Limitations of thermodilution ejection fraction: degradation of frequency response by catheter mounting of fast-response thermistors.

The present study examined whether catheter-mounting of a fast-response thermistor impaired the thermistor's ability to measure rapid temperature changes during thermodilution measurement of ejection fraction (EF). The response to a square-wave temperature change of six fast-response thermistors mounted on commercially available, pulmonary artery balloon-flotation catheters was compared to the response of a similar but unmounted fast-response thermistor. The response of the catheter-mounted fast-response thermistors recorded only 82% to 92% of a step-temperature change at 0.5 sec, and 88% to 96% of the step change at 1.0 sec. In contrast, the unmounted fast-response thermistor responded to 100% of the step change in 125 msec. The response of the catheter-mounted fast-response thermistors demonstrated an important slow component (second time constant) introduced by the catheter body, so that equilibration to a temperature change was not complete for about 6 sec. This slow equilibration lowered EF measured by thermodilution below true EF.