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EPIDURAL ANALGESIA
BRITISH JOURNAL OF ANAESTHESIA
418 all, in three of the five cases the episode was confined to the period of oxygen washout during delivery. However, the main burden of my study was to suggest a method of evaluating techniques of anaesthesia and analgesia with reference to the incidence of neonatal depression. If the substitution of 75 per cent nitrous oxide is indeed of benefit to the mother, the next step is obviously to analyze the results of a series conducted under the modified technique, and thus to discover the effect upon the infant. J. SELWYN CRAWFORD
EPIDURAL ANALGESIA
Sir,—The paper by Dr. Roy Simpson and his colleagues on epidural analgesia for postoperative pain relief (Brit. J. Anaesth., 33, 628) is an admirable presentation of the way sensory blockade prevents respiratory inhibition after upper abdominal operations; their thorough and painstaking approach commands great respect. Dr. Simpson kindly discussed some of the aspects of this paper with me when he was in Montreal recently, and there are certain areas in which our experience differed. I feel that these points should be aired more fully, since objective evidence of the sort presented in their paper is so important in attempting to reach a decision on the real merits of regional analgesia. Firstly, the effects of the drugs used. Figures for duration of action were given, and it was stated that no tachyphylaxis occurred with repeated injections of 1.5 per cent lignocaine up the epidural catheter. Pronouncements on duration of action should be treated with reserve when they are based on patient's subjective feelings and reports, and relayed through a third party at a remote part of the hospital. Subjective relief, lying quietly in bed, does not necessarily imply freedom from pain on deep breathing, or objective analgesia to pin-prick within a certain number of spinal dermatomes (and how else can one determine tachyphylaxis?). Accurate data on objective analgesia demands constant direct supervision of almost obsessional nature by one or two trained observers whose strength of testing-stimuli is similar; otherwise wildly different and erroneous results can arise from variations of stimulus and
Downloaded from http://bja.oxfordjournals.org/ at Karolinska Institutet on July 13, 2015
University of Chicago Lying-in Hospital
interpretations of segmental levels of blockade. Our observations in this hospital over the past six years have been done by a few trained observers, in the operating rooms or the adjacent recovery ward, where the patients were under constant direct surveillance by the investigators. I cannot agree that decay of effect does not occur with repeated injections of analgesic solution. Dr. R. T. Pettigrew and I carried out a study of this a few years ago with catheters in the upper lumbar region (unpublished results) and we found that a progressive decrement of blockade occurred in both space (the number of segments blocked) and time (duration) with each successive injection, although subjective relief of pain continued to be effective for some time. We attributed this to accumulation of fluid in the extradural space and waterlogging of the nerve integuments, and for want of a better word called it "epidural sog". This phenomenon is probably less obvious when smaller volumes of solution are used, as in Dr. Simpson's series. Analgesic drugs are absorbed into the general circulation from the epidural space, and there is a limit to the amount that can be given with impunity in any given period. Toxic effects become apparent when the venous concentration of lignocaine exceeds lO^g/ml, and this level will be approached if more than 3.5 g of lignocaine are given epidurally to a 10-stone man in 24 hours (Bromage and Robson, 1961). Again, this level is not likely to be reached when small volumes are injected in the thoracic region. It is the mass of drug, rather than either the volume or the concentration alone which is important (Bromage, 1962), but in some people 9 ml of 1.5 per cent lignocaine (as recommended by the authors) will produce an undesirable amount of motor blockade, and it might be safer to start with 0.8 or 1 per cent solutions in the postoperative period. The studies of respiratory mechanics are most interesting, and they appear to have been designed to answer one crucial question: does upper abdominal pain lead to a diminution of effective lung volume? We have been trying to answer the same question in a similar way, with measurements of static total compliance during operation, followed by serial lung volume and pressurevolume measurements postoperatively. Results with total respiratory compliance during operation suggest that high epidural blocks do prevent per-
CORRESPONDENCE
419
Downloaded from http://bja.oxfordjournals.org/ at Karolinska Institutet on July 13, 2015
sistent deficits of compliance after noxious stimuli paper is an excellent attempt to quantitate the (Bromage, 1958), but our postoperative studies ventilatory effects of this change, and it is just with dynamic compliance measurements have because this type of work is so important in been less clear cut. This is a very difficult field, arriving at a true evaluation of regional analgesia bedevilled by technical artefacts which can lead that we should be extremely critical of any data the investigator into erroneous conclusions, and I which appears to support our clinical impressions. suspect that these hazards have led Dr. Simpson P. R. BROMAGE and his colleagues a little astray in some of their Montreal REFERENCES compliance measurements. In table VIII figures are given which show con- Bromage, P. R. (1958). Total respiratory compliance in anaesthetized subjects and modifications prosiderable improvements of compliance after epiduced by noxious stimuli. Clin. Sci., 17, 217. dural block combined with a vigorous "stir-up" (1962). Spread of analgesic solutions in the epidural space and their site of action: a statisregime, but not after epidural block alone. Patients tical study. Brit. J. Anaesth., 34, 161. number 8, 15, 28, and 34 showed increases of Robson, J. G. (1961). Concentration of lignocaine in the blood after intravenous, intramuscular, 18.5, 26, 19, and 55 per cent respectively above epidural and endotracheal administration. Anaestheir pre-operative control readings. It is difficult thesia, 16, 461. to understand why a "stir-up" regime should in- Ferris, B. G., and Pollard, D. S. (1960). Effects of deep and quiet breathing on pulmonary complicrease compliance above pre-operative values, ance in man. /. clin. Invest., 39, 143. unless the latter were taken after a period of Howell, J. B. L. (1956). The mechanics of breathing in shallow breathing, in which case they would man. Ph.D. Thesis, London. scarcely have constituted a valid set of controls. Marshall, R. (1957). The physical properties of the lungs in relation to the subdivisions of lung The importance of the immediate volume-history volume. Clin. Sci., 16, 507. in determining respiratory mechanics is well Mead, J., and Collier, C. (1959). Relation of volume history of lungs to respiratory mechanics in known (Mead and Collier, 1959), and it is essenanaesthetized dogs. /. appl. Physiol., 14, 669. tial to preface control compliance readings with Nisell, O. I., and DuBois, A. B. (1954). Relationship between compliance and F.R.C. of the lungs in a few deep breaths to open up all peripheral cats and measurement of resistance to breathing. alveoli (Howell, 1956; Bromage, 1958). An alterAmer. J. Physiol., 178, 206. native and more likely explanation is that the vigorous activity of the "stir-up" period altered Sir,—Dr. Bromage very kindly sent us a copy of the behaviour of either the oesophageal balloon his letter, but at the time one of us was away. We or the oesophagus in some way, perhaps by change are grateful to you for giving us time to reply. of position or small air leaks in the former or Dr. Bromage's discussion is based essentially on altered tone in the latter (Ferris and Pollard, his own view of the design and purpose of our 1960), thus causing false values for transpul- work. His observations are valuable but not all monary pressures. Repeated postoperative FRC relevant; while his interest is in "arriving at a determinations (after waiting a suitable interval true evaluation of regional analgesia" ours was in for excretion of any nitrous oxide administered the welfare of patients after upper abdominal during the operation) might have given an inter- surgery. esting check on the compliance changes (Nisell We did not intend an "admirable presentation" and DuBois, 1959; Marshall, 1957). However, of any unique effect on "respiratory inhibition"— alterations of compliance are disproportionately whatever that may be. It seems to us that the large compared to acute changes of lung volume completeness of pain relief is the important thing, (Mead and Collier, 1959), and so it is unlikely and whether a centrally acting drug can ever that a rise of more than 15 per cent would have achieve the same as "sensory blockade" is still been recorded in the FRC even if the compliance unknown. increase had been real rather than apparent. Our patients' feelings were not "relayed through As the authors say so enthusiastically, effective a third party at a remote part of the hospital"; regional analgesia produces a wonderful change first-hand supervision was continuous. Furtherin the patient's appearance, and a veil seems to more, all "pronouncements on duration of action" lift as if all trace of ordeal had evaporated. Their should be "treated with reserve" whether they
418 all, in three of the five cases the episode was confined to the period of oxygen washout during delivery. However, the main burden of my study was to suggest a method of evaluating techniques of anaesthesia and analgesia with reference to the incidence of neonatal depression. If the substitution of 75 per cent nitrous oxide is indeed of benefit to the mother, the next step is obviously to analyze the results of a series conducted under the modified technique, and thus to discover the effect upon the infant. J. SELWYN CRAWFORD
EPIDURAL ANALGESIA
Sir,—The paper by Dr. Roy Simpson and his colleagues on epidural analgesia for postoperative pain relief (Brit. J. Anaesth., 33, 628) is an admirable presentation of the way sensory blockade prevents respiratory inhibition after upper abdominal operations; their thorough and painstaking approach commands great respect. Dr. Simpson kindly discussed some of the aspects of this paper with me when he was in Montreal recently, and there are certain areas in which our experience differed. I feel that these points should be aired more fully, since objective evidence of the sort presented in their paper is so important in attempting to reach a decision on the real merits of regional analgesia. Firstly, the effects of the drugs used. Figures for duration of action were given, and it was stated that no tachyphylaxis occurred with repeated injections of 1.5 per cent lignocaine up the epidural catheter. Pronouncements on duration of action should be treated with reserve when they are based on patient's subjective feelings and reports, and relayed through a third party at a remote part of the hospital. Subjective relief, lying quietly in bed, does not necessarily imply freedom from pain on deep breathing, or objective analgesia to pin-prick within a certain number of spinal dermatomes (and how else can one determine tachyphylaxis?). Accurate data on objective analgesia demands constant direct supervision of almost obsessional nature by one or two trained observers whose strength of testing-stimuli is similar; otherwise wildly different and erroneous results can arise from variations of stimulus and
Downloaded from http://bja.oxfordjournals.org/ at Karolinska Institutet on July 13, 2015
University of Chicago Lying-in Hospital
interpretations of segmental levels of blockade. Our observations in this hospital over the past six years have been done by a few trained observers, in the operating rooms or the adjacent recovery ward, where the patients were under constant direct surveillance by the investigators. I cannot agree that decay of effect does not occur with repeated injections of analgesic solution. Dr. R. T. Pettigrew and I carried out a study of this a few years ago with catheters in the upper lumbar region (unpublished results) and we found that a progressive decrement of blockade occurred in both space (the number of segments blocked) and time (duration) with each successive injection, although subjective relief of pain continued to be effective for some time. We attributed this to accumulation of fluid in the extradural space and waterlogging of the nerve integuments, and for want of a better word called it "epidural sog". This phenomenon is probably less obvious when smaller volumes of solution are used, as in Dr. Simpson's series. Analgesic drugs are absorbed into the general circulation from the epidural space, and there is a limit to the amount that can be given with impunity in any given period. Toxic effects become apparent when the venous concentration of lignocaine exceeds lO^g/ml, and this level will be approached if more than 3.5 g of lignocaine are given epidurally to a 10-stone man in 24 hours (Bromage and Robson, 1961). Again, this level is not likely to be reached when small volumes are injected in the thoracic region. It is the mass of drug, rather than either the volume or the concentration alone which is important (Bromage, 1962), but in some people 9 ml of 1.5 per cent lignocaine (as recommended by the authors) will produce an undesirable amount of motor blockade, and it might be safer to start with 0.8 or 1 per cent solutions in the postoperative period. The studies of respiratory mechanics are most interesting, and they appear to have been designed to answer one crucial question: does upper abdominal pain lead to a diminution of effective lung volume? We have been trying to answer the same question in a similar way, with measurements of static total compliance during operation, followed by serial lung volume and pressurevolume measurements postoperatively. Results with total respiratory compliance during operation suggest that high epidural blocks do prevent per-
CORRESPONDENCE
419
Downloaded from http://bja.oxfordjournals.org/ at Karolinska Institutet on July 13, 2015
sistent deficits of compliance after noxious stimuli paper is an excellent attempt to quantitate the (Bromage, 1958), but our postoperative studies ventilatory effects of this change, and it is just with dynamic compliance measurements have because this type of work is so important in been less clear cut. This is a very difficult field, arriving at a true evaluation of regional analgesia bedevilled by technical artefacts which can lead that we should be extremely critical of any data the investigator into erroneous conclusions, and I which appears to support our clinical impressions. suspect that these hazards have led Dr. Simpson P. R. BROMAGE and his colleagues a little astray in some of their Montreal REFERENCES compliance measurements. In table VIII figures are given which show con- Bromage, P. R. (1958). Total respiratory compliance in anaesthetized subjects and modifications prosiderable improvements of compliance after epiduced by noxious stimuli. Clin. Sci., 17, 217. dural block combined with a vigorous "stir-up" (1962). Spread of analgesic solutions in the epidural space and their site of action: a statisregime, but not after epidural block alone. Patients tical study. Brit. J. Anaesth., 34, 161. number 8, 15, 28, and 34 showed increases of Robson, J. G. (1961). Concentration of lignocaine in the blood after intravenous, intramuscular, 18.5, 26, 19, and 55 per cent respectively above epidural and endotracheal administration. Anaestheir pre-operative control readings. It is difficult thesia, 16, 461. to understand why a "stir-up" regime should in- Ferris, B. G., and Pollard, D. S. (1960). Effects of deep and quiet breathing on pulmonary complicrease compliance above pre-operative values, ance in man. /. clin. Invest., 39, 143. unless the latter were taken after a period of Howell, J. B. L. (1956). The mechanics of breathing in shallow breathing, in which case they would man. Ph.D. Thesis, London. scarcely have constituted a valid set of controls. Marshall, R. (1957). The physical properties of the lungs in relation to the subdivisions of lung The importance of the immediate volume-history volume. Clin. Sci., 16, 507. in determining respiratory mechanics is well Mead, J., and Collier, C. (1959). Relation of volume history of lungs to respiratory mechanics in known (Mead and Collier, 1959), and it is essenanaesthetized dogs. /. appl. Physiol., 14, 669. tial to preface control compliance readings with Nisell, O. I., and DuBois, A. B. (1954). Relationship between compliance and F.R.C. of the lungs in a few deep breaths to open up all peripheral cats and measurement of resistance to breathing. alveoli (Howell, 1956; Bromage, 1958). An alterAmer. J. Physiol., 178, 206. native and more likely explanation is that the vigorous activity of the "stir-up" period altered Sir,—Dr. Bromage very kindly sent us a copy of the behaviour of either the oesophageal balloon his letter, but at the time one of us was away. We or the oesophagus in some way, perhaps by change are grateful to you for giving us time to reply. of position or small air leaks in the former or Dr. Bromage's discussion is based essentially on altered tone in the latter (Ferris and Pollard, his own view of the design and purpose of our 1960), thus causing false values for transpul- work. His observations are valuable but not all monary pressures. Repeated postoperative FRC relevant; while his interest is in "arriving at a determinations (after waiting a suitable interval true evaluation of regional analgesia" ours was in for excretion of any nitrous oxide administered the welfare of patients after upper abdominal during the operation) might have given an inter- surgery. esting check on the compliance changes (Nisell We did not intend an "admirable presentation" and DuBois, 1959; Marshall, 1957). However, of any unique effect on "respiratory inhibition"— alterations of compliance are disproportionately whatever that may be. It seems to us that the large compared to acute changes of lung volume completeness of pain relief is the important thing, (Mead and Collier, 1959), and so it is unlikely and whether a centrally acting drug can ever that a rise of more than 15 per cent would have achieve the same as "sensory blockade" is still been recorded in the FRC even if the compliance unknown. increase had been real rather than apparent. Our patients' feelings were not "relayed through As the authors say so enthusiastically, effective a third party at a remote part of the hospital"; regional analgesia produces a wonderful change first-hand supervision was continuous. Furtherin the patient's appearance, and a veil seems to more, all "pronouncements on duration of action" lift as if all trace of ordeal had evaporated. Their should be "treated with reserve" whether they