Aa_O2_gradient_simplified v.1
The A-a O2 gradient is the O2 pressure difference between arteries and the aveoli. After calculating the A-a Gradient and determining if it is elevated or normal, one can rule out several causes of hypoxia. This gradient is calculated with the following formula: A-a O2 Gradient = [ 0.21 × (Atmospheric Pressure - H2O Pressure) - (PaCO2/0.8) ] – PaO2 from ABG where - Atmospheric pressure is the pressure of air the patient is breathing in. It is calculated using the altitude of the patient's location using the following formula: Atmospheric pressure [kPa] =101.33 kPa - 0.012 kPa * Altitude [m] - H20 pressure is the vapor pressure inside the aveoli. This depends on the temperature of the patient and it can be approximated by the Buck equation with less than +0.04% error. The Buck equation is: P [kPa] = 0.61121*exp((18.678- T[°C]/234.5)*(T[°C]/(257.14+T[°C]))) PaO2 and PaCO2 are the partial pressure of O2 and Co2 in the blood available from Arterial Blood Gases (ABG). The calculated gradient should be compared to the expected gradient for patient's age, which is calculated as: Normal Gradient Estimate in mmHg = (Age/4) + 4 The results of the gradient then can be interpreted as: Causes of Hypoxemia A-a O2 Gradient Shift V/Q Mismatch (ex: PNA, CHF, PE, ARDS, atelectasis, etc) Elevation Shunt (ex: PFO, ASD, pulmonary AVMs) Elevation Alveolar Hypoventilation (ex: interstitial lung dz, environmental lung dz, PCP PNA) Elevation Hypoventilation (ex: COPD, CNS d/o, neuromuscular dz, etc) Depression High altitude Depression
To rule out several causes of hypoxia.
Use to help determining the cause of hypoxemia in: - Patients with unexplained hypoxemia; - Patients with hypoxemia exceeding the degree of their clinical illness when the patient is not on mechanical ventilation and when you do not know air pressure. Otherwise, use the more complex formulas. Use this guideline when you absolutely know the air pressure at the location of the patient. Air pressure depends on meteorological factors, height over sea level, and whether the patient is on mechanical ventilation. Use this guideline in patients on mechanical ventilation, or having special diets. Otherwise, refer to the Simplified A-a O2 gradient guideline.
In case of hypoventillation, the large amount CO2 in the blood can mask hypoxemia. Do not use this formula if the patient is on mechanical ventilation, or when there is low or high air pressure (due to meteorological variations). Do not use this formula if the patient is on an unbalanced diet.
[1] Helmholz HF Jr. The abbreviated alveolar air equation. Chest. 1979 Jun;75(6):748 [2] McFarlane MJ, Imperiale TF. Use of the alveolar-arterial oxygen gradient in the diagnosis of pulmonary embolism. Am J Med. 1994 Jan;96(1):57-62.
OBSERVATION.aao2_gradient.v0, EVALUATION.aao2_gradient_assessment.v0, OBSERVATION.basic_demographic.v1, OBSERVATION.body_temperature.v2, OBSERVATION.lab_test-blood_gases.v1
