212
Correspondence
(OIB) are protected from rebreathing by a one-way valve. Those arterial blood carbon dioxide tensions (Paco,) which were highrepresent, therefore, alveolar hypoventilation and not re-breathing. The very wide variation in Paco, results suggest that, in wide usage, there would sometimes be even worse hypoventilation. No measurement of the gas flow through the OIB was made. A large proportion of the fresh gas, including ether, will be wasted, expelled through the Heidbrink valve. This will result in excessive consumption of anaesthetic ether, which is itself often in short supply in the third world. The cost of an AMBU-E valve is small and it is so light it costs little to airmail; the price would quickly be repaid by savings in ether consumption. If the use of a one-way valve causes concern to users, as it necessitates knowledge of the use of the magnet on the OIB, this may be eliminated when buying new anaesthetic equipment, by ordering the kit containing the Penlon Bellows (PB) rather than the OIB. The PB has only an inlet one-way valve (i.e. there is no outlet valve requiring immobilisation with a magnet) and is used both for spontaneous respiration or intermittent positive pressure ventilation (IPPV) with an AMBU-E or a Ruben valve. Whilst the article recounts a justifiable emergency technique, it would be sad if any were discouraged from using an easily obtainable, better and, in the long run, cheaper alternative. King’s College Hospital, Denmark Hill, London, SE5 9 R S
A reply from Dr Towey Thank you for the opportunity of replying to Dr. James’s letter. Rebreathing into the Oxford Inflating Bellows (OIB) using the technique we described is of course impossible with a functioning one-way valve at the outlet of the OIB: We did not suggest that this could occur. Our concern was for gas mixing in the 105cm length wide-bore tubing. If the Heidbrink valve is almost open then the escaping gas can result in hypoventilation and, if the Heidbrink valve is almost closed, a high positive end expiratory pressure can occur which may have disadvantages in the hypovolaemic patient. Dr James is quite correct in that it is wasteful of ether; it would also be wasteful of supplementary oxygen. The purpose of our study was not to discourage the use of the recommended methods of intermittent positive pressure ventilation (IPPV) using the E M 0 system but to demonstrate that satisfactory alveolar ventilation is possible when certain liberties are taken with the original E M 0 system. We think it is always reassuring to know the possibilities and limitations of our basic equipment. We fully endorse Dr. James’s views on the use of a dual purpose non-rebreathing valve with the outlet valve of the OIB immobilised with a magnet or with the Penlon bellows (PB). For IPPV this is the most satisfactory method and is the one which should be taught for general use.
R.H. JAMES St. Thomas’ Hospital, London, SEI
R.M.TOWEY
Excessive airway pressure during anaesthesia Accidents, caused by excessive airway pressure such as those described by Dr Newton and Dr Adams (Anaesthesia, 1978, 33, 689-699) are more likely to occur now that endotracheal tubes are used, almost invariably, with an air-tight cuff. The cuff robs the breathing circuit of its safety valve. Frightening overdistension of the reservoir bag, is a not infrequent occurrence and certainly it is a more common hazard than oxygen failure against which many efficient devices have recently been introduced. It is possible however to cause alveolar rupture without the use of a cuffed tube. An accident which came to my notice recently, occurred during bronchoscopy. The patient was being ventilated with oxygen using a catheter
modification of Sander’s high-pressure injector technique. The catheter, which was transparent, had been passed too far down the trachea and entered the right bronchus. No damage was done until the bronchoscope entered the bronchus alongside the catheter displacing it further and wedging it in a smaller bronchus. Alveolar rupture then became inevitable since oxygen could not escape and mediastinal emphysema was quickly followed by the appearance of the ‘Michelin’ man. Fortunately prompt treatment of the tension pneumothorax rescued the patient. Odstock Hospital, Salisbury
N. TATE
Studies on awareness during Caesarian section It has been clearly established that when a mixture of 50% nitrous oxide/50”/, oxygen is used as the sole
agent for the maintenance of anaesthesia during
Caesarean section, about 25% of patients will be aware.’ Therefore there can be no justification for conducting 16 further anaesthetics (Anaesthesia,
Correspondence
(OIB) are protected from rebreathing by a one-way valve. Those arterial blood carbon dioxide tensions (Paco,) which were highrepresent, therefore, alveolar hypoventilation and not re-breathing. The very wide variation in Paco, results suggest that, in wide usage, there would sometimes be even worse hypoventilation. No measurement of the gas flow through the OIB was made. A large proportion of the fresh gas, including ether, will be wasted, expelled through the Heidbrink valve. This will result in excessive consumption of anaesthetic ether, which is itself often in short supply in the third world. The cost of an AMBU-E valve is small and it is so light it costs little to airmail; the price would quickly be repaid by savings in ether consumption. If the use of a one-way valve causes concern to users, as it necessitates knowledge of the use of the magnet on the OIB, this may be eliminated when buying new anaesthetic equipment, by ordering the kit containing the Penlon Bellows (PB) rather than the OIB. The PB has only an inlet one-way valve (i.e. there is no outlet valve requiring immobilisation with a magnet) and is used both for spontaneous respiration or intermittent positive pressure ventilation (IPPV) with an AMBU-E or a Ruben valve. Whilst the article recounts a justifiable emergency technique, it would be sad if any were discouraged from using an easily obtainable, better and, in the long run, cheaper alternative. King’s College Hospital, Denmark Hill, London, SE5 9 R S
A reply from Dr Towey Thank you for the opportunity of replying to Dr. James’s letter. Rebreathing into the Oxford Inflating Bellows (OIB) using the technique we described is of course impossible with a functioning one-way valve at the outlet of the OIB: We did not suggest that this could occur. Our concern was for gas mixing in the 105cm length wide-bore tubing. If the Heidbrink valve is almost open then the escaping gas can result in hypoventilation and, if the Heidbrink valve is almost closed, a high positive end expiratory pressure can occur which may have disadvantages in the hypovolaemic patient. Dr James is quite correct in that it is wasteful of ether; it would also be wasteful of supplementary oxygen. The purpose of our study was not to discourage the use of the recommended methods of intermittent positive pressure ventilation (IPPV) using the E M 0 system but to demonstrate that satisfactory alveolar ventilation is possible when certain liberties are taken with the original E M 0 system. We think it is always reassuring to know the possibilities and limitations of our basic equipment. We fully endorse Dr. James’s views on the use of a dual purpose non-rebreathing valve with the outlet valve of the OIB immobilised with a magnet or with the Penlon bellows (PB). For IPPV this is the most satisfactory method and is the one which should be taught for general use.
R.H. JAMES St. Thomas’ Hospital, London, SEI
R.M.TOWEY
Excessive airway pressure during anaesthesia Accidents, caused by excessive airway pressure such as those described by Dr Newton and Dr Adams (Anaesthesia, 1978, 33, 689-699) are more likely to occur now that endotracheal tubes are used, almost invariably, with an air-tight cuff. The cuff robs the breathing circuit of its safety valve. Frightening overdistension of the reservoir bag, is a not infrequent occurrence and certainly it is a more common hazard than oxygen failure against which many efficient devices have recently been introduced. It is possible however to cause alveolar rupture without the use of a cuffed tube. An accident which came to my notice recently, occurred during bronchoscopy. The patient was being ventilated with oxygen using a catheter
modification of Sander’s high-pressure injector technique. The catheter, which was transparent, had been passed too far down the trachea and entered the right bronchus. No damage was done until the bronchoscope entered the bronchus alongside the catheter displacing it further and wedging it in a smaller bronchus. Alveolar rupture then became inevitable since oxygen could not escape and mediastinal emphysema was quickly followed by the appearance of the ‘Michelin’ man. Fortunately prompt treatment of the tension pneumothorax rescued the patient. Odstock Hospital, Salisbury
N. TATE
Studies on awareness during Caesarian section It has been clearly established that when a mixture of 50% nitrous oxide/50”/, oxygen is used as the sole
agent for the maintenance of anaesthesia during
Caesarean section, about 25% of patients will be aware.’ Therefore there can be no justification for conducting 16 further anaesthetics (Anaesthesia,
