Cardiac output provides a useful measurement of cardiovascular function and the capacity for whole-body oxygen delivery. In human medicine, this information is used extensively during anesthesia1 and to monitor critically ill patients.2 Recent advances in techniques are making similar information available to veterinary clinicians.3–5 Cardiac output measurements are also useful in describing normal physiologic responses to exercise. In horses, measurement of CO could be extremely useful in the diagnosis of potential performance-limiting cardiac abnormalities that may only manifest during exercise. A number of methods exist to measure CO in horses.6 These include the indicator dilution methods (ie, thermodilution, indocyanine green, and direct impedance), echocardiography, and the direct or indirect Fick principle.7 With the exception of the direct Fick principle, these techniques have been found to be unsuitable in maximally exercising horses. Although the Fick principle provides an accurate, reproducible measurement of CO in horses during exercise, technical requirements make it unsuitable for most uses in a clinical setting. Requirements for a system to measure O2 (a facemask, the capability of measuring expired oxygen concentrations, and, most importantly, a tractable horse), combined with the need for invasive placement of a catheter in the pulmonary artery, prevent its use in most situations other than research settings.
The development of the lithium dilution method8 may provide a possible alternative to the Fick principle to measure CO in exercising horses. The lithium dilution method is a form of indicator dilution in which lithium chloride is used as the injectate. In this method, a known volume and concentration of lithium is rapidly injected into a vein and the lithium concentration is measured from a peripheral artery by a sensor. The LiDCO computer generates a lithium concentrationtime curve, and CO is calculated from the area under the curve by use of the Stewart-Hamilton equation.9,10 Thus, the dilution of the lithium is directly proportional to the total amount of blood passing through the heart (ie, CO). The advantage of this technique over other indicator dilution techniques is its reduced invasiveness. It requires catheterization of only a peripheral vein11,12 and artery and avoids the need to catheterize the pulmonary artery. The suitability of the lithium dilution method to measure CO in human medicine has been reported,13,14 and recently, comparison with other methods in clinical veterinary patients has also been reported.3–5,15 However, the use of LiDCO during highintensity exercise in horses has yet to be investigated. Therefore, the purpose of the study reported here was to compare CO measurements obtained with the commonly used Fick principle with those obtained by use of the newer lithium dilution method in horses exercising up to maximal intensities.
Coefficient of variation
Cardiac output calculated from the Fick principle
Cardiac output determined by the lithium dilution method
Limits of agreement
Maximum oxygen consumption
Classic 4000, Walmanik Inc, Freedom, Pa.
Sigma Chemical Co, St Louis, Mo.
Physitemp Instruments Inc, Clifton, NJ.
DATAQ, Akron, Ohio.
Medical gas analyzer, model 1100, Marquette Gas Analysis Corp, St Louis, Mo.
Model M1420A, Hewlett-Packard Medical Products, Andover, Mass.
Model 200B, Teledyne Hastings-Raydst, Hampton, Va.
OSM-3 radiometer, Copenhagen, Denmark.
YSI 2300, Stat Plus, Yellow Springs, Ohio.
Model 288, Ciba-Corning, Norwood, Mass.
CM 31-01, version 3.4.2, LiDCO Ltd, London, England.
Stata, version 9.2, Stata Corp, College Station, Tex.
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