Cardiac Output Technologies with Special Reference to the Horse

Abstract

Critical illness, anesthesia, primary cardiovascular disease, and exercise may result in marked hemodynamic alterations. Measuring cardiac output (CO) is central to defining these alterations for both clinician and researcher. In the past 10 years, several new methods of measuring CO have been developed for the human medical market. Some of these methods are now validated in the horse and are in clinical use. The Fick method has been used in equine research for more than a century. It depends on simultaneous measurement of mixed venous (pulmonary arterial) and peripheral arterial oxygen content and oxygen uptake by the lungs. The technique is technically demanding, which restricts its clinical use. Indicator dilution techniques, with indocyanine green, cold (thermodilution), or lithium as the marker, have also been widely used in the horse. The indocyanine technique is cumbersome, and thermodilution requires right heart catheterization, which is not a benign procedure, making both of these methods less than ideal for clinical use. Lithium dilution requires catheterization of a peripheral artery and a jugular vein. It has recently been validated in anesthetized adult horses and neonatal foals. Doppler echocardiography is a noninvasive ultrasound-based technique. More accurate measurements are obtained with transesophageal than with transthoracic measurements; however, both methods require considerable technical expertise. Bioimpedance and pulse contour analysis are 2 new methods that have yet to be validated in the horse. With the currently available technology, lithium dilution appears to be the method of measuring CO best suited to the equine clinic.Key words: Doppler echocardiography; Fick; Hemodynamics; Indicator dilution techniques; Lithium dilution; Thermodilution. The cardiovascular system has a great capacity to adapt to the requirements for oxygen delivery created by different physiological circumstances. In athletic animals, such as the horse, this ability is very well developed, and cardiac output (CO) may be increased to more than 8 times its resting value during intense exercise.1 This ability to adapt may well be a component of the response to disease, when tissue oxygen demand may be increased. However, many disease states result in hemodynamic derangements that can interfere with this adaptive response and often result in inadequate tissue perfusion. Currently, clinical evaluation of the hemodynamic status in patients consists of repeated physical examinations, emphasizing assessment of heart rate, peripheral pulse quality, temperature of extremities, and mucous membrane color and refill time. Ancillary diagnostics, such as indirect blood pressure, hemoglobin saturation by pulse oximetry or blood gas analysis, and blood lactate measurement, are also performed. Measurement of CO would contribute to a more complete understanding of the state of the cardiovascular system but, in the past, has been difficult to perform.There are 4 main situations in which knowledge of CO would be valuable in the horse...

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“…In humans, CO and SV are indexed using calculated body surface area in m 2 , 20 which generally is not used in horses. 21 During the study period, urine output was measured with a closed collection system attached to the indwelling urinary catheter. The urine collected was analyzed for specific gravity with a hand-held refractometer.…”