Relación entre el gradiente alveolo-arterial de oxígeno y la PaO2/FiO2 introduciendo la PEEP en el modelo

objectives to determine whether the alveolar-arterial oxygen gradient (grad[a-a]o2) helps confirm the influence of peep on pafi (pao2/fio2). design: observational study; we used linear regression to perform a multivariate study to improve the pafi formula by taking peep into account. setting: tertiary hospital. patients: we included all patients who were admitted to the intensive care unit, regardless of pulmonary damage. variables: we recorded personal history, clinical judgment, intensive care data, severity scores on the first day and progression. two calculated variables: pafi and grad(a-a)o2. results: a total of 956 patients were included: 63.9% men; median age 68 years. on the first day, 31.8% did not have mechanical ventilation (mv), 13.1% had non-invasive mv and 55.1% had invasive mv. pafi values: 32.9% 0-200, 32.2% 201-300, and 34.8% >300. peep values: 0-5 69.8%, 6-10 27.5% and >10 2.6%. we observed a correlation (pearson) between grad(a-a)o2 and pafi of -0.84 (p<0.001). on performing multiple regression (dependent variable: grad[a-a]o2), the following variables were included in the model: pafi, peep, apache iv and sofa; coefficient of determination (r2) of 0.62 without peep and 0.72 with peep. we changed the pafi formula, referring to it as pafip (pafi plus peep): ln (pafi/[peep+12]). correlation index between pafip and grad(a-a)o2: -0.9 (p<0.001). we performed linear regression (dependent variable: grad[a-a]o2) and used pafip instead of pafi. only pafi remained in the model, and was discretely complemented by apache iv; r2=0.8. conclusions: by adding peep to the pafi model (pafip), we clearly improve the latter, as reflected by a better goodness of fit.