Download Update 11 - Update in Anaesthesia - WFSA

76Update in AnaesthesiaKidney function is immature at birth and although thevarious functions develop at different rates there is a rapidimprovement in the first few weeks of life. The relevanceis that fluids ,electrolytes and drugs excreted by the kidneyare handled more slowly during the early days and weeksof life. Glomerular filtration is less, the cortical tubules whichare important in sodium excretion are not fully developed,and the interstitial urea concentration in the loops of Henleis low (because the amino acids are being utilized to buildcells) and hence water reabsorption is reduced.The brain is immature. Centrally acting drugs such asmorphine and barbiturates have a greater depressant effectand thus have to be used in reduced doses, if at all.The temperature regulating centres are also immature sothat body temperature control is less efficient. This problemis aggravated in neonates because they have a relativelylarge surface area (2-2.5 times relative to weight), thinskin and subcutaneous fat so that they are poorly insulatedand their body mass is less so that the body stores lessheat. Neonates do not shiver so that they cannot respondto a cold environment. During anaesthesia the temperaturecontrol mechanisms are depressed so that methods ofmaintaining body heat must be instituted. These includeoverhead heaters, warming blankets, warming inspiredgases and fluids and covering parts of the body not beingoperated upon.Other anatomical points are important in regional andlocal anaesthesia. The spinal cord and dura mater reachlower levels in neonates (L3 and S3), the iliac crests arenot fully developed so that the line between them is onevertebral space lower. Fascia and aponeurosis are thinnerand therefore not as easily detected when used as depthmarkers during nerve blocks. They can be located moreeasily moving the needle up and down until a scratchingsensation is felt or by angling the needle so that the traversethrough the layer is thicker.An understanding of the basic sciences is helpful inoptimally managing our smallest patients duringanaesthesia. In the next section anaesthesia for somecommon operations will be considered highlighting theapplication aspects of basic sciences to the clinicalmanagement.Inguinal hernia repair is a common operation in youngchildren, especially ex-premature infants. In the latterpatients the abdominal wall is weak and the normalobliteration of the sac has not occurred. The infant bornprematurely has deficient iron and glycogen stores becausethese are laid down mainly in the last three months ofpregnancy. Thus they tend to be anaemic and susceptibleto hypoglycaemia unless glucose is administered. Inaddition, the factors which increase heat loss areexaggerated so that particular care is necessary to maintainbody temperature.There are several options for anaesthesia. Generalanaesthesia for hernia repair in infants can be used ifthere is no history of apnoea. Even if there is thiscomplication can be largely avoided postoperatively if thepatient is ventilated with air instead of nitrous oxide andPEEP of 2-3cm water is applied.The use of air prevents denitrogenation of the lungs and,together with PEEP, prevents atelectasis which results inincreased work of breathing and fatigue in ex-prematuresand is a major cause of postoperative hypoxaemia in manypatients. The anaesthetic consists of muscle relaxation,ventilation with an inhalation agent and preferably a localanaesthetic block rather than opioids so that respiratorydepression is avoided.In prematures spinal analgesia in advocated by someanaesthetists because there are fewer respiratory problemsif they are immobilised rather than being anaesthetised.The fact that the iliac crests are level with one intervertebralspace lower is fortuitous as the spinal cord also ends onespace lower. A 25 needle is often used to administerbupivacaine 0.5%. An alternative is to use caudalanaesthesia aiming to reach at least T10.The ilioinguinal block as originally described involvedplacing local anaesthetic under the external obliqueaponeurosis thus blocking the ilioinguinal andiliohypogastric nerves as they approach the skin. Thisprovides adequate surface anaesthesia but does notanaesthetise the area around the inguinal sac. This can beachieved by placing local anaesthetic in the layer betweenthe internal oblique and transversus abdominis muscles. Ifa short bevelled needle is available it makes it easierto feel the loss of resistance as the aponeurosis ispenetrated 1-2 cm medial to the anterior superior iliacspine depending on the size of the patient. If one is notavailable the aponeurosis can be located by moving theneedle horizontally as it is gradually advanced until a gratingor rough sensation is felt. The needle is then advancedthrough the aponeurosis and a pop may be felt especiallywith a short beveled needle. 0.25 ml/kg of 0.25%bupivacaine can be injected to produce surface analgesia.The needle is then advanced slowly with gentle pressureon the plunger of the syringe. It is difficult to inject into