Qp:Qs

Qr:Qs refers the the ratio of pulmonary blow flow to systemic blood flow

The shunt calculation:

There are many ways to calculate shunt fraction; one can use angiography, echocardiography or take direct measurements. This is a simple way to calculate the shunt fraction when in the OR:

Qp:Qs = S_aO2 – S_VO2 / S_PVO2 – S_PAO2

In the OR, we can take samples from the arterial line (SaO2), the SVC and IVC to determine mixed venous sats using SVO2 = ((3 × S_SVCO2) + S_IVCO2) / 4 a direct sample from the pulmonary veins (S_PVO2) and the PA to yield S_PAO2. Sometimes surgeons do not sample the PV and assume it is 100% saturated

Inter-atrial shunts: Qp:Qs is dependent primarily on the compliance differences between the two ventricles.

Inter-ventricular or great vessel level shunts: Qp:Qs is dependent primarily on the relative resistances of the pulmonary and systemic circulations. Manipulation strategies include:

To increase pulmonary blood flow: Reduce PVR (+/- increase SVR)

To decrease pulmonary blood flow: Increase PVR (+/- decrease SVR)

Factors increasing PVR

Hypoxia

Hypercarbia

Acidosis

Hypothermia

Sympathetic stimulation or catecholamines

Low or high lung volumes, or atelectasis or hyperinflation respectively

High airway pressures

Factors decreasing PVR

Increasing FiO₂

Decreasing PaCO₂

Alkalosis

Avoidance of hypothermia

Pulmonary vasodilators (iNO, Milrinone, Sildenafil)

Avoidance of catecholamine surges (pain, sympathetic stimulation)

Lung volume at functional residual capacity