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Water Vapour Pressure in Expired Air

Water Vapour Pressure in Expired Air Respiration 31: 252-261 (1974) W ater Vapour Pressure in Expired A ir1 W. Liese, W. J. W arwick- an d G. Gumming Department o f Medieine, University of Birmingham, Queen Elizabeth Medical Centre, Birmingham Abstract. The water vapour pressure in the ex- K ey Words pired air of normal subjects during rest and exercise Respired gas was measured by a mass spectrometer. The mean va- Mass spectrom eter pour pressure was found to be 37.9 mm Hg during W ater vapour pressure tidal breathing and 39.7 mm Hg after a vital capacity Exercise test manoeuvre. The pressure was reduced to 36.1 mm Hg during exercise. This decrease was not significantly different from breathing at rest. When offering warm wet air with a w ater vapour pressure of about 75 mm Hg, the end-tidal pressure was found to be 47 mm Hg. The evidence for complete saturation of expired gas is discussed. Introduction When measuring alveolar-arterial gradients for respiratory gases using the end-expiratory concentrations to estimate the alveolar pressures, it is necessary to know the water-vapour pressure in the expirate. There has been some disagreement among various investigators as to whether gas expired from the mouth is fully saturated with water vapour. B urch [1, 2] found it to be 80%> saturated, and M cC utcheon and T ayeor [10] re­ ported a value of 88% . Ingelstedt [7], however, found the expired air to be 100% saturated and pointed out the need for measuring both the tem­ perature and relative humidity simultaneously. Yet these methods are sensitive to airflow and are therefore unsatisfactory for breath-to-breath studies. 1 The authors gratefully acknowledge the help rendered by Dr. M. X. M. F itz­ G erald, Dr. R. C. Joshi, and Mr. D. Barer. 2 Departm ent of Pediatrics, University of Minnesota, Minn. (USA). Received: June 21, 1973; accepted: June 26, 1973. 253 L iesk/W arwick/C umming The respiratory mass spectrometer allows direct and continuous mea­ surement of the partial pressure of water vapour and of other gases simul­ taneously throughout expiration. G reen and N esarajah [5] found the ex­ pired water-vapour pressure to be 37.8 mm Hg at the lips and 41.9 mm Hg deep in the oral cavity. They corrected their results to an estimated temperature of 33 C at the lips. H erzog et al. [6] showed by rebreathing manoeuvres that when the respiratory apparatus is subjected to sudden changes in the temperature and water vapour pressure of the inspired air, it adapts itself gradually to the new conditions until equilibrium is reached. Experiments were designed to measure the expiratory water vapour pressure in normal subjects under different conditions. The subjects breathed into a heated valve system to avoid water condensation due to cooling. Method A Q-806 quadrupole mass spectrom eter (20th Century Electronics) was used to measure the partial pressures of all expired gases. Nitrogen was sampled at mass number 14, water vapour at mass 18, oxygen at mass 32, and C 0 2 at mass 44. All traces were recorded photographically (Electronics for Medicine DR8 Recorder). The flow rate through the heated sampling system was 30 ml/min, with a transit time of 130 msec. T he 90-percent response time for water vapour was 140 msec. To calibrate the mass spectrometer, compressed air was passed through aerators in six bottles of distilled w ater arranged in series. The bottles and all the connec­ tions were submerged in a water bath with tem perature controllable to + 0.1 °C. The saturated air was sampled directly from a wide bore glass tube (internal diame­ ter 12 mm) connected to the last wash bottle, so that the tip of the mass spectrome­ ter probe was below the surface of the water in the bath. Dry air or argon from a gas cylinder was sampled to obtain the zero point. With the sampling system closed, the mass spectrom eter gave a signal equivalent to about 5 mm Hg for w ater vapour. This was checked against a zero obtained by detuning the mass selector. The valve system was set to m aintain a tem perature above 40 C. Air saturated above this tem perature (about 80 mm Hg) was obtained from a metal column containing boil­ ing water over which the air was passed. Exercise Tests Nine subjects were given exercise tests on a bicycle ergom eter. The initial load was 100 kpm and was increased by 100 kpm every 30 sec until the pulse rate reached 170 bcats/min. This load was then maintained for a further 2 min to achieve a steady state. At the end of this period, the water-vapour pressure and the minute ventilation were measured. The pulse rate of two subjects (the only two fe- 254 I.iese/W arwick/C umming Fig. / . The output of the mass spectrom eter is negative so that concentration in­ creases downwards on the record. The vertical lines represent time intervals of 0.1 sec. The top trace shows carbon dioxide concentration changing from less than 1% in the inspired air to about 4°/o in the expired air. The next trace is that of water vapour and shows an end-tidal pressure of about 39 mm Hg. The lowest trace is that of oxygen. males in this group) did not reach 170 beats/min, and measurements were made at submaximal load. The following measurements were made: (1) water-vapour pressure in normal subjects during quiet breathing and after a vital capacity m anoeuvre; (2) w ater-va­ pour pressure in normal subjects during exercise; and (3) w ater pressure during breathing of warm wet air. The physical data of the subjects are listed in table I. Results All results are summarized in tables II-V I. (1) There were 28 normal subjects (16 male and 12 female) aged 21^13. The group was selected at random. During quiet breathing, the Water Vapour Pressure in Expired Air 255 Table I. Physical data on normal subjects No. Subject Age Sex Height, Weight. cm kg 1 WW 43 M 190 85.5 2 WL 27 M 176 80.0 3 PP 22 M 175 79.0 4 GHn 24 F 161 50.0 5 PB 23 F 168 47.0 MF 29 M 173 66.0 7 LV 21 F 155 46.5 8 FS 23 F 163 57.5 9 KP M 174 76.0 10 AF 21 M 175 59.5 11 DN 21 M 165 66.5 BD 22 F 168 63.5 13 RB 23 M 178 60.5 70.0 14 MP 21 M 184 15 DB 21 M 180 64.5 16 JW 21 M 173 67.0 17 JD 23 F 168 60.5 18 PJ 27 M 185 72.0 19 EC 22 M 178 66.5 20 KS 22 F 157 61.5 171 61.5 21 GH 24 F 22 PF 22 F 156 55.5 69.0 23 SH 20 F 170 34 M 170 70.0 24 RJ 25 NM 27 M 171 56.0 26 JB 21 F 165 68.0 27 178 76.5 R Br 22 M 28 RS 23 F 176 65.0 end-tidal water-vapour pressure was found to be 37.9 mm Hg, increasing to 39.7 mm Hg after a vital capacity manoeuvre. This difference is just significant in Student's t distribution (p<0.05) and highly significant in paired comparison (p<0.001). The comparison between males and fe­ males shows no difference either during quiet breathing (p<0.2) or after the vital capacity manoeuvre ( p < 0 . 1). No significant correlation was found between the values of end-liJal water-vapour pressure and the heights, weights, or estimated body-surface areas of the subjects. 256 Liese/W arwiok/C umming Table II. Waler vapour pressure in normal subjects1 Subject 1 11 No. 1 37.4 37.3 37.1 38.3 35.0 37.3 4 34.7 36.8 34.2 36.8 6 32.9 35.7 36.7 7 37.3 8 33.2 36.2 9 36.2 38.3 42.6 10 42.1 II 40.0 42.6 12 40.0 41.6 36.1 13 39.0 14 41.1 43.6 15 35.1 39.0 41.6 16 38.8 17 41.1 42.1 18 38.5 40.6 19 40.6 40.3 20 40.0 40.6 21 37.4 38.4 22 36.4 38.4 23 38.4 40.0 24 40.8 40.8 25 46.1 47.1 26 43.3 41.3 27 39.3 42.4 33.8 28 30.8 39.68 Mean 37.94 SD ±3.3 ± 2.9 Student’s t-test: I— I I : p<0.05. Paired com parison: 1— 11: pcO.OOI. 1 I = Normal breathing, end-tidal (mean of 3 breaths); 11 = after a vital capacity manoeuvre, end-expiratory. (2) During exercise the end-tidal water-vapour pressure was found to be 36.1 mm Hg. This decrease was not significantly different from breath­ ing at rest (p<0.3 by Student's t-test, p<0.01 by paired comparison); ex­ ercise water-vapour pressure was diminished significantly in comparison © W ater V apour Pressure in Expired Air 257 Table III. Comparison between males and females Male Female No. I III No. 11 IV end- end- end- end- tidal expiratory tidal expiratory 37.4 37.3 4 34.7 36.8 2 37.1 38.3 5 34.2 36.8 3 35.0 37.3 7 37.3 36.7 6 32.9 35.7 33.2 8 36.2 9 36.2 38.3 12 40.0 41.6 10 42.6 42.1 17 41.1 42.1 11 40.0 42.6 20 40.0 40.6 13 36.1 39.0 21 37.4 38.4 14 41.1 43.6 22 36.7 38.4 15 35.1 39.0 23 38.4 40.0 16 38.8 41.6 26 41.3 43.3 18 38.5 40.6 28 30.8 33.8 19 40.3 40.6 24 40.8 40.8 25 46.1 47.1 27 39.3 42.4 Mean 38.58 40.39 37.09 38.73 SD ±3.3 ±2.8 ±3.3 ±2.8 I— I I: p > 0 .2 ; 111- with a vital capacity manoeuvre at rest (p<0.02 Student’s t-test, p<0.001 by paired comparison). (3) The effects of breathing air saturated at high temperature were in­ vestigated in 10 normal subjects. Three breaths of normal room air were monitored, followed by a vital capacity breath, before repeating the man­ oeuvre breathing from the wet air supply. The mean end-inspiratory par­ tial pressure was 74.5 mm Hg, and the mean end-tidal partial pressure rose from 45.4 after the first breath to 47.9 mm Hg after the third breath; the partial pressure after the vital capacity manoeuvre was 48.7 mm Hg. Discussion Most work in respiratory physiology is based on the assumption that the inspired air presented to the alveoli has a temperature of 37 °C and 258 L iese/W arw ick/C umming Table IV. Water vapour pressure during rest and exercise Subject Rest Exercise No. end- end- end- tid al1 tidal expiratory III I II 15 35.8 38.1 33.2 16 35.8 39.5 34.2 24 39.9 40.9 38.1 6 40.9 40.9 39.7 2 39.9 39.9 38.4 11 35.6 39.0 33.1 26 41.0 44.3 38.6 27 39.2 41.7 37.0 28 30.8 33.2 32.3 Mean 37.66 39.72 36.08 SD ±3.4 ± 3 .0 ±2.8 Student’s t-test: I— I II : p >0.3 ; II— III: p < 0 .0 2 Paired differences: I— III : p < 0 .0 1 ; II— III : p<0.001. 1 Mean o f 4 breaths. Table V. Minute ventilation, work load, and pulse rate o f subjects Subject VE (BTPS) Work load, Pulse rate, No. 1 ■ m i n 1 kpm min-1 15 47.5 1,000 168 16 38.8 1,000 176 24 43.0 800 168 6 61.3 900 168 2 62.5 1,000 172 11 800 36.5 168 26 - 500 144 27 42.5 900 172 28 25.0 600 156 that it is saturated with water vapour at this temperature, i.e., has a va­ pour pressure of 47 mm Hg. It has been shown [11] that there is indeed a rigid thermal homeostasis of the alveolar air and of the pulmonary capil­ lary blood even under the most extreme thermal conditions. W ater Vapour Pressure in Expired Air Table VI. W ater vapour pressure for subjects breathing warm wet air Subject End- End-tidal Vc No. inspiratory manoeuvre I. breath 2. breath 3. breath 15 77.2 47.6 46.0 48.9 52.3 21 76.5 46.5 47.2 48.9 47.1 19 71.1 45.4 46.5 47.6 48.9 1 68.9 43.7 45.2 46.4 44.8 2 70.9 45.1 46.1 47.0 47.0 65.0 44.7 46.5 45.9 48.8 26 74.6 47.9 47.9 49.6 51.8 11 76.3 43.3 44.4 47.3 48.4 9 82.5 49.0 49.6 49.6 49.6 10 82.2 42.2 46.2 47.9 48.5 Mean 74.52 45.38 46.72 47.91 48.72 SD ±5.6 ±2.1 ±1.5 ±1.3 ±2.2 1 V c=V ital capacity. According to Seely [12], however, the inspiratory air cannot be com­ pletely saturated on its passage through the conducting airways for the simple reason that it does not take its moisture from distilled water but from secretion, i.e., from a salt solution normally containing l-2%> crys­ talloids which lower the vapour pressure. This factor is practically negligi­ ble according to C hristie and L oomis [3]. The relative humidity of air over a 5-percent NaCl solution is only 2-3°/o lower than that over dis­ tilled water at 30-35 °C [13]. The mean end-tidal water-vapour pressure at the lips is 37.9 mm Hg, and the end-expiratory value after a vital capacity manoeuvre is 39.7 mm Hg. These values are lower than the generally accepted level of 47 mm Hg in the respired gas. Our values are in agreement with the observations of G reen and N esarajah [5], L iese et al. [8], in a study on the humidify­ ing capacity of the nasal mucosa, found a mean end-tidal water-vapour pressure of 37.8 mm Hg on oral breathing and 40.6 mm Hg on nasal breathing. In recent studies it could be demonstrated that atropine has no effect on the end-tidal water-vapour pressure [9]. Warwick et al. [14] found that patients with chronic bronchitis had a mean end-tidal water-vapour pressure of 33.5 mm Hg. This value increased to 35.5 mm Hg after a vi­ L iese/W arw ick/C umming tal capacity manoeuvre. These values are significantly reduced as com­ pared to normal subjects. Their patients with cystic fibrosis showed nor­ mal water-vapour pressures. While breathing air with a water-vapour pressure higher than 47 mm Hg, the mean end-inspiratory partial pressure of water was found to be 75.5 mm Hg. During tidal breathing, the end-tidal water-vapour pressure increased from 45.4 to 47.7 mm Hg over a period of three breaths (table VI). The observed water-vapour tensions can be interpreted in two ways. First, assuming a temperature of respired gas of 37 °C, the water-vapour tensions represent a saturation of 80.5% during normal breathing, 84.2% after a vital capacity manoeuvre, and 76.6% during submaximal exercise. Secondly, assuming 100% saturation, the expired gas would have a tem­ perature of 33.1 C C during normal breathing, 33.9 °C after a vital capaci­ ty manoeuvre, and 32.2 °C during submaximal exercise. In the upper airways, H erzog et al. [6] reported an average tempera­ ture of 32.7 °C for expired gas, while C ole [4] found the temperature in the trachea to be 34 °C during normal breathing. Summing up, we suggest that the alveolar partial pressure for water is in the neighborhood of 47 mm Hg, but that the water vapour pressure in the expired air is less than this value, and the air at the end of an expirate is more alveolized than alveolar air. References 1 Burch, G. E.: Study of water and heat loss from the respiratory tract o f man. Arch, intern. Med. 76: 308-314 (1945). 2 Burch, G. E.: Rate of w ater and heat loss from the respiratory tract o f normal subjects in a subtropical climate. Arch, intern. Med. 76: 315-327 (1945). 3 Christie, R. V. and Loomis, A. L.: The pressure of aqueous vapour in the al­ veolar air. J. Physiol., Lond. 77: 35 (1932). 4 Cole, P.: Recording of respiratory air temperatures. J. Laryng. 68: 295-307 (1954). 5 G reen, I. D. and N esarajah, M. S.: W ater vapor pressure of end-tidal air of norm als and chronic bronchitics. J. appl. Physiol. 24: 229-331 (1968). 6 H erzog, H.; H uguenin, F.; J aeger, M.; Mathys, H., and Haab, P.: Tem perature and humidity of the alveolar air. 5th Int. Cystic Fibrosis Conf., Churchill Col­ lege, Cam bridge 1969, paper No. 14. 7 Ingelstedt, S.: Studies on the conditioning of air in the respiratory tract. Acta oto-laryng., Stockh., suppl. 131, pp. 1-80 (1956). 8 Liese, W.; Joshi, R. C., and Cumming, G.: The humidifying capacity o f the nose. Ann. Otol. Rhinol. Laryng., St. Louis 82: 330 (1973). W ater V apour Pressure in Expired Air 261 9 Liese, W. and Josm , R. C.: The effect o f atropine on hum idification of respired gas. Arch. Pharm akol. 274: 415-417 (1972). 10 McCutcheon, J. W. and T aylor, C. L.: Respiratory heat exchange with varying tem perature and hum idity of inspired air. J. appl. Physiol. 4: 121-135 (1951). 11 Rubinstein, E.; Pardee, R. C., and Eldridge, F.: Alveolar-capillary tem pera­ ture. J. appl. Physiol. 15: 10-12 (1960). 12 Seely, L. E.: Study of changes in the tem perature and w ater vapour content of respired air in the nasal cavity; in Heating, piping and air conditioning, p. 377 (1940). 13 Tables; International critical, vol. 3, pp. 369-370 (New York, 1928). 14 Warwick, W. J.; Liese, W., and Cumming, G.: Studies on the w ater vapor pres­ sure in expired air. 13th Ann. Meet. Cystic Fibrosis Club, A tlan ta 1972. Request reprints from: Dr. W. Liese, Departement Innere Medizin, Abteilung Kardiologie, Medizinische H ochschule Hannover, K arl-W iechert Allee 9, D -3 Hannover-Kleefeld (FRG) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Respiration Karger

Water Vapour Pressure in Expired Air

Liese, W.; Warwick, W.J.; Cumming, G.
Respiration , Volume 31 (3): 10 – Jan 1, 1974
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Karger
Copyright
© 1974 S. Karger AG, Basel
ISSN
0025-7931
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1423-0356
DOI
10.1159/000193114
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Abstract

Respiration 31: 252-261 (1974) W ater Vapour Pressure in Expired A ir1 W. Liese, W. J. W arwick- an d G. Gumming Department o f Medieine, University of Birmingham, Queen Elizabeth Medical Centre, Birmingham Abstract. The water vapour pressure in the ex- K ey Words pired air of normal subjects during rest and exercise Respired gas was measured by a mass spectrometer. The mean va- Mass spectrom eter pour pressure was found to be 37.9 mm Hg during W ater vapour pressure tidal breathing and 39.7 mm Hg after a vital capacity Exercise test manoeuvre. The pressure was reduced to 36.1 mm Hg during exercise. This decrease was not significantly different from breathing at rest. When offering warm wet air with a w ater vapour pressure of about 75 mm Hg, the end-tidal pressure was found to be 47 mm Hg. The evidence for complete saturation of expired gas is discussed. Introduction When measuring alveolar-arterial gradients for respiratory gases using the end-expiratory concentrations to estimate the alveolar pressures, it is necessary to know the water-vapour pressure in the expirate. There has been some disagreement among various investigators as to whether gas expired from the mouth is fully saturated with water vapour. B urch [1, 2] found it to be 80%> saturated, and M cC utcheon and T ayeor [10] re­ ported a value of 88% . Ingelstedt [7], however, found the expired air to be 100% saturated and pointed out the need for measuring both the tem­ perature and relative humidity simultaneously. Yet these methods are sensitive to airflow and are therefore unsatisfactory for breath-to-breath studies. 1 The authors gratefully acknowledge the help rendered by Dr. M. X. M. F itz­ G erald, Dr. R. C. Joshi, and Mr. D. Barer. 2 Departm ent of Pediatrics, University of Minnesota, Minn. (USA). Received: June 21, 1973; accepted: June 26, 1973. 253 L iesk/W arwick/C umming The respiratory mass spectrometer allows direct and continuous mea­ surement of the partial pressure of water vapour and of other gases simul­ taneously throughout expiration. G reen and N esarajah [5] found the ex­ pired water-vapour pressure to be 37.8 mm Hg at the lips and 41.9 mm Hg deep in the oral cavity. They corrected their results to an estimated temperature of 33 C at the lips. H erzog et al. [6] showed by rebreathing manoeuvres that when the respiratory apparatus is subjected to sudden changes in the temperature and water vapour pressure of the inspired air, it adapts itself gradually to the new conditions until equilibrium is reached. Experiments were designed to measure the expiratory water vapour pressure in normal subjects under different conditions. The subjects breathed into a heated valve system to avoid water condensation due to cooling. Method A Q-806 quadrupole mass spectrom eter (20th Century Electronics) was used to measure the partial pressures of all expired gases. Nitrogen was sampled at mass number 14, water vapour at mass 18, oxygen at mass 32, and C 0 2 at mass 44. All traces were recorded photographically (Electronics for Medicine DR8 Recorder). The flow rate through the heated sampling system was 30 ml/min, with a transit time of 130 msec. T he 90-percent response time for water vapour was 140 msec. To calibrate the mass spectrometer, compressed air was passed through aerators in six bottles of distilled w ater arranged in series. The bottles and all the connec­ tions were submerged in a water bath with tem perature controllable to + 0.1 °C. The saturated air was sampled directly from a wide bore glass tube (internal diame­ ter 12 mm) connected to the last wash bottle, so that the tip of the mass spectrome­ ter probe was below the surface of the water in the bath. Dry air or argon from a gas cylinder was sampled to obtain the zero point. With the sampling system closed, the mass spectrom eter gave a signal equivalent to about 5 mm Hg for w ater vapour. This was checked against a zero obtained by detuning the mass selector. The valve system was set to m aintain a tem perature above 40 C. Air saturated above this tem perature (about 80 mm Hg) was obtained from a metal column containing boil­ ing water over which the air was passed. Exercise Tests Nine subjects were given exercise tests on a bicycle ergom eter. The initial load was 100 kpm and was increased by 100 kpm every 30 sec until the pulse rate reached 170 bcats/min. This load was then maintained for a further 2 min to achieve a steady state. At the end of this period, the water-vapour pressure and the minute ventilation were measured. The pulse rate of two subjects (the only two fe- 254 I.iese/W arwick/C umming Fig. / . The output of the mass spectrom eter is negative so that concentration in­ creases downwards on the record. The vertical lines represent time intervals of 0.1 sec. The top trace shows carbon dioxide concentration changing from less than 1% in the inspired air to about 4°/o in the expired air. The next trace is that of water vapour and shows an end-tidal pressure of about 39 mm Hg. The lowest trace is that of oxygen. males in this group) did not reach 170 beats/min, and measurements were made at submaximal load. The following measurements were made: (1) water-vapour pressure in normal subjects during quiet breathing and after a vital capacity m anoeuvre; (2) w ater-va­ pour pressure in normal subjects during exercise; and (3) w ater pressure during breathing of warm wet air. The physical data of the subjects are listed in table I. Results All results are summarized in tables II-V I. (1) There were 28 normal subjects (16 male and 12 female) aged 21^13. The group was selected at random. During quiet breathing, the Water Vapour Pressure in Expired Air 255 Table I. Physical data on normal subjects No. Subject Age Sex Height, Weight. cm kg 1 WW 43 M 190 85.5 2 WL 27 M 176 80.0 3 PP 22 M 175 79.0 4 GHn 24 F 161 50.0 5 PB 23 F 168 47.0 MF 29 M 173 66.0 7 LV 21 F 155 46.5 8 FS 23 F 163 57.5 9 KP M 174 76.0 10 AF 21 M 175 59.5 11 DN 21 M 165 66.5 BD 22 F 168 63.5 13 RB 23 M 178 60.5 70.0 14 MP 21 M 184 15 DB 21 M 180 64.5 16 JW 21 M 173 67.0 17 JD 23 F 168 60.5 18 PJ 27 M 185 72.0 19 EC 22 M 178 66.5 20 KS 22 F 157 61.5 171 61.5 21 GH 24 F 22 PF 22 F 156 55.5 69.0 23 SH 20 F 170 34 M 170 70.0 24 RJ 25 NM 27 M 171 56.0 26 JB 21 F 165 68.0 27 178 76.5 R Br 22 M 28 RS 23 F 176 65.0 end-tidal water-vapour pressure was found to be 37.9 mm Hg, increasing to 39.7 mm Hg after a vital capacity manoeuvre. This difference is just significant in Student's t distribution (p<0.05) and highly significant in paired comparison (p<0.001). The comparison between males and fe­ males shows no difference either during quiet breathing (p<0.2) or after the vital capacity manoeuvre ( p < 0 . 1). No significant correlation was found between the values of end-liJal water-vapour pressure and the heights, weights, or estimated body-surface areas of the subjects. 256 Liese/W arwiok/C umming Table II. Waler vapour pressure in normal subjects1 Subject 1 11 No. 1 37.4 37.3 37.1 38.3 35.0 37.3 4 34.7 36.8 34.2 36.8 6 32.9 35.7 36.7 7 37.3 8 33.2 36.2 9 36.2 38.3 42.6 10 42.1 II 40.0 42.6 12 40.0 41.6 36.1 13 39.0 14 41.1 43.6 15 35.1 39.0 41.6 16 38.8 17 41.1 42.1 18 38.5 40.6 19 40.6 40.3 20 40.0 40.6 21 37.4 38.4 22 36.4 38.4 23 38.4 40.0 24 40.8 40.8 25 46.1 47.1 26 43.3 41.3 27 39.3 42.4 33.8 28 30.8 39.68 Mean 37.94 SD ±3.3 ± 2.9 Student’s t-test: I— I I : p<0.05. Paired com parison: 1— 11: pcO.OOI. 1 I = Normal breathing, end-tidal (mean of 3 breaths); 11 = after a vital capacity manoeuvre, end-expiratory. (2) During exercise the end-tidal water-vapour pressure was found to be 36.1 mm Hg. This decrease was not significantly different from breath­ ing at rest (p<0.3 by Student's t-test, p<0.01 by paired comparison); ex­ ercise water-vapour pressure was diminished significantly in comparison © W ater V apour Pressure in Expired Air 257 Table III. Comparison between males and females Male Female No. I III No. 11 IV end- end- end- end- tidal expiratory tidal expiratory 37.4 37.3 4 34.7 36.8 2 37.1 38.3 5 34.2 36.8 3 35.0 37.3 7 37.3 36.7 6 32.9 35.7 33.2 8 36.2 9 36.2 38.3 12 40.0 41.6 10 42.6 42.1 17 41.1 42.1 11 40.0 42.6 20 40.0 40.6 13 36.1 39.0 21 37.4 38.4 14 41.1 43.6 22 36.7 38.4 15 35.1 39.0 23 38.4 40.0 16 38.8 41.6 26 41.3 43.3 18 38.5 40.6 28 30.8 33.8 19 40.3 40.6 24 40.8 40.8 25 46.1 47.1 27 39.3 42.4 Mean 38.58 40.39 37.09 38.73 SD ±3.3 ±2.8 ±3.3 ±2.8 I— I I: p > 0 .2 ; 111- with a vital capacity manoeuvre at rest (p<0.02 Student’s t-test, p<0.001 by paired comparison). (3) The effects of breathing air saturated at high temperature were in­ vestigated in 10 normal subjects. Three breaths of normal room air were monitored, followed by a vital capacity breath, before repeating the man­ oeuvre breathing from the wet air supply. The mean end-inspiratory par­ tial pressure was 74.5 mm Hg, and the mean end-tidal partial pressure rose from 45.4 after the first breath to 47.9 mm Hg after the third breath; the partial pressure after the vital capacity manoeuvre was 48.7 mm Hg. Discussion Most work in respiratory physiology is based on the assumption that the inspired air presented to the alveoli has a temperature of 37 °C and 258 L iese/W arw ick/C umming Table IV. Water vapour pressure during rest and exercise Subject Rest Exercise No. end- end- end- tid al1 tidal expiratory III I II 15 35.8 38.1 33.2 16 35.8 39.5 34.2 24 39.9 40.9 38.1 6 40.9 40.9 39.7 2 39.9 39.9 38.4 11 35.6 39.0 33.1 26 41.0 44.3 38.6 27 39.2 41.7 37.0 28 30.8 33.2 32.3 Mean 37.66 39.72 36.08 SD ±3.4 ± 3 .0 ±2.8 Student’s t-test: I— I II : p >0.3 ; II— III: p < 0 .0 2 Paired differences: I— III : p < 0 .0 1 ; II— III : p<0.001. 1 Mean o f 4 breaths. Table V. Minute ventilation, work load, and pulse rate o f subjects Subject VE (BTPS) Work load, Pulse rate, No. 1 ■ m i n 1 kpm min-1 15 47.5 1,000 168 16 38.8 1,000 176 24 43.0 800 168 6 61.3 900 168 2 62.5 1,000 172 11 800 36.5 168 26 - 500 144 27 42.5 900 172 28 25.0 600 156 that it is saturated with water vapour at this temperature, i.e., has a va­ pour pressure of 47 mm Hg. It has been shown [11] that there is indeed a rigid thermal homeostasis of the alveolar air and of the pulmonary capil­ lary blood even under the most extreme thermal conditions. W ater Vapour Pressure in Expired Air Table VI. W ater vapour pressure for subjects breathing warm wet air Subject End- End-tidal Vc No. inspiratory manoeuvre I. breath 2. breath 3. breath 15 77.2 47.6 46.0 48.9 52.3 21 76.5 46.5 47.2 48.9 47.1 19 71.1 45.4 46.5 47.6 48.9 1 68.9 43.7 45.2 46.4 44.8 2 70.9 45.1 46.1 47.0 47.0 65.0 44.7 46.5 45.9 48.8 26 74.6 47.9 47.9 49.6 51.8 11 76.3 43.3 44.4 47.3 48.4 9 82.5 49.0 49.6 49.6 49.6 10 82.2 42.2 46.2 47.9 48.5 Mean 74.52 45.38 46.72 47.91 48.72 SD ±5.6 ±2.1 ±1.5 ±1.3 ±2.2 1 V c=V ital capacity. According to Seely [12], however, the inspiratory air cannot be com­ pletely saturated on its passage through the conducting airways for the simple reason that it does not take its moisture from distilled water but from secretion, i.e., from a salt solution normally containing l-2%> crys­ talloids which lower the vapour pressure. This factor is practically negligi­ ble according to C hristie and L oomis [3]. The relative humidity of air over a 5-percent NaCl solution is only 2-3°/o lower than that over dis­ tilled water at 30-35 °C [13]. The mean end-tidal water-vapour pressure at the lips is 37.9 mm Hg, and the end-expiratory value after a vital capacity manoeuvre is 39.7 mm Hg. These values are lower than the generally accepted level of 47 mm Hg in the respired gas. Our values are in agreement with the observations of G reen and N esarajah [5], L iese et al. [8], in a study on the humidify­ ing capacity of the nasal mucosa, found a mean end-tidal water-vapour pressure of 37.8 mm Hg on oral breathing and 40.6 mm Hg on nasal breathing. In recent studies it could be demonstrated that atropine has no effect on the end-tidal water-vapour pressure [9]. Warwick et al. [14] found that patients with chronic bronchitis had a mean end-tidal water-vapour pressure of 33.5 mm Hg. This value increased to 35.5 mm Hg after a vi­ L iese/W arw ick/C umming tal capacity manoeuvre. These values are significantly reduced as com­ pared to normal subjects. Their patients with cystic fibrosis showed nor­ mal water-vapour pressures. While breathing air with a water-vapour pressure higher than 47 mm Hg, the mean end-inspiratory partial pressure of water was found to be 75.5 mm Hg. During tidal breathing, the end-tidal water-vapour pressure increased from 45.4 to 47.7 mm Hg over a period of three breaths (table VI). The observed water-vapour tensions can be interpreted in two ways. First, assuming a temperature of respired gas of 37 °C, the water-vapour tensions represent a saturation of 80.5% during normal breathing, 84.2% after a vital capacity manoeuvre, and 76.6% during submaximal exercise. Secondly, assuming 100% saturation, the expired gas would have a tem­ perature of 33.1 C C during normal breathing, 33.9 °C after a vital capaci­ ty manoeuvre, and 32.2 °C during submaximal exercise. In the upper airways, H erzog et al. [6] reported an average tempera­ ture of 32.7 °C for expired gas, while C ole [4] found the temperature in the trachea to be 34 °C during normal breathing. Summing up, we suggest that the alveolar partial pressure for water is in the neighborhood of 47 mm Hg, but that the water vapour pressure in the expired air is less than this value, and the air at the end of an expirate is more alveolized than alveolar air. References 1 Burch, G. E.: Study of water and heat loss from the respiratory tract o f man. Arch, intern. Med. 76: 308-314 (1945). 2 Burch, G. E.: Rate of w ater and heat loss from the respiratory tract o f normal subjects in a subtropical climate. Arch, intern. Med. 76: 315-327 (1945). 3 Christie, R. V. and Loomis, A. L.: The pressure of aqueous vapour in the al­ veolar air. J. Physiol., Lond. 77: 35 (1932). 4 Cole, P.: Recording of respiratory air temperatures. J. Laryng. 68: 295-307 (1954). 5 G reen, I. D. and N esarajah, M. S.: W ater vapor pressure of end-tidal air of norm als and chronic bronchitics. J. appl. Physiol. 24: 229-331 (1968). 6 H erzog, H.; H uguenin, F.; J aeger, M.; Mathys, H., and Haab, P.: Tem perature and humidity of the alveolar air. 5th Int. Cystic Fibrosis Conf., Churchill Col­ lege, Cam bridge 1969, paper No. 14. 7 Ingelstedt, S.: Studies on the conditioning of air in the respiratory tract. Acta oto-laryng., Stockh., suppl. 131, pp. 1-80 (1956). 8 Liese, W.; Joshi, R. C., and Cumming, G.: The humidifying capacity o f the nose. Ann. Otol. Rhinol. Laryng., St. Louis 82: 330 (1973). W ater V apour Pressure in Expired Air 261 9 Liese, W. and Josm , R. C.: The effect o f atropine on hum idification of respired gas. Arch. Pharm akol. 274: 415-417 (1972). 10 McCutcheon, J. W. and T aylor, C. L.: Respiratory heat exchange with varying tem perature and hum idity of inspired air. J. appl. Physiol. 4: 121-135 (1951). 11 Rubinstein, E.; Pardee, R. C., and Eldridge, F.: Alveolar-capillary tem pera­ ture. J. appl. Physiol. 15: 10-12 (1960). 12 Seely, L. E.: Study of changes in the tem perature and w ater vapour content of respired air in the nasal cavity; in Heating, piping and air conditioning, p. 377 (1940). 13 Tables; International critical, vol. 3, pp. 369-370 (New York, 1928). 14 Warwick, W. J.; Liese, W., and Cumming, G.: Studies on the w ater vapor pres­ sure in expired air. 13th Ann. Meet. Cystic Fibrosis Club, A tlan ta 1972. Request reprints from: Dr. W. Liese, Departement Innere Medizin, Abteilung Kardiologie, Medizinische H ochschule Hannover, K arl-W iechert Allee 9, D -3 Hannover-Kleefeld (FRG)

Journal

Respiration – Karger

Published: Jan 1, 1974

Keywords: Respired gas; Mass spectrometer; Water vapour pressure; Exercise test

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