Am. J. Respir. Crit. Care Med.,
Volume 161, Number 6, June 2000, 2113-2114
EXHALED NITRIC OXIDE DOES NOT PROVIDE
A MARKER OF VASCULAR ENDOTHELIAL
FUNCTION IN HEALTHY HUMANS
To the Editor :
Sartori and colleagues (1) measured mixed expired NO during
spontaneous tidal breathing in eight healthy subjects before and after the inhalation or infusion of the competitive NO synthase inhibitor L-NG-monomethyl arginine (L-NMMA). They
found a 40% reduction in exhaled NO after inhaling nebulized
L-NMMA, but only at 10% reduction after intravenous infusion of L-NMMA. They concluded that exhaled NO originates
primarily from the epithelial cells of the respiratory tract as
opposed to the endothelial cells. They claim their data refutes
the hypothesis of Borland, Cox, and Higenbottam (2) that expired NO can provide a marker of vascular endothelial function.
Recent reports have improved our knowledge of the physiological factors that determine the level of NO in the airways.
These new findings suggests that the conclusions by Sartori and
coworkers (1) need modification. Silkoff and coworkers (3)
made the sentinel observation that the expired NO concentration increases with decreasing expiratory flow rates. We and
others have recently shown this finding is accurately explained
by a two-compartment lung model consisting of the conducting
airways and the alveolar airways (4, 5). Expired NO measured
during tidal breathing (as in the experiments by Sartori and coworkers) comes principally from the conducting airways, because on the order of 95% of NO produced in the alveoli rapidly diffuses into the blood in the pulmonary capillaries (6, 7).
The resultant low alveolar concentration of NO contributes little to the expired tidal breath in resting subjects. Therefore, an
inhaled aerosol of L-NMMA that likely only inhibits NO production in the conducting airways will significantly lower the
NO concentration in the tidal breath. Intravenous infusion of
L-NMMA that may lower the concentration of NO in the alveoli will only slightly decrease expired NO, because alveolar NO
makes a small contribution to expired NO during tidal breathing. Thus, the two-compartment lung model is in agreement with the findings of Sartori and colleagues (1), but this does not
mean alveolar NO production cannot be measured.
If their subjects had taken a deep breath, breathheld for
10 to 15 s, and exhaled at a number of different constant expiratory flow rates, the resultant expired NO concentrations
could have been extrapolated to the expired NO concentration present at an infinitely rapid expiration (4). The infinitely
large volume being exhaled abolishes any contribution of NO
from the conducting airways, and the extrapolated expired
NO concentration equals the concentration of NO in the alveoli. This alveolar concentration multiplied by a measurement
or estimate of the pulmonary diffusing capacity for NO equals
the NO produced by the alveoli (4, 7). If this alveolar NO
production comes mainly from the pulmonary vascular endothelium, then Borland and coworkers (2) are correct that exhaled NO can provide a marker of endothelial function in healthy adults.
Unfortunately, there is no way to determine if the NO is of
epithelial, endothelial or neuronal origin using nitric oxide synthase (NOS) inhibitors such as L-NMMA because these do not selectively inhibit the various isoforms of NOS that are present throughout the lungs (8). Therefore, it is not currently known if
exhaled NO provides a marker of endothelial, epithelial, or neuronal NO production, but it is possible to distinguish the amount
of NO being produced by the nasal passages (1, 7, 9), the conducting airways (4, 10, 11), and the alveolar airways (4, 11) from expired breaths. Does exhaled NO provide a marker of vascular endothelial function? The jury is still out.
Anthony P.
Pietropaoli,
Peter T.
Perkins,
Irene B.
Perillo,
and
Richard W.
Hyde
Environmental Medicine, Department of Medicine andUniversity of Rochester School ofMedicine and Dentistry, Rochester, New York
1.
Sartori, C.,
M. Lepori,
T. Busch,
H. Duplain,
W. Hildebrandt,
P. Bartsch,
P. Nicod,
K. J. Falke, and
U. Scherrer.
1999.
Exhaled nitric
oxide does not provide a marker of vascular endothelial function in
healthy humans.
Am. J. Respir. Crit. Med.
160:
879-882
[Abstract/Free Full Text].
2.
Borland, C.,
Y. Cox, and
T. Higenbottam.
1993.
Measurement of exhaled nitric oxide in man.
Thorax
48:
1160-1162
[Abstract/Free Full Text].
3.
Silkoff, P. E.,
P. A. McClean,
A. S. Slutsky,
H. G. Furlott,
E. Hoffstein,
S. Wakita,
K. R. Chapman,
J. P. Shalai, and
N. Zamel.
1997.
Marked
flow -dependence of exhaled nitric oxide using a new technique to exclude nasal nitric oxide.
Am. J. Respir. Crit. Care Med.
155:
260-267
[Abstract].
4.
Pietropaoli, A. P.,
I. B. Perillo,
A. Torres,
P. T. Perkins,
L. M. Frasier,
M. J. Utell,
M. W. Frampton, and
R. W. Hyde.
1999.
Simultaneous
measurement of nitric oxide production by conducting and alveolar
airways of man.
J. Appl. Physiol.
87:
1532-1542
[Abstract/Free Full Text].
5.
Tsoukias, N. M., and
S. C. George.
1998.
A two-compartment model of pulmonary nitric oxide exchange dynamics.
J. Appl. Physiol.
85:
653-666
[Abstract/Free Full Text].
6.
Hyde, R. W.,
E. J. Geigel,
A. L. Olszowka,
J. A. Krasney,
R. E. Forster II,
M. J. Utell, and
M. W. Frampton.
1997.
Determination of production of nitric oxide by lower airways of humans
theory.
J. Appl.
Physiol.
82:
1290-1296
[Abstract/Free Full Text].
7.
Geigel, E. J.,
R. W. Hyde,
I. B. Perillo,
A. Torres,
P. T. Perkins,
A. P. Pietropaoli,
L. M. Frasier,
M. W. Frampton, and
M. J. Utell.
1999.
Rate of
nitric oxide production in the lower airways of human lungs.
J. Appl.
Physiol.
86:
211-221
[Abstract/Free Full Text].
8.
Kobzik, L.,
D. S. Bredt,
C. J. Lowerstein,
J. Drazen,
B. Gaston,
D. Sugarbaker, and
J. S. Stamler.
1993.
Nitric oxide synthase in human and
rat lung: immunocytochemical and histochemical localization.
Am. J. Respir. Cell Mol. Biol.
9:
371-377
.
9.
Dubois, A. B.,
J. S. Douglas,
J. T. Stitt, and
V. Mohsenin.
1998.
Production and absorption of nitric oxide gas in the nose.
J. Appl. Physiol.
84:
1217-1224
[Abstract/Free Full Text].
10.
DuBois, A. B.,
P. M. Kelley,
J. S. Douglas, and
V. Mohsenin.
1999.
Nitric
oxide production and absorption in trachea, bronchi, bronchioles, and
respiratory bronchioles of humans.
J. Appl. Physiol.
86:
159-167
[Abstract/Free Full Text].
11.
Tsoukias, N. M.,
Z. Tannous,
A. F. Wilson, and
S. C. George.
1998.
Single-exhalation profiles of NO and CO2 in humans: effect of dynamically changing flow rate.
J. Appl. Physiol.
85:
842-852
[Abstract/Free Full Text].
From the Authors:
Dr. Pietropaoli and colleagues question our conclusion (1) because they believe the measurement method we used to assess alveolar (as opposed to airway) nitric oxide (NO) production
was not adequate, and in addition they point to the limitations
of L-NMMA administration to selectively inhibit NOS isoforms.
First of all, it is important to note that the hypothesis of
Borland and colleagues (2) and the ensuing numerous reports touting the claim that exhaled nitric oxide represents a marker of endothelial function in humans (3) were all based on
measurements of exhaled NO, obtained by techniques similar
to the one used in our studies. Our data demonstrate that systemic L-NMMA infusion that resulted in a clinically highly relevant increase in arterial pressure and inhibition of eNOS
(nNOS and/or iNOS inhibition alone would not have resulted
in arterial hypertension, as evidenced by normal arterial pressure in mice with selective disruption of the gene encoding for
these two isoforms) had only a barely detectable effect on exhaled NO. In line with these findings in humans, exhaled NO
in eNOS-deficient mice is not lower than in control littermates
(7). Does exhaled NO represent a marker of endothelial function in humans? Based on the experimental evidence available
at the end of this millennium the answer clearly is no.
The two-compartment lung model proposed by Pietropaoli
(8) and other researchers (9) is interesting. We have to bear in
mind, however, that it has been used in a very limited number of
studies. At the time of this writing there exists no direct experimental evidence for the assumptions made in this model. We do
not yet know if it will perform any better than the traditional measurement techniques in distinguishing between the contribution of alveolar and conducting airways to exhaled NO in humans (intriguingly, our reported baseline exhaled NO concentration of 4.7 ± 0.6 ppb [3.6 ± 0.5 × 10
6 mm Hg] fits surprisingly
well with the value of 2.9 × 106 mm Hg obtained after breathholding by Pietropaoli and colleagues [10]), and if it does,
whether alveolar NO contains a major fraction of endothelium-derived nitric oxide. Will exhaled nitric oxide provide a marker
of endothelial function in humans in the next millennium? The
heat is on the proponents of the two-compartment lung model to
provide experimental evidence supporting their hypothesis.
URS SCHERRER
CLAUDIO SARTORI
HERVÉ DUPLAIN
THILO BUSCH
Department of Internal Medicine
Centre Hospitalier Universitaire Vaudois
Lausanne, Switzerland
1.
Sartori, C.,
M. Lepori,
T. Busch,
H. Duplain,
W. Hildebrandt,
P. Bartsch,
P. Nicod,
K. J. Falke, and
U. Scherrer.
1999.
Exhaled nitric oxide does
not provide a marker of vascular endothelial function in healthy humans.
Am. J. Respir. Crit. Care Med.
160:
879-992
.
2.
Borland, C.,
Y. Cox, and
T. Higenbottam.
1993.
Measurement of exhaled nitric oxide in man.
Thorax
48:
1160-1162
.
3.
Sogni, P.,
P. Garnier,
A. Gadano,
R. Moreau,
J. Dall'Ava-Santucci,
A. T. Dinh-Xuan, and
D. Lebréc.
1995.
Endogenous pulmonary nitric
oxide production measure from exhaled air is increased in patients
with severe cirrhosis.
J. Hepatol.
23:
471-473
[Medline].
4.
Cailes, J. B.,
S. A. Kharitonov,
D. Yates,
P. Barnes, and
R. M. Du Bois.
1995.
Decreased endogenous nitric oxide in the exhaled air of systemic sclerosis patients.
Thorax
50:
452P
.
5.
Schilling, J.,
P. Holzer,
M. Guggenbach,
D. Gyurech,
K. Marathia, and
S. Geroulanos.
1994.
Reduced endogenous nitric oxide in the exhaled air
of smokers and hypertension.
Eur. Respir. J.
7:
467-471
[Abstract].
6.
Bucca, C.,
P. Colagrande,
L. Brussino,
G. Chiavassa,
L. Del Piano,
L. Dutto,
S. Polizzi, and
G. Rolla.
1998.
Exhaled nitric oxide in patients
with mitral valve disease (abstract).
Am. J. Respir. Crit. Care Med.
157:
A227
.
7.
Steudel, W.,
M. Kirmse,
J. Weimann,
R. Ullrich,
J. Hromi, and
W. Zapol.
1999.
Nitric oxide exhalation by mice with congenital deficiency of nitric oxide synthase isoforms (abstract).
Am. J. Respir. Crit.
Care Med.
159:
A408
.
8.
Pietropaoli, A. P.,
I. B. Perillo,
A. Torres,
P. T. Perkins,
L. M. Frasier,
M. J. Utell,
M. W. Frampton, and
R. W. Hyde.
1999.
Simultaneous
measurement of nitric oxide production by conducting and alveolar
airways of humans.
J. Appl. Physiol.
87:
1532-1542
.
9.
Tsoukias, N. M.,
Z. Tannous,
A. F. Wilson, and
S. C. George.
1998.
Single-exhalation profiles of NO and CO2 in humans: effect of dynamically changing flow rate.
J. Appl. Physiol.
85:
642-652
[Abstract/Free Full Text].
10.
Geigel, E. J.,
R. W. Hyde,
I. B. Perillo,
A. Torres,
P. T. Perkins,
A. P. Pietropaoli,
L. M. Frasier,
M. W. Frampton, and
M. J. Utell.
1999.
Rate of nitric oxide production by lower alveolar airways of human
lungs.
J. Appl. Physiol.
86:
211-221
.
