3. Physiology and Clinical Anatomy of the Infant and Neonate Flashcards
The general physiological characteristics of the infant
The general physiological characteristics of the infant (defined as a child aged between
1 month and 1 year) and neonate.
The very youngest children exemplify the differences between paediatric and adult practice
- Surface Area to Mass Ratio
The smaller the child, the larger is the ratio of surface area to mass,
so that in the neonate it is 2.5 times that of the adult.
The larger this ratio,
the greater is the required increase in metabolic rate to
maintain normal body temperature, and this is
one of the core factors that explains many
of the physiological characteristics.
Cardiovascular System
Anatomical features:
at birth there is right ventricular hypertrophy (owing to the fetal circulation).
The limbs are smaller in relation to the body, so there is less reserve
blood volume to mobilize from the periphery.
The foramen ovale closes functionally
at birth although it remains anatomically patent
This need to maintain body temperature via heat production results in a higher basal
metabolic rate (BMR) and higher tissue oxygen consumption which, at 7 ml kg1
min1, is twice that of an adult
Cardiac output, which at birth is 200 ml kg1 min1 (100 ml kg1 min1 in the
adult), increases predominantly by an increase in heart rate rather than stroke
volume.
Blood volume is 80 ml kg1 at term and 75 ml kg1 at age 2 years. The haemoglobin
concentration at birth is 160–180 g l1 (80% HbF), dropping to 100 g l1 at 3 months
and rising again to 120–140 g l1 at 1 year.
Infants demonstrate increased sensitivity to vagal stimulation.
Airway
A number of characteristics have implications for airway management.
The head is disproportionately large in comparison with adults;
the angle of the jaw is 140’ (120’in adults);
and the dimensions of the upper airway are
reduced by a larger tongue,
by lymphoid tissue and
by narrower nasal and pharyngeal passages.
The epiglottis is U-shaped and,
although it is stiffer than in the adult, lies more horizontally
(at an angle of 45). It is sometimes described as ‘infantile’.
The larynx is higher (the cricoid cartilage lies at the level of C4)
and not only lies more anteriorly but is also tilted anteriorly.
The ring of the cricoid cartilage forms the narrowest part of the
airway up to at least the age of 8 years.
The trachea is short (around 4 cm in the infant) and narrow
(about 5–6 mm in diameter).
The angles of the left and right main bronchi are approximately equal, so that left endobronchial intubation is as
likely as right
Respiration
Rate and mechanics
Respiration:
the high BMR of infants is associated with a high respiratory rate.
Respiratory compensation occurs via an increase in
respiratory frequency more than increases in tidal volume.
Infant ribs are more horizontal and so are mechanically less efficient.
The compliant chest wall is unable effectively to oppose the action of
the diaphragm to maintain the FRC, and the soft sternum retracts rather than
providing support for respiration.
Respiration is predominantly diaphragmatic,
and the intercostal and accessory muscles are relatively weak,
being deficient in type 1 muscle fibres until around the age of 2 years. (Tidal ventilation is 7 ml kg1, the same as in older children and adults.)
Infants respond to hypoxia with bradypnoea
rather than tachypnoea.
Respiration
Alveoli
Alveoli at birth number 20–50 million,
and they are structurally underdeveloped.
By 18 months they total 300 million,
and thereafter grow in size rather than number.
The FRC is small and desaturation occurs quickly.
Decreased compliance
(because of poorly developed elastic tissue) means that ventilatory
units have short time constants, so alveolar ventilation is maintained at the
expense of a high respiratory rate, high work of breathing and high oxygen consumption
(15% of the total).
Respiration
Closing capacity exceeds FRC (up to the age of 6 years), and infants generate
physiological CPAP (of around 4 cm H2O) by partial adduction of the cords during
expiration. The ‘grunting’ of a premature neonate in respiratory difficulty is an
exaggeration of this mechanism.
Pre-term infants are at risk of sudden apnoeic episodes (defined as cessation of
breathing for 15 seconds or more). This applies up to around 60 weeks of postconceptual
age, and is a manifestation of poor maturation of ventilatory control.
More than 50% of total airways resistance in infants (and children up to the age of
around 6 years) is provided by peripheral airways less than 2 mm in diameter.
This is why conditions such as bronchiolitis are so problematic in this age group.
Temperature Control
Thermoregulation is immature in the infant.
A large surface area is associated with increased heat loss,
and neonates are especially vulnerable to rapid hypothermia.
Infants aged less than 3 months do not shiver, but generate heat via non-shivering
thermogenesis from brown fat, which comprises up to 6% of the body weight of the
term fetus.
Heat is generated by the catecholamine-mediated metabolism of fatty
acids.
Energy Metabolism
Fetal, pre-term and neonatal glucose homeostasis is complex.
It should be sufficient to know that the infant does not have the same capacity to mobilize glucose as the adult.
Illness, trauma or the stress of pre-operative fasting can all combine with the
high basal metabolic rate to deplete glycogen stores and produce hypoglycaemia
(defined in this context as a blood glucose concentration below 2.2 mmol l1)
.
Restricted fat reserves also reduce the mobilization of free fatty acids and the
production of ketones (which are an important energy substrate).
Renal System
Infant kidneys have a reduced glomerular filtration rate (which, at 65 ml min1, is
half that of the adult),
diminished tubular function and sodium excretion, and a decreased concentrating ability.
Sodium loss is inevitable, and there is
limited ability either to conserve or excrete water,
so infants tolerate hypovolaemia or overtransfusion badly.
The excretory load is mitigated partially by 50% of the nitrogen that is
incorporated into growing tissue. Renal function is mature at about 2 years of age.
Central Nervous System
Neurological development continues in the early years of life with the completion of
myelination of the brain and spinal cord.
The sympathetic nervous system is also incompletely developed,
which explains the tolerance of children to the effects of
central neuraxial blockade which in adults may cause significant hypotension.
The blood–brain barrier is immature, which increases the neonate’s, and to a lesser extent the infant’s, sensitivity to opiates and other CNS depressants.
By 6 months of age the response to morphine is probably the same as in adults.
There is some concern, as yet unresolved, about possible deleterious effects of general anaesthesia on the nervous system of the young child
Gastrointestinal System
The incidence of neonatal gastro-oesophageal reflux is high
(coordination of swallowing with respiration does not mature until around 4–5 months), but this rarely proves to be a problem in clinical practice
Drug Effects
A combination of factors influences the response of the neonate and infant to drugs.
CNS depressants may have enhanced effects, both because the blood–brain barrier is
less effective and because cerebral blood flow accounts for a greater proportion of the
cardiac output.
Total body water is higher, and so water-soluble drugs have a larger
volume of distribution and may require higher initial doses.
(Suxamethonium is an example.)
Fat-soluble drugs may have a longer clinical effect because lower stores of
body fat decrease redistribution.
Plasma proteins are lower, and so free diffusible drug levels may be higher.
Enzymatic function, particularly that associated with
hepatic phase II conjugation reactions, is also immature.
This may delay metabolism
and excretion of drugs.
Fluid balance – neonates/infants
: the preceding calculation does not apply to neonates and young infants.
By the fifth day of life,
term neonates of weight >2.5 kg require 150 ml kg1 day1.
Newborns, however, have a relative excess of total body water and extracellular fluid, and in the first few days their requirements are much less.
A typical regimen would be 60 ml kg1 day1 on day 1, 75 ml kg1 day1
on day 2, 90 ml kg1 day1 on day 3, 120 ml kg1 day1 on day 4 and 150 ml kg1
day1 thereafter.
Fluid resuscitation
: the immediate management of a child with moderate or severe
volume loss is a bolus of 20 ml kg1 of colloid or NaCl 0.9% repeated as necessary.
Infants not only have high total body water of 70–80% compared with around 60% in
the adult, but they also have a higher proportion of extracellular fluid (>50% in the
neonate compared to 33% in the adult). This increases their vulnerability to
dehydration.
