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It remains a great challenge to measure systemic blood flow in critically ill newborns, especially during the transitional period with intracardiac and extracardiac shunts. Due to technical restraints, size limitations, the necessity for a relatively large amount of blood withdrawn and possible indicator toxicity, many methods of cardiac output monitoring are not feasible, hence cardiac output is generally estimated from indirect parameters of systemic blood flow. In a former study we assessed the agreement for cardiac output using the ultrasound dilution method (UDCO) and ultrasound transit time-based measurement of main pulmonary blood flow in a juvenile piglet model without shunts . In the present study we analyzed the influence of a left-to-right shunt on the agreement between UDCO and ultrasonic transit time pulmonary blood flow in a juvenile lamb model.
In this prospective, experimental animal study, which was approved by the Ethical Committee on Animal Research of the Radboud University Nijmegen, we placed a Gore-Tex® shunt between the left pulmonary artery and the descending aorta in eight randombred newborn lambs (3.5 to 8.3 kg). This aortopulmonary shunt was intermittently opened and closed while cardiac output was manipulated by creating hemorrhagic hypotension. Ultrasound dilution cardiac output (Q-UDCO) - using repeated injection of 1.0 ml/kg isotonic saline at body temperature - was compared with pulmonary blood flow (Q-MPA) invasively measured by a perivascular flow probe around the main pulmonary artery.
A total of 220 measurements were performed. Bias, defined as Q-UDCO minus Q-MPA, was calculated for each measurement. With an open shunt there was a significant left-to-right shunt (mean Qp-/Qs- ratio 1.8; range 1.6 to 2.6). Mean bias (SD) was 6.80 (18.0) ml/kg/minute with a closed shunt and 11.1 (19.8) ml/kg/minute with an open shunt. Limits of agreement (±1.96 SD) were ±35.2 ml/kg/minute and ±38.8 ml/kg/minute, respectively. Percentage error was 22.0% and 24.2% for measurements with a closed and open shunt, respectively.
Cardiac output measurement with the UDCO method is reliable and easily applicable in ventilated juvenile lambs, even in the presence of a significant left-to-right shunt.