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NEUROLEPTANAESTHESIA FOR MAJOR SURGERY
740
BRITISH JOURNAL OF ANAESTHESIA
MICHAEL JOHNSTONE
Manchester
diagnostic procedures. Lately, we have used it in a similar manner for laparoscopic tubal ligations. J. ANTONIO ALDRETE
Louisville, Kentucky REFERENCES
Aldrete, J. A., Clapp, H. W., Fishman, J., and O'Higgins, J. W. (1971). Pentazepam, a supplementary agent. Anesth. Analg. (Cleve.% 50, 498. Schoenfeld, A., Goldman, J. A., and Levy, E. (1974). Pentazocine and diazepam analgesia for minor gynaecological operations. Br. J. Anaesth., 46, 385. NEUROLEPTANAESTHESIA FOR MAJOR SURGERY
Sir,—Is it not a sad comment on our specialty that a valuable clinical study like that of Morgan, Lumley and Gillies (1974) on neuroleptanalgesia can be published without any reference of any kind to what the patients thought about it afterwards? One real disadvantage of the technique has often been described. The authors refer, for instance, to the paper by Edmonds-Seal and Prys-Roberts (1970), who wrote: "Although these drugs are generally potent tranquillizers, it must be emphasized that in some subjects acute administration of clinically effective doses may induce a state of mental restlessness and agitation. This may be completely masked to the observer by the overt appearance of tranquillity and dissociation". In my experience, about 10% of patients find the period after surgery most unpleasant because of this. Pharmacological detachment is quite different from philosophical detachment: it is a physical cutting-off of a patient's feelings so that they no longer find expression in his face nor outlet in his speech. His ordinary means of communication are detached from him and he is shut up in the private world of his own fear and pain. Where cardiovascular stability and postoperative tranquillity are essential this is an acceptable price to pay. How often are they essential in ordinary anaesthetic practice?
REFERENCES
Johnstone, M. (1961). Halothane: the first five years. Anesthesiology, 22, 591. (1972). The cardiovascular effects of oxytocic drugs. Br. J. Anaesth., 44, 826. (1974). Facial vasomotor behaviour. Br. J. Anaesth., 46, 765. Prys-Roberts, C , Lloyd, J. W., Fisher, A., Kerr, J. H., and Patterson, T. J. S. (1974). Deliberate profound hypotension induced with halothane: studies of haemodynamics and pulmonary gas exchange. Br. J. Anaesth., 46, 105. Ravent6s, J. (1961). The action of Fluothane on the autonomic nervous system. Helv. Chir. Acta, 28, 358. (1962). Action du Fluothane sur mechanisms regulateurs de la tension arterielle. Anesth. Analg. {Paris), 19, 27. ANALGESIA FOR MINOR GYNAECOLOGICAL OPERATIONS
Sir,—I write in relation to the paper on the combination of pentazocine and diazepam by Drs A. Schoenfeld, J. A. Goldman and E. Levy (1974). The combination of these two drugs is indeed a form of neuroleptanalgesia and I was pleased to find that the authors, using exactly the same doses as we did, found the same results (Aldrete et al., 1971). In our publication, the mixture given i.v. was found to be a satisfactory supplementary agent for a variety of minor surgical and
T. H. SPREADBURY
Warwick REFERENCES
Edmonds-Seal, J., and Prys-Roberts, C. (1970). Pharmacology of drugs used in neuroleptanalgesia. Br. J. Anaesth., 42, 207. Morgan, M., Lumley, J., and Gillies, I. D. S. (1974). Neuroleptanaesthesia for major surgery. Br. J. Anaesth, 46, 288. PREDICTION OF Pa C O 2 IN A CIRCLE SYSTEM
Sir,—I was interested to read the paper by Drs Scholfield and Williams reporting Pacoa values in patients ventilated using large minute volumes and a circle system without carbon dioxide absorption (Scholfield and Williams, 1974). I would like to suggest two additional explanations for the discrepancy between the predicted and observed Pacoa values that they found. First, it is possible that their assumption that elimination of carbon dioxide from the system is directly related to the fresh gas flow, is not correct. In the circuit shown in their figure 1, the spill valve is immediately adjacent to the reservoir bag. During the inspiratory phase of the ventilator, fresh gas will flow into the reservoir bag. It is possible that the composition of the gas spilt from the valve during the inspiratory phase will alter with the composition of gas in the bag,
Downloaded from http://bja.oxfordjournals.org/ at Karolinska Institutet on July 14, 2015
A feature of the arterial hypotension produced by halothane is that it is associated with bradycardia. This suggests that the baroreceptor system is inactivated by the drug. Atropine prevents the bradycardia and may modify the hypotension to some extent (Johnstone, 1961). The overall cardiovascular effects of halothane indicate that at least two factors are involved in the causation of hypotension: one is the blockade of the sympathetic efferents to the heart and the peripheral blood vessels; the other is increased vagal tone with its negative chronotropic and inotropic effects. It is not improbable that the "resistance" of some patients to the hypotensive effect of certain sympathetic blocking agents may be related to the constrictive effect of sympathetic blockade on the /3-adrenoceptive blood vessels. This is most obvious in young adults with good muscle perfusion. For obvious reasons, the alterations in peripheral vascular tone caused by variations in sympathetic activity may not be reflected by changes in the total peripheral resistance in normovolaemic subjects. Sodium nitroprusside is a vasodilator substance which acts directly on the smooth muscle of both the a- and the y3-adrenoceptive vessels. It is not a sympathetic blocking drug. In patients with intact sympathetic reflexes, the hypotensive action of nitroprusside, like that of oxytocin (Johnstone, 1972), is antagonized by brisk baroreceptor activity with reflex adrenergic stimulation of the heart producing tachycardia and an increased cardiac contractility and output, provided that the venous return is not impeded. Undoubtedly nitroprusside potentiates the hypotensive effect of halothane by dilating the constricted /8-adrenoceptive vessels. It remains to be seen whether or not this is a desirable procedure in anaesthetic practice, especially in view of the depression of the baroreceptor system by halothane. Unless considerable care is taken in their use, the combined effects of the two drugs may precipitate a serious failure of venous return in patients who are anaesthetized in the head-up position.
CORRESPONDENCE
V C O 2 = V F . FECO2
as long as V F is equal to or less than VE. If the system is in equilibrium, the carbon dioxide elimination from the patient will be the same as the elimination from the whole system, and may be expressed as: VCO 2 =(FECO 2 -FIOO 2 ) . VE
Similarly, elimination of carbon dioxide from the alveolar gas will be identical and can be expressed as: VCO 2 =(FACO 2 —Flco2) • VA
V F is the fresh gas flow; Fico 2) FECO25 and FAco2 are the carbon dioxide concentrations in inspired, mixed expired, and alveolar gases, respectively. From these three expressions for carbon dioxide production, the following prediction for alveolar carbon dioxide concentration can be obtained:
prediction of Paco2 might be obtained if the relationship between fresh gas flow and ventilation is altered so that their ratio is constant. G.
B. DRUMMOND
Edinburgh REFERENCES
Eger, E. I., and Ethans, C. T. (1969). The effects of inflow, overflow and valve placement on economy of the circle system. Anesthesiology, 30, 93. Scholfield, E. J., and Williams, N. E. (1974). Prediction of arterial carbon dioxide tension using a circle system without carbon dioxide absorption. Br. J. Anaesth., 46, 442. Suwa, K., and Yamamura, H. (1970). The effect of gas inflow on the regulation of CO2 levels with hyperventilation during anesthesia. Anesthesiology, 33, 440. USE OF THE MINIVENT WITH ELECTRIC SUCTION APPARATUS
Sir,—The Minivent, described by Cohen (1966), is a simple miniature ventilator of a group which includes the Autovent and Microvent (Carden, 1969). Driven by gases delivered from an anaesthetic machine or oxygen cylinder, it is dependent on a distended reservoir bag to provide a sufficiently high pressure to operate a magnetic valve (Carden, 1969). We have found the 0.25 h.p. motor of an ordinary suction apparatus, which is readily available, to be a good substitute for gas cylinders in isolated and difficult situations where compressed air or oxygen cylinders may not be available. The air outlet of the 0.25 h.p. motor of the suction apparatus (not the suction inlet) is connected via rubber pressure tubing to a 2-litre reservoir bag which in turn is connected to the Minivent (fig. 1).
FACO2=VCO2(1/VF+1/VA-1/VE)
This expression can be rearranged to give an expression analogous to equation (8) obtained by Suwa and Yamamura (1970) in their study of carbon dioxide elimination by circle systems. If it is assumed that carbon dioxide production is constant, that PACO2 is proportional to Paco2 and also that the ratio V D / V T (and thus the ratio VA/VE) remains constant, then the expression simplifies to: Paco2OCl/VF + K . l / V E If the ratio VE/VF is large and constant this expression can be simplified to give: Paco2OCl/VF In the study quoted, the minute volume was set to a value between two and four times the fresh gas flow. It might be reasonable to assume that when the fresh gas flow was large, the minute volume would be closer to two times the fresh gas flow, and closer to four times the fresh gas flow when the fresh gas flow was small. In this way, the value of the second factor in the expression 1/VF + K . l / V E might vary inversely with the first. Hence, measured Paco2 values would be greater than predicted in the low PaCo2 range, and vice-versa. This supposition agrees with the results obtained in the study. I would suggest, therefore, that a more consistent
'4 hp ordinary electric suction apparatus
FIG. 1. Modified circuit for the Minivent Respirator. The suction apparatus delivers a gas flow of about 8-10 litre /min from the exhaust and this is sufficient to distend the reservoir bag and operate the magnetic valve of the Minivent. Addition of oxygen is possible through an extra inlet to the reservoir bag. If a new reservoir bag is not available, an older one may be used if it is enclosed in a thick plastic bag to prevent over-distension. The suggested use of the suction apparatus is not intended as a substitute for compressed air cylinders, but it may be helpful to colleagues who are working in environments similar to ours. M. T. BHATIA S. M. PATEL G. H. SHAH
Ahmedabad, India REFERENCES
Carden, E. (1969). The Microvent. Anaesthesia, 24, 90. Cohen, A. D. (1966). The Minivent. Anaesthesia, 21, 563.
Downloaded from http://bja.oxfordjournals.org/ at Karolinska Institutet on July 14, 2015
and the volume of the bag remaining to be filled by exhaled gas. For larger fresh gas flows, more fresh gas will be spilt and carbon dioxide elimination will become less efficient. In this way, measured Paco2 values would exceed those predicted when the predicted Paco2 was low, as the results of the investigation demonstrated. However, if the spill valve were resited between the reservoir bag and the expiratory valve, only mixed expired gas would be spilt. The circuit would then resemble that described as circuit C by Eger and Ethans (1968), although, with the reduction in capacity between inflow and spill valve caused by removal of the absorber, fresh gas might be spilt. A further factor is the effect of the patient deadspace in preventing full equilibration of alveolar gas with circuit gas. A second factor leading to the discrepancy between observed and predicted Paco2 is the effect of the patient deadspace in preventing full equilibration of alveolar gas with circuit gas. If it is assumed that the ideal situation obtains, and only mixed expired gas is spilt from the circuit, the carbon dioxide elimination from the circuit will be regulated by fresh gas flow, so that:
741
BRITISH JOURNAL OF ANAESTHESIA
MICHAEL JOHNSTONE
Manchester
diagnostic procedures. Lately, we have used it in a similar manner for laparoscopic tubal ligations. J. ANTONIO ALDRETE
Louisville, Kentucky REFERENCES
Aldrete, J. A., Clapp, H. W., Fishman, J., and O'Higgins, J. W. (1971). Pentazepam, a supplementary agent. Anesth. Analg. (Cleve.% 50, 498. Schoenfeld, A., Goldman, J. A., and Levy, E. (1974). Pentazocine and diazepam analgesia for minor gynaecological operations. Br. J. Anaesth., 46, 385. NEUROLEPTANAESTHESIA FOR MAJOR SURGERY
Sir,—Is it not a sad comment on our specialty that a valuable clinical study like that of Morgan, Lumley and Gillies (1974) on neuroleptanalgesia can be published without any reference of any kind to what the patients thought about it afterwards? One real disadvantage of the technique has often been described. The authors refer, for instance, to the paper by Edmonds-Seal and Prys-Roberts (1970), who wrote: "Although these drugs are generally potent tranquillizers, it must be emphasized that in some subjects acute administration of clinically effective doses may induce a state of mental restlessness and agitation. This may be completely masked to the observer by the overt appearance of tranquillity and dissociation". In my experience, about 10% of patients find the period after surgery most unpleasant because of this. Pharmacological detachment is quite different from philosophical detachment: it is a physical cutting-off of a patient's feelings so that they no longer find expression in his face nor outlet in his speech. His ordinary means of communication are detached from him and he is shut up in the private world of his own fear and pain. Where cardiovascular stability and postoperative tranquillity are essential this is an acceptable price to pay. How often are they essential in ordinary anaesthetic practice?
REFERENCES
Johnstone, M. (1961). Halothane: the first five years. Anesthesiology, 22, 591. (1972). The cardiovascular effects of oxytocic drugs. Br. J. Anaesth., 44, 826. (1974). Facial vasomotor behaviour. Br. J. Anaesth., 46, 765. Prys-Roberts, C , Lloyd, J. W., Fisher, A., Kerr, J. H., and Patterson, T. J. S. (1974). Deliberate profound hypotension induced with halothane: studies of haemodynamics and pulmonary gas exchange. Br. J. Anaesth., 46, 105. Ravent6s, J. (1961). The action of Fluothane on the autonomic nervous system. Helv. Chir. Acta, 28, 358. (1962). Action du Fluothane sur mechanisms regulateurs de la tension arterielle. Anesth. Analg. {Paris), 19, 27. ANALGESIA FOR MINOR GYNAECOLOGICAL OPERATIONS
Sir,—I write in relation to the paper on the combination of pentazocine and diazepam by Drs A. Schoenfeld, J. A. Goldman and E. Levy (1974). The combination of these two drugs is indeed a form of neuroleptanalgesia and I was pleased to find that the authors, using exactly the same doses as we did, found the same results (Aldrete et al., 1971). In our publication, the mixture given i.v. was found to be a satisfactory supplementary agent for a variety of minor surgical and
T. H. SPREADBURY
Warwick REFERENCES
Edmonds-Seal, J., and Prys-Roberts, C. (1970). Pharmacology of drugs used in neuroleptanalgesia. Br. J. Anaesth., 42, 207. Morgan, M., Lumley, J., and Gillies, I. D. S. (1974). Neuroleptanaesthesia for major surgery. Br. J. Anaesth, 46, 288. PREDICTION OF Pa C O 2 IN A CIRCLE SYSTEM
Sir,—I was interested to read the paper by Drs Scholfield and Williams reporting Pacoa values in patients ventilated using large minute volumes and a circle system without carbon dioxide absorption (Scholfield and Williams, 1974). I would like to suggest two additional explanations for the discrepancy between the predicted and observed Pacoa values that they found. First, it is possible that their assumption that elimination of carbon dioxide from the system is directly related to the fresh gas flow, is not correct. In the circuit shown in their figure 1, the spill valve is immediately adjacent to the reservoir bag. During the inspiratory phase of the ventilator, fresh gas will flow into the reservoir bag. It is possible that the composition of the gas spilt from the valve during the inspiratory phase will alter with the composition of gas in the bag,
Downloaded from http://bja.oxfordjournals.org/ at Karolinska Institutet on July 14, 2015
A feature of the arterial hypotension produced by halothane is that it is associated with bradycardia. This suggests that the baroreceptor system is inactivated by the drug. Atropine prevents the bradycardia and may modify the hypotension to some extent (Johnstone, 1961). The overall cardiovascular effects of halothane indicate that at least two factors are involved in the causation of hypotension: one is the blockade of the sympathetic efferents to the heart and the peripheral blood vessels; the other is increased vagal tone with its negative chronotropic and inotropic effects. It is not improbable that the "resistance" of some patients to the hypotensive effect of certain sympathetic blocking agents may be related to the constrictive effect of sympathetic blockade on the /3-adrenoceptive blood vessels. This is most obvious in young adults with good muscle perfusion. For obvious reasons, the alterations in peripheral vascular tone caused by variations in sympathetic activity may not be reflected by changes in the total peripheral resistance in normovolaemic subjects. Sodium nitroprusside is a vasodilator substance which acts directly on the smooth muscle of both the a- and the y3-adrenoceptive vessels. It is not a sympathetic blocking drug. In patients with intact sympathetic reflexes, the hypotensive action of nitroprusside, like that of oxytocin (Johnstone, 1972), is antagonized by brisk baroreceptor activity with reflex adrenergic stimulation of the heart producing tachycardia and an increased cardiac contractility and output, provided that the venous return is not impeded. Undoubtedly nitroprusside potentiates the hypotensive effect of halothane by dilating the constricted /8-adrenoceptive vessels. It remains to be seen whether or not this is a desirable procedure in anaesthetic practice, especially in view of the depression of the baroreceptor system by halothane. Unless considerable care is taken in their use, the combined effects of the two drugs may precipitate a serious failure of venous return in patients who are anaesthetized in the head-up position.
CORRESPONDENCE
V C O 2 = V F . FECO2
as long as V F is equal to or less than VE. If the system is in equilibrium, the carbon dioxide elimination from the patient will be the same as the elimination from the whole system, and may be expressed as: VCO 2 =(FECO 2 -FIOO 2 ) . VE
Similarly, elimination of carbon dioxide from the alveolar gas will be identical and can be expressed as: VCO 2 =(FACO 2 —Flco2) • VA
V F is the fresh gas flow; Fico 2) FECO25 and FAco2 are the carbon dioxide concentrations in inspired, mixed expired, and alveolar gases, respectively. From these three expressions for carbon dioxide production, the following prediction for alveolar carbon dioxide concentration can be obtained:
prediction of Paco2 might be obtained if the relationship between fresh gas flow and ventilation is altered so that their ratio is constant. G.
B. DRUMMOND
Edinburgh REFERENCES
Eger, E. I., and Ethans, C. T. (1969). The effects of inflow, overflow and valve placement on economy of the circle system. Anesthesiology, 30, 93. Scholfield, E. J., and Williams, N. E. (1974). Prediction of arterial carbon dioxide tension using a circle system without carbon dioxide absorption. Br. J. Anaesth., 46, 442. Suwa, K., and Yamamura, H. (1970). The effect of gas inflow on the regulation of CO2 levels with hyperventilation during anesthesia. Anesthesiology, 33, 440. USE OF THE MINIVENT WITH ELECTRIC SUCTION APPARATUS
Sir,—The Minivent, described by Cohen (1966), is a simple miniature ventilator of a group which includes the Autovent and Microvent (Carden, 1969). Driven by gases delivered from an anaesthetic machine or oxygen cylinder, it is dependent on a distended reservoir bag to provide a sufficiently high pressure to operate a magnetic valve (Carden, 1969). We have found the 0.25 h.p. motor of an ordinary suction apparatus, which is readily available, to be a good substitute for gas cylinders in isolated and difficult situations where compressed air or oxygen cylinders may not be available. The air outlet of the 0.25 h.p. motor of the suction apparatus (not the suction inlet) is connected via rubber pressure tubing to a 2-litre reservoir bag which in turn is connected to the Minivent (fig. 1).
FACO2=VCO2(1/VF+1/VA-1/VE)
This expression can be rearranged to give an expression analogous to equation (8) obtained by Suwa and Yamamura (1970) in their study of carbon dioxide elimination by circle systems. If it is assumed that carbon dioxide production is constant, that PACO2 is proportional to Paco2 and also that the ratio V D / V T (and thus the ratio VA/VE) remains constant, then the expression simplifies to: Paco2OCl/VF + K . l / V E If the ratio VE/VF is large and constant this expression can be simplified to give: Paco2OCl/VF In the study quoted, the minute volume was set to a value between two and four times the fresh gas flow. It might be reasonable to assume that when the fresh gas flow was large, the minute volume would be closer to two times the fresh gas flow, and closer to four times the fresh gas flow when the fresh gas flow was small. In this way, the value of the second factor in the expression 1/VF + K . l / V E might vary inversely with the first. Hence, measured Paco2 values would be greater than predicted in the low PaCo2 range, and vice-versa. This supposition agrees with the results obtained in the study. I would suggest, therefore, that a more consistent
'4 hp ordinary electric suction apparatus
FIG. 1. Modified circuit for the Minivent Respirator. The suction apparatus delivers a gas flow of about 8-10 litre /min from the exhaust and this is sufficient to distend the reservoir bag and operate the magnetic valve of the Minivent. Addition of oxygen is possible through an extra inlet to the reservoir bag. If a new reservoir bag is not available, an older one may be used if it is enclosed in a thick plastic bag to prevent over-distension. The suggested use of the suction apparatus is not intended as a substitute for compressed air cylinders, but it may be helpful to colleagues who are working in environments similar to ours. M. T. BHATIA S. M. PATEL G. H. SHAH
Ahmedabad, India REFERENCES
Carden, E. (1969). The Microvent. Anaesthesia, 24, 90. Cohen, A. D. (1966). The Minivent. Anaesthesia, 21, 563.
Downloaded from http://bja.oxfordjournals.org/ at Karolinska Institutet on July 14, 2015
and the volume of the bag remaining to be filled by exhaled gas. For larger fresh gas flows, more fresh gas will be spilt and carbon dioxide elimination will become less efficient. In this way, measured Paco2 values would exceed those predicted when the predicted Paco2 was low, as the results of the investigation demonstrated. However, if the spill valve were resited between the reservoir bag and the expiratory valve, only mixed expired gas would be spilt. The circuit would then resemble that described as circuit C by Eger and Ethans (1968), although, with the reduction in capacity between inflow and spill valve caused by removal of the absorber, fresh gas might be spilt. A further factor is the effect of the patient deadspace in preventing full equilibration of alveolar gas with circuit gas. A second factor leading to the discrepancy between observed and predicted Paco2 is the effect of the patient deadspace in preventing full equilibration of alveolar gas with circuit gas. If it is assumed that the ideal situation obtains, and only mixed expired gas is spilt from the circuit, the carbon dioxide elimination from the circuit will be regulated by fresh gas flow, so that:
741