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Blood clot formation in the leg veins,
a condition known as deep vein thrombosis (DVT), is much
more likely to develop during winter months than in the summer.
Similar seasonal variation in a related,
potentially fatal condition known as pulmonary embolism, in
which blood clots travel to the blood vessels of the lung,
has already been well documented.
The researchers found a clear seasonal
variation in hospitalizations for DVT and for pulmonary embolism.
While hospitalizations for DVT were about 18% lower than average
during the summer months, they were about 18%
higher than average in the winter.
And for pulmonary embolisms, hospitalizations
dropped below the average in the summer and climbed as
much as 26% above average in the winter, the report
indicates.
Although this was not part of the study,
the researchers speculated that cold-induced
narrowing of blood vessels and reduced physical
activity during winter months may produce a reduction in blood
flow in the lower limbs.
Winter respiratory tract infections could
also influence blood coagulation.
Patients at risk of deep vein thrombosis
and/or pulmonary embolism -- and their doctors -- should be
informed of this increased risk in winter.
Because people who travel by air in cramped
seats for long hours face an increased risk of DVT, the condition
has garnered considerable media attention as "economy
class syndrome."
British
Medical Journal September 15, 2001;323:601-602
People traveling long distances by air
are more at risk of a blood clot forming and lodging in their
lungs than are people traveling shorter distances.
While airline travel is considered to
be a risk factor for developing these blood clots, researchers
had not investigated whether the risk increases with longer
flights.
According to researchers, being
immobile on a plane and having a limited space in which to
move increases the risk that blood clots will develop
and then travel to and lodge in the lungs, an event called
pulmonary embolism.
During long periods of sitting, blood
pools in the legs and gets thicker. This in turn increases
the risk of pulmonary embolism, which can be fatal. A person
may experience no symptoms, or they may feel a pain in the
calf or leg, chest pain or shortness of breath. Surface veins
may become more visible.
Over the last 30 years, about 100 cases
of pulmonary embolism after air travel have been reported.
The authors report that the number of
cases was much higher among those traveling more than 3,100
miles than among those traveling a shorter distance. The number
was higher still among those who traveled more than 6,200
miles.
According to the report, most of the
people with DVTs had traveled in economy class and only three
reported that they had left their seats during the flight.
About three-quarters said they were
completely immobile during the flight.
A relation between the duration of air
travel and the risk of pulmonary embolism is strongly suggested
by this study.
Several reports have suggested that pulmonary
embolism may develop in passengers several weeks after air
travel.
To reduce risk, the researchers advise
passengers to drink fluids, avoid
alcohol, wear loose clothing, avoid crossing their
legs, wear elastic support stockings and engage
in minor physical activity, such as walking or
moving their legs, during flight.
A related editorial calls the findings
"the tip of the iceberg" with respect to the number
of cases that may really be occurring.
The
New England Journal of Medicine September 13, 2001;779-783,
828-829
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