Changes in stroke distance in response to haemorrhage in a swine model

OBJECTIVE: To determine, using an animal model of blood loss, (1) if stroke distance, derived non-invasively from the time integral of the maximum velocity of red cells in the aorta, changed to a greater extent than heart rate and mean arterial pressure (MAP), which are recognised to be unreliable indicators of blood loss; (2) if changes in stroke distance reflected changes in stroke volume derived from thermodilution cardiac output measurements. METHODS: Eight anaesthetised swine had baseline measurements of heart rate, MAP, stroke volume, and stroke distance and were then exsanguinated at a rate of 1 ml/kg/min. Percentage changes from baseline of heart rate, MAP, stroke volume, and stroke distance were compared after 10, 20, and 30 ml/kg blood loss. The animal's blood was then reinfused at the rate of 2 ml/kg/min for 15 min, followed by normal saline 1 ml/kg/min. Percentage changes from baseline measurement of stroke volume and stroke distance over the whole experiment were evaluated by regression analysis. RESULTS: Heart rate, MAP, and stroke distance changed +7.9%, -22.5%, and -18.1% respectively (from baseline values) after 10 ml/kg blood loss; +23.2%, -44.0%, and -47.4% after 20 ml/kg blood loss; and +55.7%, -62.0%, and -69.8% after 30 ml/kg blood loss. Regression analysis of percentage changes in stroke volume and stroke distance from their baseline values at experimental time zero is stroke volume = 1.014 x stroke distance -2.156, r = 0.92, n = 54, P < 0.0001. CONCLUSIONS: (1) At maximal blood loss, stroke distance changes to a greater extent than heart rate and MAP. (2) Changes in stroke distance reflected changes in stroke volume but with less variability at lower values. Stroke distance may be a more useful measure of blood loss than heart rate and MAP.