology, Dr Wu is convinced
there could be one, for several reasons. "It's biologically
plausible that the stress from an operation may tax
an overworked cardiovascular system. And the association
between hematocrit and risk persists independently of
medical condition and type of surgical procedure, except
for emergent surgeries, which carry big risks that could
overshadow hematocrit effects," Dr Wu indicates.
"Also, the relationship is a linear
one, which you might not see if the association is spurious.
There's a good chance that hematocrit deviation explains,
by itself, the effect on post-operative outcomes." If
Dr Wu is right, this could lead to hematocrit-altering
interventions prior to surgery.
In an accompanying editorial, Dr
Farhood Farjah and Dr David Flum lauded the work as
a solid use of the large body of veteran data. At the
same time, they caution doctors not to jump the gun
and start tinkering with RBC levels through transfusions,
erythropoietics or iron supplements until we know more
about the cause and effect.
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