The feasibility of replacing a conventional mass spectrometer (MS) with a specially modified multicomponent (O2, CO2, Freon 22, and SF6) acoustic infrared and paramagnetic (IR/PM) gas analyzer in inert gas-rebreathing and metabolic gas exchange measurements has been investigated. Rebreathing variables were determined simultaneously with the MS and IR/PM analyzers in duplicate measurements at rest and during submaximal exercise in 10 subjects. The differences (means +/- SD, IR/PM - MS) were 0.028 +/- 0.048 liters [functional residual capacity (FRC)], 0.18 +/- 0.38 l/min [cardiac output (Qc)], -0.006 +/- 0.030 l/min [O2 consumption (VO2)], and -33 +/- 108 ml [combined lung tissue and capillary blood volume (Vti,c)]. The coefficients of variation on repeated estimates were 5.8% (FRC), 5.4% (Qc), 6.2% (VO2), and 17% (Vti,c) with the IR/PM analyzer and 5.9% (FRC), 4.2% (Qc), 5.0% (VO2), and 9.8% (Vti,c) with the MS. The differences (IR/PM - MS) obtained in mixed-expirate measurements were -0.006 +/- 0.020 l/min (VO2) and 0.020 +/- 0.021 l/min (CO2 production). Breath-by-breath estimates of VO2 and CO2 production with the IR/PM analyzer were, on average, 2.4 and 4.4% higher than the MS estimates, respectively. Our results demonstrate that the IR/PM gas analyzer, when appropriately modified, can substitute for a complex MS in a variety of noninvasive pulmonary gas exchange measurements.

A new method for breath-to-breath determination of the oxygen flux across the alveolar membrane is described. The principle of the method is to integrate the product of oxygen concentration and flow in the respiratory gas over an interval, which covers a complete respiratory cycle. The result is corrected for the change in oxygen content of the lungs through a formula, which, in contrast to those used in other methods, is independent of the residual capacity of the lungs. The method was evaluated with respect to repeatability by repetitive measurement of oxygen flux in twenty volunteer subjects, and with respect to accuracy by comparing the measured oxygen fluxes with those obtained by the gas collection method. The coefficient of variation was found to be 8% and the breath to breath determinations were, on an average, 6% lower than those of the gas collection method.

Analytic expressions were derived for the response time, flow dependency, and signal-to-background ratio of blood gas catheters. These expressions were utilized to optimize the design of a new catheter. The catheter was tested in vitro and in vivo, and shown to be fast responding and practically flow independent, with an acceptable signal-to-background ratio.

Stout et al. (J. Appl. Physiol. 38:913-918, 1975) suggested an open-circuit multibreath (MB) inert gas method for determining pulmonary capillary blood flow (Qc) in anesthetized dogs receiving artificial ventilation. In the present work we investigated the accuracy and reproducibility of the MB method in nine healthy human subjects at spontaneous ventilation, and we compared the MB method with the inert gas rebreathing (RB) method. Qc was calculated at rest and during exercise at 50 or 100 W, and experimental errors were evaluated in computer simulations of a two-alveoli lung model. The calculated mean Qc values of the MB method were 5.56 +/- 1.23 (SD), 10.02 +/- 0.78, and 13.2 +/- 0.84 l/min, and the mean difference (MB Qc - RB Qc) was not significantly different (P > 0.05). The variation in Qc of the MB method was found to be significantly larger than that in Qc of the RB method (P < 0.01). Random measurement errors and uneven distribution of ventilation contributed to the experimental errors. We conclude that the MB method is inferior to the RB method but that the MB method may be useful under exercise conditions.

SummaryWhen a steady-state oxygen concentration is measured with a membrane-covered probe in an open system, the oxygen consumption in the unstirred layer gives rise to an error of measurement whose seriousness depends on the kinetics of the oxygenconsuming process. First-order oxygen consumption in the sample causes a propoitional reduction in the signal so that the calibration curve remains linear. A zerothorder process causes the calibration curve to be offset from the origin, but it remains linear. Oxygen consumption according to the Michaelis-Menten equation causes the calibration curve to become nonlinear with the maximum deviation at the lower end of the scale. The error determines a lower limit for usefulness of membrane-covered probes. Steady-state kinetics at oxygen concentrations in the order of K M cannot be determined with a membrane-covered probe for enzymes with K M for oxygen lower than 0.0lp.M. In a dense culture of respiring microorganisms, no oxygen will reach the probe when the bulk concentration of oxygen is in the order of KM.