Pulmonary Nitric Oxide Uptake Reflects the Entire Diffusive Properties of the Alveolar Capillary Membrane

The following is the abstract of the article discussed in the subsequent letter:

	ABSTRACT

Borland, Colin, Bryan Mist, Mariella Zammit, and Alain Vuylsteke. Steady-state measurement of NO and CO lung diffusing capacity on moderate exercise in men. J Appl Physiol 90: 538-544, 2001.Using a rapidly responding nitric oxide (NO) analyzer, we measured the steady-state NO diffusing capacity (DL_NO) from end-tidal NO. The diffusing capacity of the alveolar capillary membrane and pulmonary capillary blood volume were calculated from the steady-state diffusing capacity for CO (measured simultaneously) and the specific transfer conductance of blood per milliliter for NO and for CO. Nine men were studied bicycling at an average O₂ consumption of 1.3 ± 0.2 l/min (mean ± SD). DL_NO was 202.7 ± 71.2 ml · min¹ · Torr¹ and steady-state diffusing capacity for CO, calculated from end-tidal (assumed alveolar) CO₂, mixed expired CO₂, and mixed expired CO, was 46.9 ± 12.8 ml · min¹ · Torr¹. NO dead space = (VT × FE_NO  VT × FA_NO)/(FI_NO  FA_NO) = 209 ± 88 ml, where VT is tidal volume and FE_NO, FI_NO, and FA_NO are mixed exhaled, inhaled, and alveolar NO concentrations, respectively. We used the Bohr equation to estimate CO₂ dead space from mixed exhaled and end-tidal (assumed alveolar) CO₂ = 430 ± 136 ml. Predicted anatomic dead space = 199 ± 22 ml. Membrane diffusing capacity was 333 and 166 ml · min¹ · Torr¹ for NO and CO, respectively, and pulmonary capillary blood volume was 140 ml. Inhalation of repeated breaths of NO over 80 s did not alter DL_NO at the concentrations used.

	LETTER

To the Editor: In their excellent study, Borland et al. (1) showed that the steady-state method is suitable for measuring the diffusing capacity of the alveolar capillary membrane (Dm) for nitric oxide (NO) and carbon monoxide (CO) during moderate exercise as well as for determining pulmonary capillary blood volume (Vc). Furthermore, they reported that repeated NO inhalations over >1 min did not modify pulmonary NO diffusing capacity (DL_NO).

Like other groups, Borland et al. (1) applied the Roughton and Forster (3) model [1/DL = 1/Dm + 1/(Vc)] to pulmonary NO uptake where  denotes the specific gas transfer conductance of blood. It is commonly assumed that, because of the extremely rapid rate of reaction of NO with hemoglobin, _NO approaches infinity and DL_NO approximates Dm_NO. Nonetheless, the authors queried this argument because of the absence of any in vivo data of the specific NO transfer conductance of blood.

In an animal study (2), we calculated the contribution of diffusion to the overall resistance to alveolar capillary NO transfer (DL_NO/Dm_NO) by determining the rates of disappearance from alveolar space of NO and singly and doubly ¹⁸O-labeled carbon dioxide. For this purpose, we applied the single-breath method to seven artificially ventilated rabbits. On the basis of the unique features of both isotopic species and a classic model of test gas uptake, we obtained DL_NO/Dm_NO = 0.98 ± 0.06 (mean ± SD), a value no different from unity.

Thus our in vivo data strongly confirm the hypothesis that pulmonary NO uptake mainly reflects the diffusive properties of the alveolar capillary membrane. This finding indeed allows us to set 1/(_NOVc)  0 or to assume the term (_NOVc) approaching infinity.

	REFERENCES

1.   Borland, C, Mist B, Zammit M, and Vuylsteke A. Steady-state measurement of NO and CO lung diffusing capacity on moderate exercise in men. J Appl Physiol 90: 538-544, 2001[Abstract/Free Full Text].

2.   Heller, H, and Schuster KD. Nitric oxide used to test pulmonary gas exchange in rabbits. Pflügers Arch 437: 94-97, 1998[Medline].

3.   Roughton, FJW, and Forster RE. Relative importance of diffusion and chemical reaction rates in determining rate of exchange of gases in the human lungs, with special reference to true diffusing capacity of pulmonary membrane and volume of blood in lung capillaries. J Appl Physiol 11: 290-302, 1957[Abstract/Free Full Text].

Hartmut Heller, Sebastian Brandt, Klaus-Dieter Schuster, Department of Physiology University of Bonn Nussallee 11 D-53115 Bonn, Germany E-mail: h.heller{at}uni-bonn.de

	REPLY

To the Editor: We are grateful to Heller, Brandt, and Schuster for drawing our attention to their interesting work comparing alveolar uptake of nitric oxide (NO) and of singly and doubly ¹⁸O-labeled carbon dioxide (4). Because of the extremely rapid carbonic anhydrase-mediated uptake of singly and doubly ¹⁸O-labeled carbon dioxide into the red blood cell, the uptake of these gases should reflect diffusion alone. They showed that the ratio of rate constants () was exactly as predicted by application of Graham's law of diffusion to the three gases (i.e., rate of diffusion is proportional to water solubility divided by square root of molecular weight) and that diffusion contributes 98% of the resistance to overall alveolar NO uptake.

This confirms in vivo that, as predicted from the extremely rapid in vitro rate of reaction of NO with hemoglobin, DL_NO is an index of diffusion alone and unlike DL_CO does not reflect the chemical reaction of the gas with hemoglobin.

To determine the membrane component of diffusion resistance, it is necessary to solve the Roughton and Forster equation for Dm: 1/DL = 1/Dm + 1/Vc. In that equation, 1/DL is the overall resistance to alveolar gas uptake of the whole lung, 1/Dm is the diffusive resistance of the alveolar capillary membrane, and 1/Vc is the resistance of the pulmonary capillary blood. 1/Vc is believed to contain terms for diffusion and reaction in the case of CO. Forster (2) quotes a value for 1/CO of 1.30 + 0.0041 (PO₂) derived from continuous-flow rapid-reaction apparatus and spectroscopy and measured at pH 4 and 37°C. The right-hand value (0.0041) represents the reaction resistance of the blood to CO uptake, whereas the left-hand value (1.30) is the diffusion resistance of the blood to CO uptake. The three groups who have studied DL_NO the most have each applied the Roughton and Forster equation to DL_NO differently. Our approach (1) has been to use rapid reaction apparatus values for both CO (2) and NO at 4.479 ml · ml¹ · mmHg¹ · min¹. Guenard et al. (3) have used the rapid reaction apparatus values for CO but have assumed NO to be infinite. Finally, Heller and Schuster (4) have used a value for NO of 14 ml · ml¹ · mmHg¹ · min¹ derived from thin blood film experiments.

Although this shows that there is still uncertainty among various workers about the exact values for CO and NO in vivo, we decided to present our data both assuming the lower value (4.479 ml · ml¹ · mmHg¹ · min¹) and also assuming that NO is infinity. The former approach will give higher values for Dm and lower for Vc, whereas the latter gives lower values for Dm and higher for Vc.

We would therefore recommend that studies using combined DL_NO and DL_CO should quote the absolute values for hemoglobin and alveolar oxygen concentration so that future readers can recalculate Dm and Vc once there is final consensus on .

	FOOTNOTES

10.1152/japplphysiol.01032.2001

	REFERENCES

1.   Borland, C, Mist B, Zammit M, and Vuylsteke A. Steady-state measurement of NO and CO lung diffusing capacity on moderate exercise in men. J Appl Physiol 90: 538-544, 2001.

2.   Forster, RE. Diffusion of gases across the alveolar membrane. In: Handbook of Physiology. The Respiratory System. Gas Exchange. Bethesda, MD: Am. Physiol. Soc., 1987, sect. 3, vol. IV, chapt. 5, p. 71-88.

3.   Guenard, H, Varene N, and Vaida P. Determination of lung capillary blood volume and membrane diffusing capacity in man by the measurements of NO and CO transfer. Respir Physiol 70: 113-120, 1987[Web of Science][Medline].

4.   Heller, H, and Schuster KD. Nitric oxide used to test pulmonary gas exchange in rabbits. Pflügers Arch 437: 94-97, 1998.

Colin Borland, Department of Medicine Hinchingbrooke Hospital Huntingdon, Cambridgeshire PE18 8NT, United Kingdom

Bryan Mist, Department of Cardiological Sciences St Georges Hospital Medical School Cranmer Terrace London SW17 0RE, United Kingdom

Alain Vuylsteke, Anaesthetic Department Papworth Hospital Papworth Everard Cambridge CB3 8RE, United Kingdom