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Fresh gas flow rate and enflurane consumption
British Journal of Anaesthesia 1995; 74: 627-632
CORRESPONDENCE
Fresh gas flow rate and enflurane consumption
S. M. COTTER D. C. WHITE
Department of Anaesthesia Northwick Park Hospital Harrow 1. Logan M. Breathing systems: effect of fresh gas flow rate on enflurane consumption. British Journal of Anaesthesia 1994; 73: 775-778. 2. Cotter SM, Petros AJ, Dore CJ, Barber ND, White DC. Lowflow anaesthesia. Practice, cost implications and acceptability. Anaesthesia 1991; 46: 1009-1012. 3. Newton DEF, Thornton C, Konieczko KM, Frith CD, Dore CJ, Webster NR, Lough NP. Level of consciousness in volunteers breathing sub-MAC concentrations of isoflurane. British Journal of Anaesthesia 1990; 65: 609-615. 4. Boulogne P, Demontoux MH, Colin D, Feiss P. Utilisation de l'isoflurane en circuit ferme. Avantages economiques. Annales Francoises D'Anesthesie el de Reanimation 1987; 6: 54-56. 5. Revell SP, Taylor DH. Isoflurane in a circle system with low gas flow. British Journal of Anaesthesia 1987; 59: 1219-1222. 6. Lockwood GG, White DC. Effect of ventilation and cardiac output on the uptake of anaesthetics from different breathing systems: a theoretical study. British Journal of Anaesthesia 1991; 66: 519-526.
M. LOGAN
Department of Anaesthetics Royal Infirmary of Edinburgh Edinburgh 1. Cotter SM, Petros AJ, Dore CJ, Barber ND, White DC. Lowflow anaesthesia. Practice, cost implications and acceptability. Anaesthesia 1991; 46: 1009-1012.
Circumcision analgesia Sir,—I read with interest the recent paper by Chambers and colleagues [1], and I agree that topical lignocaine cannot be used to provide operative analgesia as it does not penetrate intact skin; it only penetrates the intact mucous membrane undersurface of the foreskin, and the exposed nerve endings and skin edges which remain after surgery [1]. However, local anaesthetic may be applied after uncovering the mucous membrane, the exposed nerve endings and skin edges which remain after the foreskin is removed. In my article [2] "A new method of analgesia for relief of circumcision pain", lignocaine was used as topical analgesia during operation. After the foreskin was removed, lignocaine was applied topically as drops onto the foreskin undersurface. After application, it was possible to discontinue halothane and anaesthesia could be continued with 60-70% nitrous oxide in oxygen in the treated group. The treated group required significantly less pethidine after operation, and the painless period was significantly longer compared with the placebo group. The technique is simple and requires no special skills, and has none of the dorsal nerve block complications. K. H. ANDERSEN
Department of Anaesthetics Bispebjerg Hospital Copenhagen, Denmark 1. Chambers FA, Lee J, Smith J, Casey W. Post-circumcision analgesia: comparison of topical analgesia with dorsal nerve block using the midline and lateral approaches. British Journal of Anaesthesia 1994; 73: 437^139. 2. Andersen KH. A new method of analgesia for relief of circumcision pain. Anaesthesia 1989; 44: 118-120.
Sir,—We agree with Dr Andersen that topical lignocaine does not provide operative analgesia nor was it intended to do so in our study. With regard to timing, lignocaine jelly was applied to the surgical wound on removal of the foreskin and allowed to settle for 2 min so that it could penetrate the exposed nerve endings and skin edges. Dr Andersen, in his study, compared topical lignocaine with placebo, concluding that it provided superior analgesia. The results of our study show conclusively that dorsal nerve block, despite its associated risks, is superior to topical analgesia as a technique for providing postoperative analgesia. F. A. CHAMBERS
Department of Anaesthesia University of California San Francisco CA, USA
Downloaded from http://bja.oxfordjournals.org/ at Florida International University on May 29, 2015
Sir,—The amount of liquid anaesthetic agent used during inhalation anaesthesia with the vaporizer in the fresh gas flow (FGF) is directly proportional to the rate of FGF. It would therefore seem simple to calculate the savings to be expected from reducing FGF during routine clinical anaesthesia. However, in practice, several additional factors complicate the picture. Dr Logan's article [1] is a valuable addition to the literature on this subject. He found a 33 % reduction in enflurane usage when FGF was reduced from 6 to 3 litre min"1. This contrasts with our finding [2] of a 56 % reduction in enflurane usage with similar, but not identical, reductions in FGF. Dr Logan suggested that the considerable difference between his findings and ours may be attributed to differences in the FGF rates studied, but the differences inflowrate were not great. Mean FGF in our low-flow enflurane group was 2.8 litre min"1 (Logan 3 litre min*1) and in the high-flow group 6.7 litre min"1 (Logan 6 litre min"1). Differences in FGF studied may indeed be a factor in the discrepancy between his results and ours, but we believe that the chief reason is that all of Dr Logan's patients were breathing spontaneously. In our study, 55 % of the patients receiving enflurane were paralysed and undergoing artificial ventilation. Under these circumstances, lower concentrations of inhalation anaesthetics are administered than is the case with spontaneously breathing patients. This is presumably a reflection of the fact that experience has shown that 1 MAC is a higher concentration than is required to abolish consciousness. There is experimental evidence to support this view [3]. Our suggestion is supported by the results of a study performed by Boulogne and colleagues [4] who showed a 10-fold reduction in isoflurane consumption when FGF was reduced from 7 to 0.5 litre min"1 in 60 patients undergoing ventilation. In another study, Revell and Taylor [5] could only achieve a 70% reduction in volatile agent consumption for the same reduction in FGF in spontaneously breathing patients. None of Dr Logan's patients received opioids, but in our patients there was no restriction on their use. Opioids do reduce experimental volatile agent requirements, but it would not be easy to measure this effect with the relatively small doses of opioids used in routine clinical practice which do not appear to allow the anaesthetist to turn down the vaporizer. There is one circumstance in which the anaesthetic sparing effect of opioids can be seen clearly. If the vaporizer is in a circle system (VIC) and the patient is breathing spontaneously, then administration of opioids reduces tidal volume, the inspired concentration of agent decreases and the patient's uptake of anaesthetic is reduced [6].
Sir,—I am grateful for the comments of Drs Cotter and White, expanding the details of their study [1]. I agree with their additional thoughts on the potential reasons for the differences in the results of the two studies. Their use of IPPV in 55 % of their patients and especially in those given opioids will undoubtedly have reduced the amount of volatile agent required for surgical levels of anaesthesia. The rate of usage of volatile agents is influenced by many factors however, and a difference in depth of anaesthesia used for maintenance of the rate at which the required depth of anaesthesia is reached will vary with individual anaesthetist techniques.
CORRESPONDENCE
Fresh gas flow rate and enflurane consumption
S. M. COTTER D. C. WHITE
Department of Anaesthesia Northwick Park Hospital Harrow 1. Logan M. Breathing systems: effect of fresh gas flow rate on enflurane consumption. British Journal of Anaesthesia 1994; 73: 775-778. 2. Cotter SM, Petros AJ, Dore CJ, Barber ND, White DC. Lowflow anaesthesia. Practice, cost implications and acceptability. Anaesthesia 1991; 46: 1009-1012. 3. Newton DEF, Thornton C, Konieczko KM, Frith CD, Dore CJ, Webster NR, Lough NP. Level of consciousness in volunteers breathing sub-MAC concentrations of isoflurane. British Journal of Anaesthesia 1990; 65: 609-615. 4. Boulogne P, Demontoux MH, Colin D, Feiss P. Utilisation de l'isoflurane en circuit ferme. Avantages economiques. Annales Francoises D'Anesthesie el de Reanimation 1987; 6: 54-56. 5. Revell SP, Taylor DH. Isoflurane in a circle system with low gas flow. British Journal of Anaesthesia 1987; 59: 1219-1222. 6. Lockwood GG, White DC. Effect of ventilation and cardiac output on the uptake of anaesthetics from different breathing systems: a theoretical study. British Journal of Anaesthesia 1991; 66: 519-526.
M. LOGAN
Department of Anaesthetics Royal Infirmary of Edinburgh Edinburgh 1. Cotter SM, Petros AJ, Dore CJ, Barber ND, White DC. Lowflow anaesthesia. Practice, cost implications and acceptability. Anaesthesia 1991; 46: 1009-1012.
Circumcision analgesia Sir,—I read with interest the recent paper by Chambers and colleagues [1], and I agree that topical lignocaine cannot be used to provide operative analgesia as it does not penetrate intact skin; it only penetrates the intact mucous membrane undersurface of the foreskin, and the exposed nerve endings and skin edges which remain after surgery [1]. However, local anaesthetic may be applied after uncovering the mucous membrane, the exposed nerve endings and skin edges which remain after the foreskin is removed. In my article [2] "A new method of analgesia for relief of circumcision pain", lignocaine was used as topical analgesia during operation. After the foreskin was removed, lignocaine was applied topically as drops onto the foreskin undersurface. After application, it was possible to discontinue halothane and anaesthesia could be continued with 60-70% nitrous oxide in oxygen in the treated group. The treated group required significantly less pethidine after operation, and the painless period was significantly longer compared with the placebo group. The technique is simple and requires no special skills, and has none of the dorsal nerve block complications. K. H. ANDERSEN
Department of Anaesthetics Bispebjerg Hospital Copenhagen, Denmark 1. Chambers FA, Lee J, Smith J, Casey W. Post-circumcision analgesia: comparison of topical analgesia with dorsal nerve block using the midline and lateral approaches. British Journal of Anaesthesia 1994; 73: 437^139. 2. Andersen KH. A new method of analgesia for relief of circumcision pain. Anaesthesia 1989; 44: 118-120.
Sir,—We agree with Dr Andersen that topical lignocaine does not provide operative analgesia nor was it intended to do so in our study. With regard to timing, lignocaine jelly was applied to the surgical wound on removal of the foreskin and allowed to settle for 2 min so that it could penetrate the exposed nerve endings and skin edges. Dr Andersen, in his study, compared topical lignocaine with placebo, concluding that it provided superior analgesia. The results of our study show conclusively that dorsal nerve block, despite its associated risks, is superior to topical analgesia as a technique for providing postoperative analgesia. F. A. CHAMBERS
Department of Anaesthesia University of California San Francisco CA, USA
Downloaded from http://bja.oxfordjournals.org/ at Florida International University on May 29, 2015
Sir,—The amount of liquid anaesthetic agent used during inhalation anaesthesia with the vaporizer in the fresh gas flow (FGF) is directly proportional to the rate of FGF. It would therefore seem simple to calculate the savings to be expected from reducing FGF during routine clinical anaesthesia. However, in practice, several additional factors complicate the picture. Dr Logan's article [1] is a valuable addition to the literature on this subject. He found a 33 % reduction in enflurane usage when FGF was reduced from 6 to 3 litre min"1. This contrasts with our finding [2] of a 56 % reduction in enflurane usage with similar, but not identical, reductions in FGF. Dr Logan suggested that the considerable difference between his findings and ours may be attributed to differences in the FGF rates studied, but the differences inflowrate were not great. Mean FGF in our low-flow enflurane group was 2.8 litre min"1 (Logan 3 litre min*1) and in the high-flow group 6.7 litre min"1 (Logan 6 litre min"1). Differences in FGF studied may indeed be a factor in the discrepancy between his results and ours, but we believe that the chief reason is that all of Dr Logan's patients were breathing spontaneously. In our study, 55 % of the patients receiving enflurane were paralysed and undergoing artificial ventilation. Under these circumstances, lower concentrations of inhalation anaesthetics are administered than is the case with spontaneously breathing patients. This is presumably a reflection of the fact that experience has shown that 1 MAC is a higher concentration than is required to abolish consciousness. There is experimental evidence to support this view [3]. Our suggestion is supported by the results of a study performed by Boulogne and colleagues [4] who showed a 10-fold reduction in isoflurane consumption when FGF was reduced from 7 to 0.5 litre min"1 in 60 patients undergoing ventilation. In another study, Revell and Taylor [5] could only achieve a 70% reduction in volatile agent consumption for the same reduction in FGF in spontaneously breathing patients. None of Dr Logan's patients received opioids, but in our patients there was no restriction on their use. Opioids do reduce experimental volatile agent requirements, but it would not be easy to measure this effect with the relatively small doses of opioids used in routine clinical practice which do not appear to allow the anaesthetist to turn down the vaporizer. There is one circumstance in which the anaesthetic sparing effect of opioids can be seen clearly. If the vaporizer is in a circle system (VIC) and the patient is breathing spontaneously, then administration of opioids reduces tidal volume, the inspired concentration of agent decreases and the patient's uptake of anaesthetic is reduced [6].
Sir,—I am grateful for the comments of Drs Cotter and White, expanding the details of their study [1]. I agree with their additional thoughts on the potential reasons for the differences in the results of the two studies. Their use of IPPV in 55 % of their patients and especially in those given opioids will undoubtedly have reduced the amount of volatile agent required for surgical levels of anaesthesia. The rate of usage of volatile agents is influenced by many factors however, and a difference in depth of anaesthesia used for maintenance of the rate at which the required depth of anaesthesia is reached will vary with individual anaesthetist techniques.