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Hemodynamic principles: pressure measurement, cardiac output and shunt detection: Difference between revisions
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Hemodynamic principles: pressure measurement, cardiac output and shunt detection (view source)
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Cardiac output is the amount of blood per time unit ejected by a cardiac chamber. It is the product of stroke volume and heart frequency. It can be estimated on the basis of various assumptions. The two most commonly used methods are the Fick method and the thermodilution method. For comparison among patients, the cardiac index is used, a correction of cardiac output for the patient body surface area. | Cardiac output is the amount of blood per time unit ejected by a cardiac chamber. It is the product of stroke volume and heart frequency. It can be estimated on the basis of various assumptions. The two most commonly used methods are the Fick method and the thermodilution method. For comparison among patients, the cardiac index is used, a correction of cardiac output for the patient body surface area. | ||
The thermodilution technique requires an injection of saline in the proximal end of a catheter, while measuring the temperature in the distal end of a catheter. The cardiac output is related to the area under a thermodilution curve, considering the temperature and specific gravity of the fluids in the proximal and distal ends of the catheter. The thermodilution method tends to overestimate the cardiac output in patients with low outputs (<2.5L/min) . | The thermodilution technique requires an injection of saline in the proximal end of a catheter, while measuring the temperature in the distal end of a catheter. The cardiac output is related to the area under a thermodilution curve, considering the temperature and specific gravity of the fluids in the proximal and distal ends of the catheter. The thermodilution method tends to overestimate the cardiac output in patients with low outputs (<2.5L/min)<cite>Bonow</cite>. | ||
Cardiac output measured by Fick method uses the the oxygen consumption and the arteriovenous concentration difference. It assumes that the rate at which oxygen is consumed is a function of the blood flow and the rate of oxygen pick-up by the red blood cells. | Cardiac output measured by Fick method uses the the oxygen consumption and the arteriovenous concentration difference. It assumes that the rate at which oxygen is consumed is a function of the blood flow and the rate of oxygen pick-up by the red blood cells. | ||
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'''Figure 1''' | '''Figure 1''' | ||
[[File:HemodynamicCO_Fig1.svg | 300px | left | | [[File:HemodynamicCO_Fig1.svg | 300px | left | Figure 1<cite>Winniford</cite>]] | ||
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A cardiac shunt is an abnormal blood flow in the circulatory system. Normally, pulmonary blood flow equals systemic blood flow. A shunt can be right to left (from pulmonary circulation to systemic circulation), left to right or bidirectional. | A cardiac shunt is an abnormal blood flow in the circulatory system. Normally, pulmonary blood flow equals systemic blood flow. A shunt can be right to left (from pulmonary circulation to systemic circulation), left to right or bidirectional. | ||
A intracardiac shunt can be detected and localized by using blood samples with measurement of the oxygen saturation at different sites within and close to the heart, the so-called “oximetry run” . This run obtains blood samples from all right-sided locations, including the SVC, IVC, right atrium, right ventricle, and pulmonary artery. Figure 1 and 2 show an example of an oximetry run in patients with atrial en ventricular septal defects. | A intracardiac shunt can be detected and localized by using blood samples with measurement of the oxygen saturation at different sites within and close to the heart, the so-called “oximetry run”<cite>Camm</cite>. This run obtains blood samples from all right-sided locations, including the SVC, IVC, right atrium, right ventricle, and pulmonary artery. Figure 1 and 2 show an example of an oximetry run in patients with atrial en ventricular septal defects. | ||
'''Figure 1''' | |||
[[File:HemodynamicShunt_Fig1.svg | 300px | left | ‘Oximetry run’ in a patient with atrial septal defect. The ‘step-up’ detected in the right atrium (RA) identifies a left-to-right shunt at this location. ]] | |||
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'''Figure 2''' | '''Figure 2''' | ||
[[File: | [[File:HemodynamicShunt_Fig2.svg | 300px | left | ‘Oximetry run’ in a patient with ventricular septal defect. The ‘step-up’ detected in the right ventricle (RV) identifies a left-to-right shunt at this location. ]] | ||
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''Qs (L/min) = oxygen consumption (mL/min)/ systemic arterial oxygen content ‒ mixed venous oxygen content mL/L)'' | ''Qs (L/min) = oxygen consumption (mL/min)/ systemic arterial oxygen content ‒ mixed venous oxygen content mL/L)'' | ||
The shunt is then measured by the flow ratio Qp/Qs. A ratio < 1.5 indicates a small left to right shunt, and a ratio of 1.5-2.0 a moderate-size shunt. A ratio of 2.0 or more indicates a large left to right shunt and generally requires percutaneous or surgical repair to prevent pulmonary or RV complications. A flow ratio of less than 1.0 indicates a net right to left shunt. | The shunt is then measured by the flow ratio Qp/Qs. A ratio < 1.5 indicates a small left to right shunt, and a ratio of 1.5-2.0 a moderate-size shunt. A ratio of 2.0 or more indicates a large left to right shunt and generally requires percutaneous or surgical repair to prevent pulmonary or RV complications. A flow ratio of less than 1.0 indicates a net right to left shunt<cite>Bonow</cite>. | ||
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== References == | == References == | ||
<biblio> | <biblio> | ||
# | #Bonow Bonow RO, Man DL, Zipes DP and Libby P. Braunwald’s Heart Disease: A textbook of cardiovascular medicine (9th edition). Chapter 20: Cardiac catheterization. Pg 383-405. | ||
#Winniford Winniford MD, Kern MJ, Lambert CR. Blood flow measurement. In Pepine CJ, Hill JA, Lambert CR [eds]: Diagnostic and Therapeutic Cardiac Catheterization (3rd edition). Baltimore, Williams & Wilkins, 1998. Pg 400. | |||
#Camm Camm J, Luscher, TF, Serruys PW. The ESC Textbook of cardiovascular medicine (2 ed.). Chapter 8: Invasive imaging and hemodynamics. Pg 237-281. | |||
</biblio> | </biblio> | ||