Alveolar Gas Equation (Firecracker)

A-a gradient is normal during conditions of hypoxia caused by

-Hypoventilation -Decreased FiO2 (experimentally or at high altitude due to decreased PiO2)

What is H20 Vapor Pressure

-Moisture added to inspired air as it is humidified by the respiratory mucosa -Normally ~47

What is FiO2

-Percentage of O2 in the air -normally 21%

Increased A-a gradient may occur in:

-Shunting -V/Q mismatch -Diffusion impairments (ex. fibrosis or pulmonary edema) -Aging

FiO2 of room air is

.21

PiO2 with a known altitude change

1. Atmospheric pressure (760 at sea level) - H20 vapor pressure 2. Multiply the result by FiO2 (normally 21%)

PiO2 at sea level

150

PiO2 of room air is

150mmHg 1. 760-47=713 2. 713x.21= 149.73

Normal resting A-a gradient in healthy middle aged adults

5-10 mmHg

What is the A-a gradient

A value that reflects the integrity of O2 diffusion across the alveolar and pulmonary arterial membranes

R=

CO2 produced / O2 consumed

What do you use the alveolar gas equation for?

Calculate the PaO2 from an arterial gas sample, which can be used to ultimately estimate the A-a gradient

A-a gradient calculation

PAO2- PaO2 =Alveolar partial pressure of oxygen minus the arterial partial pressure of oxygen

Alveolar Gas Equation

PaO2=PiO2- (PaCO2/R)

Examples of diffusion impairments

interstitial fibrosis or pulmonary edema