From the Guidelines
The computation of Qp (pulmonary blood flow) and Qs (systemic blood flow) in hemodynamics is typically performed using the formula Qp:Qs = (SaO2 - MvO2)/(PvO2 - PaO2), where SaO2 is systemic arterial oxygen saturation, MvO2 is mixed venous oxygen saturation, PvO2 is pulmonary venous oxygen saturation, and PaO2 is pulmonary arterial oxygen saturation, as stated in the most recent guidelines 1. The Qp:Qs ratio is a crucial measurement in congenital heart disease, as it helps clinicians determine the severity of cardiac shunts and guides treatment decisions.
- A ratio of 1:1 indicates no shunt,
- while values greater than 1.5:1 typically suggest a significant left-to-right shunt that may require intervention 1. The measurement is typically performed during cardiac catheterization, where blood samples can be directly obtained from different cardiac chambers. However, non-invasive methods such as Doppler echocardiography are also being used to assess Qp:Qs, although they may be prone to inaccuracy due to inadequate data acquisition and invalid assumptions in the calculation 1. Understanding this ratio is crucial for managing patients with atrial septal defects, ventricular septal defects, patent ductus arteriosus, and other congenital heart conditions involving abnormal blood flow between the pulmonary and systemic circulations. In clinical practice, the choice of method for measuring Qp:Qs depends on the individual patient's condition and the availability of resources, with invasive oximetry being the reference standard method but also having its own set of disadvantages, including its invasive nature and potential for error propagation 1. Therefore, the most accurate method for computing Qp:Qs should be chosen based on the individual patient's needs and the clinical context, with consideration of the potential risks and benefits of each method 1.
From the Research
Computation of Qp and Qs
The computation of Qp (pulmonary blood flow) and Qs (systemic blood flow) in hemodynamics can be determined using various methods, including:
- Thermodilution technique, which involves injecting an indicator into both sides of the heart and measuring the temperature change in the pulmonary artery 2
- Oximetry method, which measures the difference in oxygen saturation between the pulmonary artery and the systemic circulation 3
- Doppler echocardiography, which uses the proximal isovelocity surface area (PISA) method to estimate ventricular septal defect flow and defect area 4
- First-pass radionuclide angiocardiography, which measures the uptake of a radioactive tracer by the lungs and the systemic circulation 3, 5
- Transthoracic echocardiography, which estimates the pulmonary to systemic flow ratio (Qp/Qs) using stored clips and previously described methods 6
Formulas for Qp and Qs
The formulas for calculating Qp and Qs vary depending on the method used. For example:
- The thermodilution technique uses the formula: Qp = heart rate x (2pir(2) x NL x VTI(VSD))/V(max) 2
- The oximetry method uses the formula: Qp/Qs = (SaO2 - SvO2) / (PvO2 - PaO2), where SaO2, SvO2, PvO2, and PaO2 are the oxygen saturations in the systemic artery, systemic vein, pulmonary vein, and pulmonary artery, respectively 3
- The Doppler echocardiography method uses the formula: Qp-Qs = heart rate x [2pir(2) x NL x VTI(VSD)]/V(max) 4
Comparison of Methods
The accuracy and concordance of these methods have been compared in several studies:
- A study comparing thermodilution and oximetry methods found a correlation coefficient of 0.54 for Qp and 0.78 for Qs 2
- A study comparing oximetry, first-pass radionuclide angiocardiography, and Doppler echocardiography found a low correlation between the methods, with a correlation coefficient of 0.52 for oximetry, 0.40 for radionuclide angiocardiography, and 0.72 for Doppler echocardiography 3
- A study comparing transthoracic echocardiography and oximetry found a poor correlation between the methods, with a correlation coefficient of 0.32 6