Paediatric anatomy and physiology

The paediatric airway is different from that of an adult and changes in size, shape and position throughout development (Subhash 2004); these differences gradually reduce as children age (Allman and Wilson 2006). Airway and respiratory problems are the most common cause of morbidity under general anaesthesia in children (Adewale 2009). Most complications are caused by laryngospasmfollowing a difficult intubation or an overdose of anaesthetic drugs (Hatfield and Tronson 2009); inability to ventilate is also important (Prasad 2012). Variation in the size of the patient, the equipment available and anaesthetic technique mean that clear communication between the anaesthetist and other theatre staff is vital to ensure patient safety. The respiratory system has five functions (Martini 2006):

1. To provide extensive surface area for gas exchange between air and circulating blood.
2. To move air to and from the exchange surfaces of the lungs along the respiratory passageways.
3. To protect respiratory surfaces from dehydration, temperature changes, other environmental changes and invading pathogens.
4. To produce sound.
5. To assist the detection of olfactory stimuli receptors in the superior nasal cavity.

Airway anatomy

Paediatric patients have (Macfarlane 2005):

• A relatively large head with a short neck.
• A prominent occiput.
• A relatively large tongue.

This large tongue decreases the amount of space available in the oral cavity and obstructs the airway more easily than in adults (Subhash 2004). The paediatric larynx sits higher in the throat than that of an adult; the narrowest part of the airway is below the chords at the cricoid.

The smaller airways of the paediatric patient mean that any oedema reduces the diameter of the trachea significantly (Hatfield and Tronson 2009). Intubation reduces the cross-sectional area of the trachea in a similar way to oedema. This causes increased airway resistance which, when coupled with the effects of volatile agents, means that a paediatric patient is likely to require controlled ventilation.

Figure 1

Infants are obligate nasal breathers (physiologically they have to breathe through the nose) with small nasal apertures; this increases the resistance to airflow and the work of breathing, contributing to difficulties in airway management under general anaesthesia (Subhash 2004). The cartilaginous support of the airway is significantly lower in paediatric patients than in adults, leaving infants and children susceptible to dynamic airway collapse when obstruction is present. Studies have shown a correlation between airway obstruction during general anaesthesia and reduction in laryngeal muscle tone (Subhash 2004).

The heart rate and respiratory rate fluctuate noticeably in paediatric patients (Table 2). The high oxygen demands in children mean a faster respiratory rate, and stiffer lungs mean that there is little capacity for the lungs to establish functional residual capacity (Hatfield and Tronson 2009). As a result of low functional residual capacity, paediatric patients desaturate rapidly, becoming hypoxic and cyanosed; a falling heart rate may provide advance warning of this. Hatfield and Tronson (2009) state that cardiac arrest is impending and brain damage has already occurred by the time hypoxia triggers bradycardia (Figure 2).

Table 2

The ventricles of the young heart are less compliant than those of an adult heart, producing less tension during contraction. This contributes to limited stroke volume, blood pressure and tissue perfusion (Macfarlane 2005). A limited stroke volume means that fluid resuscitation does not improve cardiac output in patients aged under two years in the same way as in an adult, and may not elevate a falling blood pressure. Response to falling blood pressure is decided by the anaesthetist since drug intervention is required. The drugs used to elevate the blood pressure of a paediatric patient are the same as those used for adults, but in different doses, so the operating department practitioner or anaesthetic nurse should have the usual emergency drugs available (Samuels and Wieteska 2005). Samuels and Wieteska (2005) suggest that fluid response in patients aged over two years is similar to that in adults.

Figure 2

McDougall (2013) states that blood pressure is an unreliable guide to circulatory status and Pasman and Watson (2015) suggest that infants and children are able to preserve their central blood pressure in an attempt to protect the heart and brain, reducing perfusion to the extremities. Aside from the normal response to painful stimuli, tachycardia is an early warning sign of an inability of the body to meet metabolic demands, which occurs before development of recognisable hypotension.

Therefore, the placement of electrocardiograph leads to monitor the patient is vital, and they should remain in place until the last moment possible before transfer to the recovery area. Electrocardiography is the best indication of the patient’s condition.

Metabolism

The resting metabolism of an infant is two to three times higher than that of an adult because of the infant’s high nutritional requirements (Phillips 2007). The potential for metabolic imbalance complications, for example hyperglycaemia, increases proportionately with the duration of fluid restriction (Litman 2013). Therefore, paediatric patients should be given priority on the surgical list.

Fluid and electrolyte balance

The renal system of a child generally reaches a level of performance equivalent to that of an adult by the age of two months (Hatfield and Tronson 2009). Fluid warmers and burettes should be available and used when volumes to be infused are large in relation to patient size and/or when fluid administration could result in an unwanted reduction in temperature (The Royal College of Anaesthetists 2015); small boluses of fluid may be given through a cannula. Fluid given to paediatric patients should be isotonic, for example Hartmann’s solution (compound sodium lactate), unless indicated otherwise; there is currently no specific paediatric fluid available in Europe (Disma et al 2014). Intravenous (IV) fluids should be used where blood loss is anticipated, in intra-abdominal cases and in procedures expected to exceed 30 minutes (Macfarlane 2005). There are smaller margins of safety for blood loss in the paediatric patient, and a loss of around 10% may indicate the need for replacement (Phillips 2007).

Inhalation agents can cause myocardial depression resulting in hypotension, and often this may not be noticeable until 50% of circulating volume has been lost. Hypertension is unusual in a child.

Hypovolaemia may be indicated by tachycardia, accompanied by cold hands and poor capillary refill, leading to shock (Figure 3) (Hatfield and Tronson 2009).

Figure 3

Temperature regulation

Patients lose body heat quickly under anaesthesia (Phillips 2007).

Factors effecting the maintenance of temperature

Infants under three months do not have a shiver response and have a higher heat loss rate in proportion to the amount they can generate. Infants’ thermoregulatory mechanisms are not fully developed, and their thin layer of brown insulating fat and poor vasoconstrictive mechanisms add to the effects of the general anaesthetic (Macfarlane 2005). Low body temperature can cause:

• Respiratory depression.
• Acidosis.
• Reduced cardiac output.
• Increased risk of infection.
• It can also extend the duration of action of drugs (Macfarlane 2005).

Tachycardia and tachypnoea are the initial signs of hypothermia (Figure 4). They are caused by the body’s attempt to compensate for temperature loss by circulating warm blood before the onset of bradycardia and shallow respirations as the metabolic process slows down. Hypothermia is the most common cause of blood failing to clot (Hatfield and Tronson 2009). Methods of reducing heat loss include (Allman and Wilson 2006):

Figure 4

• Warm blankets.
• Forced air warming blankets.
• Warmed fluids and solutions.
• Wrapping exposed body parts, such as the head.

All of these options should be available for use during anaesthesia (The Royal College of Anaesthetists 2015).

Fasting

Fasting times are important in preventing potential pulmonary aspiration because of general anaesthetic, but it is common that recommended fasting times are exceeded (Arun and Korula 2013). In their audit of fasting in children, Arun and Korula (2013) showed that education of nursing staff and a more co-ordinated approach by the peri-operative team could significantly reduce inappropriate fasting in the paediatric patient. The Association of Anaesthetists of Great Britain and Ireland (AAGBI) (2010) guidelines for paediatric fasting before a procedure are:

• For infants, a normal diet or formula milk up to six hours before, breast milk up to four hours before and clear liquids up to two hours before.
• For children aged one year and over, a regular diet up to six hours before.

Learning points

1. The function of the respiratory system is to provide extensive surface area for gas exchange, to move air to and from the exchange surfaces of the lungs along the respiratory passageways, to protect respiratory surfaces, to produce sound and to assist the detection of olfactory stimuli receptors.
2. Intravenous (IV) fluids should be used where blood loss is anticipated, in intra-abdominal cases and in procedures expected to exceed 30 minutes.
3. In children, renal function reaches maturity at about two months so fluid warmers and burettes should be available and used when volumes to be infused are large in relation to patient size and/or when fluid administration could result in an unwanted reduction in temperature.
4. Low body temperature in a paediatric patient can cause respiratory depression, acidosis, reduced cardiac output and increased risk of infection; it can also extend the duration of action of drugs.
5. Fasting times are important in preventing potential pulmonary aspiration because of general anaesthetic. The recommended fasting time before surgery for infants is a normal diet or formula milk up to six hours before, breast milk up to four hours before and clear liquids up to two hours before and for children aged one year and over is, a regular diet up to six hours before.