Temporal effects in the estimation of pulmonary diffusing capacity

Steady state estimates of the pulmonary diffusing capacity for carbon monoxide (DLCO) require measurement of the uptake and the average alveolar partial pressure of carbon monoxide (PACO). The expired alveolar sample obtained by different experimental methods and/or breathing patterns rarely represents the actual PACO. It is widely accepted that nonuniform distribution of ventilation, diffusion and perfusion causes discrepancies in the measurement of diffusing capacity. tan additional source of error in choosing PACO arises in the sampling time chosen by the experimental method. A theoretical study of a ramp-with-pause and a square breathing pattern demonstrates that the sample-time error exists even in the homogeneous lung. The study shows for the homogeneous lung that the correct fractional concentration of alveolar carbon monoxide (FAV) occurs at a time (TAV), one-half of a breathing period after the effective inspiration time (TI) for the two very different breathing patterns. TI is well-defined in relation to any breathing pattern which can be approximated by ramps and pauses. If TAV and the sample time chosen by the experimental method are known, then the measured DLCO can be corrected to the actual diffusing capacity (DL). The theory agrees with experimental results and computer simulations of inhomogeneous lungs from the literature. This agreement suggests that the theory for the homogeneous lung is also relevant to the inhomogeneous lung.     

Kinetics of CO uptake and diffusing capacity in transition from rest to steady-state exercise.

In the transition from rest to steady-state exercise, O2 uptake from the lungs (VO2) depends on the product of pulmonary blood flow and pulmonary arteriovenous O2 content difference. The kinetics of pulmonary blood flow are believed to be somewhat faster than changes in pulmonary arteriovenous O2 content difference. We hypothesized that during CO breathing, the kinetics of CO uptake (VCO) and diffusing capacity for CO (DLCO) should be faster than VO2 because changes in pulmonary arteriovenous CO content difference should be relatively small. Six subjects went abruptly from rest to constant exercise (inspired CO fraction = 0.0005) at 40, 60, and 80% of their peak VO2, measured with an incremental test (VO2peak). At all exercise levels, DLCO and VCO rose faster than VO2 (P less than 0.001), and DLCO rose faster than VCO (P less than 0.001). For example, at 40% VO2peak, the time constant (tau) for DLCO in phase 2 was 19 +/- 5 (SD), 24 +/- 5 s for VCO, and 33 +/- 5 s for VO2. Both VCO and DLCO increased with exercise intensity but to a lesser degree than VO2 at all exercise intensities (P less than 0.001). In addition, no significant rise in DLCO was observed between 60 and 80% VO2peak. We conclude that the kinetics of VCO and DLCO are faster than VO2, suggesting that VCO and DLCO kinetics reflect, to a greater extent, changes in pulmonary blood flow and thus recruitment of alveolar-capillary surface area. However, other factors, such as the time course of ventilation, may also be involved.(ABSTRACT TRUNCATED AT 250 WORDS)     

Morphometric estimation of oxygen diffusing capacity for the air-sac in the catfish Heteropneustes fossilis

A detailed morphometric study has been made of the air-sacs of this air-breathing catfish using whole mounts, light and electron microscopy, of six specimens, body weight 40±2 g. Measurements of surface areas of the gas exchange and non-respiratory surfaces have taken into account foldings of the surface at macro and ultramicroscopic levels. Area of the gas exchange surface was estimated as 23.915cm² (=0.598cm²/g) which is 67%of the total surface area of the two air-sacs. Significant differences were found in some morphometric parameters which were related to the three antero-posterior regions into which air-sacs were divided. Harmonic mean thickness of the tissue component of the air/blood barrier was estimated for the whole air-sac as 0.342 μ m. These and other measurements enabled the diffusing capacity for the air-sacs to be calculated as 0.0638 m1O₂/min/mmHg/kg.
These results show that Heteropneustes has an air-breathing organ which is superior to that of Amphipnous cuchia , similar to that of Lepidosiren , but less well developed than that of Protopterus . In addition, Heteropneustes is well adapted to obtain oxygen directly from water by means of its gills and skin as indicated by both morphometric and physiological measurements which also correlate with its life in ponds and streams which are Iiable to dry up.     

Dynamic measurements of CO diffusing capacity using discrete samples of alveolar gas

It has been shown that measurements of the diffusing capacity of the lung for CO made during a slow exhalation [DLCO(exhaled)] yield information about the distribution of the diffusing capacity in the lung that is not available from the commonly measured single-breath diffusing capacity [DLCO(SB)]. Current techniques of measuring DLCO(exhaled) require the use of a rapid-responding (less than 240 ms, 10-90%) CO meter to measure the CO concentration in the exhaled gas continuously during exhalation. DLCO(exhaled) is then calculated using two sample points in the CO signal. Because DLCO(exhaled) calculations are highly affected by small amounts of noise in the CO signal, filtering techniques have been used to reduce noise. However, these techniques reduce the response time of the system and may introduce other errors into the signal. We have developed an alternate technique in which DLCO(exhaled) can be calculated using the concentration of CO in large discrete samples of the exhaled gas, thus eliminating the requirement of a rapid response time in the CO analyzer. We show theoretically that this method is as accurate as other DLCO(exhaled) methods but is less affected by noise. These findings are verified in comparisons of the discrete-sample method of calculating DLCO(exhaled) to point-sample methods in normal subjects, patients with emphysema, and patients with asthma.     

Characteristics of pulmonary diffusing capacity for CO at rest and during exercise

Characteristic patterns of changes in pulmonary diffusing capacity (DL) at rest and during exercise were investigated and characteristics of normal DL values concerned on sex, age, and ethnic groups were examined by viewing our studies and other reports. The relation of DL and pulmonary capillary blood volume (Vc) was represented as a logarithmic regression at rest and as a linear regression during exercise. The curve relation at rest is considered to show that the increase in Vc mainly reflects the process of transport from pulmonary capillary recruitment to pulmonary capillary dilation. The increasing rate of DL was not decreased during exercise, which seemed to be due to an increase in pulmonary blood flow accompanying exercise. The linear regression was also found between DL and oxygen intake during exercise and the slope was always constant among individuals and among subject groups. The general results concerned with sex difference in Japanese or ethnic difference between Japanese and Caucasians in both sexes could show that DL per stature was greater in males or Caucasians than in females or Japanese in young adults, however, the sex or ethnic difference disappeared in middle or old aged group. DL per alveolar volume which showed no sex or ethnic difference in young adults, was greater in middle or old aged group of females or Japanese than in that of males or Caucasians.     

Acute embolization of the uteroplacental circulation: uterine blood flow and placental CO diffusing capacity

In an effort to determine to what extent the fetal sequalae following repeated embolization result from decreased area of placental exchange or from decreased uterine blood flow, we injected microspheres into the uterine circulation of the pregnant ewe. We measured total UBF continuously and sampled fetal blood gases in 6 chronically instrumented ewes following repeated injections of 1 to 2 million 25 mu microspheres into the common internal iliac artery at 30 min intervals. Embolization resulted in an immediate 25 to 30% drop in uterine flow, with partial recovery to about 85% of its control value within 30 min after injection. A linear relation existed between uterine blood flow and fetal O2 tension. A slightly accelerated decrease in O2 content with a more rapid increase in CO2 tension and [H+] was seen when uterine flow decreased below 150 ml X min-1 X kg fetal wt-1. Following repeated injections fetal descending aortic O2 tension and content decreased 34 and 82% respectively, while PCO2 and [H+] increased 28 and 84% respectively. Placental diffusing capacity for CO increased 117% after repeated embolization. Most of this increase could be accounted for by the fetal hypoxia and acidosis, although some of it may have resulted from distension or recruitment of vessels in the placental exchange area, or a more uniform distribution of placental blood flows. These studies suggest that the acute changes in fetal blood gas values following embolization result from a reduction in blood flow rather than from a reduced placental exchange area.