Extracorporeal Arteriovenous Ultrasound Measurement of Cardiac Output in Small Children

Theodor S. Sigurdsson, M.D.; Anders Aronsson, M.D.; Lars Lindberg, M.D., Ph.D.

Disclosures

Anesthesiology. 2019;130(5):712-718. 

In This Article

Discussion

In the present study, we found that repeated CO measurements by the tested method, which uses saline bolus injections and ultrasound detection of the expected blood dilution (COUD), showed a similar precision as our reference method, a perivascular flow probe around the aorta (COPVFP). We believe that CO measurement with a periaortic flow probe is the most accurate technique to be used for comparison studies using repeated CO measurements in vivo. There was a small but significant bias of the measured CO between COUD and COPVFP according to the Bland–Altman analysis of 0.08 l/min. This finding of a lower COPVFP compared with COUD is consistent with a coronary blood flow of approximately 4 to 10% of the measured CO, which agrees with suggestions from earlier publications.[27]

Earlier studies using perivascular flow probes in animals have applied the flow probe around the pulmonary artery as a measurement of total CO, which is also measured by COUD. This had been our intention, but we discovered that the peripulmonary flow probe tended to compress the right coronary vessels, resulting in a reduction of CO. This led us to abandon its use for the simultaneously repeated CO measurements and rely solely on the blood flow probe around the aorta, although the total CO missed the drain by the coronary blood flow.

There will always be a physiologic variability in CO caused by ventilation and cardiac filling, which result in various degree of imprecision during the CO measurements. The COPVFP analysis recorded beat-to-beat fluctuations in CO caused by the ventilation, changes in cardiac filling, and changes in coronary blood flow. This can be the reason for a slightly but not significantly higher coefficient of error (2.5%) compared with CO measured with COUD (1.8%). In addition, the dilution curve, which is used by the COUD to calculate CO, reflects a mean of several heart beats, which may give a more stable value. We suspected that five consecutive repeated injections of 0.5 to 1.0 ml/kg of saline could decrease the precision of the COUDanalysis, because of a potential dilutional effect on the blood by multiple saline injections, but this did not affect the precision of our analysis. Because we conducted a trial with a high number (n = 5) of repeated measurements and did not remove any measurements, we believe that our analysis is accurate and indicates that the dilution of the blood with bolus doses of saline and detection by ultrasound sensors in a constant extracorporeal arteriovenous loop flow gives stable CO measurements compared with, for example, thermodilution, which still is regarded to be the gold standard for CO measurement in children and a reference method in validation studies.[28,29]

Although our analysis was negatively influenced by the bias caused by the coronary blood flow between COUD and COPVFP, our results show a percentage error of 26.6%, which is less than the 30% limit, that has been concluded by Critchley and Crithchley[26] as an acceptable limit of a new technique to be equivalent to the reference method.

One limitation to the tested technology is that the arterial blood pressure monitoring has to be closed during the CO measurement. The device, in its present form, only allows for intermittent but not continuous CO measurements as would be preferable for a monitoring device. In addition, one central venous line is occupied for the injection and the loop circulation during the measurement. However, there is no patient blood loss because all blood in the arteriovenous loop is flushed back into the circulation after measurements. Only small-volume (0.5 to 1.0 ml/kg) body-temperature saline boluses are needed to do the measurements, and there is no drop in heart rate as often is seen during measurements with a thermodilution catheter because of cold saline boluses.

In summary, the tested technology, which uses a saline bolus into the venous side and ultrasound sensors on an extracorporeal loop to detect the blood dilution both on the venous and arterial side for calculation of CO, has a precision comparable with the precision obtained by continuous CO measurement by a periaortic flow probe in young children. A Bland–Altman plot shows that the tested method detects a small expected bias between COUD and COPVFP caused by the coronary blood flow. It is easy to apply clinically in children with arterial and central venous catheters in place. The technology could potentially be a promising alternative as a reference method for comparison studies of CO in young children.

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