Cardiac output can be defined as the volume of blood moved by the heart per unit of time.1,2 Measurement of CO also allows for calculation of oxygen delivery, oxygen consumption, and systemic vascular resistance (Appendix).3 Monitoring of CO can be used to guide treatment, improve clinical outcome in critically ill patients, and monitor patients during anesthesia and has prognostic value.4 Changes in CO are not detected by means of routine hemodynamic monitored variables (eg, arterial blood pressure, central venous pressure, heart rate, and urine output). Furthermore, studies4,5 have revealed that those hemodynamic monitored variables were the worst predictors of survival. On the other hand, hemodynamic variables such as CI, systemic vascular resistance, and oxygen consumption have good specificity and sensitivity with regard to predicting survival and aid in early goal-directed treatment.4
Cardiac output is routinely measured in humans undergoing anesthesia in critical care units. Horses have intrinsic characteristics (body size, temperament, and unique anatomic features) that make routine use of CO in this species a challenge. Cardiac output measurement in veterinary patients is not routinely performed because of the invasiveness of currently validated procedures.3,6 The clinical gold standard of CO measurement is the TDCO method, which requires the use of a pulmonary artery catheter. Use of pulmonary artery catheters is associated with major risks including development of arrhythmias, cardiac endothelial damage, infection, knotting of the catheter in the pulmonary artery, pulmonary thromboembolism, and pulmonary artery rupture.2,3 Consequently, there has been an interest in less invasive yet accurate methods of CO measurement including the LiDCO method, esophageal Doppler ultrasonography, thoracic electrical bioimpedance assessment, and arterial pulse analysis.7–9 The values of CO provided by the LiDCO technique have excellent agreement with values of CO provided by the TDCO technique in horses, dogs, and cats.2,10,11 The LiDCO method is widely applied in veterinary medicine and is the most common technique used to study CO in anesthetized foals.9–11
The UDCO method is a novel technique that eliminates some of the drawbacks of the TDCO and LiDCO methods. The UDCO method is minimally invasive, requiring placement of only peripheral venous and arterial catheters; also, it uses a physiologic noncumulative indicator, saline (0.9% NaCl) solution.12,13 Although numerous repeated LiDCO measurements can result in considerable blood loss, there is minimal blood loss associated with numerous repeated UDCO measurements. The UDCO measurements are performed by injecting a bolus of isotonic saline solution (0.5 to 2 mL/kg) into the venous circulation, which results in a transient hemodilution and change in the velocity of the blood detected by the ultrasound sensors.
To evaluate a new CO monitoring technique, a comparative study should be conducted in the species of interest during various clinical scenarios and over a wide physiologic range of CO of interest to demonstrate that the measurements obtained are in agreement with those obtained with validated techniques. Use of the LiDCO method has been validated and compared with the TDCO method in neonatal foals and other species.14,15 The UDCO technique was recently evaluated in anesthetized euvolemic neonatal foals (1 to 3 days of age).6 There is, however, significant variation in CO values among horses of differing ages. Foals have high heart rates and CI values because of their high metabolic rates and immature sympathetic nervous system.16 Mean ± SD CI in 2-hour-old foals is 155.2 ± 8.1 mL/kg/min, and in 14-day-old foals, the value is 222 ± 21.6 mL/kg/min; in adult horses, CI is typically 72 mL/kg/min.16–18 The purpose of the study reported here was to assess the accuracy of a UDCO method, compared with that of the LiDCO method, for determination of CO (and CI calculated on the basis of the CO values) in juvenile horses with experimentally induced hypovolemia. Our hypothesis was that values of CI derived by use of the UDCO technique would have good agreement with values determined by use of the LiDCO technique in juvenile horses with hypovolemia induced via controlled hemorrhage.
Lithium dilution cardiac output
Mean arterial blood pressure
Ultrasound velocity dilution cardiac output
Thermodilution cardiac output
IsoFlo, Abbott Laboratories, North Chicago, Ill.
Mila International Inc, Denver, Colo.
Terumo Surflow 20G Terumo Med Corp, Summerset, NJ.
A/S5 Gas Calibration Canister. Datex-Ohmeda Division, Helsinki, Finland.
ABL 725 Radiometer Inc, Bronshoj, Denmark.
Lithium Chloride, LiDCO Limited, London, England.
CM 31-01, LiDCO Ltd, London, England.
Gleed RD, Kislukhin V, Krivitski N, et al. The effect of injection temperature on cardiac output determination by ultrasound dilution in an anesthetized sheep (abstr), in Proceedings, 7th World Cong Vet Anaesthesiol 2000;15.
Ultrasound COStatus flow probe, Transonic System Inc. Ithaca, NY.
CoStatus Roller pump, Transonic Systems Inc, Ithaca, NY.
HFW1000, Transonic Systems Inc, Ithaca, NY.
CPDA-1 blood collection bag, Terumo Corp Teruflex, Tokyo, Japan.
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Equations commonly used to determine CO and other variables.
|CO = Heart rate × stroke volume|
|Systemic vascular resistance = (MAP–central venous pressure) × 80/CO|
|Oxygen delivery = CO × arterial oxygen concentration|
|Oxygen consumption = arterial oxygen concentration – venous oxygen concentration × CO|