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pulmonary membrane diffusing capacity
membrane diffusing capacity
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Colin D R Borland, consultant physician
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colin.borland{at}talk21.com Colin D R Borland
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Sir May I comment on two aspects of the paper by Tamhane et al (Chest 2001;120:1850-1856) In their introduction they state that DLNO and DLCO have not previously been compared during exercise. In fact in our original paper (1) describing the technique we studied three individuals at increasing levels of exercise using the single breath technique. Like Tamhane et al. we also observed a rise in DLNO with exercise but proportionately less than for DLCO so that DLNO / DLCO actually fell from 4.4 to 3.6.A possible reason for this fall is the inevitably lower PAO2 during 7.5 seconds apnoea at maximal exercise compared to rest. DLNO/DLCO is known to be reduced by hypoxia (2). Interestingly they observed that DLNO/DMCO was 2.5 whereas they expected a ratio of 2.One of the explanations offered was error in their assumed value for èCO which was Roughton and Forster’s 1957 value (1/ èCO = 0.73 + 5.8.10-3 (Pc O2) )(3).Forster re-estimated èCO (1/ èCO = 1.3 + 4.110-3 (Pc O2) ) in 1987 (4) and this has a major effect on calculated DMCO. To illustrate this point I append a worked example using data from subject 1 in our first paper (1): Row Alveolar PAO2 DLNO DLCO DMCO Comment (mmHg) (ml/min/mmHg) 1 128 179 39 90 DMCO=DLNO/2 2 453 176 25 67 Using 1957 èCO 3 149 Using 1987 èCO 4 154 Using 1987 èCO and èNO DMCO in the first row is calculated assuming that DLNO equals DMNO and is twice DLCO.DMCO in the second row is calculated using the classic Roughton and Forster method (3) using the 1957 value for èCO. In agreement with Tamhane et al DLNO/DMCO is 2.7.Using the 1987 value for èCO gives a twofold greater value for DMCO compared to using the 1957 value. We should predict from this value for DMCO that DLNO would be 298 ml/min/mmHg, substantially greater than DLNO implying that the pulmonary capillary blood conductance to NO is less than infinity. In fact estimates of èNO are available using a variety of techniques and we have used a value from 1955 (2) using a rapid reaction apparatus similar to that used for èCO .Solving the Roughton and Forster equation for DMNO using DMNO = (èNO – 2. ÈCO)/( èNO/DLNO – ÈCO/DLCO) =2DMCO yields the value for DMCO in row 4.The similarity of values for DMCO in rows 3 and 4 may be coincidence but is consistent with the view that the pulmonary capillary membrane offers minimal resistance to gas transfer. There is an urgent need to establish the correct values for èCO and èNO if the maximum potential for experimental and clinical interpretation of pulmonary diffusing capacity is to be realised. Colin Borland Department of Medicine Hinchingbrooke Hospital Huntingdon Cambridgeshire PE18 8NT United Kingdom 1. Borland, C.D.R., and T.W. Higenbottam. A simultaneous single breath measurement of pulmonary diffusing capacity with nitric oxide and carbon monoxide.Eur Respir J.1989; 2:56-63 2.Borland, C.D.R., and Y. Cox. Effect of varying alveolar oxygen partial pressure on diffusing capacity for nitric oxide and carbon monoxide, membrane diffusing capacity and lung capilary blood volume.Clin. Science 1991;81:759-765. 3.Roughton FJW and Forster RE Relative effects of diffusion and chemical reaction rates in determining the rate of exchange of gases in the human lung, with special reference to true diffusing capacity of the pulmonary membrane and volume of blood in lung capillaries.J Appl Physiol 1957;11:290-302 4.Forster R.E.Diffusion of gases across the alveolar membrane.In: Farhi LE and Tenney SM eds.Handbook of Physiology.Respiration, Bethesda , MD :American Physiological Society 1987, pp71-88 |
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Colin D R Borland, Consultant Physician Hinchingbrooke Hospital Huntingdon Cambs UK
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colin.borland{at}talk21.com Colin D R Borland
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Sir Unfortunately theta and a table got distorted during submission of our earlier letter so this is a revised version May I comment on two aspects of the paper by Tamhane et al (Chest 2001;120:1850-1856) In their introduction they state that DLNO and DLCO have not previously been compared during exercise. In fact in our original paper (1) describing the technique we studied three individuals at increasing levels of exercise using the single breath technique but admittedly not with the detail of Tamhane et al’s work. We also observed a rise in DLNO with exercise but less than for DLCO so that DLNO / DLCO actually fell from 4.4 to 3.6.A possible reason for our observation is the inevitably lower PAO2 during 7.5 seconds apnoea at maximal exercise compared to rest. DLNO/DLCO is known to be reduced by hypoxia (2). Interestingly they observed that DLNO/DMCO was 2.5 whereas they expected a ratio of 2.One of the explanations offered was error in their assumed value for thetaCO which was Roughton and Forster’s 1957 value (1/ thetaCO = 0.73 + 5.8.10-3 Pc O2 )(3).Forster re-estimated thetaCO (1/ thetaCO = 0.73 + 5.8.10-3 Pc O2 )(1987)(4) and this has a major effect on calculated DMCO. To illustrate this point I append a worked example using data from subject 1 in our first paper (1) :Using DMCO=DLNO/2 gives a value for DMCO of 90 ml/min/mmHg. Using DLCO at 128mmHg = 39 ml/min/mmHg and at 453 mmHg = 25.4 ml/min/mmHg and the 1957 value for theta CO gives DMCO of 67 ml/min/mmHg . Our measured DLNO is 176 ml/min/mmHg.In agreement with Tamhane et al DLNO/DMCO is 2.7.Using the 1987 value for theta CO and DLCO measured at two oxygen tensions gives DMCO of ml/min/mmHg 149 Using the 1987 value for thetaCO gives a twofold greater value for DMCO compared to using the 1957 value.We should predict from this value for DMCO that DMNO would be 298 ml/min/mmHg, substantially greater than DLNO implying that the pulmonary capillary blood conductance to NO was less than infinity.In fact estimates of thetaNO are available using a variety of techniques and we have used a value from 1955 (2) using a rapid reaction apparatus similar to that used for thetaCO.Solving the Roughton and Forster equation for DMNO using DMNO = (thetaNO – 2. thetaCO)/ ( thetaNO/DLNO – thetaCO/DLCO) = 2DMCO yields a value for DMCO of 154 ml/min/mmHg.The similarity of values for DMCO measured using the classic Roughton and Forster technique and via DLNO/DLCO (above) may be coincidence but is consistent with the view that the pulmonary capillary membrane offers minimal resistance to gas transfer. There is an urgent need to establish the correct values for thetaCO and thetaNO if the maximum potential of experimental and clinical interpretation of pulmonary diffusing capacity is to be realised. Colin Borland Department of Medicine Hinchingbrooke Hospital Huntingdon Cambridgeshire PE18 8NT United Kingdom 1. Borland, C.D.R., and T.W. Higenbottam. A simultaneous single breath measurement of pulmonary diffusing capacity with nitric oxide and carbon monoxide.Eur Respir J.1989; 2:56-63 2.Borland, C.D.R., and Y. Cox. Effect of varying alveolar oxygen partial pressure on diffusing capacity for nitric oxide and carbon monoxide, membrane diffusing capacity and lung capilary blood volume.Clin. Science 1991;81:759-765. 3.Roughton FJW and Forster RE Relative effects of diffusion and chemical reaction rates in determining the rate of exchange of gases in the human lung, with special reference to true diffusing capacity of the pulmonary membrane and volume of blood in lung capillaries.J Appl Physiol 1957;11:290-302 4.Forster R.E.Diffusion of gases across the alveolar membrane.In: Farhi LE and Tenney SM eds.Handbook of Physiology.Respiration, Bethesda , MD :American Physiological Society 1987, pp71-88 |
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