Plasma prostanoids in neonates with pulmonary hypertension treated with conventional therapy and with extracorporeal membrane oxygenation

J

THORAC CARDIOVASC SURG

1991;101:973-83

Plasma prostanoids in neonates with pulmonary hypertension treated with conventional therapy and with extracorporeal membrane oxygenation Tbromboxane may be a mediator of pulmonary hypertension in the neonate. Acute thromboxanemediated pulmonary hypertension has been described in sheep receiving extracorporeal membrane oxygenation, which raises concerns about a potential thromboxane-mediated exacerbation of p~ nary hypertension in human neonates with severe pulmonary hypertension who are treated with extracorporeal membrane oxygenation. We measured plasma levels of thromboxane, prostaglandin F2m and 6-keto-prostaglandin F la in infants with pulmonary hypertension, some of whom were treated medically and some of whom were treated with extracorporeal membrane oxygenation. Plasma levels of all three prostanoids were elevated in infants with pulmonary hypertension and decreased with time, whether the neonates were treated with extracorporeal membrane oxygenation or with medical management alone. In infants treated with extracorporeal membrane oxygenation, we collected samples simultaneously from preoxygenator sites, postoxygenator sites, and umbilical artery catheter. We could demonstrate no significant difference in plasma prostanoid levels across the oxygenator. In two patients, plasma thromboxane and prostaglandin F2a levels measured shortly after a platelet transfusion were distinctly higher in the umbilical artery catheter than in venous samples.

Kim Chi Bui, MD, Cathy Hammerman, MD, Ronald B. Hirschl, MD, Valery Hill, BS, Sandy M. Snedecor, MLS, Robert Schumacher, MD, and Robert H. Bartlett, MD, Chicago, Ill .. and Ann Arbor, Mich.

Rrsistent pulmonary hypertension of the newborn (PPHN) is a severe condition that affects both term and preterm infants and is associated with significant mortalityand morbidity.!" It is characterized by pulmonary vasoconstriction in early postnatal life with pulmonary artery pressure equal to or above systemic pressure, I, 2 resulting in right-to-left shunting of deoxygenated blood at the patent foramen ovale or patent ductus arteriosus with consequent desaturation of arterial blood. In the absence of a specific pulmonary vasodilator, medical management of these infants is both difficult and From the Department of Pediatrics, Wyler Children's Hospital, University of Chicago, Pritzker School of Medicine, Chicago, 111., and Department of Surgery, University of Michigan, Ann Arbor, Mich. Received for publication Nov. 21,1989. Accepted for publication May 23, 1990. Address for reprints: Kim Chi Bui, MD, Division of Neonatology and Pediatric Pulmonology, Childrens Hospital Los Angeles, 4650 Sunset Blvd., Bin No. 83, Los Angeles, CA 90027. 12/1/23651

challenging."? Extracorporeal membrane oxygenation (ECMO) has improved survival among infants who do not respond to conventional therapy.f'" Thromboxane and leukotrienes have been implicated as mediators of pulmonary hypertension in animal modelsl"?! and in human infants. 22.24 Thromboxane-mediated pulmonary hypertension has been described after initiation of ECMO in sheep,25-2? raising concerns about a potential thromboxane-mediated exacerbation of pulmonary hypertension in the human infant with PPHN who undergoes ECMO. We undertook this study to describe the evolution of plasma levels of the vasoconstrictors thromboxane B2 (TxB2) and prostaglandin F 2a (PGF2a) and of the vasodilator prostacyclin (6-keto-PGF l a ) in patients with PPHN who were treated with ECMO and in patients with PPHN who were treated medically. We started with two major hypotheses. (1) Infants with PPHN who are treated medically will show a decrease in plasma levels of vasoconstrictor over time, with improvement in their conditions. (2) Infants with PPHN who are treated with EeMO will show an acute increase in vas973

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Age in Days Fig. 1. Plasma prostanoids in infants with PPHN who were treated medically . A, TxB 2 . B, 6-Keto- PG Fla' C, PG F2a . Serial data points for the same patient are joined by a line. The shaded area is the upper limit of the normal range.

Fig. 1. Coot'd. Plasma prostanoids in individual patients treated with ECMO, in relation to postnatal age. D, TxB 2 . E, 6-Keto-PGF]a. F, PGF2a . The first data point is the baseline (before ECMO), the second data point is at 3 days of ECMO, and the third data point is after ECMO.

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Table I. Clinical data Birth weight Patient I

2 3 4 5 6 7 8 9 10

Sex F M F M M F F M

12 13 14

M F F F F M

1 2 3 4 5 6 7 8 9 10

M M M F F F M M F M

II

(kg)

Gestational age (wk)

Apgar scores

Diagnoses

Clinical characteristics of patients with PPHN treated with conventional therapy 2.8 HMO 35 8/8 3.76 NA 40 MAS 3.13 42 Perinatal asphyxia 1/2 3.07 40 MAS 5/8 2.81 Congenital anomalies 40 3/6 2.61 37 Idiopathic PPHN 1/6 2.86 40 MAS 1/7 3.26 Abruptio placentae, birth asphyxia, 40 0/3 encephalomalacia, blood aspiration 2.4 34 PNTX, pneumopericardium 7/8 2.43 GBSS 38 3/5 3.02 43 MAS 3/6 NA 2.06 35 HMO, born in toilet bowl 3.33 43 MAS, perinatal asphyxia 0/1 2.89 37 GBSS 9/9 Clinical characteristics of patients with PPHN treated with ECMO 2.7 37 7/5 COH 3.3 34 9/9 Idiopathic PPHN 3.7 41 6/7 MAS 3.9 38 7/8 Septic PPHN 3.74 38 5/7 Pneumonia 1/3 MAS 3.4 41 2.32 35 1/3 COH, MAS 3.76 41 1/8 MAS 3.21 41 1/8 MAS, GBSS 3.96 42 1/8 MAS

HMO, Hyaline membranedisease; NA,not available;MAS, meconium aspiration syndrome; PNTX, pneumothorax;GBSS. group Bstreptococcus sepsis: CDH. congenital diaphragmatic hernia.

oconstrictor levels after initiationof ECMO, followed by a decreaseovertime with resolution of PPHN, similarto that of infants treated medically. Methods Patient population. Newborn infants of birth weight greater than 2 kg and gestational age of at least 34 weeks were entered in the study after parental consent was obtained. There were two study groups. Patients with PPHN who were to be treated medically were enrolled in the study at the University of Chicago, where ECM0 was not an option. Fourteen patients with PPHN were enrolled between November 1983 and May 1987. Nine infants had serial determinations of prostanoid levels and five had a single determination. Assays for all three prostanoids were not done on all samples because of the limited volume of the blood samples. All blood samples were drawn from the umbilical artery catheter (UAC). Patients with PPHN who were to be treated with ECMO were enrolled in the study at the University of Michigan, Ann Arbor, in accordance with that institution's ECMO entry criteria." Eleven patients were enrolled in the ECMO group between November 1986 and May 1988. One patient experienced necrotizing enterocolitis and required bowel resection while undergoing ECMO. Because of his rather unusual clinical situation, we decided to exclude that patient's data from

analysis, We therefore were left with 10 patients in the ECMO group. Baseline blood samples were drawn before cannulation from the UAC and from the blood prime in the circuit. After bypass was initiated blood samples were drawn from the preoxygenator and postoxygenator sites and from the UAC at 10 minutes, 30 minutes, 1 hour, 2 hours, 4 hours, 8 hours, 24 hours, and then daily while the patient was undergoing ECMO. Blood samples were also collected from the UAC in the first 24 hours and at 48 hours after ECMO had been terminated. Nine of the 10 patients in the ECMO group underwent ECMO for at least 3 days. We noticed no significant change in plasma prostanoid levels in the patients who continued to be treated with ECMO on days 4,5,6, or 7, and we therefore chose to report only data obtained before ECMO, during the first 3 days of ECMO, and after ECMO. Six patients had one data point missing because of mishandling of samples. Patient 4 in the ECMO group had a complete set of UAC samples but no preoxygenator and postoxygenator samples drawn before 8 hours of ECMO, and was therefore not included in the comparison of plasma prostanoid levels among sampling sites. Each sample of blood (1.5 ml) was drawn into a heparinized syringe when the patient was not undergoing bypass and into a nonheparinized syringe when the patient was heparinized during bypass. The blood was immediately placed in a plastic tube containing indomethacin in a final concentration of 2 X 10- 5 mol/L and kept on ice until it was centrifuged at 2000 g for 10 minutes at 4° C. The supernatant plasma was then separated

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Plasmaprostanoids in PPHN 977

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Table II. Plasma levels of TxB 2 from UAC, venous, and postoxygenator samples (means for patients 3 through 10) Baseline

10 min

30 min

I hr

2 hr

4 hr

8 hr

24 hr

2 days

3 days

After ECMO

330 ± 68 275 ± 48 338 ± 25 426 ± 31 463 ± 35 351 ± 79 268 ± 50 209 ± 68 130 ± 23 163 ± 8 115 ± 33 (n = 7) (n = 7) (n = 7) (n = 7) (n = 6) (n = 6) (n = 6) (n = 6) (n = 7) (n = 7) (n = 7) Venous (pgjml) 196 ± 16 281 ± 36 266 ± 38* 280 ± 48* 334 ± 71 270 ± 49 194 ± 37 185 ± 23 133 ± 22 (n = 7) (n = 7) (n = 7) (n = 7) (n = 6) (n = 5) (n = 5) (n = 5) (n = 6) Postoxygenator 185 ± 32 273 ± 23 333 ± 38 338 ± 68 337 ± 80 264 ± 55 174 ± 43 243 ± 74 146 ± 15 (n = 7) (n = 7) (n = 7) (n = 6) (n = 5) (n = 6) (n = 5) (pgjml) (n = 7) (n = 7)

UAC (pgjml)

Values arc mean ± standard error of the mean. 'Venous level of TxB 2 was lower than UAC level of TxB 2 at (p

< 0.05).

and frozen at -60 0 C until the assay was performed. Samples collectedat the University of Michigan were sent packed in dry ice to the University of Chicago by overnight delivery. The diagnosis of PPHN was established by the presence of one or more of the following criteria: 1. A positive hyperoxic hyperventilation test in which the infant's lungs were ventilated with 100% inspired oxygen concentration to a critical arterial carbon dioxide tension (generally less than 30 torr, occasionally less than 20 torr); a positive test was defined by an increase in arterial oxygen tension (P02) to at least 100 torr 2. Proof of a transductal right-to-left shunt through simultaneous measurement of pre- and postductal arterial blood samples,in which a difference in arterial P0 2 > 15 torr was found 3. Positive echocardiographic Doppler study or contrast echocardiogram with 2 ml saline solution injected into the umbilical or peripheral vein; the study was considered positive if the Doppler results delineated a right-to-left bloodflowshunt, or if microbubble contrast was visualized simultaneously in the right ventricular outflow tract and the left atrium 4. Cardiac catheterization revealing pulmonary artery or right ventricular pressures increased above systemic pressures Clinical characteristics. Pertinent clinical data, such as birth weight, gestation, diagnoses, and Apgar scores, were recorded. Age at ECMO, criteria for ECMO with pertinent oxygenationindexes,last arterial bloodgas values before bypass, and the duration of bypass were also recorded. We recorded platelet counts and arterial P02 corresponding to each prostanoid sampling time. Alveolar-arterial oxygen gradient (A-aDo2) was calculated from the arterial blood gas values obtained at each sampling time in the infants treated medically and before ECMO in infants treated with ECMO. ECMO procedure. Venoarterial perfusion was used for all patients in the ECMO group. The techniques have been previously described.v 28 All patients received a I-unit platelet transfusionafter initiation of bypass and were systemically heparinizedfor the duration of ECMO. The patients were weaned from ECMO support gradually, with mixed venous oxygen saturation maintained above 75%. Radioimmunoassay. Plasma TxB 2, 6-keto-PGF lm and PGF2a weredetermined by radioimmunoassay performed at the University of Chicago. Duplicate plasma samples were incubated with tritiated tracer (Du Pont Diagnostic Imaging Division, North Billerica, Mass.) and TxB 2, PGF2m and 6-keto-

PGFjaantisera (Seragen Diagnostics, Indianapolis, Ind.) for 16 to 20 hours at 4 0 C. The free ligand was then adsorbed with dextranjcharcoal solution, and the radioactivity of the supernatant was measured with a liquid scintillation counter. Standard concentrations ofTxB2, PG F2a,and 6-keto-PGFla (Sigma Diagnostics, S1. Louis, Mo.) were used to generate standard curves by logit transformation. The standards were diluted in plasma protein fraction (Plasmanate) to simulate plasma with minimal basal prostaglandin levels. Nonspecific binding in Plasmanate most closely approximates that of neonatal plasma (9.6% in Plasmanate and 9.4% in neonatal plasma, compared with 13.1% for adult plasma and 4.6% for buffer). The accuracy of this method for detection of a known amount of prostaglandin added to Plasmanate ranges from 92% to 114%. Intraassay and interassay coefficients of variation were 5% to 15% and 11% to 15%, respectively. Minimum detectable level ofTxB2 and PGF2a was 25 pgjml; thatof6-keto-PGF la was 50 pgjml. Values reported were the means of duplicate determinations performed on each sample. For values below our limit of detection, we assumed values of 25 pgjml for TxB 2 and PGF2a and 50 pgjml for 6-keto-PGF la in subsequent data analysis. Plasma levels of TxB2 and 6-keto-PGF ta were measured in all patients in the ECMO group. Plasma levels of PGF2awere measured in only the first six patients in the ECMO group. We attempted to measure PGE2leveis in both the PPHN and ECMO groups, but these levels were below the limit of detection of our assay, which was 50 pgjml. Obviously, normal infants do not have UACs placed, so "normal" arterial levelscannot be defined. Normal plasma levels reported for TxB 2 in human adults are variable: 39 ± 24 pgjmlaccordingtoMcCannandcoworkers,29180 ± 70pgjml according to Davies and associates,30 or less than 100 pgjml according to Sell and coworkers." Published normal values for PGF2a in adults are 83 ± 13 pgjml (range, 62 to 102 pgjml).32 Normal values reported by Kaapa and colleagues'? for 6-ketoPGF ja in infants are 397 ± 25 pgjml the first day of life, decreasing to 191 ± 20 pgj ml by 1 week of age. Normal plasma levelsobtained in our labora tory by venipuncture of normal infants less than 1 week old are 224 ± 140 pgjml for 6keto-PGF lm with a range from less than 25 to 447 pgjml (n = 17); for PGF2a, normal values range from less than 25 to 178 pgjml, with a median less than 25 pgjml (n = 16). We have attempted to define normal values for TxB 2 in neonates; however, we believe that they were falsely elevated because the peripheral venous sampling technique, which used a tourniquet, a small 25-gauge needle, and sporadic squeezing of the extremity, possibly caused hemolysis, local tissue hypoxia, endothelial

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Surgery

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Fig. 2. Positive correlation between plasma TxB2and A-aDo 2. The lines join serial samples in the same patient. Overall, TxB 2 decreases with decreasing A-aDo2. (r = 0.663, P < 0.001). injury, and platelet activation, with TxB 2 production and release. Plasma levelsof TxB 2 in stable infants after ECMO but before the UAC was removed ranged from less than 25 to 127 pgjml. Levels in recovering infants with PPHN who had an A-aDo 2less than 150 were 66 to 125 pgjml and comparable to normal values reported in the literature. Statistical methods. Data are presented as mean ± the standard deviation, exception the figures, which represent data as mean ± the standard error of the mean. Student's t test was used to compare plasma prostanoids in the ECMO group from different sites at each time studied and changes over time, with multiple-comparisons correction. Statistical significance was accepted at a p value of less than 0.05. Correlations between plasma TxB2 and A-aDo 2, plasma PGF2a and A-aDo 2, TxB 2 and arterial P02, plasma PGF2a and arterial Po 2,and TxB 2 and platelet count were sought by least-squares linear regression.

Results Clinical characteristics. Clinical characteristics of patients with PPHN are summarized in Table I. Birth weight was 3.40 ± 0.54 kg for patients in the ECMO group and 2.89 ± 0.43 kg for medically treated infants with PPHN. Gestational age was 39 ± 3 weeks for the ECMO group and 39 ± 3 weeksfor the medically treatedgroup.AgeatinceptionofECMOwas44 ± 47 hours (range, 17 to 174 hours), and duration of ECMO was 119 ± 41 hours (range, 73 to 192 hours). Oxygenation index before ECMO was 56 ± 19. A-aDo2 before ECMO was638 ± 14.A-aDo2 was 623 ± 27 at diagnosisfor medically treated patients with PPHN whounderwent initial study at 1 day of age or earlier. Medically treated infants with PPHN. Infants with PPHN whowere treated medically had high plasma levels of TxB2, 6-keto-PGFjw and PGF2" initially. The levelsof all three prostanoids decreased with time (Fig. 1, A through C) as lung recovery progressed. Thromboxane

levels decreased with decreasing A-aDo2 (Fig. 2, r = 0.663, n = 22,p < 0.001).The correlation coefficient between TxB2and arterial P0 2 was -0.253. The correlationcoefficient between PGF2" and A-aDo2was0.29,and the correlation coefficient between PGF2" and arterial P02 was -0.167. Infants undergoingECMO therapy. Arterialsamples from infants treated with ECMO showed a transientrise inplasmalevels ofTxB2duringthe first2 hoursofECMO therapy (Fig. 3). Otherwise, individual patients in the ECMO group showed a decreaseovertime of UAC plasma levels of TxB2, 6-keto-PGFj" and PGF2" (Fig. 1, D through F). Mean level of TxB2 after ECMO was 70% lowerthan levels beforeECMO (p < 0.01). Mean levels of 6-keto-PGF1" at 3 days and after ECMO were lower than levels before ECMO (p < 0.05). Although plasma levels ofPGF2" after ECMO in individual patients were 54% to 98% lower than baseline values obtained before ECMO, mean levels ofPGF 2" during and after ECMO were not statistically different from baseline values becauseof the wide range in observed baseline values. There were no significant differences in our ECMO population among samplesfrom the three sampling sites for any of the three prostanoids (Fig. 3). However, when we exclude patients 1 and 2 (as explained below), the mean UAC plasma level of TxB2 in the remaining ECMO population (n = 7) at 1 and 2 hours was higher than that of venous samples (p < 0.05, Table II), although not significantly differentfrom that of postoxygenator samples. These data suggest that there may be somereleaseof TxB2 in the circuit in additionto production or release by the babies' heart and lungs. Abrupt elevations in TxB2 and PGF 2" at 1 hour and 2 days in Fig. 3 are accountedfor by twopatients (patients 1 and 2, Fig. 4), in whom samples were drawn within minutes of a platelet transfusion. Although platelet transfusions were given to all patients, only in patients I and 2 were samples drawn immediately after such a transfusion. The acute rise in PGF 2" and TxB2 in these two patientsoccurredin the UAC samplesand waslikely the resultof production in the lungs. There wasno correlation between level of TxB2 and either plateletcount or arterial P02 in our ECMO population. Discussion Previous studies have implicated thromboxane as a pulmonary vasoconstrictor.lv-l-" Elevated levels of 6-keto-PGFl" have been reported in hypoxic pulmonary vasoconstriction'v-" and in response to hyperventilation alkalosis.'? Our present data show that plasma levels of TxB2, PGF2w and 6-keto-PGF1" are initially elevated in

Volume 101 Number 6 June 1991

Plasma prostanoids in PPHN 979

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Fig. 3. Comparison of plasma levels of (A) TxB2 (n = 9), (B) 6-keto-PGF 1a (n = 9), and (C) PGF2a (n = 5) from UAC, venous, and postoxygenator sites. Time points are counted from initiation of bypass (time 0). Blood prime level was 70 ± 48 pgjml for TxB 2, 130 ± 33 pgjml for 6-keto-PGF 1a> and 25 ± 8 pgjml for PGF2a.

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2200 , - - - - - - - - - - - - - - - - , - - - - - - - - , 2000 1BOO

1600 1400 pg/ml 1200 1000 800 600 400 200

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minutes

hours

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Fig. 4. Effect of platelet transfusion on arterial plasma TxB2 and PGF2a levels in two patients undergoing treatment with ECMO. A, Patient I. B, Patient 2.

patients with PPHN and decreaseovertime as lung condition improves. We also observed a positive correlation between plasma levels of TxB2 and A-aDo2. Hyperventilation alkalosis was standard therapy in our unit in Chicago, and the decrease in plasma level of 6-keto-PGFla was probably associatedwith resolution of hypoxic vasoconstriction rather than reduction in lung stretch. We speculated that if thromboxane and PGF2amediate pulmonary hypertensionin neonates, then the most severely affected infants would have the highest circulating levels of these prostanoids. We enteredinto a collaboration with the University of Michigan, where infants with severe PPHN that did not respond to medical treatment were referred for ECMO. To our surprise, plasma prostanoid levels before ECMO in those moribund infants were not higher than those observedin our infants who responded to medical management. Of our 14 infants with PPHN who were managed medically, two responded to hyper-

ventilation alone, one received dopamine as the only vasoactive agent, four responded to tolazoline infusion alone, one responded to tolazoline and dopamine, three responded to tolazoline and epinephrine infusions, and three required tolazoline, dopamine, and epinephrine infusions. Of the sevendeaths, three wereattributed to a rapid pulmonary decompensation (one patient was referred to an ECMO center but died during transport) and four deaths were attributed to neurologic complications.This was not a randomizedstudy and we recognize its limitations. We compareda grouptreated medically in an institution where ECMO was not available (with a prospective mortality rate of 50%) to group treated with ECMO (with estimated 80%or greater prospective mortality rate), all of whom survived. The lung diseasecould have been lesssevere in the group of infants treated medically, sincemost of them responded to medicalmanagement, whereasthose in the ECMO group had uniformly failed to respond to medical therapy before ECMO rescue. We monitoredthe plasmalevels of all three prostanoids during and after ECMO, becausealterations in levels of thromboxaneand 6-keto-PGFla have been reportedduring cardiopulmonarybypass, both in animals26, 27, 38 and in human subjects,30. 39-44 with an acute pulmonary hypertension described after initiation of ECMO in sheep.25-27,45 In humans undergoing bypass for heart operations, circulating levels of 6-keto-PGFla increase immediatelyafter thoracotomy" and cannulationof the great vessels beforebypass,40-44 risefurther withinitiation of bypass, and decrease after bypass. Ylikorkala and coworkers" and Fleming and associatesf reported persistentlyhigh levels of 6-keto-PGFla throughout bypass, whereas Watkins and coworkers'?and Faymonville and colleagues" describeda progressive decrease in levels of 6-keto-PGFla during rewarming and reperfusion of the lungs. Higher levels were found with pulsatilethan with nonpulsatile bypass." This increase has been attributed to prostacyclin release from vascular endothelium in response to vessel wall injury and surgical manipulation of the heart structures rather than in response to bypass itself,sinceGreely and associates" showed similarintraoperative increases in plasma levels of 6-keto-PGFla in children with congenital heart disease who underwent palliative operations without bypass and those who had surgical repair with bypass. We did not observe an increase in levels of 6-keto-PGFla during ECMO. Instead, our samplesat 10 minutes had lowerlevels than pre-ECMO baselinesamples,although the difference was not statistically significant. Baseline (pre-ECMO) levels of 6-keto-PGFla in our neonates with PPHN were very high comparedto relatively lowbaselinevaluesdescribed

Volume 101 Number 6 June 1991

in children43,44 and adults39-42 with heart disease. Any effect of ECMO cannulation and perfusionon releaseof 6-keto-PGF 1a may have been minimized by the lesser vascularmanipulationor the use of partial bypassat normothermia instead of total bypass with hypothermia. It also may not have been demonstrable in our neonates becauseof their already high baselinevalues,in the range of peak values previously reported for patients undergoing bypass. We and others40,44 did not find a difference in concentration of 6-keto-PGF1a across the extracorporealcircuit.Ourpatients'Ievels of'e-keto-Ptff'c.at 3 days ofECMO werecomparableto baselinevaluesreportedby others.We speculatethat the decreasein levels of 6-ketoPGF'a during ECMO reflects resolution of pulmonary hypertension, similar to that in our patients with PPHN who were treated medically. TxB2 elevation has also been reported during cardiopulmonary bypass, both immediately after onset of bypass3o,42 and at the end of bypass during rewarming and lung reperfusion,40,41 with a return to baselineafter bypass. Stolar and colleagues'? reported an increase in plasma TxB2 at initiation of ECMO in an infant with diaphragmatic hernia and PPHN, whereas Sell and associates".noted no increasein plasmaTxB2in neonates undergoing ECMO. We observed an increase in plasma levels of TxB2 during the first 2 hours of ECMO in our population, although it was not statistically significant, followed by a gradual decreaseover time with continued ECMO support. This pattern of decrease in plasma levels of TxB2 over time was similar to that observed in infantswith PPHN who were treated medically. No significant difference was found between venous and left ventricular'? or arterial'? plasma levels in adults undergoing cardiopulmonary bypass. In contrast, our neonates with PPHN had higher mean arterial levels than venous levels at I and 2 hours of ECMO. A temporal relationship between decreased platelet count and elevation in plasma TxB2 was described in animal experiments.P?? and it has been suggested that TxB2 elevation resulted from platelet release and consumption from interaction with the extracorporealcircuit and particularly with the membrane oxygenator. However, Faymonville and coworkers'? showed that in a closedcircuit primed with freshhuman blood, thromboxanelevels remain undetectable even after 2 hours of bypass, suggesting that the membraneoxygenatordoes not playa direct role in productionof TxB2. We, like Faymonville and coworkers'? and Greely and colleagues.r' did not find a significant difference in plasma concentration of TxB2 across the extracorporeal circuit. All our ECMO-treated patients weregiven transfusions to maintain a plateletcountabove 70,OOO/mm3, whichmay have contributed to the general

Plasma prostanoids in PPHN 9 8 I

levels. However, two patients had blood samples drawn within minutes of a platelet transfusion. We observed in them an abrupt rise in plasma levels of TxB 2, accompaniedbya risein levels ofPGF2a. The higherlevels of'Txls, and PGF2a foundin UAC samples(compared with postoxygenator samples) suggest that these vasoconstrictors were producedby the baby. Since UAC bloodis sampled from the descending aorta after the blood from the ECMO circuit has mixed with the bloodthat had passed through the baby's own heart and lungs, one would assumethat the enrichmentin plasma level of TxB2 in the UAC samples comes from production or release by the baby's heart or lungs. This TxB2 gradient was also reported by Peterson and colleagues.l? who found higher aortic levels than pulmonary artery levels of TxB2 in sheepduring extracorporealperfusion. Cardiopulmonary bypass is associated with complement activation" and alteration in leukocyte and platelet function.Fr" with aggregationand sequestrationparticularly in the lungs. It is tempting to speculate that the elevationin TxB2 levels reflects a release by leukocytes and platelets sequestered in the lungs and vascular release from capillary injury. We studied these three prostanoidsbecause TxB2 has beenimplicatedas pulmonaryvasoconstrictor in previous studies and because PGF2a and 6-keto-PGF1a may be released under similar stimuli. We found that patients with PPHN had elevatedplasma levels of TxB2' 6-ketoPGF 1m and PGF2a' Whether they were treated medically or with ECMO, all patients showeddecreased plasma prostanoids levels. Decreasesin plasma levels correspond chronologically to resolution of lung injury and pulmonary hypertension. Further studies are needed to clarify the role of these vasoactive substancesin the human neonate. K.B. thanks Jerry Hric and Mary Jane Aramburo for participation in data collection, and David Warburton for advice and criticism during the preparation of the manuscript. REFERENCES 1. Fox W, Duara S. Persistent pulmonary hypertension in the neonate: diagnosis and management. J Pediatr 1983; 103:505-14. 2. Yeh TF, Luken JA, Lilien LD, Pildes RS. Persistent pulmonary hypertension of the newborn. In: Yeh TF, ed. Drug therapy in the neonate and small infant. Chicago, Illinois: Year Book Medical Publishers, 1985:89-104. 3. Klesh KW, Murphy TF, Scher MS, et al. Cerebral infarction in persistent pulmonary hypertension of the newborn. Am J Dis Child 1987;141:852-7. 4. Ferrara B, Johnson DE, Chang P-N, Thompson TR. Efficacy and neurologic outcome of profound hypocapneic alkalosis for the treatment of persistent pulmonary hypertension in infancy. J Pediatr 1984;105:457-61.

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