Introduction		The Brain
Ventilation		Temperature
Intubation		Other Anatomical Points
Blood		Inguinal Hernia
Body Water		Ilioinguinal Block
Extra Cellular Space		Circumcison
Kidneys		Consequences of Hypovolemia

Introduction

This paper will attempt to show how a sound knowledge of anatomy, physiology, pharmacology and psychology of infants and children and how they differ from adults helps to improve their care during anaesthesia and the perioperative period.

A baby can be taken from the parent without undue distress up to 6 to 7 months of age while an older infant or young child will become very distressed and hence they should be accompanied by a parent to induction of anaesthesia provided the parent doesn't exhibit undue anxiety. In such cases it may be better to give the child some premedication. Older children can cope but many like to be accompanied by a parent. Sometimes children, particularly boys of 8 to10 years old, appear well adjusted when seen beforehand but show signs of extreme apprehension when they reach the induction room - this presents as almost invisible veins so that venepuncture is difficult. Again, premedication should be considered. Midazolam 0.2-0.3 mg/kg given orally 30 to 45 minutes before anaesthesia usually has a tranquilising effect. If a child is very distressed on arrival at the operating theatre 0.2mg/kg can be squirted into the nose. It may sting but it usually has an effect within about 10 minutes. In older children diazepam (0.3 mg/kg) or temazepam can be given an hour before induction. This may be accompanied by an analgesic such as paracetamol (30mg/kg orally). An apprehensive patient has an increased cardiac output, which is largely redistributed to muscle so that the injected or inhaled induction agent does not reach the target organ - the brain - unless substantially increased doses are given. The increased ventilation through crying does not necessarily speed induction - the increased uptake merely compensates for the drug which has been redistributed to muscle.

It is often said that children are like small adults. This is not so - for a start their proportions differ. Infants have a relatively large head and therefore brain. It must receive a greater proportion of cardiac output as a consequence. The surface area is larger and thus heat loss is increased when it is exposed, especially in neurosurgery.

The surface area:body weight ratio is double in infants compared to adults resulting in greater heat loss. Oxygen consumption relative to body weight is also double (6-7 mls/kg/min). This is another key to working out important differences because if they need double the volume of oxygen then they have to double the amount taken in and transported. This means the alveolar ventilation must be increased which is largely achieved by increasing respiratory rate. Cardiac output must also be doubled to carry the oxygen around the body - this is achieved by increasing heart rate as babies have a very limited ability to increase stroke volume. Thus heart rates of 120 - 160 are common. The increased work of doing this is minimized by having a lower vascular resistance so that babies systolic blood pressures are lower ( 70 - 80 mmHg ).

The fixed stroke volume in infants is also important because anything that causes bradycardia such as hypoxia, deep halothane anaesthesia, or reflex bradycardia due to vagal stimulation, such as occurs during laryngoscopy, will result in a decrease in cardiac output. When combinations of these occur serious decreases in output can result.

Cardiac output can be assessed clinically with a stethoscope because heart sounds become softer as the output decreases. Normally blood flow into the ventricles or into the aorta and pulmonary artery causes expansion followed by an elastic recoil which slams the valves closed resulting in loud heart sounds. If the volume decreases the recoil is diminished and the resulting heart sounds become soft. When the cause is corrected, such as by giving blood or fluids in hypovolaemia, one can hear the sounds becoming louder. The stethoscope is thus a very useful and sensitive monitor with which it is also easy to differentiate between patient and equipment problems when a monitor such as an oximeter gives abnormal readings. 

Ventilation

Ventilation is greatly influenced by the anatomical differences, especially the structure of the chest wall. The ribs in neonates are more horizontal limiting antero- posterior expansion of the chest and they lack the bucket handle movement of the middle ribs that allows lateral expansion of the thoracic cage in older patients. The consequence is that ventilation is much more dependent on diaphragmatic movement and hence anything that restricts it (abdominal distention or compression) will cause respiratory difficulties. This includes inflation of the stomach with gas which can occur during ventilation with a mask when too high a pressure is applied or the bag is squeezed too fast thereby forcing gas down the oesophagus as well as the trachea. In patients with oesophageal atresia stomach distention is more likely with positive pressure ventilation when there is a large fistula. This can been assessed beforehand with a lateral chest X ray which shows the air containing fistula. Beware if this is more than 2.5mm in diameter. Patients in the lithotomy position have their abdominal contents compressed forcing the diaphragm up and restricting ventilation. 

Intubation

Intubation technique is important because infants have a higher oxygen consumption (6-7ml/kg/minute compared to 3 in an adult ). This results in there being a shorter time before hypoxia begins to develop when a paralysed baby is not being ventilated. There are anatomical differences in the airway which are relevant. The larynx is situated at a higher level relative to the vertebrae - C3 in the infant compared to C6 in the adult; the epiglottis is U shaped and relatively longer, the angle of the mandible is greater (120 degrees) and the trachea has an anterior inclination. In addition the relatively large head does not need to be on a pillow but needs to be stabilized. This can be done by slightly extending the neck, rolling the thenar eminence of the right hand on to the forehead to stabilize it, then opening the mouth with the index finger and inserting the laryngoscope with the left hand down the right hand side of the mouth so that the tongue is kept out of the way. If the laryngoscope is held between the thumb and index finger the little finger of the left hand can reach to press the larynx backwards thus bringing the larynx into view (figures 1 - 3). The tube can then be passed from the right corner of the mouth so that it does not obstruct the view of the larynx. The important anatomical points in relation to the tube are that the cricoid cartilage forms the narrowest part of the larnyx before puberty and because it is circular an uncuffed tube can be used until 10 -12 years of age. Another convenient point is that the nose accommodates the same size of tube as the larynx before puberty. Tracheal length is often quoted to be 4cms but Anneke Meursing showed that the mean length is 4.5cms in a 3 kg baby. The importance of tracheal length is to appreciate how far the tube can be passed without going into the bronchus. The problem is that there are occasional babies who have short tracheas. It is thus important always to check after intubation that both lungs are being ventilated.

Blood

Blood volume and haemoglobin are greater in newborns. The volume is about 80-85 ml.

Haemoglobin at full term is about 180-200gm/l decreasing to about 110gm/l at 3-6 months. This haemoglobin is predominantly fetal with alpha and gamma chains which enable it to take up oxygen at low tensions such as exist in the placenta but do not release it as readily to the tissues. Gradually it changes to adult haemoglobin (alpha and beta chains). As lowering PaCO2 shifts the oxygen dissociation curve to the left, hyperventilation further reduces oxygen delivery to the tissues so that excessive hyperventilation should be avoided even by increasing dead space in the circuit - not shortening the endotracheal tube will help.

In neonates with a high haemoglobin, albumin rather than blood can be used for early transfusion, when needed. In premature infants haemoglobin tends to be low because most of the iron stores are laid down in the last three months of pregnancy.

Several factors should be considered in deciding that it is necessary to start a blood transfusion. The haemoglobin should be at a level above that which supplies minimal oxygen requirements for metabolism. In infants metabolic rate is higher, the haemoglobin level may be higher in full term babies but lower in prematures and between 3-6 months so that the tolerated blood loss will vary. A 20% loss is usually well tolerated provided fluids are given to maintain the circulating volume. At that point one might consider whether blood loss is likely to continue and, if haemoglobin was low to start with, then blood may be started. On the other hand, clinical signs such as a rising pulse, when apparently adequate fluids have been given or a bolus does not reduce the pulse in a patient who also looks pale, will usually suggest that it is time to start blood. 

Body Water

The total body water is about 80% of body weight at birth, gradually decreasing with age to 60-65 % in adults. Premature infants have relatively more, making fluid loss an even more critical problem to them. When neonates and infants become dehydrated they initially lose extracellular water. Because the extracellular space is relatively larger at this age (about 50% of body weight) the losses will be proportionately greater.The relatively smaller intracellular compartment then has less fluid to shift to the extracellular space when losses occur resulting in a much sicker infant than an adult might be in similar circumstances. 

Extracellular Space

The other consequence of the relatively large extracellular space is that drugs predominantly distributed in the extracellular space will have to be given with a larger loading dose. Also, extracellular electrolytes such as chloride will have to be given in larger amounts to correct deficits which occur, for example in pyloric stenosis, because of the larger extracellular compartment. 

(Continued ...)

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