CHAPTER 6: DEEP VEIN THROMBOSIS, SEATING
AND STRESS
6.1 Four main medical concerns
have arisen from our Inquiry. Of these, we and many of those who
submitted evidence consider the most significant to be deep vein
(or venous) thrombosis (DVT). This is discussed in this Chapter[77],
together with associated issues of seating, comfort and stress.
The three other main medical concerns (the transmission of infection,
the effects of the cabin environment on vulnerable individuals
and the handling of in-flight medical emergencies) are considered
in Chapter 7.
Deep
vein thrombosis
BLOOD CIRCULATION
6.2 Blood provides the principal
means of transporting metabolic necessities round the body and
collecting wastes. Considering for simplicity only the respiratory
cycle mentioned in paragraphs 4.2ff, oxygen-rich blood from the
lungs is pumped by the heart through a network of arteries. When
the blood reaches the organ or tissue served by a particular artery,
it is diffused through a mass of small blood vessels ("capillaries")
through which the blood supplies oxygen to the tissues and carries
away the waste carbon dioxide. Through a reverse network of veins,
the blood then returns to the heart and lungs for re-oxygenation
and elimination of carbon dioxide.
6.3 The pressure at which the heart pumps blood through
the arteries is substantially dissipated by the capillaries, making
the return through the veins less vigorous. The heart is located
in the chest, and blood thus has a long way to return from the
feet - from which the flow is also hampered by the effects of
gravity. To assist that return blood flow, the main leg veins
are deep inside the leg muscles and contain a series of non-return
valves enabling muscle action to augment the pumping action of
the heart.
DESCRIPTION
AND TERMINOLOGY
6.4 DVT
is a condition
in which a
small blood clot
or thrombus (thrombi in the plural) forms
mainly in the
deep veins of
the legs[78].
Such clots can be present without symptoms
or signs, but may give rise
to swelling
of the affected
leg, sometimes accompanied by pain (particularly when the
foot is flexed hard upwards) and local tenderness. Such swelling
is not to be confused with the commonly experienced swelling of
both lower legs during and after a long flight, which is due to
inactivity and soon disappears after leaving the aircraft.
6.5 DVT is
not dangerous
in itself but complications
arising from
it may occasionally
be life-threatening.
Complications occur when a
thrombus breaks away
from the wall
of the vein
to which it is attached and is
carried along with the flow of the blood as what is termed an
embolus. If the embolus reaches a blood vessel through which it
cannot pass, it blocks the vessel. The consequence of such a blockage
is called an embolism. The most serious of these occurs in the
lungs (a pulmonary
embolism) giving
rise to
chest pain and
breathing difficulties and,
in the worst cases, death from
respiratory failure.
6.6 The
full syndrome
is properly known
as venous thrombo-embolism
(VTE)
as used in
some of our
evidence. However, we
follow the more common usage of DVT
throughout this
Report as it is
the initial
deep vein thrombosis
which is the
main concern in
relation to the aircraft
cabin environment,
VTE being a complication occasionally
arising from it.
NATURAL
HISTORY
6.7 It has been known for
many years that clotting in blood vessels (thrombosis) is associated
with:
- poor circulation or stagnation of the blood;
- excessive coagulability (thickening leading to
increased tendency to clot) of the blood; and
- abnormalities in, or damage to, the walls of
the blood vessels.
6.8 The presence of one or more of these factors
(known medically as Virchow's Triad) leads to an increased
possibility of blood clotting in an otherwise healthy person.
DVT is a frequent complication of major surgical procedures, occurring
in about a third of such patients, and much of the present knowledge
of the condition has been obtained from studies on post-surgical
patients. As pointed out by Dr Kesteven (p 246), that may not
necessarily reflect the natural history of travel-related DVT
(see paragraph 6.14).
6.9 It is thought that anaesthesia and enforced immobility
may induce the formation of small thrombi around the venous valves
in the deep veins of the calf. If there are additional factors
present tending to augment the potential of the blood to clot
(thrombotic tendency), a thrombus may grow and extend to other
veins, may amalgamate with other clots, and may spread into the
larger deep veins of the upper leg. When this occurs the risk
of a large thrombus breaking away and giving rise to a serious
pulmonary embolism is much higher than if the thrombi remain small
and limited to the calves. The time interval between the development
of the initial small thrombi and such an embolism may be anything
from days to weeks.
6.10 Pulmonary embolism is a relatively rare complication
of DVT, Professor Kakkar and Dr De Lorenzo (p 181) giving an incidence
of about 1 in 100 post-surgical patients who have suffered a DVT.
However, many more patients with DVT, about 30% according to the
Royal College of Physicians of Edinburgh (p 280), will develop
local complications in the affected leg (known as post-thrombotic
leg or post-phlebitic syndrome). This may lead to skin deterioration,
venous ulceration, and physical disability, and particularly to
a substantially greater risk of further DVT in future adverse
circumstances. Mr Scurr (p 283) estimated the cost to the United
Kingdom of dressings alone for these complications at in excess
of £300 million per year.
6.11 To keep this in proportion,
it has been concluded from post-mortem studies that up to one
half of all people with DVT and/or pulmonary embolism will show
no signs or symptoms at all (pp 181 & 246), and only one in
three cases of suspected DVT is subsequently confirmed (p 181).
6.12 Possible links between
DVT and air travel have been suggested in published case reports
with increasing frequency during the past 50 years or so. Professor
Mohler's evidence (p 251) included his own paper referring to
many of these reports. However, as noted by Dr Janvrin of CAA
(Q 23) and Dr Morgan Williams (Appendix 4 - who kindly sent us
a copy of the original paper), the first strong evidence that
pulmonary embolism was causally linked with spatial confinement,
immobility and constrained seating conditions was published 60
years ago by the late Professor Keith Simpson, an expert in forensic
medicine[79].
He noted a sharp increase in deaths from pulmonary embolism among
people who had spent long periods in air-raid shelters, and found
that such deaths were six times higher among those who had sat
in hard-edged deckchairs than among those who had not. Many of
the subsequent reports cite this important paper in their linking
of DVT with enforced immobility and unsuitable seating in cars,
coaches, trains and, in particular, aircraft.
RISK FACTORS
6.13 From studies on surgical
patients, various factors have been recognised as contributing
to a raised risk of DVT - see Box 2. As discussed in paragraph
6.17, there are also factors in the aircraft cabin environment
which may be risk factors for DVT in themselves, or which may
augment the risk from the predisposing factors. Most of the factors
shown in Box 2 have been identified only by retrospective studies,
though some would be expected from the known natural history of
the condition.
6.14 Views differ as to the
relative significance of individual factors, and some are disputed,
particularly in relation to air or other travel (as discussed
below), but the overall contribution of known risk factors to
the incidence of DVT is high. In studies cited by the Royal College
of Physicians of Edinburgh (p 280) and Professor Mohler (p 251),
it was found that only 11% of cases of post-surgical DVT had no
risk factors identifiable by non-laboratory tests, and that 40%
of new acute DVT cases were associated with inheritable clot-enhancing
conditions.
6.15 The evidence relating
to these risk factors is generally derived from surgical studies.
We note that, in his 1940 report (see paragraph 6.12), Professor
Simpson commented upon clear pathological differences between
clots found in the lungs of post-surgical cases and of the air-raid
shelter cases. When considering any possible contributions of
predisposing or environmental risk factors to the development
of DVT in travellers (see paragraphs 6.17ff), it must be borne
in mind that the natural history of the latter - about which we
have no specific knowledge - may differ significantly from that
of post-surgical DVT.
| Box 2
Predisposing factors for DVT
Generally agreed by witnesses
• Increasing age above 40 years
• Pregnancy
• Former or current malignant disease
• Blood disorders leading to increased clotting tendency
• Inherited or acquired impairment of blood clotting mechanisms
• Some types of cardiovascular disease or insufficiency
• Personal or family history of DVT
• Recent major surgery or injury, especially to lower limbs or abdomen
• Oestrogen hormone therapy, including oral contraception
• Immobilisation for a day or more
• Depletion of body fluids causing increased blood viscosity
Witnesses' views variable
• Varicose veins
• Obesity
• Current tobacco smoking
Sources: Dr Giangrande (p 234), Professor Kakkar & Dr De Lorenzo (p 181), Dr Kesteven (p 246), Professor Meade (p 174), Professor Mohler (p 251), Royal College of Physicians of Edinburgh (p 280), Mr Scurr (p 283)
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GENERAL INCIDENCE
6.16 Up to 20% of the total
population is thought to have some degree of increased clotting
tendency, largely from inherited or acquired biochemical, metabolic
or blood abnormalities, and this indicates that there might be
a "natural" underlying incidence of DVT in the general
population associated with those factors alone. There are no published
prospective (forward-looking) studies of the development of DVT
in normal populations, but retrospective studies (finding cases
which have already occurred in a defined population) indicate
that the "natural" incidence increases rapidly with
age, averaging overall about one case per thousand people per
year. Thus, in those who have recently travelled by air, we might
expect DVT to occur at an average rate of about one per thousand
per year, with a lower rate in the young and a higher rate in
the old - although, as noted in paragraph 6.11, in many travellers
the DVT may not be apparent.
DVT IN AIR TRAVELLERS
6.17 From the incidence among
the general population, it is certain that a number of people
will develop DVT while travelling or soon after they have travelled
by air. The fundamental question yet to be answered is whether
this number is greater than the number from an equivalent population
who would have developed DVT if they had not recently flown. The
time-scale between flight and the development of DVT is important
because the anecdotal evidence indicates that flight-related DVT
may develop, or be diagnosed, at any time from in-flight to many
days post-flight. As discussed in paragraph 6.12, prolonged immobility
and cramped seating are known to be causative factors for DVT
in susceptible individuals. It is a matter of fact that these
conditions are readily experienced in the aircraft cabin.
6.18 The actual incidence
of DVT in those who have recently travelled by air is not known,
because the appropriate scientifically rigorous epidemiological
studies have not yet been carried out. We note that Mr Scurr (p
283) and Dr Kesteven (p 246) are both carrying out research in
relation to this, and we look forward with much interest to seeing
their reports. However, there are currently no authoritative data
to show any clear difference in the incidence of DVT between those
who have recently travelled and those who have not. Thus it is
also not yet known whether air travel increases the incidence
of DVT above that found in the general population or above that
found in other travellers. Indeed, as we were finalising this
Report, a study was reported[80]
in which the authors concluded that there was no increased risk
of DVT among travellers.
6.19 Nevertheless, a wealth of anecdotal evidence,
a few case-collection reports, and a case-control study - all
cited widely by our witnesses - have led Professor Mohler, the
Consumers' Association and others to conclude that there is a
clear causal relationship between air travel itself and an increased
incidence of DVT (pp 59 & 251). However, Mr Scurr, Professor
Meade and Professor Kakkar think that any such relationship has
not yet been established (pp 283, 174 & 181), and Dr Kesteven
and CAA (pp 246 & 16 ) as well as the airlines think that
any increased DVT incidence is related to long-distance travel
itself, rather than with the means of transport.
6.20 There are various circumstances associated with
long-distance travel by air, road, or rail, which are postulated
as risk factors for an increased incidence of DVT in travellers.
There are factors related primarily to the aircraft cabin environment
itself which might act similarly; and there are also predisposing
personal factors which could contribute adversely in relation
to DVT in air and other travellers: see Box 3. When considering
the possible contributions of any of these factors to a given
case of DVT, it is the multiple contribution of a number of them
acting together that is important rather than the impact of a
single entity. However, it must be reiterated here that there
are no data currently available by which the contribution of air
travel to the overall risk of DVT from any of these factors, singly
or in combination, can be estimated.
6.21 Views on some of the
factors given in Box 2 differ. Obesity, for instance, is considered
as a well-recognised risk factor for DVT by Professor Meade (p
174), but not by Dr Giangrande (p 234) except in terms of restricting
mobility in aircraft. More controversially, perhaps, smoking is
viewed by Professor Kakkar and Dr De Lorenzo (p 181) as predisposing
to DVT, while Dr Giangrande (p 234) considers that it may actually
be protective. We note in Box 2 that, while age over 40 is a risk
factor for DVT in general, travel-related DVT seems to strike
the young as well as the old. We consider that, in women, this
may be related in part to the widespread use of oestrogen hormones
in oral contraception, and we draw attention to this and hormone
replacement therapy (HRT) in Box 4.
6.22 The postulated travel-associated factors given
in Box 3 are largely theoretical, being deduced from consideration
of what is known about the natural history of DVT itself, and
how travel environments might impact upon it. We have already
excluded low cabin humidity as a potential contributor to central
dehydration in our discussion about the significance to health
of cabin RH in paragraphs 5.30-5.36. When considering the possible
contribution of any of these factors to the risk of DVT in travellers,
it is vital to be aware that interactions between them are likely
to be much more important than any individual factors, as we conclude
more generally in paragraphs 6.58ff.
6.23 The term "economy-class
syndrome" first appeared in 1977 in a paper by Symington
and Stack[81]
and is widely used to refer to flight-related DVT. It is misconceived
in suggesting that the possibility of DVT need not concern business[82]
and first class air travellers - or those using other forms of
long-distance transport. We recommend that health professionals
and others stop using the seriously misleading term "economy-class
syndrome". "Flight-related DVT" or "traveller's
thrombosis" would be more appropriate.
6.24 If there is an increased
risk of DVT solely from flying, it is small. Anecdotal and case-series
reports (see paragraph 6.16) indicate that anything from 0% to
50% of all cases of DVT/VTE may have an association with recent
travel, and the case-control study (Ferrari et al, 1999),
cited by Professor Meade (p 174) and others, centres this at about
25%. However in this study, only one quarter of the cases were
associated with air travel, most of the rest being linked with
road travel. Thus, the current estimates of the added contribution
of recent long-distance travel to the 1:1000 per year risk of
DVT in the general population vary from zero to 0.4:1000. Again
taking the centred figure of some 0.2:1000, this implies that,
amongst every million people taking a long journey by any mode
once per year, at least one thousand cases of clinically detectable
DVT will be found, because that is the general population incidence,
plus possibly another 200 because of the additional risk of travelling.
Many of the latter will have additional risk factors (see paragraph
6.13), so for healthy individuals, the risk of getting a clinically
significant DVT solely because they are taking a flight
seems to be exceedingly small. For those who are already at risk
because they are subject to predisposing factors, there may be
an additional risk from flying, but it is not currently quantifiable.
INFORMATION
AND RESEARCH NEEDS
6.25 We are very conscious
that the remarks above on incidence can only be tentative. Firm
conclusions cannot be reached from the available evidence. This
lack of knowledge is particularly important in terms of the ability
of people to make informed judgements on their own position in
relation to any risk of incurring DVT through flying or other
activities. It also affects the soundness and quality of advice
that can be given to them by their medical advisers regarding
the risk and any precautions or preventive measures they should
take to alleviate it. It is imperative that the current paucity
of data on DVT be remedied and we recommend that an epidemiological
research programme of the case-control type[83]
outlined by Professor Meade (p 174 and
Q 496) be commissioned by DoH as soon as practicable.
6.26 We also agree with Professor
Meade that other research into any relationship between flying
and DVT, as suggested by a number of witnesses, seems premature
until the statistical facts of any link have been established,
in which the outcome of current studies by Mr Scurr and Dr Kesteven,
referred to in paragraph 6.18, may be very helpful. As there is
a public health issue here that is wider than air travel, Professor
Kakkar's suggestion of a National Register for travel-associated
DVT (Q 498) has considerable merit but, to avoid what might prove
to be unnecessary expenditure of scarce resources, we see no case
for pursuing this in advance of the outcome of the recommended
research by case-control studies.
PEOPLE AT
RISK
6.27 From the predisposing
factors given in Box 2 it is clear that certain groups of people
are at increased risk of developing DVT, and it is possible that
their susceptibility may be increased further by association with
some or all of the postulated travel risk factors given in Box
3. Some would-be travellers will have more than one of the predisposing
factors, and each may be compounded by association with relevant
travel risk factors. It is known from post-surgical studies that
the greater the number of predisposing factors present, the greater
the risk of developing DVT (p 251). This is likely to be true
also of the travel risk factors: the risk of getting DVT as a
result of travelling probably increases as more factors are present.
| Box 3
Postulated risk factors for DVT associated specifically with long-distance travel
Travel by air, road and rail
• Increasing age above childhood
• Increasing duration of travel
• Increasing frequency of long-distance travel
• Immobility, elected or enforced
• Seating constraints, particularly leg-room
• Seated posture, including when asleep
• Wearing of tight undergarments or movement-restricting clothing
Within the aircraft cabin
• Reduced pressure leading to abdominal distension acting against venous return from the legs
• Reduced oxygen and/or pressure leading to increased blood clotting tendency
• Low humidity affecting body fluid content (not unique to the aircraft cabin)
• Excessive consumption of alcohol and coffee leading to dehydration (not unique to the aircraft cabin)
• Safety procedures compounding immobility
• Cabin crew activities discouraging mobility
• Increasing duration of non-stop flight sectors
Personal risk factors compounded by the travel environment
• Altered physical environment affecting predisposing factors (unique to the aircraft cabin)
• Obesity compounding immobility and seating comfort
• Height compounding mobility, seating and posture constraints
• For smokers, enforced non-smoking altering physiology and psychology
Sources: The Consumers' Association (p 59), Dr Giangrande (p 234), Professor Kakkar & Dr De Lorenzo (p 181), Dr Kesteven (p 246), Professor Meade (p 174), Professor Mohler (p 251), Royal College of Physicians of Edinburgh (p 280), Mr Scurr (p 283), Bendz B (et al) 2000, Lancet 356: 1657-8
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6.28 Intending travellers
who know they have, or suspect they might have, any of the predisposing
factors noted in Box 2 should take advice from knowledgeable authorities
to enable them to decide firstly, whether and how to travel, and
secondly, what precautionary measures they should take. We make
general proposals in paragraphs 9.5-9.8 to improve the availability
of sound advice. In the meantime, the immediately following paragraphs
make proposals to bridge the gap with regard to DVT. We should,
however, emphasise that intending passengers who are on medication
which might influence susceptibility to DVT (such as anti-coagulants,
steroids and cholesterol-reducing agents) must take specialist
advice before travelling.
| Box 4
Interim precautionary and preventive advice concerning air travel and DVT
Those with no known predisposing factors:
• Move around in seat and in cabin as much as practicable
• Exercise calf muscles whilst seated by half-hourly flexing and rotating of ankles for a few minutes
• Avoid excess of alcohol and caffeine-containing drinks, both before and during flight
• Drink only water or non-caffeinated soft drinks or juices when thirsty or feeling dry
• Observe and act on advice given in in-flight media
Those at minor risk - i.e. meeting one or more of the following conditions
aged over 40
very tall, very short, or obese
previous or current leg swelling from any cause
recent minor leg injury or minor body surgery
extensive varicose veins
• As above plus the following:
• Avoid leg discomfort whilst seated
• Avoid alcohol and caffeine-containing drinks, both before and during flight
• Take only short periods of sleep, unless normal sleeping position can be attained
• Do not take sleeping pills
• Consider the need to wear support stockings
Those at moderate risk - i.e. meeting one or more of the following conditions
recent heart disease
pregnant or on any hormone medication - particularly the
contraceptive pill and HRT
recent major leg injury or leg surgery
family history of DVT
• All the above plus the following:
• Take professional medical advice about the risks involved
• Take pre-flight low dose aspirin as advised by doctor unless contra-indicated
• Take professional advice about the need to wear compression stockings
Those at substantial risk - i.e. meeting one or more of the following conditions
previous or current DVT
known clotting tendency
recent major surgery or stroke
current malignant disease or chemotherapy
paralysed lower limb(s)
• Consider avoiding or postponing flight, taking medical advice if unsure
• If travelling, all the above but have low molecular weight heparin prescribed instead of aspirin
Sources: Dr Dawood (p 220), Dr Giangrande (p 234), Dr Kesteven (p 246), Professor Mohler (p 251), Virgin Atlantic (p 107)
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PREVENTIVE AND PRECAUTIONARY
MEASURES
6.29 The current lack of
sound information makes it difficult for individuals to make reasoned
judgements about their personal DVT risk and, consequently, the
precautions to take. To help meet those highly understandable
needs, we have summarised the relevant material submitted by our
witnesses during the Inquiry. As an interim measure pending
the development of more authoritative guidance[84],
we recommend airlines, their agents and others with consumer interests
to repackage the summary indicative and precautionary advice on
DVT in Box 4, together with the summary information on predisposing
and risk factors in Boxes 2 and 3, and make it widely available
to the general public. This will enable those who have no access
to other advice to make preliminary decisions about their travel
and the risk of DVT.
6.30 We can understand the
airlines' reluctance to accept suggestions that there might be
factors specific to the aircraft cabin environment that lead to
an increase in the overall risk of DVT. Although, as noted in
paragraph 6.24, any additional risk is likely to be small,
it is not in doubt that the risk factors of prolonged immobility
and cramped seating are present in aircraft. As noted in Q 315,
however, these circumstances are not limited to aircraft, and
we recommend the Government to consider tackling the issues on
a wider travel-related front or, indeed, as a general public health
matter.
6.31 In relation to air travel
alone, however, and applying the precautionary principle used
in other fields where health risks are considered possible but
are not well defined or quantified, there are measures which could
be taken to improve information and alleviate concerns about flying
and DVT, and to encourage preventive activities. We recommend
that airlines and their associates reappraise their current practices
in relation to not only the provision of information for passengers
but also the design of the cabin[85]
and cabin service procedures. The following are the main
points:
(a) easily available authoritative information
on DVT and travel;
(b) high-profile pre- and in-flight preventive
advice;
(c) active encouragement of in-flight mobility
and preventive leg exercises;
(d) restraints and constraints on seating and
eating, and on sleeping comfort;
(e) improved seat space and leg-room to a healthy
minimum, together with increased availability of pre-bookable
more spacious seating at reasonable additional cost;
(f) freedom and space
to move around, with minimum limitations on aisle access due to
cabin service activities; and
(g) reduced availability of alcohol and other
dehydrating beverages with increased availability of re-hydrating
drinks.
6.32 We also recommend
the Government, aviation regulators, trade groups and consumer
representatives to consider what action they should take in relation
to the above points.
77 In our discussion on DVT, we have drawn heavily
on the written evidence submitted by our expert witnesses identified
in Box 2 on page 46. We do not cite them individually in the text
except where there are specific attributions to be made. Back
78
The condition commonly known as varicose veins is not causally
linked to DVT (Q 483), although it is considered by some as a
risk factor (pp 181 & 251). Back
79
Shelter deaths from pulmonary embolism, Lancet, December
1940. Back
80
Kraaijenhagen R A et al (2000), Travel and risk of venous
thrombosis, Lancet 356: 1492, 28 October 2000 Back
81
Symington I S and Stack B H R, Pulmonary thromboembolism
after travel, Br J Chest, 1977. Back
82
Lord Graham of Edmonton suffered his DVT following a business
class flight (Q 84). Back
83
A study which involves identifying people ("cases")
who have DVT and comparing their recent travel histories and medical
backgrounds with a closely matched series of "control"
subjects without DVT. Back
84
As a starting point for which we offer the material in Boxes 2-4. Back
85
See also paragraph 5.40 about cabin design implications for the
provision of air nozzles under individual passenger control. Back
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