VOL21 NO4 ANESTHESIA OCTOBER 1966 The use of pre-mixed nitrous oxide and oxygen in dental anaesthesia A clinical trial of the 50,/50 mixture in general dental practice JOHN LATHAM, FFARCS Consultant Anzsthetist and Clinical Lecturer GEOFFREY D. PARBROOK, FFARCS Senior Registrar and Clinical Tutor in Anzsthetics The Royal Infirmary and the University of Aberdeen Pre-mixed cylinders of nitrous oxide and oxygen in a 50/50 mixture are now available for use by midwives for the relief of pain in labour. This study was undertaken to see whether the mixture might also provide the basis of a more convenient and safer system of dental anasthesia. Much of the credit for the original idea of this use of pre-mixed gases must go to our colleague Dr M. E. Tunstall who used the mixture in dental practice in 1962 and encouraged us to conduct this trial. APPARATUS Two machines were investigated; the one was basically a demand flow apparatus (figure 1) and the other continuous flow (figure 2). The demandjow system This was analogous to the conventional Walton gas machine; there was provision for superimposed continuous flow (figure 1). An aqualung demand regulator (Siebe Gorman Merlin valve ) was attached to the cylinder. Minor modifications necessary included provision of a cylinder pressure gauge and an arrangement for the production of a continuous flow. Initially, continuous flow was achieved by means of a balloon on the diaphragm of the valve and later by means of a special screw through the cover of the diaphragm. The flow rate was not monitored but the flow was adjusted to keep the reservoir bag full. 472
Vol21 ANESTHESIA 473 FIGURE 1 Demand flow system. A spring-loaded screw on the diaphragm of the demand valve allows one to superimpose a continuous flow. The apparatus may be used without a reservoir bag, provided a halothane vaporiser of low internal resistance is used, and the Braun nosepiece may be replaced by a Goldinan nasal mask. (The expiratory valve proximal to the nasal tubings is turned off except when the apparatus is used with standard anasthetic face masks.) The continuousflow system This resembled the conventional arrangement of the Boyles ansesthetic machine (figure 2). A standard Adams reducing valve, cylinder gauge and fiowmeter were used. In both cases the apparatus was mounted on to a 50/50 nitrous oxide oxygen cylinder and a Fluothane vaporiser capable of giving up to 4% halothane was incorporated. The gases were delivered to the patients via a reservoir bag through either a Goldman nasal mask or a Braun nosepiece.
474 ANESTHESIA v0121 FIGURE 2 Continuous flow system. The gases are metered through a rotameter to the halothane vaporiser and a reservoir bag must be incorporated in the gas delivery system. (The expiratory valve proximal to the nasal tubings is turned off when the apparatus is used with dental nose pieces.) The demand system TECHNIQUE The demand system with superimposed continuous flow was first tried. It was used to anasthetise eight hundred patients in some ten general dental practices in Aberdeen over a period of four months. The patients undergoing dental extractions were unselected and consecutive. All the eight hundred cases had had no food for a minimum
Vol21 ANESTHESIA 475 of four hours before the operation. Resuscitative drugs, syringes, laryngoscope, endotracheal tubes and aspirators of the high speed type were available in all surgeries. The number of teeth extracted varied from one to total clearances. The particular technique of anasthesia used depended on the age of the patient. Patients over twelve years old. Of the eight hundred patients three hundred and eighty were over twelve years, the eldest being a man of eighty years; these patients were induced with methohexitone and then maintained on the premixed 50/50 nitrous oxide supplemented by halothane. The average dose of methohexitone for females was 70mg of 1 % solution and for males 80mg. Patients under twelve years old. Four hundred and twenty of the patients were under twelve years, the youngest being three years old. These children were induced with the gas-oxygen mixture and halothane. ControZ series with a BoyZe s apparatus. Following this study a comparable series of a hundred consecutive cases were anasthetised with a standard Boyle s machine to see if any special differences of technique or of the amounts of drugs used occurred. Continuouspow system. A further one thousand consecutive cases were ansesthetised using the continuous flow system (figure 2). The age distribution and technique was similar to that described for the demand system. RESULTS Assessment was by comparison with the results obtained with the standard Boyle s machine used in the trial and by comparison with previous experience of many machines and systems of dental anathesia. Advantages The advantages of this method were found to be as follows. The portability of the apparatus due to its minimal size and light weight. The accuracy of the percentage composition of the gas-oxygen mixture and consequent increased safety. The simplicity of the apparatus and consequent lower cost of a production model compared with conventional apparatus. The economy of the method. The versatility of the system which may be modified to meet the anzesthetist s preference for demand or continuous flow or for high or low flow rates.
476 ANESTHESIA Vol21 The light weight is one of the main advantages for the anathetist who may need to visit several dental surgeries. The total weight of the apparatus is 131b (6kg) (cylinder included) in the demand system and 151b (7kg) in the continuous flow system. The accuracy of the gas composition gives added safety and eliminates the risk of any accidentally prolonged administration of pure nitrous oxide. Modern dental machines such as the Walton 52 and Cyprane AE3 provide a satisfactory degree of accuracy and will always be in demand for larger dental clinics. The older machines at present in use in many dental surgeries may be very inaccurate and dangerous 5. The itinerant dental anasthetist and those working in small dental surgeries will find that cylinders of pre-mixed gas provide a safer system of amesthesia than the use of old and inaccurate machines. One simple but important safety precaution must be taken with the pre-mixed cylinders. If the cylinders have been left outside in conditions of severe frost then they must be rewarmed and inverted a few times to mix the contents because separation of the mixture into nitrous oxide and oxygen can occur if the cylinders are exposed to temperatures of less than minus 7 C 1. The simplicity and versatility of this system of dental anzsthesia requires no further comment but special records were made to compare the relative economy of the pre-mixed gas system and the conventional system of dental ansesthesia. When the records of the amounts of halothane and methohexitone used on the one hundred patients ansesthetised with the Boyle s machine were compared with the other series of patients receiving pre-mixed gas it was found that the halothane and methohexitone were comparable. It was found that a 500 litre capacity cylinder of pre-mixed gas costing 15 shillings would anesthetise an average of forty cases whereas to anasthetise forty cases using gas and oxygen in separate cylinders cost 18 shillings. DISCUSSION There have been two epochs of dental anasthesia, firstly the Anoxic Epoch in which the patient was rendered cyanotic by the administration of nitrous oxide with 0 to 5% oxygen and secondly the Hypoxic Epoch in which percentages of oxygen of the order of 15 %, which is just enough to prevent the appearance of cyanosis is used. We have now entered the Pre-mixed Gas Epoch and it may, at first sight, appear odd that the considerable increase of oxygen concentration to 50% is now advocated.
voz 21 ANSSTHESIA 477 Mild degrees of hypoxia, equivalent to about 14% oxygen in nitrogen at sea level may occur in aircraft passengers. Although this degree of hypoxia might be considered permissible in an aircraft it is not necessarily acceptable in the case of the anzsthetised patient. It has been shown that approximately 35 % inspired oxygen is required to maintain normal levels of oxygen tension in the arterial blood in spontaneously respiring patients anasthetised with halothane 6. Alterations of pulmonary physiology and mild respiratory depression due to the anaesthetic are possible factors in this need for the higher oxygen percentage. Cyanosis is an unreliable index of hypoxia7 and its absence cannot be regarded as evidence that oxygenation is satisfactory during a dental anaesthetic. It is more difficult in dental anaesthesia than in general anaesthesia to guarantee a continuously unobstructed airway. Even if a wide bore delivery tube and modern apparatus are used to reduce the resistance to breathing caused by the anasthetic machine to a minimum, slight temporary arrests of respiration may still occur. These may be caused by such factors as depression of the chin during extractions, breath-holding during induction or following intravenous injections. The time required before serious hypoxia develops in such arrests is increased if a higher percentage oxygen has previously been inspired8 and for this reason 50% oxygen appears preferable in dental anasthesia in the chair to the usual 30 % to 35 % used in in-patient anasthesia. If a slight degree of rebreathing is allowed to occur during dental anasthesia then a higher oxygen percentage is needed than if a non-rebreathing circuit is used. This is a further problem as some dental anasthetists find a non-rebreathing system uneconomical where gas supply is concerned. It should be noted that if such a concentration of oxygen is used in the form of pre-mixed cylinders of nitrous oxide, it ceases to be necessary to stock extra cylinders of oxygen for emergency resuscitation as the 50 % oxygen-nitrous oxide cylinder will suffice. The 50% nitrous oxide component in the cylinder has the three beneficial actions of analgesia, sedation and compressibility. The analgesia and sedative effect of nitrous oxide are both marked with the 50 % mixture. Even 25 % nitrous oxide is a better analgesic than morphine9 and the degree of analgesia increases with concentration of the gas. In in-patient anzsthesia it has been shown that the nitrous oxide is a more important factor than the premedication in reducing the concentration of halothane which is required10 and by using nitrous oxide one can lower the halothane concentration needed and patients will awaken quicker. The nitrous oxide component of the pre-mixed cylinder gives the added benefit of greater compressibility and permits one to carry
478 ANESTHESIA Vol21 50% more gas in the cylinder than would be the case with oxygen alone. Disadvantages An alleged disadvantage of the use of pre-mixed gas is that the patient may not be induced so rapidly as with higher concentrations of nitrous oxide and that the smell of halothane is therefore more noticeable. However, 50% nitrous oxide has a very marked effect on all modalities of sensation and therefore, if the patient breathes 50% nitrous oxide for a short period before halothane is introduced, the smell of the halothane will not be noticed. Alternatively adult patients may be induced with methohexitone as in this trial. CONCLUSIONS Pre-mixed 50/50 nitrous oxide oxygen cylinders give prospects of a new era of better and safer dental anaesthesia and may also prove valuable in other out-patient situations. They provide an ideal basic anaesthetic mixture in compact form and can be used with an absolute minimum of light-weight, ancillary anzsthetic apparatus. SUMMARY Pre-mixed 50/50 nitrous oxide oxygen was used to anaesthetise 1800 patients using a light-weight, portable apparatus. The following benefits over conventional apparatus were noted. (1) Extreme portability. (2) Increased accuracy and safety of gas composition. (3) Simplicity and low cost of apparatus. (4) Economy of anaesthetic gases. (5) Versatility of the system which can be usedwith demand or continuous flow. The advantages of the use of 50% oxygen in dental anmthesia over conventional 10 % to 20 % are discussed. 50 % oxygen removes many of the risks of hypoxia inherent in the established dental anzesthetic techniques while the 50% nitrous oxide component of the cylinder gives analgesia, sedation and compressibility. Acknowledgements We wish to thank Dr M. E. Tunstall for the original suggestion which prompted this work. This paper is based upon our reports to the Third Conference on Dental Anzsthesia at the Eastman Dental Hospital on 11 June 1965 and we are grateful to Dr Victor Goldman for permission to reproduce the data we presented. Dr Parbrook currently holds a grant from the Medical Research Council for investigation of pre-mixed gases.
Vol21 ANWSTHESIA 479 References TUNS STALL, M.E. (1963). Br. med. J., 2, 915 SMITH, W.D. (1961). Br. J. Anaesth., 33,440 3HUNTER, J.D. and FRASER, A.C. (1959). Br. J. Anaesth., 31, 367 4GOLDMAN, v. (1958). Bri. dent.j., 105, 160 5PARBROOK, G.D. (1964). Br. dent.j., 117,115 ~NUNN, J.F. (1964). Br. J. Anaesth., 36, 327 'ICOMROE, J.H. and BOTELHO, S. (1947). Am. J. med. SC., 214, 1 ~WEITZNER, S.W., KING, B.D. and IKEZONO, E. (1959). Anesthesiology, 20, 624 gparbrook, G.D., REES, G.A.D. and ROBERTSON, G.S. (1964). Br. rned. J., 2,480 ~OSAIDMAN, L.J. and EGER, E.I. (1964). Anesthesiology, 25, 302