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Department of Physiology, University of Bonn, 53115 Bonn, Germany
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ABSTRACT |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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The contribution of reaction resistance to
overall resistance to pulmonary carbon monoxide (CO) uptake
[DLCO/(
CO · Vc),
where DLCO is lung CO diffusing capacity,
CO is CO uptake conductance of erythrocytes, and Vc is
pulmonary capillary blood volume] was determined in 10 anesthetized, paralyzed, and artificially ventilated rabbits. On the
basis of the classical double-reciprocal equation of F. G. W. Roughton
and R. E. Forster (J. Appl. Physiol. 11: 290-302, 1957),
DLCO/(CO · Vc)
was obtained by solving the relation DLCO/(CO · Vc) = 1 
2/(DLNO/DLCO),
where
DLNO/DLCO
represents the ratio between the respective single-breath diffusing
capacities (DL) of nitric
oxide (NO) and CO pulmonary capillary blood. The lungs of
eight rabbits were inflated, starting from residual volume, by using 55 ml of indicator gas mixture (0.2% CO and 0.05% NO in nitrogen).
DL values were calculated by
taking the end-tidal partial pressures of CO and NO as analyzed by
using a respiratory mass spectrometer. The overall value was
DLCO/(CO · Vc) = 0.4 ± 0.025 (mean ± SD). Because of the use of
O2-free indicator gas mixtures,
the end-tidal O2 partial pressures
were ~21 Torr. In one other rabbit, the application of 0.2% CO and
0.001% NO yielded DLCO/(CO · Vc) = 0.39; in the tenth rabbit, however, inspiratory volume was varied,
and an identical value was found at functional residual capacity. We
conclude that the contribution of reaction resistance to overall
resistance to pulmonary CO uptake is independent of the inspiratory NO
concentration used, including, with respect to the pertinent
literature, the conclusion that in rabbits, dogs, and humans this
contribution amounts to 40% when determined at functional residual
capacity.
single-breath diffusing capacity; nitric oxide
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INTRODUCTION |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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ADOPTING THE FAMILIAR concept of Roughton and Forster
(10), the overall pulmonary resistance to carbon monoxide (CO) uptake (1/DLCO)
can be partitioned into the sum of diffusion
(1/DmCO) and reaction
[1/(CO · Vc)]
resistances
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(1) |
CO · Vc),
which is given as the product of the specific CO uptake conductance of
erythrocytes (CO) and the
pulmonary capillary blood volume (Vc). In particular, the in vitro
determination of CO, derived
from the initial rates of CO uptake by erythrocytes, has not yet been
conclusively evaluated because measurements of erythrocyte reaction
kinetics may have been disturbed by unstirred layers in the external
medium (3, 12), constituting an additional diffusion resistance to CO
uptake. Therefore, the relative contributions of the diffusion and
reaction resistances to
1/DLCO
remain unclear, aggravating the transformation of alveolar-capillary CO
uptake conditions to respired gases.
A promising novel approach to circumventing such methodical problems has been the additional determination of the pulmonary diffusing capacity of nitric oxide (DLNO) by the single-breath technique performed on humans (1, 4, 6, 7) and by application of the rebreathing method to dogs (8). On the basis of the unique features of nitric oxide (NO), it was always expected that reaction kinetics would limit the pulmonary uptake of NO less than that of CO and, therefore, that the overall resistance to pulmonary NO uptake (1/DLNO) would be almost exclusively caused by diffusion resistance of the alveolar-capillary membrane (1/DmNO). Because the diffusion resistances to NO and CO uptake should be related to each other in inverse proportion to the Krogh diffusion constant ratio of both gases (KNO/KCO), Eq. 1 has been transformed into
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(2) |
CO · Vc)
represents the contribution of reaction resistance to overall
resistance to CO uptake, and
KNO/KCO = 2. However, by applying Eq. 2 to the
studies mentioned above, significantly diverse values of
DLCO/(CO · Vc)
are obtained in humans [0.58 ± 0.04 (mean ± SD) of 4 studies] and in dogs (0.38). Despite species-specific
differences, this deviation could have also been caused by the
different methods used. Nevertheless, the separation of diffusion and
reaction resistances to pulmonary CO uptake remains uncertain.
The present study was thus designed to provide additional values of
DLCO/(CO · Vc)
by applying the single-breath method to rabbits to determine the
pulmonary diffusing capacity
(DL) for the two gases, NO
and CO.
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METHODS |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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Single-breath maneuvers were carried out on 10 anesthetized, paralyzed, supine rabbits (mean body wt, 3.4 kg; range,
3.0-3.9 kg). Experiments were approved by the local animal ethics
committee. The animals were anesthetized by using pentobarbital sodium
(19 mg · kg
1 · h1
iv), paralyzed by alcuronium (0.1 mg · kg1 · h1
iv), intubated by using a cuffed endotracheal tube, and artificially ventilated by using a self-made ventilatory servo system.
Mass spectrometry. We used a respiratory magnetic sector mass spectrometer (M3; Varian MAT, Bremen, Germany) to detect the relevant gases (CO, NO, O2, CO2, and Ar) at ion collectors set at the following mass-to-charge ratios (m/e): 12 (CO), 30 (NO), 32 (O2), and 44 (CO2). Ar (m/e = 40) was recorded at the CO2 44-ion collector by repeatedly changing the accelerating voltage (peak jump). Because the signal at m/e = 12 also included C+ generated from CO2, this contribution was electronically subtracted. The cross talk between both signals was further reduced and drift errors during CO and NO measurements were avoided by repeatedly comparing the alveolar sample gas against a reference gas which differed only in its CO and NO content. The signal-to-noise ratios for both indicator gases were 680:1 at 2,000 parts/million (ppm) CO; 3,400:1 at 500 ppm NO; and 110:1 at 10 ppm NO.
Gas mixtures. To avoid the oxidation of NO and to facilitate the comparison of DLNO and DLCO values, three gas mixtures (0.2% CO in N2, 0.05% NO in N2, and 0.2% CO + 0.05% NO in N2) were prepared by leading the respective pure gases into a gastight flexible aluminum bag which had been washed out repeatedly with N2. NO, especially, was led through diluted KOH to purify it from traces of NO2 or N2O4.
Experimental protocol.
After the onset of anesthesia in each animal, pressure-volume curves
were recorded. For this purpose, the lungs were inflated and deflated
in definite steps of volume, and the airway pressure was measured
during short breath holds by using a differential pressure transducer.
The residual volume (VR) was
defined as the resting lung volume attained at 20
cmH2O of airway pressure and was
calculated from the Ar dilution induced by inflating the lungs with
N2, averaging
VR = 13.5 ± 1.2 ml.
Calculations for DL. Taking the end-tidal partial pressures of CO and NO, we calculated DLNO and DLCO values on the basis of three equations, defining gas transfer during inflation, breath holding, and deflation. DL values were obtained by applying a trial-and-error approximation method. A detailed description of the underlying model and the derivation of the three equations is given elsewhere (11).
Data analysis.
Results are given as means ± SD. To assess the influence of NO on
pulmonary CO uptake,
DLCO values
obtained from the separate and simultaneous application of CO and NO
were compared by using Student's paired
t-test. The level of significance was
set at P < 0.05. Ratios of
DLCO/DLNO
and
DLCO/(CO · Vc)
as determined in rabbits A and
B were compared with those obtained in
rabbits C-J by using the one-tailed
t-test (level of significance,
P < 0.05).
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RESULTS |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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NO does not affect measurements of
DLCO.
Results of measurements of
DLCO and
DLNO in
rabbits C-J are summarized for each
animal in Table 1. Despite both separate and simultaneous applications
of CO and NO,
DLCO
values did not vary significantly, averaging 0.66 ± 0.06 ml · Torr1 · min1
without NO, and 0.65 ± 0.05 ml · Torr1 · min1
in the case of a simultaneous application of NO. The
DLNO mean values also remained unchanged. Because we used
O2-free indicator gas mixtures,
the respective values of end-tidal partial pressure of oxygen
(PETO2) were ~21 ± 1.5 Torr during the three sets of experiments.
Contribution of reaction resistance to overall resistance to CO
uptake.
Values of
DLCO/(CO · Vc),
as calculated from applying Eq. 2 to
DLNO/DLCO
of rabbits C-J, are reported in Table
2.
DLNO/DLCO was 3.31 ± 0.15 (mean ± SD) for the separate measurements and 3.33 ± 0.13 when CO and NO were applied simultaneously.
Thus reaction resistance contributed to overall resistance to CO uptake
by 40 ± 2.5% [coefficient of variation (i.e., SD/mean)
±6.3%].
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1 · min1
and DLCO = 0.61 ± 0.04 ml · Torr1 · min1,
leading to
DLNO/DLCO = 3.28 and
DLCO/(CO · Vc) = 0.39. These values are no different from the corresponding mean
values of rabbits C-J.
By increasing VL in rabbit
B (weight, 3.5 kg,
VR, 13.7 ml; inspiratory gas
mixture, 0.2% CO + 0.05% NO in
N2) from FRC (intrapulmonary pressure, 0 cmH2O) to higher
values, we obtained a transient increase of
DLNO/DLCO
and
DLCO/(CO · Vc),
which was followed by a clear decrease in both ratios, approaching
control values at an alveolar volume of 1.7 × FRC (intrapulmonary pressure = 8 cmH2O; see Fig. 1).
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DISCUSSION |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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The present study was designed to determine
DLNO/DLCO
ratios and, hence, to provide additional data for quantifying the
contribution of reaction resistance
[1/(CO · Vc)]
to the overall resistance to pulmonary CO uptake
(1/DLCO).
As a crucial point of reference, we assumed that
NO is very high, allowing us to
set
1/(NO · Vc)
0, so as to develop Eq. 2.
Although this procedure has been used previously (8), it should be
judged by estimating DmNO on the
basis of data in the literature. However, very diverse details about the specific NO blood uptake conductance (given as
ml · ml1 · Torr1 · min1)
are obtained from determinations of
O2 (5, 12)
and NO (2), revealing
NO values of 4 (2), 3.9 (12),
and 14 (5). Because both the solubility and the reaction rate with hemoglobin are greater for NO than for
O2,
O2 is
expected to represent an underestimate of
NO. Because Heidelberger and Reeves (5) investigated
O2-transfer kinetics in a
unicellular thin layer of whole blood, the discrepancy
between the NO estimates may be
caused by diffusion limitation from unstirred layers (3, 5, 8) which
could have seriously biased the measurements performed by applying both
the rapid-mixing technique (2) and the stopped-flow technique (12).
Therefore, we used NO = 14 ml · ml1 · Torr1 · min1.
By assuming that Vc = 3 ml and
DLNO = 2.18 ml · Torr1 · min1,
as found in our animals, we obtained
DmNO 
2.27 ml · Torr1 · min1
(that is, only 4% higher than the overall
DLNO).
Thus Eq. 2 appears to provide a close
estimate of the contribution of reaction resistance to overall
resistance to CO uptake.
The main finding of the present study, that
DLCO/(CO · Vc)
amounts to 0.4 ± 0.025, corresponds to the result reported by Meyer
et al. (8) for rebreathing experiments in dogs but is far below the
mean
DLCO/(CO · Vc),
or 0.58 ± 0.04, as calculated from the single-breath studies
performed on humans by Borland and Higenbottam (1), Guénard et
al. (4), and Manier et al. (6, 7).
A possible explanation for this difference may be the
fact that in the present study
DLNO and
DLCO were
measured during normoxic ventilatory conditions but by using
O2-free indicator gas mixtures (PETO2, 21 Torr); however,
in the studies on humans, O2-containing gas mixtures
(FIO2 = 0.21) were used to perform the single-breath maneuvers. However, Meyer et al. (8) found
similar values of
DLCO/(CO · Vc)
even at various levels of oxygenation
[DLCO/(CO · Vc) = 0.38 for hypoxia
(PETO2 = 24 Torr,
as respectively determined at second 8 of rebreathing), and
DLCO/(CO · Vc) = 0.41 for normoxia (PETO2 = 67 Torr)]. Only from their rebreathing experiments during
hyperoxia (PETO2 = 390 Torr)
did Meyer et al. obtain
DLCO/(CO · Vc) = 0.64, which is similar in magnitude when compared with the studies
carried out in humans. Therefore, within 24 Torr < PETO2 < 67 Torr,
determinations of the contribution of reaction resistance to pulmonary
CO uptake appear to be rather independent of the respective level of
oxygenation.
Another explanation for the inequality between the values of
DLCO/(CO · Vc)
may be the use of different
FINO
in the animal studies [600 and 500 ppm NO, Meyer et al. (8) and
present study, respectively] and in the investigations performed
on humans [40 ppm NO, Borland and Higenbottam (1); 8 ppm,
Guénard et al. (4) and Manier et al. (6, 7)]. To examine
such an influence of
FINO, we
applied 0.001% NO (10 ppm) and 0.2% CO simultaneously to
rabbit A with no significant
difference in the values of
DLNO/DLCO or of
DLCO/(CO · Vc)
compared with experiments using 0.05% NO and 0.2% CO. Thus we found
that, in rabbits, reaction resistances equally contributed to overall
resistance to CO uptake, even at such diverse levels of
FINO as 10 or 500 ppm NO.
Borland and Higenbottam (1) investigated the effect of variations in
VL on the ratio
DLNO/DLCO.
They showed little change in
DLCO with a
75% increase of VL but a
distinct increase in
DLNO, leading to a rise of
DLNO/DLCO
values from 3.3 to 4.3. If we apply Eq. 2 to their data,
DLCO/(CO · Vc)
would nevertheless have increased by 36% of the control values. This
clearly contrasts with the study of Meyer et al. (8), who reported that
DLNO/DLCO
was slightly increased but was later significantly decreased through a
71% increase of VL, which also
applies to
DLCO/(CO · Vc).
Their findings are confirmed by the results in rabbit
B in the present study. Although the
mechanisms involved are not readily apparent, one decisive
methodological aspect may provide an explanation despite
species-specific differences. To investigate pulmonary gas exchange in
anesthetized, paralyzed animals, we inflated the lungs by increasing
intrapulmonary pressure. Thus, two opposing mechanisms counteractively
influence vascular volume, i.e., vascular volume is expected to
increase with increasing inflation but is simultaneously reduced with
increasing intrapulmonary pressure. In contrast, it is expected that at
the end of a single inspiration actively carried out by humans,
intrapulmonary pressure will attain ~0
cmH2O, and vascular volume should
rise with increasing inspiration. These confusing inconsistencies can
be ignored by comparing
DLNO/DLCO values which have been determined at similar levels of alveolar volume
(e.g., at FRC, 0 cmH2O of
intrapulmonary pressure). Applying these experimentally determined
values to the data of Borland and Higenbottam (1) and Meyer et al. (8),
as well as to rabbit B of the present
study, one obtains
DLNO/DLCO
ratios of equal magnitude (3.3, 3.0, and 3.26, respectively) and, on the basis of Eq. 2, similar values for the contribution of
DLCO/(CO · Vc)
(39, 33, and 39%, respectively). Thus, simultaneous application of CO
and NO does provide similar values of
DLCO/(CO · Vc),
even when determined in different species.
In all studies dealing with
DLNO and
DLCO
determinations, the model used to calculate the
DLNO and
DLCO,
respectively, was always based on the assumption of a
functionally homogeneous lung. This also holds true in the present
study. The consistency in DLCO/(CO · Vc)
values in investigations performed on dogs and humans is all the more
astonishing, because sufficiently large functional inhomogeneities have
been shown to affect the ratio of
DLNO/DLCO
considerably (8). However, it is not the aim of the present study to
evaluate the role of inhomogeneities in producing directional errors in
the determination of single-breath diffusing capacity, particularly
because this has already been carried out in detail by Piiper and
Sikand (9).
In conclusion, the present study has confirmed that reaction resistance contributes to overall resistance to pulmonary CO uptake by a value of ~40% (even at PETO2 of 21 Torr). This finding applies to rabbits, dogs, and humans, at least when obtained at FRC.
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ACKNOWLEDGEMENTS |
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The authors wish to thank Bernd Eixmann, Barbara Schreiber, and Christa Pusch for expert technical assistance.
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FOOTNOTES |
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Address for reprint requests: H. Heller, Dept. of Physiology, Univ. of Bonn, Nussallee 11, 53115 Bonn, Germany.
Received 27 March 1997; accepted in final form 9 February 1998.
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REFERENCES |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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1.
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A simultaneous single breath measurement of pulmonary diffusing capacity with nitric oxide and carbon monoxide.
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