Respiratory Physiology - The Shunt Equation

Question 1

Explain the shunt equation

Answer 1

DO2 (Oxygen Flux in ml/min) = Q (Flow) x C (Content)

Q = L/Min (Volume of blood pumped through the lungs)
C = ml/L (Volume of O2 that can be dissolved in a L of blood)

This equation would be simple if gas exchange was 100% efficient, however, a proportion of blood does not pick up oxgen, and is referred to as being shunted

Breaking the flow into different components:
Qt - The total blood pumped out by the heart. Split into:
Qs - The shunted component.
Qa - The component perfusing the alveoli
These then mix again in the left atrium, adding back up to Qt.

Adding oxygen content into the equation,
Qt - Blood in the RV has venous oxygen content QtCvO2
Qs - Fails to pick up any O2, remains at QsCvO2
Qc - Fully saturated with O2 QaCcO2 This is assumed to be the same as PAO2 (From the alveolar gas equation)
As above, these mix to the total of Qt, with an arterial oxygen content QtCaO2

Thus
Qt=Qs+Qc
QtCaO2=QsCvO2 + QcCcO2
QtCaO2=QsCvO2+(Qt-Qs)CcO2
QtCaO2=QsCvO2+QtCcO2-QsCcO2
QsCcO2-QsCvO2=QtCcO2-QtCaO2
Qs(CcO2-CvO2)=Qt(CcO2-CaO2)

(Qs/Qt)=(CcO2-CaO2)/(CcO2-CvO2)

A normal shunt ratio is approximately 0.3, increasing in pathological states

https://youtu.be/WXPLMcf9z5c

Question 2

Explain Shunts

Answer 2

Blood that passes through the lungs without participating in gas exchange
1. Physiological shunts:
Bronchial circulation drains deoxygenated blood into pulmonary veins
Thebesian veins of the heart drain directly into the LV
1. Pathological (Anatomical shunts)
Respiratory
Pneumonia
Oedema
ARDS
Bronchial obstruction
PE
Cardiovascular
Pulmonary AV fistula
Cyanotic CHD

Question 3

What effect does shunt have?

Answer 3

PaO2 decreases as shunted blood mixes with oxygenated blood
There may be a transient increase in PaCO2, but this is quickly detected by central chemoreceptors, with an increase in MV to correct it.

Question 4

What is the effect of increasing FiO2 in context of shunts?

Answer 4

Hypoxaemia caused by shunt responds poorly to an increase in FiO2
1. Shunted blood is not exposed to the higher FiO2
2. In areas where blood is being oxygenated, Hb is already maximally saturated, so the only improvement is an increase in PaO2, which has negligible effect on overall carrying capacity (Henry’s Law)

In practice, improvement in the patient’s hypoxia can be seen, as poorly ventilated areas of lung may receive improved oxygenation, and shunt is a continuum rather than all-or-nothing (for instance impaired diffusion in pulmonary oedema).

Question 5

What is Henry’s Law

Answer 5

The amount of dissolved gas in a liquid is directly proportional to its partial pressure above the liquid.

Thus a higher FiO2 allows for a higher PaO2.