Flick and thermodilution principles
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Basics on Fick´s and Thermodilution principles
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Flick and thermodilution principles
- 1. Methods to Calculate Cardiac Output By: Samuel Moreno
- 2. Fick´s Principle • Oxigen Consumption Rate • Relationship between CV function and Metabolism Adolf Fick125mL/m2
- 3. Thermodilution Principle • Fegler 1954 Shunts TR <<<Qs
- 4. Modified Stewart-Hamilton • Blood and indicator temperature • Specific gravity and volume of the injected • Area under the thermodilution curve.
Editor's Notes
- https://www.sciencedirect.com/topics/medicine-and-dentistry/thermodilution
- Método de Fick: De acuerdo al principio de Fick, la velocidad a la que se consume el oxígeno es el flujo sanguíneo por la velocidad con la que los hematíes captan oxígeno. Partiendo de que el flujo de sangre en un periodo dado es igual a la cantidad de sustancia que entra en el flujo en ese mismo periodo, dividido por la diferencia entre las concentraciones de la sustancia en la sangre anterior y posterior a su punto de entrada en la circulación. En condiciones normales, el mismo número de hematíes que entra en el pulmón debe dejarlo, salvo que existan cortocircuitos intracardiacos. Por lo que conociéndose ciertos parámetros, se puede determinar el flujo de los hematíes que pasan a través del pulmón, de manera que el gasto cardiaco seria igual a To calculate CO, the oxygen contents of arterial and venous blood samples are measured, and at the same time, whole body oxygen consumption is measured by analyzing expired air. The blood concencentration of oxygen is expressed as ml O2/ml blood, and the VO2 is expressed in units of ml O2/min. If CaO2 and CvO2 are 0.2 ml and 0.15 ml O2/ml blood, respectively, and VO2 is 250 ml O2/minute, then CO = 5000 ml/min, or 5 L/min. Ventricular stroke volume would simply be the cardiac output divided by the heart rate. The consumption of oxygen by a patient can be measured using a covered hood in the cardiac catheterization laboratory, and the A-V difference can be measured by obtaining blood gases from a systemic artery and from the PA. This method is more accurate than the thermodilution method in patients with atrial fibrillation, tricuspid regurgitation, and low CO. Common sources of error include improper collection of blood samples. At the bedside, use of a covered hood can be cumbersome and impractical. For this reason, some laboratories assume that resting oxygen consumption is 125 mL/m2 and calculate CO based on an assumed Fick equation
- Termodilución (cateter Swan-Ganz): Requiere inyectar un bolo de líquido (suero fisiológico) por el puerto proximal del catéter y a través de un termistor montado en el extremo distal se determina el cambio resultante de la temperatura, de manera que el gasto cardiaco se relaciona inversamente con el área bajo la curva de termodilución entre la temperatura y el tiempo. Por ello cuanto mayor es el área, menor es el gasto cardiaco, porque el líquido tarda más en calentarse. Esta técnica tiende a sobreestimar el gasto cardiaco, y está muy artefactada en insuficiencias tricúspideas severas y cuando el gasto cardiaco es muy bajo. 2 Lumen Catheter Termister at the tip (detects temp) 5-10mL of room temp saline is injected to the RA port Thermistor at PA detects the temp This procedure is repeated 3 to 5 times Several direct and indirect techniques for measurement of cardiac output are available. The thermodilution technique uses a special thermistor-tipped catheter (Swan-Ganz catheter) that is inserted from a peripheral vein into the pulmonary artery. A cold saline solution of known temperature and volume is injected into the right atrium from a proximal catheter port. The injectate mixes with the blood as it passes through the ventricle and into the pulmonary artery, thus cooling the blood. The blood temperature is measured by a thermistor at the catheter tip, which lies within the pulmonary artery, and a computer is used to acquire the thermodilution profile; that is, the computer quantifies the change in blood temperature as it flows over the thermistor surface. The cardiac output computer then calculates flow (cardiac output from the right ventricle) using the blood temperature information, and the temperature and volume of the injectate. The injection is normally repeated a few times and the cardiac output averaged. Because cardiac output changes with respiration, it is important to inject the saline at a consistent time point during the respiratory cycle. In normal practice this is done at the end of expiration. Left-to-right shunt causes recirculation of the injectate, resulting in underestimated measured CO. Right-to-left shunt allows for injectate to bypass the thermistor, resulting in overestimation of CO. Tricuspid or pulmonic valve regurgitation leads to prolonged decay time and unreliable measurement that depends on severity of regurgitation and typically underestimates CO In a low-cardiac-output state, blood is rewarmed by the walls of the cardiac chambers and surrounding tissue, resulting in an overestimation of CO.
- in which CO is the cardiac output (L/min), V is the volume of injectate (mL), TB is the initial blood temperature (degrees Celsius), TI is the initial injectate temperature (degrees Celsius), K1 is the density factor, K2 is the computation constant, and is the integral of blood temperature change over time. A computer that integrates the area under the temperature versus time curve is used to perform the calculation. CO is inversely proportional to the area under the curve. The temperature-versus-time curve is the crux of this technique, and any circumstances that affect it have consequences for the accuracy of the CO measurement. Specifically, anything that results in less “cold” reaching the thermistor, more “cold” reaching the thermistor, or an unstable temperature baseline will adversely affect the accuracy of the technique. Less “cold” reaching the thermistor would result in overestimation of the CO, which could be caused by a smaller amount of indicator, an indicator that is too warm, a thrombus on the thermistor, or partial wedging of the catheter. Conversely, underestimation of the CO will occur if excessive volume of injectate or injectate that is too cold is used to perform the measurement. In patients with large intracardiac shunts, PAC-derived thermodilution CO is not recommended for accurate CO measurement. Box 10.13 lists common errors in PAC thermodilution CO measurements.