Indices of pulmonary oxygenation in pathological lung states: an investigation using high-fidelity, computational modelling

• Published in: British journal of anaesthesia : BJA Vol. 103; no. 2; pp. 291 - 297
• Main Authors: Kathirgamanathan, A., McCahon, R.A., Hardman, J.G.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01-08-2009; Oxford University Press; Oxford Publishing Limited (England)
• Subjects: Anesthesia; Anesthesia. Intensive care medicine. Transfusions. Cell therapy and gene therapy; Biological and medical sciences; Carbon Dioxide - blood; Computer Simulation; gas exchange; Hemoglobins - metabolism; Humans; lung; lung damage; lung, respiratory distress syndrome; mathematical; measurement techniques; measurement techniques, gas exchange; Medical sciences; model, lung damage; model, mathematical; Models, Biological; Oxygen - blood; Oxygen Consumption - physiology; Partial Pressure; Pulmonary Gas Exchange - physiology; Respiration; respiratory distress syndrome; Respiratory Distress Syndrome, Adult - blood; Respiratory Distress Syndrome, Adult - physiopathology; lung, respiratory distress syndrome; measurement techniques, gas exchange; model, lung damage; model, mathematical; lung; measurement techniques; respiratory distress syndrome; Respiratory disease; Measurement technique; Anesthesia; Respiratory distress; model; Gas exchange; Oxygenation; lung damage; mathematical
• ISSN: 0007-0912; 1471-6771
• DOI: 10.1093/bja/aep140

Existing indices of pulmonary oxygenation vary misleadingly with external factors such as inspired oxygen fraction (Fio2), arterial carbon dioxide tension (Paco2), and haemoglobin (Hb). Previous work suggested that some indices may be acceptably useful in particular scenarios such as acute respiratory distress syndrome (ARDS) or where Fio2>60%. However, it is not possible to identify such scenarios in most clinical contexts; therefore we aimed to examine the induced variability of existing indices in a population of patients with a variety of lung defects.
We configured nine virtual patients within the Nottingham Physiology Simulator, each with a unique pulmonary configuration but identical arterial blood gases at Fio2 30%, Paco2 6.0 kPa and Hb 8.0 g dl−1. Factors (Fio2, Pco2, Hb) were varied independently and indices of oxygenation including calculated venous admixture (Qs/Qt), arterial oxygen tension (Pao2/Fio2), arterio-alveolar gas tension gradient (Pa–ao2), and respiratory index (Pa–ao2/Pao2) were recorded.
All indices varied with Fio2, with greatest variation with lung defects having least true (absolute) shunt. Calculated Qs/Qt resisted induced variation best of all the indices, but varied by 30% of its mean value during Fio2 variation. Pao2/Fio2 varied greatly, especially during variation in Fio2 (up to 74% of its average value), and most markedly in defects with little true (absolute) shunt. Paco2 and Hb variation caused small, consistent changes in all indices that were similar between lung-states.
No existing index of oxygenation adequately describes the severity of gas exchange defect. Existing indices of oxygenation vary with disease severity, disease type, and external factors such as Fio2. A novel and robust index is needed.