PCO2 levels apparently differed by less than 5 mm Hg. Fowler [1954] and. Godfrey and Campbell [1969] have shown that it is possible to resume a

Q. Ji exp. Physiol. (1969) 54, 129-140 THE INFLUENCE OF LUNG SHRINKAGE ON BREATH HOLDING TIME. By S. GODFREY, R. H. T. EDWARDS and D. A. WARRELL. From the Department of Medicine, Royal Postgraduate Medical School, London. (Received for publication 13th June 1969) The influence of lung shrinkage on the breath holding time was studied in three normal males. The rate of shrinkage was increased two to three fold either by slow expiration or by reducing gas density in a hypobaric chamber. For any given lung volume the relationship between breath holding time and pco2 was defined by a rebreathing method which allowed breath holds to be performed at progressively increasing levels of pco2. Hypoxia was avoided throughout and the ventilatory response to CO2 remained constant irrespective of ambient pressure. The breath holding time for any given pco2 was usually longer the larger the lung volume. Increasing the rate of lung shrinkage by either method did not alter the relationship between breath holding time and pco2. There was no constant relationship between the pco2 and the lung volume at the breaking point of a breath hold. These findings are discussed in the light of previous work and it is concluded that the sensation arising during breath holding cannot be fully explained in terms of any combination of pco2, lung volume and lung shrinkage. IT is well known that the breath holding time is shorter the smaller the lung volume [Muxworthy, 1951]. It has also been shown by Mithoefer [1959] that the lung volume of a breath hold interacts with the pco2 and PO2 to determine the breaking point under the conditions of his experiment. The range of pco2 covered in his work was relatively small and the initial PCO2 levels apparently differed by less than 5 mm Hg. Fowler [1954] and Godfrey and Campbell [1969] have shown that it is possible to resume a breath hold by rebreathing at breaking point, even though the pco2 was higher at the start of the resumed breath hold than at the end of the first breath hold. These experiments must cast some doubt on the apparent unique relationship between lung volume at breaking point and pco2, and we therefore determined to explore this problem another way. During a breath hold the lungs shrink [Stevens et al., 1946] because absorption of 02 continues while the excretion of CO2 is blocked by the loss of the tension difference between mixed venous blood and alveolar gas. We have increased this shrinkage either by having our subjects slowly expire throughout the breath hold, or by conducting the experiment at lowered ambient pressure. At a lower ambient pressure gas density is reduced so that a given number of 02 molecules absorbed occupies a larger volume; CO2 excretion is blocked as before and therefore the lungs shrink faster. We have then used the rebreathing-breath holding method described in the previous paper [Godfrey and Campbell 1969], to define the relationships between breath holding time and pco2 under conditions of normal or increased rate of shrinkage. 129

130 Godfrey, Edwards and Warrell METHODS The experiments were performed in a decompression chamber at the Institute of Aviation Medicine, Royal Aircraft Establishment, Farnborough, England. Breath holding time and the ventilatory response to C02 were measured at total barometric pressures of 753 mm Hg and 380 mm Hg. This corresponds to sea level and an altitude of approximately 18,000 feet (5,500 m). Three subjects were studied. S.G. had a good knowledge of the physiology of breath holding, D.W. had a little knowledge and I.B. was a lay airman. All communication between the observer in the chamber and those outside was by telephone from which the subject was excluded. The method of defining the relationship between breath holding time and PCO2 was that described in the previous paper [Godfrey and Campbell, 1969]. The subject rebreathed a C02/02 mixture from a 6 1. bag and the initial gas composition was adjusted to have a PCO2 of approximately 54 mm Hg and a P02 of 290 mm Hg, irrespective of ambient pressure. This mixture ensured that rapid equilibrium was established between mixed venous blood, alveolar gas and bag gas, and that after recirculation the PCO2 of the blood-lung-bag system rose at a uniform rate. The 02 content of the system was sufficient to avoid hypoxia at any time during rebreathing or breath holding. Between experiments the subjects breathed from aviation masks set to deliver gas with a P02 of 150 mm Hg irrespective of ambient pressure. Gas was sampled at the mouth for analysis by a mass spectrometer (M.S.4, AEI Ltd.) outside the chamber. It was calibrated frequently with dry gas mixtures and the actual readings were corrected for water vapour pressure. Ventilation was recorded by means of a Bernstein spirometer. All information was displayed on a Devices thermal recorder. Each single experiment lasted about 3 to 4 min during which time the subject rebreathed and held his breath alternately, so that a series of breath holds were obtained at progressively higher levels of PCO2. A graph was then constructed relating breath holding time to pco2. Each subject performed the experiment at both ambient pressures, the breath holds being at residual volume. Two subjects also performed the experiments at both altitudes holding their breath at 1.3 1. above residual volume. These subjects performed an additional experiment at 753 mm Hg in which each 'breath hold' begun at 1-3 1. above residual volume consisted of a slow controlled expiration. The expiration was sufficient to keep the spirometer bell horizontal while gas was leaked from it at a rate of 380 ml. per min. Thus for these two subjects the rate of shrinkage was increased either by lowering ambient pressure or by slow expiration. Ventilation-CO2 response curves were obtained for each subject by the method of Read [1967] using the same circuit. The shrinkage of the lung-bag system was obtained from the fall in level of the spirometer each time the subject expired to residual volume during breath holding experiments, and by having him expire to residual volume periodically during the rebreathing ventilation-co2 response experiments. There was insufficient time, because of the adjustment required, to obtain the shrinkage directly during breath holding experiments at the larger lung volumes. RESULTS Gas mixtures; ventilatory response. -An aviation mask was used between experiments and rebreathing mixtures were chosen so as to avoid hypoxia and hyperventilation at all stages. The relatively constant end-tidal PCO2 for any subject before the start of an experiment (Table I) attested the

Lung Shrinkage and Breath Holding 131 Subject D.W. S.G. I.B. TABLE I. Type of Experiment Barometric Pressure BH-RV BH-SV BH-Leak Vent BH-RV BH-SV BH-Leak Vent H-RV Vent RANGE OF END OF TIDAL CURVES End tidal gases before experiment (mm Hg) pco2 P02 753 380 753 380 Composition of bag at end of experiment (mm Hg) PCO2 P02 753 380 753 380 37-5 34-0 109 104 70 0 66-8 120 160 38-6 36-2 104 116 68-6 66-8 120 160 39.2 112 70-0 128 37-5 39-0 105 107 662 61-6 152 184 40-0 43-3 200 138 40-0 42-3 159 137 42-9 200 40-6 424 176 140 42*9 42*2 116 140 42-9 43-0 108 133 70-8 71-2 141 189 74-5 74-5 137 191 75-2 156 69-9 68-5 176 207 67-0 70-1 177 209 75.5 70-4 146 213 BH-RV =Breath hold at residual volume; BH-SV =Breath hold at approximately 1-3 1. above residual volume; BH-Leak =Breath hold at 1-3 1. above residual volume with a leak of 380 ml./min from the system; Vent =ventilation-co2 response experiment. The final pco2 or P02 refers to the end of the last breath hold or the end of rebreathing in a ventilation-co2 experiment. Where the breath hold was broken by an inspiration, the values have been calculated as described in the text. The values in this Table show the maximum variation in gas tension encountered by the subjects during the experiment. TABLE II. C02, SHRINKAGE AND VENTILATION. Rate of Rise Rate of Type of of pco2 Shrinkage Subject Experiment (mm Hg/min) (ml. /min) D.W. S.G. I.B. Ventilatory Response Slope Intercept (1. /min/mm Hg) (mm Hg) Barometric Pressure 753 380 753 380 753 380 753 380 BH-RV BH-SV BH-Leak Vent BH-RV BH-SV BH-Leak Vent BH-RV Vent 5-0 5-0 6 0 4 6 5.4 5.4 6-0 5.1 4-8 5.7 4 2 6-4 6-6 5.4 344 752 406 768 4 69 3 91 49-0 44.4 520 983 422 804 510 618 517 52-2 6-9 6-5 317 683 6-9 7.5 375 705 2-96 2-92 52-3 50-0 The symbols used are the same as those in Table I. The slopes of the calculated regression lines are given for rate of rise of pco2 and rate of shrinkage of lung-bag system. Both the slope (S) and the intercept (B) of the ventilation-co2 response curve are given.

132 Godfrey, Edwards and Warrell D.W. S.G. 0/ / -i / / I- = ui I.- J2 0. - 0 Pco2 (mm.hg) I I I i 50 55 60 Pco2 (mm.hg.) 65 70 100-1. B. * At 753 mm.hg. - - 0 At 380 mm.hg. _80_-X i I 19I O s I 60 40 _ 0~ I t 20 _ 55 60 65 70 75 80 Pco 2 (mm.hg.) Fig. 1. Ventilatory response to CO2 for each subject, at normal and low ambient pressure. The lines were calculated by the method of least squares. For any one subject the two lines are not significantly different.

Lung Shrinkage and Breath Holding 133 success of this control. In fact the lowest P02 recorded was 104 mm Hg and the lowest level at the end of a breath hold was 120 mm Hg (Table I). This latter value is above the threshold for a hypoxic effect on the breath holding time [Douglas and Haldane, 1909] and the vast majority of our breath holds ended well above this level. For those breath holds ending in an inspiration, the final P02 and pco2 were calculated from the measured rates of change during breath holds ending in expiration. The rate of rise of pco2 was calculated for all experiments in all subjects (Table II). Although there was a little inter-subject variation, for any one subject the rate of rise of pco2 was relatively constant and especially so for any one type of experiment at either ambient pressure. Such differences as occurred were not significant and would not have affected the results. The 'leak' experiment was not performed at 380 mm Hg. In any study on breath holding in which the subject's environment is changed it is important to ensure that the sensitivity to CO2 has not altered. The ventilatory response to CO2 by the rebreathing method was recorded in each of our subjects at both ambient pressures. The regression lines are shown in fig. 1 and the slopes and intercepts in Table II. Differences between altitudes for any one subject were not significant. Volume shrinkage and breath holding time. - During rebreathing or breath holding the volume of the bag-lung system fell. The shrinkage measured by the spirometer for each subject is shown in fig. 2 where the lines are drawn by eye to represent a mean rate of shrinkage at each ambient pressure. The slopes of the individual regression lines are given in Table II. The outstanding feature is the doubling of the shrinkage at low ambient pressure. The rate of shrinkage during the ventilatory response curve was usually a little higher than during the breath holding response curve, possibly due to the greater effort involved. No data are available for the shrinkage during breath holding at the larger lung volume, but there seems little reason to believe that the results could be very different. Indeed, from the 'openloop' nature of the system, rebreathing could well be considered analogous to breath holding at a moderate lung volume as far as shrinkage is concerned. The actual shrinkage during breath holding must in fact be a little faster than that indicated in Table II which represents combined breath holding-rebreathing shrinkages, since the lungs alone (breath holding) will have to supply all the 02 consumed. We measured the residual volume of D.W. in a body plethysmograph and were able then to calculate the actual rates of shrinkage during breath holding. At 380 mm Hg his actual rate of shrinkage was 2-6 times that at 753 mm Hg. The ratio calculated from the data in Table II is 2'2. This small difference does not affect the main objectives of this study. The rate of lung shrinkage was also increased by leaking gas from the system, thus requiring the subject to expire throughout the breath hold. The normal shrinkage at 753 mm Hg was therefore increased by 380 ml./min. This is shown as the dotted lines in fig. 2 which lie very close to the shrinkage at the lower ambient pressure. VOL. LIV, NO. 2. 1969 9

134 Godfrey, Edwards and Warrell 1-- LA, C/7 -J J CX7 -I cm 2: TIME (sec.) TIME (sec.) = LJ * Rebreathing at 753 mm.hg. * Breath holding at 753 mm.hg. o Rebreathing at 380 mm.hg. o Breath holding at 380 mm. Hg. + Breath holding at 753 mm. Hg. with leak from system LJ 100 150 TIME (sec.) 250 Fig. 2. Loss of volume from the lung-bag system during simple rebreathing or alternating rebreathing and breath holding, at normal and low ambient pressure. For simplicity one solid line was drawn by eye through all points for any one pressure. The dashed lines represent the shrinkage when voluntary expiration at a rate of 380 ml./min was added at 758 mm Hg. Both lowering ambient pressure and voluntary expiration increased shrinkage by some 2-2 times.

Lung Shrinkage and Breath Holding 135 70rK D.W. S. G. 60 - (- @3 LUJ ~-- 50 - '40 _ cj = LA co 30-20 _ 10 - n.. v. _ It- I I I rli -i 145 50 55 60 65 50 55 Pco2 (mm.hg.) 60 65 Pco2 (mm.hg.) a 70 I. B. * Breath holding at 753 mm.hg. O Breath holding at 380 mm.hg. CD) u-c L.j 30- -j 20k m LUJ 10 n I I I I %55 60 65 70 75 Pco2 (mm.hg.) 11 u L-. I Fig. 3. Breath holding time at residual volume is plotted against PCO2 at the start of the breath hold. The results for each subject at each ambient pressure are given. There is no consistent difference between the two lines for any one subject.

136 Godfrey, Edwards and Warrell The breath holding time for each subject was found to depend upon the pco2 whether measured at the beginning or the end of the breath hold. The higher the pco2 the shorter the breath holding time, the actual time varying from subject to subject. Either value of pco2 could be used for constructing breath holding-co2 response curves, but we have chosen to use the initial pco2 because the final pco2 depends upon the initial pco2 and B00 O.W. S.G. 60 \ Breath hold at 753mm.Hg. \\\\\o0 50 Breath hold @50 \ \\ - \\\ at 380mm.Hg. \\a\i \\* Breath hold _ 0 p \ p \ X \> at 753 mm.hg. 40 l with leak C= 30 70- ~20-10 I I I I I 5 50 55 60 65 55 60 65 70 75 INITIAL Pco2 (mm. Hg.) Fig. 4. Breath holding time at 1-3 1. above residual volume is plotted against pco2 at the start of the breath hold. The lines represent experiments with a normal rate of lung shrinkage at 758 mm Hg and with a two- to three-fold increase in shrinkage produced by (a) voluntary expiration throughout the breath hold, and (b) by lowering ambient pressure to 380 mm Hg. There is no consistent difference in the lines for any one subject. the breath holding time is therefore a dependent variable [Godfrey and Campbell, 1968]. For breath holds begun at residual volume, the breath holding time at any given pco2 was little changed by ambient pressure (fig. 3) and hence by rate of lung shrinkage. The breath holding time at low ambient pressure was actually slightly higher in two subjects, one of whom was entirely unaware of the significance of the result. All three subjects noted a crushing sensation in the chest as the lungs contracted below residual volume, especially at low ambient pressure. In a similar way, the duration of breath holds begun 13 1. above residual volume was not consistently affected by an increased rate of shrinkage, whether produced by slow expiration or by low ambient pressure (fig. 4). In the past a good deal of attention has been given to the relationship

Lung Shrinkage and Breath Holding 137 between breaking point pco2 and breaking point lung volume [Mithoefer, 1959]. We have been able to calculate the whole volume/pco2 history of the breath holds for one of our subjects (S.G.) whose residual volume was measured in a body plethysmograph. By choosing breath holds begun at different lung volumes and different levels of PCO2 (from the curves shown in figs. 3 and 4) and from the rate of rise of pco2 and rate of shrinkage (Table II) 75 -A, 70 S.G. A - 753 mm.hg. 4-380 mm. Hg. E 65 * E 60 _ 55 (D 507 I l 10 15 2-0 2-5 30 3-5 LUNG VOLUME (l. BIPS.) Fig. 5. The changes in lung volume and PCO2 during the course of a series of breath holds are shown for one subject (S.G.) at both ambient pressures. The arrows indicate the direction of change between the points representing the start and breaking point of selected breath holds. It can be seen that there is no constant relationship between the breaking point PCO2 and lung volume. The implications of this observation with reference to the labelled breath holds are discussed in the text. the plots shown in fig. 5 were obtained. Each line joins the initial volume/ PCO2 point to the final volume/pco2 point for a single breath hold, the arrow indicating the direction of change. The point of this plot is to show that there is no constant relationship between breaking point volume and pco2 provided that a sufficiently wide range of initial values is used. Thus while a comparison of breath holds A with B or C suggests that the larger the final volumes the higher the final pco2, a comparison between C and B shows that the final volume may be smaller and yet the final pco2 may be higher. It can also be seen that it was possible to begin breath hold C at a higher pco2 and smaller volume than existed at the breaking point of B. A period of rebreathing, which enables a sufficiently wide range of PCO2 to be studied, totally disproves any constant relationship between final volume and pco2.

138 Godfrey, Edwards and Warrell DIscusSION In any experiment on breath holding it is important to consider the question of individual variability and the nature of the sensation generated. Although we only used three subjects, each served as his own control, and each breath holding-co2 response curve was based on three or more breath holds. We have found that the relationship between breath holding time and initial PCO2 remains very constant for any one subject, with a coefficient of variation for the slope of 8-6 per cent and for the intercept of 2-8 per cent [Godfrey and Campbell, 1969]. The data on which these calculations were based, were obtained on one of the subjects (D.W.) of the present study. Under these circumstances, we believe it is reasonable to accept the similarity of the curves under different conditions of shrinkage as meaning that the rate of shrinkage does not significantly affect the breath holding time. The simplest method of increasing lung shrinkage is to expire slowly during the breath hold. However, during slow expiration the respiratory muscles are active and alveolar pressure nearly atmospheric, while during breath holding the activity of the muscles is intermittent and the alveolar pressure varies. Because of the known effect of voluntary movements such as swallowing (which we allowed) on breath holding, it is obviously important to confirm that voluntary expiration does not act similarly. The only method by which lung shrinkage can be increased under otherwise identical conditions is by lowering ambient pressure. Most previous studies at high altitude have been done under hypoxic conditions so that the pco2 was lowered by hyperventilation. Acclimatisation caused subsequent changes in C02 sensitivity [Otis et al., 1948; Severinghaus et al., 1963]. In one subject studied by Otis et al., [1948], they commented that the breath holding time was not shortened by acute exposure to low ambient pressure while breathing 02- We were careful to avoid levels of P02 likely to affect our experiments. The experiments described here, in the previous paper [Godfrey and Campbell, 1969] and by Fowler [1954] all argue against specific chemical or lung volume thresholds for the breaking point of breath holding, or any constant relationship between final pco2 and volume. The earlier finding of such a relationship [Mithoefer, 1959] can be explained by the fact that all his breath holds were begun at approximately the same pco2 and with a larger lung volume the breath holding time increased and hence the final pco2 was higher. The effect of hyperventilation [Mithoefer, 1959] was to prolong breath holding time but not apparently to affect the relationship between final pco2 and volume, although the actual figures were not given. Hyperventilation lowers the initial pco2, so that the rise in pco2 during the breath hold only brings the final pco2 up to relatively low levels, but during the prolonged breath hold the lungs shrink to a relatively small volume. This means that the relatively low final pco2 will correlate with the relatively small lung volume, and both values might well fit on a 'normal' curve.

Lung Shrinkage and Breath Holding 139 Although we did not systematically investigate the effect of lung volume on breath holding time, it can be seen from figs. 3 and 4 that the breath holding time for any given pco2 was always longer at the larger lung volume for S.G. and usually larger for D.W. Muxworthy [1951] studied this subject using a wide range of volumes and showed a nearly linear relationship. The greatest increase in shrinkage over control shrinkages in our experiments was of the order of 300 400 ml. (for a breath hold lasting one minute). This is very small compared with the difference in volume of some 2000 ml. needed by subject S.G. to prolong his breath holding time in the experiments at different volumes. D.W. was even less sensitive to the static volume effect. This argument suggests that the absolute shrinkage during a breath hold can hardly be expected to be large enough to influence holding time. This was found by Mithoefer [1959] in his breath holding experiment after hyperventilation; in this breath hold the amount of shrinkage was large, yet the breath holding time was long. We have been able to show that breath holding is affected by neither the magnitude nor the rate of shrinkage (which was unchanged in Mithoefer's experiment). This makes it very unlikely that the prolongation of breath holding time by vagal blockade [Guz et at., 1966] can be due to the removal of impulses from deflation receptors. The explanation remains obscure. We conclude that large changes in lung volume and moderate changes in pco2 do affect the breath holding time, but the magnitude and rate of lung shrinkage do not. No constant pco2 or lung volume is found at breaking point, and almost any combination of pco2 and volume can be produced by suitable manoeuvres. ACKNOWLEDGEMENTS We are grateful to Wing Commander J. Ernsting and the Ministry of Defence for permitting us to use the altitude chamber at Farnborough. We thank Mr. W. J. Tonkins and Corporal R. Russel for their help in running the experiments, and Junior Technician I. E. Butcher for volunteering to be one of our subjects. We also thank Dr. E. J. M. Campbell for his help in the preparation of this paper. R.H.T.E. was supported from a grant by the Medical Research Council. REFERENCES DOUGLAS, J. G., and HALDANE, J. S. (1909). 'The regulation of normal breathing', J. Physiol. (Lond.), 38, 420-440. FOWLER, W. S. (1954). 'Breaking point of breath holding', J. appi. Physiol., 6, 539-545. GODFREY, S., and CAMPBELL, E. J. M. (1968). 'The control of breath holding', Re&sp. Physiol., 5, 385-400. GODFREY, S., and CAMPBELL, E. J. M. (1969). 'Mechanical and chemical control of breath holding', Q. Jl exp. Physiol., 54, 117-128.

140 Godfrey, Edwards and Warrell Guz, A., NOBLE, M. I. M., WIDDICOMBE, J. G., TRENCHARD, D., MusHIN, W. W., and MAKEY, A. R. (1966). 'The role of vagal and glossopharyngeal afferent nerves in respiratory sensation, control of breathing and arterial pressure regulation in conscious man', Clin. Sci., 30, 161-170. MITHOEFER, J. C. (1959). 'Mechanisms of pulmonary gas exchange and CO2 transport during breath holding', J. appl. Physiol., 14, 701-705. MUXWORTHY, J. F., (1951). 'Breath holding studies: Relationship to lung volume', A. F. Tech. Rep. 6528, 452-456. Wright-Patterson Air Force Base, Dayton, Ohio. OTIS, A. B., RARN, H., and FENN, W. 0. (1948). 'Alveolar gas exchange during breath holding', Am. J. Physiol., 152, 674-686. READ, D. J. C. (1967). 'A clinical method for assessing the ventilatory response to carbon dioxide', Aust. Ann. Med., 16, 20-32. SEVERINGHAUS, J. W., MITCHELL, R. A., RICHARDSON, B. W., and SINGER, M. (1963). 'Respiratory control at high altitude suggesting active transport regulation of C.S.F. ph', J. appl. Physiol., 18, 1155-1166. STEVENS, C. D., FERRIS, E. B., WEBB, J. P., ENGEL, G. L., and LOGAN, M. (1946). Voluntary breath holding: 1. 'Pulmonary gas exchange during breath holding', J. Clin. Invest., 25, 723-728.