Abstract
If one were to choose a single object to epitomise the discipline of Design, it could easily be the chair. Most buildings, vehicles or machines contain them. Many of us find ourselves compelled to sit in seats or chairs for eight hours, or more, every day. However, the health implications of this extended chair use are not well understood. Particularly, there is little quantitative research on how the act of sitting affects venous blood flow in the legs.
In 2001, the Joint Aviation Authorities of the European Union asked for research that would test how venous physiology responded to different seat designs. They wondered if current aircraft seat design could adversely affect blood flow in the legs, possibly causing the increased risk of venous thrombo-embolic diseases (VTE) found in air travellers.
A 2007 study found a significant correlation between travelling by air with a 320% increased risk of developing VTE disease. Moreover, this study also showed that shorter travellers, when compared to averaged sized travellers, had a further 263% increased risk of developing VTE.
For this present study, fifteen healthy adult subjects with uncomplicated popliteal/femoral veins were recruited. They sat at three different seat depths and with three degrees of foot support: (1) unsupported (dependent), (2) extended and (3) supported. Doppler ultrasound was used to record cross-sectional diameters of the popliteal/ femoral vein at seven points distributed from the popliteal fossa, behind the knee, to a point 2 cm distal to the sapheno/femoral junction, in the upper thigh. The peak systolic velocities (PSVs) and time averaged mean velocities (TAM) of the venous blood were recorded and the volume flow rate of blood, calculated.
The femoral vein must pass through he adductor hiatus, a small rectangular opening in the adductor magnus tendons. The adductor hiatus was discovered to be the site of dramatic decreases in lumen diameter and corresponding large increases in PSV in every subject as they sat with their legs unsupported. The constriction extended at least 5 cm distal and proximal to the adductor hiatus. There was a 68% mean decrease in venous volume flow rate at one measurement point, distal to the adductor hiatus.
One subject was excluded from the main study because she had incompetent valves in her calf muscle. When she sat with unsupported legs, blood flow ceased in a 12 cm section of her popliteal/ femoral vein. This was unexpected and concerning.
When the legs of the 15 subjects were unsupported, an acute narrowmg formed in thefemoral/popliteal vein. Conversely, when their legs were correctly supported, normal hemodynamics were restored. This may help explain the increased risk of VTE experienced by airline passengers. The current design of economy class seats may preclude many passengers from sitting with fully supported legs. Therefore, it is recommended that seating designers, engineers, manufacturers, researchers and consumers become conscious of the effects that seat designs and seating postures have on the return of the blood to the heart. The resultant seat designs should allow unimpeded venous blood flow in the legs of people, regardless of their height.