Gary F. Maruschak

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 thermistors 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.

Overestimation of pediatric cardiac output by thermal indicator loss.

The accurate measurement of pediatric cardiac output by thermodilution requires that the quantity of cold indicator introduced into the central circulation be known. This study defines an important source of error in the correction factor for the amount of heat gained by small volumes of cold injectate during passage through pediatric catheter systems. This error may result in significant overestimation of cardiac output (as much as 59%) when blood at body temperature is withdrawn into the injection lumen of the pediatric catheter before the injection.

Correction factor for thermodilution determination of cardiac output in children.

The authors determined the correction factor (Ct) for three sizes of commonly available central venous catheters. These catheters are used as the injectate catheter when a 2.5 Fr transthoracic thermistor is used to determine cardiac output by thermodilution. The valves for Ct were highly reproducible. These determinations can be used conveniently in children to determine cardiac output by thermodilution.