FEATURE
Features
Planes, Trains, and VTEs
Simple recommendations can help
mitigate risk for travel-related blood clots
In November 2011, hematologist Suely Rezende, MD,
PhD, deplaned from a flight from Lisbon to São Paulo,
where she was scheduled to speak at a national hematolo-
gy conference. She fainted almost immediately and woke
up with a heart rate of about 130 beats per minute. The
paramedics at the airport struggled to make a diagnosis,
so she assessed her symptoms: “I decided that either I had
a heart problem or this was a pulmonary embolism (PE),”
she recounted on World Thrombosis Day. 1
Dr. Rezende experienced what many people fear ahead
of long flights: a travel-related venous thromboembolism
(VTE). Nearly eight years later, she has not suffered any
complications from the event, but she started to take extra
precautions when planning long-distance travel.
“If this were just a regular person, especially not a very
demanding person, he or she would probably be sent home
and feel bad for a few more days,” she opined. “This would
cause terrible complications such as chronic pulmonary
hypertension and cardiac failure if [the patient] did not see
a medical doctor who could diagnose it.”
While stories like Dr. Rezende’s help to raise aware-
ness about this potentially fatal complication, the actual
absolute risk of a travel-related VTE is quite small – and
one that the average traveler does not need to worry
about – according to Susan Kahn, MD, MSc, FRCPC,
professor of medicine at McGill University and a Canada
Research Chair in venous thromboembolism. “Billions of
people travel by plane every year, and the vast majority
do not get a VTE.” For her part, Dr. Rezende said that she
was lucky, considering that she “did everything that we
recommend patients don’t do,” she said. 1 “I was very tired
in economy class on the plane. I had a boot that came to
my knee, and I didn’t move at all. I slept the whole flight,
and I was very stuck to the seat.”
VTE, including both deep vein thrombosis (DVT)
and PE, occurs in approximately one per 1,000 adults per
year in the general population. After prolonged travel –
flights of four hours or longer – that risk is estimated to
increase by about two- to fourfold, for an absolute risk of
an event within four weeks of the flight of about one in
4,600 flights. 2 This risk could increase in people with other
predisposing factors for thrombosis (see SIDEBAR , page 36).
Still, much more media attention is given to flight-
related thrombosis than other risk factors for thrombosis,
such as having a cast or undergoing orthopedic surgery.
Concerns about travel-related VTE are so prevalent that
the American Society of Hematology (ASH) chose to
include recommendations for long-distance travelers in
its guidelines for VTE prophylaxis in hospitalized and
non-hospitalized medical patients, which were released in
late 2018. 3
“Millions of people travel every day and travel is a
mild risk factor for VTE,” said Mary Cushman, MD,
medical director of the Thrombosis and Hemostasis
Program at the University of Vermont Medical Center in
ASHClinicalNews.org
Burlington. Dr. Cushman was chair of the guideline panel
that developed the guidelines, on which Dr. Kahn also
served as a member. “Our patients are concerned about
the risk posed by travel, so recommendations on preven-
tion of VTE during travel are necessary.”
ASH Clinical News spoke with Dr. Kahn and Dr. Cush-
man as members of the guideline-writing panel, as well
as other experts in thrombosis, about the misconceptions
about travel-related VTE, practical recommendations for
patients seeking peace of mind, and the continuing debate
about prevention.
Cabin Fever
Risk factors for travel-related VTE can be broken down
into two categories: air cabin risk factors and patient risk
factors, according to Casey O’Connell, MD, associate
professor in the Jane Anne Nohl Division of Hematology at
the Keck School of Medicine of the University of Southern
California.
Many elements inherent to air travel also can promote
venous stasis, Dr. O’Connell explained. Flights typically
involve sitting in a cramped position where the seat may
press against the popliteal vein behind the knee, a common
site of clots. Travelers’ ability to move around the cabin
might be hampered by flight conditions such as turbulence,
their location on the plane, or flight attendants’ use of the
aisles. For example, one study of air travel–associated PE
found that, although the overall incidence of PE was low,
most passengers diagnosed with a severe PE were com-
pletely immobile during flight. 4
Dehydration may also play a role in risk for VTE,
but evidence is inconsistent. Factors such as low cabin
humidity, decreased fluid intake, and the effect of alcohol
or coffee consumption during a flight can all contribute
to dehydration. 5
Commercial flights also expose passengers to pres-
surized cabins and hypobaric hypoxia, a condition that
impairs the body’s ability to transfer oxygen from the
lungs to the bloodstream. In studies of simulated flights,
researchers found mixed effects of hypobaric hypoxia on
coagulation activity, but one study examining passengers
after a transatlantic flight showed significant reductions
in activated partial thromboplastin time and tissue plas-
minogen activator, with a rise in plasminogen-activator
inhibitor-1 (PAI-1), suggesting active coagulation and
suppressed fibrinolysis. 6
On the whole, though, it is difficult to say conclusively
that long-haul air travel puts someone at any increased
risk compared with long-distance car travel, according
to Mark Crowther, MD, MSc, FRCPC, the Leo Pharma
Chair in Thromboembolism Research at McMaster
University Medical Centre in Ontario, Canada.
“It is not really known that air travel is riskier than
car travel; there is a lot of witchcraft to this and only a
little bit of science,” Dr. Crowther said. “The data in this
area are really poor.”
Factors that put patients at an increased VTE risk,
irrespective of travel, include age, active cancer diagnosis,
pregnancy, oral contraceptive use, recent surgery, obesity,
and certain hereditary conditions. 7,8
In its recommendations for travel-associated VTE, the
ASH guideline panel considered patients with these fac-
tors at greater risk for travel-related VTE, but the panel
noted that little evidence exists showing that patients with
prior VTE or those with total joint arthroplasty were at
any increased risk for travel-related VTE. 3
However, the guideline authors noted that the findings
might be biased “if travelers took precautions to reduce
their risk of VTE” before flying. Other studies have shown
that cancer, plaster casts, hormone replacement therapy,
oral contraceptives, and pregnancy increased risk for VTE
several-fold.
Reading Between the Guidelines
In ASH’s guidelines, authors issued three recommenda-
tions for the prevention of travel-related VTE, all of
which were “conditional,” meaning that the right decision
will vary for individual patients and that “clinicians must
help each patient arrive at a management decision consis-
tent with the patient’s values and preferences.” 3
“We suggested all conditional recommendations
because the surrounding evidence is quite poor in terms
of the amount and the quality of it,” said Dr. Kahn. Con-
versely, recommendations are scored as “strong” if the
guideline panel authors believe all patients should follow
the recommended course of action.
For travelers with no additional risk factors for VTE,
the authors suggested foregoing the use of graduated
compression stockings, low-molecular weight heparin
(LMWH), or aspirin for VTE prophylaxis – preventive
measures that many providers receive questions about
from their patients.
“For the average healthy person taking a long-haul
flight, we do not recommend anything special beyond
the usual commonsense measures, such as getting up
and walking around the plane and staying hydrated,” Dr.
Kahn said.
However, travelers are considered at higher risk for
VTE if they have undergone recent surgery, have a his-
tory of VTE, are postpartum, have an active malignancy,
or have at least one of these factors and are taking hor-
mone replacement therapy, have obesity, or are pregnant.
For these high-risk travelers, the guidelines suggest use of
graduated compression stockings or prophylactic LMWH
on flights longer than four hours.
Evidence for this recommendation was taken from
a 2016 review of the use of compression stockings to
prevent DVT in airline passengers. 9 The review in-
cluded nine trials, with about half of the 2,637 travelers
randomly assigned to wear compression stockings on a
ASH Clinical News
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