A thermodilution method for quantification of bidirectional shunts

Cardiac output technologies with special reference to the horse

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.

Thermodilution cardiac output measurement with a large left-to-right shunt

Cardiac output was measured by the thermodilution method in a patient with a left-to-right shunt undergoing cardiac catheterization. It appeared that the thermodilution method measured systemic rather than pulmonary blood flow. This occurred because of the slow injection of injectate in the presence of a large left-to-right shunt. Theoretical thermodilution cardiac output curves are provided to illustrate the interaction of these two factors when three different durations of injection and four different shunt sizes are used.

Rapid method for determination of shunt ratio using a thermodilution technique

A new technique using thermodilution by which shunt ratio can be rapidly and conveniently determined in patients with left-to-right intracardiac shunts is described. A flow-directed balloon-tip thermistor catheter was positioned in the pulmonary artery to measure cardiac output in 20 patients with left-to-right intracardiac shunts. Shunt ratio measured by the Fick technique (Qp:QSF) varied between 1.3:1 and 3.6:1. Early recirculation was seen on the downslope of the thermodilution curves (TC). The first-pass curve before the point of early recirculation was extrapolated to the baseline and the area inscribed by this portion of the TC (A) was measured by planimetry. The area inscribed by the entire TC (A + B) was also measured by planimetry. Shunt size was determined by calculating the ratio (A + B):A. Shunt ratio determined by this technique (Qp:QSTh) correlated well with Qp:QSF (r = 0.89). The thermodilution technique provides a simple, rapid, and accurate method for determining the magnitude of left-to-right intracardiac shunts.