Qp/Qs
Qp/Qs describes the ratio of pulmonary and systemic blood flow in single
ventricle physiology. The portion of total cardiac output directed Qp or Qs depends on
the specific heart lesion (various degrees of obstruction) and the vascular resistance to
flow in the pulmonary and systemic circuits. Through manipulation of the Fick
equation, one can calculate Qp/Qs.
Qp/Qs = (SaO 2 – SmvO 2 ) / (SpvO 2 – SaO 2 ),
Where SaO 2 = oxygenation saturation of arterial blood,
SmvO 2 = oxygen saturation of mixed venous blood,
SpvO 2 = oxygen saturation of pulmonary venous blood
Obviously, not all of these variables can be easily measured in an infant prior to
cardiac surgery. However, an estimate of Qp/Qs can be obtained with a pulse
oximetry measurement and a few assumptions. If the patient has healthy lungs, then
one can assume that the SpvO 2 on room air approaches 100%. If the patient is not
severely anemic or septic, and has good cardiac function, one can also assume that
the systemic arterial-venous oxygenation difference (SaO 2 – SmvO 2 ) will be about
25%. Thus the above equation can be simplified to:
Qp/Qs = 25 / (100 - SaO 2 ).