Download Update 11 - Update in Anaesthesia - WFSA

78Update in AnaesthesiaVOLATILE ANAESTHETIC AGENTSProfessor P Fenton , Queen Elizabeth Central Hospital, Blantyre, MalawiOne of the prominent features of anaesthetic practice indeveloping countries is the widespread use of volatileanaesthetic agents. This is surprising, as they are relativelyexpensive. Even modest supplies of halothane, forexample, can cost several times more than the salary ofthe person using it but despite this burden on limitedbudgets, in most government hospitals cases are done usinggeneral anaesthesia with halothane and no other drug.However, many mission hospitals favour spinal anaesthesiafor reasons of cost.The demise of inhalation anaesthesia is sometimespredicted, partly because of cost and partly because ofpollution of the atmosphere. Total intravenous anaesthesiamay one day replace it. This event is probably far awayand volatile agents will remain a central part of anaesthesiapractice for many years to come.An important safety feature of all volatile agents is thatmost of what goes into the patient via the lungs shouldcome out the same way. Therefore the anaesthetic effectis reversible, as long as the patient is breathing. Also, withspontaneous breathing, the patient adjusts his or her own“dose “ and respiratory depression will reduce the amountof vapour taken up and help prevent overdose. Withcontrolled ventilation it is very easy to give an overdose.A typical general anaesthetic (GA) using halothane orether and nothing else is “bumpy”, often unpleasant forthe patient during induction and recovery, but reasonablysafe.The cost of some of the newer agents is very great andthey are not generally used in developing countries. Thecheaper, older agents, like ether, though widely used inpoorer countries, are hardly ever used in the west. Mostanaesthetists in the western world today have never givenether anaesthesia.How do volatile agents work?An agent breathed into the lungs will dissolve first in theblood and then be carried to all parts of the body anddissolve in the tissues. The agent that dissolves in the brainproduces the state of anaesthesia. The brain, being mostlyfat, absorbs a lot of the agent. Many theories have beenconsidered to explain how anaesthesia is produced. Onesuggests that the fat in the cell wall swells up. This reducesthe ability of the nerves to conduct impulses to each otherand activity is reduced, or stopped altogether if you givean overdose. Fortunately, the higher centres controllingconsciousness are the first affected and the vital centressuch as the respiratory and vasomotor centres are moreresistant to this effect. Thus we take it almost for grantedthat the anaesthetised patient will go on breathing with anear-normal pulse and blood pressure.There are four broad physical properties of any agent thatwill tell the anaesthetist how it behaves in and out of thebody and, therefore, how to use it to best advantage.1. Solubility and Uptake. The blood solubility of an agentis related to its blood-gas partition coefficient. The partitioncoefficient is a simple ratio of amounts: eg. the blood/gascoefficient is the ratio of the amount dissolved in blood tothe amount in the same volume of gas in contact with thatblood. The more blood-soluble the agent (high bloodgaspartition coefficient), the slower the onset of effectand the slower the patient goes to sleep. Thus a very solubleagent eg. ether will dissolve in large quantities in bloodbefore the brain levels can rise sufficiently to produceanaesthesia. To understand this concept, think of thecirculating blood volume as a large pool, soaking up agentand not allowing the brain to have any.An anaesthetic agent does not “target” the brain: itdissolves in all tissues according to the tissue/gas partitioncoefficient for the particular agent in a tissue type. Theblood flow to that tissue and the mass of tissue presentwill also determine the amount of agent reaching it andaccumulating there. Fat stores, like the brain, have a veryhigh affinity for anaesthetic agents. Luckily for the inductionof anaesthesia, body fat has a very poor blood flow andduring a short or medium length operation, only a limitedamount of agent will have dissolved there.Similarly, a high cardiac output such as may be found infever or fear will cause more agent to be dissolved in bloodand tissues other than brain, thus delaying the onset ofCNS effects. In all these instances, there is said to be ahigh uptake of the agent into the body, i.e. the venousblood returning to the heart has a low concentration of theagent and there is room for lots more. Paradoxically, thougha high uptake means a lot of agent is disappearing intothe body, blood levels rise slowly and the patient takes along time to go to sleep by inhalation.