Characterization of PaO₂/FiO₂ and SpO₂/FiO₂ oxygenation indices at 2,240 meters above sea level

Luis E. Santos-Martínez, Departamento de Hipertensión Pulmonar y Corazón Derecho, Unidad Médica de Alta Especialidad, Hospital de Cardiología, Centro Médico Nacional Siglo XXI, Instituto Mexicano del Seguro Social, Ciudad de México, México
Julio E. López-Nuño, Departamento de Hipertensión Pulmonar y Corazón Derecho, Unidad Médica de Alta Especialidad, Hospital de Cardiología, Centro Médico Nacional Siglo XXI, Instituto Mexicano del Seguro Social, Ciudad de México, México
Marissa Gámez-Escamilla, Departamento de Hipertensión Pulmonar y Corazón Derecho, Unidad Médica de Alta Especialidad, Hospital de Cardiología, Centro Médico Nacional Siglo XXI, Instituto Mexicano del Seguro Social, Ciudad de México, México
Francisco J. González-Ruiz, Departamento de Cuidados Intensivos Posquirúrgicos Cardiovasculares, Secretaría de Salubridad y Asistencia, Instituto Nacional de Cardiología Ignacio Chávez, Ciudad de México, México
Jacobo J. Cruz-Sánchez, Departamento de Cuidados Intensivos Posquirúrgicos Cardiovasculares, Secretaría de Salubridad y Asistencia, Instituto Nacional de Cardiología Ignacio Chávez, Ciudad de México, México
Eduardo I. Arteaga-Chan, Departamento de Cuidados Intensivos Posquirúrgicos Cardiovasculares, Secretaría de Salubridad y Asistencia, Instituto Nacional de Cardiología Ignacio Chávez, Ciudad de México, México

Objective: To describe the oxygenation indices adjusted for barometric pressure in healthy subjects at 2,240 meters above sea level. Method: Subjects aged 20 to 65 years living in Mexico City without cardiopulmonary disease were studied using a pilot cross-sectional analytical design. Their demographic variables and arterial blood gas measurements were determined, and their oxygenation indices adjusted for barometric pressure were calculated. Differences between variables were calculated using the t test for independent variables, Pearson’s correlation coefficient for the relationship between them, and the intraclass correlation coefficient to estimate concordance. A p < 0.05 was considered significant. Results: A total of 220 subjects aged 51 ± 10 years were studied, with the most frequent occupation being domestic services (66, 30%). Arterial blood gas analysis showed no differences by gender. The unadjusted PaO₂/FiO₂ ratio was 325 ± 23 and the adjusted ratio 250 ± 18 (p < 0.05) for barometric pressure. Pearson’s r between the two 0.843 (p = 0.0001), and the intraclass correlation coefficient 0.671 (0.571-0.748), with substantial agreement. The unadjusted SpO₂/FiO₂ ratio 446 ± 8 and the adjusted ratio 343 ± 6 (p < 0.05). Pearson’s r between both 0.844 (p < 0.05) and the intraclass correlation coefficient 0.682 (0.586-0.756), with substantial agreement; coefficient of determination 71%. Conclusions: The PaO₂/FiO₂ and SpO₂/FiO₂ ratios behave at 2,240 meters above sea level towards a decrease in healthy subjects due to barometric pressure. Unlike PaO₂/FiO₂, the change in SpO₂/FiO₂ is useful and generalizable for use at moderate altitudes.

Keywords: Altitude. Barometric pressure. Pulse oximetry. Partial pressure of oxygen. Acute respiratory distress syndrome.