Seasonal variation in the incidence of deep vein thrombosis

Two double-blind controlled clinical trials of the effectiveness of prophylactic low dose subcutaneous calcium heparin (dose based on body weight) in the prevention of deep vein thrombosis (DVT) have been completed. The first was concerned with upper abdominal operations in 242 patients over 21 years of age, and the second with 50 patients presenting with a fracture of the neck of the femur. There was no increase in the incidence of bleeding or wound complications in the patients given heparin. In each trial, the incidence of DVT as diagnosed by 125I-labelled fibrinogen was significantly reduced in the treated group. The incidence of DVT in the control groups varied significantly during the period of the trials. The incidence was much higher in the cold half of the year than in the hot months. In the first trial, this variation in incidence was directly correlated with the average temperature and the diurnal variation in temperature in the perioperative period. These results may help to explain the considerable variation in the incidence of postoperative DVT reported from various parts of the world, and also from within Australia.