aa_o2_gradient_complex 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. It is calculated with the following formula: A-a O2 Gradient = [ (FiO2) × (Atmospheric Pressure - H2O Pressure) - (PaCO2/RQ) ] – PaO2 from ABG where FiO2 is the percentage of O2 in the air breathed in. In normal conditions, it is 21% Atmospheric pressure is the pressure of air the patient is breathing in. It depends on the location (height over sea level and meteorological conditions). The average air pressure at sea level is 101.33 kPa.* 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]))) RQ is the respiratory quotient of the patient. This can be approximated by 0.8 in most of the cases (except some special diets). 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

Daniel Keszthelyi

models@cambiocds.com

Cambio Healthcare Systems

To rule out several causes of hypoxia.

Use to help determining the cause of hypoxemia; it pinpoints the location of the hypoxia as intra- or extra-pulmonary in: - Patients with unexplained hypoxemia; - Patients with hypoxemia exceeding the degree of their clinical illness. Use this guideline when you are absolutely sure about the inputs and the simpler guidelines are not enough, i.e. due to meteorological conditions, low or high air pressure is present when doing the calculations. 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.

[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