Respiratory failure

In medicine, respiratory failure is "failure to adequately provide oxygen to cells of the body and to remove excess carbon dioxide from them."

Diagnosis

PaO₂/FiO2 ratio (PF ratio)

${P/F\ ratio}=\left({\frac {PaO_{2}}{Fi0_{2}}}\right)\times 100$

• Normal is 500
• ARDS is < 200

This measure is easier to calculate. Comparative studies suggest it correlates better with pulmonary shunts than does the A-a gradient.

Alveolar-arterial oxygen (A-a) gradient (alveolar-arterial oxygen difference - AVO₂D)

${\mbox{A-a gradient}}={PAO_{2}}\ -\ {PaO_{2}}$

${PAO_{2}}={Fi0_{2}}*\left({760-47}\right)\ -\ {\frac {PaCO_{2}}{0.8}}$

• Normal is < 10 mm Hg

The A-a gradient is harder to calculate, but accounts for changes in respiration as measured by the partial pressure of carbon dioxide. However, this calculation relies on the respiratory quotient being constant in the prediction of alveolar CO₂ When compared to the PF ratio, the A-a gradient is found to correlate less well with pulmonary shunting.

Among outpatients with possible pulmonary embolism, the A-a gradient may be a better test.

An online calculator for the A-a gradient is at http://www.mdcalc.com/aagrad.

Treatment

Extracorporeal membrane oxygenation

Extracorporeal membrane oxygenation (ECMO) is "application of a life support system that circulates the blood through an oxygenating system, which may consist of a pump, a membrane oxygenator, and a heat exchanger. Examples of its use are to assist victims of smoke inhalation injury, respiratory failure, and cardiac failure." ECMO may increase survival in respiratory failure.