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Changes in the functional residual capacity with deep breathing
AND DISEASES OF T H E C H E S T
3
CHANGES IN THE FUNCTIONAL RESIDUAL CAPACITY WITH DEEP BREATHING IN N O R M A L AND E M P H Y S E M A T O U S SUBJECTS BY R. ST.J. BUXTON Department of Physiology,King's College, London MEASUREMENT of the functional residual capacity is less easy to perform than spirometry, so that it is not undertaken as often as the estimation of the vital capacity in the assessment of lung function. It is however of importance, since the functional residual capacity represents the volume by which the tidal air is diluted, and so changes in its volume may be expected to affect the rate of gas mixing in the lungs. Bernstein (1954) has shown that inflation of the rabbit's lung in vivo can lead to an opening up of previously closed alveoli. Anaesthetists are familiar with this phenomenon in man. It was therefore of interest to discover whether the functional residual capacity was increased after a subject took a few vital capacity breaths. METHOD
The technique used followed that of Bates and Christie (i 95 o) with certain modifications which have been described by Buxton and D'Silva (1956). All subjects were examined in the sitting position. The normal group consisted of 20 healthy medical students aged i8-27 years, and in addition 20 patients with pulmonary emphysema were studied whose ages ranged from 35-63 years (mean 48.8, S.D. 7"4). EXPERIMENTAL RESULTS
Normal Subjects The mean functional residual capacity of the normal group of 20 subjects was 3.76 litres (range 2.2-5-7, S.D. 0.78 ). The time taken for " 63.2 per cent. mixing "--i.e., the time constant of the mixing curve (Buxton and D'Silva, I956)--was significantly (r=o.54) though not closely related to the size of the functional residual capacity (Fig. I). It has been demonstrated that the minute ventilation and the time constant are closely re!ated (Buxton and D'Silva, 1956 ) in a hyperbolic form for the group of normal subjects breathing with minute ventilations ranging from 6-45 litres. If the results are plotted not as total minute ventilation but as minute ventilation per litre of functional residual capacity, the same type of relationship holds (Fig. 2) with a correlation coefficient of 0.96. One subject performed the mixing test under the standard conditions. At the end of the experiment the helium percentage was read and the subject, still connected to the circuit, was asked to take several vital capacity breaths. (Receivedfor publication October 2, I956.)
4
THE BRITISH J O U R N A L OF TUBERCULOSIS FUNCTIONAL RESIDUAL CAPACITY ' in litre=
Fro. 1 . - - T h e relationship between the functional residual capacity and the time constant of the mixing curve in normal subjects, o9 Q~
3-
D
0
io
~,0
TIME
30
CONSTANT
o
IN SECONDS
MINUTE VENTILATION per litr¢ of functiona!
residual copocity in lltrcs III-
IO,
Fro. 2 . - - T h e relationship between the minute ventilation per litre of functional residual capacity and the time constant of the mixing curve. The crosses represent the results when the subject was breathing normally and the full circles were obtained when the breathing was controlled.
e m
2. O
C,-
O
T
ao
~
TIME C O N S T A N T
T
30
IN S E C O N D S
~o
AND DISEASES OF T H E C H E S T
5
After these deep breaths he resumed normal breathing, and a few minutes later the helium percentage was again read. From these two readings two values for the functional residual capacity were calculated. The test was repeated on successive days on the same subject so that ten pairs of observations were available. It was found that the mean functional residual capacity as usually performed was 3"84 litres (S.D.o.26); this was increased to 4" 74 litres (S.D. 0.38 ) by the deep breathing. The difference between the two figures is highly significant ( P ~ o . o I ) . The mean values for the ratio of the residual capacity to the total lung capacity under the same conditions were 33"4 per cent. (S.D. 3"1) and 46"4 per cent. (S.D. 4"8) respectively, a difference which was very highly significant ( P ~ o ' o o I ) . When each of 16 medical students, taken in chronological order from the 20
NORMAL
rln
<2o ~.s" C~
<~
7
3
(~. (,,5
o
NUMBER OF SUBJECTS
RESIDUAL CAPACITY x IOOTO'I:'ALLUNG CAPACITY
7 NUMBEOF
.____._El_
sUbjECTS
AFTER DEEP BREATHING
(~.o (~;
(~o
Or
(4o
(4~"
o
RESIDUAL CAPACI;rY x IOO TOTAL LUNG CAPACITY
FIO. 3.--The figure shows the distribution of the percentage ratio of the residual capacity to the total lung capacity in the normal group. The upper histogram was obtained from tests when breathing was normal, the lower one after deep breathing.
in the group, performed the test once, the mean value for the functional residual capacity rose from 3.65 litres (S.D.o.57) to 4"28 litres (S.D. o-58), a difference which was highly significant ( P ~ o . o I ) . The ratio of residual capacity to total lung capacity increased from 28.6 per cent. (S.D. 4"7) to 37"2 per cent. (S.D. 5"2), which was a very highly significant difference (P~o.ooi). This change is illustrated in Fig. 3.
Patients with Emphysema The 20 patients with emphysema had a mean functional residual capacity of 4" 75 litres (S.D.I. 76), which was probably significantly greater (o.o5~P ~ o.o2) than that of the normal controls. The time constants of the mixing curves of the patients were related to the size of the functional residual capacity (r=o'5) as closely as in the young normal group, though the time constants
THE BRITISH JOURNAL
OF T U B E R C U L O S I S
were longer and the values for the functional residual capacity were somewhat greater. But these patients differed from the young control group in one important respect: when the minute ventilation per litre of functional residual capacity was plotted against the time constant, the observations did not fall on the curve for normal subjects (Fig. 4). It is reasonable to deduce from this that the inefficiency of mixing cannot adequately be explained on the basis of an increased functional residual capacity alone. In severe cases of emphysema, deep breathing involves considerable effort. This makes it difficult to obtain results comparable to the normal group for the effect of vital capacity breaths on the functional residual capacity. From those experiments which were technically satisfactory, there was a small increase in the functional residual capacity of o. I8 litre: this was not statistically significant. MINUTE V ENTILATION per litre of fut~ctlonol r~slduQt copoclty in lltr¢,
k
o
2o
3o
~o
TIME CONSTANT
~'o
-go
70
Ito
$~O
IN sECONO$
FIG. 4 . - - T h e observations for the minute ventilation per litre of the functional residual capacity and the time constant of the mixing curve in the patients with emphysema. The curve~obtained from the normal subjects and shown in Fig. ~ is superimposed.
Discussion There is no evidence to suggest that in normal subjects deep breathing alters the expiratory reserve volume, and since deep breathing increases the functional residual capacity, it follows that the residual capacity must be affected. The significance of this lies in the fact that the residual capacity expressed as a percentage of the total lung capacity is often used as an aid in the diagnosis of chest disease, particularly of emphysema. From this series of experiments on young subjects, the upper limit of normality of this fraction is 38.2 per cent., a figure which is similar to those quoted by Hurtado and Boller (I933) , Bates and Christie (I95O), and Needham, Rogan and McDonald (I954) , amongst others. If, however, the fraction residual capacity divided by total lung capacity was determined soon after the subject had breathed deeply,
AND DISEASES O F T H E
CHEST
7
over 4o per cent. of the present young controls would have appeared to be abnormal. An increase in the volume of the functional residual capacity may be expected to retard the process of gas mixing. Reference to Fig. 2 shows that when the minute ventilation per litre of functional residual capacity is 4 litres, any increase in ventilation will be of little significance in improving the efficiency of gas mixing, but any decrease will lead to an impairment of efficiency. It is a reasonable supposition that the purpose of this increase in the functional residual capacity may be to provide a greater respiratory surface for gas exchange in response to a ventilatory demand. The fact that the functional residual capacity does not increase with deep breathing in patients with emphysema may be regarded as an expression of the absence of any pulmonary reserve in these cases. Although the volume of the functional residual capacity at rest may be increased, and the observations presented suggest that the range is wide, our results are in agreement with the view that the volume change is not the major cause of the impairment of gas mixing in emphysema.
Summary A group of 2o young normal subjects and 2o patients with emphysema was studied. On deep breathing the thnctional residual capacity was significantly increased in the young control group but not in the patients with emphysema. The significance of this finding is discussed. These investigations were undertaken during the tenure of a London Hospital postgraduate Fellowship. It gives me great pleasure to acknowledge the help of ProfessorJ. L. D'Silva. The author is indebted to Mr. G. Walter and the many subjects who have assisted him. REFERENCES BAT,S, D. V., and CHPaSTm,R. V. (195o): Clin. Sci., 9, 17. B~.RNSTEIN,L. (1954): J. Physiol., 125, 38P. BUXTON,R. ST.J., and D'SILVA,J. L. (I956): Tubercle (Lond.), 37, 264. HURTADO,A., and BOLLER,C. (I933) : J. din. Invest., 12, 793. NZEDHAM,C. D,, ROGAN,MARYC., and McDONALD,I. (1954): Thorax, 9, 313.
3
CHANGES IN THE FUNCTIONAL RESIDUAL CAPACITY WITH DEEP BREATHING IN N O R M A L AND E M P H Y S E M A T O U S SUBJECTS BY R. ST.J. BUXTON Department of Physiology,King's College, London MEASUREMENT of the functional residual capacity is less easy to perform than spirometry, so that it is not undertaken as often as the estimation of the vital capacity in the assessment of lung function. It is however of importance, since the functional residual capacity represents the volume by which the tidal air is diluted, and so changes in its volume may be expected to affect the rate of gas mixing in the lungs. Bernstein (1954) has shown that inflation of the rabbit's lung in vivo can lead to an opening up of previously closed alveoli. Anaesthetists are familiar with this phenomenon in man. It was therefore of interest to discover whether the functional residual capacity was increased after a subject took a few vital capacity breaths. METHOD
The technique used followed that of Bates and Christie (i 95 o) with certain modifications which have been described by Buxton and D'Silva (1956). All subjects were examined in the sitting position. The normal group consisted of 20 healthy medical students aged i8-27 years, and in addition 20 patients with pulmonary emphysema were studied whose ages ranged from 35-63 years (mean 48.8, S.D. 7"4). EXPERIMENTAL RESULTS
Normal Subjects The mean functional residual capacity of the normal group of 20 subjects was 3.76 litres (range 2.2-5-7, S.D. 0.78 ). The time taken for " 63.2 per cent. mixing "--i.e., the time constant of the mixing curve (Buxton and D'Silva, I956)--was significantly (r=o.54) though not closely related to the size of the functional residual capacity (Fig. I). It has been demonstrated that the minute ventilation and the time constant are closely re!ated (Buxton and D'Silva, 1956 ) in a hyperbolic form for the group of normal subjects breathing with minute ventilations ranging from 6-45 litres. If the results are plotted not as total minute ventilation but as minute ventilation per litre of functional residual capacity, the same type of relationship holds (Fig. 2) with a correlation coefficient of 0.96. One subject performed the mixing test under the standard conditions. At the end of the experiment the helium percentage was read and the subject, still connected to the circuit, was asked to take several vital capacity breaths. (Receivedfor publication October 2, I956.)
4
THE BRITISH J O U R N A L OF TUBERCULOSIS FUNCTIONAL RESIDUAL CAPACITY ' in litre=
Fro. 1 . - - T h e relationship between the functional residual capacity and the time constant of the mixing curve in normal subjects, o9 Q~
3-
D
0
io
~,0
TIME
30
CONSTANT
o
IN SECONDS
MINUTE VENTILATION per litr¢ of functiona!
residual copocity in lltrcs III-
IO,
Fro. 2 . - - T h e relationship between the minute ventilation per litre of functional residual capacity and the time constant of the mixing curve. The crosses represent the results when the subject was breathing normally and the full circles were obtained when the breathing was controlled.
e m
2. O
C,-
O
T
ao
~
TIME C O N S T A N T
T
30
IN S E C O N D S
~o
AND DISEASES OF T H E C H E S T
5
After these deep breaths he resumed normal breathing, and a few minutes later the helium percentage was again read. From these two readings two values for the functional residual capacity were calculated. The test was repeated on successive days on the same subject so that ten pairs of observations were available. It was found that the mean functional residual capacity as usually performed was 3"84 litres (S.D.o.26); this was increased to 4" 74 litres (S.D. 0.38 ) by the deep breathing. The difference between the two figures is highly significant ( P ~ o . o I ) . The mean values for the ratio of the residual capacity to the total lung capacity under the same conditions were 33"4 per cent. (S.D. 3"1) and 46"4 per cent. (S.D. 4"8) respectively, a difference which was very highly significant ( P ~ o ' o o I ) . When each of 16 medical students, taken in chronological order from the 20
NORMAL
rln
<2o ~.s" C~
<~
7
3
(~. (,,5
o
NUMBER OF SUBJECTS
RESIDUAL CAPACITY x IOOTO'I:'ALLUNG CAPACITY
7 NUMBEOF
.____._El_
sUbjECTS
AFTER DEEP BREATHING
(~.o (~;
(~o
Or
(4o
(4~"
o
RESIDUAL CAPACI;rY x IOO TOTAL LUNG CAPACITY
FIO. 3.--The figure shows the distribution of the percentage ratio of the residual capacity to the total lung capacity in the normal group. The upper histogram was obtained from tests when breathing was normal, the lower one after deep breathing.
in the group, performed the test once, the mean value for the functional residual capacity rose from 3.65 litres (S.D.o.57) to 4"28 litres (S.D. o-58), a difference which was highly significant ( P ~ o . o I ) . The ratio of residual capacity to total lung capacity increased from 28.6 per cent. (S.D. 4"7) to 37"2 per cent. (S.D. 5"2), which was a very highly significant difference (P~o.ooi). This change is illustrated in Fig. 3.
Patients with Emphysema The 20 patients with emphysema had a mean functional residual capacity of 4" 75 litres (S.D.I. 76), which was probably significantly greater (o.o5~P ~ o.o2) than that of the normal controls. The time constants of the mixing curves of the patients were related to the size of the functional residual capacity (r=o'5) as closely as in the young normal group, though the time constants
THE BRITISH JOURNAL
OF T U B E R C U L O S I S
were longer and the values for the functional residual capacity were somewhat greater. But these patients differed from the young control group in one important respect: when the minute ventilation per litre of functional residual capacity was plotted against the time constant, the observations did not fall on the curve for normal subjects (Fig. 4). It is reasonable to deduce from this that the inefficiency of mixing cannot adequately be explained on the basis of an increased functional residual capacity alone. In severe cases of emphysema, deep breathing involves considerable effort. This makes it difficult to obtain results comparable to the normal group for the effect of vital capacity breaths on the functional residual capacity. From those experiments which were technically satisfactory, there was a small increase in the functional residual capacity of o. I8 litre: this was not statistically significant. MINUTE V ENTILATION per litre of fut~ctlonol r~slduQt copoclty in lltr¢,
k
o
2o
3o
~o
TIME CONSTANT
~'o
-go
70
Ito
$~O
IN sECONO$
FIG. 4 . - - T h e observations for the minute ventilation per litre of the functional residual capacity and the time constant of the mixing curve in the patients with emphysema. The curve~obtained from the normal subjects and shown in Fig. ~ is superimposed.
Discussion There is no evidence to suggest that in normal subjects deep breathing alters the expiratory reserve volume, and since deep breathing increases the functional residual capacity, it follows that the residual capacity must be affected. The significance of this lies in the fact that the residual capacity expressed as a percentage of the total lung capacity is often used as an aid in the diagnosis of chest disease, particularly of emphysema. From this series of experiments on young subjects, the upper limit of normality of this fraction is 38.2 per cent., a figure which is similar to those quoted by Hurtado and Boller (I933) , Bates and Christie (I95O), and Needham, Rogan and McDonald (I954) , amongst others. If, however, the fraction residual capacity divided by total lung capacity was determined soon after the subject had breathed deeply,
AND DISEASES O F T H E
CHEST
7
over 4o per cent. of the present young controls would have appeared to be abnormal. An increase in the volume of the functional residual capacity may be expected to retard the process of gas mixing. Reference to Fig. 2 shows that when the minute ventilation per litre of functional residual capacity is 4 litres, any increase in ventilation will be of little significance in improving the efficiency of gas mixing, but any decrease will lead to an impairment of efficiency. It is a reasonable supposition that the purpose of this increase in the functional residual capacity may be to provide a greater respiratory surface for gas exchange in response to a ventilatory demand. The fact that the functional residual capacity does not increase with deep breathing in patients with emphysema may be regarded as an expression of the absence of any pulmonary reserve in these cases. Although the volume of the functional residual capacity at rest may be increased, and the observations presented suggest that the range is wide, our results are in agreement with the view that the volume change is not the major cause of the impairment of gas mixing in emphysema.
Summary A group of 2o young normal subjects and 2o patients with emphysema was studied. On deep breathing the thnctional residual capacity was significantly increased in the young control group but not in the patients with emphysema. The significance of this finding is discussed. These investigations were undertaken during the tenure of a London Hospital postgraduate Fellowship. It gives me great pleasure to acknowledge the help of ProfessorJ. L. D'Silva. The author is indebted to Mr. G. Walter and the many subjects who have assisted him. REFERENCES BAT,S, D. V., and CHPaSTm,R. V. (195o): Clin. Sci., 9, 17. B~.RNSTEIN,L. (1954): J. Physiol., 125, 38P. BUXTON,R. ST.J., and D'SILVA,J. L. (I956): Tubercle (Lond.), 37, 264. HURTADO,A., and BOLLER,C. (I933) : J. din. Invest., 12, 793. NZEDHAM,C. D,, ROGAN,MARYC., and McDONALD,I. (1954): Thorax, 9, 313.