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Aorta-pulmonary shunts in growing pigs
Aorta-pulmonary shunts In growing pigs Functional and structural assessment of the changes in the pulmonary circulation Aorta-pulmonary shunts were produced in growing pigs, 4 to /2 weeks of age, by anastomosing the thoracic descending aorta to the pulmonary trunk, and the animals were followed for periods of / to 3 months. Correlation between the hemodynamic findings and the lung structure, analyzed by quantitative methods, showed that the young group (operated upon at 4 weeks of age) developed more severe pulmonary hypertension with increased musculariration of arteries of all sizes and reduction in the size of those running with respiratory bronchioli and beyond. Since this response is similar to that seen in cases of left-to-right shunt associated with congenital heart defects and pulmonary hypertension early in life, it seems that the present experiments reproduce the early response of the growing lung to such hemodynamic disturbance.
Antonio Rendas, M.D., Ph.D., Stuart Lennox, B.S., F.R.C.S., and Lynne Reid,* M.D., F.R.C.Path., F.R.C.P., Boston, Mass., and London, England
Experimental attempts to produce pulmonary hypertension by anastomosing a systemic artery to a pulmonary artery have been reported frequently since the initial work by Levy and Blalock.' These studies were performed in adult animals, and met with little success whether the anastomosis was to the trunk" or to one of the main pulmonary arteries" 4; on the other hand, pulmonary hypertension was produced when a systemic artery was grafted to a branch supplying a lobe or less.':" This failure to develop pulmonary hypertension in the adult lung has been attributed to the low vascular resistance of the lung, which allows left-to-right shunts large enough to produce left ventricular failure without a significant increase in pulmonary artery pressure. To simulate conditions of left-to-right shunts due From the Departments of Experimental Pathology and Surgery, Cardiothoracic Institute, Brompton Hospital, London, England. Supported in part by the Board of Governors, Brompton Hospital, and the Gulbenkian Foundation, Lisbon, through a scholarship for Dr. Rendas (January, 1974, to November, 1976). Part of this work was submitted by Dr. Antonio Rendas as a doctoral thesis, University of London, 1977. Received for publication June 14, 1978. Accepted for publication Aug. 14, 1978. Address for reprints: Lynne Reid, M.D., Department of Pathology, Children's Hospital Medical Center, Boston, Mass. 02115. *Present address: Department of Pathology, Children's Hospital Medical Center, Harvard Medical School, Boston, Mass. 02115.
to congenital cardiac defects, Hawe and his colleagues" suggested that growing animals should be used in the hope that like man, they would be prone to develop vascular changes. Morphometric studies have established that the newborn lung is not a rninature of the adult lung, and that considerable vascular remodeling occurs after birth, particularly in the respiratory zone"; furthermore, it has been demonstrated that infants with ventricular septal defects and severe pulmonary hypertension show an impairment of the normal growth process without the structural features of pulmonary vascular obstructive disease." 10 Few attempts have been made to produce pulmonary hypertension in growing animals. Systemic-to-pulmonary shunts in puppies were reported to cause a slight rise in pulmonary artery pressure, as in adult animals!': pneumonectomies in l-month-old dogs led to pulmonary hypertension according to some reports but not others 12, )3; ligation of the right pulmonary artery in newborn calves produced pulmonary hypertension, whereas no effect occurred when the left artery was ligated!"; left pneumonectomy produced medial hypertrophy in 2-month-old mini pigs but not in I-monthold piglets. 15, 16 These conflicting results reflect differences between species and breeds, but since few comparisons between human and animal lung development have been made, it is difficult to relate the experimental
0022-5223179/010109+ 10$01.00/0 © 1979 The C. V. Mosby Co.
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The Journal of Thoracic and Cardiovascular
I 10 Rendas, Lennox, Reid
Surgery
Table I. Aorta-pulmonary shunts in growing pigs by age, body weight, and duration offollow-up period Age at operation (wk.) 4
Group * Age at sacrifice (wk.) (no. of 1 - - - - - , - - - - - , - - - - animals) /6 8 /2
I
S (8) C (2)
N S (6)
,I ,,, ,
,,
, ,,
,, ,, Pulmonary Artery Fig. 1. Aorta-pulmonary graft.
results to the features of pulmonary hypertension in children. In our early studies'? it was demonstrated that during growth, similar structural changes occurred in pig and human lungs, except that the growth rate of the pig was faster and changes seen in human childhood and adolescence were telescoped into the first 2 months of life in the pig. Aorta-pulmonary shunts, pulmonary trunk to the thoracic descending aorta, have been established in pigs aged I, 2, and 3 months, and the animals were studied for I to 3 months after operation. Cardiac catheterization and lung function studies were performed, together with a structural analysis of the lung, particularly of the pulmonary circulation, using quantitative techniques. Material and methods Aorta-pulmonary anastomosis was performed in 28 Large White pigs between 4 and 12 weeks of age; eight died of pulmonary edema during the first postoperative day. The present study is concerned with the 20 surviving animals. Details of age, sex, body weight and duration of the follow-up period are listed in Table I. Surgical protocol. All animals were deprived of food and water for the 12 hours before operation. Premedication was administered intramuscularly and consisted of fentanyl (29 meg. per kilogram of body weight) and droperidol (1.4 mg. per kilogram) together with atropine (50 meg. per kilogram). 17 Fifteen minutes later, anesthesia was induced with a mixture of halothane, 1.5 to 2 percent, and 50 percent nitrous
8
C(2)
12
S (2) N
N
23 ± 1.4 (2)t 19 20 ± 2.8 (4)
I
28 ± 3.1 (3) 38 ± 2.1 (2)
43 ± 3.2 (3) 40 59 ± 1.0 (3)
33±7.0(3) 50 ± 4.7 (3) 41 26 38 ± 2.1 (2) 59 ± 1.0 (3) 49 ± 1.4 59 ± 1.0 (3)
*S, Shunted animals. C, Operated controls. N, Normals." tBody weights in kilograms with number of animals in parentheses.
oxide in oxygen administered by face mask. Endotracheal intubation with a cuffed oral tube was carried out during spontaneous breathing. The animals were given d-tubocurarine, 0.5 mg. per kilogram, intravenously and the respiration was controlled using a BromptonMainley ventilator that delivered 50 percent nitrous oxide in oxygen at a tidal volume of 40 to 50 c.c. per kilogram. Anesthesia was maintained by an intravenous infusion of fentanyl (0.5 mg.) and droperidol (12.5 mg.) in saline (100 ml.) administered via an ear vein at a rate of I to 2 ml. per minute. Atropine was administered intramuscularly every 30 minutes. Halothane (0.5 to I percent) was used intermittently during the operation but was discontinued immediately after the shunt had been established. Measurements at operation. After opening the chest, both aortic and pulmonary arterial pressures were measured by direct needle puncture. This was repeated when the shunt was established using a silicone rubber prosthesis, * 6 mm. inner diameter for the 4-week-old animals and 8 mm. inner diameter for the older ones (Fig. I); graft flow was then measured with a sine-wave electromagnetic flowmeter, SEM 275. t The cuff-head transducers were calibrated in vitro over a range of gravity flow rates of 100 to 1,000 ml. per minute. All measurements were taken at end expiration with the ventilator stopped for a short period: Zero was taken as midchest level. Arterial blood samples were collected immediately before and after each study and were stored on ice to be analyzed later for blood gas tension and pH using a BMS-3 electrode *This material was supplied to us by Professor F. Ashton. Department of Surgery, University of Birmingham, England. See Ashton F, Lightwood R, Hardman J: A silicone rubber arterial prosthesis. Br J Surg 54:709-712, 1967. tSe Laboratories, Middlesex, England.
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Aorta-pulmonary shunts
Number 1
January, 1979
system; * blood hematocrit was measured in a Hawkesley centrifuge. A six-channel system, EMMA, twas used for amplification and display of the signals, and data were recorded on an ultraviolet oscillograph, 3006DL.t Follow-up studies. The follow-up period ranged between I and 3 months; no studies were made in the first IS days after operation. Serial measurements of lung function were performed for each case on at least three occasions. Each animal was submitted to right-side cardiac catheterization before being put to death. Right atrial and ventricular, pulmonary and systemic arterial pressures were measured; details of the techniques used in these studies have been described elsewhere."? The left-to-right shunt was assessed from the pulmonaryto-systemic flow ratio (Qp/Qs) calculated from the oxygen content of blood samples taken from the right ventricle (02C m V), the pulmonary artery (02C PA), and a systemic artery (02C SA) and measured in a Lex-Oj-Con analyzert using the formula": 02C SA - O2C m V 02C SA - 02C PA
Structural studies. The methods used for study of structure have been described previously in a report on the normal development of the pig lung;" They include: injection into the pulmonary vasculature, either arteries or veins, of a barium sulphate-gelatin mixture at a constant pressure of 100 ern. of H 20 maintained for 5 to 7 minutes to allow complete distention of the vessel walls and more accurate comparison between specimens; quantitative analysis of vessels, including muscularity, size, and number related to position in the vascular tree; and assessment of the ventricular weights, left ventricle plus septum (L V + S) and right ventricle. Results In most animals, a continuous murmur was heard in the precordium after the anastomosis, the murmur being loudest at the upper left sternal border. In Cases I, 6, 10, and 18, a right-side cardiac catheterization was performed 15 days after operation because no murmur was audible. This demonstrated that the shunt was closed, since there was no increase in the oxygen content of the blood between the right ventricle and the pulmonary artery. These animals were also studied as "operated" controls. The remaining 16 were divided *Radiometer, AIS, Copenhagen, Denmark. tSe Laboratories, Middlesex, England. :j:Lexington Instruments Corp., Waltham, Mass.
I I I
into two groups: those operated upon at 4 weeks of age (eight animals), subsequently described as the "young group," and those operated upon between 8 and 12 weeks of age (eight animals), described as the "older group." All the animals showed a normal increase in body weight during the follow-up period. Functional studies. Pulmonary hemodynamics. In all animals the pulmonary arterial pressure (millimeters of mercury) rose immediately after the shunt was established, from a mean value of 9 ± 1.4 (S.D.) to 13 ± 2.4, whereas systemic arterial pressure fell from 58 ± 7.2 to 41 ± 6.5, the differences in both were highly significant (p < 0.00 I). In most animals the heart rate increased, but the change was significant only in the young group. Blood flow through the shunt was continuous during the cardiac cycle. The flow wave was pulsatile, with a peak coinciding with the systolic component of both aortic and pulmonary pressure waves (Fig. 2). Mean shunt flow ranged between 257 and 987 mi. per minute in the series; when adjusted to body weight the range was similar in both age groups (20 to 48.3 rnl. per minute per kilogram). No constant relationship was observed between shunt flow and the immediate pressure changes despite the fact that animals with the larger shunts tended to show a greater increase in pulmonary arterial pressure, particularly in the young group. Paco"was 42 ± 3.9 (S.D.), with a Pao" that always exceeded 98 mm. Hg and a hematocrit of 29 ± 2.5. Cardiac catheterization performed at the end of the follow-up period showed a highly significant increase in mean pulmonary arterial pressure (millimeters of mercury) in all animals with a patent shunt (29 ± 5.2, S.D.) when compared with the normal (II ± 2.6) and the operated control groups (10 ± 2.3) (p < 0.001). The young group showed higher values (32 ± 5.6) than the older group (26 ± 3. I), the difference between the two being significant (p < 0.05). In the young group (Fig. 3) the pulmonary artery pressure values increased in the follow-up period: 27 mm. Hg in the first month to 36 mm. Hg in second month and 34 mm. Hg in the third month. In the older group the increase was significant after the first postoperative month (Fig. 4). The increase in right ventricular pressure closely followed that in the pulmonary artery, the young group showing higher mean values (39 mm. Hg) than the older one (33 mm. Hg) (p < 0.05). The heart rate was also increased only in the young group (137 beats per minute) but not in the older one (99 b.p.m.-p < 0.01). Mean systemic arterial and right atrial pressures were not significantly different in
The Journal of
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Rendas, Lennox, Reid
Thoracic and Cardiovascular Surgery
(i)
-
, 60 "''''Hg.
(ii)
[
. )0
(iii)
1OO1Ill/ .... [
. 3SO
[20 _ "
(iv)
10
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,
70
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,
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Fig. 2. A, Simultaneous recording of electrocardiogram. Lead DII (i), aortic pressure (ii), phasic shunt flow (iii), and pulmonary artery pressure (iv). Notice that shunt flow is normal. B, The effect on aortic pressure (ii) and pulmonary artery pressure (iv) of brief occlusion of graft. i, ii, and iv, Same as in A. iii, Mean shunt flow.
either shunted or operated control or normal animals. The Qp/Qs ranged between 1.5 and 2.4 with no significant differences between the two age groups. Mean values for Paco. and pH were 40 and 7.39, respectively, with a Pao. that exceeded 82 mm. Hg. Lungfunction. The shunted animals did not show the increase in dynamic compliance that normally occurs during growth; the ratio between dynamic compliance and body weight was lower in the shunted series than in either normal or operated control groups (p < 0.001), which had similar values. Values for the young group were significantly lower than for the older animals (p < 0.01) (Fig. 5).
Throughout the follow-up period respiratory frequency (breaths per minute) did not differ significantly in the shunted (range of 15 to 38), operated control (range of 14 to 40), and normal animals (range of 10 to 36). Tidal volume increased with age, and although in the young group neither shunted nor operated controls increased their tidal volumes at the normal rate for age and body weight during the first postoperative month. by the end of the second month they had achieved the expected level. Measurements of thoracic gas volume (TGV) showed that the young group, whether shunted or operated control, had values higher than those for normal animals, suggesting that after operation some
Volume 77
Aorta-pulmonary shunts
Number 1 January, 1979
A-P SHUNT ARTERIES Pre-acinar Intra-acinar
Size WI Size WI Ext. No.
A-P SHUNT
(4 WEEKS OF AGE)
N
N
N
t
t
tt
I
I
II
N
t
t t
tt
N
N
N
I~
20
Intra-acinar
L_------1
40
Size Wt. Ext. No.
8
It
N
N
N
t
t t
N
N
t
Qp/Qs -1'7-2-4
I---I
I
4
(8 WEEKS OF AGE)
t t
Size
WI.
t
Qp/QS-l·5-24 40
ARTERIES Pre-acinar
I I3
4
12
8
Follow- up (weeks)
Follow-up (weeks)
Fig. 3. Summary of structural and functional findings in the young group. Wt., Wall thickness. Ext., Distal extension of muscle along the peripheral arterial pathway. No., Arterial number per unit area of lung tissue (square centimeters ). PAP, Mean pulmonary arterial pressure. Qp/Qs, Pulmonary-to-systemic flow ratio. Fig. 4. Summaryof structural and functional findings in the older group. See legend to Fig. 3 for abbreviations. air was trapped; when expressed in terms of body weight (BW) these differences were not significant (TGV/BW).
Structural studies. General features. The lung volumes in the shunted animals did not differ significantly from those in the normal and operated control animals. Lung growth (length) was decreased in the shunted and operated control animals of the young group at the first postoperative month, suggesting that if performed early in life thoracotomy affects the normal increase in length; on the other hand, these changes were transitory because animals from the young group showed normal lung growth (length), according to age and body weight, at the second and third postoperative months. Macroscopically, the lungs showed no collapse: The pleural surface appeared normal, except for some thickening over the left upper and middle lobes, resulting from chest wall adhesions from the thoractomy wound. The microscopic architecture of the lung was not altered in shunted and operated control animals; the alveolar walls and connective tissue septa appeared normal and there was no evidence of perivascular or peribronchiolar edema. Pulmonary arteries. At the end of the first postoperative month the arteriograms of the older group showed dilation of the main artery and its branches; this
-----. - - -. - ----- --- ----. --- - ---- •• 2 SO
------'lHHI"----
5
10
Mean (Normall
15
AGE (weeks)
Fig. 5. Measurements of dynamic compliance during the follow-up period; notice that although there is a decrease in all animals, the young group (_) shows lower values than the older one (e). feature was never evident in the young group, even 3 months after operation. Although the diameter of the axial pathway was not increased, the lateral pathways were dilated (Fig. 6). In each age group the arteriograms of the animals followed for the longest periods
The Journal of
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Rendas, Lennox. Reid
Thorac ic and Cardiovascular Surgery
Fig. 6. Pulmonary arteriograms of three animals at 16 week s of age . Center. Normal. Left. Shunt at 8 weeks. Right. Shunt at 4 week s. Notice the dilation of the main vessels in the anim al operated upon later in life.
showed a reduction in the degree of background haze, suggesting a reduction in the number or size of the intra-acinar arteries. The medial wall of the main pulmonary artery was thicker in all cases; this change was already apparent by the end of the first postoperative month, but the increase was always more significant in the young group (276.4 ~m ± 32.6 S .D .) than in the older one (221.9 ~m ± 34 .8) (p < 0.01). The number of elastic laminae in the media was not increased in the operated animals, but fibers were more continuous and some degree of patchy intimal thickening was found; these features were not found in the normal and operated control groups. In all shunted animal s, percentage wall thickness of the small muscular arteries (15 to 200 ~m in diameter) showed no significant increase after the first postoperative month . In the second postoperative month, the percentage wall thickness was increased in both young (7 .8 ± 1.2 S .D.) and older groups (6 .1 ± 1.1) when compared with the control (4 .8 ± 1.6) and normal (4.7 ± 1.4) animals . In the young group, animal s followed for 3 months continued to show an increased percentage wall thickness, 8.5 ± 1.7. A similar pattern of change was found in external diameter of the larger muscular arteries (200 to I ,000 ~m) ; nevertheless, the increase in percentage wall thickness was greater (p < 0.005) in the young group (4.9 ± 0 .6) than in the older group (3 .8 ± 0 .2) of animals followed for the same period of time , i.e ., 2 months.
In the shunted cases, the proportion of small , muscular and partially muscular arteries was relatively increased as compared with the normal proportion of nonmuscular arteries. In the young group this shift was already apparent by the end of the first postoperative month, with muscular arteries present in the vessels with an external diameter as small as 20 ~m; whereas in the normal and operated control groups, muscular arteries were seen only in those with an external diame ter of 50 to 60 ~m . This change was less apparent in the older group. In all shunted animals , fully muscularized arteries were found in the 10 to 20 ~m diameter group 2 months after operation . Analy sis of the structural types of intra-acinar arteries in relation to the accompanying airway s (i.e., alveolar ducts and respiratory and terminal bronchioli) also established that in the shunted animals, muscle extended further along the peripheral arterial pathway; although the normal pig already has muscularized arteries at alveolar duct level by the fifth week of life, their proportion as related to partially muscular and nonmuscular arteries was increased in all shunted animals: In animals followed for 8 weeks, the differences from the normals and controls were more apparent. In the young group the size of the intra-acinar arteries did not increa se normally during the follow-up period, i. e., the external diameter of arteries accompanying respiratory bronchioli measured 58 ~m ± 3.6 S .D . (n > 20 in each case) by the end of the first month of follow -up; whereas for the normals and operated
Volume 77 Number 1
Aorta-pulmonary shunts
I I5
January. 1979
controls it was 73 JLm ± 5.2 (n > 20 in each case) (p < 0.001). Although some increase in size occurred during the folIowing 2 months the values continued to be lower than those expected on the basis of age (Fig. 3). There was no decrease in arterial size in the older group (Fig. 4). The assessment of the intra-acinar arterial number per unit of lung tissue (square centimeter) did not show a significant change, although some reduction was detected in alI shunted animals followed for 2 or more months. The reduction of background haze seen in the arteriograms of the shunted cases was probably due more to the reduction in the size of the intra-acinar arteries than to an effective reduction in arterial concentration. Pulmonary veins. The venograms from the shunted animals were similar to those for the normal and operated control groups, with no significant change in size, branching pattern, or background haze. Microscopically, the percentage wall thickness of veins 15 to 1,000 JLm in external diameter was also similar to values for normal and operated control groups. Ventricular weights. Total ventricular weight was increased in all animals that underwent operation, but it is unclear whether this reflected the effect of the thoracotomy or of the left-to-right shunt, which in the controls was found to be occluded 15 days later. In the shunted animals, the increase in total ventricular weight, adjusted to body weight (grams per kilogram), was significant only in the young group at 12 (4.3 ± 0.4 S.D.) and 16 (4.0 ± 0.3) weeks of age when compared with the control group (2.9 ± 0.7). The (LV + S)/R V ratio was not changed in the operated control group (3.8 ± 0.4 S.D.) when compared with the normal animals (3.7 ± 0.4). Nevertheless, in the young group, the longer the duration of the folIowup, the greater the progressive decrease in the (LV + S)/R V ratio, which became significant by the second month of follow-up (3.1 ± 0.2) and continued to fall at 3 months (2.8 ± 0.1). This suggests that in the young group, right ventricular hypertrophy was occurring during alteration of the structure of the pulmonary arteries. Discussion In these experiments, pulmonary hypertension was produced in growing pigs, 4, 8, and 12 weeks of age, by anastomosing the thoracic descending aorta to the pulmonary trunk with a silicone graft. The longer the follow-up period, the more severe the changes in the lungs. Functionally, there was an increase in pulmonary arterial and right ventricular pressures and a de-
crease in lung dynamic compliance in alI cases, the changes being more significant in the young group (the animals operated upon at 4 weeks of age); the heart rate was increased in the young group and was normal in the older group, the animals operated upon at 8 and 12 weeks of age. Structurally, all pulmonary arteries were thicker than normal, from hilum to periphery; in addition, muscle extension to smaller intra-acinar arteries was greater than estimated on the basis of age. There was left ventricular hypertrophy, with the more hypertensive animals having right ventricular hypertrophy; these features were also more significant in the young animals. The luminal diameter of the main arteries was increased in the older group and was normal in the young animals; at the intra-acinar level, the arteries were normal in size in the older group and small for age in the young group. In all cases, the arterial and alveolar concentrations were normal for age, as was the thickness of the pulmonary veins. The development of more severe pulmonary hypertension in the younger animals may reflect structural differences between the various age groups, particularly the interaction between muscularization and changes in vascular cross-sectional area during growth. A l-month-old animal still has somewhat immature lungs in that, in the elastic compartment of the arterial vasculature, the elastic laminae in the medial wall are thicker and more continuous than they become later. In the muscular compartment the vessels between 200 to 1,000 JLm in external diameter are still thicker than those in the adult, the latter values not being reached until 8 weeks of age. Extension of muscle into small intra-acinar arteries is incomplete at 4 weeks of age, i.e., more arteries with a complete muscular coat accompany respiratory airways at 8 weeks than at 4 weeks; these arteries are smaller and less numerous than those in the older group, indicating that the vascular cross-sectional area in a respiratory unit increases with age. Comparison between the present and other experimental studies on pulmonary hypertension, produced by either pneumonectomy or systemic-to-pulmonary shunt, is difficult, since in most of previous works, adult animals have been used and changes occurred only if systemic arteries were anastomosed to a small part of the pulmonary circulatory structure.v ;;-7 Such studies demonstrated the importance of the height of the blood pressure, duration of shunt, and shunt size in the production of arterial changes, particularly increased muscularization. It is difficult to apply such findings to clinical conditions of left-to-right shunts associated with congenital cardiac effects, where, usu-
The Journal of
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Rendas, Lennox, Reid
ally, the whole of the pulmonary vascular bed is submitted to the hemodynamic disturbance during the period of postnatal growth. Pulmonary hypertension does occur in young children with left-to-right shunts associated with congenital cardiac defects, particularly in cases of a direct communication between the two arterial systems. Nevertheless, when patients less than 2 years of age have symptoms, they tend to have higher flow ratios than older children for similar increases in mean pulmonary artery pressure. The severity of the pulmonary vascular disease increases with age, but most often, only in cases of a mean pulmonary arterial pressure higher than 50 mm. Hg are the more severe, ultimately irreversible changes seen. 19 - 2 2 In cases of mild to moderate increase in pulmonary vascular resistance (PVR), the structural basis for the pulmonary hypertension is considered to be thickening of the muscular coat, and therefore the disease is reversible; in cases of a severe increase in PVR, intimal fibrosis with dilation lesions has been considered the basis for irreversible "obstructive" disease. 2:1 More recently, morphometric techniques have been used to analyze the pulmonary circulation in infants with ventricular septal defects, taking into account the present knowledge oflung development and growth.": 10 These studies have shown that in cases of a mild increase in PVR, there is increase in arterial muscularity without features indicating impaired growth. In other patients, particularly those with a high PVR, the clinical criteria for irreversibility (a ratio of pulmonary to systemic resistance greater than 0.7) can be present without the structural stigmata of "obstructive" disease, i.e., intimal fibrosis and dilation lesions. In these cases, quantitative study shows a hypoplastic circulation with thicker, smaller, and fewer vessels than expected on the basis of age. It is now generally accepted that the changes in the lung in cases of simple left-to-right shunts occur postnatally by, e.g., patent ductus arteriosus, ventricular septal defects, and aortopulmonary window. Although in the present experiment the shunt is produced at a stage when the rapid postnatal fall in PVR has already occurred, the young group is close enough to the newborn period to develop a response similar to that in clinical cases of a lesion present at birth. Although a pulmonary arterial pressure equaling more than half the systemic values was not recorded in the experimental cases, the structural features approximate human ones, save that in the pigs a reduction in arterial concentration was not found. This resembles results of studies in man at postmortem biopsy which show that reduction in
Thoracic and Cardiovascular Surgery
arterial concentration occurs only in cases of significant pulmonary hypertension (pulmonary artery pressure> 20 mm. Hg and PVR of 3 U. per square meter) and only after 4 months of age. 9. 10.24 In the pigs, the differences between the two age groups may resemble such differences with left-to-right shunts associated with congenital cardiac defects. Symptoms in the young group may resemble those found in infants who have a moderate increase in pulmonary arterial pressure and show some impairment of postnatal growth; the older animals presented with symptoms similar to those accompanying normal arterial concentration and size but excessive muscle in the arterial wall. In both age groups the decrease in dynamic compliance was progressive, the lowest values occurring in the young group and coinciding with the highest pulmonary artery pressures and the greatest structural impairment of lung growth. This suggests a close relationship between respiratory airways and arteries, the latter affecting the mechanical properties of the lungs, probably because of increased muscularity. This finding confirms previous studies on lung mechanics in cases of left-to-right shunts that showed a decrease in dynamic compliance in infants with severe pulmonary hypertension.": 26 Both clinical and experimental studies have emphasized the greater importance of the increase in blood pressure, when compared to flow to the lungs, in the pathogenesis of the pulmonary vascular lesions; other features that influence the response are increases in left atrial and ventricular end-diastolic pressures; the altered flow profiles seen in ventricular septal defects, with the systolic portion of the wave showing a more peaked effect than atrial septal defects; and at least in the acute experimental conditions of a large aorto-pulmonary shunt, an increase in diastolic flow velocity. 27-:10 Individual variability may also contribute to the degree of pulmonary vascular damage.:" Only recently have growth and development of the pulmonary circulation been investigated in clinical cases of left-to-right shunts associated with congenital cardiac defects, but it has already been shown that its impairment is an important feature of the response to the hemodynamic disturbance. The present study shows that the changes are progressive and that they affect the whole arterial bed and the heart: Increased muscularization probably arises from the increased vascular tension that is maintained throughout the cardiac cycle, the contractile elements failing to relax. 32 In the hypoxic adult rat, extension of muscle to smallerthan-normal intra-acinar arteries has been shown by Meyrick and Reid'" to represent differentiation of precursor cells, the pericyte and intermediate cell, nor-
Volume 77
Aorta-pulmonary shunts
Number 1
January. 1979
mally present in the wall of the nonmuscular segment of the arterial system. It seems possible that this cell differentiation is triggered not only by hypoxia, but also by other stimuli such as changes in pulmonary blood flow and pressure. The decrease in size of the intra-acinar arteries, only found in the young animals, must effectively reduce the vascular cross-sectional area at the level of the respiratory airways; this feature represents the earliest impairment in the growth process. The reduction in arterial concentration is not seen in the experimental animals, probably because the increase in pulmonary artery pressure is only moderate. Left ventricular hypertrophy occurs early, reflecting the changes in both preload, caused by the increase in pulmonary venous return, and in afterload, as a result of direct communication between the aorta and the main pulmonary artery. Right ventricular hypertrophy occurs late and is secondary to the pulmonary vascular changes and to the rise in pulmonary arterial pressure. The age-related differences may reflect the balance between vascular reactivity and reserve capacity during growth'": regarding these two features it is important to emphasize the role played not only by the peripheral muscular arteries, but also by the larger ones, because our study suggests the existence of an elastic compartment that is less distensible in the young animals than in the older ones. Infants and children with a moderate increase in pulmonary artery pressure have some impairment of growth of the intra-acinar pulmonary circulation; but when clinical improvement occurs, it is probably due to adaptation by the circulation, via increasing vessel size and concentration, early in life; deterioration occurs if these mechanisms do not operate. A rise in pulmonary vascular resistance early in childhood probably means that compensation of the hemodynamic disturbances does not occur and that a truly hypoplastic circulation emerges. Surgical correction of left-to-right shunts early in life creates the anatomic basis for the establishment of normal hemodynamic conditions, perhaps allowing lung growth to restart or even catch up and thus improve prognosis. We are grateful to Mr. D. Guerreiro and Miss C. Lloyd for technical assistance. REFERENCES Levy SE, Blalock A: Experimental observations on the effects of connecting by suture the left main pulmonary artery to the systemic circulation. 1 Thorac Surg 8:525530, 1939 2 Hawe A, Tsakiris AG, Rastelli GC, Titus lL, McGoon DC: Experimental studies on the pathogenesis of pulmo-
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nary vascular obstructive disease. 1 THORAC CARDIOVASC 63:652-669, 1972 3 Eppinger EG, Burwell CS, Gross RE: The effect of the patent ductus arteriosus on the circulation. 1 Clin Invest 20:127-143, 1941 4 Leeds SE: The effects of occlusion of experimental chronic patent ductus arteriosus on the cardiac output, pulse and blood pressure of dogs. Am 1 Physiol 139:451-459, 1943 5 Damman lF lr: The relationship of flow and pressure in various types of congenital heart disease particularly those associated with pulmonary hypertension, Pulmonary Circulation, WR Adams, I Veith, ed., New York and London, 1959, Grune & Stratton, Inc., pp 220-226 6 Damman lF Jr , Baker lP, Muller WH lr: Pulmonary vascular changes induced by experimentally produced pulmonary arterial hypertension. Surg Gynecol Obstet 105: 16-26, 1959 7 Ferguson Dl, Varco RL: The relation of blood pressure and flow to the development and regression of experimentally induced pulmonary arteriosclerosis. Circ Res 3: 152-158, 1955 8 Hislop A, Reid L: Growth and development of the respiratory system, Scientific Foundations of Paediatrics, lA Davis, 1 Dobbing, eds., London, 1974, William Heinemann Medical Books, Ltd., pp 214-254 9 Hislop A, Haworth SG, Shinebourne EA, Reid L: Quantitative structural analysis of pulmonary vessels in isolated ventricular septal defects in infancy. Br Heart 1 37: 10141021, 1975 10 Haworth SG, Sauer U, Biihlmeyer K, Reid L: Development of the pulmonary circulation in ventricular septal defect. A quantitative structural study. Am 1 Cardiol 40:781-788, 1977 II Richardson rw, Phillips CE, DeWeese lA, Manning lA, Mahoney EB: Influence of pulmonary vascular maturity on response to subclavian pulmonary arterial anastomosis. Circulation 24:1020, 1961 12 Rudolph AM, Neuhauser EBD, Golinko Rl, Auld PAN: Effects of pneumonectomy on pulmonary circulation in adult and young animals. Circ Res 9:856-861, 1961 13 Massion WH, Schilling lA: Physiological effects of lung resection in adult and puppy dogs. 1 THORAC CARDIOVASC SURG 48:239-249, 1964 14 Vogel lHK, McNamara DG, Hallman G, Rosenberg H: Effects of mild chronic hypoxia on the pulmonary circulation in calves with reactive pulmonary hypertension. Circ Res 21:661-669, 1967 15 Kato H, Kidd L, Olley PM: Effects of hypoxia on pulmonary vascular reactivity in pneumonectomized puppies and minipigs. Circ Res 28:397-402, 1971 16 Friedli B, Kent G, Kidd LSC: The effect of increased pulmonary blood flow on the pulmonary vascular bed in pigs. Pediatr Res 9:547-553, 1975 17 Rendas A, Branthwaite M, Reid L: Growth of the pulmonary circulation in the normal pig-Structural analysis SURG
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19
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and aspects of cardiopulmonary function. J Appl Physiol (in press) Yang S, Bentivoglio LG, Maranhao V, Goldberg H: From cardiac catheterization to hemodynamic parameters, Philadelphia, 1972, F. A. Davis Co., pp 137-156 Weidman WH, Gersony WM, Nugent EW, DuShane JW, Rowe RD: Indirect assessment of severity in ventricular septal defect. Circulation 56:Suppl 1:24-33, 1977 Weidman WH: Indirect assessment of severity in ventricular septal defect. Summary and conclusions. Circulation 56:Suppl 1:34-35, 1977 Weidman WH, Blount SG Jr, DuShane JW, Gersony WM, Hayes CJ, Nadas AS: Clinical course in ventricular septal defect. Circulation 56:Suppl 1:56-69, 1977 Nadas AS: Clinical course in ventricular septal defect. Summary and conclusions. Circulation 56:Suppll:70-71, 1977 Heath D, Edwards JE: The pathology of hypertensive pulmonary vascular disease. A description of six grades of structural changes in the pulmonary arteries with special reference to congenital cardiac defects. Circulation 18:533-547, 1958 Rabinovitch M, Haworth SG, Castaneda AR, Nadas AS, Reid L: The lung biopsy in congenital heart disease. A morphometric approach to pulmonary vascular disease. Circulation 56:Suppl 3: 193, 1977 Davies H, Williams J, Wood P: Lung stiffness in states of abnormal pulmonary blood flow and pressure. Br Heart J 24:129-143, 1962 Griffin AJ, Ferrara JP, Lax JO, Cassels DF: Pulmonary
27
28
29
30
31
32
33
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compliance. An index of cardiovascular status in infants. Am J Dis Child 123:89-95, 1972 Lucas CL, Wilcox BR, Coulter NA Jr: Contrasting pulmonary blood flow profiles in children with atrial and ventricular septal defects. Cardiovasc Res 10: 1-12, 1976 Ellison LT, Hall DP, Yeh T, Mobarhan H, Rossi 1, Ellison RG: Physiological alteration in increased pulmonary blood flow with and without pulmonary hypertension. J Appl Physiol 16:305-308, 1961 Linde LM, Goldberg SJ, Takahashi M, Momma K, Groverman G: Pulmonary vasoreactivity in animals with relatively increased pulmonary blood flow. Cardiovasc Res 4:99-104, 1970 Rudolph AM, Scarpelli EM, Golinko RJ, Gootman NG: Hemodynamic basis for clinical manifestations of patent ductus arteriosus. Am Heart J 68:447-458, 1964 Grover RF, Vogel JHK, Averill KH, Blount SG Jr: Pulmonary hypertension. Individual and species variability relative to vascular reactivity. Am Heart J 66:1-3,1966 Rodbard S: Negative feedback mechanisms in the architecture and function of the connective tissue and cardiovasculartissues. Perspect Bioi Med 13:507-527,1970 Meyrick B, Reid L: The effect of continued hypoxia on rat pulmonary arterial circulation. An ultrastructural study. Lab Invest 38: 188-200, 1978 Hoffman J1E: The normal pulmonary circulation, Pulmonary Physiology of the Fetus, Newborn and Child, EM Scarpelli ed., Philadelphia, 1975, Lea & Febiger, Publishers, pp 259-272
Antonio Rendas, M.D., Ph.D., Stuart Lennox, B.S., F.R.C.S., and Lynne Reid,* M.D., F.R.C.Path., F.R.C.P., Boston, Mass., and London, England
Experimental attempts to produce pulmonary hypertension by anastomosing a systemic artery to a pulmonary artery have been reported frequently since the initial work by Levy and Blalock.' These studies were performed in adult animals, and met with little success whether the anastomosis was to the trunk" or to one of the main pulmonary arteries" 4; on the other hand, pulmonary hypertension was produced when a systemic artery was grafted to a branch supplying a lobe or less.':" This failure to develop pulmonary hypertension in the adult lung has been attributed to the low vascular resistance of the lung, which allows left-to-right shunts large enough to produce left ventricular failure without a significant increase in pulmonary artery pressure. To simulate conditions of left-to-right shunts due From the Departments of Experimental Pathology and Surgery, Cardiothoracic Institute, Brompton Hospital, London, England. Supported in part by the Board of Governors, Brompton Hospital, and the Gulbenkian Foundation, Lisbon, through a scholarship for Dr. Rendas (January, 1974, to November, 1976). Part of this work was submitted by Dr. Antonio Rendas as a doctoral thesis, University of London, 1977. Received for publication June 14, 1978. Accepted for publication Aug. 14, 1978. Address for reprints: Lynne Reid, M.D., Department of Pathology, Children's Hospital Medical Center, Boston, Mass. 02115. *Present address: Department of Pathology, Children's Hospital Medical Center, Harvard Medical School, Boston, Mass. 02115.
to congenital cardiac defects, Hawe and his colleagues" suggested that growing animals should be used in the hope that like man, they would be prone to develop vascular changes. Morphometric studies have established that the newborn lung is not a rninature of the adult lung, and that considerable vascular remodeling occurs after birth, particularly in the respiratory zone"; furthermore, it has been demonstrated that infants with ventricular septal defects and severe pulmonary hypertension show an impairment of the normal growth process without the structural features of pulmonary vascular obstructive disease." 10 Few attempts have been made to produce pulmonary hypertension in growing animals. Systemic-to-pulmonary shunts in puppies were reported to cause a slight rise in pulmonary artery pressure, as in adult animals!': pneumonectomies in l-month-old dogs led to pulmonary hypertension according to some reports but not others 12, )3; ligation of the right pulmonary artery in newborn calves produced pulmonary hypertension, whereas no effect occurred when the left artery was ligated!"; left pneumonectomy produced medial hypertrophy in 2-month-old mini pigs but not in I-monthold piglets. 15, 16 These conflicting results reflect differences between species and breeds, but since few comparisons between human and animal lung development have been made, it is difficult to relate the experimental
0022-5223179/010109+ 10$01.00/0 © 1979 The C. V. Mosby Co.
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I 10 Rendas, Lennox, Reid
Surgery
Table I. Aorta-pulmonary shunts in growing pigs by age, body weight, and duration offollow-up period Age at operation (wk.) 4
Group * Age at sacrifice (wk.) (no. of 1 - - - - - , - - - - - , - - - - animals) /6 8 /2
I
S (8) C (2)
N S (6)
,I ,,, ,
,,
, ,,
,, ,, Pulmonary Artery Fig. 1. Aorta-pulmonary graft.
results to the features of pulmonary hypertension in children. In our early studies'? it was demonstrated that during growth, similar structural changes occurred in pig and human lungs, except that the growth rate of the pig was faster and changes seen in human childhood and adolescence were telescoped into the first 2 months of life in the pig. Aorta-pulmonary shunts, pulmonary trunk to the thoracic descending aorta, have been established in pigs aged I, 2, and 3 months, and the animals were studied for I to 3 months after operation. Cardiac catheterization and lung function studies were performed, together with a structural analysis of the lung, particularly of the pulmonary circulation, using quantitative techniques. Material and methods Aorta-pulmonary anastomosis was performed in 28 Large White pigs between 4 and 12 weeks of age; eight died of pulmonary edema during the first postoperative day. The present study is concerned with the 20 surviving animals. Details of age, sex, body weight and duration of the follow-up period are listed in Table I. Surgical protocol. All animals were deprived of food and water for the 12 hours before operation. Premedication was administered intramuscularly and consisted of fentanyl (29 meg. per kilogram of body weight) and droperidol (1.4 mg. per kilogram) together with atropine (50 meg. per kilogram). 17 Fifteen minutes later, anesthesia was induced with a mixture of halothane, 1.5 to 2 percent, and 50 percent nitrous
8
C(2)
12
S (2) N
N
23 ± 1.4 (2)t 19 20 ± 2.8 (4)
I
28 ± 3.1 (3) 38 ± 2.1 (2)
43 ± 3.2 (3) 40 59 ± 1.0 (3)
33±7.0(3) 50 ± 4.7 (3) 41 26 38 ± 2.1 (2) 59 ± 1.0 (3) 49 ± 1.4 59 ± 1.0 (3)
*S, Shunted animals. C, Operated controls. N, Normals." tBody weights in kilograms with number of animals in parentheses.
oxide in oxygen administered by face mask. Endotracheal intubation with a cuffed oral tube was carried out during spontaneous breathing. The animals were given d-tubocurarine, 0.5 mg. per kilogram, intravenously and the respiration was controlled using a BromptonMainley ventilator that delivered 50 percent nitrous oxide in oxygen at a tidal volume of 40 to 50 c.c. per kilogram. Anesthesia was maintained by an intravenous infusion of fentanyl (0.5 mg.) and droperidol (12.5 mg.) in saline (100 ml.) administered via an ear vein at a rate of I to 2 ml. per minute. Atropine was administered intramuscularly every 30 minutes. Halothane (0.5 to I percent) was used intermittently during the operation but was discontinued immediately after the shunt had been established. Measurements at operation. After opening the chest, both aortic and pulmonary arterial pressures were measured by direct needle puncture. This was repeated when the shunt was established using a silicone rubber prosthesis, * 6 mm. inner diameter for the 4-week-old animals and 8 mm. inner diameter for the older ones (Fig. I); graft flow was then measured with a sine-wave electromagnetic flowmeter, SEM 275. t The cuff-head transducers were calibrated in vitro over a range of gravity flow rates of 100 to 1,000 ml. per minute. All measurements were taken at end expiration with the ventilator stopped for a short period: Zero was taken as midchest level. Arterial blood samples were collected immediately before and after each study and were stored on ice to be analyzed later for blood gas tension and pH using a BMS-3 electrode *This material was supplied to us by Professor F. Ashton. Department of Surgery, University of Birmingham, England. See Ashton F, Lightwood R, Hardman J: A silicone rubber arterial prosthesis. Br J Surg 54:709-712, 1967. tSe Laboratories, Middlesex, England.
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January, 1979
system; * blood hematocrit was measured in a Hawkesley centrifuge. A six-channel system, EMMA, twas used for amplification and display of the signals, and data were recorded on an ultraviolet oscillograph, 3006DL.t Follow-up studies. The follow-up period ranged between I and 3 months; no studies were made in the first IS days after operation. Serial measurements of lung function were performed for each case on at least three occasions. Each animal was submitted to right-side cardiac catheterization before being put to death. Right atrial and ventricular, pulmonary and systemic arterial pressures were measured; details of the techniques used in these studies have been described elsewhere."? The left-to-right shunt was assessed from the pulmonaryto-systemic flow ratio (Qp/Qs) calculated from the oxygen content of blood samples taken from the right ventricle (02C m V), the pulmonary artery (02C PA), and a systemic artery (02C SA) and measured in a Lex-Oj-Con analyzert using the formula": 02C SA - O2C m V 02C SA - 02C PA
Structural studies. The methods used for study of structure have been described previously in a report on the normal development of the pig lung;" They include: injection into the pulmonary vasculature, either arteries or veins, of a barium sulphate-gelatin mixture at a constant pressure of 100 ern. of H 20 maintained for 5 to 7 minutes to allow complete distention of the vessel walls and more accurate comparison between specimens; quantitative analysis of vessels, including muscularity, size, and number related to position in the vascular tree; and assessment of the ventricular weights, left ventricle plus septum (L V + S) and right ventricle. Results In most animals, a continuous murmur was heard in the precordium after the anastomosis, the murmur being loudest at the upper left sternal border. In Cases I, 6, 10, and 18, a right-side cardiac catheterization was performed 15 days after operation because no murmur was audible. This demonstrated that the shunt was closed, since there was no increase in the oxygen content of the blood between the right ventricle and the pulmonary artery. These animals were also studied as "operated" controls. The remaining 16 were divided *Radiometer, AIS, Copenhagen, Denmark. tSe Laboratories, Middlesex, England. :j:Lexington Instruments Corp., Waltham, Mass.
I I I
into two groups: those operated upon at 4 weeks of age (eight animals), subsequently described as the "young group," and those operated upon between 8 and 12 weeks of age (eight animals), described as the "older group." All the animals showed a normal increase in body weight during the follow-up period. Functional studies. Pulmonary hemodynamics. In all animals the pulmonary arterial pressure (millimeters of mercury) rose immediately after the shunt was established, from a mean value of 9 ± 1.4 (S.D.) to 13 ± 2.4, whereas systemic arterial pressure fell from 58 ± 7.2 to 41 ± 6.5, the differences in both were highly significant (p < 0.00 I). In most animals the heart rate increased, but the change was significant only in the young group. Blood flow through the shunt was continuous during the cardiac cycle. The flow wave was pulsatile, with a peak coinciding with the systolic component of both aortic and pulmonary pressure waves (Fig. 2). Mean shunt flow ranged between 257 and 987 mi. per minute in the series; when adjusted to body weight the range was similar in both age groups (20 to 48.3 rnl. per minute per kilogram). No constant relationship was observed between shunt flow and the immediate pressure changes despite the fact that animals with the larger shunts tended to show a greater increase in pulmonary arterial pressure, particularly in the young group. Paco"was 42 ± 3.9 (S.D.), with a Pao" that always exceeded 98 mm. Hg and a hematocrit of 29 ± 2.5. Cardiac catheterization performed at the end of the follow-up period showed a highly significant increase in mean pulmonary arterial pressure (millimeters of mercury) in all animals with a patent shunt (29 ± 5.2, S.D.) when compared with the normal (II ± 2.6) and the operated control groups (10 ± 2.3) (p < 0.001). The young group showed higher values (32 ± 5.6) than the older group (26 ± 3. I), the difference between the two being significant (p < 0.05). In the young group (Fig. 3) the pulmonary artery pressure values increased in the follow-up period: 27 mm. Hg in the first month to 36 mm. Hg in second month and 34 mm. Hg in the third month. In the older group the increase was significant after the first postoperative month (Fig. 4). The increase in right ventricular pressure closely followed that in the pulmonary artery, the young group showing higher mean values (39 mm. Hg) than the older one (33 mm. Hg) (p < 0.05). The heart rate was also increased only in the young group (137 beats per minute) but not in the older one (99 b.p.m.-p < 0.01). Mean systemic arterial and right atrial pressures were not significantly different in
The Journal of
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Rendas, Lennox, Reid
Thoracic and Cardiovascular Surgery
(i)
-
, 60 "''''Hg.
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Fig. 2. A, Simultaneous recording of electrocardiogram. Lead DII (i), aortic pressure (ii), phasic shunt flow (iii), and pulmonary artery pressure (iv). Notice that shunt flow is normal. B, The effect on aortic pressure (ii) and pulmonary artery pressure (iv) of brief occlusion of graft. i, ii, and iv, Same as in A. iii, Mean shunt flow.
either shunted or operated control or normal animals. The Qp/Qs ranged between 1.5 and 2.4 with no significant differences between the two age groups. Mean values for Paco. and pH were 40 and 7.39, respectively, with a Pao. that exceeded 82 mm. Hg. Lungfunction. The shunted animals did not show the increase in dynamic compliance that normally occurs during growth; the ratio between dynamic compliance and body weight was lower in the shunted series than in either normal or operated control groups (p < 0.001), which had similar values. Values for the young group were significantly lower than for the older animals (p < 0.01) (Fig. 5).
Throughout the follow-up period respiratory frequency (breaths per minute) did not differ significantly in the shunted (range of 15 to 38), operated control (range of 14 to 40), and normal animals (range of 10 to 36). Tidal volume increased with age, and although in the young group neither shunted nor operated controls increased their tidal volumes at the normal rate for age and body weight during the first postoperative month. by the end of the second month they had achieved the expected level. Measurements of thoracic gas volume (TGV) showed that the young group, whether shunted or operated control, had values higher than those for normal animals, suggesting that after operation some
Volume 77
Aorta-pulmonary shunts
Number 1 January, 1979
A-P SHUNT ARTERIES Pre-acinar Intra-acinar
Size WI Size WI Ext. No.
A-P SHUNT
(4 WEEKS OF AGE)
N
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Follow- up (weeks)
Follow-up (weeks)
Fig. 3. Summary of structural and functional findings in the young group. Wt., Wall thickness. Ext., Distal extension of muscle along the peripheral arterial pathway. No., Arterial number per unit area of lung tissue (square centimeters ). PAP, Mean pulmonary arterial pressure. Qp/Qs, Pulmonary-to-systemic flow ratio. Fig. 4. Summaryof structural and functional findings in the older group. See legend to Fig. 3 for abbreviations. air was trapped; when expressed in terms of body weight (BW) these differences were not significant (TGV/BW).
Structural studies. General features. The lung volumes in the shunted animals did not differ significantly from those in the normal and operated control animals. Lung growth (length) was decreased in the shunted and operated control animals of the young group at the first postoperative month, suggesting that if performed early in life thoracotomy affects the normal increase in length; on the other hand, these changes were transitory because animals from the young group showed normal lung growth (length), according to age and body weight, at the second and third postoperative months. Macroscopically, the lungs showed no collapse: The pleural surface appeared normal, except for some thickening over the left upper and middle lobes, resulting from chest wall adhesions from the thoractomy wound. The microscopic architecture of the lung was not altered in shunted and operated control animals; the alveolar walls and connective tissue septa appeared normal and there was no evidence of perivascular or peribronchiolar edema. Pulmonary arteries. At the end of the first postoperative month the arteriograms of the older group showed dilation of the main artery and its branches; this
-----. - - -. - ----- --- ----. --- - ---- •• 2 SO
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5
10
Mean (Normall
15
AGE (weeks)
Fig. 5. Measurements of dynamic compliance during the follow-up period; notice that although there is a decrease in all animals, the young group (_) shows lower values than the older one (e). feature was never evident in the young group, even 3 months after operation. Although the diameter of the axial pathway was not increased, the lateral pathways were dilated (Fig. 6). In each age group the arteriograms of the animals followed for the longest periods
The Journal of
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Rendas, Lennox. Reid
Thorac ic and Cardiovascular Surgery
Fig. 6. Pulmonary arteriograms of three animals at 16 week s of age . Center. Normal. Left. Shunt at 8 weeks. Right. Shunt at 4 week s. Notice the dilation of the main vessels in the anim al operated upon later in life.
showed a reduction in the degree of background haze, suggesting a reduction in the number or size of the intra-acinar arteries. The medial wall of the main pulmonary artery was thicker in all cases; this change was already apparent by the end of the first postoperative month, but the increase was always more significant in the young group (276.4 ~m ± 32.6 S .D .) than in the older one (221.9 ~m ± 34 .8) (p < 0.01). The number of elastic laminae in the media was not increased in the operated animals, but fibers were more continuous and some degree of patchy intimal thickening was found; these features were not found in the normal and operated control groups. In all shunted animal s, percentage wall thickness of the small muscular arteries (15 to 200 ~m in diameter) showed no significant increase after the first postoperative month . In the second postoperative month, the percentage wall thickness was increased in both young (7 .8 ± 1.2 S .D.) and older groups (6 .1 ± 1.1) when compared with the control (4 .8 ± 1.6) and normal (4.7 ± 1.4) animals . In the young group, animal s followed for 3 months continued to show an increased percentage wall thickness, 8.5 ± 1.7. A similar pattern of change was found in external diameter of the larger muscular arteries (200 to I ,000 ~m) ; nevertheless, the increase in percentage wall thickness was greater (p < 0.005) in the young group (4.9 ± 0 .6) than in the older group (3 .8 ± 0 .2) of animals followed for the same period of time , i.e ., 2 months.
In the shunted cases, the proportion of small , muscular and partially muscular arteries was relatively increased as compared with the normal proportion of nonmuscular arteries. In the young group this shift was already apparent by the end of the first postoperative month, with muscular arteries present in the vessels with an external diameter as small as 20 ~m; whereas in the normal and operated control groups, muscular arteries were seen only in those with an external diame ter of 50 to 60 ~m . This change was less apparent in the older group. In all shunted animals , fully muscularized arteries were found in the 10 to 20 ~m diameter group 2 months after operation . Analy sis of the structural types of intra-acinar arteries in relation to the accompanying airway s (i.e., alveolar ducts and respiratory and terminal bronchioli) also established that in the shunted animals, muscle extended further along the peripheral arterial pathway; although the normal pig already has muscularized arteries at alveolar duct level by the fifth week of life, their proportion as related to partially muscular and nonmuscular arteries was increased in all shunted animals: In animals followed for 8 weeks, the differences from the normals and controls were more apparent. In the young group the size of the intra-acinar arteries did not increa se normally during the follow-up period, i. e., the external diameter of arteries accompanying respiratory bronchioli measured 58 ~m ± 3.6 S .D . (n > 20 in each case) by the end of the first month of follow -up; whereas for the normals and operated
Volume 77 Number 1
Aorta-pulmonary shunts
I I5
January. 1979
controls it was 73 JLm ± 5.2 (n > 20 in each case) (p < 0.001). Although some increase in size occurred during the folIowing 2 months the values continued to be lower than those expected on the basis of age (Fig. 3). There was no decrease in arterial size in the older group (Fig. 4). The assessment of the intra-acinar arterial number per unit of lung tissue (square centimeter) did not show a significant change, although some reduction was detected in alI shunted animals followed for 2 or more months. The reduction of background haze seen in the arteriograms of the shunted cases was probably due more to the reduction in the size of the intra-acinar arteries than to an effective reduction in arterial concentration. Pulmonary veins. The venograms from the shunted animals were similar to those for the normal and operated control groups, with no significant change in size, branching pattern, or background haze. Microscopically, the percentage wall thickness of veins 15 to 1,000 JLm in external diameter was also similar to values for normal and operated control groups. Ventricular weights. Total ventricular weight was increased in all animals that underwent operation, but it is unclear whether this reflected the effect of the thoracotomy or of the left-to-right shunt, which in the controls was found to be occluded 15 days later. In the shunted animals, the increase in total ventricular weight, adjusted to body weight (grams per kilogram), was significant only in the young group at 12 (4.3 ± 0.4 S.D.) and 16 (4.0 ± 0.3) weeks of age when compared with the control group (2.9 ± 0.7). The (LV + S)/R V ratio was not changed in the operated control group (3.8 ± 0.4 S.D.) when compared with the normal animals (3.7 ± 0.4). Nevertheless, in the young group, the longer the duration of the folIowup, the greater the progressive decrease in the (LV + S)/R V ratio, which became significant by the second month of follow-up (3.1 ± 0.2) and continued to fall at 3 months (2.8 ± 0.1). This suggests that in the young group, right ventricular hypertrophy was occurring during alteration of the structure of the pulmonary arteries. Discussion In these experiments, pulmonary hypertension was produced in growing pigs, 4, 8, and 12 weeks of age, by anastomosing the thoracic descending aorta to the pulmonary trunk with a silicone graft. The longer the follow-up period, the more severe the changes in the lungs. Functionally, there was an increase in pulmonary arterial and right ventricular pressures and a de-
crease in lung dynamic compliance in alI cases, the changes being more significant in the young group (the animals operated upon at 4 weeks of age); the heart rate was increased in the young group and was normal in the older group, the animals operated upon at 8 and 12 weeks of age. Structurally, all pulmonary arteries were thicker than normal, from hilum to periphery; in addition, muscle extension to smaller intra-acinar arteries was greater than estimated on the basis of age. There was left ventricular hypertrophy, with the more hypertensive animals having right ventricular hypertrophy; these features were also more significant in the young animals. The luminal diameter of the main arteries was increased in the older group and was normal in the young animals; at the intra-acinar level, the arteries were normal in size in the older group and small for age in the young group. In all cases, the arterial and alveolar concentrations were normal for age, as was the thickness of the pulmonary veins. The development of more severe pulmonary hypertension in the younger animals may reflect structural differences between the various age groups, particularly the interaction between muscularization and changes in vascular cross-sectional area during growth. A l-month-old animal still has somewhat immature lungs in that, in the elastic compartment of the arterial vasculature, the elastic laminae in the medial wall are thicker and more continuous than they become later. In the muscular compartment the vessels between 200 to 1,000 JLm in external diameter are still thicker than those in the adult, the latter values not being reached until 8 weeks of age. Extension of muscle into small intra-acinar arteries is incomplete at 4 weeks of age, i.e., more arteries with a complete muscular coat accompany respiratory airways at 8 weeks than at 4 weeks; these arteries are smaller and less numerous than those in the older group, indicating that the vascular cross-sectional area in a respiratory unit increases with age. Comparison between the present and other experimental studies on pulmonary hypertension, produced by either pneumonectomy or systemic-to-pulmonary shunt, is difficult, since in most of previous works, adult animals have been used and changes occurred only if systemic arteries were anastomosed to a small part of the pulmonary circulatory structure.v ;;-7 Such studies demonstrated the importance of the height of the blood pressure, duration of shunt, and shunt size in the production of arterial changes, particularly increased muscularization. It is difficult to apply such findings to clinical conditions of left-to-right shunts associated with congenital cardiac effects, where, usu-
The Journal of
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Rendas, Lennox, Reid
ally, the whole of the pulmonary vascular bed is submitted to the hemodynamic disturbance during the period of postnatal growth. Pulmonary hypertension does occur in young children with left-to-right shunts associated with congenital cardiac defects, particularly in cases of a direct communication between the two arterial systems. Nevertheless, when patients less than 2 years of age have symptoms, they tend to have higher flow ratios than older children for similar increases in mean pulmonary artery pressure. The severity of the pulmonary vascular disease increases with age, but most often, only in cases of a mean pulmonary arterial pressure higher than 50 mm. Hg are the more severe, ultimately irreversible changes seen. 19 - 2 2 In cases of mild to moderate increase in pulmonary vascular resistance (PVR), the structural basis for the pulmonary hypertension is considered to be thickening of the muscular coat, and therefore the disease is reversible; in cases of a severe increase in PVR, intimal fibrosis with dilation lesions has been considered the basis for irreversible "obstructive" disease. 2:1 More recently, morphometric techniques have been used to analyze the pulmonary circulation in infants with ventricular septal defects, taking into account the present knowledge oflung development and growth.": 10 These studies have shown that in cases of a mild increase in PVR, there is increase in arterial muscularity without features indicating impaired growth. In other patients, particularly those with a high PVR, the clinical criteria for irreversibility (a ratio of pulmonary to systemic resistance greater than 0.7) can be present without the structural stigmata of "obstructive" disease, i.e., intimal fibrosis and dilation lesions. In these cases, quantitative study shows a hypoplastic circulation with thicker, smaller, and fewer vessels than expected on the basis of age. It is now generally accepted that the changes in the lung in cases of simple left-to-right shunts occur postnatally by, e.g., patent ductus arteriosus, ventricular septal defects, and aortopulmonary window. Although in the present experiment the shunt is produced at a stage when the rapid postnatal fall in PVR has already occurred, the young group is close enough to the newborn period to develop a response similar to that in clinical cases of a lesion present at birth. Although a pulmonary arterial pressure equaling more than half the systemic values was not recorded in the experimental cases, the structural features approximate human ones, save that in the pigs a reduction in arterial concentration was not found. This resembles results of studies in man at postmortem biopsy which show that reduction in
Thoracic and Cardiovascular Surgery
arterial concentration occurs only in cases of significant pulmonary hypertension (pulmonary artery pressure> 20 mm. Hg and PVR of 3 U. per square meter) and only after 4 months of age. 9. 10.24 In the pigs, the differences between the two age groups may resemble such differences with left-to-right shunts associated with congenital cardiac defects. Symptoms in the young group may resemble those found in infants who have a moderate increase in pulmonary arterial pressure and show some impairment of postnatal growth; the older animals presented with symptoms similar to those accompanying normal arterial concentration and size but excessive muscle in the arterial wall. In both age groups the decrease in dynamic compliance was progressive, the lowest values occurring in the young group and coinciding with the highest pulmonary artery pressures and the greatest structural impairment of lung growth. This suggests a close relationship between respiratory airways and arteries, the latter affecting the mechanical properties of the lungs, probably because of increased muscularity. This finding confirms previous studies on lung mechanics in cases of left-to-right shunts that showed a decrease in dynamic compliance in infants with severe pulmonary hypertension.": 26 Both clinical and experimental studies have emphasized the greater importance of the increase in blood pressure, when compared to flow to the lungs, in the pathogenesis of the pulmonary vascular lesions; other features that influence the response are increases in left atrial and ventricular end-diastolic pressures; the altered flow profiles seen in ventricular septal defects, with the systolic portion of the wave showing a more peaked effect than atrial septal defects; and at least in the acute experimental conditions of a large aorto-pulmonary shunt, an increase in diastolic flow velocity. 27-:10 Individual variability may also contribute to the degree of pulmonary vascular damage.:" Only recently have growth and development of the pulmonary circulation been investigated in clinical cases of left-to-right shunts associated with congenital cardiac defects, but it has already been shown that its impairment is an important feature of the response to the hemodynamic disturbance. The present study shows that the changes are progressive and that they affect the whole arterial bed and the heart: Increased muscularization probably arises from the increased vascular tension that is maintained throughout the cardiac cycle, the contractile elements failing to relax. 32 In the hypoxic adult rat, extension of muscle to smallerthan-normal intra-acinar arteries has been shown by Meyrick and Reid'" to represent differentiation of precursor cells, the pericyte and intermediate cell, nor-
Volume 77
Aorta-pulmonary shunts
Number 1
January. 1979
mally present in the wall of the nonmuscular segment of the arterial system. It seems possible that this cell differentiation is triggered not only by hypoxia, but also by other stimuli such as changes in pulmonary blood flow and pressure. The decrease in size of the intra-acinar arteries, only found in the young animals, must effectively reduce the vascular cross-sectional area at the level of the respiratory airways; this feature represents the earliest impairment in the growth process. The reduction in arterial concentration is not seen in the experimental animals, probably because the increase in pulmonary artery pressure is only moderate. Left ventricular hypertrophy occurs early, reflecting the changes in both preload, caused by the increase in pulmonary venous return, and in afterload, as a result of direct communication between the aorta and the main pulmonary artery. Right ventricular hypertrophy occurs late and is secondary to the pulmonary vascular changes and to the rise in pulmonary arterial pressure. The age-related differences may reflect the balance between vascular reactivity and reserve capacity during growth'": regarding these two features it is important to emphasize the role played not only by the peripheral muscular arteries, but also by the larger ones, because our study suggests the existence of an elastic compartment that is less distensible in the young animals than in the older ones. Infants and children with a moderate increase in pulmonary artery pressure have some impairment of growth of the intra-acinar pulmonary circulation; but when clinical improvement occurs, it is probably due to adaptation by the circulation, via increasing vessel size and concentration, early in life; deterioration occurs if these mechanisms do not operate. A rise in pulmonary vascular resistance early in childhood probably means that compensation of the hemodynamic disturbances does not occur and that a truly hypoplastic circulation emerges. Surgical correction of left-to-right shunts early in life creates the anatomic basis for the establishment of normal hemodynamic conditions, perhaps allowing lung growth to restart or even catch up and thus improve prognosis. We are grateful to Mr. D. Guerreiro and Miss C. Lloyd for technical assistance. REFERENCES Levy SE, Blalock A: Experimental observations on the effects of connecting by suture the left main pulmonary artery to the systemic circulation. 1 Thorac Surg 8:525530, 1939 2 Hawe A, Tsakiris AG, Rastelli GC, Titus lL, McGoon DC: Experimental studies on the pathogenesis of pulmo-
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nary vascular obstructive disease. 1 THORAC CARDIOVASC 63:652-669, 1972 3 Eppinger EG, Burwell CS, Gross RE: The effect of the patent ductus arteriosus on the circulation. 1 Clin Invest 20:127-143, 1941 4 Leeds SE: The effects of occlusion of experimental chronic patent ductus arteriosus on the cardiac output, pulse and blood pressure of dogs. Am 1 Physiol 139:451-459, 1943 5 Damman lF lr: The relationship of flow and pressure in various types of congenital heart disease particularly those associated with pulmonary hypertension, Pulmonary Circulation, WR Adams, I Veith, ed., New York and London, 1959, Grune & Stratton, Inc., pp 220-226 6 Damman lF Jr , Baker lP, Muller WH lr: Pulmonary vascular changes induced by experimentally produced pulmonary arterial hypertension. Surg Gynecol Obstet 105: 16-26, 1959 7 Ferguson Dl, Varco RL: The relation of blood pressure and flow to the development and regression of experimentally induced pulmonary arteriosclerosis. Circ Res 3: 152-158, 1955 8 Hislop A, Reid L: Growth and development of the respiratory system, Scientific Foundations of Paediatrics, lA Davis, 1 Dobbing, eds., London, 1974, William Heinemann Medical Books, Ltd., pp 214-254 9 Hislop A, Haworth SG, Shinebourne EA, Reid L: Quantitative structural analysis of pulmonary vessels in isolated ventricular septal defects in infancy. Br Heart 1 37: 10141021, 1975 10 Haworth SG, Sauer U, Biihlmeyer K, Reid L: Development of the pulmonary circulation in ventricular septal defect. A quantitative structural study. Am 1 Cardiol 40:781-788, 1977 II Richardson rw, Phillips CE, DeWeese lA, Manning lA, Mahoney EB: Influence of pulmonary vascular maturity on response to subclavian pulmonary arterial anastomosis. Circulation 24:1020, 1961 12 Rudolph AM, Neuhauser EBD, Golinko Rl, Auld PAN: Effects of pneumonectomy on pulmonary circulation in adult and young animals. Circ Res 9:856-861, 1961 13 Massion WH, Schilling lA: Physiological effects of lung resection in adult and puppy dogs. 1 THORAC CARDIOVASC SURG 48:239-249, 1964 14 Vogel lHK, McNamara DG, Hallman G, Rosenberg H: Effects of mild chronic hypoxia on the pulmonary circulation in calves with reactive pulmonary hypertension. Circ Res 21:661-669, 1967 15 Kato H, Kidd L, Olley PM: Effects of hypoxia on pulmonary vascular reactivity in pneumonectomized puppies and minipigs. Circ Res 28:397-402, 1971 16 Friedli B, Kent G, Kidd LSC: The effect of increased pulmonary blood flow on the pulmonary vascular bed in pigs. Pediatr Res 9:547-553, 1975 17 Rendas A, Branthwaite M, Reid L: Growth of the pulmonary circulation in the normal pig-Structural analysis SURG
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