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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)
Revision as of 14:47, 8 September 2014
, 8 September 2014no edit summary
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''Arteriovenous oxygen difference = A VO 2 (arterio-venous oxygen saturation difference) x 1.36 x Hb x 10'' | ''Arteriovenous oxygen difference = A VO 2 (arterio-venous oxygen saturation difference) x 1.36 x Hb x 10'' | ||
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A schematic illustration of this flow measurement is shown in figure 1. Fluid with a certain oxygen concentration enters the system at flow rate Q, while O2 is added at rate V, raising the oxygen concentration (Cout). The rate of the indicator QxCout must equal the QxCin, plus the rate at which O2 is added. The blood flow is then the rate at which oxygen is added (oxygen consumption) divided by the difference in oxygen content of in this case the arterial and mixed venous system. | A schematic illustration of this flow measurement is shown in figure 1. Fluid with a certain oxygen concentration enters the system at flow rate Q, while O2 is added at rate V, raising the oxygen concentration (Cout). The rate of the indicator QxCout must equal the QxCin, plus the rate at which O2 is added. The blood flow is then the rate at which oxygen is added (oxygen consumption) divided by the difference in oxygen content of in this case the arterial and mixed venous system. | ||
[[File: | '''Figure 1''' | ||
[[File:HemodynamicCO_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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== Shunt detection == | |||
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. | |||
'''Figure 2''' | |||
[[File:HemodynamicShunt_Fig1.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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A shunt can also be quantified. This is based on measurement of pulmonary (Qp in L/min) and systemic cardiac output (Qs in L/min). | |||
''Qp (L/min) = oxygen consumption (mL/min) / pulmonary venous oxygen content ‒ pulmonary arterial 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. | |||