Download Update 11 - Update in Anaesthesia - WFSA

74Update in AnaesthesiaIt is often said that children are like small adults. This isnot so - for a start their proportions differ. Infants have arelatively large head and therefore brain. It must receive agreater proportion of cardiac output as a consequence.The surface area is larger and thus heat loss is increasedwhen it is exposed, especially in neurosurgery.The surface area: body weight ratio is double in infantscompared to adults resulting in greater heat loss. Oxygenconsumption relative to body weight is also double(6-7 mls/kg/min). This is another key to working outimportant differences because if they need double thevolume of oxygen then they have to double the amounttaken in and transported. This means the alveolar ventilationmust be increased which is largely achieved by increasingrespiratory rate. Cardiac output must also be doubled tocarry the oxygen around the body - this is achieved byincreasing heart rate as babies have a very limited abilityto increase stroke volume. Thus heart rates of 120 - 160are common. The increased work of doing this is minimizedby having a lower vascular resistance so that babiessystolic blood pressures are lower ( 70 - 80 mmHg ).The fixed stroke volume in infants is also important becauseanything that causes bradycardia such as hypoxia, deephalothane anaesthesia, or reflex bradycardia due to vagalstimulation, such as occurs during laryngoscopy, will resultin a decrease in cardiac output. When combinations ofthese occur serious decreases in output can result.Cardiac output can be assessed clinically with astethoscope because heart sounds become softer as theoutput decreases. Normally blood flow into the ventriclesor into the aorta and pulmonary artery causes expansionfollowed by an elastic recoil which slams the valves closedresulting in loud heart sounds. If the volume decreases therecoil is diminished and the resulting heart sounds becomesoft. When the cause is corrected, such as by giving bloodor fluids in hypovolaemia, one can hear the soundsbecoming louder. The stethoscope is thus a very usefuland sensitive monitor with which it is also easy todifferentiate between patient and equipment problems whena monitor such as an oximeter gives abnormal readings.Ventilation is greatly influenced by the anatomicaldifferences, especially the structure of the chest wall. Theribs in neonates are more horizontal limiting anteroposteriorexpansion of the chest and they lack the buckethandle movement of the middle ribs that allows lateralexpansion of the thoracic cage in older patients. Theconsequence is that ventilation is much more dependenton diaphragmatic movement and hence anything thatrestricts it (abdominal distention or compression) will causerespiratory difficulties. This includes inflation of the stomachwith gas which can occur during ventilation with a maskwhen too high a pressure is applied or the bag is squeezedtoo fast thereby forcing gas down the oesophagus as wellas the trachea. In patients with oesophageal atresiastomach distention is more likely with positive pressureventilation when there is a large fistula. This can beenassessed beforehand with a lateral chest X ray whichshows the air containing fistula. Beware if this is more than2.5mm in diameter. Patients in the lithotomy position havetheir abdominal contents compressed forcing thediaphragm up and restricting ventilation.Intubation technique is important because infants have ahigher oxygen consumption (6-7ml/kg/minute comparedto 3 in an adult ). This results in there being a shorter timebefore hypoxia begins to develop when a paralysed babyis not being ventilated. There are anatomical differences inthe airway which are relevant. The larynx is situated at ahigher level relative to the vertebrae - C3 in the infantcompared to C6 in the adult; the epiglottis is U shapedand 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 beon a pillow but needs to be stabilized. This can be doneby slightly extending the neck, rolling the thenar eminenceof the right hand on to the forehead to stabilize it, thenopening the mouth with the index finger and inserting thelaryngoscope with the left hand down the right hand sideof the mouth so that the tongue is kept out of the way. Ifthe laryngoscope is held between the thumb and indexfinger the little finger of the left hand can reach to pressthe larynx backwards thus bringing the larynx into view(figures 1 - 3). The tube can then be passed from the rightcorner of the mouth so that it does not obstruct the viewof the larynx. The important anatomical points in relationto the tube are that the cricoid cartilage forms thenarrowest part of the larnyx before puberty and becauseit is circular an uncuffed tube can be used until 10 -12years of age. Another convenient point is that the noseaccommodates the same size of tube as the larynx beforepuberty. Tracheal length is often quoted to be 4cms butAnneke Meursing showed that the mean length is 4.5cmsin a 3 kg baby. The importance of tracheal length is toappreciate how far the tube can be passed without goinginto the bronchus. The problem is that there are occasionalbabies who have short tracheas. It is thus important alwaysto check after intubation that both lungs are being ventilated.