Calculation of QP:QS Ratio

Understanding the QP:QS Ratio

The QP:QS ratio is a fundamental hemodynamic measurement used in cardiology, especially in the evaluation of congenital heart diseases. It quantifies the relationship between pulmonary blood flow (QP) and systemic blood flow (QS). This ratio helps clinicians understand the magnitude and direction of intracardiac or extracardiac shunts, which are abnormal connections between the systemic and pulmonary circulations.

What are QP and QS?

• QP (Pulmonary Blood Flow): Represents the total volume of blood flowing through the lungs per minute. In a healthy heart without shunts, QP is equal to QS.
• QS (Systemic Blood Flow): Represents the total volume of blood flowing through the systemic circulation (the rest of the body) per minute.

In the absence of a shunt, QP:QS should ideally be 1:1, meaning the same amount of blood is flowing through both the lungs and the body.

The Significance of Cardiac Shunts

A shunt occurs when blood flows directly from one side of the heart to the other, or from a great vessel to another, bypassing its normal pathway. These can be:

• Left-to-Right Shunt: Oxygenated blood from the left side of the heart or aorta flows into the right side of the heart or pulmonary artery. Examples include Atrial Septal Defect (ASD), Ventricular Septal Defect (VSD), and Patent Ductus Arteriosus (PDA). This leads to increased blood flow to the lungs (QP > QS).
• Right-to-Left Shunt: Deoxygenated blood from the right side of the heart or pulmonary artery flows into the left side of the heart or aorta. Examples include Tetralogy of Fallot or Eisenmenger syndrome. This leads to decreased oxygenation of systemic blood (QS > QP).

The Fick Principle and Oxygen Saturations

The QP:QS ratio is derived from the Fick principle, which states that oxygen consumption by the body is equal to the product of cardiac output and the arteriovenous oxygen content difference. By applying this principle to both systemic and pulmonary circulations, and making some simplifying assumptions (like constant hemoglobin and oxygen carrying capacity), the ratio can be approximated using oxygen saturation values:

QP:QS = (Systemic Arterial O2 Saturation - Mixed Venous O2 Saturation) / (Pulmonary Venous O2 Saturation - Pulmonary Artery O2 Saturation)

Or, in terms of the variables used in our calculator:

QP:QS = (SaO2 - SvO2) / (SpvO2 - SpaO2)

Understanding the Variables:

• SaO2 (Systemic Arterial O2 Saturation): This is the oxygen saturation of blood in a systemic artery (e.g., aorta or femoral artery). It reflects the oxygenation of blood being delivered to the body.
• SvO2 (Mixed Venous O2 Saturation): This is the oxygen saturation of mixed venous blood returning to the right side of the heart, typically sampled from the superior vena cava, inferior vena cava, or right atrium before any shunted blood mixes in. It reflects the overall oxygen extraction by the body's tissues.
• SpvO2 (Pulmonary Venous O2 Saturation): This is the oxygen saturation of blood in the pulmonary veins, after it has been fully oxygenated in the lungs. It is often assumed to be 98-100% in individuals with healthy lungs.
• SpaO2 (Pulmonary Artery O2 Saturation): This is the oxygen saturation of blood in the pulmonary artery. In the presence of a left-to-right shunt, this value will be higher than the true mixed venous saturation (SvO2) because oxygenated blood from the left side has mixed with the deoxygenated blood in the right side before entering the pulmonary artery.

Interpreting the QP:QS Ratio

• QP:QS = 1.0 (or close to 1.0): Indicates no significant shunt. Blood flow through the lungs and the body is balanced.
• QP:QS > 1.0 (e.g., 1.5, 2.0): Suggests a left-to-right shunt. The higher the ratio, the larger the shunt. A ratio of 1.5:1 means that 1.5 times more blood is flowing through the lungs than through the systemic circulation. Significant shunts (typically > 1.5:1 to 2.0:1) can lead to pulmonary hypertension, heart failure, and require intervention.
• QP:QS < 1.0 (e.g., 0.8, 0.5): Suggests a right-to-left shunt. This means deoxygenated blood is bypassing the lungs and entering the systemic circulation, leading to cyanosis (bluish discoloration of the skin due to low oxygen levels).

Limitations and Clinical Considerations

While the QP:QS ratio is an invaluable tool, it's important to understand its limitations:

• Measurement Accuracy: The accuracy of the ratio heavily relies on precise measurements of oxygen saturations from specific sites within the cardiovascular system, typically obtained via cardiac catheterization.
• Assumptions: The simplified formula assumes constant hemoglobin levels and oxygen carrying capacity, and ignores dissolved oxygen, which may not always be perfectly true.
• Complex Shunts: In very complex congenital heart defects with multiple shunts or bidirectional shunts, interpretation can be more challenging and require more advanced hemodynamic calculations.
• Clinical Context: The ratio must always be interpreted within the broader clinical picture of the patient, including symptoms, physical examination findings, and other diagnostic tests.

This calculator provides a quick way to compute the QP:QS ratio, but it should be used for educational and informational purposes only. Clinical decisions should always be made by qualified healthcare professionals.