Altitude-adaptive model for the interpretation of oxygen saturation

Normal oxygen saturation (SpO2) is dependent on altitude. We propose a computer model based on the oxygen cascade to predict SpO2 values between 0 and 4000 meters above sea level. The model was evaluated on pediatric permanent residents of Peru.

Tüshaus L, Moreo M, Zhang J, Hartinger SM, Mäusezahl D, Karlen W. Physiologically driven, altitude-adaptive model for the interpretation of pediatric oxygen saturation at altitudes above 2,000 m a.s.l. J Appl Physiol 127: 847–857, 2019. doi: 10.1152/japplphysiol.00478.2018

Background

The use of pulse oximeters and supplemental oxygen in clinical applications has shown to drastically reduce death rates. However, in countries where these devices are needed most, access to these life saving technologies is difficult. The interpretation of SpO₂ values for hypoxemia is challenging, especially for health personnel not familiar with respiratory physiology and the measurement principles of pulse oximeters. Altitude has a direct influence on SpO₂ as the air pressure decreases. Therefore, we introduce an altitude-adaptive SpO₂ model
and propose a model-derived altitude-adaptive abnormal SpO₂ threshold to facilitate the interpretation of measured SpO₂ values.

Oxygen Cascade

The oxygen cascade model describes the pathway of oxygen throughout the cardio-respiratory body compartments (Figure) using physiological equations. The model was originally developed to estimate the “virtual shunt”, describing the overall loss of oxygen content between the alveolar gas and arterial blood compartments. It uses SpO₂ and inspired oxygen (FIO₂) values as input parameters. An increase of virtual shunt is one of the main causes of hypoxemia. Virtual shunt spans over two components (Figure):

incomplete capillary diffusion (diffusion defect between the alveolar and end-capillary compartments, VS_diff)
incomplete perfusion with intrapulmonary shunt (perfusion defect, VS_perf).

Impact

Our model describes the altitude-dependent decrease of SpO2 in healthy pediatric residents based on physiological equations and can be adapted based on measureable clinical parameters. The proposed altitude-specific abnormal SpO2 threshold might be more appropriate than rigid guidelines for administering oxygen that currently are only available for patients at sea level. We see this as a starting point to discuss and adapt oxygen administration guidelines.

Downloads

Altitude-adaptive SpO₂ model (code)

The software for estimating SpO2 based on altitude is open source (matlab scripts, MIT license):

doi.org/10.5905/ethz-1007-205

Paediatric SpO₂ model (data)

Output values of models developed (csv files, Figure 2)

doi.org/10.3929/ethz-b-000344084

Citing

If you are reusing the model, please cite

Tüshaus L, Moreo M, Zhang J, Hartinger SM, Mäusezahl D, Karlen W. Physiologically driven, altitude-adaptive model for the interpretation of pediatric oxygen saturation at altitudes above 2,000 m a.s.l. J Appl Physiol 127: 847–857, 2019. doi: 10.1152/japplphysiol.00478.2018 BioRxiv: doi.org/10.1101/334482

Partners

In collaboration with the Household Health Systems Research Group at SwissTPH and the Universidad Peruana Cayetano Heredia, Lima, Peru, we collected the data for developing this model. Dr. Jose Rojas-Camayo (Lima, Peru) kindly shared the centiles of his valuable data set. Masimo International facilitated access to the pulse oximetry devices.