Anaesthesia

16. Paediatric Anaesthesia

16. Paediatric Anaesthesia In paediatric anaesthesia, two main points should be remembered: (a) the difference between th e youn g developin g chil d an d th e matur e adul t i n relationshi p t o th e structure and function o f some of the important systems, (b) the differences i n the respective responses to the administered drugs and their significance (Chan and Lett, 1974). Anatomy and Physiology 1. General Considerations A normal neonat e has a weight of approximately 5% , a body are a o f approxi mately 11 % and a length of approximately 33 % of that of an adult. The neonate's head is large in relationship to the body. Also, because of the relatively large liver and small pelvis, the abdomen is usually full and protuberant. 2. Respiratory System The larynx. At birth it occupies a more cephalad position. It is located at the level of the 4th cervical vertebra. The adult larynx is at level of cervical vertebra 6. The epiglottis. It is folded and inclined at an angle of 45 degrees to the base of the tongue. Thus different apparatu s and intubation techniques are required in the newborn . The trachea. I n the newborn it has an average diameter o f 6 mm, while the adult trachea measures about 15 mm in diameter. Bronchi. In children there are proportionately more glands in the major bronchi, the cartilage and connective tissue, but there is significantly les s muscle, particularly in children below the age of one year. (This might help to explain the relative lack of response to bronchodilating agents in small children). The alveoli. At birth there are about 8 million alveoli, increasing to about 300 million by the age of 8 years (close to the adult number). At birth the mean alveolar surface area is about 0.4 mm2 . This size does not change much until late childhood. The adult alveolus measures about 1 mm2 . Growth in lung volume in the early years is 286 Anaesthesia and Other Specialties therefore mainly related to the increase in the number of alveoli, while in adolescence it is related mainly to increase in alveolar size. Respiration. Whe n resting , infants normall y breath e throug h thei r nose . Therefore , any nasa l obstructio n wil l mak e respiratio n difficult . Th e coug h refle x i s normall y active at birth, as is Head's reflex (baby takes a gasp when pressure is applied t o the chest). Infants weighin g less than 2 kg exhibit 'periodic breathing' (period of breathing associated with periods of apnoea lasting 5-10 seconds) . This is probably due to immaturity of the respiratory centre . The average minute ventilation in the newborn is 220 ml/kg (a s compared t o 10 0 ml/kg o r s o in the adult). The tidal volume s an d deadspace are proportionally simila r to those in adults (tidal volume 6.6 ml/kg an d deadspace 2.2 ml/kg). Because th e rib s ar e place d mor e horizontall y an d th e ri b cag e i s quit e sof t i n infants, ther e i s only a relativel y smal l increas e i n intrathoraci c volum e durin g in spiration . Occasionall y on e see s paradoxica l movemen t o f th e ri b cag e durin g in spiration , no t onl y durin g respirator y obstruction . Becaus e respiratio n i s largel y diaphragmatic, anythin g that interfere s wit h the descent of the diaphragm (e.g . gastric distension, etc.) will impede adequate respiratory exchange . As newborn have a high oxygen consumption (6. 5 ml/kg as compared t o 3.5 ml/kg in adults) and also a higher respiratory rate, the respiratory reserve in the newborn is rather limited . A newbor n o f 3 kg has a deadspac e o f 6-7 m l (approx.). Usin g a face-mas k fo r anaesthesia woul d ad d anothe r 1 0 ml to this , increasing th e deadspace b y approxi mately 150% ! It might, for this reason, be better to employ endotracheal anaesthesi a for infant s unde r 3 months . However , a s th e endotrachea l tub e itsel f ha s t o b e narrow, i t will add significantl y t o airway...