Surfactant decreases pulmonary vascular resistance and increases pulmonary blood flow in the fetal lamb model of congenital diaphragmatic hernia

Surfactant Decreases Pulmonary Vascular Resistance and Increases Pulmonary Blood Flow in the Fetal Lamb Model of Congenital Diaphragmatic Hernia l3y Stuart J. O’Toole,

Hratch L. Karamanoukian, Frederick C. Morin III, Bruce A. Holm, Edmond Richard G. Azizkhan, and Philip L. Glick Buffalo, New York

l Introduction: Experiments using animal models of neonatal respiratory distress syndrome have shown a decrease in pulmonary vascular resistance (PVR) with surfactant replacement, whereas studies with the lamb model of congenital diaphragmatic hernia (CDH) have demonstrated improvement in oxygenation and lung mechanics with this therapy. The aim of the present study was to measure the effects of surfactant replacement therapy on the pulmonary hemodynamics of the lamb model of CDH. Methods: Ten lambs with surgically created CDH and five control iambs were instrumented at term, with the placental circulation intact. Ultrasonic flow probes were positioned around the main pulmonary artery and the common origin of the left and right pulmonary arteries to record total lung and main pulmonary artery blood flow. Catheters were inserted to record systemic, pulmonary, and left atrial pressure. Five CDH animals received 50 mg/kg of surfactant by tracheal instillation just before delivery. All 15 animals were then ventilated for 4 hours. Results: Correcting the surfactant deficiency in the CDH lamb resulted in a significant increase in pulmonary blood flow, a decrease in PVR, and a reduction in right-to-left shunting. These improvements in hemodynamics were associated with a significant improvement in gas exchange over 4 hours. Conclusion: The fetal lamb model of CDH has elevated PVR in comparison to controls. Prophylactic surfactant therapy reduces this resistance and dramatically increases pulmonary blood flow while reducing extrapulmonary shunt. A surfactant deficiency may be partially responsible for the persistent pulmonary hypertension in neonates with CDH. Copyright o 7996 by W. B. Saunders Company

INDEX WORDS: nary hypertension,

Congenital surfactant.

diaphragmatic

hernia,

pulmo-

T

HE MORTALITY RATE associated with congenital diaphragmatic hernia (CDH) has remained unchanged despite advances in neonatal intensive care and the advent of extracorporeal membrane oxygenation. Q Traditionally, the high mortality rate has been attributed to the presence of a lethal degree of pulmonary hypoplasia.3,4 In recent years, persistence of the fetal circulatory pattern and the development of pulmonary hypertension have been recognized as critical pathophysiological factors in the prognosis of CDH. G Morphometric studies have shown not only that CDH lungs are hypoplastic but also that they have decreased cross-sectional area of the vascular bed.7 Some investigators have suggested that the critical determinant of survival in these babies relates to the degree to which the crosssectional area of the pulmonary vascular bed is Journaloffediatric

Surgery,

Vol31,

No 4 (April),

1996: pp 507-511

A. Egan,

reduced.8 However, there is also strong evidence for anomalous development of the surfactant system in CDH9-l6 and increasing evidence to suggest that a surfactant deficiency can influence pulmonary hemodynamics. Studies in babies and baboons with neonatal respiratory distress syndrome indicate that surfactant replacement therapy can decrease pulmonary vascular resistance (PVR) and increase pulmonary blood flo~.‘~-~OTherefore, we hypothesized that the surfactant deficiency present in CDH may be partially responsible for the elevated PVR and reduced pulmonary blood flow associated with CDH.21-23 The present study was designed to determine the pulmonary hemodynamic effects of correcting the surfactant deficiency present in the fetal lamb model of CDH. MATERIALS

AND METHODS

Study Design The study was approved by the State University of New York at Buffalo Laboratory Animal Care Committee. Three groups of lambs were studied (five in each group). Ten lambs had diaphragmatic hernia created in utero, at 78 days’ gestation, and were instrumented at term with the placental circulation intact. Group 1 (CDH) did not receive surfactant therapy, Group 2 (CDH + Surf) received surfactant before delivery. Group 3 (control) was instrumented to demonstrate the status of lambs without CDH.

Creation of the Animal Model The lamb model was created as described previously.” With the ewe under general anesthesia, a left-sided diaphragmatic hernia was created surgically at 78 days’ gestation. The pregnancy was continued to term.

From the Buffalo Institute of Fetal Therapy (BIFT), The Children’s Hospital of Buffalo, and the Departments of Surgery, Pediatrics, and Obstetrics and Gynecology, State University of New York at Buffalo, School of Medicine and Biomedical Sciences, Buffalo, NY. Presented at the 42nd Annual International Congress of the British Association of Paediattic Surgeons, Shefield, England, July 25-28, 199.5. Supported in part by grants from the American Lung Association, the Women and Children’s Health Research Foundation, National Institute of Health grants (HL 49977, HL 36543), and the U.S. Surgical Corporation, Norwalk, CT. Address reprint requests to Phi&p L. Giick, MD, The Buffalo Institute of Fetal Therapy, Department of Pediatric Surgety, The Children’s Hospital of Buflalo, 219 Bvant St, Buffalo, NY14222. Copyright o 1996 by W B. Saunders Company

0022-3468/9613104-~0009$03.00l0 507

508

O’TOOLE

Surgical Instrumentation At 141 days’ gestation, the ewes underwent a second hysterotomy. The head and upper torso of the fetus were delivered, and an endotracheal tube (internal diameter, 4 mm) was inserted and secured via a tracheostomy. The tube was occluded to prevent breathing movements. Polyvinyl catheters were threaded into the right internal carotid artery and right jugular vein. The left mediastinum was exposed via a thoracotomy in the fourth intercostal space. Catheters were placed in the pulmonary artery and the left atrium. In the fetal lamb, the left and right pulmonary arteries originate from a common origin that is approximately 1 cm long.24 This enabled us to position two ultrasonic transit time flow transducers (Transonic Systems Inc, Ithaca, NY)-one around the common origin of the left and right pulmonary artery (proximal to the bifurcation) to measure total lung blood flow, the other around the main pulmonary artery to measure right ventricular output. Infasurf (50 mg/kg) (Ony Inc, Buffalo, NY) was instilled into the trachea of five animals in the CDH group. The umbilical cord was tied and the animal delivered. The lamb was dried, weighed, and placed on a warming blanket beneath a radiant warmer wrapped in plastic. A rectal thermistor was positioned to continuously monitor body temperature, which was maintained at 39°C 2 2°C.

Table

1. Mean Body

Weight

ET AL

and Lung Weight

Body Weight (kg)

Lung Weight(g)

CDH (n = 5) CDH + Surf (n = 5)

3.82 + 0.32 3.51 k 0.36

55 k 4.8 53 -t 4.5

Control

3.74 + 0.54

*Control

(n = 5) versus

CDH and versus

92 f 21*

CDH + Surf,/’

= ,026.

All data are expressed as mean ? standard error (SEM). The groups were compared using analysis of variance, with time as a dependent variable. Individual time points were compared using unpaired f tests. The control group was not included in the statistical analysis, and data are provided for comparison only. RESULTS

There were no significant differences in body weight between the groups (Table 1). The lung weights were similar between the CDH and CDH + Surf groups, but the lung weights of controls were significantly higher (Table 1). All five CDH + Surf lambs survived *

Neonatal Resuscitation Protocol The CDH and CDH + Surf lambs were ventilated at a rate of 60 breaths per minute, at a peak inspiratory pressure of 30 cm Hz0 and a positive end-expiratory pressure (PEEP) of 4 cm Hz0 throughout the experiment (Servo ventilator 900~; Siemens Elema, Solna, Sweden). The control lambs were ventilated at these settings initially, but subsequently the rate was adjusted according to arterial Pco2. Tris Hydroxymethyl amino methane (THAM) was administered to maintain a base deficit of greater than -5 mEq. Blood glucose was monitored every 30 minutes. Anesthesia and paralysis was maintained with ketamine (8 mg/kg intravenously) and pancuronium bromide (0.1 mgikg intravenously).

180 160 s

140-

=

izoioo-

E 4-l 802

6040ZO-

1

1407

T

T

-

Measurements Phasic pulmonary artery pressure (PAP), left atria1 pressure (LAP), systemic blood pressure, superior vena cava pressure, and pulmonary artery and total lung blood flow were monitored continuously (Physiologic Amplifier Recorder System, Gould Electronics, Cleveland, OH). Mean pressures and flows were measured every 30 minutes, and these values were used for subsequent calculations. In addition, carotid and umbilical arterial blood was analyzed for pH, Paco2, and Pao2 every 30 minutes (Acid Base Laboratory 3; Radiometer Medical A/S, Copenhagen Denmark). Baseline measurements were taken while on the placental circulation and every 30 minutes thereafter up to 4 hours.

4oJ 7.31

*

*

T

T

*

*

*

Calculations and Statistical Analysis PVR was calculated, per 100 g of lung weight, according to the formula: PAP - LAP PVR = Total lung blood flow/l00 g lung

1

6.8

,

/ 0

The percentage of blood flow from right to left across the ductus arteriosus was calculated as follows: % Ductal shunt= Right venticular output - Total lung blood flow x 100 Right ventricular output

I

I 60 TIME

/

I 120 IN MINUTES

I

/ 180

1

1 240

Fig 1. Preductal arterial blood gases and pH. *CDH + Surf versus CDH, P c .Ol. There are no significant differences at time 0. Arterial gases for the control lambs at 1 hour were: pH, 7.41 f 0.09; PO,, 407 -+ 30.2; Pco~, 28.8 + 7.6. These amounts did not vary significantly throughout the remainder of the experiment.

SURFACTANT

IMPROVES

PULMONARY

BLOOD

FLOW

509

IN CDH

F 600 .E E -: 500g

0.6h I 0.5f .E $ 0.4I"

z4003 m 8 300c 3 p 200-0 8 z ioo-

Eo.3F I

m 0.28 r g O.l-

0

30

60

90 Time

120 150 in minutes

180

210

240

I 90 Time

1 aFig I!. Pulmonary blood flow standardized per 100 g of lung weight. There were no significant differences between the groups at time 0. Correcting the surfactant deficiency present in CDH results in pulmonary blood flow (per 100 g of lung weight) that is comparable to that of control animals and three times that of untreated animals. *CDH versus CDH + Surf, P < .Ol. The control group has normal values of pulmonary blood flow normalized for lung weight.

until the end of the experiment, as did all the control lambs. Of the CDH lambs that did not have surfactant, three lambs survived the 4 hours, one died after 210 minutes, and one died after 150 minutes. The latter two lambs had severe hypoxemia, hypercapnia, and respiratory acidosis at the time of death. The CDH + Surf group had significant improvement in Paz and Pcoz compared with the untreated CDH group (Fig 1). Untreated CDH animals had profound respiratory acidosis (Fig 1) after aggressive correction of the base deficit. Blood flow per 100 g of lung weight was similar for CDH + Surf and control lambs (Fig 2). Their pulmonary blood flow value was three times that of the untreated animals. This increase in blood flow was associated with a dramatic decrease in PVR (Fig 3). This decrease in PVR led to a reduction in right-to-left shunting of blood across the ductus arteriosus (Fig 4). In the CDH + Surf group 90% of the right ventricular output went to the lungs. In control lambs, as expected, there was a large left-toright shunt of blood across the ductus arteriosus. DISCUSSION

The prognosis of babies with CDH has been associated with the. degree of pulmonary hypoplasia present and with the subsequent development of pulmonary hypertension .3-8The pattern of persistent fetal circulation occurs if the pulmonary vascular bed is incapable of accepting the right ventricular output at birth. It is these infants who will continue to have

CDH

--h-

I

I

120 150 in minutes

Cont

-o-

I 180

I 210

CDH+Surf

1

240

1

Fig 3. Pulmonary vascular resistance (PVR) standardized per 100 g of lung weight. There were no significant differences at time 0. Surfactant replacement resulted in a significant decrease in PVR (compared with the untreated group). *CDH versus CDH + Surf, P s: .Ol. The control group has normal values of PVR normalized for lung weight.

pulmonary hypertlension after birth.s However, the absolute size of the pulmonary vascular bed is not the only factor that determines whether it can accept right ventricular output at birth. Therefore it is not the sole arbiter of which infants will have a poor prognosis. Our results indicate that by correcting the surfactant deficiency present in the fetal lamb model of CDH, we can dramatically decrease PVR, increase pulmonary blood flow, and reduce the amount of

;

'g

20

22

0 -20

$2: I-0,

-40 -60

0 I-----a-

30

60

LDH

150

180

Time in minutes -oCDH+Surf

90

120

A--

210

240

Cont

Fig 4. Right-to-left ,flow of blood across the ductus arteriosus, expressed as a percentage of pulmonary artery flow. The large standard errors reflect the dramatic changes in central blood flow that occur at birth in controls, and to a lesser extent in CDH + Surf animals. Untreated CDH lambs show no such change. *CDH versus CDH + Surf,P< .Ol.

510

O’TOOLE

pulmonary artery flow shunting across the ductus arteriosus. These changes in blood flow are associated with a significant improvement in oxygenation and ventilation in this model.20 The CDH lamb model has been found to have the same degree of alveolar vascularization as the controls. This indicates that pulmonary capillary load normalized for lung mass should be similar for the two groups. 25 If the entire capillary bed can be recruited for gas exchange, pulmonary hypertension, hypoxemia, and hypercarbia should be improved. In the normal fetus, there is a dramatic decrease in PVR with the onset of ventilation at birth. This is caused by an increase in oxygen tension and mechanical distension of the 1ung.26-29Correcting a surfactant deficiency is known to increase total lung capacity (TLC), improve alveolar stability, and increase compliance.30-32It is likely that these improvements in lung mechanics result in better alveolar recruitment, ventilation, and alveolar PO*, which lead to an increase in

ET AL

pulmonary blood flow. Recent work with surfactant replacement therapy in the lamb model of CDH has confirmed that lungs treated prophylactically with Infasurf have increased alveolar recruitment, reduced ventilator injury, and greater TLC.2o,33 These results demonstrate that persistent pulmonary hypertension in the CDH lamb model is caused in part by a surfactant deficiency. This deficiency prevents adequate alveolar recruitment, which leads to alveolar hypoxia and elevated PVR.33 Therapies aimed at correcting the surfactant deficiency16 or reducing surface tensioS4 should improve alveolar ventilation and decrease the incidence of persistent pulmonary hypertension, We speculate that in many neonates with CDH there is actually adequate pulmonary parenchyma for survival.35 However, the dysfunction of the surfactant system prevents the efficient use of the available lung tissue and increases the incidence of persistent pulmonary hypertension.

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SURFACTANT

IMPROVES

PULMONARY

BLOOD

FLOW

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IN CDH

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