Modeling the impacts of assumptions and nonpulmonary factors on the performance and reliability of indices of oxygenation

AbstractAssessment of oxygenation is fundamental to the care of patients. Numerous indices of oxygenation have been developed that entail variable degrees of invasiveness, complexity and physiologic underpinning. The clinical reliability of these indices has been questioned. This theoretical study uses a steady-state model of blood gas physiology to study the assumptions and nonpulmonary factors that have been hypothesized to impact index performance. A model derived from cardiac and pulmonary Fick expressions was used to calculate the effects of the physiological parameters —shunt, dead space, cardiac output, ventilation, oxygen extraction, carbon dioxide elimination, hematocrit, temperature and base excess—on predicted arterial, mixed-venous and post-capillary oxygen contents and arterial and alveolar oxygen and carbon dioxide partial pressures. Values of these pa rameters were determined over a range of shunt from 0 to 50% and then used to calculate (1) estimated shunt with the shunt equation and FShunt, and (2) the alveolar-arterial partial pressure of oxygen difference (A-a\({O}_{2}\) gradient), and the arterial partial pressure of oxygen to fraction of inspired oxygen (Pa/Fi) ratio. Calculations were performed either treating parameters as fixed (assuming several values) or as random variables. Assumptions of constant arterio-venous oxygen content and of alveolar and arterial partial pressures of carbon dioxide being equal were shown to fail in certain settings wher...
Source: Journal of Clinical Monitoring and Computing - Category: Information Technology Source Type: research