Simple Formula Predicts Blood Needs in Heart Surgery
DURHAM, N.C. -- Duke University Medical Center researchers
have developed a simple formula that will enable
anesthesiologists to predict, based on individual patient
characteristics, how much blood to have on hand in the
operating room prior to coronary artery bypass surgery.
The new formula not only can save hospital resources and
staff time, but also can lead to a more rational allocation of
banked blood, according to the researchers.
Before a bypass procedure, blood typically is delivered to
the operating room in case it will be needed to replace blood
lost during surgery. But not just any blood will do.
reserve blood must be matched with the patient's blood type,
and it also must be "cross-matched." In cross-matching, a small
amount of the patient's blood is mixed with a small portion of
banked blood to test for adverse immunological reactions.
In their study, the Duke researchers found that the patients
who are most likely to need the most blood during or
immediately following their surgery are those who are over the
age of 75, those who have impaired kidney function and those
who weigh less than 121 pounds.
"At most U.S. hospitals, four units of typed and
cross-matched blood are routinely sent to the operating room at
the time of surgery," said George Lappas, MD, a
cardiovascular anesthesiology fellow who presented the Duke
team's results on May 1, 2006, at the annual scientific
sessions of the Society of Cardiovascular Anesthesiologists.
"This is the arbitrary amount that has been used for many years
in operating rooms nationwide.
"We thought there needed to be a better way to estimate
blood needs, since blood is a scarce commodity that takes staff
times and resources to prepare," Lappas said. "Also, with blood
being held in operating rooms 'just in case,' it is not
available for other patients who might need it."
The savings to hospitals and blood banks would be realized
primarily in the reduced costs and manpower involved in the
cross-matching process, Lappas said. On average, it costs about
$55 to cross-match each unit of blood, meaning that $220 is
typically spent for every surgery, whether the blood is used or
not, he said.
The team's research was supported by Duke's Department of
In their study, the researchers analyzed the charts of 5,402
consecutive patients who received a bypass procedure at Duke
University Hospital between 1993 and 2002.
blood use during the first 24 hours after surgery with 13
different patient characteristics, such as anemia before
surgery, age, gender, weight, heart and kidney function,
tobacco use and other accompanying illnesses.
For the first two-thirds of the patients, they determined
which characteristics led to greater blood use. They then
developed the formula based on those findings and tested its
predictive abilities on the last third of patients.
"We confirmed our hypothesis that a model based on a
patient's characteristics prior to surgery could predict how
much blood would need to be cross-matched prior to surgery,"
Lappas said. "This approach would appear to be a more rational
and scientifically based approach for predicting blood
The simplified scoring system devised by the team would
identify risk factors for transfusion. For example, 1.5 units
of cross-matched blood would be made available for patients
older than 75, weighing less than 121 pounds or having a
creatinine level of more than 1.4.
Creatinine is a normal
byproduct of metabolism; higher-than-normal levels in the blood
indicate an impairment of the kidney's filtering ability,
because kidneys normally filter creatinine out of the blood and
excrete it in the urine.
Other factors, such as age older than 65, female sex or
anemia (a hematocrit of less than 36 percent), would need one
additional unit of cross-matched blood. Hematocrit is a
measurement of the percentage of oxygen-carrying red blood
cells in the blood; normal hematocrit ranges between 36 percent
and 40 percent.
An additional half unit of blood would be
supplied for patients who have creatinine levels of greater
than 1.2, who weigh less than 187 pounds, or whose heart
function is moderately impaired.
To determine how many units of blood to set aside for an
individual about to undergo surgery, physicians would add up
all of the amounts for that person's applicable
"We believe this model could easily be used by any
hospital's blood bank to allocate blood in a more safe and
cost-effective manner, Lappas said. "The formula is simple and
easy to use."
Other members of the Duke team were Barbara Phillips-Bute
and Mark Stafford-Smith.