Which statement about the Qp:Qs ratio is true?

Which statement about the Qp:Qs ratio is true?

• A. A Qp:Qs ratio greater than 1 indicates a left-to-right shunt.
• B. A Qp:Qs ratio greater than 1 indicates a right-to-left shunt.
• C. Qp:Qs must always equal 1.
• D. Qp:Qs is calculated from systemic flow only.

Answer: (A)

Qp:Qs compares how much blood is pumped to the lungs (pulmonary flow) versus to the rest of the body (systemic flow). When there is a left-to-right shunt, oxygenated blood from the left heart is diverted back into the pulmonary circuit, which increases pulmonary blood flow (Qp) while systemic flow (Qs) does not rise accordingly. That makes the ratio exceed 1, indicating a left-to-right shunt. If the shunt were right-to-left, the pulmonary flow would be reduced relative to systemic flow, giving a ratio less than 1. The ratio is derived from both flows, typically using Doppler measurements of stroke volumes across the pulmonary and aortic (or LVOT) outflows, not from systemic flow alone. Therefore, a Qp:Qs greater than 1 best reflects a left-to-right shunt.

Qp:Qs compares how much blood is pumped to the lungs (pulmonary flow) versus to the rest of the body (systemic flow). When there is a left-to-right shunt, oxygenated blood from the left heart is diverted back into the pulmonary circuit, which increases pulmonary blood flow (Qp) while systemic flow (Qs) does not rise accordingly. That makes the ratio exceed 1, indicating a left-to-right shunt. If the shunt were right-to-left, the pulmonary flow would be reduced relative to systemic flow, giving a ratio less than 1. The ratio is derived from both flows, typically using Doppler measurements of stroke volumes across the pulmonary and aortic (or LVOT) outflows, not from systemic flow alone. Therefore, a Qp:Qs greater than 1 best reflects a left-to-right shunt.