ABC of preterm birth : Care in the early newborn period
The first week after birth
is a time of major metabolic and physiological adaptation for newborn
infants. Preterm infants have to cope with additional stresses because most
of their organ systems are immature or because of associated illnesses,
such as congenital infection. Very preterm infants (<32 weeks’
gestation) or ill infants often need intensive monitoring and support
during this critical period of postnatal adaptation.
Preterm infants are susceptible to heat and fluid loss
because
- High surface area to volume ratio
- Thin non-keratinised skin
- Lack of insulating subcutaneous fat
- Lack of thermogenic brown fat
- Inability to shiver
Temperature control and fluid balance
Preterm infants are susceptible to heat and fluid loss,
especially immediately after delivery and in the first few days after
birth. Hypothermia is associated with increased morbidity and mortality.
Trials in the 1950s showed that reducing heat loss improves survival for
preterm and low birthweight infants. Measures to prevent cold stress should
start immediately after delivery—for example, resuscitating newborns
under radiant heaters, drying them, and wrapping them in warmed towels
straight away. A randomised controlled trial showed that wrapping the
infant in polyethylene immediately (without drying) is at least as
effective in reducing evaporative heat loss in extremely preterm infants
(<28 weeks’ gestation).
Risk factors for respiratory distress syndrome
Potential adverse consequences of hypothermia
- High oxygen consumption can lead to hypoxia
- High use of glucose can lead to hypoglycaemia
- High energy expenditure can cause reduced rate
of growth
- Low surfactant production can cause respiratory
distress
- Vasoconstriction may cause poor perfusion or
metabolic acidosis
-
Delayed adjustment from fetal to newborn circulation
Heat and fluid loss
After admission to the neonatal unit unnecessary
oxygen and energy consumption must be minimised. Several options are
available for nursing preterm infants in a neutral thermal environment.
Bigger and more mature infants (weighing > 1800 g) can usually maintain
their body temperatures in open cots with clothing (including a hat),
covers, and possibly a heated mattress. Smaller and less mature infants,
particularly very preterm infants, are usually cared for in air heated
Perspex incubators or in open cots, where they are placed under clear
polyethylene blankets and there are overhead radiant heaters. The air
temperature of the incubator or the power of the overhead heater can be set
to respond to changes in the temperature of the infant’s abdominal
wall to try to maintain the infant’s temperature at 36.5°C to
37°C.
Closed incubators allow adjustment of the ambient
humidity, and this further reduces heat and fluid evaporation.
Consequently, incubator care is associated with less insensible water loss,
and lower fluid requirements, than nursing infants in open cots under
radiant heaters. Both closed incubator and open cot care have other
potential advantages. Environmental noise and light can be reduced with
incubator care and this may improve sleep patterns. Open cots, however,
allow easy access for carers. Additionally, parents might find it easier to
bond with their babies if they are nursed in an open cot rather than in a
closed incubator. At present there are insufficient data to determine
whether open cots or incubators confer more beneficial effects on important
clinical outcomes.
Preterm infants have higher fluid requirements than
term infants, especially in the first week after birth, mainly because they
lose more fluid through the skin and through breathing. As fluid input for
preterm infants must take into account these variable losses, prescriptions
are usually tailored to individual infants. Additionally, preterm infants
have immature renal tubular function in the first few days after birth.
This is associated with an inadequate capacity to excrete sodium and so
preterm infants have a lower sodium requirement than term infants. Fluid
and electrolyte balance must be monitored frequently to avoid dehydration
or excessive fluid input.
The neutral thermal environment is the ambient
temperature at which oxygen consumption and energy expenditure is at the
minimum to sustain vital activities.
Risk factors for increased insensible fluid loss
- Short gestation
- High ambient temperature
- Radiant heater
- Phototherapy lamp
- Low ambient humidity
Glucose homeostasis
Hypoglycaemia is common in preterm infants, with risk
inversely related to gestational age. Very preterm infants must maintain
high energy output to overcome thermal stress and to support respiratory
efforts. Growth restricted preterm infants are at great risk of
hypoglycaemia because they have limited fat and glycogen reserves at the
time of delivery.
The level or duration of hypoglycaemia that is harmful
to a preterm infant’s developing brain is not known. Hypoglycaemia is
a potentially more serious complication for preterm infants than term
infants because preterm infants have a relatively impaired ability to
produce alternative brain fuels, such as ketones. Interventions, such as
giving more milk or starting an intravenous glucose infusion, are necessary
when the laboratory measured blood glucose level remains <2.0 mmol/l.
Conversely, very preterm infants are also susceptible
to hyperglycaemia and glycosuria, which can disturb fluid balance by
inducing an osmotic diuresis. If glycosuria persists despite reducing the
glucose input, insulin may be needed.
TMeta-analysis of need for surgical ductus ligation in trials of prophylactic indometacin in very low birthwight infants.Adapted from fowlie PW etal.
Cochrane Database Syst Rev 2003;(4):CD000174
Variables that should be monitored in the very preterm
infant
- Temperature: core and peripheral
- Heart rate
- Respiratory rate
- Peripheral oxygen saturation
- Blood pressure
- Urine output
- Partial pressure of oxygen and carbon dioxide
- Acid-base status
- Electrolyte balance
- Weight gain or loss
Haemodynamic status
Hypotension is associated with adverse outcomes,
particularly intraventricular haemorrhage and periventricular leucomalacia.
Hypotension and suboptimal systemic perfusion can be secondary to several
problems Management should be directed at treating the underlying cause
(for example, giving volume replacement or antibiotics) and should include
measures to improve systemic perfusion, such as inotrope support.
Variables that should be monitored in the very preterm
infant
Optimal arterial blood pressure for preterm infants
meets perfusion needs for vital organs. Reference ranges for blood pressure
in healthy preterm infants in the first week after birth have been
published. As a rule of thumb, the mean blood pressure (mm Hg) should not
be lower than the number of weeks of the infant’s gestational age.
Risk factors for hypotension and poor perfusion
Although relatively easy to measure and monitor,
arterial blood pressure does not correlate well with cardiac output and
systemic perfusion in preterm infants. Other variables including heart
rate, peripheral oxygen saturation, acid-base status, and urine output can
be measured. These too are relatively poor measures of organ perfusion.
Doppler ultrasonography assessments of systemic perfusion might be more
useful for determining when to intervene and which intervention is most
appropriate. Currently, these techniques are not widely available.
Risk factors for hypotension and poor perfusion
- Short gestational age
- Lack of antenatal steroids
- Positive pressure ventilation
- Patent ductus arteriosus
- Perinatal asphyxia
- Systemic infection
Patent ductus arteriosus
In the first few days after birth, patency of the
ductus arteriosus is a major cause of hypotension and poor perfusion. Over
one quarter of very preterm infants develop a clinically important patent
ductus arteriosus. The risk of this is greatest in infants with severe
respiratory distress syndrome. The clinical consequences are related to
shunting of blood through the patent ductus from the aorta to the pulmonary
arterial circulation. This “left to right” shunt alters the
blood flow distribution to vital organs. Increased pulmonary blood flow can
damage the preterm lungs. Preterm infants with a patent ductus arteriosus
are at higher risk of more severe and prolonged respiratory distress
syndrome, chronic lung disease, intraventricular haemorrhage, and death
than similar infants whose ductuses have closed.
Aetiology of anaemia of prematurity
- Frequent blood sampling
- Low reticulocyte levels
- Low levels of endogenous erythropoietin
- Poor response to endogenous erythropoietin
- Shortened life span of neonatal erythrocytes
The patent ductus arteriosus may be closed surgically,
with transthoracic ligation, or pharmacologically, with prostaglandin
synthase inhibitors, such as indometacin or ibuprofen. Current data on
overall benefits and harms are insufficient to determine if surgical or
medical treatment is the better initial treatment for symptomatic patent
ductus arteriosus in preterm infants. In most centres, surgical ligation is
reserved for instances where the ductus remains open despite
pharmacological treatment. However, retrospective studies show that
surgical ligation may be a better firstline treatment in extremely preterm
infants, particularly if the ductus is large. Further randomised controlled
trials are needed to clarify these issues.
Risks of blood transfusion in preterm infants
- Fluid overload
- Transfusion associated infection: Hepatitis B
and C viruses Human immunodeficiency virus Cytomegalovirus
- Haemolytic transfusion reactions
- Immune mediated transfusion reactions
- Extravasation injury
- Graft versus host disease (rare)
Prophylaxis with indometacin
Prophylactic use of indometacin in very low
birthweight infants confers short term benefits, including a fall in the
incidence of symptomatic patent ductus arteriosus, a reduced need for
surgical ligation, and a reduced incidence of intraventricular haemorrhage.
Prophylactic indometacin does not, however, improve survival or longer term
neurodevelopmental outcomes. The decision to use prophylactic indometacin
will depend on the values that parents and carers attach to the short term
benefits. In neonatal units without ready access to cardiac surgical
services, a reduction in the need for surgical ligation may be considered a
greater benefit than in units with these services.
Measures to reduce the risk of infection associated
with transfusion
- Screen donors for transmissible viruses
- Limit exposure to multiple
donors—multiple paediatric packs from single adult donor
- Use cytomegalovirus antibody negative blood
- Irradiate transfusion packs
- Use leucocyte depletion filters (removes
cytomegalovirus)
Aetiology of anaemia of prematurity
Anaemia is common in very preterm infants. Evidence
exists that delaying umbilical cord clamping until 30-60 seconds after
birth facilitates fetoplacental transfusion and reduces the need for blood
transfusions in the early neonatal period. Further large trials are needed
to clarify whether this practice improves important outcomes, such as
longer term neurodevelopment for very preterm infants. Postnatally,
repeated blood sampling is a major cause of anaemia of prematurity. Very
preterm infants can lose 10-25% of their blood volume each week through
blood sampling. Although transfusion with packed cells can replace these
losses, uncertainty exists over the most appropriate indications for
replacement transfusion. Given the potential complications, blood
transfusions should be limited to the minimum needed to maintain optimal
oxygen delivery to vital organs. Recombinant erythropoietin is an
alternative to blood transfusion. Little evidence exists, however, to show
that its use reduces the number of blood transfusions needed in extremely
preterm infants—the population at greatest risk of anaemia of
prematurity.
Doppler colour flow of patent ductus arteriosus with left to right shunt that can change blood flow distribution to organs.Courtesy of Drs N Evans and G Malcom,Royal Prince Alfred Hospital Sydney
Conclusion
As well as respiratory and nutritional support, optimal
care for preterm infants in the early neonatal period demands attention to
several key inter-related issues, including temperature control, fluid and
electrolyte balance, glucose homeostasis, and haemodynamic status.
Maintaining metabolic and physiological stability at this time may have an
important impact on survival and neurodevelopmental outcomes.
Further reading
-
- Flenady VJ,
Woodgate PG. Radiant warmers versus incubators for regulating body
temperature in newborn infants. Cochrane
Database Syst Rev 2003;(4):CD000435
- Cornblath M,
Hawdon JM, Williams AF, Aynsley-Green A, Ward-Platt MP, Schwartz R, et al.
Controversies regarding definition of neonatal hypoglycemia: suggested
operational thresholds. Pediatrics 2000;105:1141-5
- Malviya M,
Ohlsson A, Shah S. Surgical versus medical treatment with cyclo-oxygenase
inhibitors for symptomatic patent ductus arteriosus in preterm infants. Cochrane Database Syst Rev 2003;(3):
CD003951
- Fowlie PW, Davis
PG. Prophylactic intravenous indomethacin for preventing mortality and
morbidity in preterm infants. Cochrane Database
Syst Rev 2003;(4):CD000174
- Rabe H, Reynolds
G, Diaz-Rossello J. Early versus delayed umbilical cord clamping in preterm
infants. Cochrane Database Syst Rev 2003;(3):CD003248
- Evans NJ, Malcolm G. Practical
echocardiography for the neonatologist (CD Rom) (search via
www.cs.nsw.gov.au/rpa/neonatal)
William McGuire, senior lecturer in neonatal medicine, Tayside Institute of Child Health, Ninewells Hospital and Medical School, University of Dundee
Peter Fowlie, consultant paediatrician, Perth Royal Infirmary and Ninewells Hospital and Medical School, Dundee
Peter McEwan, specialist registrar, Ninewells Hospital and Medical School, Dundee
studentBMJ 2005;13:309-352 September ISSN 0966-6494
