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Large radio-opaque clots introduced into the pulmonary circulation of anaesthetized dogs cause no reflex cardiovascular or respiratory disturbance at the time of impaction in the pulmonary arteries, and subsequent radiographs of the chest and histological studies show rapid absorption of the clot (Allison, Dunnill, and Marshall, 1960). The structure of a thrombus formed in a blood vessel in vivo in the presence of flowing blood is different from that of a clot formed in a test tube. Whereas a clot consists of a fibrin mesh enclosing red cells, a thrombus consists mainly of layers of fibrin and platelets. The immediate effect of impaction and the natural history of a thrombus might be different from that of a clot. The present study was undertaken to observe the effects of embolization by large thrombi in dogs and to follow the changes in lung function over the succeeding days. Information was also sought on methods which could be used to detect the occurrence of pulmonary embolism in man. METHODSMongrel dogs of 10 to 19 5 kg. were used. For the operative procedures and for the breathing tests they were premedicated with morphine, 11 mg./kg., and chlorpromazine, 1-0 mg./kg., and anaesthetized with pentobarbitone. Thrombi were produced in the inferior venae cavae by painting the wall of the vessel with pure phenol. The inferior vena cava was exposed through a mid-line abdominal incision, and all tributaries but one between the iliac veins and the renal veins were tied off. A small polythene catheter was passed through the remaining tributary into the inferior vena cava and tied into place. A clamp was placed across the inferior vena cava just above the iliac veins, the inferior vena cava was emptied of blood, and another clamp was placed just below the renal veins. One millilitre of pure phenol was introduced into the inferior vena cava through the poly-1 Assistant professor of surgery, Johns Hopkins Hospital 2 Nuffield Foundation Dominion travelling fellow thene catheter, massaged around the lumen for one minute, and then withdrawn. The vessel was washed out with several changes of saline, the catheter was removed, and the clamps were released. Studies on another series of dogs have shown that after phenol treatment of the inferior vena cava thrombus formation is present by the following day and usually causes occlusion of the vein in five days (Sheil and Sabiston, to be published). Twenty dogs were used in the present series. A large thrombus was produced in the inferior vena cava after one application of phenol in 11 dogs, and in a further three dogs a second application was required to produce the thrombus. In six dogs only a thread-like thrombus was produced and the experiments were not continued. Of the 14 dogs in which a good thrombus was formed, follow-up breathing tests were not made on the first three, mainly because bronchospirometric studies at the time when the clot was passed on did not show any appreciable changes in differential oxygen uptake.

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