LEIDEN, NETHERLANDS—Researchers from Leiden University Medical Center have invented a new thermodilution method—the single duration-controlled injection technique—that simplifies cardiac output measurements without compromising precision.[1] It involves giving one thermodilution injection at a constant velocity over a ventilatory cycle.

The new technique is therefore less complex than the conventional phase-controlled method, which requires giving four injections (spread equally over a ventilatory cycle) and averaging the resulting cardiac output measurements. Jansen et al recently compared their new thermodilution technique to the conventional one and found it to be just as accurate—and more repeatable. They asserted, therefore, that its clinical use is feasible.

SERIES OF MEASUREMENTS

The study included 33 adults, 24 of whom were scheduled for coronary artery bypass graft surgery. Four were scheduled for valvular surgery, and five were being treated in the intensive care unit (ICU).

Each patient underwent seven cardiac output measurements during a period of about 10 minutes; the first one and the last two were obtained with the new technique, whereas the remaining four were obtained conventionally. The researchers let five to seven ventilatory cycles elapse between measurements. Thermodilution injections consisted of 10 mL of iced 5% glucose to obtain a maximal signal-to-noise ratio.

To calculate cardiac output, the researchers used a version of the Stewart-Hamilton equation adapted to reduce errors from temperature baseline variations and ventilator-associated blood flow changes. Cardiac output measurements were always taken while patients were hemodynamically stable, which was defined as no more than a 5% difference in average heart rate and arterial blood pressure between the first and last measurements. When that difference exceeded 5%, the patient was considered unstable; the measurements were repeated after hemodynamic stability was restored.

SIMILAR DEGREE OF ACCURACY

There was no statistically significant difference between the averaged cardiac output values obtained conventionally and the measurements derived with the new technique. Moreover, the new technique produced highly consistent results; the mean standard deviation was only 0.15 L/min and the coefficient of variance was only 3%.

Cardiac output was higher (as measured with both techniques) in the ICU patients than in the surgical patients, the researchers noted. However, because of the small number of patients in their study, they were unable to determine whether the results with the new technique were as reliable in subgroups as they were in the patients overall. More research is needed to address this issue, as well as the accuracy of the new technique under less controlled conditions.

The study has other limitations as well, stated Michael R. Pinsky, MD, in an accompanying editorial.[2] First, most ICUs do not use 10 mL of iced injectate for thermodilution. “Thus, it is not clear if [the new] technique would maintain its accuracy if 5 mL of room temperature injectate were used instead,” said Dr. Pinsky, a Professor of Anesthesiology and Critical Care Medicine at the University of Pittsburgh.

Also, most ICUs do not keep patients sedated, paralyzed, and on controlled ventilation, as they were in this study. And, the researchers measured cardiac output only at one point in time and thus did not address whether the new technique can measure cardiac output changes.

Perhaps the most important question concerns not the new technique but the lack of a clear definition of cardiac output. “We have no specific target values for cardiac output, and there is no proof that any specific value improves survival,” Dr. Pinsky told PULMONARY REVIEWS. “So, what we have got is a more accurate way to measure a parameter that we do not know how to use.” Nevertheless, he believes that the study by Jansen et al is important because it provides a simple technique for obtaining accurate cardiac output measurements at the bedside—the first step towards answering questions regarding the value of cardiac output measurements.

—Timothy Begany

References
1. Jansen JRC, Schreuder JJ, Punt KD, et al. Mean cardiac output by thermodilution with a single controlled injection. Crit Care Med. 2001;29:1868-1873.
2. Pinsky MR. A rose by any other name: cardiac output. Crit Care Med. 2001;29:2021-2022.

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