A NEW METHOD OF CLOSED CIRCUIT ANAESTHESIA UTILISING HALOTHANE AND METHOXYFLURANE R. E. CUST,B.V.Sc., D.D.A., M.A.I.A.S. “
Warlaby”, Oaklands Junction, Victoria, 3047
Introduction
This paper describes experiences with 1003 small animal surgical cases involving cats and dogs and in which the anaesthesia was maintained using low doses of methoxyflurane and halothane with a basal gas flow of 100% oxygen in a closed circuit system. Weaver (1 964), Rex (1 968) and other workers have stated that a balanced anaesthetic technique must be capable of providing sleep, analgesia and muscle relaxation followed by a quiet pain-free recovery. The use of combinations of anaesthetic agents and drugs with specific action frequently reduces the required dosage of individual drugs and further reduces anaesthetic risk factors. Many anaesthetic techniques utilise apparatus with out of circle vaporisers (VOC), to provide a known concentration of anaesthetic gas but using high flow rates of nitrous oxide and oxygen. By way of comparison, the administration of anaesthetic gas from a vaporiser located within the closed circuit (VIC), has also been well documented (O’Brien 1966). More recently, some human and veterinary anaesthetists have been experimenting by combining more than 1 volatile anaesthetic agent by using the *Komesaroff anaesthetic machine (Figures 1, 2), used in this series of cases, which utilises a closed circuit technique with a basal gas flow of 100% oxygen entering the circuit. Dual in series low resistance vaporisers with volume calibrated vaporiser bowls (Figure 3), are situated in the inspiratory limb of the patients circuit (VIC) enabling 2 volatile anaesthetic agents for example, halothane and methoxyflurane to be used concurrently to produce safe, easily managed, economical and balanced anaesthesia. The apparatus also incorporates a sphygmomanometer and electronic pulse monitor to facilitate patient monitoring during anaesthesia. Method
mazine maleate to produce a tranquil animal for the induction of anaesthesia. Prior to induction about 10 mls of methoxyflurane were placed in the bowl of the vaporiser nearest the carbon dioxide absorber, and 10 mls of halothane were placed in the bowl of the distal vaporiser. The exact volumes of the anaesthetic agents are read from the scale on the glass vaporiser bowls (Figure 3). An indwelling cannula or butterfly needle was inserted into the cephalic vein, covered with an alcohol soaked gauze pleget and taped into position. This procedure provides a readily accessible venipuncture site for the administration of anaesthetic inducing agents, muscle relaxants, intravenous fluids and blood transfusions throughout surgery, and into the post-operative recovery period. A blood pressure cuff (infant size) was positioned around the radius and ulna and the photoelectric sensor of the pulse monitor was taped over a small shaved area of the volar metacarpal region. A cardiac oscilloscope was also
*Medical Developments Pty. Ltd., 364 Londale St., Melbourne, Australia.
Figure 1. The Komesaroff Anaesthetic Machine (Flowmeter Model). The closed circuit arrangement is shown with the two bidirectional vaporisers in the circuit. The autoclavable polycarbonate absorber and cross valve are shown on the right. The flowmeter is graduated to measure low flows with scale readings 0-2% litres per minute. Higher flows can be obtained by rotating the white valve further. The new pulsemonitor (shown at the left rear) functions with conventional batteries, is unaffected by diathermy, and has a very sensitive but well protected sensor. The base may be either fibreglass (shown) or steel. The fibreglass model has a fitting cover.
32
Australian Veterinary Journal, Vol. 51, January, 1975
A thorough preoperative clinical examination was conducted in all cases and suitable preoperative medication was selected, usually 0.02 mg/kg atropine sulphate and 0.1 mg/kg acepro-
used to monitor cardiac function during anaesthesia. In most cases anaesthesia was induced by the administration of a solution of short acting or ultra short acting barbiturate, for example, thiopentone sodium 2.5% (10 mg/kg), via the previously prepared cephalic venipuncture. Following induction, an endotracheal tube was passed into the trachea and was connected to the anaesthetic machine; the rebreathing bag was approximately half filled with oxygen. Anaesthesia was then maintained by administering halothane and methoxyflurane with oxygen. As the exact volume of each anaesthetic agent placed in each vaporiser at the start of anaesthesia was known, the rate of vaporisation of each agent and hence an accurate determination of the amount or dose of anaesthetic agent taken up by the patient could be made at predetermined intervals by direct observation of the scale on the vaporiser bowls. For spontaneous ventilation the vaporiser containing methoxyflurane was advanced to the fully ON position while the halothane vaporiser was advanced gradually to the fully ON position over 3 to 4 minutes. It was occasionally necessary to increase the rate of vaporisation of the voIatiIe anaesthetic agents by manually assisting respiration by gentle compression of the rebreathing bag. The depth of anaesthesia was assessed by observing the patients responses, and the vaporisers were set to maintain the required depth. The oxygen flow rate was set to maintain the patients metabolic oxygen requirements and calculated on the basis of 10 ml oxygen/kg bodyweight/minute. For some procedures lasting 45 to 60 minutes it was found that the halothane vaporiser could be turned off after 15 to 20 minutes and the methoxyflurane vaporiser turned off approximately 15 minutes prior to the end of the procedure. The use of methoxyflurane prolonged the recovery period but the recovery was smoother than that following halothane, and nitrous oxide anaesthesia.
For controlled ventilation techniques the induction of anaesthesia and intubation were similar to spontaneous ventilation techniques. However, the patient was then ventilated using either a mixture of halothane and methoxyflurane with oxygen or preferably ventilated with oxygen and methoxyflurane only. If it was necessary to use halothane during intermittent positive pressure ventilation, the halothane vaporiser setting did not exceed notch 1. The selected muscle relaxant, usually gallamine triethiodide (1 mg/kg) was administered. The flow of oxygen was regulated to maintain the rebreathing bag approximately three quarters full. It is essential to ensure that the patient remains adequately anaesthetised during surgery while under the influence of muscle relaxants. The rate of vaporisation of methoxyflurane is controlled by the vaporiser setting and the rate and depth of artificial ventilation. If
,FLUID ..LEVELS
MAGNIFICATION =
obrorber
W
P.egulotor
Venturi Suction
Figure 3. Calibrated Vaporiser Bowl. A reference mark is shown on the housing of the vaporiser If fluid level difference 2 mi Volume = - 1.3 mm Vertical Distance = - 10 mm Lateral Distance Magnification 10
...
Flow c o n t r o l L> Bypols
unit
H = Halothone V a p o r i z e r M = Methoxylluione Vaporizer
Figure 2. The Circle System VIC
Austrulian Veterinary Journal. Vol. 51, January, 1975
c= 8 1.3
_-
1.3
= 8 times (approx.) x = Intersections of the fluid meniscus with the sloping line, which is at an angle of 70° 40’ to the horizontal.
33
TABLE 1
It is important to appreciate that anaesthesia with closed circuit VIC apparatus is regulated by the vaporiser setting and the patients minute and tidal volume. At a given setting of the vaporiser the concentration of anaesthetic vapor during Drum Position Average Range spontaneous ventilation tends to remain constant as shown in Table 1 (Marrett 1957; Newman 1 0. 5% 0.4-0 75% 1958; Mushin and Galloon 1960; Gurubatham 2 1.25% 1.0-1 5% 3 2. 5 % 2.0-4 0% 1971; Komesaroff 1973). Should the patients tidal 4 4. 0 % 3.5-5 0% volume increase or decrease during surgery, the rate of vaporisation of the anaesthetic agents will anaesthesia became too light as indicated by an correspondingly increase or decrease producing a increase in pulse rate, systolic blood pressure and self regulating system. Manual compression of the lacrimation, or by movement of the patient, the rebreathing bag however increases the rate of depth of anaesthesia was increased by advancing vaporisation and hence the depth of anaesthesia. the methoxyflurane vaporiser setting, and in- Therefore in a closed circuit with VIC and concreasing the rate of manual compression of the trolled ventilation, the halothane vaporiser should rebreathing bag. Care was taken to ensure that be in the OFF position or at least not exceed the the fresh gas flow of oxygen was not excessive, No 1 setting otherwise it is possible to increase as high oxygen flow rates dilute the inspired dangerously the rate of halothane vaporisation, vapors. Alternatively the level of anaesthesia was because of its high vapor pressure (241 mm Hg deepened by the addition of small increments of at 20°C). It is safe to use methoxyflurane with the induction agent, for example, thiopentone. At controlled ventilation in these circumstances bethe conclusion of surgery, the muscle relaxant cause of its low vapor pressure ( 2 3 mm Hg at effect was reversed with neostigmine (0.02 mg/ 21°C). No significant changes were observed in blood kg). Atropine sulphate (0.02 mg/kg) was administered with the neostigmine to prevent side affects pressure, pulse rate or ECG waveform. The blood pressure was measured by using the pulse monitor such as excessive salivation and bradycardia. in conjunction with a sphygmomanometer both of The endotracheal tube was not removed until which are fixed to the anaesthetic machine. The mild swallowing movements were observed. The author advocates blood pressure measurements as oropharynx was then cleared with suction and the a routine in anaesthesia of small animals. patient extubated and placed in the recovery room The combined use of methoxyflurane and halofor further observation. thane provided a safe and easily controlled anaesThe application of this technique is similar to thesia. In the majority of cases the anaesthetic that used by Komesaroff and WilIiamson (1968) administered produced satisfactory muscle relaxaand by Komesaroff (1973) for human anaesthesia. tion without the use of muscle relaxant drugs. The use of these 2 volatile anaesthetic agents elimiResults nated the need for the addition of premixed gases In this series 1003 anaesthetics were admini- (02/N20) to balance the anaesthesia. Methoxystered to cats and dogs. The ages of the patients flurane was found satisfactory for obstetrical survaried from 2 weeks to 15 years and they weighed gery and was not observed to depress foetal between 2.0 kg and 65.0 kg. The surgery per- respiration during Caesarean section. In selected formed covered a wide range of abdominal, thora- cases this method may be preferred to the use of cic, orthopaedic and obstetrical procedures. A neuroleptanalgesics or epidural anaesthesia. In total of 840 patients were anaesthetised with aged patients this method of anaesthesia was spontaneous ventilation techniques, and in 163 extremely well tolerated. No post-operative compatients controlled ventilation was used. In a 16 plications were attributed to the method of anaeskg dog the rate of vaporisation of halothane was thesia or to the agents used. In all cases the 1.O-1.5 ml/hr and the vaporisation of methoxy- recovery phase was uneventful with the patient flurane was 0.5-1 -0ml/hr for spontaneous ventila- generally responding to arousal stimuli 10-12 tion. With controlled ventilation techniques the minutes after the completion of surgery. Recovery rate of vaporisation of methoxyfiurane was 1.0- was smooth in all cases and the animal was tran1.25 ml/hr. The flow rate of oxygen was 75-150 quil in the post-operative period. The animals apcc/minute for both spontaneous and controlled peared to be free from pain and would generally ventilation techniques. lapse into an apparently comfortable post-operaConcentration of Halothane (VIC) iri the Inspiratory Limb in 20 Patients During C h e d Circuit Anaesthesia with the Modified Bidirectional Vaporiser in the Circuit (Komesaroff 1973)
34
Australian Veterinary Journal. Vol. 51, January, 1975
tive sleep from which they could be aroused. concentration of gas entering the inspiratory limb The post-operative analgesic effect appears to be is immediately diluted by the patients exhalations a major advantage of methoxyffurane anaesthesia. and therefore early in anaesthesia is considerably Recovery time can be prolonged, if indicated, by less concentrated than indicated by the vaporiser
setting. Tn contrast, with VIC techniques the anaesthetic vapor delivered to the patient depends on the vaporiser setting and on the patients minute and tidal volume (Newman 1958). It is of interest to note that during spontaneous respiration, the concentration of gas in the inspiratory limb tends to remain constant. The aim of closed circuit anaesthesia is to administer to the patient the basal requirements of oxygen and to remove the expired carbon dioxide, and to rely on the patients minute volume either spontaneously or artificially controlled to provide an adequate concentration Discussion of inspired anaesthetic. This differs from VOC Closed circuit halothane techniques with basal oxygen have been used in veterinary surgery very techniques where high gas flow rates are used to satisfactorily for some years (O’Brien 1966; introduce an adequate concentration of inspired O’Brien and Heath 1968), but not without some anaesthetic following dilution by the patients exdisadvantages. Halothane, because of its high halations. The VTC technique described in this vapor pressure, is most suited to procedures where paper provides the patient with its metabolic oxyventilation is spontaneous and should only be used gen requirements, thereby eliminating the exhaust in controlled ventilation techniques when under of anaesthetic vapors to the atmosphere. It is the control of an experienced anaesthetist. In important that the patients tidal and minute contrast, methoxyflurane which has a low vapor volume should not be confused with the patients pressure, is an ideal anaesthetic agent for use in oxygen consumption. The former refers to the procedures requiring controlled ventilation. It is total volume of gas excursions during respiration safe and it is almost impossible to produce while the latter refers to the actual metabolic dangerous levels of anaesthesia. Methoxyflurane consumption by the patients. The utilisation of a basal gas flow of 100% is used in both spontaneous and controlled venoxygen compared with combinations of other tilation techniques to provide hypnosis and analgesia as does nitrous oxide, while halothane gases for example, oxygen and nitrous oxide, provides powerful hypnotic effects. It is therefore has a number of advantages. Firstly, high oxygen a logical consequence to combine these 2 volatile concentrations are essential in seriously ill, anaesthetics, and this is conveniently done with shocked or aged patients where pulmonary and 2 vaporisers in series placed in the patients in- cardiac function may be depressed. Secondly, incidents have been reported of the oxygen supspiratory limb (VIC). An important factor in the technique described ply expiring unnoticed. Thirdly, the risk of difin this paper is that the dose of anaesthetic taken fusion hypoxia following prolonged nitrous oxide up by the animal can be measured. This is in con- administration is eliminated post-operatively when trast to techniques where the vaporiser is out of oxygen is the sole gas used (Fink 1955). Pollution of the operating theatre by anaesthetic the circuit (VOC) and anaesthesia is regulated by controlling the concentration. High flow rates are gases and its subsequent effects on the health of used and the excess is exhausted to the atmos- theatre staff is a problem which is becoming more phere. Hence the amount of anaesthetic taken up frequently publicised (Bruce et a1 1968; Linde by the animal is unknown. By using a closed and Bruce 1969; Cohen et a1 1971; Corbett et al circuit VIC system and specially calibrated vapor- 1973; Mackay and McLean 1973). The low flow iser bowls, the exact dose by volume can be rates of oxygen used in this anaesthetic apparatus measured. This is a more logical approach to prevents the escape of harmful anaesthetic vapors administering anaesthesia, and in addition is easier into the operating theatre as the system is completely closed. to understand and teach. With expensive temperature compensating anThe cost of using halothane and methoxyflurane aesthetic vaporisers situated out of the circuit in this closed system is about one quarter to one (VOC), the indicated concentration is not the third the cost of running the more conventional actual concentration that the patient inhales. The circle absorber apparatus with VOC techniques maintaining the anaesthesia with methoxyflurane until the end of surgery. A minor disadvantage of the closed circuit VIC system is that water vapor tends to accumulate in the vaporiser bowls. This was more pronounced when larger breeds of dogs were lightly anaesthetised and their respiration more rapid particularly in colder conditions. It was doubtful if the accumulating water vapor had any significant effect on the rate of vaporisation of the anaesthetic agents.
Australian Veterinary Journal, Voi. 5 1 , January, 1975
35
and the oxygen consumption is reduced. The apparatus is safe and easy to use and maintain. Airway resistance of the polycarbonate absorber and cross valve at flows of 60 I/minute is only 1.5 cm H 2 0 (Komesaroff 1973). Patients weighing less than 2.0 kg have been maintained under anaesthesia for periods well in excess of 1 hour with this closed circuit apparatus. There are some minor disadvantages associated with closed circuit VIC anaesthesia. It is essential that care be taken during controlled ventilation with halothane in the circuit, and the vaporiser setting 1 should not be exceeded. In general it is recommended that only methoxyflurane be used for controlled ventilation with or without relaxants. On the other hand anaesthesia with a combination of halothane and methoxyflurane is recommended during spontaneous ventilation. The rate of soda lime exhaustion is increased, due primarily to the circuit being completely closed and the increased efficiency of carbon dioxide absorption. For several years nephrotoxicity produced by methoxyflurane and related to dose administered has been a subject for concern in human anaesthesia (Crandell et a1 1966; Mazze et a1 1971, 1972; Cousins et a1 1972; Cousins and Mazze 1973; Hetrick et a1 1973; Komesaroff 1973). In our experience no post-operative clinical signs of nephrotoxicity have been observed. The renal responses to halothane and methoxyflurane are similar (Messick 1972). According to Messick (1972), the post-operative renal dysfunction ascribed to methoxyflurane anaesthesia in man does not occur during acute exposure in the dog. The absence of evidence of renal dysfunction in this present series may be due to the extremely low volumes of methoxyflurane employed. Summary
Experiences with a new technique of closed circuit anaesthesia used for 1003 surgical cases involving cats and dogs are described. The technique utilises the combined vapors of methoxyflurane and halothane from dual, low-resistance
36
vaporisers situated in series, and supported by low flow rates of 100% oxygen in the inspiratory limb of the anaesthetic machine. This anaesthetic circuit can be adapted for many surgical procedures involving both controlled and spontaneous ventilation techniques and for young and old animals. The technique is considered safe, economical and easily managed and pollution of the operating theatre is eliminated by the use of the completely closed cirucuit. References Bruce, D. L., Eide, K. A. and Linde, H. W. (1968)Anaesthesiology 29: 565. Cohen, E. N., Belleville, J. W. and Brown, B. W. (1971)-Anaesthesio/ogy 35: 343. Corbett, T. H., Cornell, R. G., Lieding, K. and Endres, J. L. (1973)-Anaesthesiology 38: 260. Cousins, M. J. and Mazze, R. I. (1973jAnaesthesiu and intensive care 1: 355. Cousins, M. J., Nishimura, T . C. and Mazze, R. I. (1972)-Anaestlzesiology 36: 286. Crandell, W. B., Pappas, S. G. and Macdonald, A. (1966)-Anaesthesiology 27: 591. Fink, B. R. (1955t-Anaesthesiotogy 16: 51 1 . Gurubatham, A. I. (197l)--Anaesthesia 26: 1 . Hetrick, W. D., Wolfson, B., Garcia, D. A. and Siker, E. S. (1973)-Anaesthesiology 38: 30. Komesaroff, D. (1973)-Anaesth. Analg. curr. Res. 52: 605. Komesaroff, D. and Williamson, W. (1968?--Med. J . Anst. 2: 391. Linde, H.W. and Bruce, D. L. (1969)-Anaesthesiology 30: 363. Mackay, P. and McLean, A. G. (1973)-Seminar Manual “Occupational Injuries”. Royal Australasian College of Surgeons. Marrett, H. R. ( 1 9 5 7 b B r . Med. J . 2: 3 3 1 . Mazze, R. I., Cousins, M. J . and Kosek, J. C. (1972) Anaesthesiology 36: 571. Mazze, R. I., Shue, G. L. and Jackson, S. H. (1971)I . A m . med. Ass. 216: 278. Messick, J. M. ( 1 9 7 2 j A n a e s t h . Analg. curr. Res. 51: 933. Mushin, W. W. and Galloon, S . ( 1 9 6 0 F B r . I . Anaesth. 32: 324. Newman, H. C. (1958)-Br. J . Anaesth. 30: 555. O’Brien, J. J. ( 1 9 6 6 h V e t . Rec. 79: 526. OBrien, J. J. and Heath, T. J. (1968)-Br. J . Anaesth. 40: 853-863. Rex, M. A. E. (1968)-N.Z. vet. J . 16: 39. Weaver, B. (1964)--Vet. Rec. 70: 719. (Received for publication I 1 October 1973)
Australian Veterinary Journal, Vol. 51, January, 1975
Warlaby”, Oaklands Junction, Victoria, 3047
Introduction
This paper describes experiences with 1003 small animal surgical cases involving cats and dogs and in which the anaesthesia was maintained using low doses of methoxyflurane and halothane with a basal gas flow of 100% oxygen in a closed circuit system. Weaver (1 964), Rex (1 968) and other workers have stated that a balanced anaesthetic technique must be capable of providing sleep, analgesia and muscle relaxation followed by a quiet pain-free recovery. The use of combinations of anaesthetic agents and drugs with specific action frequently reduces the required dosage of individual drugs and further reduces anaesthetic risk factors. Many anaesthetic techniques utilise apparatus with out of circle vaporisers (VOC), to provide a known concentration of anaesthetic gas but using high flow rates of nitrous oxide and oxygen. By way of comparison, the administration of anaesthetic gas from a vaporiser located within the closed circuit (VIC), has also been well documented (O’Brien 1966). More recently, some human and veterinary anaesthetists have been experimenting by combining more than 1 volatile anaesthetic agent by using the *Komesaroff anaesthetic machine (Figures 1, 2), used in this series of cases, which utilises a closed circuit technique with a basal gas flow of 100% oxygen entering the circuit. Dual in series low resistance vaporisers with volume calibrated vaporiser bowls (Figure 3), are situated in the inspiratory limb of the patients circuit (VIC) enabling 2 volatile anaesthetic agents for example, halothane and methoxyflurane to be used concurrently to produce safe, easily managed, economical and balanced anaesthesia. The apparatus also incorporates a sphygmomanometer and electronic pulse monitor to facilitate patient monitoring during anaesthesia. Method
mazine maleate to produce a tranquil animal for the induction of anaesthesia. Prior to induction about 10 mls of methoxyflurane were placed in the bowl of the vaporiser nearest the carbon dioxide absorber, and 10 mls of halothane were placed in the bowl of the distal vaporiser. The exact volumes of the anaesthetic agents are read from the scale on the glass vaporiser bowls (Figure 3). An indwelling cannula or butterfly needle was inserted into the cephalic vein, covered with an alcohol soaked gauze pleget and taped into position. This procedure provides a readily accessible venipuncture site for the administration of anaesthetic inducing agents, muscle relaxants, intravenous fluids and blood transfusions throughout surgery, and into the post-operative recovery period. A blood pressure cuff (infant size) was positioned around the radius and ulna and the photoelectric sensor of the pulse monitor was taped over a small shaved area of the volar metacarpal region. A cardiac oscilloscope was also
*Medical Developments Pty. Ltd., 364 Londale St., Melbourne, Australia.
Figure 1. The Komesaroff Anaesthetic Machine (Flowmeter Model). The closed circuit arrangement is shown with the two bidirectional vaporisers in the circuit. The autoclavable polycarbonate absorber and cross valve are shown on the right. The flowmeter is graduated to measure low flows with scale readings 0-2% litres per minute. Higher flows can be obtained by rotating the white valve further. The new pulsemonitor (shown at the left rear) functions with conventional batteries, is unaffected by diathermy, and has a very sensitive but well protected sensor. The base may be either fibreglass (shown) or steel. The fibreglass model has a fitting cover.
32
Australian Veterinary Journal, Vol. 51, January, 1975
A thorough preoperative clinical examination was conducted in all cases and suitable preoperative medication was selected, usually 0.02 mg/kg atropine sulphate and 0.1 mg/kg acepro-
used to monitor cardiac function during anaesthesia. In most cases anaesthesia was induced by the administration of a solution of short acting or ultra short acting barbiturate, for example, thiopentone sodium 2.5% (10 mg/kg), via the previously prepared cephalic venipuncture. Following induction, an endotracheal tube was passed into the trachea and was connected to the anaesthetic machine; the rebreathing bag was approximately half filled with oxygen. Anaesthesia was then maintained by administering halothane and methoxyflurane with oxygen. As the exact volume of each anaesthetic agent placed in each vaporiser at the start of anaesthesia was known, the rate of vaporisation of each agent and hence an accurate determination of the amount or dose of anaesthetic agent taken up by the patient could be made at predetermined intervals by direct observation of the scale on the vaporiser bowls. For spontaneous ventilation the vaporiser containing methoxyflurane was advanced to the fully ON position while the halothane vaporiser was advanced gradually to the fully ON position over 3 to 4 minutes. It was occasionally necessary to increase the rate of vaporisation of the voIatiIe anaesthetic agents by manually assisting respiration by gentle compression of the rebreathing bag. The depth of anaesthesia was assessed by observing the patients responses, and the vaporisers were set to maintain the required depth. The oxygen flow rate was set to maintain the patients metabolic oxygen requirements and calculated on the basis of 10 ml oxygen/kg bodyweight/minute. For some procedures lasting 45 to 60 minutes it was found that the halothane vaporiser could be turned off after 15 to 20 minutes and the methoxyflurane vaporiser turned off approximately 15 minutes prior to the end of the procedure. The use of methoxyflurane prolonged the recovery period but the recovery was smoother than that following halothane, and nitrous oxide anaesthesia.
For controlled ventilation techniques the induction of anaesthesia and intubation were similar to spontaneous ventilation techniques. However, the patient was then ventilated using either a mixture of halothane and methoxyflurane with oxygen or preferably ventilated with oxygen and methoxyflurane only. If it was necessary to use halothane during intermittent positive pressure ventilation, the halothane vaporiser setting did not exceed notch 1. The selected muscle relaxant, usually gallamine triethiodide (1 mg/kg) was administered. The flow of oxygen was regulated to maintain the rebreathing bag approximately three quarters full. It is essential to ensure that the patient remains adequately anaesthetised during surgery while under the influence of muscle relaxants. The rate of vaporisation of methoxyflurane is controlled by the vaporiser setting and the rate and depth of artificial ventilation. If
,FLUID ..LEVELS
MAGNIFICATION =
obrorber
W
P.egulotor
Venturi Suction
Figure 3. Calibrated Vaporiser Bowl. A reference mark is shown on the housing of the vaporiser If fluid level difference 2 mi Volume = - 1.3 mm Vertical Distance = - 10 mm Lateral Distance Magnification 10
...
Flow c o n t r o l L> Bypols
unit
H = Halothone V a p o r i z e r M = Methoxylluione Vaporizer
Figure 2. The Circle System VIC
Austrulian Veterinary Journal. Vol. 51, January, 1975
c= 8 1.3
_-
1.3
= 8 times (approx.) x = Intersections of the fluid meniscus with the sloping line, which is at an angle of 70° 40’ to the horizontal.
33
TABLE 1
It is important to appreciate that anaesthesia with closed circuit VIC apparatus is regulated by the vaporiser setting and the patients minute and tidal volume. At a given setting of the vaporiser the concentration of anaesthetic vapor during Drum Position Average Range spontaneous ventilation tends to remain constant as shown in Table 1 (Marrett 1957; Newman 1 0. 5% 0.4-0 75% 1958; Mushin and Galloon 1960; Gurubatham 2 1.25% 1.0-1 5% 3 2. 5 % 2.0-4 0% 1971; Komesaroff 1973). Should the patients tidal 4 4. 0 % 3.5-5 0% volume increase or decrease during surgery, the rate of vaporisation of the anaesthetic agents will anaesthesia became too light as indicated by an correspondingly increase or decrease producing a increase in pulse rate, systolic blood pressure and self regulating system. Manual compression of the lacrimation, or by movement of the patient, the rebreathing bag however increases the rate of depth of anaesthesia was increased by advancing vaporisation and hence the depth of anaesthesia. the methoxyflurane vaporiser setting, and in- Therefore in a closed circuit with VIC and concreasing the rate of manual compression of the trolled ventilation, the halothane vaporiser should rebreathing bag. Care was taken to ensure that be in the OFF position or at least not exceed the the fresh gas flow of oxygen was not excessive, No 1 setting otherwise it is possible to increase as high oxygen flow rates dilute the inspired dangerously the rate of halothane vaporisation, vapors. Alternatively the level of anaesthesia was because of its high vapor pressure (241 mm Hg deepened by the addition of small increments of at 20°C). It is safe to use methoxyflurane with the induction agent, for example, thiopentone. At controlled ventilation in these circumstances bethe conclusion of surgery, the muscle relaxant cause of its low vapor pressure ( 2 3 mm Hg at effect was reversed with neostigmine (0.02 mg/ 21°C). No significant changes were observed in blood kg). Atropine sulphate (0.02 mg/kg) was administered with the neostigmine to prevent side affects pressure, pulse rate or ECG waveform. The blood pressure was measured by using the pulse monitor such as excessive salivation and bradycardia. in conjunction with a sphygmomanometer both of The endotracheal tube was not removed until which are fixed to the anaesthetic machine. The mild swallowing movements were observed. The author advocates blood pressure measurements as oropharynx was then cleared with suction and the a routine in anaesthesia of small animals. patient extubated and placed in the recovery room The combined use of methoxyflurane and halofor further observation. thane provided a safe and easily controlled anaesThe application of this technique is similar to thesia. In the majority of cases the anaesthetic that used by Komesaroff and WilIiamson (1968) administered produced satisfactory muscle relaxaand by Komesaroff (1973) for human anaesthesia. tion without the use of muscle relaxant drugs. The use of these 2 volatile anaesthetic agents elimiResults nated the need for the addition of premixed gases In this series 1003 anaesthetics were admini- (02/N20) to balance the anaesthesia. Methoxystered to cats and dogs. The ages of the patients flurane was found satisfactory for obstetrical survaried from 2 weeks to 15 years and they weighed gery and was not observed to depress foetal between 2.0 kg and 65.0 kg. The surgery per- respiration during Caesarean section. In selected formed covered a wide range of abdominal, thora- cases this method may be preferred to the use of cic, orthopaedic and obstetrical procedures. A neuroleptanalgesics or epidural anaesthesia. In total of 840 patients were anaesthetised with aged patients this method of anaesthesia was spontaneous ventilation techniques, and in 163 extremely well tolerated. No post-operative compatients controlled ventilation was used. In a 16 plications were attributed to the method of anaeskg dog the rate of vaporisation of halothane was thesia or to the agents used. In all cases the 1.O-1.5 ml/hr and the vaporisation of methoxy- recovery phase was uneventful with the patient flurane was 0.5-1 -0ml/hr for spontaneous ventila- generally responding to arousal stimuli 10-12 tion. With controlled ventilation techniques the minutes after the completion of surgery. Recovery rate of vaporisation of methoxyfiurane was 1.0- was smooth in all cases and the animal was tran1.25 ml/hr. The flow rate of oxygen was 75-150 quil in the post-operative period. The animals apcc/minute for both spontaneous and controlled peared to be free from pain and would generally ventilation techniques. lapse into an apparently comfortable post-operaConcentration of Halothane (VIC) iri the Inspiratory Limb in 20 Patients During C h e d Circuit Anaesthesia with the Modified Bidirectional Vaporiser in the Circuit (Komesaroff 1973)
34
Australian Veterinary Journal. Vol. 51, January, 1975
tive sleep from which they could be aroused. concentration of gas entering the inspiratory limb The post-operative analgesic effect appears to be is immediately diluted by the patients exhalations a major advantage of methoxyffurane anaesthesia. and therefore early in anaesthesia is considerably Recovery time can be prolonged, if indicated, by less concentrated than indicated by the vaporiser
setting. Tn contrast, with VIC techniques the anaesthetic vapor delivered to the patient depends on the vaporiser setting and on the patients minute and tidal volume (Newman 1958). It is of interest to note that during spontaneous respiration, the concentration of gas in the inspiratory limb tends to remain constant. The aim of closed circuit anaesthesia is to administer to the patient the basal requirements of oxygen and to remove the expired carbon dioxide, and to rely on the patients minute volume either spontaneously or artificially controlled to provide an adequate concentration Discussion of inspired anaesthetic. This differs from VOC Closed circuit halothane techniques with basal oxygen have been used in veterinary surgery very techniques where high gas flow rates are used to satisfactorily for some years (O’Brien 1966; introduce an adequate concentration of inspired O’Brien and Heath 1968), but not without some anaesthetic following dilution by the patients exdisadvantages. Halothane, because of its high halations. The VTC technique described in this vapor pressure, is most suited to procedures where paper provides the patient with its metabolic oxyventilation is spontaneous and should only be used gen requirements, thereby eliminating the exhaust in controlled ventilation techniques when under of anaesthetic vapors to the atmosphere. It is the control of an experienced anaesthetist. In important that the patients tidal and minute contrast, methoxyflurane which has a low vapor volume should not be confused with the patients pressure, is an ideal anaesthetic agent for use in oxygen consumption. The former refers to the procedures requiring controlled ventilation. It is total volume of gas excursions during respiration safe and it is almost impossible to produce while the latter refers to the actual metabolic dangerous levels of anaesthesia. Methoxyflurane consumption by the patients. The utilisation of a basal gas flow of 100% is used in both spontaneous and controlled venoxygen compared with combinations of other tilation techniques to provide hypnosis and analgesia as does nitrous oxide, while halothane gases for example, oxygen and nitrous oxide, provides powerful hypnotic effects. It is therefore has a number of advantages. Firstly, high oxygen a logical consequence to combine these 2 volatile concentrations are essential in seriously ill, anaesthetics, and this is conveniently done with shocked or aged patients where pulmonary and 2 vaporisers in series placed in the patients in- cardiac function may be depressed. Secondly, incidents have been reported of the oxygen supspiratory limb (VIC). An important factor in the technique described ply expiring unnoticed. Thirdly, the risk of difin this paper is that the dose of anaesthetic taken fusion hypoxia following prolonged nitrous oxide up by the animal can be measured. This is in con- administration is eliminated post-operatively when trast to techniques where the vaporiser is out of oxygen is the sole gas used (Fink 1955). Pollution of the operating theatre by anaesthetic the circuit (VOC) and anaesthesia is regulated by controlling the concentration. High flow rates are gases and its subsequent effects on the health of used and the excess is exhausted to the atmos- theatre staff is a problem which is becoming more phere. Hence the amount of anaesthetic taken up frequently publicised (Bruce et a1 1968; Linde by the animal is unknown. By using a closed and Bruce 1969; Cohen et a1 1971; Corbett et al circuit VIC system and specially calibrated vapor- 1973; Mackay and McLean 1973). The low flow iser bowls, the exact dose by volume can be rates of oxygen used in this anaesthetic apparatus measured. This is a more logical approach to prevents the escape of harmful anaesthetic vapors administering anaesthesia, and in addition is easier into the operating theatre as the system is completely closed. to understand and teach. With expensive temperature compensating anThe cost of using halothane and methoxyflurane aesthetic vaporisers situated out of the circuit in this closed system is about one quarter to one (VOC), the indicated concentration is not the third the cost of running the more conventional actual concentration that the patient inhales. The circle absorber apparatus with VOC techniques maintaining the anaesthesia with methoxyflurane until the end of surgery. A minor disadvantage of the closed circuit VIC system is that water vapor tends to accumulate in the vaporiser bowls. This was more pronounced when larger breeds of dogs were lightly anaesthetised and their respiration more rapid particularly in colder conditions. It was doubtful if the accumulating water vapor had any significant effect on the rate of vaporisation of the anaesthetic agents.
Australian Veterinary Journal, Voi. 5 1 , January, 1975
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and the oxygen consumption is reduced. The apparatus is safe and easy to use and maintain. Airway resistance of the polycarbonate absorber and cross valve at flows of 60 I/minute is only 1.5 cm H 2 0 (Komesaroff 1973). Patients weighing less than 2.0 kg have been maintained under anaesthesia for periods well in excess of 1 hour with this closed circuit apparatus. There are some minor disadvantages associated with closed circuit VIC anaesthesia. It is essential that care be taken during controlled ventilation with halothane in the circuit, and the vaporiser setting 1 should not be exceeded. In general it is recommended that only methoxyflurane be used for controlled ventilation with or without relaxants. On the other hand anaesthesia with a combination of halothane and methoxyflurane is recommended during spontaneous ventilation. The rate of soda lime exhaustion is increased, due primarily to the circuit being completely closed and the increased efficiency of carbon dioxide absorption. For several years nephrotoxicity produced by methoxyflurane and related to dose administered has been a subject for concern in human anaesthesia (Crandell et a1 1966; Mazze et a1 1971, 1972; Cousins et a1 1972; Cousins and Mazze 1973; Hetrick et a1 1973; Komesaroff 1973). In our experience no post-operative clinical signs of nephrotoxicity have been observed. The renal responses to halothane and methoxyflurane are similar (Messick 1972). According to Messick (1972), the post-operative renal dysfunction ascribed to methoxyflurane anaesthesia in man does not occur during acute exposure in the dog. The absence of evidence of renal dysfunction in this present series may be due to the extremely low volumes of methoxyflurane employed. Summary
Experiences with a new technique of closed circuit anaesthesia used for 1003 surgical cases involving cats and dogs are described. The technique utilises the combined vapors of methoxyflurane and halothane from dual, low-resistance
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vaporisers situated in series, and supported by low flow rates of 100% oxygen in the inspiratory limb of the anaesthetic machine. This anaesthetic circuit can be adapted for many surgical procedures involving both controlled and spontaneous ventilation techniques and for young and old animals. The technique is considered safe, economical and easily managed and pollution of the operating theatre is eliminated by the use of the completely closed cirucuit. References Bruce, D. L., Eide, K. A. and Linde, H. W. (1968)Anaesthesiology 29: 565. Cohen, E. N., Belleville, J. W. and Brown, B. W. (1971)-Anaesthesio/ogy 35: 343. Corbett, T. H., Cornell, R. G., Lieding, K. and Endres, J. L. (1973)-Anaesthesiology 38: 260. Cousins, M. J. and Mazze, R. I. (1973jAnaesthesiu and intensive care 1: 355. Cousins, M. J., Nishimura, T . C. and Mazze, R. I. (1972)-Anaestlzesiology 36: 286. Crandell, W. B., Pappas, S. G. and Macdonald, A. (1966)-Anaesthesiology 27: 591. Fink, B. R. (1955t-Anaesthesiotogy 16: 51 1 . Gurubatham, A. I. (197l)--Anaesthesia 26: 1 . Hetrick, W. D., Wolfson, B., Garcia, D. A. and Siker, E. S. (1973)-Anaesthesiology 38: 30. Komesaroff, D. (1973)-Anaesth. Analg. curr. Res. 52: 605. Komesaroff, D. and Williamson, W. (1968?--Med. J . Anst. 2: 391. Linde, H.W. and Bruce, D. L. (1969)-Anaesthesiology 30: 363. Mackay, P. and McLean, A. G. (1973)-Seminar Manual “Occupational Injuries”. Royal Australasian College of Surgeons. Marrett, H. R. ( 1 9 5 7 b B r . Med. J . 2: 3 3 1 . Mazze, R. I., Cousins, M. J . and Kosek, J. C. (1972) Anaesthesiology 36: 571. Mazze, R. I., Shue, G. L. and Jackson, S. H. (1971)I . A m . med. Ass. 216: 278. Messick, J. M. ( 1 9 7 2 j A n a e s t h . Analg. curr. Res. 51: 933. Mushin, W. W. and Galloon, S . ( 1 9 6 0 F B r . I . Anaesth. 32: 324. Newman, H. C. (1958)-Br. J . Anaesth. 30: 555. O’Brien, J. J. ( 1 9 6 6 h V e t . Rec. 79: 526. OBrien, J. J. and Heath, T. J. (1968)-Br. J . Anaesth. 40: 853-863. Rex, M. A. E. (1968)-N.Z. vet. J . 16: 39. Weaver, B. (1964)--Vet. Rec. 70: 719. (Received for publication I 1 October 1973)
Australian Veterinary Journal, Vol. 51, January, 1975
