Improved accuracy and precision of thermodilution cardiac output measurement using a dual thermistor catheter system
Kenneth G. Lehmann, MD, FACCa and
Miriam S. Platt*
a Sections of Cardiology, University of Washington School of Medicine, Seattle, Washington, USA
* Veterans Affairs Puget Sound Health Care System, Seattle, Washington, USA
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Figure 1 Schematic depiction of set-up used and location of the injectate temperature sensor (thermistor) for each group. For cooled injections using the standard technique (top), the thermistor was either: 1) affixed to the outside of the cooled injectate bag which was then suspended in an ice slurry (Cooled Group 1); 2) suspended in an ice slurry adjacent to cooled injection syringes prefilled with injectate (Cooled Group 2); or 3) mounted inside the injection syringe apparatus to record the temperature of the cooled injectate as it exited the syringe (Cooled Group 3). For ambient temperature injections using the standard technique (not shown), the injectate thermistor was placed in a water bath maintained at room temperature. For all injections using the dual thermistor catheter technique (bottom), the injectate thermistor was mounted in contact with the injectate lumen of the thermodilution catheter to allow measurement of temperature just before the solution entered the right atrium.
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Figure 2 Plot of the error expected from averaging a varying number of serial cardiac output measurements to arrive at a single value. A value based on averaging five serial measurements was assigned an error of 0%. Only data derived from ambient temperature injections were used for this plot. Regression lines represent second-order polynomial functions.
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Figure 3 Each horizontal line represents a single injection with the two points defining the ends of each line representing the two simultaneous thermodilution measurements (one using the standard technique, one using the dual thermistor technique). The values on the x-axis are computed as the difference between the single injection and the mean of all five injections for an individual patient. The 240 ambient temperature injections are shown on the left graph, and the 240 cooled injections are shown on the right. Note that many injections are represented by short lines that don’t cross the x = 0 vertical axis (that is, the two measurements are similar in magnitude and are either both positive or both negative). With these injections the two techniques provide similar results. In contrast, the injections represented by lines that are long and/or cross x = 0 reveal substantial divergence between the two measurements. Also note that the lines in the right graph tend to be shorter in length and more clustered near the midline, reflecting the slightly better reproducibility between techniques for cooled injections.
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Figure 4 Histogram depicting the distribution of warming of the injectate as it traveled down the length of the tubing and thermodilution catheter to the injection port positioned in the right atrium. Only the 240 ambient temperature injections are shown on this graph.
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Figure 5 Scatter plot showing the temperature increase of the injectate during its travel from point of measurement outside the body to the point of injection into the right atrium. Only cooled injections are shown with thin lines representing each of the 240 individual injections and long thick lines the means of each group.
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