Oxygenation and Respiratory Assessment
Published October 11, 2026 · Last updated October 11, 2026
The respiratory physiology calculators: the A–a oxygen gradient, PaO₂/FiO₂ ratio, oxygen index, ROX index, and BODE index — what each measures about gas exchange, lung injury, or COPD prognosis.
Respiratory calculators convert blood gas and ventilator values into measures of gas exchange and lung injury, and one composite index summarises COPD prognosis. They are used in critical care and respiratory clinics where objective oxygenation metrics complement the clinical assessment of distress.
This guide describes what each of the five respiratory calculators measures.
Gas exchange: the A–a oxygen gradient
The alveolar-arterial (A–a) oxygen gradient compares calculated alveolar oxygen with measured arterial oxygen. A widened gradient suggests that hypoxaemia is not explained by hypoventilation alone and points to impaired gas exchange such as ventilation-perfusion mismatch or shunt.
ARDS severity: the PaO₂/FiO₂ ratio and oxygen index
The PaO₂/FiO₂ (P/F) ratio quantifies the degree of oxygenation impairment and supports classification of ARDS severity under the Berlin definition. The oxygen index combines FiO₂, mean airway pressure, and PaO₂ to gauge the severity of oxygenation failure in mechanically ventilated patients, a measure used particularly in paediatric respiratory failure.
High-flow therapy: the ROX index
The ROX index combines oxygenation (SpO₂/FiO₂) with respiratory rate to estimate the likelihood that high-flow nasal cannula therapy will succeed in hypoxaemic respiratory failure, supporting decisions about when to escalate support.
COPD prognosis: the BODE index
The BODE index is a multidimensional prognostic measure for COPD that combines body mass index, airflow obstruction, dyspnoea, and exercise capacity. It is used to predict mortality and inform decisions such as transplant evaluation, and it is read over time rather than as an isolated value.
Reading oxygenation numbers
Oxygenation indices depend on the FiO₂ and ventilator settings at the moment of measurement, so trends within a stable setting mean more than single values. They are decision-support measures that complement, not replace, bedside assessment and the broader clinical trajectory.
Calculators in this guide
Related guides
References
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- West JB. Respiratory Physiology: The Essentials. 11th ed. Wolters Kluwer; 2022.
- ARDS Definition Task Force; Ranieri VM, et al. Acute respiratory distress syndrome: the Berlin definition. JAMA. 2012;307(23):2526-2533.
- Pediatric Acute Lung Injury Consensus Conference Group. Pediatric Acute Respiratory Distress Syndrome: Consensus Recommendations From the Pediatric Acute Lung Injury Consensus Conference. Pediatr Crit Care Med. 2015;16(5):428-439.
- Roca O, et al. Predicting success of high-flow nasal cannula in pneumonia patients with hypoxemic respiratory failure: The utility of the ROX index. J Crit Care. 2016;35:200-205.
- Celli BR, Cote CG, Marin JM, et al. The Body-Mass Index, Airflow Obstruction, Dyspnea, and Exercise Capacity Index in Chronic Obstructive Pulmonary Disease. N Engl J Med. 2004;350(10):1005-1012.
- Global Initiative for Chronic Obstructive Lung Disease (GOLD). Global Strategy for the Diagnosis, Management, and Prevention of Chronic Obstructive Pulmonary Disease. 2024 Report.