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Pulmonary function indices in spontaneously breathing subjects


Assessment of pulmonary gas exchange and adequacy of oxygen supply is complex. At least five different pulmonary indices are available in clinical practice. Modern blood gas analysers allow for calculations of these pulmonary indices. A prerequisite for accurate calculation is a precise determination of the inspired O2 fraction [FO2(I)]. In spontaneously breathing patients it is difficult to estimate FO2(I). The Venturi mask delivers a precise FO2(I) (fixed performance system), whereas the Large nasal catheter (variable performance system) delivers a variable FO2(I), which is dependent on the patient's ventilatory pattern. The purpose of this study is to determine the influence of FO2(I) on five different pulmonary indices.


(i) To calculate five different pulmonary indices, presuming the end-tidal O2 tension (pO2(ET)] to be the best expression of alveolar gas composition. (ii) To generate computer calculations of the five indices referring to the fluctuations of FO2(I) when a variable performance system delivers O2.

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The pO2(ET) in the airway was continously recorded during the use of different O2supply systems and flows in 10 healthy spontaneously breathing volunteers.


Oxygen was supplied through two different supply systems: Venturi mask and Large nasal catheter. The Brüel & Kjær gas monitor 1304 was used to measure the gas tensions in the airway. The sampling tube of the monitor was connected to a soft 12 CH PVC catheter placed in the subject's upper airway. Simultaneously with the pO2(ET) recordings blood was drawn from a radialis and analysed by the ABL 520 (Radiometer, Medical).


The SD of the measured FO2(I) was small when O2 was supplied through the Venturi mask (40%, SD = 0.9%) and when the subject breathed atmospheric air (SD = 1.1%) but increased with increasing O2 flows with the use of the Large nasal catheter (30 IO2, SD = 6.5%). The mean values of the pulmonary indices for the different O2 flows are summarised in the Table. Normal values: pO2(a/A)>0.75; pO2[(A-a)/a] = 0.1– 0.37; pO2(a)/FO2(I) = 42–70; FShunt<0.15.


As expected pO2(ET-a) increases with higher FO2(I) values. The increase in FO2(I) docs not influence the calculated indices. The FShunt is defined by assuming a fixed-mixed-venous oxygen concentration difference of 2.3 mmol/1. which may not be the case in some clinical situations.


When FO2 is measured in the airway in a healthy subject the corresponding calculated indices turn out to be in the normal range. Suggestion for the optimal pulmonary index in healthy volunteers, based on estimation of FO2(I), will be presented by computer generation.


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Waldau, T., Larsen, V., Bonde, J. et al. Pulmonary function indices in spontaneously breathing subjects. Crit Care 1 (Suppl 1), P065 (1997).

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