76
Correspondence
tumour. No myasthenic syndrome was clinically detected or obtained from the history. However in both cases the non-depolarizing muscle relaxant had a prolonged action and the patients needed ventilatory support for 3-4 days postoperatively. Edrophonium testing was positive and treatment with pyridostigmine was started a few days after the operation. The histological report of the tumour was thymoma. Therefore a latest myasthenic syndrome, which cannot be excluded in the cases with mediastinal tumour, will be manifested as prolonged muscle paralysis even after a ‘judicious dose’ of non-depolarizing muscle relaxant. On the other hand to abolish spontaneous respiration in patients with respiratory problems may lead to postoperative difficulty with ventilation. Mediactinoscopy is a short procedure and, a s the patient’s condition is often poor, we prefer to carry out the anaesthesia using intermittent doses of succinylcholine. In this technique respiration is not abolished altogether but it is assisted. We believe that spontaneous ventilation under general anaesthesia in these patients is not usually satisfactory. In addition the avoidance of nondepolarizing muscle relaxants avoids complications if a latent myasthenic syndrome is present. Atropine is always given as premedication and sodium thiopentone for induction, since intravenous induction is more comfortable for the patient. The patient is given routinely 100% 0,for a few minutes before the induction of anaesthesia; none the less, we are reluctant to give the patient 100% oxygen at the end of the procedure. We extubate while the patient is breathing 50% O2 and 50% N,O and after that he is given an oxygen mask (usually Ventimask); 50% oxygen is sufficient and 100% oxygen favours collapse of the alveoli postoperatively.
muscle blocking agents are better avoided though this view may not be universally accepted. The recommended use of intermittent succinylcholine for mediastinoscopy in this communication seems to depend on the duration of the procedure and the possibility of a latent myasthenic syndrome. My experience with mediastinoscopy is that the minimum time taken for both induction and procedure usually exceeds 30-40 minutes (if this is followed by thoracotomy then this time may be considerably increased). Whereas succinylcholine may well suit those patients with latent myasthenia (two in 3 years), the remainder of the patients may well end up with other difficulties which may require postoperative ventilation with possible allied dysrhythmia, muscle pains and electrolyte problems. In addition, I presume that before subsequent increments of succinylcholine are given, the patie’nt must show some sign that the effect of the drug has worn off. If the mediastinascope is in situ and coughing or struggling should occur a t this instant, the intrathoracic damage that may occur may well increase morbidity and possibly, if it is of a serious nature, mortality. The problem with the judicious use of non-depolarizing relaxants in the two patients mentioned, was that they required post-operative ventilation. The use of thiopentone and succinylcholine for induction of patients, who may have respiratory problems due to a mediastinal tumour, could lead to serious consequences. Consequently, in these cases, I firmly believe that the principle of avoiding any muscle relaxant before airway control is established must be followed. I am also rather puzzled by the reference to assisted ventilation with suxamethonium chloride, the patient’s respiration not being abolished. Surely, after succinylcholine there must be a period when respiration is abolished thereby requiring artificial respiration. A. FASSOULAKI As always, there are many points of view apperHellenic Aniicancer Institute St Savas, taining to the management of any anaesthetic procedure. However, I feel in this particular 173 Alexandras Ace., Athens 603, instance, that catering for the few rare cases may Greece cause far more problems with the majority of patients who undergo mediastinoscopy. A reply from Dr Vaughan
I would agree that where a myasthenic syndrome is present preoperatively, the use of nondepolarizing
University Hospital of Wales Heath Park, Cardiff
R.S. VAUGHAN
Occupational hazards of anaesthesia A question of sex
I agree wholeheartedly with Professor Vessey (Anaesthesia, 1978, 33, 430) that studies of occupational hazards of anaesthesia are beset with method-
ological difficulties; I refer particularly to the problems of response rate to postal questionnaires and of emotional overlay when questions such as still-birth and foetal abnormality are raised. One questionnaire quoted by Dr Vessey was introduced
Correspondence
on the basis that only married women need answer the question on obstetric history-very gallant but not very scientific. I must take exception, however, to the statement that no study has shown a significant change in sex ratio at birth. In a study of the (then) Sheffield region, Dr A. Murray Wilson and I showed a significant increase in the number of female live-born infants conceived while their fathers were in anaesthetic practice.’ As far as I know, this has been the only study aimed specifically a t this particular aspect. A 75% first time response rate was obtained, without emotional bias, and populations identical geographically and temporally were used for comparison. Given the known basic experimental facts such as the effects of inhalational anaesthetic agents on microtubular function, one might expect sex ratio change to be a useful, sensitive means of detecting genetic effects of chromic exposure to inhalational anaesthetic agents,’ and one amenable to the harvesting of large numbers for statistical analysis. The folk-lore of other disciplines, in which the breathing of gas mixtures other than that intended by our Creator is practised, contains suggestions of changes in the sex ratio of children, for example professional deep-sea divers and military air crew. My family and I know how much better I feel a t the end of a working day since I eliminated all but the occasional use of nitrous oxide, alone amongst the inhalational agents, from my practice. Professor Vessey’s review should be interpreted as the equivalent of a ‘not-proven’ verdict under
77
Scottish law rather than an exoneration of chronic exposure to inhalational anaesthetics as a cause of aberrations of mitosis and sperm motility. The Croby Road Hospital, Croby Road, Leicester LE3 9QE
RICHARD WYATT
References 1. WYATT, R . & WILSON, A. M. (1973) Children
2.
of Anaesthetists. British Medical Journal, 1, 675. WYATT, R., WILSON,A. & M U R R A Y , K. (1973) Anaesthetic contamination. Brirish Medical Journal, 2, 548.
A reply from Professor Vessey
In writing my review, I fear I overlooked the letter which Dr Wyatt and Dr Murray Wilson had published in the British Medical Journal. In it they describe 157 children born to male anaesthetists of whom 89 (56.8%) were girls. This percentage is just about significantly different at the 5% level from the corresponding percentage (48.6%) for England and Wales. I personally would not regard this result as convincing evidence of an alteration in sex ratio among infants born to anaesthetists. University Department of Social and Community Medicine, 8 Keble Road, Oxford OX1 3 Q N
M.P. VESSEY
An evaluation of a passive system for theatre pollution control
The method of passive disposal of waste anaesthetic gases in use at Salisbury was described in a paper in Anaesthesia in 1976 (31, 259). Some aspects of the system as installed in the two hospitals have now been assessed quantitatively, namely the resistance of the pipework and the pressures developed therein. Resistance. At Odstock, 4.2 m of 32.5-mm internal diameter pipework with three right angle bends has a resistance of 2 Pa at 30 litre/min; at the Infirmary 9.8 m of pipe with five bends, 10 Pa at 30 litrelmin. The resistance of 5 m of 22 m m corrugated hose was 35 Pa when straight (7 Pa/m) or 70 Pa (14 Pa/m) in a 0.7 m coil. These figures would appear to be quite satisfactory for clinical use, though some might prefer to use 28 mm hose. Pressures. Recordings of pressure developed within the fixed pipework were made during a period of three months. Figure 1 shows transients developed during gale force winds (29.8 m/s, 67 mph) with the operating theatre end of the pipe plugged in order to
demonstrate spikes containing small amounts of energy (the ‘worst-case’ situation). The mean wind pressure of 20 Pa was always subatmospheric and, when added to the operating theatre ventilation pressure of 20-30 Pa, gave a pressure of 50 Pa tending to assist the removal of gases. Recordings were similar in both hospitals, and on no occasion did the pressure in the pipe become positive. The second part of the figure shows a recording made immediately afterwards next to the exhaust valve with the system in use. The operating theatre pressurisation is shown dropping to zero when the doors were opened. The DHSS’ suggests that the negative pressure applied to a patient’s airway should not exceed 100 Pa. This usually applies only to the expiratory phase of a patient on a ventilator; so long as there is gas in the reservoir bag during anaesthesia with spontaneous respiration, negative pressures cannot be applied to the patient’s airway. The effect on a coaxial Mapleson ‘A’ circuit of negative pressures of
Correspondence
tumour. No myasthenic syndrome was clinically detected or obtained from the history. However in both cases the non-depolarizing muscle relaxant had a prolonged action and the patients needed ventilatory support for 3-4 days postoperatively. Edrophonium testing was positive and treatment with pyridostigmine was started a few days after the operation. The histological report of the tumour was thymoma. Therefore a latest myasthenic syndrome, which cannot be excluded in the cases with mediastinal tumour, will be manifested as prolonged muscle paralysis even after a ‘judicious dose’ of non-depolarizing muscle relaxant. On the other hand to abolish spontaneous respiration in patients with respiratory problems may lead to postoperative difficulty with ventilation. Mediactinoscopy is a short procedure and, a s the patient’s condition is often poor, we prefer to carry out the anaesthesia using intermittent doses of succinylcholine. In this technique respiration is not abolished altogether but it is assisted. We believe that spontaneous ventilation under general anaesthesia in these patients is not usually satisfactory. In addition the avoidance of nondepolarizing muscle relaxants avoids complications if a latent myasthenic syndrome is present. Atropine is always given as premedication and sodium thiopentone for induction, since intravenous induction is more comfortable for the patient. The patient is given routinely 100% 0,for a few minutes before the induction of anaesthesia; none the less, we are reluctant to give the patient 100% oxygen at the end of the procedure. We extubate while the patient is breathing 50% O2 and 50% N,O and after that he is given an oxygen mask (usually Ventimask); 50% oxygen is sufficient and 100% oxygen favours collapse of the alveoli postoperatively.
muscle blocking agents are better avoided though this view may not be universally accepted. The recommended use of intermittent succinylcholine for mediastinoscopy in this communication seems to depend on the duration of the procedure and the possibility of a latent myasthenic syndrome. My experience with mediastinoscopy is that the minimum time taken for both induction and procedure usually exceeds 30-40 minutes (if this is followed by thoracotomy then this time may be considerably increased). Whereas succinylcholine may well suit those patients with latent myasthenia (two in 3 years), the remainder of the patients may well end up with other difficulties which may require postoperative ventilation with possible allied dysrhythmia, muscle pains and electrolyte problems. In addition, I presume that before subsequent increments of succinylcholine are given, the patie’nt must show some sign that the effect of the drug has worn off. If the mediastinascope is in situ and coughing or struggling should occur a t this instant, the intrathoracic damage that may occur may well increase morbidity and possibly, if it is of a serious nature, mortality. The problem with the judicious use of non-depolarizing relaxants in the two patients mentioned, was that they required post-operative ventilation. The use of thiopentone and succinylcholine for induction of patients, who may have respiratory problems due to a mediastinal tumour, could lead to serious consequences. Consequently, in these cases, I firmly believe that the principle of avoiding any muscle relaxant before airway control is established must be followed. I am also rather puzzled by the reference to assisted ventilation with suxamethonium chloride, the patient’s respiration not being abolished. Surely, after succinylcholine there must be a period when respiration is abolished thereby requiring artificial respiration. A. FASSOULAKI As always, there are many points of view apperHellenic Aniicancer Institute St Savas, taining to the management of any anaesthetic procedure. However, I feel in this particular 173 Alexandras Ace., Athens 603, instance, that catering for the few rare cases may Greece cause far more problems with the majority of patients who undergo mediastinoscopy. A reply from Dr Vaughan
I would agree that where a myasthenic syndrome is present preoperatively, the use of nondepolarizing
University Hospital of Wales Heath Park, Cardiff
R.S. VAUGHAN
Occupational hazards of anaesthesia A question of sex
I agree wholeheartedly with Professor Vessey (Anaesthesia, 1978, 33, 430) that studies of occupational hazards of anaesthesia are beset with method-
ological difficulties; I refer particularly to the problems of response rate to postal questionnaires and of emotional overlay when questions such as still-birth and foetal abnormality are raised. One questionnaire quoted by Dr Vessey was introduced
Correspondence
on the basis that only married women need answer the question on obstetric history-very gallant but not very scientific. I must take exception, however, to the statement that no study has shown a significant change in sex ratio at birth. In a study of the (then) Sheffield region, Dr A. Murray Wilson and I showed a significant increase in the number of female live-born infants conceived while their fathers were in anaesthetic practice.’ As far as I know, this has been the only study aimed specifically a t this particular aspect. A 75% first time response rate was obtained, without emotional bias, and populations identical geographically and temporally were used for comparison. Given the known basic experimental facts such as the effects of inhalational anaesthetic agents on microtubular function, one might expect sex ratio change to be a useful, sensitive means of detecting genetic effects of chromic exposure to inhalational anaesthetic agents,’ and one amenable to the harvesting of large numbers for statistical analysis. The folk-lore of other disciplines, in which the breathing of gas mixtures other than that intended by our Creator is practised, contains suggestions of changes in the sex ratio of children, for example professional deep-sea divers and military air crew. My family and I know how much better I feel a t the end of a working day since I eliminated all but the occasional use of nitrous oxide, alone amongst the inhalational agents, from my practice. Professor Vessey’s review should be interpreted as the equivalent of a ‘not-proven’ verdict under
77
Scottish law rather than an exoneration of chronic exposure to inhalational anaesthetics as a cause of aberrations of mitosis and sperm motility. The Croby Road Hospital, Croby Road, Leicester LE3 9QE
RICHARD WYATT
References 1. WYATT, R . & WILSON, A. M. (1973) Children
2.
of Anaesthetists. British Medical Journal, 1, 675. WYATT, R., WILSON,A. & M U R R A Y , K. (1973) Anaesthetic contamination. Brirish Medical Journal, 2, 548.
A reply from Professor Vessey
In writing my review, I fear I overlooked the letter which Dr Wyatt and Dr Murray Wilson had published in the British Medical Journal. In it they describe 157 children born to male anaesthetists of whom 89 (56.8%) were girls. This percentage is just about significantly different at the 5% level from the corresponding percentage (48.6%) for England and Wales. I personally would not regard this result as convincing evidence of an alteration in sex ratio among infants born to anaesthetists. University Department of Social and Community Medicine, 8 Keble Road, Oxford OX1 3 Q N
M.P. VESSEY
An evaluation of a passive system for theatre pollution control
The method of passive disposal of waste anaesthetic gases in use at Salisbury was described in a paper in Anaesthesia in 1976 (31, 259). Some aspects of the system as installed in the two hospitals have now been assessed quantitatively, namely the resistance of the pipework and the pressures developed therein. Resistance. At Odstock, 4.2 m of 32.5-mm internal diameter pipework with three right angle bends has a resistance of 2 Pa at 30 litre/min; at the Infirmary 9.8 m of pipe with five bends, 10 Pa at 30 litrelmin. The resistance of 5 m of 22 m m corrugated hose was 35 Pa when straight (7 Pa/m) or 70 Pa (14 Pa/m) in a 0.7 m coil. These figures would appear to be quite satisfactory for clinical use, though some might prefer to use 28 mm hose. Pressures. Recordings of pressure developed within the fixed pipework were made during a period of three months. Figure 1 shows transients developed during gale force winds (29.8 m/s, 67 mph) with the operating theatre end of the pipe plugged in order to
demonstrate spikes containing small amounts of energy (the ‘worst-case’ situation). The mean wind pressure of 20 Pa was always subatmospheric and, when added to the operating theatre ventilation pressure of 20-30 Pa, gave a pressure of 50 Pa tending to assist the removal of gases. Recordings were similar in both hospitals, and on no occasion did the pressure in the pipe become positive. The second part of the figure shows a recording made immediately afterwards next to the exhaust valve with the system in use. The operating theatre pressurisation is shown dropping to zero when the doors were opened. The DHSS’ suggests that the negative pressure applied to a patient’s airway should not exceed 100 Pa. This usually applies only to the expiratory phase of a patient on a ventilator; so long as there is gas in the reservoir bag during anaesthesia with spontaneous respiration, negative pressures cannot be applied to the patient’s airway. The effect on a coaxial Mapleson ‘A’ circuit of negative pressures of
