The Effect of FiO2 on PaO2/FiO2 Ratio

Overview

The PaO2/FiO2 ratio is frequently used to determine the severity of lung injury in mechanically ventilated patients. However, several mathematical models have shown that PaO2/FiO2 ratio depends on FiO2. The relationship is complex and depends on numerous physiological variables, including shunt fraction, and arterio-venous oxygen difference. The nonlinear relation between PaO2/FiO2 and FiO2 underlines the limitations describing the intensity of hypoxemia using PaO2/FiO2 and is thus of major importance for the clinician. Surprisingly, this relationship has only been assessed mathematically. Obviously, the accuracy of the mathematical relationship depends on the input variables used. The current study is designed to assess the PaO2/FiO2 vs FiO2 relation in mechanically ventilated patients without ARDS (n =10) and with mild or moderate ARDS (n =10). In order to explain the dependency of the PaO2/FiO2 on FiO2, shunt fraction and alveolar - arterial oxygen difference ((A-a)DO2) will be determined in these patients.

Rationale: The PaO2/FiO2 ratio is frequently used to determine the severity of lung injury in mechanically ventilated patients. However, several mathematical models have shown that PaO2/FiO2 ratio depends on FiO2. The relationship is complex and depends on numerous physiological variables, including shunt fraction, and arterio-venous oxygen difference. The nonlinear relation between PaO2/FiO2 and FiO2 underlines the limitations describing the intensity of hypoxemia using PaO2/FiO2 and is thus of major importance for the clinician. Surprisingly, this relationship has only been assessed mathematically. Obviously, the accuracy of the mathematical relationship depends on the input variables used. The current study is designed to assess the PaO2/FiO2 vs FiO2 relation in mechanically ventilated patients without ARDS (n =10) and with mild or moderate ARDS (n =10). In order to explain the dependency of the PaO2/FiO2 on FiO2, shunt fraction and alveolar - arterial oxygen difference ((A-a)DO2) will be determined in these patients. Objective: To study the relation between PaO2/FiO2-ratio and FiO2 Study design: An unblinded, prospective, interventional study Study population: Mechanically ventilated patients \> 18 years, admitted to the Intensive Care Unit of VU-Medical Center post cardiac surgery without ARDS (n = 10) and with mild to moderate ARDS (n = 10). Intervention: Two interventions will be performed: 1. Modulation of FiO2: FiO2 will be reduced to 21% or until peripheral oxygen saturation of 92%, whatever occurs first. Subsequently FiO2 will be increased up to 100%. 2. Withdrawal of blood: Blood will be withdrawn from the indwelling arterial line and pulmonary artery catheter. No catheters will be inserted for the study. The maximum number of time points is 7. At each time point 1.5 ml of blood will be withdrawn from both the arterial and pulmonary artery catheter for blood gas analysis. At the start and the end of the study period 2 additional blood samples of 5 ml each will be drawn. Accordingly, the maximum amount of blood obtained will be less than 50 ml. Main study parameters/endpoints: At each level of FiO2 the following parameters will be assessed: * Arterial blood gas analysis, including SaO2 * Central Venous oxygen saturation and content * Hemoglobin * Body temperature * End tidal CO2 * VO2 and VCO2 * Ventilatory parameters (tidal volume, respiratory rate, Ppeak Paw) * Hemodynamics (Bloodpressure and Heart Rate)