Leon E. Farhi

Elimination of inert gas by the lung

Abstract When an inert gas G which is not part of the inspirate is present in the mixed venous blood at a partial pressure PVG, the partial pressure of G in an alveolus, PaG, or in the blood returning from this alveolus will be governed by several factors. These are λ, the Ostwald partition coefficient for that gas, a , the ventilation of the alveolus, and , its perfusion, according to the equation Pa = rmP r mV . λ/λ + V a / Q . The clearance of such a gas, Cl, is given by ( a . )/( a + λ), indicating that when a / is much higher than λ, the clearance is dictated mainly by the ventilation, while at a / considerably lower than γ, the perfusion is the determining factor. The fractional elimination of the gas is given by Cl/, and increases with a / but decreases with an increase in λ. As a result, the lung acts as a filter, retaining selectively the gases having a high solubility. When chemical transport of O2 and CO2 is taken into account, the behavior of these gases follows the same general pattern of inert gas exchange.

Cardiopulmonary readjustments during graded immersion in water at 35 °C

Six normal male volunteers, aged 25 to 34, suspended vertically in a harness that allowed them to completely relax their postural muscles, were studied in four randomly ordered conditions, namely in air at 28 degrees C, and immersed in water at 35 degrees C to the level of the hips, the xiphoid, or the chin. In each situation, several variables were measured by noninvasive techniques. Cardiac output rose from 5.11 min-1 (air) to 8.31-min-1 (chin), the increase in each of the three steps being significant at the 0.001 level. Heart rate dropped from 76 to 68 min-1 (P less than 0.001) from air to xiphoid immersion, but appeared to rise again (P less than 0.02) during chest immersion. Functional residual capacity decreased marginally during lower limb submergence, and considerably in each of the following stages. Pulmonary capillary blood volume rose significantly only during abdomen immersion. The arterial-endtidal PCO2 difference was minimally reduced as water reached hip level and then remained steady. Mixed venous PO2 increased during abdomen submergence, and PVCO2, was unaltered throughout. Analysis of the step-to-step changes demonstrates that some variables are set by a combination of processes which may counteract each other, and explains the difference between results obtained by previous investigators.

A system of digital computer subroutines for blood gas calculations.

Abstract A system of digital subroutines which considers all possible relationships between the gas constituents of blood is described. The subroutines apply to blood samples with any level of hemoglobin and base excess. Two specific applications of the subroutines are given: 1) a program that will draw a Dill nomogram of the blood, and 2) a program that constructs Va/q lines.

Cardiac output determination by simple one-step rebreathing technique.

We have developed a rebreathing technique for measuring cardiac output in resting or exercising subjects. The data needed are the subjects CO2 dissociation curve, the initial volume and CO2 fraction of the rebreathing bag, and a record of CO2 at the mouth during the maneuver. From these one can obtain all the values required to solve the Fick equation. The combined error due to inaccuracy in reading the tracings and to the simplifying assumptions was found to be small (mean = 0.5%, SD ;.5%). Cardiac output values determined with this technique in normal subjects were on the average 2% higher than those obtained simultaneously with an acetylene rebreathing method (n = 49, SD = 11%). Among the advantages of the technique are that it requires analysis of a single gas, takes less than thirty seconds per determination, allows one to obtain repeated measurements at rapid intervals, is not affected by the ability of lung tissue to store CO2, and eliminates many of the assumptions usually made in non-invasive measurements of cardiac output.

Fluctuations in alveolar CO2 and in base excess during the menstrual cycle1

End-tidal PCO2 and base excess were measured on 10 healthy females throughout a total of 16 menstrual cycles. A corresponding decrease was observed in both variables during the luteal phase of the cycle. The curve for PCO2 was essentially that reported by previous authors, the tension dropping from a mean value of 39.8 to 36.7, 10 days after ovulation. The parallel drop in BE lowered the variable from -0.5 to -2.6. The resulting arterial pH throughout the cycle was calculated to be between 7.38 and 7.40. Since no time lag could be observed between the change in PCO2 and that of base excess, the primary disturbance could be either a ventilatory or metabolic response to an increase in progesterone during the luteal phase. In either case, a considerable decrease in body CO2 stores must result from these changes.

Effects of ventilation-perfusion inequality on elimination of inert gases☆

Abstract Since elimination of an inert gas by the lung depends on the a / ratio, elements which have different ratios will show a different pattern of elimination. Alveoli having a high a / will contribute to the “alveolar dead space.” However, the extent of this contribution depends on the gas considered, with gases of low solubility being better suited for detecting these hyperventilated elements. Alveoli with a low a / contribute to the “venous admixture,” the effect being more pronounced with gases of high solubility. Consideration of the simultaneous elimination of two different inert gases allows to draw a a / line for these two gases. This can be used to assess a / inhomogeneity in terms of a two-compartment model.

Alveolar to mixed venous Pco2 difference under conditions of no gas exchange

Abstract Under conditions in which no gas exchange occurs across the mammalian lung a steady state difference between alveolar and mixed venous P CO 2 (ΔP CO 2 ) was found, the alveolar being higher. The ΔP CO 2 appeared to be related to the H + and HCO 3 − concentrations of the mixed venous blood and to the pulmonary blood flow rate. A model is presented involving a negatively charged capillary membrane and coupling of viscous and diffusional flows allowing the reaction H + + HCO 3 H 2 CO 3 H 2 0 + CO 2 to be shifted toward CO 2 and causing the CO 2 tension to be higher in the region of the capillary wall than in the bulk phase of capillary blood. Evidence, in support of the theoretical model, that steady state differences of the undissociated form of the weak acid DM0 can occur across the lung and that under certain conditions P CO 2 differences can occur across charged artificial membranes, is also presented.

Determination of mixed venous O2 and CO2 tensions and cardiac output by a rebreathing method

Abstract Rebreathing from a bag containing a volume approximating the functional residual capacity of 8–9 % CO 2 in N 2 results in alveolar O 2 and CO 2 tensions which become stable four to eight sec after the beginning of rebreathing and are maintained for approximately six sec. These tensions are believed to reflect mixed venous blood tensions. Ranges at rest are from 35 to 52 mm for P O 2 and 44 to 47 mm for P CO 2 . During exercise, P O 2 decreases and at an oxygen uptake of two 1/min is of the order of 30 mm Hg. Cardiac output values calculated from these data agree well with figures obtained with another method.

On mathematical analysis of gas transport in the lung

Abstract The process of gas transport in the lung. involving two mechanisms, i.e., mass convection and molecular diffusion, may be analyzed mathematically. Several such analyses, taking the classical approach, the random walk approach and a nodal analysis, are reviewed. A detailed comparison, based on the pliysical model, the mathematical representation of the physical model, the method of solution, md the final results, is made for these analyses. The underlying assumptions of these analyses are also critically examined and suggestions for possible improvement are made.

Study of ventilation-perfusion ratio distribution in the anesthetized dog by multiple inert gas washout☆

Abstract In order to determine the a / distribution in the anesthetized, artificially ventilated, supine dog, the animal was first allowed to breathe for 20 min a mixture of methane, ethane, nitrous oxide (having Ostwald partition coefficients of 0.04, 0.11, and 0.52, respectively) and oxygen. During the subsequent washout the concentration of the tracer gases was measured at intervals in the expired gas and in the arterial and mixed venous blood, using a gas Chromatograph. When the results were analyzed in terms of a two-compartment system, a distribution pattern, which would be difficult to demonstrate by O 2 and CO 2 pressure differences, was detected. The methane-ethane system indicated the existence of a compartment having a a / of less than 0.10. This low a / compartment has a ventilation of less than 2.4% of the overall alveolar ventilation, but receives 10–29% of the total blood flow.