ObjectiveTo assess the applicability of a new technology in neonates. Transtracheal Doppler and extravascular Doppler determinations of stroke volume and cardiac output were compared with thermodilution measurements at various states of volume loading in an animal model.DesignProspective, descriptive study.SettingAnimal research laboratory at a university medical center.SubjectsFourteen newly weaned piglets, weighing 2.8 to 6.5 kg.InterventionsDoppler probes were placed on the endotracheal tube tip (transtracheal Doppler) and directly on the aortic adventitia (extravascular Doppler). A 4-Fr thermodilution catheter was inserted in the pulmonary artery. Stroke volume and cardiac output determinations were recorded at baseline, after a 15-mL/kg volume load and after successive 15-mL/kg blood withdrawals to exsanguination or a systolic blood pressure of less than 20 mm Hg.Measurements and Main ResultsTranstracheal and extravascular Doppler measurements of cardiac output were not significantly different from thermodilution at any physiologic state. These techniques were able to measure stroke volumes and cardiac outputs at the low levels seen in severe hemorrhagic shock.ConclusionsTranstracheal Doppler and extravascular Doppler measurements of cardiac output compare favorably with thermodilution. These methods effectively followed trends from alterations in intravascular volume, even at very high heart rates and small stroke volumes. Transtracheal Doppler and extravascular Doppler should yield useful information in critically ill neonatal patients, where data regarding stroke volume and cardiac output may be useful in clinical management.(Crit Care Med 1995; 23:2015-2022)