Meconium Aspiration Syndrome: Have We Made a Difference?

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Meconium

Aspiration

Syndrome:

Have

We

Made

a Difference?

Thomas

E. Wiswell,

LTC,

MC; Joseph

M. Tuggle,

MAJ,

MC; and

Barbara

S. Turner,

LTC, AN

From the Department of Pediatrics, Walter Reed Army Medical Center, Washington, DC, and the Department of Nursing, Madigan Army Medical Center, Tacoma, Washington

ABSTRACT. Meconium aspiration syndrome (MAS) and

its associated complications are reviewed from the period before the routine use of intubation and suctioning to the present (1973 through 1987). Of the 176 790 neonates born during this period, the amniotic fluid was stained in

21472 (12.15%). Subsequently, MAS developed in 1162

(5.41%) of the meconium-stained neonates. Male

neo-nates were more prone to the disorder than female neo-nates (P = .022). There were no racial predilections for

MAS. The incidence of MAS significantly decreased dur-ing the 15 years (P = .043). Of the neonates with MAS,

49 (4.22%) died as a direct consequence of the disorder. The death rate significantly declined during the study period (P = .041). Ofthe neonates with MAS, 345 (29.7%)

required mechanical ventilation, and 134 (11.53%) had

pneumothoraxes. Among neonates with

MAS,

the

re-quirement for mechanical ventilation, as well as the

incidence of pneumothoraxes, did not decrease from 1973

through 1987. The incidence of MAS has declined since

the advent of combined obstetric and pediatric suctioning

of the oropharynx and trachea. Furthermore, there are

significantly fewer deaths from the disorder. These de-dines were likely influenced by other improvements in perinatal care, which have occurred since the early 1970s.

The results do not support the contention that severe

MAS

and resultant deaths can be prevented altogether. Pediatrics 1990;85:715-721; meconium aspiration syn-drome, persistent pulmonary hypertension, mechanical ventilation, pneumothorax.

born through meconium-stained amniotic fluid

sub-sequently suffer respiratory distress.5 In addition,

up to 46% of neonates with MAS reportedly die from the disorder.6 Although immediate tracheal suctioning of meconium-stained neonates was

mi-tially suggested in 1960,2 it was not until the mid-1970s that intrapartum oropharyngeal suctioning, as well as immediate postnatal intubation and di-rect tracheal suctioning, were advocated as effective measures in preventing MAS and its complica-tions.79 Some authors believe that no deaths should

be caused by MAS and that the occurrence of this

disorder should be rare.915 Such beliefs could raise medicolegal questions about physician competency should there be deaths associated with MAS.’6’17 However, a number of recent investigators have reported that MAS is not uncommon and that deaths attributed to the disorder still occur.6”23 To date, there have been no reviews of MAS that cover the period before the routine use of suctioning and intubation to the present. The objective of this investigation was to examine the incidence of MAS and its associated complications from 1973 through

1987.

METHODS

Historically, the meconium aspiration syndrome

(MAS) has been a major cause of perinatal

morbid-ity and mortality.1 As many as 62% of neonates

Received for publication Mar 20, 1989; accepted Jul 7, 1989.

The opinions or assertions contained herein are the private views of the authors and are not to be construed as official or as reflecting the views of the Department of the Army or the

Department of Defense.

Reprint requests to (T.E.W.) Dept of Pediatrics, Walter Reed

Army Medical Center, Washington, DC 20307-5001.

Our study population consisted of all neonates born in the seven United States Army Medical Centers that have both pediatric and obstetric res-idency programs, as well as level III newborn

inten-sive care units. We chose these facilities because we knew that sick neonates were unlikely to be

(2)

1432

77.46

Range/y (%)

11 050-12 405

2244-1669 (10.53-13.94)

51-109 (3.36-8.16)

0-7 (0-9.58)

Male Female

Deaths attributed to

MAS (% of

neo-nates with

MAS)

Age at death (d)t 3.75 1-23

Duration of hospital- 11.51 3-118

ization among survivors of

MAS (d)

Mechanical ventila- 345 (29.7) 15-29 (25.7-33.3)

tion

Pneumothoraxes 134 (11.53) 4-15 (5.55-22.2)

* Significant at P = .022.

t Median age at death was 2 days; 24 neonates died on the first day of life.

:1:Median duration of hospitalization was 7 days.

and Biostatistics Activity in Ft Sam Houston (TX)

consisted of the total number of live-born neonates, the number of neonates whose deliveries were com-plicated by meconium-stained amniotic fluid, and

the number of these neonates in whom MAS sub-sequently developed. We evaluated selected data from the last group. The data from each neonate

with MAS included sex, race, in-hospital diagnoses,

procedures performed, death if it occurred as well as the attributed cause of death, the dates of birth and disposition (discharge or death), and the loca-tion of each neonate’s birth. We verified this infor-mation by comparing it with separate data bases kept at several of the facilities during the study

period.

We queried pediatricians and neonatologists who had worked at the seven hospitals during the 15-year period about the initial management of the meconium-stained neonate. Meconium aspiration syndrome was consistently defined as respiratory

distress in a neonate who was meconium stained,

who had compatible chest roentgenographic find-ings, and whose symptoms could not be otherwise explained. Before 1975, few neonates were either suctioned by obstetricians or intubated by pediatri-cians. During the mid-1970s, the standard of care was changed to include routine intubation and

tra-cheal suctioning of all meconium-stained neonates. During the late 1970s, intubation was usually

pre-ceded by the performance of oropharyngeal suction-ing by the obstetrician before delivery of the

neo-nate’s shoulders. Finally, since the mid-1980s, vig-orous babies and/or those with meconium-stained

fluid of thin consistency may not have been univer-sally intubated and suctioned by pediatricians.

Phy-sicians-in-training (obstetricians and pediatricians) performed the bulk of the intrapartum and post-partum suctioning of the neonates.

Data were evaluated for significance, where ap-propriate, with linear regression, ANOVA, the

x2

test for linear trends, and

x2

analysis.24 A P value

.05 was considered to be statistically significant.

RESULTS

From 1973 through 1987, there were 176 790 live-born neonates (an average of 11 786/year).

Char-acteristics of the study population are presented in

Table 1. Of the total number of deliveries, 21 472 (12.15%) were complicated by meconium-stained amniotic fluid. The frequency of meconium-stained amniotic fluid was stable during the period of the study (Fig 1). Of the meconium-stained neonates, 1162 (5.41%) had a diagnosis of MAS. The racial

TABLE 1. Characteristics of 176 790 Neonates Born From 1973 Through 1987

Feature

Total live-born neonates Meconium stained

Subsequent MAS if

meconium stained Race of neonates

with

MAS

White Black Other

Sex of neonates with

MAS

Total (%) Mean/y

176790 11786

21 472 (12.15) 1162 (5.41)

856 (73.7)

170 (14.6) 136 (11.7)

(3)

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N N N

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Fig 1. Incidence of meconium-stained from 1973 through 1987.

a,

amniotic fluid

‘n ‘0 C

N N N.

a, o 2! a’

8 Meconium Aspiration Syndrome (1973-1987)

e 1#{176}

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a, a, C, O a,

Fig 2. Incidence of meconium aspiration syndrome

(MAS) from 1973 through 1987. A, Annual incidence of

MAS per 1000 live births. B, Annual number of cases of

MAS per 100 meconium-stained neonates.

TABLE 2. Causes of Deaths Among 1162 Neonates

With Meconium Aspiration Syndrome

Cause No. of

Deaths

Directly attributed to meco-nium aspiration syndrome (n = 49)

Respiratory failure/aspiration 28

Persistent pulmonary hyper- 16

tension

Pulmonary air leaks 4

Bacterial pneumonia 1

Chronic lung diseaes 0

“Other” deaths (n = 20)

Asphyxia unresponsive to re- 8

suscitative efforts

Lethal congenital malforma- 8

tions

Hemorrhagic diathesis 3

Congenital herpes simplex in- 1

fection Meconium Aspiration Syndrome

(1973-1987)

Meconium Aspiration Syndrome (1973-1987)

breakdown of neonates with MAS was virtually identical with that of the entire live-born

popula-tion. Male neonates were more prone to MAS than female neonates (P = .022). The incidence of MAS

significantly declined during the 15 years of the investigation (P = .043; Fig 2). Ofthe 1162 neonates

with MAS, 69 died. Of the 69 deaths, 49 (4.22% of

those with MAS) were ascribed to the disorder or

its complications (Table 2). The death rate attrib-uted to MAS significantly declined during the period from 1973 through 1987 (P = .041; Fig. 3).

Of the 49 neonates who died of MAS or its

compli-cations, half succumbed on the first day of life.

Among survivors, the median duration of hospital-ization was 7 days. We found no temporal trends in the incidence of MAS or subsequent deaths (eg, increased frequencies in July to October, when

phy-sicians were beginning their training). Of the neo-nates with MAS, 345 (29.7%) required mechanical

ventilation and 134 (11.5%) had pneumothoraxes. The requirement for mechanical ventilation (Fig 4) did not decline among neonates with MAS during

the study period. In addition, there was an apparent rise in the incidence of pneumothoraxes among neonates with MAS (Fig 5). However, this trend

was not statistically significant. No neonates with the disorder were transferred to other medical

cen-ters for additional management (eg, for high-fre-quency ventilation or extracorporeal membrane

ox-ygenation).

We were able to ascertain the Apgar scores at 1 minute of 103 of the 1162 neonates in whom MAS

subsequently developed. In 40.8% of the cases, the

neonates had Apgar scores of 8 to 10 at 1 minute. In addition, of 357 consecutive meconium-stained neonates born at one of the medical centers from

1985 through 1987, 20.1% had Apgar scores of 6

at 1 minute, and 23.8% had scores of 9 or 10.

(4)

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9

8

7

6

5

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8 #{149}

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6 #{149}

4

- 2t

ol i i I

in co a’ C’

N N N n

a, a, a, a, a,

Fig 5. Annual number of neonates with

pneumotho-raxes per 100 cases of meconium aspiration syndrome

(MAS).

A

a-Ji

p,, a’ r’i

N N N

! ?i ! 2

Usconium Asoiration Syndrome

B

(1973-1987)

10

(no

l, 7

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Fig 3. Annual death rates attributed to meconium

as-piration syndrome

(MAS)

from 1973 through 1987. A,

Annual death rate per 10000 live births. B, Annual number of deaths per 100 cases of MAS.

C

.2 40

a

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:

25

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N N N

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Fig 4. Annual number of neonates requiring mechanical ventilation per 100 neonates with meconium aspiration

syndrome

(MAS).

DISCUSSION

We have found the incidence of MAS and deaths

caused by the disorder to have significantly declined during the years since routine intrapartum oropha-ryngeal and postpartum intratracheal suctioning

have come into use. Nevertheless, MAS still occurs,

as do deaths caused by the entity. Moreover, of

neonates with MAS, many still require mechanical ventilation or have clinical courses complicated by pulmonary air leaks.

We expected different shapes for the curves that depict the annual incidences of MAS and deaths caused by the disorder (Figs 2 and 3). We antici-pated the curves would demonstrate higher baseline rates from 1973 through 1975, followed by sharp

downward trends from 1976 through 1980, with low,

flat baseline rates thereafter. However, because the annual number of cases of MAS (mean 77.5/year)

and deaths (mean 3.3/year) were relatively low,

small annual changes of 10 to 20 cases of MAS or

2 to 3 deaths could markedly alter the shapes of the

graphs. The apparent marked decline in deaths from 1976 through 1978 (there were 2, 1, and 1 deaths, respectively, per year) reflects this phenom-enon. Nevertheless, we believe the overall

down-ward trends in the incidences of MAS and associ-ated deaths are real. In addition, we were surprised

at the apparently increasing risk for the develop-ment of pneumothoraxes (Fig 5). We suspect that this may be due to different ventilatory techniques in the management of MAS in neonates who have concomitant persistent pulmonary hypertension. The latter disorder is frequently treated with high peak inspiratory pressures, as well as hyperventi-lation, in an effort to lower the arterial partial pressure of carbon dioxide. These ventilatory tech-niques, particularly if the expiratory time intervals are insufficient, may combine with the “ball-valve” obstructive nature of meconium aspiration to lead to a higher risk for pulmonary air leaks.

How does our study population compare with

those of previous reports? Most of the characteris-tics as we describe are comparable with those in the literature (Table 3)#{149}3_9121417_21.2535 Although there

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TABLE 3. Current Findings on Meconium Aspiration Syndrome Compared With Find-ings in Previous Reports

Characteristic Frequency in

This Report*

Range (Median Value) in

Pre-vious Reports*

Reference No.

Deliveries with meconium-stained amniotic fluid Meconium-stained neonates

with subsequent MAS

Neonates with MAS who died of the disorder Neonates with MAS

requir-ing mechanical ventilation

Neonates with MAS and

pneumothoraxes

Meconium-stained neonates “depressed” at birth (usu-ally Apgar score 6 at 1

mm)

12.15

5.41

4.22

29.70

11.53

20.1

6.8-22 (12.5)

0.37-62 (5.6)

0-46 (12.0)

0-59 (33.0)

0-41 (23.8)

21-53 (33)

3,6-9,18,20,25-31

3,5-9,17,18,20,21,25-28, 30,31

3,5-9,13,14,18-20,25,27, 28,31-33 6-8,12,13,18,19,27,32

5-8,13,18,25,27,32,34

4,5,7,8,17,21,28,30,35

* Values are percentages.

the frequencies from our population approximate the median figures.

The reports of Gregory et al,7 Ting and Brady,8 and Carson et al#{176}led to the widespread use of intrapartum oropharyngeal suctioning and

postpar-tum intratracheal suctioning of meconium-stained newborn infants. A number of authors suggest that MAS should rarely occur and that virtually no

deaths should result from the disorder.#{176}’5 However,

several reports subsequent to those of Gregory et al,7 Ting and Brady,8 and Carson et al9 have

con-firmed neither diminutions in the incidence of MAS nor the complete avoidance of deaths.6’1823 We have

found a decline in the incidence of MAS, but we have found the disorder still to occur in a substan-tial proportion of meconium-stained neonates. Fur-thermore, although the rate of deaths caused by

MAS has abated, deaths have not been entirely

eliminated. We believe the decline in MAS is pri-manly due to routine oropharyngeal suctioning by obstetricians and subsequent postpartum intratra-cheal suctioning. We presume the decline in deaths

from MAS has similarly been influenced by oro-phryngeal and intratracheal suctioning. However, it is likely that the numerous advances in perinatal care, particularly improvements in ventilator

man-agement, have also played a major role in the lower mortality rates. Nevertheless, because this is a ret-rospective review, our interpretation of the trends

is speculative.

We believe that the oropharynges of all

meco-nium-stained neonates should be suctioned before delivery of their shoulders and the onset of breath-ing. Unfortunately, there is no consensus about

which neonates will benefit from subsequent intu-bation and suctioning. The recommendations in

major neonatology/perinatology textbooks, as well

as in the medical literature, are diverse. The

majority of authors advise intratracheal

intuba-tion and suctioning of all meconium-stained neo-nates.5’#{176}”3’15’20’27’3545 Some maintain intubation

should be performed only if heavy, particulate me-conium is present.”31’4652 Still other physicians

rec-ommend intubation only if meconium is present in

the oropharynx or at the vocal cords.5’9’23’32’47’3’54 Finally, several authors have suggested that if a neonate is vigorous and apparently healthy, intu-bation may not be necessary.”5’25’36 Linder and

colleagues25 recently prospectively evaluated the last hypothesis. Unfortunately, there are a myriad

of problems with these authors’ methodology and interpretation that render their conclusions sus-pect.55’56 Thus, it remains unclear whether or not

all “healthy” meconium-stained neonates should be intubated and suctioned. Of the 103 neonates for

whom this information was available from our

pop-ulation, 40.8% had “good” Apgar scores (8) at 1 minute. Indeed, these neonates were less likely to require mechanical ventilation or to have

pulmo-nary air leaks. Nevertheless, 2 ofthese 103 neonates died as a result of complications of MAS.

Despite intrapartum suctioning before delivery of the neonate’s shoulders, meconium is found

be-low the vocal cords in 7% to 56% of

meconium-stained neonates.6’7’25’26’3’ Meconium aspiration

syndrome will develop in 10% to 33% of neonates with meconium in the trachea.7’31 Furthermore, Burke-Strickland and Edwards5 found that MAS

subsequently developed in more than 50% of neo-nates born through meconium despite “clear” oro-pharynges (these neonates were not intubated). The

factors that apparently place the meconium-stained

neonate at highest risk for MAS include

(6)

“pea soup” consistency meconium, moderate to se-vere fetal heart rate decelerations, and low scalp pH.’ Examination of the oropharynx and vocal

cords to assess the need for intubation has not been shown to contribute to the assessment of risk for

development of MAS.1

Thick (particulate or pea soup consistency) me-conium has been associated with a greater likeli-hood for the development of MAS, particularly more severe 45152527.2831 However, there are many

reports of MAS occurring in neonates born through

“thin” meconium-stained amniotic fluid.7’27’28’3’

Me-conium consistency may be a subjective matter, particularly for inexperienced health care pro-viders. There is no conclusive evidence that thin meconium does or does not need to be suctioned from the trachea.’5 Our current recommendations are to intubate and suction all

asphyxiated/de-pressed meconium-stained neonates. We advocate

intubation of vigorous neonates if the meconium is thick. We remain unsure about the appropriate management of vigorous neonates born through thin, meconium-stained amniotic fluid.

Despite a significant decline in the mortality rate from MAS in our population, deaths still occurred. There may always be a baseline number of deaths

associated with the disorder that are probably not

preventable and may be due to in utero aspiration or to a form of persistent pulmonary hypertension. During the last decade, a number of reports have appeared describing stillborn and live-born neo-nates who died after what appeared to be in utero aspiration of meconium.6’2023’54’57’58 Block et al59 reported in utero aspiration in a baboon model. When aspiration occurs before parturition and the meconium has been present in the most distal

air-ways for a period of time, conventional methods of

therapy may prove to be ineffective. Furthermore,

Murphy and colleagues33 have reported a close as-sociation of fatal MAS with persistent pulmonary hypertension. The pulmonary vasculature of the described neonates was structurally different from that of normal neonates; the severe narrowing of the pulmonary arterioles made affected neonates

refractory to treatment. Persistent pulmonary hy-pertension is frequently associated with

MAS.a13ls2oal33m Neonates with MAS make up a large percentage of those who have been treated with extracorporeal membrane oxygenation.#{176} As such, they are treated with extracorporeal mem-brane oxygenation when their predicted mortality is >80% compared with historical controls. Meco-nium aspiration syndrome may well represent evi-dence of prenatal stress or abnormality rather than a response to perinatal events.61 A severe form of persistent pulmonary hypertension, particularly that associated with structural vascular changes, is

conceivably the major cause of death among

neo-nates with MAS. The recognition that meconium aspiration is not necessarily a neonatal event, and that associated deaths may be neither preventable

nor specifically due to the disorder, are important for their medicolegal implications.

We believe there are a number of unresolved questions regarding MAS. Do all meconium-stained

neonates require tracheal intubation and suction-ing? Is abnormal pulmonary vasculature (causing persistent pulmonary hypertension) the major cause of death associated with MAS? Will other therapies (eg, high-frequency ventilation or

extra-corporeal membrane oxygenation) decrease the fre-quency of deaths associated with MAS even

fur-ther? We hope future investigations will clarify these issues.

We conclude that the incidence of MAS, as well as deaths caused by the disorder, has significantly declined since the advent of combined obstetric and pediatric suctioning of the oropharynx and trachea. Among neonates with MAS, the requirement for

mechanical ventilation and the occurrence of

pneu-mothoraxes have not been affected. Our results do not support the contention that MAS and deaths

associated with the disorder can be prevented

al-together.

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Pediatrics

Thomas E. Wiswell, Joseph M. Tuggle and Barbara S. Turner