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