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FLOW REVERSAL THROUGH A MARK III HALOTHANE VAPORIZER
British Journal of Anaesthesia 1993; 71: 303-304
APPARATUS
FLOW REVERSAL THROUGH A MARK III HALOTHANE VAPORIZER A. S. GREGG, R. S. JONES AND S. L. SNOWDON
SUMMARY
KEY WORDS Equipment vaporizer, reverse flow.
The effects of intermittent back pressure on vaporizer function have been reported extensively. Hill and Lowe found that concentrations delivered by susceptible vaporizers during controlled or assisted ventilation were considerably greater than when the vaporizer was used with free flow [1]. The effect is greatest at small gas flows [2]. However, little information has been published on the potential hazards of reversing the fresh gas flow rate through the vaporizer. Marks and Bullard concluded that flowing gases in a reverse direction resulted in approximately double the output indicated on the vaporizer dial [3]. This paper describes inadvertent reversal of flow through a vaporizer used for anaesthesia in two equine patients. When the fault had been discovered and rectified, the vaporizer was subjected to laboratory investigation. CASE REPORTS
Two horses required general anaesthesia for orthopaedic surgery, the first for an elective procedure and the second for emergency fracture repair. Preanaesthetic examination revealed no evidence of concurrent disease. All anaesthetic equipment was checked by the anaesthetist before use and the standard anaesthetic practice of this hospital was followed in each case. Cardiovascular and ventilatory variables were monitored by methods currently accepted in this species [4,5] and the vaporizer setting adjusted accordingly. Despite this, in each of the animals there was a sudden deterioration in cardiovascular
Laboratory investigation Halothane vapour concentration was measured using an on-line organic anaesthetic analyser (Wetenschappelijk Technische Instrumentatie B.V. Anaesthetic Gas Analyser (W.T.I.)). This system uses a specially designed hydrogen flame-ionization detector (FID) [6]. It is sensitive only to organic agents, in this case the anaesthetic used; inorganic components, such as oxygen, are not detected. The ionization current, measured with an electrometer amplifier, is proportional to the amount of organic anaesthetic agent that reaches the FID per unit time. Step response-time is approximately 0.2 s, and the small dimension of the capillary transport tube ensures that there is no mixing between successive samples. The large dynamic range, excellent stability and high sensitivity of this detector are well known from gas chromatography applications [7]. The unit was first calibrated using a W.T.I. Anaesthetic Gas Calibrator, designed specifically for this purpose [7, 8]. The first part of the investigation involved measurement of vaporizer output with the flow in the forward direction. The vaporizer (Fluotec Mark A. S. GREGG, B.V.SC. (HONS), CERT, V.A., M.R.C.V.S.; R. S. JONES, M.V.SC., D.VET.M.E.D., D.V.SC., D.V.A., F.I.BIOL., F.R.C.V.S.; S. L .
SNOWDON, M.B., CH.B., F.R.c.A.; Department of Anaesthesia, Royal Liverpool University Hospital, Prescot Street, PO Box 147, Liverpool L69 3BX. Accepted for Publication: March 4, 1993.
Downloaded from http://bja.oxfordjournals.org/ at Karolinska Institutet on May 25, 2015
A fault in the assembly of a Matrix Large Animal Circle anaesthetic machine resulted in reversal of fresh gas flow through the vaporizer. The fault was discovered only after the sudden development of excessive depth of anaesthesia in two equine patients. Laboratory investigations were conducted to determine the effect of flow reversal on vaporizer output. Results indicated that output concentration was approximately doubled under these conditions. (Br. J. Anaesth. 1993; 7 1 ; 303-304)
and respiratory status, and a loss of ocular reflexes after being moved to the operating table. The cause of this dramatic increase in depth of anaesthesia was not apparent at the time. Appropriate resuscitative therapy was successful in each case. The animal undergoing the elective procedure was allowed to recover immediately after resuscitation without surgical intervention. Subsequent investigation revealed that the fresh gas flows had been routed inadvertently through the vaporizer in the reverse direction. The apparatus involved was a new Large Animal Circle (Matrix) with a Fluotec Mark III vaporizer which had not been used previously. Two other large animal anaesthetic machines, including a mechanical ventilator (North American Drager) have been in regular use at the hospital for several years. Although the incidents occurred 2 weeks apart, this particular anaesthetic machine was not used in the intervening time.
BRITISH JOURNAL OF ANAESTHESIA
304 TABLE I. Halothane concentrations delwered at various gas flow settings during normal and reversed flows Delivered concentration Flow rate Dial setting (litre min"')(%) Normal flow Reverse flow 09 1.75 2.9 4.0 4.9 1.0 20 3.2 4.1 4.85 0.75 1.6 2.6 3.45 40
1.25 2.40 3.95 5.05 6.0 1.6 3.3 5.05 6.1 8.05 2.1 4.2 6.6 7.6 8.2
DISCUSSION
The anaesthetic machine standard requires that the inlet of a vaporizer be male and the outlet female, the direction of gas flow marked and the inlet and outlet parts labelled [9]. Whilst all these requirements were met in the Large Animal Circle (Matrix) used in both of these
ACKNOWLEDGEMENTS We are grateful to Mr Martin Jones for technical assistance.
REFERENCES 1. Lowe HJ, Beckham LM, Han YH, Evers JL. Vaporiser performance: closed circuit fluothane anesthesia. Anesthesia and Analgesia 1962; 41: 742-754. 2. Hill DW, Lowe HJ. Comparison of concentration of halothane in closed and semi-closed circuits during controlled ventilation. Anesthesiology 1962; 23: 291-298. 3. Marks WE, Bullard JR. Another hazard of free-standing vaporizers, increased concentration with reversed flow of vaporizing gas. Anesthesiology 1976; 45: 445-446. 4. Riebold TW. Monitoring equine anaesthesia. In: Turner AS, Riebold TW, eds. The Veterinary Climes of North America. Equine Practice. The Principles and Techniques of Equine Anaesthesia. Philadelphia: Saunders, 1990; 607-623. 5. Hubbell JAE. Monitoring. In: Hubbell JAE, Muir WW, eds. Equine Anaesthesia Monitoring and Emergency Therapy. St Louis: Mosby Year Book Inc. 1991; 153-179. 6. Cramers CA, Trimbos HF. Development of an on-line analyzer and organic anaesthetics in inspiratory and end-tidal gases. Journal of Chromatography 1976; 119: 71-84. 7. Operating Manual for W.T.I. Anaesthetic Gas monitor Type AG 101 (P3). W.T.I. Wetenschappelijk Technische Instrumentatie B.V. Postbus 45, 2690 AA-'s-Gravenzande (Holland). 8. Operating Manual for W.T.I. Anaesthetic Gas Calibrator Type AC 201 (PI)- W.T.I. Wetenschappelijk Technische Instrumentatie B.V. P.O. Box 45, 2690 AA-'s-Gravenzande (Holland). 9. Dorsch JA, Dorsch SE. Understanding Anaesthesia Equipment—Construction, Care and Complications, 2nd Edn. Baltimore: Williams & Wilkins, 1984; 130.
Downloaded from http://bja.oxfordjournals.org/ at Karolinska Institutet on May 25, 2015
III) was connected in a free-standing manner to a standard anaesthetic trolley (British Oxygen Company Ltd, BOC). A short length of corrugated tubing, which was bent to ensure adequate mixing of the vapour, was attached to the vaporizer outlet. The capillary transport tube from the W.T.I, analyser was placed into this tubing for sampling. The position and orientation of the sampling needle remained constant throughout the investigation. Results were displayed continuously using a chartrecorder. Using an initial fiowmeter setting of 1 litre min"1, oxygen was delivered through the vaporizer in the forward direction. The vaporizer dial setting was adjusted in 1 % increments from 1 to 5 % halothane, and the output at each setting recorded. The procedure was repeated using fiowmeter settings of 4 litre min"1 and then 8 litre min"1. Each test was performed twice and mean values calculated (table I). The analyser was re-calibrated between successive tests to prevent drift. The second part of the investigation involved measurement of vaporizer output with the flow in the reverse direction. For this purpose, the inlet and outlet pipes were transposed and the procedure repeated exactly as before. Temperature and pressure remained constant (24 °C and 760 mm Hg, respectively). As the purpose of this investigation was to demonstrate the grossness of the error, only two readings were taken at each vaporizer setting and a mean value calculated (table I). No attempt was made to define a calibration curve for the vaporizer under these conditions.
anaesthetics, the rubber tubing designed to deliver fresh gases from the flowmeters to the vaporizer inlet and that leading from the vaporizer outlet to the breathing system are of the same appearance and diameter. In addition, hose connections had not been crimped by the manufacturers. These factors allow the possibility of incorrect assembly, with the gas flowing through the vaporizer in the reverse direction. The American design of this particular circle system, with the vaporizer sited to the left of the flowmeters, is such that it may not be immediately obvious to the observer that such a mistake has been made. The machine was used exactly as it was supplied by the manufacturers and the routine preoperative equipment check did not reveal that our apparatus had been assembled incorrectly. It cannot be over-emphasized that anaesthetists should be familiar with any piece of equipment before they attempt to use it, and that a thorough check of all apparatus is made before each use. However, we do feel that the design of this particular model might be modified in such a way as to make it impossible to connect the tubing to the wrong male/female connectors in this manner. Marks and Bullard also drew attention to the fact that connecting a free-standing vaporizer in reverse is easily done and easily overlooked [3]. Where portable vaporizers are used, special care must be taken to ensure that gases flow through them in the correct direction because, although inlet and outlet sides of the vaporizer are marked, the labelling is not conspicuous and the same size of tubing fits each side.
APPARATUS
FLOW REVERSAL THROUGH A MARK III HALOTHANE VAPORIZER A. S. GREGG, R. S. JONES AND S. L. SNOWDON
SUMMARY
KEY WORDS Equipment vaporizer, reverse flow.
The effects of intermittent back pressure on vaporizer function have been reported extensively. Hill and Lowe found that concentrations delivered by susceptible vaporizers during controlled or assisted ventilation were considerably greater than when the vaporizer was used with free flow [1]. The effect is greatest at small gas flows [2]. However, little information has been published on the potential hazards of reversing the fresh gas flow rate through the vaporizer. Marks and Bullard concluded that flowing gases in a reverse direction resulted in approximately double the output indicated on the vaporizer dial [3]. This paper describes inadvertent reversal of flow through a vaporizer used for anaesthesia in two equine patients. When the fault had been discovered and rectified, the vaporizer was subjected to laboratory investigation. CASE REPORTS
Two horses required general anaesthesia for orthopaedic surgery, the first for an elective procedure and the second for emergency fracture repair. Preanaesthetic examination revealed no evidence of concurrent disease. All anaesthetic equipment was checked by the anaesthetist before use and the standard anaesthetic practice of this hospital was followed in each case. Cardiovascular and ventilatory variables were monitored by methods currently accepted in this species [4,5] and the vaporizer setting adjusted accordingly. Despite this, in each of the animals there was a sudden deterioration in cardiovascular
Laboratory investigation Halothane vapour concentration was measured using an on-line organic anaesthetic analyser (Wetenschappelijk Technische Instrumentatie B.V. Anaesthetic Gas Analyser (W.T.I.)). This system uses a specially designed hydrogen flame-ionization detector (FID) [6]. It is sensitive only to organic agents, in this case the anaesthetic used; inorganic components, such as oxygen, are not detected. The ionization current, measured with an electrometer amplifier, is proportional to the amount of organic anaesthetic agent that reaches the FID per unit time. Step response-time is approximately 0.2 s, and the small dimension of the capillary transport tube ensures that there is no mixing between successive samples. The large dynamic range, excellent stability and high sensitivity of this detector are well known from gas chromatography applications [7]. The unit was first calibrated using a W.T.I. Anaesthetic Gas Calibrator, designed specifically for this purpose [7, 8]. The first part of the investigation involved measurement of vaporizer output with the flow in the forward direction. The vaporizer (Fluotec Mark A. S. GREGG, B.V.SC. (HONS), CERT, V.A., M.R.C.V.S.; R. S. JONES, M.V.SC., D.VET.M.E.D., D.V.SC., D.V.A., F.I.BIOL., F.R.C.V.S.; S. L .
SNOWDON, M.B., CH.B., F.R.c.A.; Department of Anaesthesia, Royal Liverpool University Hospital, Prescot Street, PO Box 147, Liverpool L69 3BX. Accepted for Publication: March 4, 1993.
Downloaded from http://bja.oxfordjournals.org/ at Karolinska Institutet on May 25, 2015
A fault in the assembly of a Matrix Large Animal Circle anaesthetic machine resulted in reversal of fresh gas flow through the vaporizer. The fault was discovered only after the sudden development of excessive depth of anaesthesia in two equine patients. Laboratory investigations were conducted to determine the effect of flow reversal on vaporizer output. Results indicated that output concentration was approximately doubled under these conditions. (Br. J. Anaesth. 1993; 7 1 ; 303-304)
and respiratory status, and a loss of ocular reflexes after being moved to the operating table. The cause of this dramatic increase in depth of anaesthesia was not apparent at the time. Appropriate resuscitative therapy was successful in each case. The animal undergoing the elective procedure was allowed to recover immediately after resuscitation without surgical intervention. Subsequent investigation revealed that the fresh gas flows had been routed inadvertently through the vaporizer in the reverse direction. The apparatus involved was a new Large Animal Circle (Matrix) with a Fluotec Mark III vaporizer which had not been used previously. Two other large animal anaesthetic machines, including a mechanical ventilator (North American Drager) have been in regular use at the hospital for several years. Although the incidents occurred 2 weeks apart, this particular anaesthetic machine was not used in the intervening time.
BRITISH JOURNAL OF ANAESTHESIA
304 TABLE I. Halothane concentrations delwered at various gas flow settings during normal and reversed flows Delivered concentration Flow rate Dial setting (litre min"')(%) Normal flow Reverse flow 09 1.75 2.9 4.0 4.9 1.0 20 3.2 4.1 4.85 0.75 1.6 2.6 3.45 40
1.25 2.40 3.95 5.05 6.0 1.6 3.3 5.05 6.1 8.05 2.1 4.2 6.6 7.6 8.2
DISCUSSION
The anaesthetic machine standard requires that the inlet of a vaporizer be male and the outlet female, the direction of gas flow marked and the inlet and outlet parts labelled [9]. Whilst all these requirements were met in the Large Animal Circle (Matrix) used in both of these
ACKNOWLEDGEMENTS We are grateful to Mr Martin Jones for technical assistance.
REFERENCES 1. Lowe HJ, Beckham LM, Han YH, Evers JL. Vaporiser performance: closed circuit fluothane anesthesia. Anesthesia and Analgesia 1962; 41: 742-754. 2. Hill DW, Lowe HJ. Comparison of concentration of halothane in closed and semi-closed circuits during controlled ventilation. Anesthesiology 1962; 23: 291-298. 3. Marks WE, Bullard JR. Another hazard of free-standing vaporizers, increased concentration with reversed flow of vaporizing gas. Anesthesiology 1976; 45: 445-446. 4. Riebold TW. Monitoring equine anaesthesia. In: Turner AS, Riebold TW, eds. The Veterinary Climes of North America. Equine Practice. The Principles and Techniques of Equine Anaesthesia. Philadelphia: Saunders, 1990; 607-623. 5. Hubbell JAE. Monitoring. In: Hubbell JAE, Muir WW, eds. Equine Anaesthesia Monitoring and Emergency Therapy. St Louis: Mosby Year Book Inc. 1991; 153-179. 6. Cramers CA, Trimbos HF. Development of an on-line analyzer and organic anaesthetics in inspiratory and end-tidal gases. Journal of Chromatography 1976; 119: 71-84. 7. Operating Manual for W.T.I. Anaesthetic Gas monitor Type AG 101 (P3). W.T.I. Wetenschappelijk Technische Instrumentatie B.V. Postbus 45, 2690 AA-'s-Gravenzande (Holland). 8. Operating Manual for W.T.I. Anaesthetic Gas Calibrator Type AC 201 (PI)- W.T.I. Wetenschappelijk Technische Instrumentatie B.V. P.O. Box 45, 2690 AA-'s-Gravenzande (Holland). 9. Dorsch JA, Dorsch SE. Understanding Anaesthesia Equipment—Construction, Care and Complications, 2nd Edn. Baltimore: Williams & Wilkins, 1984; 130.
Downloaded from http://bja.oxfordjournals.org/ at Karolinska Institutet on May 25, 2015
III) was connected in a free-standing manner to a standard anaesthetic trolley (British Oxygen Company Ltd, BOC). A short length of corrugated tubing, which was bent to ensure adequate mixing of the vapour, was attached to the vaporizer outlet. The capillary transport tube from the W.T.I, analyser was placed into this tubing for sampling. The position and orientation of the sampling needle remained constant throughout the investigation. Results were displayed continuously using a chartrecorder. Using an initial fiowmeter setting of 1 litre min"1, oxygen was delivered through the vaporizer in the forward direction. The vaporizer dial setting was adjusted in 1 % increments from 1 to 5 % halothane, and the output at each setting recorded. The procedure was repeated using fiowmeter settings of 4 litre min"1 and then 8 litre min"1. Each test was performed twice and mean values calculated (table I). The analyser was re-calibrated between successive tests to prevent drift. The second part of the investigation involved measurement of vaporizer output with the flow in the reverse direction. For this purpose, the inlet and outlet pipes were transposed and the procedure repeated exactly as before. Temperature and pressure remained constant (24 °C and 760 mm Hg, respectively). As the purpose of this investigation was to demonstrate the grossness of the error, only two readings were taken at each vaporizer setting and a mean value calculated (table I). No attempt was made to define a calibration curve for the vaporizer under these conditions.
anaesthetics, the rubber tubing designed to deliver fresh gases from the flowmeters to the vaporizer inlet and that leading from the vaporizer outlet to the breathing system are of the same appearance and diameter. In addition, hose connections had not been crimped by the manufacturers. These factors allow the possibility of incorrect assembly, with the gas flowing through the vaporizer in the reverse direction. The American design of this particular circle system, with the vaporizer sited to the left of the flowmeters, is such that it may not be immediately obvious to the observer that such a mistake has been made. The machine was used exactly as it was supplied by the manufacturers and the routine preoperative equipment check did not reveal that our apparatus had been assembled incorrectly. It cannot be over-emphasized that anaesthetists should be familiar with any piece of equipment before they attempt to use it, and that a thorough check of all apparatus is made before each use. However, we do feel that the design of this particular model might be modified in such a way as to make it impossible to connect the tubing to the wrong male/female connectors in this manner. Marks and Bullard also drew attention to the fact that connecting a free-standing vaporizer in reverse is easily done and easily overlooked [3]. Where portable vaporizers are used, special care must be taken to ensure that gases flow through them in the correct direction because, although inlet and outlet sides of the vaporizer are marked, the labelling is not conspicuous and the same size of tubing fits each side.