Clinical experience with the use of computers for calculation of cardiac output

Underestimation of cardiac output by thermodilution in patients with tricuspid regurgitation

INTRODUCTION

This study was done to assess the accuracy and reliability of the thermodilution technique in measuring cardiac output in patients with tricuspid regurgitation.

PATIENTS AND METHODS

In 30 subjects (17 men, 13 women, aged 50 +/- 14 [mean +/- SD] years), cardiac output was measured in close temporal proximity by thermodilution as well as Fick or indocyanine green dye, after which the presence and severity of tricuspid regurgitation were assessed by contrast right ventriculography or pulsed Doppler echocardiography.

RESULTS

In the 13 patients without tricuspid regurgitation, there was excellent agreement between the results of thermodilution and Fick or indocyanine green dye cardiac output determinations (4.95 +/- 1.19 liters/minute by thermodilution, 4.90 +/- 1.11 liters/minute by Fick or indocyanine green dye; NS). In contrast, in the 17 patients with tricuspid regurgitation, the results of thermodilution were consistently lower than those of Fick or indocyanine green dye (4.22 +/- 1.45 liters/minute by thermodilution, 4.99 +/- 1.67 liters/minute by Fick or indocyanine green dye; p less than 0.001).

CONCLUSION

Thus, the thermodilution technique of measuring cardiac output is inaccurate in patients with tricuspid regurgitation, yielding results that are consistently lower than the actual outputs.

Oxygen consumption in adult patients during cardiac catheterization

The determination of cardiac output by the Fick principle requires the quantitation of the arteriovenous oxygen (O2) content difference across the pulmonary bed and the O2 consumption. Since a correct measurement of O2 consumption may be difficult, an assumed value is sometimes substituted into this equation. Despite this practice, there is no appropriate reference standard for O2 consumption applicable to the adult catheterization laboratory. Therefore, we determined O2 consumption in 108 patients (69 men and 39 women, mean age, 49 years) at the time of catheterization by multiplying the arteriovenous content difference by the indicator dilution cardiac output, thereby eliminating errors related to the collection and analysis of the timed collection of expired air. O2 consumption index (O2 consumption/m2 of body surface area) averaged 126 +/- 26 ml/min/m2 (mean +/- SD), but there was wide variability(65-250 ml/Min/m2). O2 consumption was not affected by patient age or sex but was lower in those who were mildly sedated before catheterization (sedated, 121 +/- 22 ml/Min/m2; nonsedated, 132 +/- 28 ml/min/m2; p = 0.023). Thus, O2 consumption varies greatly among adults at the time of cardiac catheterization, so that any attempt at estimating this variable is fraught with major potential error.

Computer evaluation of human circulation based on non-invasive methods

A discriminating program was developed on the basis of time, dynamic and simply calculated parameters of non-invasive tracings recorded in the supine position. Data were derived from ECG, PCG and the indirect carotid pulse curve. The optimal program, formed after 40 experimental processes, was in 85% agreement with the clinical diagnosis. To improve the decision process, we created a new 'test again' group, in addition to the healthy and sick groups. The 'test again' group included 16.5% of the examined subjects. At the same time, there was 75.6% agreement with the clinical diagnosis, and 7.9% disagreement. The risk factors, which could be demonstrated as part of the 'errors' called attention to undetected heart failure. The descriminating function found to be best, was fed into a small computer (R-10). Records for evaluation were entered on magnetic tape to the computer which measured automatically the necessary parameters and printed out the 'decision': 'healthy', 'test again!', or 'cardiac patient', as well as other data, such as systolic time intervals, etc. There is a wide potential application for automated computer system based on non-invasive parameters.

Multiple cardiac output measurements in man. Evaluation of a new closed-system thermodilution method

Cardiac output estimates by a new closed-system automatic injection thermodilution method (COI-TD) were compared serially with the direct Fick technique (COFICK) and the standard open-system manual injection thermodilution method (COTD). Comparison with cardiac outputs determined simultaneously by the direct Fick technique in 100 measurements involving ten patients showed close agreement with the new closed system method using both 25 degrees C and 3 degrees C injectates. The cardiac output range was between 1.9 to 11.6 L/min. The open-system manual injection thermodilution method under identical conditions produced a wide scatter of measurements when compared to the direct Fick technique. Reproducibility of simultaneous COI-TD and COTD measurements was examined in quadruplicate. The reproducibility of measurements was within 1.9 percent with COI-TD and 5.9 percent with the COTD method using both 25 degrees C and 3 degrees C injectates. The COI-TD method eliminates the technical problems of recirculation, unstable indicator baseline changes, thermal continuity, and reproducibility encountered with the current COTD method. Analysis of cardiac output by the closed-system automatic injection thermodilution method provides a simple, rapid, reproducible, and highly accurate method for multiple cardiac output measurements suitable for use at the bedside.

Automatic data processing in the cardiac catheterization laboratory

A review of automatic data processing in the cardiac catherization lab is presented. The emphasis placed on on-line manometry, indicator dilution and off-line quantitative ventriculography. The system organization is described and several specific examples given to illustrate the level of detail necessary to specify such a system. The clinical use of the system is described together with an evaluation of the original design goals in terms of the actual performance of the system. A few unexpected benefits have emerged with respect to quality control and reliability, but one of the original design goals, speeding up the catheterization procedure, has proven to be unrealistic. The next logical steps in the software and hardware evolution are described with the emphasis placed on making the system a more effective tool for the clinician.

Effect of nitroglycerin on cardiovascular function at fixed and at variable heart rate

Images