Prolonged immobilization increases risk of DVT -- how do you define prolonged immobilization?

Acquired risk factors for deep vein thrombosis (DVT) can be summarized in the mnemonic SICC PT:

Surgery
Immobilization
Cancer
Contraceptives

Pregnancy
Trauma

Prolonged immobilization can cause circulatory stasis which is one of the predisposing factors for DVT described by Virchow in his famous triad: endothelial injury, stasis and hypercoagulability.

How do you define prolonged immobilization?

According to the literature search I did:

- For travelers, prolonged immobilization is defined as a flight longer than 6-10 hours (most studies use a cut-off of 8 hours)

- For nursing home residents, the definition of prolonged immobilization varies between less than 15 days (increased risk of DVT) and/or longer than 3 months (no increased risk)

Nursing home residents with DVT

According to one study, the most powerful risk factor for DVT in patients older than 65 years was prolonged bed rest for up to 14 days, with a 5.6 times greater risk of DVT.

Travel-associated DVT

In a trial of previously healthy patients who traveled at least 8 hours per flight (median duration 24 hours), duplex ultrasound showed an asymptomatic DVT in 10 % of participants. In other studies, the reported risk of symptomatic DVT after flights of more than 12 hours was 0.5%. According to a 2006 Lancet study, activation of coagulation occurs in some individuals after an 8-hour flight.

Prevention of travel-associated DVT

A review of 10 randomized trials concluded that the use of compression stockings on flights lasting 4 hours or more led to a substantial reduction in asymptomatic DVT. There is no evidence that aspirin is helpful in this setting.

References

Risk Factors for Deep Vein Thrombosis in Inpatients Aged 65 and Older: A Case-Control Multicenter Study. Journal of the American Geriatrics Society 52 (8), 1299–1304, 2004.
Overview of the causes of venous thrombosis. UpToDate, 15.2, accessed 08/28/2007.
Literature Review of Deep Vein Thrombosis in Air Travellers. The Internet Journal of Surgery. 2007. Volume 10 Number 1.
Coagulation May Be Activated by Prolonged Air Travel. Medscape, 2006.
Travel and the risk of symptomatic venous thromboembolism. Thromb Haemost. 2003 Mar;89(3):499-505.
Is prolonged immobilization a risk factor for symptomatic venous thromboembolism in elderly bedridden patients? Results of a historical-cohort study. Thromb Haemost. 2004 Mar;91(3):538-43.
Image source: Saphenous vein, Gray's Anatomy, 1918 (public domain)

Further reading

Massive Lower Extremity DVT Treated with Thrombolysis
Suspected DVT - To Treat or Not to Treat?
INR 17 and Hematuria: What To Do?
Who is at risk for venous thrombosis on long haul flights?. Retired doc's thoughts.

The risk of VTE (blood clots) is 3 times higher in passengers on long-distance flights than in the general population http://goo.gl/Tk45Z
Exercises for air travel — Cleveland Clinic Journal of Medicine http://goo.gl/YYZHy