J. Physiol. (1975), 249, pp. 435-443 With 1 plate and 3 text-figurem Printed in Great Britain
435
BRONCHIAL HYSTERESIS IN EXCISED LUNGS
-By J. M. B. HUGHES, HAZEL A. JONES AND A. G. WILSON From the Departments of Medicine and Radiology, Royal Postgraduate Medical School, Hammersmith Hospital, London W12 OHS
(Received 24 September 1974) SUMMARY
1. Intrapulmonary bronchi in excised dog lungs were outlined with tantalum dust and stereoscopic radiographs taken during deflation and inflation of the lung with air, saline, Ringer or EDTA solutions. Dimensions of airways as a percentage of their values at full inflation were calculated from measurements of the stereoscopic X-ray images. 2. The mean deflation-inflation diameter difference at a transpulmonary pressure of 5 cm H20 was 20 % in the air-filled lung, 9 % in the salinefilled preparation and 2 % after filling with EDTA in saline. 3. These results show that the intrapulmonary bronchi have an intrinsic hysteresis separate from the hysteresis imposed on them by the expansion of the surrounding parenchyma. This intrinsic hysteresis is mainly due to the tone of the smooth muscle in the bronchial wall. INTRODUCTION
Hysteresis of intact (Mead, Whittenberger & Radford, 1957) and excised lungs (McIlroy, 1952) is well known. At the same lung volume, lung recoil pressure during inflation from low volumes is greater than on deflation from high volumes. Since intrapulmonary airways and alveoli form a continuous elastic system, the forces distending the lungs will also distend the airway. Thus, changes of airway volume might be expected to follow changes of lung volume in a passive manner. On the other hand, airways dissected free from lung tissue (Martin & Proctor, 1958) exhibit hysteresis in a similar direction to that of lungs. Therefore the greater bronchial wall recoil on inflation will resist the increase in lung distending pressure. In addition, the local tissue recoil pressures acting on the bronchi are not necessarily the same as those of the whole lung, particularly if the tone in the bronchial wall alters, or if the pressure in the bronchial lumen becomes significantly different from that in the surrounding alveoli. At a constant lung volume any change in bronchial diameter will be accompanied by 17.2
436 J. M. B. HUGHES, H. A. JONES AND A. G. WILSON a change in local tissue forces per unit area of bronchial wall (Mead, Takishima & Leith, 1970). Hysteresis is particularly marked in freshly excised air-filled lungs. We used this phenomenon to try to dissect the contribution made by (a) the outward-acting forces of the lung parenchyma and (b), the inward-acting forces of the bronchial wall on the calibre of intrapulmonary airways. By filling lungs with saline instead of air, the hysteresis of lung parenchyma (Radford, 1957) is virtually abolished, but there should be little effect upon the hysteresis of the bronchial wall. In this way, airway hysteresis due to the intrinsi& properties of the bronchial wall may be compared with that imposed on the bronchus by the surrounding tissue. METHODS Ten greyhound dogs weighing 20-30 kg were anaesthetized with intravenous thiopentone sodium (25 mg/kg body weight). The jugular veins were cannulated and the dogs killed by exsanguination after the administration of heparin. The lungs were excised immediately after death and the left lungs of the first three and the right middle lobes of the remaining seven isolated. The left main bronchus or the lobar bronchus was cannulated and the lung inflated by positive pressure to 30 cm H20 and held at this pressure for a short time until all areas were inflated. Under fluoroscopic control tantalum powder (nominal particle size 5#s) was insufflated into the airways with the lung held at a distending pressure of 8-10 cm H20 until a satisfactory bronchogram was obtained (Plate 1 a). The lobe was inflated to a pressure of 30 cm H20 with a pump and held at this pressure while a radiograph was exposed on non-screen film (Kodak, Crystallex) using a fine focal spot (0-28 x 0-40 mm) and peak kilovoltage (kVp) 47 (Plate 1 a). A tube shift technique (Hughes, Hoppin & Wilson, 1972) was used to enable accurate calculations of bronchial length and diameter to be made. Air was then withdrawn from the lobe into a syringe until a pressure of 10 cm H20 was reached and the volume removed noted. After the pressure had stabilized (ca. 5-10 sec), a stereoscopic radiograph was exposed. Further air was removed to reach a pressure first of 5 cm H20 and then zero; radiographs were obtained when the pressure was steady. The lobe was then inflated from the syringe to 5 cm H20, 10 cm H20 and in some cases to 15 cm H20, and radiographs exposed at each point. Following a complete deflation of the lobe during which the volume removed was recorded, the residual gas volume was calculated from the lobe weight and the displacement volume. Total gas volume at a pressure of 30 cm H20 in the air-filled preparation was taken as maximum lung volume. All air volumes were corrected for gas compression. The first seven lobes were then degassed in a vacuum chamber and filled with normal saline at room temperature, to a volume similar to the total lung volume calculated from the air-filled preparation (P1. 1 b) and the pressure noted. The reference level for pressure was the bronchial cannula. As for the air-filled preparation, volume was removed until pressures of 10 cm H20, 5 cm H20 and zero on deflation and inflation were reached, with radiographs exposed at each point. The kVp was increased to 60. The removal of saline was slow so that the tantalum was not washed out of the airways. Most lobes were studied surrounded by room air; saline leaking through the surface was collected with a syringe so that volume could be
AIR WA Y HYSTERESIS
437
measured accurately. However, the shape of the unsupported saline-filled lobes differs from those filled with air and in two lobes the measurements were repeated with the preparation filled and surrounded by saline. Changes in geometry between the two situations were assessed by gluing metal markers on the lobe surface (P1. 1). In an additional three experiments, after degassing, the lobe was filled with a smooth muscle Ringer solution (NaCi 117 mm; KCl 4-75 mm; CGCl2 2-82 mm; KH2PO4 1-19 mM; MgSO4 1-19 mm; NaHCO3 24-6 mm; and glucose 5 mm) through which 95% 02 with 5 % C02 was bubbled. In two lobes the Ringer was subsequently replaced with a 4 mM solution of the disodium salt of ethylenediaminetetra-acetic acid (EDTA). The same protocol was followed as for the saline-filled preparation. Airway lengths and diameters were measured without prior knowledge of the deflation or inflation status from the radiographs using calipers accurate to 0-005 cm. The airway was measured for both images of the stereo-pair and also the separation of the images. The dimensions were computed using Method A of Hughes, Hoppin & Wilson (1972). The lengths of the segments selected ranged from 1 to 3 cm at full inflation and the diameters were between 0-1 and 0-6 cm. 100
Air
I---
60
4)0
E
0
c bo
40 J
20
20 Transpulmonary pressure (cm H20) 10
30
Text-fig. 1. Lung volume (as percent of that at Ptp 30 cm H20 in the airfilled lung) plotted against transpulmonary pressure, for air-filled and salinefilled preparations in seven experiments. Filled symbols represent measurements during deflation (-air, A saline) and open symbols during inflation (O air, A saline). Bars represent 1 s.E. of the mean.
438
J. M. B. HUGHES, H. A. JONES AND A. G. WILSON RESULTS
The relationship between lung volume and transpulmonary pressure (Ptp) for the lobes in which air-filling was followed by saline-filling, is shown in Text-fig. 1. The air-filled preparation demonstrates hysteresis, a greater pressure being required to inflate the lobes to a volume corresponding to that on deflation. Within a single lobe the deflation and inflation curves showed little variation with repeated measurements, although some lobes did trap gas progressively, and retained an increasing volume of air at a Ptp of zero cm H20 with each deflation. The variation in the pressurevolume relationship was however more marked between lobes, especially during inflation. 100 90
80 L 'I.
o
0
E
70 60
50 40 0
10 20 Transpulmonary pressure (cm H20)
30
Text-fig. 2. Airway diameter (as percent of that at PtP 30 cm H20 in the air-filled lung) plotted against transpulmonary pressure, PtP, for the airfilled and saline-filled preparations in seven experiments. Filled symbols represent measurements during deflation (0 air, A saline) and open symbols, during inflation (0 air, A saline). Bars represent 1 s.E. of the mean.
Following degassing, when the lobes were filled with saline, a smaller distending pressure was required for inflation to maximum volume, this being reached at a pressure of 12 cm H20 compared with the 30 cm H20 pressure chosen in the air-filled preparation. There was virtually no difference between the pressure-volume relationship with deflation and inflation volume histories, in contrast with the behaviour of the same
439 AIR WAY HYSTERESIS lobes filled with air. In the pressure range of 5 to zero cm H20 the curves were essentially superimposed on the deflation limb of the curve for the air-filled preparation. This appears to be a feature of the dog lung, as in the cat the curves for the lungs filled with saline are displaced to the left (Radford, 1964). TABTLE 1. Bronchial dimensions (diameter and length) and cube root of excised lung volume, as a percentage of values at full inflation, at transpulmonary pressure 5 cm H20 No. lobes Diameter % Length % Volume* % studied Air deflation 86-0 88-0 7 83-0 s.E. of mean Air inflation s.E. of mean Saline deflation s.E. of mean Saline inflation s.E. of mean Ringer deflation s.E. of mean Ringer inflation s.E. of mean EDTA deflation s.E. of mean EDTA inflation s.. of mean
1.76 66.0 2.37 86.0 1-74 77 0 2-67 65-7 3-61 55.9 2-41
90*5 4-84 88-5 4-73
1-04 72.0 1-54 88.0
1P16 85.5 1.57 78-4 300 75-6
2X92 88-7 1-84 87-6 1-68
1-25 65.5 4.97
83'0 1P67 83.0 203 76-3 2-38 73-3 2X15 808 1-24
7
3
2
78*3
1*75
In the air-filled preparation, measurements of airway diameter when plotted against transpulmonary pressure (Text-fig. 2) show a marked hysteresis and although no hysteresis of lung volume was seen in the salinefilled lobes, hysteresis of airway diameter remains. If these results are considered at isopressure (Table 1) the difference (as per cent maximum), between deflation and inflation is greater for airway diameter than for the cube root of lung volume. (The volume is expressed as a linear function since this is more valid for comparison with diameter and length.) In the saline-filled preparation a deflation-inflation diameter difference is still present, whereas the volume deflation-inflation difference is non-existent. The deflation curves for diameter in the air-filled and the saline-filled preparation coincide at all pressures. The saline-filled lobes surrounded by saline were not significantly different in behaviour; lung volume, airway diameter and length were similar to those for the unsupported lobes. The pressure-volume relationship for the lobes filled with Ringer solution differed from that of the saline-filled lobes in that both the deflation and
440 J. M. B. HUGHES, H. A. JONES AND A. G. WILSON inflation volumes were lower at the same distending pressure, although the deflation-inflation difference was minimal (Table 1). Similarly, at isopressure, the diameters were smaller although the deflation-inflation difference was the same as that for the lobes filled with saline. Filling the lobes with EDTA solution resulted in a pressure-volume relationship essentially the same as that for saline-filling, but reduced the deflation-inflation diameter difference to 2 %. DISCUSSION
Air-filled lungs have long been known to exhibit hysteresis (McIlroy, 1952). The major reason for the increased recoil pressure on inflation is the hysteresis of the surface active alveolar lining fluid. Recruitment of small airways and alveoli with different opening pressures is an additional factor. Saline-filling abolishes the air-liquid interface and hence the effect of surface tension. The saline pressure-volume relationship for the lung (Text-fig. 1) shows minimal hysteresis. In air-filled lungs much of the marked hysteresis shown by the bronchi in the plot of diameter against pressure (Text-fig. 2) must be related to hysteresis of lung volume, because the airway hysteresis is considerably reduced in the lungs filled with saline. A certain amount of airway hysteresis remains when the hysteresis of lung tissue is absent, demonstrating that the bronchus exhibits an intrinsic hysteresis in addition to its tendency to follow lung tissue hysteresis passively. It is unlikely that saline-filling per se had any major effect on the bronchial wall because airway diameters on deflation for air and salinefilled lobes were similar. The decrease in lung volume at a pressure of 5 cm H20 in the lungs filled with Ringer (Table 1) is probably due to an increase in smooth muscle tone, especially in the alveolar ducts and terminal airways (Colebatch & Mitchell, 1971) since these account for 15-20 % of the lung volume (Weibel, 1963). The marked decrease in airway diameter in Ringer solution is partially due to the decrease of lung volume for the same transpulmonary pressure; in addition, there was probably an increase in the airway smooth muscle tone since, at 76 % maximum of the cube root of volume, the diameter on deflation in the saline and air-filled preparations was 80 % but in Ringer-filled lobes only 66 %. However, the deflation-inflation difference of airway diameter was not increased by filling with Ringer and so it appears that the increase of smooth muscle tone caused by volume history was independent of the initial tone of the preparation. To determine the influence of lung volume on bronchial calibre, diameter has been plotted against a linear function of lung volume, for both the air-filled and saline-filled lobes (Text-fig. 3). On inflation, both curves
AIR WA Y HYSTERESIS 441 differ from the deflation curve, the diameter being smaller on inflation although the lung volume is the same. In the saline-filled preparation where the deflation and inflation pressures are the same at isovolume, the difference between the curves gives an indication of the contribution of bronchial wall hysteresis alone. 100
90 X
80
-
J.
60 _ 70 0
6U
so
40 40
50
90 60 70 80 100 Lung Volume, (%) Text-fig. 3. Airway diameter as (percent of that at PtP 30 cm H20 in the air-filled lung) plotted against the cube root of lung volume (also as percent of that at PtP 30 cm H20 in the air-filled lung) for the air-filled and salinefilled preparations in seven experiments. Filled symbols represent measurements during deflation (@ air, A saline) and open symbols, during inflation (O air, A saline).
Text-fig. 3 shows that diameters on inflation in the air-filled lobe are very similar to those in the saline-filled preparation on inflation indicating that the large differences in lung distending pressure shown in Text-fig. 1 appear to have little influence on diameter when compared with the effect of lung volume. This finding is in agreement with Text-fig. 7 of Hughes, Hoppin & Mead (1972). The difference between airway diameters on inflation in air and saline-filled preparations may be related to differences in stretch history, as minimum volume in the saline-filled lobes was higher than that for the lobes inflated with air. Cabezas, Graf & Nadel (1971) showed that in the dog, with the vagus cut, bronchial dimensions were not increased by sympathetic stimulation or the administration of isoprenaline. They concluded that airway smooth
442 J. M. B. HUGHES, H. A. JONES AND A. G. WILSON muscle in the vagotomized dog had no resting tone. Nevertheless, in the excised lung the presence of airway smooth muscle activity can be demonstrated in the absence of autonomic nervous system stimulation. This was shown in this preparation by the administration of EDTA which removes calcium ions (Keatinge, 1968) and abolishes all electrical activity in the muscle. Table 1 shows that EDTA greatly increased the lung volumes and airway dimensions of lobes previously filled with Ringer solution. The increase in airway diameter was greater than that expected for the increase in volume. In addition, EDTA completely removed airway hysteresis. EDTA had only a mild bronchodilating effect on the air and saline-filled preparations on deflation, suggesting that airways in the excised lung on deflation from maximum volume show very little smooth muscle activity. Filling the lung with Ringer solution or inflating the lung from low volumes increases bronchial smooth muscle tone. The effect of stretch which increases smooth muscle tension at any given length (Bulbring, 1955) is presumably responsible for the intrinsic airway hysteresis shown in these lungs. The hysteresis of the airways in the functioning lung is thus the sum of the extrinsic hysteresis imposed on the airways by the surrounding lung parenchyma and, to a lesser extent, the intrinsic hysteresis of the bronchial wall; even in the absence of the autonomic nervous system, airway smooth muscle plays a part in determining bronchial dimensions. REFERENCES
BuLBRING, E. (1955). Correlation between membrane potential, spike discharge and tension in smooth muscle. J. Phy8iol. 128, 200-221. CABEZAS, G. A., GRAF, P. D. & NADEL, J. A. (1971). Sympathetic versus parasympathetic nervous regulation of airways in dogs. J. apple. Phy8iol. 31, 651-655. COLEBATCH, H. J. H. & MITCHELL, C. A. (1971). Constriction of isolated living liquid-filled dog and cat lungs with histamine. J. apple. Phy8iol. 30, 691-702. HUGHES, J. M. B., HoPPIN, F. G. & MEAD, J. (1972). Effect of lung inflation on bronchial length and diameter in excised lungs. J. apple. Phy8iol. 32, 25-35. HUGHES, J. M. B., Hoppr, F. G. & WILSON, A. G. (1972). Use of stereoscopic X-ray pairs for measurements of airway length and diameter in situ. Br. J. Radiol. 45, 477-485. KEATINGE, W. R. (1968). Ionic requirements for arterial action potential. J. Physiol. 194, 169-182. McILRoY, M. B. (1952). The physical properties of normal lungs removed after death. Thorax 7, 285-290. MARTIN, H. B. & PROCTER, D. F. (1958). Pressure-volume measurements on dog bronchi. J. apple. Physiol. 13, 337-343. MEAD, J., TAKSHIMA, T. & LEITH, D. (1970). Stress distribution in lungs: a model of pulmonary elasticity. J. apple. Physiol. 28, 596-608. MEAD, J., WHITTENBERGER, J. L. & RADFORD, E. P., JR. (1957). Surface tension as a factor in pulmonary volume-pressure hysteresis. J. apple. Physiol. 10, 191-196.
The Journal of Physiology, Vol. 249, No. 3
J. M. B. HUGHES, H. A. JONES AND A. G. WILSON
Plate 1
(Facing p. 443)
AIRWAY HYSTERESIS
443
RADFORD, E. P., JR. (1957). Recent studies of mechanical properties of mammalian lungs. In Tissue Elasticity, ed. REMINGTON, J. W., pp. 117-190. Washington D.C.; American Physiological Society. RADFORD, E. P., JR. (1964). Static mechanical properties of mammalian lungs. In Handbook of Physiology, section 3: Respiration, ed. FENN, W. 0. & RAHN, H. chapter 15, pp. 429-449. Washington D.C.: American Physiological Society. WEIBEL, E. R. (1963). Morphometry of the Human Lung, p. 139. Berlin: SpringerVerlag. EXPLANATION OF PLATE
Stereoscopic tantalum bronchograms of right middle lobe of dog lung, with metal markers glued to the lung surface: (a) air-filled: (b) saline-filled.
435
BRONCHIAL HYSTERESIS IN EXCISED LUNGS
-By J. M. B. HUGHES, HAZEL A. JONES AND A. G. WILSON From the Departments of Medicine and Radiology, Royal Postgraduate Medical School, Hammersmith Hospital, London W12 OHS
(Received 24 September 1974) SUMMARY
1. Intrapulmonary bronchi in excised dog lungs were outlined with tantalum dust and stereoscopic radiographs taken during deflation and inflation of the lung with air, saline, Ringer or EDTA solutions. Dimensions of airways as a percentage of their values at full inflation were calculated from measurements of the stereoscopic X-ray images. 2. The mean deflation-inflation diameter difference at a transpulmonary pressure of 5 cm H20 was 20 % in the air-filled lung, 9 % in the salinefilled preparation and 2 % after filling with EDTA in saline. 3. These results show that the intrapulmonary bronchi have an intrinsic hysteresis separate from the hysteresis imposed on them by the expansion of the surrounding parenchyma. This intrinsic hysteresis is mainly due to the tone of the smooth muscle in the bronchial wall. INTRODUCTION
Hysteresis of intact (Mead, Whittenberger & Radford, 1957) and excised lungs (McIlroy, 1952) is well known. At the same lung volume, lung recoil pressure during inflation from low volumes is greater than on deflation from high volumes. Since intrapulmonary airways and alveoli form a continuous elastic system, the forces distending the lungs will also distend the airway. Thus, changes of airway volume might be expected to follow changes of lung volume in a passive manner. On the other hand, airways dissected free from lung tissue (Martin & Proctor, 1958) exhibit hysteresis in a similar direction to that of lungs. Therefore the greater bronchial wall recoil on inflation will resist the increase in lung distending pressure. In addition, the local tissue recoil pressures acting on the bronchi are not necessarily the same as those of the whole lung, particularly if the tone in the bronchial wall alters, or if the pressure in the bronchial lumen becomes significantly different from that in the surrounding alveoli. At a constant lung volume any change in bronchial diameter will be accompanied by 17.2
436 J. M. B. HUGHES, H. A. JONES AND A. G. WILSON a change in local tissue forces per unit area of bronchial wall (Mead, Takishima & Leith, 1970). Hysteresis is particularly marked in freshly excised air-filled lungs. We used this phenomenon to try to dissect the contribution made by (a) the outward-acting forces of the lung parenchyma and (b), the inward-acting forces of the bronchial wall on the calibre of intrapulmonary airways. By filling lungs with saline instead of air, the hysteresis of lung parenchyma (Radford, 1957) is virtually abolished, but there should be little effect upon the hysteresis of the bronchial wall. In this way, airway hysteresis due to the intrinsi& properties of the bronchial wall may be compared with that imposed on the bronchus by the surrounding tissue. METHODS Ten greyhound dogs weighing 20-30 kg were anaesthetized with intravenous thiopentone sodium (25 mg/kg body weight). The jugular veins were cannulated and the dogs killed by exsanguination after the administration of heparin. The lungs were excised immediately after death and the left lungs of the first three and the right middle lobes of the remaining seven isolated. The left main bronchus or the lobar bronchus was cannulated and the lung inflated by positive pressure to 30 cm H20 and held at this pressure for a short time until all areas were inflated. Under fluoroscopic control tantalum powder (nominal particle size 5#s) was insufflated into the airways with the lung held at a distending pressure of 8-10 cm H20 until a satisfactory bronchogram was obtained (Plate 1 a). The lobe was inflated to a pressure of 30 cm H20 with a pump and held at this pressure while a radiograph was exposed on non-screen film (Kodak, Crystallex) using a fine focal spot (0-28 x 0-40 mm) and peak kilovoltage (kVp) 47 (Plate 1 a). A tube shift technique (Hughes, Hoppin & Wilson, 1972) was used to enable accurate calculations of bronchial length and diameter to be made. Air was then withdrawn from the lobe into a syringe until a pressure of 10 cm H20 was reached and the volume removed noted. After the pressure had stabilized (ca. 5-10 sec), a stereoscopic radiograph was exposed. Further air was removed to reach a pressure first of 5 cm H20 and then zero; radiographs were obtained when the pressure was steady. The lobe was then inflated from the syringe to 5 cm H20, 10 cm H20 and in some cases to 15 cm H20, and radiographs exposed at each point. Following a complete deflation of the lobe during which the volume removed was recorded, the residual gas volume was calculated from the lobe weight and the displacement volume. Total gas volume at a pressure of 30 cm H20 in the air-filled preparation was taken as maximum lung volume. All air volumes were corrected for gas compression. The first seven lobes were then degassed in a vacuum chamber and filled with normal saline at room temperature, to a volume similar to the total lung volume calculated from the air-filled preparation (P1. 1 b) and the pressure noted. The reference level for pressure was the bronchial cannula. As for the air-filled preparation, volume was removed until pressures of 10 cm H20, 5 cm H20 and zero on deflation and inflation were reached, with radiographs exposed at each point. The kVp was increased to 60. The removal of saline was slow so that the tantalum was not washed out of the airways. Most lobes were studied surrounded by room air; saline leaking through the surface was collected with a syringe so that volume could be
AIR WA Y HYSTERESIS
437
measured accurately. However, the shape of the unsupported saline-filled lobes differs from those filled with air and in two lobes the measurements were repeated with the preparation filled and surrounded by saline. Changes in geometry between the two situations were assessed by gluing metal markers on the lobe surface (P1. 1). In an additional three experiments, after degassing, the lobe was filled with a smooth muscle Ringer solution (NaCi 117 mm; KCl 4-75 mm; CGCl2 2-82 mm; KH2PO4 1-19 mM; MgSO4 1-19 mm; NaHCO3 24-6 mm; and glucose 5 mm) through which 95% 02 with 5 % C02 was bubbled. In two lobes the Ringer was subsequently replaced with a 4 mM solution of the disodium salt of ethylenediaminetetra-acetic acid (EDTA). The same protocol was followed as for the saline-filled preparation. Airway lengths and diameters were measured without prior knowledge of the deflation or inflation status from the radiographs using calipers accurate to 0-005 cm. The airway was measured for both images of the stereo-pair and also the separation of the images. The dimensions were computed using Method A of Hughes, Hoppin & Wilson (1972). The lengths of the segments selected ranged from 1 to 3 cm at full inflation and the diameters were between 0-1 and 0-6 cm. 100
Air
I---
60
4)0
E
0
c bo
40 J
20
20 Transpulmonary pressure (cm H20) 10
30
Text-fig. 1. Lung volume (as percent of that at Ptp 30 cm H20 in the airfilled lung) plotted against transpulmonary pressure, for air-filled and salinefilled preparations in seven experiments. Filled symbols represent measurements during deflation (-air, A saline) and open symbols during inflation (O air, A saline). Bars represent 1 s.E. of the mean.
438
J. M. B. HUGHES, H. A. JONES AND A. G. WILSON RESULTS
The relationship between lung volume and transpulmonary pressure (Ptp) for the lobes in which air-filling was followed by saline-filling, is shown in Text-fig. 1. The air-filled preparation demonstrates hysteresis, a greater pressure being required to inflate the lobes to a volume corresponding to that on deflation. Within a single lobe the deflation and inflation curves showed little variation with repeated measurements, although some lobes did trap gas progressively, and retained an increasing volume of air at a Ptp of zero cm H20 with each deflation. The variation in the pressurevolume relationship was however more marked between lobes, especially during inflation. 100 90
80 L 'I.
o
0
E
70 60
50 40 0
10 20 Transpulmonary pressure (cm H20)
30
Text-fig. 2. Airway diameter (as percent of that at PtP 30 cm H20 in the air-filled lung) plotted against transpulmonary pressure, PtP, for the airfilled and saline-filled preparations in seven experiments. Filled symbols represent measurements during deflation (0 air, A saline) and open symbols, during inflation (0 air, A saline). Bars represent 1 s.E. of the mean.
Following degassing, when the lobes were filled with saline, a smaller distending pressure was required for inflation to maximum volume, this being reached at a pressure of 12 cm H20 compared with the 30 cm H20 pressure chosen in the air-filled preparation. There was virtually no difference between the pressure-volume relationship with deflation and inflation volume histories, in contrast with the behaviour of the same
439 AIR WAY HYSTERESIS lobes filled with air. In the pressure range of 5 to zero cm H20 the curves were essentially superimposed on the deflation limb of the curve for the air-filled preparation. This appears to be a feature of the dog lung, as in the cat the curves for the lungs filled with saline are displaced to the left (Radford, 1964). TABTLE 1. Bronchial dimensions (diameter and length) and cube root of excised lung volume, as a percentage of values at full inflation, at transpulmonary pressure 5 cm H20 No. lobes Diameter % Length % Volume* % studied Air deflation 86-0 88-0 7 83-0 s.E. of mean Air inflation s.E. of mean Saline deflation s.E. of mean Saline inflation s.E. of mean Ringer deflation s.E. of mean Ringer inflation s.E. of mean EDTA deflation s.E. of mean EDTA inflation s.. of mean
1.76 66.0 2.37 86.0 1-74 77 0 2-67 65-7 3-61 55.9 2-41
90*5 4-84 88-5 4-73
1-04 72.0 1-54 88.0
1P16 85.5 1.57 78-4 300 75-6
2X92 88-7 1-84 87-6 1-68
1-25 65.5 4.97
83'0 1P67 83.0 203 76-3 2-38 73-3 2X15 808 1-24
7
3
2
78*3
1*75
In the air-filled preparation, measurements of airway diameter when plotted against transpulmonary pressure (Text-fig. 2) show a marked hysteresis and although no hysteresis of lung volume was seen in the salinefilled lobes, hysteresis of airway diameter remains. If these results are considered at isopressure (Table 1) the difference (as per cent maximum), between deflation and inflation is greater for airway diameter than for the cube root of lung volume. (The volume is expressed as a linear function since this is more valid for comparison with diameter and length.) In the saline-filled preparation a deflation-inflation diameter difference is still present, whereas the volume deflation-inflation difference is non-existent. The deflation curves for diameter in the air-filled and the saline-filled preparation coincide at all pressures. The saline-filled lobes surrounded by saline were not significantly different in behaviour; lung volume, airway diameter and length were similar to those for the unsupported lobes. The pressure-volume relationship for the lobes filled with Ringer solution differed from that of the saline-filled lobes in that both the deflation and
440 J. M. B. HUGHES, H. A. JONES AND A. G. WILSON inflation volumes were lower at the same distending pressure, although the deflation-inflation difference was minimal (Table 1). Similarly, at isopressure, the diameters were smaller although the deflation-inflation difference was the same as that for the lobes filled with saline. Filling the lobes with EDTA solution resulted in a pressure-volume relationship essentially the same as that for saline-filling, but reduced the deflation-inflation diameter difference to 2 %. DISCUSSION
Air-filled lungs have long been known to exhibit hysteresis (McIlroy, 1952). The major reason for the increased recoil pressure on inflation is the hysteresis of the surface active alveolar lining fluid. Recruitment of small airways and alveoli with different opening pressures is an additional factor. Saline-filling abolishes the air-liquid interface and hence the effect of surface tension. The saline pressure-volume relationship for the lung (Text-fig. 1) shows minimal hysteresis. In air-filled lungs much of the marked hysteresis shown by the bronchi in the plot of diameter against pressure (Text-fig. 2) must be related to hysteresis of lung volume, because the airway hysteresis is considerably reduced in the lungs filled with saline. A certain amount of airway hysteresis remains when the hysteresis of lung tissue is absent, demonstrating that the bronchus exhibits an intrinsic hysteresis in addition to its tendency to follow lung tissue hysteresis passively. It is unlikely that saline-filling per se had any major effect on the bronchial wall because airway diameters on deflation for air and salinefilled lobes were similar. The decrease in lung volume at a pressure of 5 cm H20 in the lungs filled with Ringer (Table 1) is probably due to an increase in smooth muscle tone, especially in the alveolar ducts and terminal airways (Colebatch & Mitchell, 1971) since these account for 15-20 % of the lung volume (Weibel, 1963). The marked decrease in airway diameter in Ringer solution is partially due to the decrease of lung volume for the same transpulmonary pressure; in addition, there was probably an increase in the airway smooth muscle tone since, at 76 % maximum of the cube root of volume, the diameter on deflation in the saline and air-filled preparations was 80 % but in Ringer-filled lobes only 66 %. However, the deflation-inflation difference of airway diameter was not increased by filling with Ringer and so it appears that the increase of smooth muscle tone caused by volume history was independent of the initial tone of the preparation. To determine the influence of lung volume on bronchial calibre, diameter has been plotted against a linear function of lung volume, for both the air-filled and saline-filled lobes (Text-fig. 3). On inflation, both curves
AIR WA Y HYSTERESIS 441 differ from the deflation curve, the diameter being smaller on inflation although the lung volume is the same. In the saline-filled preparation where the deflation and inflation pressures are the same at isovolume, the difference between the curves gives an indication of the contribution of bronchial wall hysteresis alone. 100
90 X
80
-
J.
60 _ 70 0
6U
so
40 40
50
90 60 70 80 100 Lung Volume, (%) Text-fig. 3. Airway diameter as (percent of that at PtP 30 cm H20 in the air-filled lung) plotted against the cube root of lung volume (also as percent of that at PtP 30 cm H20 in the air-filled lung) for the air-filled and salinefilled preparations in seven experiments. Filled symbols represent measurements during deflation (@ air, A saline) and open symbols, during inflation (O air, A saline).
Text-fig. 3 shows that diameters on inflation in the air-filled lobe are very similar to those in the saline-filled preparation on inflation indicating that the large differences in lung distending pressure shown in Text-fig. 1 appear to have little influence on diameter when compared with the effect of lung volume. This finding is in agreement with Text-fig. 7 of Hughes, Hoppin & Mead (1972). The difference between airway diameters on inflation in air and saline-filled preparations may be related to differences in stretch history, as minimum volume in the saline-filled lobes was higher than that for the lobes inflated with air. Cabezas, Graf & Nadel (1971) showed that in the dog, with the vagus cut, bronchial dimensions were not increased by sympathetic stimulation or the administration of isoprenaline. They concluded that airway smooth
442 J. M. B. HUGHES, H. A. JONES AND A. G. WILSON muscle in the vagotomized dog had no resting tone. Nevertheless, in the excised lung the presence of airway smooth muscle activity can be demonstrated in the absence of autonomic nervous system stimulation. This was shown in this preparation by the administration of EDTA which removes calcium ions (Keatinge, 1968) and abolishes all electrical activity in the muscle. Table 1 shows that EDTA greatly increased the lung volumes and airway dimensions of lobes previously filled with Ringer solution. The increase in airway diameter was greater than that expected for the increase in volume. In addition, EDTA completely removed airway hysteresis. EDTA had only a mild bronchodilating effect on the air and saline-filled preparations on deflation, suggesting that airways in the excised lung on deflation from maximum volume show very little smooth muscle activity. Filling the lung with Ringer solution or inflating the lung from low volumes increases bronchial smooth muscle tone. The effect of stretch which increases smooth muscle tension at any given length (Bulbring, 1955) is presumably responsible for the intrinsic airway hysteresis shown in these lungs. The hysteresis of the airways in the functioning lung is thus the sum of the extrinsic hysteresis imposed on the airways by the surrounding lung parenchyma and, to a lesser extent, the intrinsic hysteresis of the bronchial wall; even in the absence of the autonomic nervous system, airway smooth muscle plays a part in determining bronchial dimensions. REFERENCES
BuLBRING, E. (1955). Correlation between membrane potential, spike discharge and tension in smooth muscle. J. Phy8iol. 128, 200-221. CABEZAS, G. A., GRAF, P. D. & NADEL, J. A. (1971). Sympathetic versus parasympathetic nervous regulation of airways in dogs. J. apple. Phy8iol. 31, 651-655. COLEBATCH, H. J. H. & MITCHELL, C. A. (1971). Constriction of isolated living liquid-filled dog and cat lungs with histamine. J. apple. Phy8iol. 30, 691-702. HUGHES, J. M. B., HoPPIN, F. G. & MEAD, J. (1972). Effect of lung inflation on bronchial length and diameter in excised lungs. J. apple. Phy8iol. 32, 25-35. HUGHES, J. M. B., Hoppr, F. G. & WILSON, A. G. (1972). Use of stereoscopic X-ray pairs for measurements of airway length and diameter in situ. Br. J. Radiol. 45, 477-485. KEATINGE, W. R. (1968). Ionic requirements for arterial action potential. J. Physiol. 194, 169-182. McILRoY, M. B. (1952). The physical properties of normal lungs removed after death. Thorax 7, 285-290. MARTIN, H. B. & PROCTER, D. F. (1958). Pressure-volume measurements on dog bronchi. J. apple. Physiol. 13, 337-343. MEAD, J., TAKSHIMA, T. & LEITH, D. (1970). Stress distribution in lungs: a model of pulmonary elasticity. J. apple. Physiol. 28, 596-608. MEAD, J., WHITTENBERGER, J. L. & RADFORD, E. P., JR. (1957). Surface tension as a factor in pulmonary volume-pressure hysteresis. J. apple. Physiol. 10, 191-196.
The Journal of Physiology, Vol. 249, No. 3
J. M. B. HUGHES, H. A. JONES AND A. G. WILSON
Plate 1
(Facing p. 443)
AIRWAY HYSTERESIS
443
RADFORD, E. P., JR. (1957). Recent studies of mechanical properties of mammalian lungs. In Tissue Elasticity, ed. REMINGTON, J. W., pp. 117-190. Washington D.C.; American Physiological Society. RADFORD, E. P., JR. (1964). Static mechanical properties of mammalian lungs. In Handbook of Physiology, section 3: Respiration, ed. FENN, W. 0. & RAHN, H. chapter 15, pp. 429-449. Washington D.C.: American Physiological Society. WEIBEL, E. R. (1963). Morphometry of the Human Lung, p. 139. Berlin: SpringerVerlag. EXPLANATION OF PLATE
Stereoscopic tantalum bronchograms of right middle lobe of dog lung, with metal markers glued to the lung surface: (a) air-filled: (b) saline-filled.
