790
Correspondence
Relief of tension pneumothorax A . J. Asbury, MB, FFARCS
801
Theatre pollution control A. J. Asbury, MB, FFARCS & A . J. Hancox, BSc
802
The tail of a bag: a hazard C . G. M . Lynch, BM, FFARCS
803
An aid to the diagnosis of cerebral death John Pollard, MB, FFARCS
804
Lord of the flies Alan Gilston, MB, FFARCS
805
Research into child blindness G. Bradley, Esq.
805
More problems with piped gases A continuing task of the British and International Standards Committees is to consider reported and potential hazards associated with equipment design and construction and to decide whether they are of sufficient importance to merit attempts to legislate against their occurrence when drafting future standards. (This counsel of perfection is seldom attainable and, if attained, sometimes introduces unforeseen new hazards.) This letter describes examples of design features currently being discussed from this point of view. They are reported here as the possible hazards involved may not be so widely known as, for example, those likely to lead to misconnection of breathing system or piped gas supply hoses. While it is easy to determine that gas will flow from a pipeline, a gauge is required to confirm that it is being supplied at the correct pressure. It has, therefore, become increasingly common practice to fit a low pressure ‘pipeline’ gauge reading up to 700 kPa (100 Ib/in.2)on the anaesthetic machine. If the machine has no reserve cylinders (arguably a hazardous practice) there can be no confusion about the gauge-it will record the pipeline pressure. The situation in machines with reserve cylinders is, however, far from straightforward and, in some circumstances, the ‘pipeline’ gauge reading may be positively misleading. Such machines have a check valve in the pipeline inlet system to prevent gas escaping when a cylinder is opened without the pipeline being connected. The ‘pipeline’ pressure gauge may be connected either to the pipeline side of this check valve or to the machine side. It is not always possible to determine the method of connection by inspection-even when the gauge is mounted on top of the inlet connection block. For example, on the B.O.C. series ‘E’, ‘F‘ and ‘H’ machines the connections are on the pipeline side, while on the ‘K’, ‘L’, ‘M’ and ‘International’ models almost identical gauges in the same position are connected to the machine side of the check valve. In the latter case, however, the ‘pipeline’ gauge indicates the ‘system’ pressure within the machine, i.e. it monitors the cylinder reducing valve outlet pressure. While this gauge will indeed record any failure of the pipeline supply pressure, it will only do so if the cylinders are turned off, for as soon as a full cylinder is opened, the gauge will indicate the sameor almost the same pressure as in the pipeline. It also follows that before checking the pipeline pressure, it is essential to close the cylinders and open the flowmeter control to empty the system before connecting the pipeline. Thus, if the pipeline is imperfectly connected or if the supply has unknowingly failed and the cylinder is turned on, the anaesthetist may not appreciate that his gas is being supplied by the cylinder rather than the pipeline as he might reasonably suppose.
Correspondence
79 1
Although no serious accident has been reported from misinterpreting a ‘pipeline’ pressure gauge, a fatality has occurred in very similar circumstances where there was, however, no such gauge on the machine. In this instance, a damaged hose connector, although apparently connected correctly, failed to supply oxygen which was, therefore, drawn from the reserve cylinder until the latter emptied without being noticed. (A similar potential hazard exists with a pipeline wall outlet hose connector fitted with a ‘secondary lock’ which will retain a male Schrader type probe in its socket but without the gas supply being connected). It might, therefore, be inferred that the gauge should be connected on the pipeline side of the check valve-and preferably in such a way that it can easily be seen to be so connected (e.g. as illustrated in HMI 38/75 where the gauge is on a pin-index block used as a hose connection). Unfortunately, however, even when thus connected the gauge may still be misleading in that, although correctly indicating the pipeline pressure, the gas may be being drawn from the reserve cylinder. This will occur if the cylinder is turned on and if the pipeline supply pressure is lower than that of gas delivered from the cylinder reducing valve, either because the pipeline pressure has fallen or because the cylinder reducing valve output pressure is higher than it should be. Neither fault is unknown and each could lead to inadvertent emptying of the reserve cylinder. It is, therefore, essential for the user to ascertain to which side of the check valve such ‘pipeline’ gauges are connected and manufacturers’ literature is not always explicit on this point. Though the balance of advantages probably lies with the gauge being connected to the pipeline side-a case can be made for saying that no ‘pipeline’ gauges should be fitted to machines, For these reasons, with most machines used in the U K , it is a wise precaution always to turn all cylinders off (having previously tested them) when using a pipeline supplywhether or not a pipeline gauge is fitted. There is, however, an alternative and, at first sight, attractive design philosophy, though it does not affect the problem just discussed of correctly interpreting any ‘pipeline’ pressure gauge. Many machines, particularly of US manufacture, are so designed that the gas is drawn preferentially from the pipeline supply and that only when the latter fails is gas drawn from the cylinders: indeed, in some machines with more than one reserve cylinder these too are designed to empty sequentially. These sequential gas supply systems are, however, by no means universally favoured. One argument against them is that they are complex and hence potentially unreliable. The outlet pressures of the reducing valves must be very accurately adjusted and it is difficult to ensure their consistent performance unless the difference between each outlet pressure is approximately 28 kPa (4 Ib/in.2). As the outlet pressure from the cylinder reducing valves is thus inevitably-and sometimes appreciably-lower than the pipeline pressure, the performance of some devices, particularly ventilators, may then be so affected that they will not function satisfactorily. The sequential system has a further disadvantage in that it is more difficult for the user to check that it is functioning properly than it is for him to ascertain that a simple reserve cylinder contains some gas. Furthermore, as such a system-per se-will not give total protection in the event of pipeline supply failure-an accident similar to that described earlier could still occur with it-its advantages are more apparent than real. The closely related problems of oxygen supply pressure failure warning and protection devices have recently been described in Anaesthesia (1976,31,36) and need not be considered here, though these too are a major concern of the Standards Committees. Irrespective of the outcome of such Committees’ deliberations on these (and many other) problems of ‘safe’ design, many existing types of machine will for practical and economic reasons long remain in use-with or without ‘pipeline’ pressure gauges, sequential gas supply systems or oxygen failure warning or protection devices. Similar considerations also apply to pipeline installations and connections. While undoubtedly the design of much currently used equipment can be improved, the possibility of designing an absolutely fool proof gas supply system is debatable, as is the cost
792
Correspondence
effectiveness of some of the attempts so to do. Frequent observation of the flowmeters, reservoir bag (and/or ventilator) and not least, of the patient, is the essential check on the integrity of the gas supplies-it will probably always remain so. The Middlesex Hospital, Mortimer Street, London, W1
0. P. DINNICK Association of Anaesthetists Liaison Officer with BSI
Safety of piped gas supplies Although we approve of the sentiments expressed in Dr Zorab’s letter (Anaesthesia, 1976, 31,570)we find some of his justifications wanting. We would stress that it does not require a Swedish Savant to propose such an obvious solution to the problem of hose indexing, but Dr Zorab appears to have ignored the proposal for size differentiation of the hoses contained in Dr Cundy’s letter (Anaesthesia, 1976,31,109) which we are satisfied can easily be defeated. We agree that the hallowed excuse of ‘lack of antistatic properties’ is a poor justification for the continuance of an expensive and probably unnecessary practice. In one hospital we examined, it would be financially advantageous, to retire the single Consultant Anaesthetist addicted to the use of an explosive agent on a full salary rather than maintain, albeit inadequately, antistatic precautions in the operating theatres. The BSI pipeline Sub-Committee has designated that the nitrous oxide hose should be invested throughout its length with a blue plastic sheave and need not be antistatic. Hoses for other services will be antistatic. Transposition of the nitrous oxide hose has been the cause of some of the tragedies and logically must be the one to be immediately recognizable. Antistatic precautions have bedevilled the BSI pipeline Sub-committee deliberation, and it would have been easy to have adopted the confrontation technique advocated by Dr Zorab. As we get older (or is it more mature?) we find it simpler and quicker to attempt to improve safety within the existing regulations than try the lengthy process of getting regulations changed. We hope that eventually such regulations then will suffer disuse atrophy. If a pipeline standard is delayed whilst the legal ramifications of Dr Zorab’s proposals are sorted out, then we doubt that we will see a BSI Pipeline Standard in our lifetime. Scientific subterfuge is sometimes successful. University Department of Anaesthetics, JOHNS . ROBINSON Queen Elizabeth Hospital, Birmingham, B1S 2TH. J. M.THOMPSON The effect of posture on dental anaesthetic mortality On a previous occasion we commented on the difficulty of drawing valid conclusions from incomplete data of dental anaesthetic mortality.’ In spite of the difficulty we have been studying for some years the details of deaths associated with dental treatment with the help of the Registrar General’s Office and many coroners. At the moment our studies are incomplete and thus not ready for publication, but one point has emerged which seems of sufficient topical importance to justify an early comment. Its significance will be obvious from the conflicting and in some respects ignorant views expressed in the press and reported in the British Dental Journal’ following a recent death in the dental chair. The point may also release practitioners from unjustifiable pressure to use the supine position for general anaesthesia irrespective of the dictates of their own clinical judgement in any particular situation. In 1974,the latest year for which the Registrar General’s returns are available, there were 17 deaths associated with dental treatment and 13 of them were classified as connected with
Correspondence
Relief of tension pneumothorax A . J. Asbury, MB, FFARCS
801
Theatre pollution control A. J. Asbury, MB, FFARCS & A . J. Hancox, BSc
802
The tail of a bag: a hazard C . G. M . Lynch, BM, FFARCS
803
An aid to the diagnosis of cerebral death John Pollard, MB, FFARCS
804
Lord of the flies Alan Gilston, MB, FFARCS
805
Research into child blindness G. Bradley, Esq.
805
More problems with piped gases A continuing task of the British and International Standards Committees is to consider reported and potential hazards associated with equipment design and construction and to decide whether they are of sufficient importance to merit attempts to legislate against their occurrence when drafting future standards. (This counsel of perfection is seldom attainable and, if attained, sometimes introduces unforeseen new hazards.) This letter describes examples of design features currently being discussed from this point of view. They are reported here as the possible hazards involved may not be so widely known as, for example, those likely to lead to misconnection of breathing system or piped gas supply hoses. While it is easy to determine that gas will flow from a pipeline, a gauge is required to confirm that it is being supplied at the correct pressure. It has, therefore, become increasingly common practice to fit a low pressure ‘pipeline’ gauge reading up to 700 kPa (100 Ib/in.2)on the anaesthetic machine. If the machine has no reserve cylinders (arguably a hazardous practice) there can be no confusion about the gauge-it will record the pipeline pressure. The situation in machines with reserve cylinders is, however, far from straightforward and, in some circumstances, the ‘pipeline’ gauge reading may be positively misleading. Such machines have a check valve in the pipeline inlet system to prevent gas escaping when a cylinder is opened without the pipeline being connected. The ‘pipeline’ pressure gauge may be connected either to the pipeline side of this check valve or to the machine side. It is not always possible to determine the method of connection by inspection-even when the gauge is mounted on top of the inlet connection block. For example, on the B.O.C. series ‘E’, ‘F‘ and ‘H’ machines the connections are on the pipeline side, while on the ‘K’, ‘L’, ‘M’ and ‘International’ models almost identical gauges in the same position are connected to the machine side of the check valve. In the latter case, however, the ‘pipeline’ gauge indicates the ‘system’ pressure within the machine, i.e. it monitors the cylinder reducing valve outlet pressure. While this gauge will indeed record any failure of the pipeline supply pressure, it will only do so if the cylinders are turned off, for as soon as a full cylinder is opened, the gauge will indicate the sameor almost the same pressure as in the pipeline. It also follows that before checking the pipeline pressure, it is essential to close the cylinders and open the flowmeter control to empty the system before connecting the pipeline. Thus, if the pipeline is imperfectly connected or if the supply has unknowingly failed and the cylinder is turned on, the anaesthetist may not appreciate that his gas is being supplied by the cylinder rather than the pipeline as he might reasonably suppose.
Correspondence
79 1
Although no serious accident has been reported from misinterpreting a ‘pipeline’ pressure gauge, a fatality has occurred in very similar circumstances where there was, however, no such gauge on the machine. In this instance, a damaged hose connector, although apparently connected correctly, failed to supply oxygen which was, therefore, drawn from the reserve cylinder until the latter emptied without being noticed. (A similar potential hazard exists with a pipeline wall outlet hose connector fitted with a ‘secondary lock’ which will retain a male Schrader type probe in its socket but without the gas supply being connected). It might, therefore, be inferred that the gauge should be connected on the pipeline side of the check valve-and preferably in such a way that it can easily be seen to be so connected (e.g. as illustrated in HMI 38/75 where the gauge is on a pin-index block used as a hose connection). Unfortunately, however, even when thus connected the gauge may still be misleading in that, although correctly indicating the pipeline pressure, the gas may be being drawn from the reserve cylinder. This will occur if the cylinder is turned on and if the pipeline supply pressure is lower than that of gas delivered from the cylinder reducing valve, either because the pipeline pressure has fallen or because the cylinder reducing valve output pressure is higher than it should be. Neither fault is unknown and each could lead to inadvertent emptying of the reserve cylinder. It is, therefore, essential for the user to ascertain to which side of the check valve such ‘pipeline’ gauges are connected and manufacturers’ literature is not always explicit on this point. Though the balance of advantages probably lies with the gauge being connected to the pipeline side-a case can be made for saying that no ‘pipeline’ gauges should be fitted to machines, For these reasons, with most machines used in the U K , it is a wise precaution always to turn all cylinders off (having previously tested them) when using a pipeline supplywhether or not a pipeline gauge is fitted. There is, however, an alternative and, at first sight, attractive design philosophy, though it does not affect the problem just discussed of correctly interpreting any ‘pipeline’ pressure gauge. Many machines, particularly of US manufacture, are so designed that the gas is drawn preferentially from the pipeline supply and that only when the latter fails is gas drawn from the cylinders: indeed, in some machines with more than one reserve cylinder these too are designed to empty sequentially. These sequential gas supply systems are, however, by no means universally favoured. One argument against them is that they are complex and hence potentially unreliable. The outlet pressures of the reducing valves must be very accurately adjusted and it is difficult to ensure their consistent performance unless the difference between each outlet pressure is approximately 28 kPa (4 Ib/in.2). As the outlet pressure from the cylinder reducing valves is thus inevitably-and sometimes appreciably-lower than the pipeline pressure, the performance of some devices, particularly ventilators, may then be so affected that they will not function satisfactorily. The sequential system has a further disadvantage in that it is more difficult for the user to check that it is functioning properly than it is for him to ascertain that a simple reserve cylinder contains some gas. Furthermore, as such a system-per se-will not give total protection in the event of pipeline supply failure-an accident similar to that described earlier could still occur with it-its advantages are more apparent than real. The closely related problems of oxygen supply pressure failure warning and protection devices have recently been described in Anaesthesia (1976,31,36) and need not be considered here, though these too are a major concern of the Standards Committees. Irrespective of the outcome of such Committees’ deliberations on these (and many other) problems of ‘safe’ design, many existing types of machine will for practical and economic reasons long remain in use-with or without ‘pipeline’ pressure gauges, sequential gas supply systems or oxygen failure warning or protection devices. Similar considerations also apply to pipeline installations and connections. While undoubtedly the design of much currently used equipment can be improved, the possibility of designing an absolutely fool proof gas supply system is debatable, as is the cost
792
Correspondence
effectiveness of some of the attempts so to do. Frequent observation of the flowmeters, reservoir bag (and/or ventilator) and not least, of the patient, is the essential check on the integrity of the gas supplies-it will probably always remain so. The Middlesex Hospital, Mortimer Street, London, W1
0. P. DINNICK Association of Anaesthetists Liaison Officer with BSI
Safety of piped gas supplies Although we approve of the sentiments expressed in Dr Zorab’s letter (Anaesthesia, 1976, 31,570)we find some of his justifications wanting. We would stress that it does not require a Swedish Savant to propose such an obvious solution to the problem of hose indexing, but Dr Zorab appears to have ignored the proposal for size differentiation of the hoses contained in Dr Cundy’s letter (Anaesthesia, 1976,31,109) which we are satisfied can easily be defeated. We agree that the hallowed excuse of ‘lack of antistatic properties’ is a poor justification for the continuance of an expensive and probably unnecessary practice. In one hospital we examined, it would be financially advantageous, to retire the single Consultant Anaesthetist addicted to the use of an explosive agent on a full salary rather than maintain, albeit inadequately, antistatic precautions in the operating theatres. The BSI pipeline Sub-Committee has designated that the nitrous oxide hose should be invested throughout its length with a blue plastic sheave and need not be antistatic. Hoses for other services will be antistatic. Transposition of the nitrous oxide hose has been the cause of some of the tragedies and logically must be the one to be immediately recognizable. Antistatic precautions have bedevilled the BSI pipeline Sub-committee deliberation, and it would have been easy to have adopted the confrontation technique advocated by Dr Zorab. As we get older (or is it more mature?) we find it simpler and quicker to attempt to improve safety within the existing regulations than try the lengthy process of getting regulations changed. We hope that eventually such regulations then will suffer disuse atrophy. If a pipeline standard is delayed whilst the legal ramifications of Dr Zorab’s proposals are sorted out, then we doubt that we will see a BSI Pipeline Standard in our lifetime. Scientific subterfuge is sometimes successful. University Department of Anaesthetics, JOHNS . ROBINSON Queen Elizabeth Hospital, Birmingham, B1S 2TH. J. M.THOMPSON The effect of posture on dental anaesthetic mortality On a previous occasion we commented on the difficulty of drawing valid conclusions from incomplete data of dental anaesthetic mortality.’ In spite of the difficulty we have been studying for some years the details of deaths associated with dental treatment with the help of the Registrar General’s Office and many coroners. At the moment our studies are incomplete and thus not ready for publication, but one point has emerged which seems of sufficient topical importance to justify an early comment. Its significance will be obvious from the conflicting and in some respects ignorant views expressed in the press and reported in the British Dental Journal’ following a recent death in the dental chair. The point may also release practitioners from unjustifiable pressure to use the supine position for general anaesthesia irrespective of the dictates of their own clinical judgement in any particular situation. In 1974,the latest year for which the Registrar General’s returns are available, there were 17 deaths associated with dental treatment and 13 of them were classified as connected with
