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Can an intermittent pneumatic compression system monitor venous filling in the leg? OBJECTIVE: Intermittent pneumatic compression (IPC) systems are used for prophylaxis of venous thromboembolism. Both legs are wrapped with inflatable sleeves connected to a pneumatic controller to allow compression of the legs causing expulsion of venous blood. Venous refill between inflation periods causes leg expansion, which can be tracked by measuring pressure changes in the sleeve. The aim of our study, which utilized the SCD RESPONSE compression system in conjunction with an independent pressure transducer, was to investigate whether factors such as temperature changes within the sleeves during inflation and deflation affect the measured venous refill time (VRT). METHODS: Transducers were used to measure air pressure in the middle chamber of the sleeve. A thermocouple was also inserted into the bladder to measure temperature changes. Inflation, deflation and refill measurements were made with the sleeves around model systems (static, rigid plastic pipes or compliant paper rolls, and dynamic, latex tubes inserted between a rigid pipe and the sleeve to simulate veins) and on 10 subjects in semi-recumbent, supine and sitting positions. RESULTS: In all the experiments the maximum temperature change was 0.023 degrees C. With the static model systems, the pressure in the venous refill measuring bladder fell from the inflation pressure of 40 - 50 mmHg to 9 +/- 1 mmHg, but then rose by 2.1 +/- 0.2 mmHg (rigid pipes) and 1.4 +/- 0.2 mmHg (paper rolls). These pressure changes were associated with reported 'filling times' of 21 - 24 s (rigid pipes) and 22 - 29 s (paper rolls). In experiments on dynamic filling of the latex tube, there was a strong linear relationship between the filling time indicated by the SCD system and the time to empty the filling reservoir. In 170 measurements on human subjects, there were only three VRTs less than 30 s and 36 less than 35 s. VRT increased in all subjects when going from supine (34.6 +/- 1.8 s) to semi-recumbent (38.9 +/- 1.9 s) to sitting (42.6 +/- 0.9 s) positions. DISCUSSION: In all cases, temperature changes during the refill phase were too small to result in significant pressure changes that would affect VRT. The pressure increases observed with the static models after deflation appeared to be due to viscoelastic relaxation. Viscoelastic responses were present in human subjects, but the effect on VRT was negligible. This indicates that the increased VRT observed in humans is due to blood return. Body position affected VRTs, indicating the system's ability to detect changes in filling times and venous blood volume. |
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