What is the significance of a QP (pulmonary flow)/QS (systemic flow) ratio greater than 1.5 in hemodynamic shunt calculation?

Significance of QP/QS Ratio > 1.5 in Hemodynamic Shunt Calculation

A QP/QS ratio greater than 1.5 indicates a hemodynamically significant left-to-right shunt that warrants intervention in most cases, provided other hemodynamic parameters are favorable. 1, 2, 3

Definition and Clinical Importance

• QP/QS represents the ratio of pulmonary blood flow to systemic blood flow, which normally equals 1:1 in healthy individuals without cardiac shunts 3
• A ratio > 1 indicates a left-to-right shunt, with blood flowing from the systemic to pulmonary circulation 2
• A ratio > 1.5 is the established threshold for defining a "hemodynamically significant shunt" according to the American Heart Association and American College of Cardiology guidelines 1, 2
• The higher the QP/QS ratio, the larger the shunt volume and greater the potential for physiological consequences 3

Clinical Implications and Management Decisions

• QP/QS ≥ 1.5 combined with right heart chamber enlargement is a key criterion for recommending surgical or device closure of atrial septal defects 1
• For ventricular septal defects, a QP/QS ≥ 1.5 with evidence of left ventricular volume overload indicates need for closure 1
• In partial anomalous pulmonary venous connections, a QP/QS ≥ 1.5 is a threshold for recommending surgical repair 1
• The ratio helps quantify the hemodynamic burden on the pulmonary circulation and right heart, which directly impacts morbidity and mortality 2, 3

Measurement Methods

• The reference standard for QP/QS measurement is invasive oximetry during cardiac catheterization 2, 4
• Cardiac MRI has become the non-invasive gold standard using phase-contrast techniques, with excellent correlation to invasive measurements (r = 0.61-0.84) 4
• Doppler echocardiography can estimate QP/QS by measuring flow at the pulmonary and aortic valves, though with more variability 5, 6

Clinical Decision-Making Framework

1. QP/QS < 1.5: Generally considered hemodynamically insignificant; observation is typically recommended 1, 2

2. QP/QS ≥ 1.5 with favorable pulmonary pressures:

   • Pulmonary arterial pressure < 50% of systemic pressure
   • Pulmonary vascular resistance < 1/3 of systemic resistance
   • Recommendation: Surgical or device closure is indicated 1

3. QP/QS ≥ 1.5 with borderline pulmonary pressures:

   • Pulmonary arterial pressure ≥ 50% but < 2/3 of systemic pressure
   • Pulmonary vascular resistance > 1/3 but < 2/3 of systemic resistance
   • Recommendation: Closure may be considered but with higher risk 1

4. Any QP/QS with severe pulmonary hypertension:

   • Pulmonary arterial pressure > 2/3 of systemic pressure
   • Pulmonary vascular resistance > 2/3 of systemic resistance
   • Recommendation: Closure is contraindicated due to risk of right heart failure 1

Important Caveats and Pitfalls

• QP/QS ratio should never be used in isolation; it must be interpreted alongside other clinical and hemodynamic parameters 2
• In Eisenmenger syndrome, the QP/QS ratio becomes < 1 as the shunt reverses to right-to-left, contraindicating closure 3
• Measurement technique matters - studies show variability between methods, with correlation coefficients ranging from 0.40 to 0.85 between different techniques 7, 5
• Valvular regurgitation can affect QP/QS calculations and should be accounted for when interpreting results 6
• Serial measurements using the same technique are more reliable for following changes over time than comparing values obtained by different methods 5