Shunt equation

In Medicine shunt may also refer to Cardiac shunts, Cerebral shunts, Lumbar-peritoneal shunts, or Peritoneovenous shunts, for additional shunts in medicine see Shunt (medical).

The Shunt equation quantifies the extent to which venous blood bypasses oxygenation in the capillaries of the lung.

“Shunt” and “dead space“ are terms used to describe conditions where either blood flow or ventilation do not interact with each other in the lung, as they should for efficient gas exchange to take place. These terms can also be used to describe areas or effects where blood flow and ventilation are not properly matched, though both may be present to varying degrees. Some references refer to “shunt-effect” or “dead space-effect” to designate the ventilation/perfusion mismatch states that are less extreme than absolute shunt or dead space.

The following equation relates the percentage of blood flow that is not exposed to inhaled gas, called the shunt fraction $Q_{s}/Q_{t}$, to the content of oxygen in venous, arterial, and pulmonary capillary blood.

$Q_{s}/Q_{t}=(Cc_{O_{2}}-Ca_{O_{2}})/(Cc_{O_{2}}-Cv_{O_{2}})$

Where:

Q_s = Pulmonary Physiologic Shunt (mL/min)

Q_t = Cardiac Output (mL/min)

C_CO2 = End-pulmonary-capillary Oxygen Content

C_aO2 = Arterial oxygen content

C_VO2 = Mixed Venous Oxygen Content

Derivation

The blood entering the pulmonary system will have oxygen flux $Q_{t}\cdot Cv_{O_{2}}$, where $Cv_{O_{2}}$ is oxygen content of the venous blood and $Q_{t}$ is the total cardiac output.

Similarly, the blood emerging from the pulmonary system will have oxygen flux $Q_{t}\cdot Ca_{O_{2}}$, where $Ca_{O_{2}}$ is oxygen content of the arterial blood.

This will be made up of blood that bypassed the lungs ($Q_{s}$) and that which went through the pulmonary capillaries ($Q_{c}$). We can express this as
$Q_{t}=Q_{s}+Q_{c}$.

We can solve for $Q_{c}$:
$Q_{c}=Q_{t}-Q_{s}$.

If we add the oxygen content of Qs to Qc we get the oxygen content of Qt:

$Q_{t}\cdot Ca_{O_{2}}=Q_{s}\cdot Cv_{O_{2}}+(Q_{t}-Q_{s})\cdot Cc_{O_{2}}$
Substitute Qc as above, CcO2 is the oxygen content of pulmonary (alveolar) capillary blood.

$Q_{t}\cdot Ca_{O_{2}}=Qs\cdot Cv_{O_{2}}+Q_{t}\cdot Cc_{O_{2}}-Qs\cdot Cc_{O_{2}}$
Multiply out the brackets.
$Q_{s}\cdot Cc_{O_{2}}-Qs\cdot Cv_{O_{2}}=Q_{t}\cdot Cc_{O_{2}}-Qt\cdot Ca_{O_{2}}$
Get the Qs terms and the Qt terms on the same side.
$Q_{s}\cdot (Cc_{O_{2}}-Cv_{O_{2}})=Q_{t}\cdot (Cc_{O_{2}}-Ca_{O_{2}})$
Factor out the Q terms.

${\dfrac {Q_{s}}{Q_{t}}}={\dfrac {Cc_{O_{2}}-Ca_{O_{2}}}{Cc_{O_{2}}-Cv_{O_{2}}}}$
Divide by Qt and by (CcO2 - CvO2).

See also

• Pulmonary shunt
• Ventilation/perfusion ratio
• Pulmonary contusion