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Paediatric Considerations in Critical Care 681

CARDIOVASCULAR SYSTEM
Practice tip In infants, approximately 70% of the haemoglobin is
fetal haemoglobin (HbF), allowing greater amounts of
If paediatric oxygen masks are not available, an adult sized oxygen to be carried for any given PaO 2 . Circulating
mask, including a partial non-rebreather mask, can be used in blood volume per kilogram decreases with age; in the
an emergency. Place the nose section under the child or infant’s infant, circulating volume is approximately 85 mL/kg,
chin in the ‘upside-down’ position.
with total body water accounting for 70% of body mass,
adjusting to the adult values of 65 mL/kg and total
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CENTRAL NERVOUS SYSTEM body water of 60%. The apex beat is heard at the fourth
intercostal space, mid-clavicle, and by around seven years
Many central nervous system functions, such as locomo- of age the left ventricle has grown and the apex beat can
tion and hand–eye coordination, will take from months be heard at the fifth intercostal space, as in adults. An
to years, to fully develop. Functions of the cerebral cortex infant’s cardiac output is approximately 500 mL/min,
are particularly underdeveloped, with myelination of all which, relative to body weight, is about twice that of an
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major nerve tracts continuing throughout infancy. Con- adult. Heart rate is a major determinant of cardiac
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sequently, assessment and management priorities will be output in infants and young children, as there is limited
dictated by the level of neurological maturity of the infant ability to increase stroke volume. Tachycardia is an early
or child. As with adults, neurological dysfunction in sign of distress, but bradycardia is an ominous sign in
infants and children may be primary or secondary. The infants and young children, as they are more dependent
plasticity inherent in the brain of the infant may compen- on a high heart rate to maintain cardiac output. In infants,
sate for injury more readily than older children and bradycardia requires resuscitation. 13
adults in some circumstances, with other areas of the
infant’s brain taking over function. Because the eight Arterial blood pressure should be appropriate for age,
cranial bones are not yet fused, infants’ skulls cope with weight and clinical condition. Mean arterial pressure is
both birth and ongoing growth, which is greatest in the generally used. Monitoring blood pressure using correct
first two years of life. In the first year, the cartilaginous cuff sizes is important because incorrect cuff size is a
sutures fuse at two points to form the posterolateral fon- common cause of inaccurate blood pressure readings in
tanelle. The larger anterior fontanelle closes during the children. Diastolic blood pressure is recorded at Korot-
second year as bone is laid down. By around five years of koff sound 5 (K5); age-related parameters for mean blood
age, the sutures of the child’s skull are completely fused. pressure are displayed in Table 25.3. Tachycardia in the
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However the thinner skull will provide less protection to absence of fever is a more reliable sign than hypotension,
underlying tissues than the adult skull. as up to 25% of the child’s circulating volume may be
lost before hypotension occurs. Hypotension is thus a
A common misconception is that the Monro-Kellie doc- late sign in children and may indicate late decompen-
trine (see Chapter 16) does not apply to young children sated shock, particularly following fluid delivery. 14
and infants with a more compliant skull. While slow rises
in intracranial volume may be accommodated over time Paediatric Considerations for
in children under three years of age, they will usually be Cardiovascular Assessment
accompanied by growing head circumference, making
routine measurement of head circumference in children Cardiovascular assessment in children includes clinical
under three years of age an important assessment. parameters that are similar to those observed in
However, the less rigid skull of the older child will not adults. The normal values are, however, age and weight
compensate for acute rises in intracranial volume, and the dependent. Urine volume in infants should average
child will display symptoms of neurological compro-
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mise. Normal ranges of intracranial pressure (ICP) and
cerebral perfusion pressure (CPP) have not been formally TABLE 25.3 Age-related mean blood pressure
studied in infants and children, but are presumed to be
lower than in adults, reaching adult range by adolescence. Age Mean BP (mmHg)
Values that are commonly used to guide treatment are Term 40–60
age-related and are displayed in Table 25.2.
3 months 45–75
6 months 50–90
TABLE 25.2 Target cerebral perfusion pressure (CPP) 1 year 50–90
by age
3 years 50–90
Age Desirable minimum CPP 7 years 60–90
Infants under 1 year 45–55 mmHg 10 years 60–90
Children 1–10 years >55 mmHg 12 years 65–95
Children over 10 years >65 mmHg 14 years 65–95
Adapted from (9). Adapted from (9).