Cardiac Output Measurement by Partial Carbon Dioxide Rebreathing, 2‐Dimensional Echocardiography, and Lithium‐Dilution Method in Anesthetized Neonatal Foals

The objective of this study was to assess 2 noninvasive methods of measuring cardiac output (CO) in neonatal foals by comparing results to that of the lithium‐dilution method. Ten neonatal foals were anesthetized and CO was manipulated by varying the depth of anesthesia and infusion of dobutamine. Concurrent CO measurements were obtained by lithium dilution (reference method), partial carbon dioxide (CO 2 ) rebreathing, volumetric echocardiography (cubic, Teichholz, Bullet, area‐length, and single and biplane modified Simpson formulas), and transthoracic Doppler echocardiography. Thirty pairs of lithium‐dilution/noninvasive CO measurements were taken from the 10 foals. For each method, relative bias was calculated as a percentage of the average CO. Lithium determinations of CO ranged between 3.09 and 11.1 L/min (mean ± SD = 6.39 ± 2.1 L/min), resulting in cardiac indices ranging between 79.0 and 209 mL/kg/min (mean ± SD = 131 ± 35.9 mL/kg/min). Relative bias of Doppler echocardiography significantly increased ( P < .05), whereas that of partial CO 2 rebreathing significantly decreased ( P = .03) with increasing CO. Other methods were not influenced by the level of CO. Among methods not influenced by the level of CO, relative bias of the Bullet method (‐4.2 ± 20.9%; limits of agreement ‐45.2 to 36.7%) was significantly lower ( P < .05) than that of each of the other noninvasive methods evaluated. Volumetric echocardiography using the Bullet method provides an accurate and noninvasive estimate of CO in anesthetized neonatal foals and warrants investigation in critically ill conscious foals.