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NEW APPARATUS: THE NICHOLSON VENOUS PRESSURE STAND
SIS
SIMPLIFIED GAS STERILIZATION STERILISATION GAZEUSE SIMPLIFIEE: UNE NOUVELLE SOLUTION A UN PROBLEME ANCIEN
VEREINFACHTE GASSTERILISATION: EINE NEUE ANTWORT AUF EIN ALTES PROBLEM
SOMMAIRE
Die Wirksamkeit der Sterilisation mit Athylenoxydgas ist dokumentarisch gut belegt. Die anhangenden Schwierigkeiten beziiglich der Zeit und der Kosten, die zusarzliche Anwendung von Hitze und/oder Druck erwiesen sich jedoch fur einen universellen Gebrauch der Methode als abschrekkend. Es wird ein vereinfachtes Verfahren, das 84 Prozent Athylenoxyd ohne Erhitzung oder Druck benotigt, aufgezeigt. Geeignete Kontrollen mit Kulturen wurden durchgefuhrt. Seit der Entwicklung der richtigen Technik im November 1966 waren alle routinemafligen wochentlichen Kulturen negativ. Da dieses System keine Warning, Energiequelle oder Installation erforderlich macht, ist es fur fast jede Situation einschliefilich Katastrophenfalle geeignet.
CORRESPONDENCE NEW APPARATUS: THE NICHOLSON VENOUS PRESSURE STAND
Sir,—This apparatus as illustrated is designed for use in conjunction with the various commercially available saline venous pressure manometers to measure the peripheral or central venous pressure. It has two main functions. It acts as a stand to hold the venous manometer vertically and also as a sighting device to enable the scale incorporated in the instrument to be aligned with any reference point of the patient acceptable to the rlinirian concerned. The instrument is designed to permit alignment by eithrr mechanical (Sykes, 1963) or optical methods (Bethune et al., 1966). Mechanically this is accomplished by the use of a pivoted stainless steel beam, which can swing to a right-angle at either side of the vertical manometer stand, and so placed in this extended position as to be in contact with the reference point of the patient. Optical alignment is provided for use when the patient is inaccessible, and is carried out by the use of a simple gunsight-rype device. When not in use a clamp is provided to hold the beam in the vertical position. The apparatus is manufactured in two models—the "Theatre" and the "Standard"—which, though similar in principle, have several important differences. The "Theatre" model, which is the instrument illustrated, has in addition lo the features already described a screw-activated parallel-arm-type movement which permits movement of the apparatus vertically, without displacing the manometer laterally. This mechanism is absent from the "Standard" model which, as it is much lighter, can be moved as a whole by loosening th; retaining screw and sliding the instrument up and down the support. Construction. The construction of both models is mainly of stainless steel. The scale on the venous manometer is marked in black on a white background, and extends from —10 cm to +45 cm. Spring steel clips retain the tubing flat against the scale. The design is aimed at simplicity of operation and the minimum weight consistent with adequate strength.
The "Theatre" model, which offers certain refinements, is intended, because of its weight, mainly for static installation The absence of these refinements from the "Standard" model has produced a much lighter instrument which is readily portable and which can also be manufactured inexpensively.
A.
LEGEND
THE UPRIGHT MANOMETER SCALE
-1
C.
B. IS THE MOVING BEAM
C. IS THE CLIP TO HOLD THE BEAM VERTICALLY WHEN NOT IN USE
D. SHOWS THE POSITION OF THE OPTICAL DEVICE (GUN-SIGHT) FOR ALIGNING THE ZERO POINT WHEN THE PATIENT IS NOT ACCESSIBLE. AS THIS IS CUT INTO T W O SIDES OFA BRACKET IT IS NOT VISIBLE IN THE ILLUSTRATION
FIG. 1 The Nicholson venous pressure stand. In this illustration, the beam is in the measuring position.
Downloaded from http://bja.oxfordjournals.org/ at University of New South Wales on September 6, 2015
L'efficacit6 de l'oxyde d'e'thyline dans la sterilisation gazeuse est bien documentee. Toutefois le probleme du temps d'exposition et du prix de revient, la question de l'addition de chaleur et/ou de pression ont iti des obstacles a l'adoption universelle de cette mdthode. On presente une methode simplified qui utilise de l'oxyde d'e'thylene a 84% sans chaleur ni pression. Des contrfiles de culture approprifc ont et6 poursuivis. Une fois que la technique itait bien au point, tous les contrdles h£bdomadaires de routine ont 6t6 n£gatifs depuis Novembre 1966. Comme ce systeme ne demande pas de travail, ni de source d'toergie ou d'installation particulicre, on peut l'adapter a tous les milieux y compris les conditions catastrophiques.
ZUSAMMENFASSUNG
BRITISH JOURNAL OF ANAESTHESIA
916
BRIAN R. EUSTACE
Shotley Bridge, Durham REFERENCES
Bethune, D. W., Gillett, G. B., Watson, A. C, and Crighton, T. C (1966). A device for simplifying the measurement of central venous pessurc. Lancet, 2, 684. Sykcs, M. K. (1963). Venous pressure as a clinical indication of adequacy of transfusion. Ann. roy. Coll. Surg. Engl., 33, 185. BLOOD LOSS DURING MAJOR VAGINAL SURGERY
Sir,—I have read with interest the article of Dr. D. D. Moir (Brit. J. Anaesth. (1968), 40, 233) and wish to bring to your attention a simple method which I have adopted from the practice of my European-trained gynaecological colleagues. It is the custom in many centres to infiltrate the subcutaneous tissues with saline to facilitate dissection. We now have a series of over 500 cases where the surgeon injects the subcutaneous tissues with a solution of 0.5 per cent lignocaine to which is added 1/250,000 adrenaline, never exceeding 30 ml. Prior to injection, the patient is anaesthetized lightly with thiopentone, nitrous oxide, oxygen sequence. The local infiltration in addition to facilitating surgery, produces analgesia, reduces reflex activity and produces haemostasis. I have not measured the blood loss, but have never found a blood transfusion necessary. Regarding side effects, using a pulse and routine blood pressure measurement, we have not found any side effects. This may be due to the low dosage of low anaesthetic agent and adrenaline used. j BmKHAN Haifa, Israel A SURFACE MARKING FOR CAUDAL BLOCK
Sir,—In the British Journal of Anaesthesia (Vol. 40, No. 8, August 1968, Correspondence, page 633) Michael Kerr, MD., Chelmsford, states in the second paragraph of his letter, "Unfortunately, location of the sacral cornua has invalidated its execution in some 20 per cent of cases, but it is the purpose of this communication to record a surface marking which obviates this disadvantage." This is not a true statement. It is correct that in 20-25 per cent of patients there is some deformity of the sacrum. Such deformities do not always involve the sacral cornua. In spite of such deformities, caudal block can be executed in most patients using the usual
techniques described in textbooks. Our incidence of successful caudal block using such landmarks is in excess of 95 per cent—we do approximately 900 caudals per year.
DANIEL C. MOORE
Seattle SYMPOSIUM ON DENTAL ANAESTHESIA
Sir,—I should like to congratulate the Editors and the many contributors for the Symposium on Dental Anaesthesia appearing in the British Journal of Anaesthesia, 1968, volume 40 (March). This compilation of current concepts and techniques can only prove of great value to all involved in dental anaesthesia and, particularly, to those of us who teach the subject. I should like to draw the attention of your readers, however, to an error which occurs in one of the articles. I refer to the description and accompanying diagram of the circuit of the A.E. Gas/Oxygen machine (fig. 8, p. 170) given by Dr. P. W. Thompson. In the text and diagram he shows the pressure of a nitrous-oxide safety cut-off valve. This valve is not a standard fitting to this machine and is only fitted to the machine at the official request of the purchaser by the manufacturers, Messrs. Cyprane of Keighley, Yorkshire. I suspect that the circuit diagram shown is that of the same firm's standard obstetric gas/oxygen machine which does have this valve fitted on all models. I feel this point is sufficiently important to bring to your attention because a false security created by the belief that such a valve is present on all these modern machines may well lead to the unfortunate assumption that oxygen is always available when this apparatus is functioning apparently normally. This particular hazard of intermittent flow machines would be obviated entirely by the more general use of continuous flow machines in dental work, losing the very dubious advantages of "economy", and ability to alter gas mixtures rapidly, provided by the traditional dental machine. JAMES WILSON Edinburgh Dr. Thompson has sent the following reply: Sir,—The description and diagram referred to are indeed of the A.E. gas-oxygen dental apparatus (fitted with nitrous oxide cut-out valve) and not the obstetric apparatus. The manufacturers kindly supplied circuit diagrams of both models of their A.E. gas-oxygen dental apparatus. I agree with Dr. Wilson's comments about the safety value of a nitrous oxide cut-off and I am so convinced of this that I had no hesitation in selecting for description the model which incorporated it. This decision was reinforced by the fact that the actual apparatus to which I had access was fitted with the nitrous oxide cut-off and air-inlet valves, and I was therefore able to correlate the machine drawing with the apparatus so as to ensure complete accuracy. I think I may claim that I fairly made the point about more general use in the future of continuousflow machines in dental work on page 168 and in figures 6 and 7.
PETER W. THOMPSON
Cardiff Sir,—Since writing my original letter the following appeared in the British Journal of Anaesthesia, 40, p. 696, 1968: "On the Walton 5, the A.E. and some McKesson dental anaesthetic machines the nitrous
Downloaded from http://bja.oxfordjournals.org/ at University of New South Wales on September 6, 2015
Both models are designed to fit on drip stands but there is a very wide range of adjustment in the retaining clamp to allow fitting to the metal rails of many hospital beds when postoperative venous pressure monitoring is desired. Both these instruments have been extensively used by the author and colleagues and their value in clinical practice established. The instruments were originally manufactured as "one off" models for our use, as there was nothing at that time commercially available in the lower price range. As a result of our experience the manufacturing company has now decided to make them available by going into series production. The venous pressure stands described will be shortly available from: Matburn Surgical Equipment Ltd., Fitzherbert Road, Farlington, Portsmouth, Hants.
CORRESPONDENCE
917
oxide is interrupted if the oxygen fails . . .". This appears to reiterate the mistake I have alluded to above. I feel that the recurrence of this mistake makes it more important for us to realize that many of these machines in use today are without this safety device. JAMES WILSON
THE POSITION OF THE WRIGHT ANEMOMETER IN THE CIRCLE ABSORBER SYSTEM
Sir,—The Wright anemometer is often used with the circle absorber system to monitor respiratory volumes and its use has increased since the introduction of the models K and L Boyle apparatus. It is usually placed in accordance with the recommendations of the British Oxygen Company, on the expiratory side of the circle, just proximal to the expiratory valve. While using the anemometer in this way, the recorded minute volumes have seemed unexpectedly high for the corresponding inflation pressures. This may have been due to the position of the anemometer in the circle system and an investigation was carried out to find out if greater accuracy could be obtained by placing the instrument at a different site in the system.
POSfTION "B"
RESERVOIR BAG OR CYCLATOR
Diagram of Boyle Mark III circle absorber system showing positions of anemometers.
Results. During spontaneous breathing, the minute volumes recorded at the expiratory valve (position "A") are markedly greater than those measured near the endotracheal tube or mask (position "B"). The differences are similar but more pronounced during controlled breathing. These greater differences may be due to compression of the gases in the circle during inspiration. The results are summarized in the table. The magnitude of the difference is dependent on the fresh gas inflow, being approximately three-quarters of this volume. TABLE
Results. Minute volume (l./min) measured 'by anemometers placed just proximal to expiratory valve (position A) and attached to catheter mount or mask (position B). Averages of all results: differences in minute volumes (l./min) (1) Spontaneous breathing: Fresh gas flow (l./min) Difference
0.50 0.11
(2) Controlled ventilation: Fresh gas flow (l./min) Difference
0.50 2.00 4.00 0.60 1.77 3.55
2.00 4.00 8.00 1.24 2.65 5.95 8.00 6.14
Discussion. These differences in recorded volumes can be explained by considering in detail the movement of gases in each phase of the respiratory cycle. During inspiration the patient inhales partly fresh and partly rebreathed gas. During expiration the patient breathes out through the anemometers recording the volume. Also during expiration, fresh gas is coming into the circuit and must join the exhaled gas at the T-junction and be measured by the anemometer next to the expiratory valve. It will not, however, be measured by the second anemometer as this is proximal to the T-junction and this gas does not pass through it. As inspiration normally occupies one-quarter and expiration three-quarters of the respiratory cycle, threequarters of the fresh gas flow must pass round the circuit without being inspired by the patient, which would account for the difference found. It would appear that an anemometer placed in the recommended position next to the expiratory valve would not record
Downloaded from http://bja.oxfordjournals.org/ at University of New South Wales on September 6, 2015
Edinburgh Editorial comment.—Dr. Ward informs us that in pursuance of the recent British Standard 4272, Part I, which deals with this subject, Cyprane Ltd. now fit a nitrous oxide cut-off, which operates if the oxygen fails, to all their dintal machines. In the past this has been an optional extra. It is, however, a relatively simple matter to fit this safety device to any older machines which lack it.—ED.
Method. The subjects were unselected patients between the ages of 14 and 69, 8 male and 4 female, undergoing routine surgery. Induction with thiopentone 200350 mg and suxamethonium 40-60 mg (if intubated) was followed by nitrous oxide and oxygen with halothane 0.5^1 per cent if breathing spontaneously, or tubocurarine for controlled ventilation. A type L Boyle machine was used, with a Mark III circle absorber and two Wright anemometers, one in the usual position just proximal to the expiratory valve and the other attached to the catheter mount or mask (see diagram). Fresh gas flows of 0.5, 2, 4, and 8 l./min were used and the volume of gases passing through each anemometer in the same minute was recorded. Three readings were taken for each fresh gas volume, the meters interchanged and the readings repeated.
BRITISH JOURNAL OF ANAESTHESIA
918 true respiratory volumes except in a fully closed circuit with low fresh gas flow. Recorded ventilation volumes are higher than true volumes and could lead clinically to underventilation. It would seem preferable to place the anemometer on the patient's side of the T-junction, unless there is a low input of fresh gas. R j pu^^x Norwich AN EXPIRATORY RESISTANCE VALVE
CONTROL KNOB CLOCKWISE ROTATION DECREASES FLOW
BODY
•SPRING TO STIFFEN CONTROL THUS PREVENTING ACCIDENTAL MOVEMENT
VALVE DISC
FIG. 1 The expiratory resistance valve.
FROM PATIENT
FIG. 2 (right) Diagram of valve showing principal parts. exposed lung but also in the lower lung even though the lower pleura is intact. The result is a gradual falling away of the mediastinum accompanied by acid comments from the surgeon! By inserting a variable resistance to flow in the expiratory path, the airway pressure during expiration can be adjusted in such a way as to maintain the lungs in any desired state of expansion. In practice, this entails the use of a variable orifice in the expiratory limb which can be varied according to the airway pressure or by direct observation of the lungs.
•KING TO PREVENT INCORRECT FITTING
Downloaded from http://bja.oxfordjournals.org/ at University of New South Wales on September 6, 2015
Sir,—When surgery involves opening one or both pleural cavities, the anaesthetist is frequently faced with the problem of gradual lung collapse. This is particularly so when a mechanical ventilator is being used. The ventilator provides the patient with an inspired tidal volume of, say, 500 ml. The expiratory valve then opens and passive expiration to atmospheric pressure occurs. In many instances, where one or both pleurae are open, the expired tidal volume will slightly exceed the inspired tidal volume, say 510 ml, with the result that gradual lung collapse will occur until the inspired and expired tidal volumes are equal. With the patient in the lateral position, during thoracotomy, this effect may be seen not only in the upper
In some ventilators, such an adjustment is already available. In others, a simple modification of the circuit may be required. Figure 1 shows an attachment designed for use with the Cape ventilator or CapcWaine anaesthetic ventilator. It is also suitable for use on other ventilators. The attachment is fitted to the port on the ventilator normally receiving the expiratory limb from the patient, this limb being plugged into the attachment. Expired gases from the patient are thus led through the attachment into the ventilator. The control knob on the top allows variation in the size of the orifice within the attachment and so allows variation in a.irway pressure during expiration (fig. 2). Three safety factors have been incorporated in the design. (a) Total occlusion of the expiratory line is impossible. (b) Accidental movements of the control knob are prevented by the spring-stiffened action. (c) Connection of the attachment to the inspiratory port of the ventilator is prevented by a ring on the female port of the attachment. This attachment has been designed primarily for use during thoracotomy and no disturbances of cardiovascular dynamics have been observed during its use.
It has also been of value in the ventilation of patients with a flail chest and in the postoperative ventilation of patients following pectoplasty. The attachment is available from the Cape Engineering Company, Warwick, and is provided with the new standard tapers. JOHN S. M. ZORAB
Frenchay Hospital, Bristol D. E. R. Fox Cape Engineering Company Ltd., Warwick
Printed la Great Britain br John Sherratt * Son Ltd.. Park Road. Altrincluun
SIMPLIFIED GAS STERILIZATION STERILISATION GAZEUSE SIMPLIFIEE: UNE NOUVELLE SOLUTION A UN PROBLEME ANCIEN
VEREINFACHTE GASSTERILISATION: EINE NEUE ANTWORT AUF EIN ALTES PROBLEM
SOMMAIRE
Die Wirksamkeit der Sterilisation mit Athylenoxydgas ist dokumentarisch gut belegt. Die anhangenden Schwierigkeiten beziiglich der Zeit und der Kosten, die zusarzliche Anwendung von Hitze und/oder Druck erwiesen sich jedoch fur einen universellen Gebrauch der Methode als abschrekkend. Es wird ein vereinfachtes Verfahren, das 84 Prozent Athylenoxyd ohne Erhitzung oder Druck benotigt, aufgezeigt. Geeignete Kontrollen mit Kulturen wurden durchgefuhrt. Seit der Entwicklung der richtigen Technik im November 1966 waren alle routinemafligen wochentlichen Kulturen negativ. Da dieses System keine Warning, Energiequelle oder Installation erforderlich macht, ist es fur fast jede Situation einschliefilich Katastrophenfalle geeignet.
CORRESPONDENCE NEW APPARATUS: THE NICHOLSON VENOUS PRESSURE STAND
Sir,—This apparatus as illustrated is designed for use in conjunction with the various commercially available saline venous pressure manometers to measure the peripheral or central venous pressure. It has two main functions. It acts as a stand to hold the venous manometer vertically and also as a sighting device to enable the scale incorporated in the instrument to be aligned with any reference point of the patient acceptable to the rlinirian concerned. The instrument is designed to permit alignment by eithrr mechanical (Sykes, 1963) or optical methods (Bethune et al., 1966). Mechanically this is accomplished by the use of a pivoted stainless steel beam, which can swing to a right-angle at either side of the vertical manometer stand, and so placed in this extended position as to be in contact with the reference point of the patient. Optical alignment is provided for use when the patient is inaccessible, and is carried out by the use of a simple gunsight-rype device. When not in use a clamp is provided to hold the beam in the vertical position. The apparatus is manufactured in two models—the "Theatre" and the "Standard"—which, though similar in principle, have several important differences. The "Theatre" model, which is the instrument illustrated, has in addition lo the features already described a screw-activated parallel-arm-type movement which permits movement of the apparatus vertically, without displacing the manometer laterally. This mechanism is absent from the "Standard" model which, as it is much lighter, can be moved as a whole by loosening th; retaining screw and sliding the instrument up and down the support. Construction. The construction of both models is mainly of stainless steel. The scale on the venous manometer is marked in black on a white background, and extends from —10 cm to +45 cm. Spring steel clips retain the tubing flat against the scale. The design is aimed at simplicity of operation and the minimum weight consistent with adequate strength.
The "Theatre" model, which offers certain refinements, is intended, because of its weight, mainly for static installation The absence of these refinements from the "Standard" model has produced a much lighter instrument which is readily portable and which can also be manufactured inexpensively.
A.
LEGEND
THE UPRIGHT MANOMETER SCALE
-1
C.
B. IS THE MOVING BEAM
C. IS THE CLIP TO HOLD THE BEAM VERTICALLY WHEN NOT IN USE
D. SHOWS THE POSITION OF THE OPTICAL DEVICE (GUN-SIGHT) FOR ALIGNING THE ZERO POINT WHEN THE PATIENT IS NOT ACCESSIBLE. AS THIS IS CUT INTO T W O SIDES OFA BRACKET IT IS NOT VISIBLE IN THE ILLUSTRATION
FIG. 1 The Nicholson venous pressure stand. In this illustration, the beam is in the measuring position.
Downloaded from http://bja.oxfordjournals.org/ at University of New South Wales on September 6, 2015
L'efficacit6 de l'oxyde d'e'thyline dans la sterilisation gazeuse est bien documentee. Toutefois le probleme du temps d'exposition et du prix de revient, la question de l'addition de chaleur et/ou de pression ont iti des obstacles a l'adoption universelle de cette mdthode. On presente une methode simplified qui utilise de l'oxyde d'e'thylene a 84% sans chaleur ni pression. Des contrfiles de culture approprifc ont et6 poursuivis. Une fois que la technique itait bien au point, tous les contrdles h£bdomadaires de routine ont 6t6 n£gatifs depuis Novembre 1966. Comme ce systeme ne demande pas de travail, ni de source d'toergie ou d'installation particulicre, on peut l'adapter a tous les milieux y compris les conditions catastrophiques.
ZUSAMMENFASSUNG
BRITISH JOURNAL OF ANAESTHESIA
916
BRIAN R. EUSTACE
Shotley Bridge, Durham REFERENCES
Bethune, D. W., Gillett, G. B., Watson, A. C, and Crighton, T. C (1966). A device for simplifying the measurement of central venous pessurc. Lancet, 2, 684. Sykcs, M. K. (1963). Venous pressure as a clinical indication of adequacy of transfusion. Ann. roy. Coll. Surg. Engl., 33, 185. BLOOD LOSS DURING MAJOR VAGINAL SURGERY
Sir,—I have read with interest the article of Dr. D. D. Moir (Brit. J. Anaesth. (1968), 40, 233) and wish to bring to your attention a simple method which I have adopted from the practice of my European-trained gynaecological colleagues. It is the custom in many centres to infiltrate the subcutaneous tissues with saline to facilitate dissection. We now have a series of over 500 cases where the surgeon injects the subcutaneous tissues with a solution of 0.5 per cent lignocaine to which is added 1/250,000 adrenaline, never exceeding 30 ml. Prior to injection, the patient is anaesthetized lightly with thiopentone, nitrous oxide, oxygen sequence. The local infiltration in addition to facilitating surgery, produces analgesia, reduces reflex activity and produces haemostasis. I have not measured the blood loss, but have never found a blood transfusion necessary. Regarding side effects, using a pulse and routine blood pressure measurement, we have not found any side effects. This may be due to the low dosage of low anaesthetic agent and adrenaline used. j BmKHAN Haifa, Israel A SURFACE MARKING FOR CAUDAL BLOCK
Sir,—In the British Journal of Anaesthesia (Vol. 40, No. 8, August 1968, Correspondence, page 633) Michael Kerr, MD., Chelmsford, states in the second paragraph of his letter, "Unfortunately, location of the sacral cornua has invalidated its execution in some 20 per cent of cases, but it is the purpose of this communication to record a surface marking which obviates this disadvantage." This is not a true statement. It is correct that in 20-25 per cent of patients there is some deformity of the sacrum. Such deformities do not always involve the sacral cornua. In spite of such deformities, caudal block can be executed in most patients using the usual
techniques described in textbooks. Our incidence of successful caudal block using such landmarks is in excess of 95 per cent—we do approximately 900 caudals per year.
DANIEL C. MOORE
Seattle SYMPOSIUM ON DENTAL ANAESTHESIA
Sir,—I should like to congratulate the Editors and the many contributors for the Symposium on Dental Anaesthesia appearing in the British Journal of Anaesthesia, 1968, volume 40 (March). This compilation of current concepts and techniques can only prove of great value to all involved in dental anaesthesia and, particularly, to those of us who teach the subject. I should like to draw the attention of your readers, however, to an error which occurs in one of the articles. I refer to the description and accompanying diagram of the circuit of the A.E. Gas/Oxygen machine (fig. 8, p. 170) given by Dr. P. W. Thompson. In the text and diagram he shows the pressure of a nitrous-oxide safety cut-off valve. This valve is not a standard fitting to this machine and is only fitted to the machine at the official request of the purchaser by the manufacturers, Messrs. Cyprane of Keighley, Yorkshire. I suspect that the circuit diagram shown is that of the same firm's standard obstetric gas/oxygen machine which does have this valve fitted on all models. I feel this point is sufficiently important to bring to your attention because a false security created by the belief that such a valve is present on all these modern machines may well lead to the unfortunate assumption that oxygen is always available when this apparatus is functioning apparently normally. This particular hazard of intermittent flow machines would be obviated entirely by the more general use of continuous flow machines in dental work, losing the very dubious advantages of "economy", and ability to alter gas mixtures rapidly, provided by the traditional dental machine. JAMES WILSON Edinburgh Dr. Thompson has sent the following reply: Sir,—The description and diagram referred to are indeed of the A.E. gas-oxygen dental apparatus (fitted with nitrous oxide cut-out valve) and not the obstetric apparatus. The manufacturers kindly supplied circuit diagrams of both models of their A.E. gas-oxygen dental apparatus. I agree with Dr. Wilson's comments about the safety value of a nitrous oxide cut-off and I am so convinced of this that I had no hesitation in selecting for description the model which incorporated it. This decision was reinforced by the fact that the actual apparatus to which I had access was fitted with the nitrous oxide cut-off and air-inlet valves, and I was therefore able to correlate the machine drawing with the apparatus so as to ensure complete accuracy. I think I may claim that I fairly made the point about more general use in the future of continuousflow machines in dental work on page 168 and in figures 6 and 7.
PETER W. THOMPSON
Cardiff Sir,—Since writing my original letter the following appeared in the British Journal of Anaesthesia, 40, p. 696, 1968: "On the Walton 5, the A.E. and some McKesson dental anaesthetic machines the nitrous
Downloaded from http://bja.oxfordjournals.org/ at University of New South Wales on September 6, 2015
Both models are designed to fit on drip stands but there is a very wide range of adjustment in the retaining clamp to allow fitting to the metal rails of many hospital beds when postoperative venous pressure monitoring is desired. Both these instruments have been extensively used by the author and colleagues and their value in clinical practice established. The instruments were originally manufactured as "one off" models for our use, as there was nothing at that time commercially available in the lower price range. As a result of our experience the manufacturing company has now decided to make them available by going into series production. The venous pressure stands described will be shortly available from: Matburn Surgical Equipment Ltd., Fitzherbert Road, Farlington, Portsmouth, Hants.
CORRESPONDENCE
917
oxide is interrupted if the oxygen fails . . .". This appears to reiterate the mistake I have alluded to above. I feel that the recurrence of this mistake makes it more important for us to realize that many of these machines in use today are without this safety device. JAMES WILSON
THE POSITION OF THE WRIGHT ANEMOMETER IN THE CIRCLE ABSORBER SYSTEM
Sir,—The Wright anemometer is often used with the circle absorber system to monitor respiratory volumes and its use has increased since the introduction of the models K and L Boyle apparatus. It is usually placed in accordance with the recommendations of the British Oxygen Company, on the expiratory side of the circle, just proximal to the expiratory valve. While using the anemometer in this way, the recorded minute volumes have seemed unexpectedly high for the corresponding inflation pressures. This may have been due to the position of the anemometer in the circle system and an investigation was carried out to find out if greater accuracy could be obtained by placing the instrument at a different site in the system.
POSfTION "B"
RESERVOIR BAG OR CYCLATOR
Diagram of Boyle Mark III circle absorber system showing positions of anemometers.
Results. During spontaneous breathing, the minute volumes recorded at the expiratory valve (position "A") are markedly greater than those measured near the endotracheal tube or mask (position "B"). The differences are similar but more pronounced during controlled breathing. These greater differences may be due to compression of the gases in the circle during inspiration. The results are summarized in the table. The magnitude of the difference is dependent on the fresh gas inflow, being approximately three-quarters of this volume. TABLE
Results. Minute volume (l./min) measured 'by anemometers placed just proximal to expiratory valve (position A) and attached to catheter mount or mask (position B). Averages of all results: differences in minute volumes (l./min) (1) Spontaneous breathing: Fresh gas flow (l./min) Difference
0.50 0.11
(2) Controlled ventilation: Fresh gas flow (l./min) Difference
0.50 2.00 4.00 0.60 1.77 3.55
2.00 4.00 8.00 1.24 2.65 5.95 8.00 6.14
Discussion. These differences in recorded volumes can be explained by considering in detail the movement of gases in each phase of the respiratory cycle. During inspiration the patient inhales partly fresh and partly rebreathed gas. During expiration the patient breathes out through the anemometers recording the volume. Also during expiration, fresh gas is coming into the circuit and must join the exhaled gas at the T-junction and be measured by the anemometer next to the expiratory valve. It will not, however, be measured by the second anemometer as this is proximal to the T-junction and this gas does not pass through it. As inspiration normally occupies one-quarter and expiration three-quarters of the respiratory cycle, threequarters of the fresh gas flow must pass round the circuit without being inspired by the patient, which would account for the difference found. It would appear that an anemometer placed in the recommended position next to the expiratory valve would not record
Downloaded from http://bja.oxfordjournals.org/ at University of New South Wales on September 6, 2015
Edinburgh Editorial comment.—Dr. Ward informs us that in pursuance of the recent British Standard 4272, Part I, which deals with this subject, Cyprane Ltd. now fit a nitrous oxide cut-off, which operates if the oxygen fails, to all their dintal machines. In the past this has been an optional extra. It is, however, a relatively simple matter to fit this safety device to any older machines which lack it.—ED.
Method. The subjects were unselected patients between the ages of 14 and 69, 8 male and 4 female, undergoing routine surgery. Induction with thiopentone 200350 mg and suxamethonium 40-60 mg (if intubated) was followed by nitrous oxide and oxygen with halothane 0.5^1 per cent if breathing spontaneously, or tubocurarine for controlled ventilation. A type L Boyle machine was used, with a Mark III circle absorber and two Wright anemometers, one in the usual position just proximal to the expiratory valve and the other attached to the catheter mount or mask (see diagram). Fresh gas flows of 0.5, 2, 4, and 8 l./min were used and the volume of gases passing through each anemometer in the same minute was recorded. Three readings were taken for each fresh gas volume, the meters interchanged and the readings repeated.
BRITISH JOURNAL OF ANAESTHESIA
918 true respiratory volumes except in a fully closed circuit with low fresh gas flow. Recorded ventilation volumes are higher than true volumes and could lead clinically to underventilation. It would seem preferable to place the anemometer on the patient's side of the T-junction, unless there is a low input of fresh gas. R j pu^^x Norwich AN EXPIRATORY RESISTANCE VALVE
CONTROL KNOB CLOCKWISE ROTATION DECREASES FLOW
BODY
•SPRING TO STIFFEN CONTROL THUS PREVENTING ACCIDENTAL MOVEMENT
VALVE DISC
FIG. 1 The expiratory resistance valve.
FROM PATIENT
FIG. 2 (right) Diagram of valve showing principal parts. exposed lung but also in the lower lung even though the lower pleura is intact. The result is a gradual falling away of the mediastinum accompanied by acid comments from the surgeon! By inserting a variable resistance to flow in the expiratory path, the airway pressure during expiration can be adjusted in such a way as to maintain the lungs in any desired state of expansion. In practice, this entails the use of a variable orifice in the expiratory limb which can be varied according to the airway pressure or by direct observation of the lungs.
•KING TO PREVENT INCORRECT FITTING
Downloaded from http://bja.oxfordjournals.org/ at University of New South Wales on September 6, 2015
Sir,—When surgery involves opening one or both pleural cavities, the anaesthetist is frequently faced with the problem of gradual lung collapse. This is particularly so when a mechanical ventilator is being used. The ventilator provides the patient with an inspired tidal volume of, say, 500 ml. The expiratory valve then opens and passive expiration to atmospheric pressure occurs. In many instances, where one or both pleurae are open, the expired tidal volume will slightly exceed the inspired tidal volume, say 510 ml, with the result that gradual lung collapse will occur until the inspired and expired tidal volumes are equal. With the patient in the lateral position, during thoracotomy, this effect may be seen not only in the upper
In some ventilators, such an adjustment is already available. In others, a simple modification of the circuit may be required. Figure 1 shows an attachment designed for use with the Cape ventilator or CapcWaine anaesthetic ventilator. It is also suitable for use on other ventilators. The attachment is fitted to the port on the ventilator normally receiving the expiratory limb from the patient, this limb being plugged into the attachment. Expired gases from the patient are thus led through the attachment into the ventilator. The control knob on the top allows variation in the size of the orifice within the attachment and so allows variation in a.irway pressure during expiration (fig. 2). Three safety factors have been incorporated in the design. (a) Total occlusion of the expiratory line is impossible. (b) Accidental movements of the control knob are prevented by the spring-stiffened action. (c) Connection of the attachment to the inspiratory port of the ventilator is prevented by a ring on the female port of the attachment. This attachment has been designed primarily for use during thoracotomy and no disturbances of cardiovascular dynamics have been observed during its use.
It has also been of value in the ventilation of patients with a flail chest and in the postoperative ventilation of patients following pectoplasty. The attachment is available from the Cape Engineering Company, Warwick, and is provided with the new standard tapers. JOHN S. M. ZORAB
Frenchay Hospital, Bristol D. E. R. Fox Cape Engineering Company Ltd., Warwick
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