Respiratory distress in the newborn

Respiratory distress in the newborn Pulmonary mechanics following fluid instillation in the lungs of neonatal lambs

JOH~

W . C.

JOH~SON,

M.D .

E . E. FARIDY, M . D. Baltimore, Maryland

I N PRE V I 0 1; s studies we found that the instillation of allantoic fluid into the lungs of term newborn lambs prior to the first breath produced a neonatal respiratory disturbance which, in some respects, was similar to the respiratory distress syndrome of the human newborn. l These lambs exhibited tachypnea, labored respirations with rib retraction, cyanosis, periodic apnea, and usually spontaneous death. At autopsy the lungs were found to be boggy and congested. Microscopically, there was relative atelectasis. In addition, extracts of these lungs were found to have abnormally elevated surface tensions. These findings indicated that increased quantities of fluid in the newborn's lung might interfere with the establishment of adequate ventilation and might indeed be of significance in the etiology of the respiratory distress syndrome. The present studies represent further attempts to define the mechanisms responsible for the respiratory disturbances seen in these lambs. The effect of instilled amniotic fluid has been studied, and the clastic recoil of the lungs of these fluid-instilled lambs has been further evalu-

ated. Since differences between lamb allantoic, lamb amniotic, and human amniotic fluids might have a direct bearing upon the noted pulmonary changes following the instillation of these fluids, we have also compared some of the properties of these various fluids. Finally, saline filling studies have been performed on the lungs of several newborn lambs in an effort to determine some of the mechanical factors involved in intrauterine aspiration.

Methods Fifteen pregnant ewes (Dorset or Hampshire breed) and their 24 lambs were used for this study. There were 9 sets of twins. Gestational ages were estimated from known tupping dates. The ewes were anesthetized, and the lambs were delivered by cesarean section and prepared essentially as previously described.! Immediately after delivery, with the umbilical circulation still intact, 13 lambs had 30 to i5 C.c. of their own amniotic fluid instilled into their tracheas which were cannulated. It had been previously decided to instill a fluid volume equal to one fifth of the total lung volume, which was estimated to be 16 m!. per kilogram of body weight. Therefore, the fluid volume to be instilled was determined by an estimate of lamb body weight. In eleven lambs the tracheas were cannulated, but no amniotic fluid was instilled. Following these preparations, the

From the Departments of Gynecology and Obstetrics and Envir()1lmental Medicine, The Johns Hopkins University School of Medicine. This work was supported in part by Public Health Service Postdoctoral Fellowship 2-F2-HD-17, 328 and Public Health Service Grant AP-00208.

253

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Johnson and Faridy

umbilical cords were ligated and divided, and the lambs were secured in the supine position. In a number of experiments, maternal arterial and fetal umbilical venous samples were obtained at the time of cord ligation and were subsequently analyzed for pH, pC0 2 , and p02 on an Epsco medical blood parameter analyzer, Model 101. The lambs usually gasped shortly after the cords were clamped. A positive pressure respirator was often used to assist ventilation, particularly when apnea or feeble respirations were noted. In addition, 100 per cent or 40 per cent O 2 was employed if extreme respiratory distress was noted. Despite these measures, most of the lambs that had fluid instilled into them died within hours after delivery. The remaining lambs were sacrificed at comparable times by the intravenous or intra-arterial injection of 2 to 4 m!. of saturated potassium chloride solution. In an effort to analyze alveolar clearing mechanisms, the rate of albumin clearance from the lungs was studied in 14 of these lambs. In 3 lambs, 70 to 75 p.c of radioiodinated human serum albumin (P31-HSA) was added to the amniotic fluid prior to instillation. Six lambs that did not have amniotic fluid instilled had 75 p.c of p31_ HSA instilled into their tracheas through a small polyethylene catheter, and 2 m!. of isotonic saline were then instilled in an attempt to clear the syringe and catheter of residual isotope. After the onset of ventilation, blood samples were taken from each lamb at predetermined intervals and analyzed for the presence of P31_HSA by the previously described techniques. 1 Immediately following the death of each lamb, the elastic recoil of the lungs was studied by performing deflation pressurevolume studies. These studies were performed with the lungs still within the thorax, and it was necessary to subject the entire lamb to a temporary vacuum in order to rid the lungs of gas. Otherwise, the apparatus and technique used in obtaining the pressurevolume data and the methods used in ana-

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lyzing these data were essentially the same as have been described elsewhere." Pressure-volume values were often unobtainable because of lung leaks, but technically satisfactory studies were accomplished in 8 lambs that had amniotic fluid instilled and in 9 control lambs. In previous experiments in which allantoic fluid was instilled, 5 lambs had satisfactory pressurevolume studies, and those results arc given for the first time in this report. The pressure-volume characteristics were evaluated by 2 methods, both of which were designed to permit the comparison of lungs of different sizes. 2 In the first method, the lung volume at a transpulmonary pressure of 40 em. H 2 0 was designated as 100 per cent, and the volumes noted during sub~e­ quent deflation were then expressed as a percentage of this value. Corresponding pressures and volumes were plotted. This method was used to detect the presence of air spaces that closed at relatively high transpulmonary pressures. In the second method, the total lung volume per gram of estimated normal lung was plotted at different transpulmonary pressures. Previous studies have indicated that the newborn lamb with well aerated lungs has an average lung weight to body weight ratio of 0.016. 1 The product of the observed body weight and 0.016 was used as the predicted normal lung weight for each lamb. The difference between the predicted normal lung weight and the observed lung weight was assumed to represent the excess fluid volume, with a specific gravity of 1.000. This estimated excess fluid volume and the observed air volume gave the estimated total volume present at a given transpulmonary pressure. These pressure-volume data were expressed as the total volume per gram of predicted normal lung weight to afford a means of comparing the distensibility of these lungs. After the pressure-volume studies were completed, lung extract surface tension and lung histology were studied in the same manner as in the previous study.l Samples were always taken from the lower lobes of

Volume 92 ~umber 2

the lungs, with care taken to exclude any grossly hemorrhagic areas. In 16 of these experiments, samples of lamb amniotic and allantoic fluid were obtained by needle aspiration before delivery of the lamb. In addition, samples of human amniotic fluid were obtained by transabdominal aspiration in 14 pregnancies. Total protein concentrations were determined on the autoanalyzer. The Patwin flame photometer of the internal standard type was used to determine sodium and potassium concentrations. Lipid studies were done on the lamb fluids only. Total lipids were determined by gravimetric analysis following extraction by the Bloor method. 3 A modification of Youngburg's method" was used for phospholipid analysis, and the total cholesterol concentration was analyzed by a modification" adapted for the autoanalyzer. The surface tensions of these fluids were determined as for lung extracts,l except that undiluted specimens were placed in the trough of the modified Wilhelmy surface tension balance. Aliquots of these specimens were then kept in containers with screw on caps at ] 0° C. until viscosity studies could be performed. These studies were performed with the fluids at 25° C. with the Ostwald viscosimeter. Viscosity values were reported as the average of 3 determinations relative to the average of 3 preliminary determinations for water. Saline filling studies were performed on the lungs of 2 lambs not included in the other experiments. These lambs were sacrificed immediately upon delivery, and their chests were left closed for this study. Their tracheas were cannulated under saline and then connected to an isotonic saline reservoir. The entire lamb was then suspended in an isotonic saline ba tho Changes in transpulmonary pressure were produced by altering the level of the saline within the reservoir with respect to the level of saline within the bath, and any volume changes in the reservoir were recorded as lung volume changes. A dye was added to the isotonic saline used to fill the lungs so that any leaks from the system might be detected. No leaks into the

Respiratory distress

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surrounding bath or into the pleural spaces were noted during these studies. Transpulmonary pressures were not altered until reservoir volumes were stable for at least 15 seconds. Results

The blood gas values for the samples obtained at the time of umbilical cord severance are summarized in Table r. The mean values and standard deviations arc given for the arterial blood samples of 6 ewes and for the umbilical venous blood samples of their 9 lambs. These values are very similar to the baseline values for ewe arterial and fetal lamb arterial blood samples reported by Harned and his associates. 6 The specific details and findings of the amniotic fluid-instilled lambs and the control lambs are summarized in Table II. Twelve of the 13 lambs that had amniotic fluid instilled died spontaneously. In general, these lambs exhibited varying degrees of respiratory difficulty characterized by tachypnea and rib retraction. Cyanosis, bradycardia, and episodes of apnea were usually noted prior to death. Lamb i'Jo. 20 initially had tachypnea and rib retraction, but began to improve several hours after birth and presumably would have survived if it had not been sacrificed. As a group, the 11 lambs that were not instilled with amniotic fluid exhibited fewer respiratory difficulties, although 5 of them died spontaneously. The incidence of spontaneous death was found to be significantly higher in the fluid-instilled lambs (p < .02) by the chi square analysis. Lambs llA, lIB, and 17B did not have fluid added to their lungs, and they were sacrificed at birth. The differences between the observed lung weights and predicted

Table I. Comparison of maternal arterial and fetal umbilical vein blood - - - _ .._- - - - - - - - _ . pH pO,

mm. Hg peo, mm. Hg

Ewe

Fetus

7.46:t .13* 71.5 :t 26.6 37.1 :t 10.9

7.30:t .09 21.5 :t B.9 49.4 :tl0

o)lo± Standard deviation.

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May 15, 1965 Am. J. Obst. & Gynec.

Johnson and Faridy

Table II. Summary of clinical course and findings

Lamb

8 9A 9B lOt llA llB 12t 13t 14At 14Bt 15Aft 15Bt 16At 16Btt 17At 17B 18At 18Bt 19A 19Bt 20tt 21Att 21BH 22ft

Gestational age (days)

Amount of amniotic fluid instilled ( cubic centimeters)

131 136 136 143 137 137 140 139 142 142 139 139 139 139 143 143 139 139 147 147 143 143 143 140

70 30 30 75 0 0 70 70 0 0 0 0 0 0 70 0 75 0 75 0 75 30 30 65

Death*

Age at death (hours)

Body weight (grams)

Lung weight (grams)

Estimated residual fluid (milliliters)

Spont. Spont. Spont. Spont. Induced Induced Spont. Spont. Spont. Spont. Induced Induced Induced Induced Spont. Spont. Spont. Spont. Spont. Spont. Induced Spont. Spont. Spont.

3.0 0.2 5.0 11.1 0.0 0.0 1.9 3.4 8.3 5.9 9.0 12.0 12.0 12.0 0.1 0.2 14.0 5.0 0.0 6.0 6.0 5.0 2.5 4.7

2,549 1,478 1,564 3,065 2,150 2,739 4,044 2.335 2,412 2,946 1,800 3,738 2,142 2,484 3,495 3,047 3,384 3,295 3,395 3,120 5,080 1,720 1,816 3,438

143 64 65 85 80 113 133 89 79 97 94 87 110 116 181 116 108 111 160 121 149 82 87 151

102 40 40 36 46 69 68 52 40 50 65 27 76 76 125 67 54 58 106 71 58 54 58 96

*Spont. = Spontaneous. tHad l'31-HSA instilled. tHad blood pH, pO., and peo. studies.

Fig. 1. Photomicrographs of lungs from Iamb 22 (left) and Iamb 15B (right) at the same magnification (x35). Both sections were fixed at zero transpulmonary pressure. Lamb 22 had amniotic fluid instilled, and the alveoli are noted to be relatively atelectatic.

Lung extract surface tensions (dynes/ centimeter) Maximum

48 39 39 37 36 37 32 44 34 40 28 32 36 35 36 35 35 32 35 38 36 42 40 39

J

Minimum

30 29 23 15 12 9 4 26 16 18 2 4 3 2 6 2 4 2 2 12 17 22 21 20

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2

Table III. Final outcome of all studies ---

No. of lambs

Death spontan eo us

Death duced

Amniotic or allantoic fluid instilled

18

16

2

4.9

1'0 fluid instilled

14

5

9

6.2

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livered by cesarean section in the present amniotic fluid study, as well as the preceding allantoic fluid study, is summarized in Table III. The higher incidence of spontaneous death observed in the fluid-instilled lambs was found to be very significant by the chi square analysis (p < .001). The average survival time, regardless of whether death was spontaneous or induced, was 4.9 hours for all of the fluid-instilled animals and 6.2 hours for all of the animals that did not have fluid instilled into them. Satisfactory air pressure-volume studies were obtained in 4 of the lambs that had allantoic fluid instilled, in 8 of the lambs that had amniotic fluid instilled, and in 10 of the lambs that had neither fluid instilled. The results of these studies are given in terms of absolute air volumes in Table IV. Figs. 2, 3, and 4 consist of comparative studies between the lambs that had amniotic or allantoic fluid instilled and the lambs that had neither of these fluids instilled. With the exception of lamb No.3, the surface tension values and pressure-volume values of the lambs that had allantoic fluid instilled were not significantly different from

lung weights of these animals suggest that the lamb born by cesarean section has a fluid volume within its lungs of approximately 22 ml. per kilogram of body weight. This is equivalent to an estimated 18 to 26 per cent of the total lung volume, and it is larger than a similar estimate made in newborn lambs by A very and Cook. 7 Photomicrographs of the lungs of lambs 15H and 22 are shown in Fig. 1. The alveoli of fluid-instilled lamb No. 22 are more collapsed or atelectatic. Vascular congestion and a relative dilatation of the terminal air passages are also present. The final outcome of all the lambs de-

Average age at death (hr.)

In

Table IV. Result of pressure-volume studies

Lamb

2

3

4

6A 7 9B IIA lIB 12 13 14A 14B 15B 16A 16B 17A 17B 19A 19B 20 21A 22

Fluid instilled* ( cubic centimeter)

75 50 75 0 75 30 0 0 70 70 3 3 0 0 3 70 0 75 3 75 30 65

AI. AI. AI. AI. Am. Am. Am. Isotope Isotope Isotope Am. Am. Isotope Am. Am. Am.

Air volume in milliliters per centimeter H,Q transpulmonary pressure 40

287 360 250 297 225 28 130 165 175 43 122 85 352 155 222 113 205 133 195 315 52 205

I

30

I

269 347 237 289 210 25

125 156 16:~

35 115 72 340 150 214 105 198 120 186 296 50 195

*Fluids instilled: AI, allantoic; Am, amniotic; Isotope,

p31~HSA

20

15

246 310 215 271 193 21 117 143 150 25 103 54 315 142 202 91 183 101 172 263 41 172

225 285 200 256 180 17 III 133 140 22 94 45 295 133 201 85 172 90 160 235

34

158

and normal saline.

I

10

5

198 257 182 233 165 12 103 117 121 16 85 36 255 123 171

114 204 137 169 110 8 85 80 92 12 68

77

158 76 143

178 28 141

22

185 86 119 61 132 47 108 128 20 100

I

0

65 65 50 90 25 1 45 27 18 0 30 3 134 32 60 55 103 26 46 82 10 26

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May 15, 1963 Am. 1. Ob" . & Gynec.

Johnson and Faridy

MAXIMUM SURFACE TENSION

55

P < .01

50 45 40 35 30

MINIMUM SURFACE TENSION P<.OOI

::i: <.> "en

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20 15

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00

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5

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• •• •

00 0

0

0000

0

" ALLANTOIC FLUID ADDEO • AMNIOTIC FLUID ADDEO

Fig. 2. Extract surface tensions of the lungs of the lambs that did not have fluid instilled compared to those of the lambs that did have fluid instilled. The solid bars represent the mean values.

100

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P <.1 < .05 <.05 <.05

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TRANSPULMONARY PRESSURE - em H2 0

10

20

30

40

Fig. 3. A comparison of the mean pressure-volume values observed during deflation and expressed as the percentage of maximum air volume for fluid-instilled and non fluid-instilled lungs.

those values of the lambs that had amniotic fluid instilled. Therefore, all of these lambs are included together in the fluid-instilled group, except lamb No.3. This lamb has been excluded from these comparative studies since it was exceptional because it had the least amount of fluid instilled for its body weight, and it was apparently well at the time it was sacrificed. Fig. 2 is a scattergram of the extract surface tension findings. The mean values for the minimum and maximum surface tensions are represented by the solid bars, and they are significantly higher in the fluid-instilled lambs. In Fig. 3 the mean pressure-volume values are expressed as percentages of maximum air inflation volumes. The mean values for the lungs of fluid-instilled lambs are represented by the dotted line and are significantly less at 20, 15, 10, and 5 em. H 2 0 trans pulmonary pressure (p < .05). The proportionately smaller volumes remaining in these fluid-instilled lungs at these pressures indicate that these lungs have air spaces that are predisposed to closure at relatively high transpulmonary pressures. The pressure-volume data are expressed as the estimated total volume per gram of lung tissue in Fig. 4. The mean values for the fluid-instilled lungs are again indicated by the dotted lines, and the mean values for the nonfluid instilled lambs are represented by the solid lines. The upper curves represent the total air and fluid volumes for the two groups of lambs and the lower horizontal lines represent the proportions of the total volumes attributed to residual fluid. The decreased distensibility of the fluid-instilled lungs is indicated by the fact that the mean total volume is significantly less (p < .05) at the maximum transpulmonary pressure (40 cm. H 2 0). Also, the mean air volumes per gram of estimated normal lung weight for the fluid-instilled lungs were significantly less at all transpulmonary pressures. In Fig. 5 we have tested the correlation between the minimum extract surface tension and the percentage of maximum volume

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Volume 92 Number 2

6

4

3

I I S.E. NO FLUID ADDED • __ . FLUID ADDED

2

II"I~

ESTIMATED FLUID VOLUME

Fig. 4. A comparison of the mean pressure-volume values observed during deflation and expressed as the total volume per gram of estimated normal lung- weight, for fluid-instilled and nonfluid-instilled lungs. The cross hatched areas represent the proportions of total volume attributed to residual fluid.

9 o

••

0

,

,

0

6

70

,

•

In

the newborn

259

remaining in the lungs at 15 cm. H"O transpulmonary pressure for all of the lambs that had both studies satisfactorily performed. A significant negative correlation was found (r ~ -.57; P < .01), which indicates that a predisposition to close at high transpulmonary pressures was associated with an elevated minimulll surface tension in these lungs. The results of the albumin clearance studies are depicted graphically in Fig. 6. The estimated percentages of P:l1-HSA absorbed are plotted along the ordinate against time along the abscissa, and the dotted lines represent those lambs that had fluid instilled. The rate of P:l1-HSA absorption was not significantly different in the 2 groups of lambs. This was an unexpected finding, which we are unable to explain at this time. The mean values for the surface tensions, the viscosities, and the electrolyte concentrations of the lamb amniotic, lamb allantoic, and human amniotic fluids are tabulated ill Table V with the results of the lipid analysis. For purposes of comparison, several of the findings of other investigators are also tabulated ..--. ". 10 The results of the present study are in general agreement with the results of other studies.'" Il-1:\ Lamb allantoic fluid has a decreased sodium and increased potassium con-

40

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60

35 ~

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vi

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(j)

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50

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-;- A ALLANTOIC FLUID ADDED .J • AMNIOTIC FLUID ADDED 0 > r = - .57 0:: p < .01

20

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"~

15


. 0

25

o NO FLUID

10

MINIMUM SURFACE TENSION DYNES/CM

5

10

15

20

25

30

35

Fig. 5. Correlation between percentage lung volumes remaining at 15 em. H 2 0 pressure (V 15 per cent) and minimum surface tensions of lung extracts for the lambs that had both studies.

-·:;;0 ::- .......;10 ADDED . AMN!CTK:: f. LUID ADO!:.:; At L ANTCIC >="LUID ADDED

Fig. 6. Estimated rates of clearance of radioiodinated human serum alhumin from lungs.

260 Johnson and Faridy

~fay

Am.

J. Obs!.

13, 1
Table V. Comparison of sheep and human fluids Lamb allantoic Maximum surface tension ( dynes/em,) Minimum surface tension (dynes/em.) Viscosity Total protein (Gm, % ) Osmolarity (mOsm./L.) Sodium (mEq.lL.) Potassium (mEq./L.) Total lipids (Gm. % ) Total cholesterol (mg. % ) Phospholipids (mg.%)

47.6 ± 4* 30.9 ± 5.6 1.27 ± .21 1.13 ± .48 278 ± 13.8t 65.7 ± 35 41.8±2.9 < .02 < 15

o

Lamb amniotic

44 ± 3.9 23.2 ± 2.2 1.46±,37 0.62 ± .72 275 ± 13.6t 127 ± 17.5 8.5 ±4.1 < .05 < 15

Human amniotic

43.1 ± 7.6 18.4 ± 8.3 1.2 ± .08 0.53t 265.5t 125.3t 4.0t

o

*± Standard deviation. tScc references, 8, 9, and 10.

140 120 100

and the arrow pointing to the left represents emptying. Under the quasistatic conditions of this study, relatively small changes in transpulmonary pressure were found to produce considerable changes in the fluid volume of the lung. On the emptying to 0 cm. saline transpulmonary pressure, a relatively large quantity of fluid remained within the lung.

SALINE FILLING -NEWBORN LAMB ~

z -i ~

80

2 TRANSPULM. PRESS,-cm, N/S

o

5

10

15

Fig. 7. Fluid-filling characteristics of lung of newborn lamb. Arrow pointing to reader's right, filling. Arrow pointing to reader's left, emptying. ~ote the net increase in resting volume,

centration, but otherwise it is similar to the other fluids. By the methods used, only small quantities of lipids were detected within the lamb fluids. These fluids appeared to be quite similar, and one would not expect from these findings that the instillation of lamb amniotic fluid would have a different effect upon the lungs of the lamb than the instillation of allantoic fluid. Human amniotic fluid appears to be quite similar to lamb amniotic fluid. The results of the saline-filling study performed on one of the newborn lambs is shown in Fig. 7, Saline volume is plotted along the ordinate against transpulmonary pressure along the abscissa. The arrow pointing to the reader's right represents filling,

Comments In these studies, the pressure-volume data are indicative of the elastic properties of the lung and chest wall combined. A few lambs were studied before and after opening the chest wall, and abnormal findings which were present in the elastic properties of the lung-chest wall system were invariably reflected in the properties of the lungs alone. For this reason and because it seems unlikely that changes in the elastic properties of the chest wall occurred during these experiments, differences between the pressurevolume values for the lung-chest wall system are attributed entirely to differences in the elastic properties of the lungs. The differences noted between the mean pressure-volume values for the 2 groups of lambs indicate that those fluid-instilled lambs which died spontaneously had lungs with an increased clastic recoil. Since the surface tension of the alveolar lining at large lung volumes contributes approximately two thirds of the total elastic recoil,14 these changes might well be due to elevations in alveolar surface tension.

Respiratory distress

Volume 92 Number 2

The minimum surface tension of an extract is thought to be related to alveolar surface tension during deflation.';; Therefore, an elevated minimum surface tension would indicate an increased tendency to deflate, or a decreased volump. at a given transpulmonary pressure during deflation. Such a relationship was demonstrated in the present study (Fig. 5). The maximum surface tension of an extract is thought to be most closely related to the alveolar surface tension during inflation.'" An elevation of this value would oppose inflation and cause a decreased lung volume at a given transpulmonary pressure ( 40 cm. H~O). Our results show that the total lung volumes at 40 cm. H 2 0 transpulmonary pressure (V max) tended to be inversely related to the maximum surface tensions (r -: -.42; P < .1 ) . This relationship was statistically significant (r :.. .:: -.52; p < .02) when the estimated fluid content was taken into consideration by an analysis of the partial correlation coefficients of these three variables. The relationships between lung volumes and extract surface tensions support the hypothesis that the increased elastic recoil of these fluid-instilled lungs is the result of an elevation in alveolar surface tension. The increased elastic recoil of the fluidinstilled lungs might be due to other factors. Cook and his colleagues 'n have suggested that the presence of fluid in alveoli might decrease the radii of alveolar gas liquid surfaces and thereby result in an increase in retractive forces, as indicated by the Laplace

..

relatIOnship: pressure

= 2--x _surface .-. tension --radIUS

If this were the mechanism responsible for the changes in the present study, we would expect those lungs with the most fluid to have the greatest increase in retractive forces. However, we were unable to demonstrate any significant relationship between the V 15 per cent or V maximum volumes and the estimated fluid contC'nt of these lungs. The mechanisms responsible for the high

In

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261

surface tension in the lungs of the fluidinstilled lambs must still be determined. Lambs over 115 days' gestation have been reported to have lungs with normally low surface tension,17 attributed to the presence of a specific lipoprotein or "surfactant" at the alveolar surface. This lipoprotein is thought to be produced by the alveolar cells, I S and to be expended at the alveolar surface during ventilation. 19 All of our lambs were thought to be over 115 days' gestational age, and normal lung surface tension at delivery was demonstrated in lambs 11 A, lIB, 17 A, and 19A. The finding of high surface tensions in the fluid-instilled lambs that did not die immediately after birth suggests that some mechanism was operating in thes(~ lambs during the neonatal period to produce a deficiency of surfactant at the alveolar lining. Such an acquired deficiency of surfactant might result from the following mechanisms: (1) a decrease in production, (2) an increased rate of removal or utilization, and ( 3) an inhibition or inactivation of surfactant. Finley 211 has reported that an impaired pulmonary circulation may result in decreased surfactant. He has also noted that saline washing is injurious to alveolar cells .~ l In the present study, either of these mechanisms may have produced alveolar cell injury which resulted in a decreased production of smfactant. In earlier studies, we found that foam production occurred in fluid-instilled lungs that were ventilated. 2 Foam affords a large air-fluid surface for the absorption of surfactant, a nd this could result in the physical removal of large quantities of this lipoprotein from the alveoli. Such an increased rate of removal would soon leave the alveoli depleted of surfactant. Plasma is known to contain inhibitors of surfactant, such as various lipid fractions.~~ We have found that in excised lungs which are not subsequently ventilated, instilled plasma produces striking elevations in lung surface tension properties, whereas the effect of amniotic fluid is not as marke(F ~ In the present experiments, plasma exudation might

262

May D , 196:' Alii . ] . Obst. & Gyncc.

Johnson and Faridy

have occurred as a result of an increase in capillary permeability secondary to hypoxia/I or an altered osmotic gradient across the alveolar membrane secondary to the presence of intra alveolar fluid ."'; Quite possibly all three of these mechanisms contributed to the changes noted in the present study. However, the rapidity and potency with which plasma inactivates surfactant suggests that the exudation of plasma may have been the main mechanism responsible for the alterations in surface activity and in the pressure-volume characteristics. In ascertaining the clinical significance of the changes seen in these lambs, it would be of interest to determine if an increase in pulmonary fluid can occur spontaneously. In the fetus, an increase in intraalveolar fluid could result from a transudation into alveolar spaces or from intrauterine respiratory efforts. Transudation has been demonstrated in the lung of the lamb/ and it may occur in the human fetus . Respiratory movements of the fetus have also been noted. 2fl • 2, In such cases, fluid might be aspirated, but the elastic recoil of the fetal lung might result in a return to the original volume and prevent a net increase in fluid content. However, the results of our saline studies (Fig. 7), which are in general agreement with the findings reported by Agostoni and his associates 28 for newborn animals and by Clements and co-workers 29 for the newborn infant indicate that fluid instilled into the lung is not completely expelled by elastic recoil. By inference, fluid introduced by fetal intra-

uterine respiratory movements might not be completely expelled in utero. This study was designed to test the hypothesis that increased fluid in the lung of the newborn predisposes to the respiratory distress syndrome. The results are compatible with this hypothesis, and suggest that the careful removal of fluid from the airways of the newborn at delivery:lOmay be a desirable preventive measure. Summary

Further studies III newborn Iambs have demonstrated that the pulmonary instillation of amniotic fluid produces a respiratory disturbance very similar to that previously observed after the instillation of allantoic fluid. These lambs exhibited labored respirations with a high incidence of spontaneous death, and their lungs were found to have elevated surface tensions and microscopic atelectasis. In addition, the fluid-instilled lambs were found to have lunr,,'S with increased retractive forces that appeared to be related to elevations in lung extract surface tensions. The possible mechanisms by which excess intraalveolar fluid might produce these changes and their relevance to the respiratory distress syndrome of the human newborn have been discussed. We wish to thank Dr. John G . McAfee for the J Ial-IISA determinations and Dr. R. L. Riley for reviewing the manuscript. In addition, we would like to acknowledge the assistance of Mr. S. Przyborowski, Mr. H. West, and Miss J. Himmelhcbcr in the preparation and carc of the animals.

REFERENCES

1. Johnson, J. W. C., Salem, E., and Holzman, G. B.: AM. J. OBST. & Gy;o.;EC. 89: 481, 1964. 2. Johnson, J. W. C., Permutt, S., Sipp!e, J. H., and Salem, E. S.: J. Appl. Physio!. 19: 769, 1964. 3. Bloor, W. R.: J BioI. Chern . 77: 53 , 1928. 4. Fiske, G. H., and Subbarow, Y.: J BioI. Chern. 66: 375, 1925. 5. Zlatkis, A., Zak, B., and Boyle, A. J : J. Lab. & Clin. Med. 4: 486, 1953 . 6. Harned, H. S., Rowshan, G., MacKinney,

7. 8.

9. 10.

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Respiratory distress

Volume 92 :-
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