There are a number of key differences in physiology and anatomy between neonates and adults, with a spectrum of difference as children mature. There is are also significant changes between foetal life and neonatal physiology. An awareness of these is essential for paediatric anaesthetic practice. These are discussed in a systematic manner.
Notable anatomical differences in infants include.
Relatively large tongue
Relatively smaller jaw
Smaller oral cavity
Larger nasal cavity
Longer, floppier epiglottis
Narrowest part of larynx is at cricoid
Infants have a relatively large head compared with adults, due the size of the developing brain, which is more developed in comparison to the rest of the body at birth. The large occiput can mean that there is a natural tendency for flexion of the neck and airway obstruction.
The jaw is relatively small compared to adults, whilst the tongue is in comparison larger, making the oral cavity smaller. In comparison, the nasal cavity is larger, and without turbinates or sinuses. This means that infants are termed obligate nasal breathers, and can struggle with their breathing when there is mucosal inflammation and mucus (particularly when feeding).
The laryngeal anatomy is different in infants. The larynx is situated higher (more cephalad) making straight laryngoscope blades more useful than curved blades. The epiglottis is longer and floppier, making it harder to control. Laryngoscopy technique usually involved lifting the epiglottis with the laryngoscope blade, rather than placing the tip in the vallecula. The infant larynx is more funnel-shaped, with the narrowest part at the level of the cricoid cartilage. As such, uncuffed tubes are commonly used, designed to fit snugly at this level. The change to a normal adult airway is complete around the age of puberty.
Infants start to develop their first teeth around the age of 6 months, being complete by 2 years of age.. There are 20 deciduous (milk) teeth, which begin to be replaced by permanent adult teeth around the age of 6 years old. Deciduous teeth are less well rooted than adult teeth, but sharper. Adults have 32 permanent teeth.
Significant differences in the infant’s respiratory system make it less efficient than in the adult:
Narrower airways - greater resistance to flow
More compliant chest wall
Diaphragm more fatigable
Higher oxygen demand
Potential for inadequate pulmonary surfactant in prematurity
The airways are smaller and narrower, producing greater resistance to airflow (as per the Hagen-Poiseuille equation). The airways are also more compliant in the adult. This is combined with a notably more compliant chest wall (cartilaginous ribs and underdeveloped musculature). The result is a less effective generation of negative intrathoracic pressure with inspiration, and functional airways collapse.
The diaphragm plays a greater role in ventilation in infants. However, it does not have the same composition of type 1 muscle fibres as found in adults (and children from around 2 years). As such, it is more susceptible to fatigue, especially when increased demands are placed on the respiratory system.
The lungs are insufficiently developed to support extrauterine life until 24-26 weeks gestation. There is continued growth in the number of alveoli until around the age of 8 years. Pulmonary surfactant is essential to prevent alveolar collapse in neonates, and these levels can be inadequate in premature infants.
In addition, children have a greater oxygen demand in proportion to adults (2-3 times), because of their high metabolic activity (of growing). This factor, on top of the features of decreased efficiency of the respiratory system, explain the greater risk of respiratory compromise from a more minor insult.
It is also important to be aware of the significant changes that occur within the respiratory system as part of the transition from intrauterine life, that may be very relevant in the neonatal period. The lungs are initially collapsed and filled with amniotic fluid, before opening with the first few breaths after delivery. The respiratory centres may also be immature at birth.
Major cardiovascular differences include:
Transition from foetal circulation
Underdeveloped LV myocardium
Poorly compliant ventricle at birth
Rate dependent cardiac output
As with the respiratory system, and perhaps more so, there are significant changes that occur in the CVS to allow transition from foetal to neonatal life. The major changes involve closure of the main shunts present in foetal life - the foramen ovale and ductus arteriosus. This change is not irreversible, and instead occurs because of the effects of pressure changes and change in physiological conditions (e.g. notable increase in pO2). As such, there may be a reversal of these changes in the neonate, for example if there is notable hypoxia or acidosis, potentially leading to a vicious circle of deterioration as the neonate flips back to (a now useless) foetal circulation.
The myocardial mass of the neonatal left ventricle is nearly the same at the right ventricle. With the increase in systemic vascular resistance after birth, there is hypertrophy of the contractile elements, with the LV increasing in size. There is also an increase in the distensibility of the ventricles. Initially, the ventricles have poor compliance, and cardiac output is very rate dependent.
The blood volume decreases from 90ml/kg at birth to around 80ml/kg in the infant.
Cardiac output is around 3.5L/min at birth. This drops to around 3L/min by 1 year of age - a significant drop in terms of proportion to body weight.
The calcium storage system in the neonate is also relatively immature. This means that cardiac contractility is more susceptible to changes in systemic calcium levels.
Central Nervous System
Important differences include:
Spinal cord ends at lower level
Immature pain pathways
Notable psychological differences
The spinal cord ends at a lower level in infants. The conus medullaris is at around L3 level in the neonate. It is at the adult level of L1/2 by the age of 1 year old. The dural sac ends at S2 in the neonate, elevating to S1/2 by 1 year old.
Nociceptive pathways are present from early gestation. However, many of the components of adult life are not yet fully mature at birth e.g. descending inhibition pathways.
The foetus has a different structure to the acetylcholine (ACh) receptor. It has a gamma subunit instead of an epsilon one. They have increased sensitivity to ACh, with a more prolonged opening time. By 31 weeks gestation they have been almost entirely replaced by the adult form. The neuromuscular junction is still immature in the infant, probably with reduced stores of presynaptic ACh, which can manifest as fade on repeat nerve stimulation.
Functionally immature renal system before 2 years old
The neonate has the full number of nephrons at birth, but these are functionally immature. The GFR is about ⅓ of that of an adult. Renal maturity has been reached by around the age of 2 years.
Neonates have a relatively high sodium loss compared to adults, but less capacity to excrete potassium. The reduced renal function limits water loss.
The half life of drugs with renal elimination will be prolonged.
Large but functionally immature liver at birth
At birth, the liver comprises of around 5% of the neonate’s body weight. This reduces to around 2% by 1 year. The liver receives around 25% of the cardiac output at birth.
Hepatic function is also fairly immature at birth. Phase 1 reactions function at about ⅓ of the adult level. The biochemistry of phase 2 reactions is different with sulphation taking on a much more active role.
HbF is replaced by adult HB in the first months of life
Hb synthesis switches from the liver to the bone marrow after delivery
The neonate has high levels of foetal haemoglobin (HbF), which provided an advantage for oxygen transport in the hypoxic conditions of the womb. The p50 of HbF is 2.0 kPa, compared to 3.3 kPa in the adult.
The foetus also has a notably higher Hb concentration, at around 16-18 g/dl, again to aid oxygen transport. In the foetus, haemoglobin and RBC synthesis occurs in the liver. Following delivery this switches to the bone marrow. As this transition occurs there is a drop in total circulating Hb to about 10g/dl (at around 3 months), before the production of adult Hb has built up significantly.
High susceptibility to heat loss (BSA to weight ratio)
High total body water percentage at birth
Children have a relatively large head compared with adults, due the size of the developing brain.
Children have a large body surface area to weight ratio, meaning that they are particularly susceptible to heat loss and hypothermia. This can put an increased strain on the metabolic system of the foetus.
At birth, neonates have a total body water percentage of around 80%. This has dropped to around 60% at 1 year. A significant proportion of this total body is represented as a higher percentage of extracellular fluid.
Links & References
Best, C. Developmental anatomy. e-LFH. 2012.
Cote, C. Paediatric anesthesia. In: Miller’s Anaesthesia (7th ed.)
Barker, I. Adaptation to extrauterine life. e-LFH. 2012
Barker, I. Changes in major organs, blood and biochemistry. e-LFH. 2014
Meakin, G. Development and the neuromuscular system. e-LFH. 2014.