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THE EFFECT OF REPAIRS ON THE PERFORMANCE OF THE WRIGHT RESPIROMETER
Brit. J. Anaesth. (1970), 42, 1127
THE EFFECT OF REPAIRS ON THE PERFORMANCE OF THE WRIGHT RESPIROMETER BY J. N . LUNN AND E. K. HlIXARD
t t r
r
[ t
The Wright respirometer* is widely used in the practice of clinical anaesthesia and intensive care. The clear advantage which the respirometer has over all other available devices is that of compactness. It is well recognized that this small size results in a high incidence of damage by dropping. In order to keep respirometers available at all times repairs must be prompt. Ideally there should be a stock from which substitute respirometers in use in this 400-bed hospital and our carried out at convenient intervals or even sent elsewhere. We have never had an unlimited supply of respirometers and this paper reports our solution to this problem. REPAIR SERVICE
This anaesthetic department repairs the respirometers in use in this 400-bed hospital and our experience, gained over nearly 10 years, may be of interest to others. The work occupies one technician for between 4 and 8 hours each week. Damaged parts are replaced as necessary and spares cost about £150 annually. Table I shows the incidence of repairs compared with the numbers of respirometers available. TABLE I
Incidence of repairs. Year 1965 1966 1967 1968 1969
Total No. of meters in the hospital 12 16 25 32 38
• British Oxygen Co. Ltd.
No. of referrals 42 59 74 53 79
The reason for referral of the respirometer to the department is always because it has ceased to work at all, but only about one-third of them are known to have been dropped recently. In the majority of cases it is impossible to ascertain the precise cause of failure. A few have ceased to work because of corrosion and these are stripped and cleaned. Table II sets out the site of the mechanical failure and the actual cause of that TABLE II
Mechanical defects found in respirometers. Figures in brackets are for 1969. Location of defect
Diagnosis
Hands
(29)
Lost, bent, fallen off
Zero mechanism
(14)
Wear of cam, snapped screws
Casing
(14)
Bezel bent, glass misplaced, ports deformed
Vane
(15)
Pinion broken, vane blade torn
(7)
Jewel smashed, cogwheel pivot bent or broken
Watch movement
failure. It will be realized that many of these could be directly attributed to damage by dropping. Damage to the vane itself is probably caused by examining fingers. A mercury seal is incorporated between the pneumatic and the mechanical (watch mechJOHN N . LUNN, MJ3.(LOND.), F.F.A.R.C.S.; E. K. H D >
LARD, L-LB.S.T.; Department of Anaesthetics, Welsh National School of Medicine, Royal Infirmary, Cardiff, CF2 1SZ.
Downloaded from http://bja.oxfordjournals.org/ at University of Iowa Libraries/Serials Acquisitions on May 24, 2015
SUMMARY
Respirometers are repaired in this department They do not always retain the makers' specifications but they are clinically useful. A random selection of those in clinical use has shown that, on the average, the respirometers read 2 per cent low at low tidal volumes and 4 per cent high at high tidal volumes. Individual meters differed from the mean by up to ±15 per cent. Damage, caused by dropping, is responsible for the majority of technical failures.
1128
BRITISH JOURNAL OF ANAESTHESIA
LABORATORY ASSESSMENT
It is clear that an accurate assessment of the performance of the respirometers repaired by us would reveal not only the value of this servicing but also the accuracy of the resultant clinical tool. Most anaesthetists working in Cardiff always use a respirometer for monitoring
automatic ventilation but only rarely during spontaneous ventilation. It seemed unlikely that the accuracy claimed under ideal circumstances (Nunn and Ezi-Ashi, 1962) would be achieved by our respirometers in the environment of a routine ward or operating theatre. The following is a report of some tests of accuracy performed in the laboratory on respirometers temporarily taken out of clinical use. ME IHOD
On two different days random samples of ten respirometers were collected into the laboratory. Continuous flow. On the first day two continuous flows of dry oxygen, 10 l./min and 30 l./min, from two 10 inch Rotameters, were passed through each respirometer successively for exactly 1 minute timed by a stopwatch. (The catheter mount contained a baffle.) WRIGHT RESPIROMETER
PNEUMOWCHO GflAPH
FIG. 1 Circuit diagram of apparatus used to test the respirometers.
Intermittent flow. On the second day a Beaver ventilator (see fig. 1) was used to provide an intermittent flow of air. It was connected, with an Ambu inflating valve, to a drygas-meter (Parkinson-Cowan) and thence to each respirometer. A pneumotachograph was included in the circuit just before the respirometer. This demonstrated that no reverse-flow occurred and that the flow of air occupied a little more than one-third of the cycle. The accuracy of the dry-gas-meter was checked with a wet-test meter and all readings corrected appropriately. The ventilator frequency was kept constant at 19/min. Two stroke vol-
Downloaded from http://bja.oxfordjournals.org/ at University of Iowa Libraries/Serials Acquisitions on May 24, 2015
anism) sections of the device. The purpose of this seal is to prevent corrosion of the latter mechanism by moisture in the expired air. However, when the respirometers reach our department all the mercury has escaped from these seals and none has ever been found lurking in any other part of the apparatus. Furthermore, corrosion has never been an important cause of failure. Therefore, because reintroduction of the mercury correctly is time-consuming, we no longer replace it. As a result of this we accept an increased risk of corrosion. This is made greater by our insistence that ventilation measurements should be made during expiration. Nevertheless corrosion only accounts for 7 per cent of the known causes of failure. Nurses are, however, encouraged to remove the respirometer from the ventilator circuit after making a measurement. We have no reason to believe that our experience with these seals is unrepresentative and it seems likely to us that when a respirometer is introduced into clinical service, although initially in pristine condition, it loses its mercury seal very soon and thus becomes similar to the respirometers which we use. Indeed we have recently examined two new respirometers; one in use for only two weeks, another in service but not yet actually used; neither had mercury seals. Following servicing each respirometer is connected to a 10 l./min continuous flow of oxygen to reveal gross inaccuracies. The flow rate at which the vanes start to rotate is also recorded. If the respirometer reads very low, or if the starting flow is greater than 4 l./min, then the mechanism is rechecked until the tests are satisfactory. The maker's specification for starting flow is that this should be 2.0 l./min. We have not found the "walking" test (BOC Handbook) of much value.
EFFECT OF REPAIRS ON PERFORMANCE OF WRIGHT RESPIROMETER
RESULTS
The results are given in table HI. These respirometers under-read at low continuous flow and apparently over-read at high continuous flows, confirming previous work (Nunn and Ezi-Ashi, 1962).
+ 13 per cent for small stroke volumes) of the true ventilation. DISCUSSION
The tests reported above, and those of other workers, have never fully mimicked the clinical situation in which the respirometer is most often used, i.e. during automatic ventilation. Most automatic ventilators, in Great Britain at least, are pressure generators during the expiratory phase. That is, during this phase the lungs are connected directly to the atmosphere. During the test the flow rate of gas should therefore decline exponentially. One new respirometer has been used in a model which is designed to behave in this way. The accuracy of this respirometer was no less, in response to a flow declining exponentially,
TABLE i n
Results. Flow meter (l./min)
Mean rcspirometer reading (l./min)
Mean difference as % of respirometer
SE % of mean
SD % of observations
10 30
8.78 31.6
-13.8 + 5.1
2.73 1.88
8.5 5.9
Stroke volume
Dry-gas meter (L/min)
Mean respirometer reading (L/min)
Mean difference as % of respirometer
SE % of mean
SD % of observations
Small Large
10.25 10.23
10.02 10.65
-2.4
1.99 1.97
6.47 6.19
CONTINUOUS FLOW
INTERMITTENT FLOW
For intermittent flows the small systematic error of this technique of measurement is given by the mean difference. The 95 per cent confidence limits of the mean, expressed as percentage of the mean, are ±4.5 per cent (9 d.f.; *=2.26), for both stroke volumes indicating the degree of uncertainty about the mean difference. The 95 per cent confidence limits of the population are ± 14.6 per cent and ± 14.0 per cent for small and large stroke volumes respectively. This means that only 1 in 20 of all respirometers would give results which differed from the mean in table III by more than ± 15 per cent (i.e. - 11 per cent to +19 per cent for the large stroke volumes and — 17 per cent to
J-3.9
than that when assessed by a continuous or intermittent flow (Ng and Mapleson, personal communication). So despite the apparent inadequacy of continuous, intermittent or sinewave flow the actual performance of the respirometer when used in association with automatic ventilators is unlikely to be very different from that obtained in the response to these test-flow patterns. CONCLUSIONS
Respirometers can be satisfactorily repaired by the technical staff of a clinical department. This practice results in prompt return to use though the work is time-consuming. Some
Downloaded from http://bja.oxfordjournals.org/ at University of Iowa Libraries/Serials Acquisitions on May 24, 2015
umes, approximately 150 ml and 500 ml, were used in order to simulate the effect of different minute-volume ventilations (approximately 3 l./min and 10 L/min). For each flow rate the total gas volume delivered was 10 litres (i.e. one complete revolution of the drygas-meter). Two experiments at each flow rate were made with each respirometer. The arithmetic mean of the two results was used in the statistical calculation.
1129
BRITISH JOURNAL OF ANAESTHESIA
1130 check of accuracy after each repair should be made, and if unsatisfactory results are obtained the respirometer should be re-examined. These arrangements seem to work well in this hospital and respirometers are therefore always available when required. Useful clinical information can be obtained without resort to more expensive apparatus although this would clearly be necessary for more precise measurements.
L'EFFET DE REPARATIONS SUR LA PERFORMANCE DU RESPIROMETRE DE WRIGHT
ACKNOWLEDGEMENTS
DIE WIRKUNG VON REPARATUREN AUF DIE LEISTUNG DES RESPIROMETERS NACH WRIGHT
REFERENCE
Nunn, J. F., and Ezi-Ashi, T. I. (1962). The accuracy of the respirometer and ventigrator. Brit. J. Anaesth., 34, 422.
ZUSAMMENFASSUNG
Reparaturen an Respirometern werden an unserer Abtcilung selbst durchgefuhrt. Nicht immer bleibt dabei die vom Hersteller angegebenc Leistung erhalten, die Gerate sind jedoch weiterhin in der Klinik brauchbar. Eine zufaUige Auswahl von Geraten im klinischen Gebrauch hat gezeigt, dafi die Anzeige im Durchschnitt bei niedrigem Atemvolumen 2 % zu tief und bei grofiem Atemvolumen 4 % zu hoch liegt Einzelne Mefigerate wichen vom Mittelwert bis zu ±15 % ab. Beschadigungen btim Fallenlassen der Gerate sind die haufigste Ursache technischer Defekte.
Downloaded from http://bja.oxfordjournals.org/ at University of Iowa Libraries/Serials Acquisitions on May 24, 2015
It is a pleasure to acknowledge the encouragement and helpful criticism of Professor W. W. Mushin and Dr W. W. Mapleson. Dr B. M. Wright has also kindly commented on the manuscript. We are grateful to Mr R. Saunders, departmental technician, for his careful work and record-keeping without which this article could not have been written.
SOMMAIRE
Les respirometres sont repares dans ce department. Ils ne conservent pas toujours les specifications de performance du fabriquant, mais demeurent cliniquement utilisables. Une selection au hasard des appareils en usage dinique a montre que les respirometres indiquent en moyenne un taux trop bas de 2 pourcent pour les volumes courants peu eleves et un taux trop grand de 4 pourcent pour les volumes courants eleves. IndividueUement les metres different dc la moyenne jusqtfa ±15 pourcent. L'endommagement par chute est responsable de la majorite des defauts techniques.
THE EFFECT OF REPAIRS ON THE PERFORMANCE OF THE WRIGHT RESPIROMETER BY J. N . LUNN AND E. K. HlIXARD
t t r
r
[ t
The Wright respirometer* is widely used in the practice of clinical anaesthesia and intensive care. The clear advantage which the respirometer has over all other available devices is that of compactness. It is well recognized that this small size results in a high incidence of damage by dropping. In order to keep respirometers available at all times repairs must be prompt. Ideally there should be a stock from which substitute respirometers in use in this 400-bed hospital and our carried out at convenient intervals or even sent elsewhere. We have never had an unlimited supply of respirometers and this paper reports our solution to this problem. REPAIR SERVICE
This anaesthetic department repairs the respirometers in use in this 400-bed hospital and our experience, gained over nearly 10 years, may be of interest to others. The work occupies one technician for between 4 and 8 hours each week. Damaged parts are replaced as necessary and spares cost about £150 annually. Table I shows the incidence of repairs compared with the numbers of respirometers available. TABLE I
Incidence of repairs. Year 1965 1966 1967 1968 1969
Total No. of meters in the hospital 12 16 25 32 38
• British Oxygen Co. Ltd.
No. of referrals 42 59 74 53 79
The reason for referral of the respirometer to the department is always because it has ceased to work at all, but only about one-third of them are known to have been dropped recently. In the majority of cases it is impossible to ascertain the precise cause of failure. A few have ceased to work because of corrosion and these are stripped and cleaned. Table II sets out the site of the mechanical failure and the actual cause of that TABLE II
Mechanical defects found in respirometers. Figures in brackets are for 1969. Location of defect
Diagnosis
Hands
(29)
Lost, bent, fallen off
Zero mechanism
(14)
Wear of cam, snapped screws
Casing
(14)
Bezel bent, glass misplaced, ports deformed
Vane
(15)
Pinion broken, vane blade torn
(7)
Jewel smashed, cogwheel pivot bent or broken
Watch movement
failure. It will be realized that many of these could be directly attributed to damage by dropping. Damage to the vane itself is probably caused by examining fingers. A mercury seal is incorporated between the pneumatic and the mechanical (watch mechJOHN N . LUNN, MJ3.(LOND.), F.F.A.R.C.S.; E. K. H D >
LARD, L-LB.S.T.; Department of Anaesthetics, Welsh National School of Medicine, Royal Infirmary, Cardiff, CF2 1SZ.
Downloaded from http://bja.oxfordjournals.org/ at University of Iowa Libraries/Serials Acquisitions on May 24, 2015
SUMMARY
Respirometers are repaired in this department They do not always retain the makers' specifications but they are clinically useful. A random selection of those in clinical use has shown that, on the average, the respirometers read 2 per cent low at low tidal volumes and 4 per cent high at high tidal volumes. Individual meters differed from the mean by up to ±15 per cent. Damage, caused by dropping, is responsible for the majority of technical failures.
1128
BRITISH JOURNAL OF ANAESTHESIA
LABORATORY ASSESSMENT
It is clear that an accurate assessment of the performance of the respirometers repaired by us would reveal not only the value of this servicing but also the accuracy of the resultant clinical tool. Most anaesthetists working in Cardiff always use a respirometer for monitoring
automatic ventilation but only rarely during spontaneous ventilation. It seemed unlikely that the accuracy claimed under ideal circumstances (Nunn and Ezi-Ashi, 1962) would be achieved by our respirometers in the environment of a routine ward or operating theatre. The following is a report of some tests of accuracy performed in the laboratory on respirometers temporarily taken out of clinical use. ME IHOD
On two different days random samples of ten respirometers were collected into the laboratory. Continuous flow. On the first day two continuous flows of dry oxygen, 10 l./min and 30 l./min, from two 10 inch Rotameters, were passed through each respirometer successively for exactly 1 minute timed by a stopwatch. (The catheter mount contained a baffle.) WRIGHT RESPIROMETER
PNEUMOWCHO GflAPH
FIG. 1 Circuit diagram of apparatus used to test the respirometers.
Intermittent flow. On the second day a Beaver ventilator (see fig. 1) was used to provide an intermittent flow of air. It was connected, with an Ambu inflating valve, to a drygas-meter (Parkinson-Cowan) and thence to each respirometer. A pneumotachograph was included in the circuit just before the respirometer. This demonstrated that no reverse-flow occurred and that the flow of air occupied a little more than one-third of the cycle. The accuracy of the dry-gas-meter was checked with a wet-test meter and all readings corrected appropriately. The ventilator frequency was kept constant at 19/min. Two stroke vol-
Downloaded from http://bja.oxfordjournals.org/ at University of Iowa Libraries/Serials Acquisitions on May 24, 2015
anism) sections of the device. The purpose of this seal is to prevent corrosion of the latter mechanism by moisture in the expired air. However, when the respirometers reach our department all the mercury has escaped from these seals and none has ever been found lurking in any other part of the apparatus. Furthermore, corrosion has never been an important cause of failure. Therefore, because reintroduction of the mercury correctly is time-consuming, we no longer replace it. As a result of this we accept an increased risk of corrosion. This is made greater by our insistence that ventilation measurements should be made during expiration. Nevertheless corrosion only accounts for 7 per cent of the known causes of failure. Nurses are, however, encouraged to remove the respirometer from the ventilator circuit after making a measurement. We have no reason to believe that our experience with these seals is unrepresentative and it seems likely to us that when a respirometer is introduced into clinical service, although initially in pristine condition, it loses its mercury seal very soon and thus becomes similar to the respirometers which we use. Indeed we have recently examined two new respirometers; one in use for only two weeks, another in service but not yet actually used; neither had mercury seals. Following servicing each respirometer is connected to a 10 l./min continuous flow of oxygen to reveal gross inaccuracies. The flow rate at which the vanes start to rotate is also recorded. If the respirometer reads very low, or if the starting flow is greater than 4 l./min, then the mechanism is rechecked until the tests are satisfactory. The maker's specification for starting flow is that this should be 2.0 l./min. We have not found the "walking" test (BOC Handbook) of much value.
EFFECT OF REPAIRS ON PERFORMANCE OF WRIGHT RESPIROMETER
RESULTS
The results are given in table HI. These respirometers under-read at low continuous flow and apparently over-read at high continuous flows, confirming previous work (Nunn and Ezi-Ashi, 1962).
+ 13 per cent for small stroke volumes) of the true ventilation. DISCUSSION
The tests reported above, and those of other workers, have never fully mimicked the clinical situation in which the respirometer is most often used, i.e. during automatic ventilation. Most automatic ventilators, in Great Britain at least, are pressure generators during the expiratory phase. That is, during this phase the lungs are connected directly to the atmosphere. During the test the flow rate of gas should therefore decline exponentially. One new respirometer has been used in a model which is designed to behave in this way. The accuracy of this respirometer was no less, in response to a flow declining exponentially,
TABLE i n
Results. Flow meter (l./min)
Mean rcspirometer reading (l./min)
Mean difference as % of respirometer
SE % of mean
SD % of observations
10 30
8.78 31.6
-13.8 + 5.1
2.73 1.88
8.5 5.9
Stroke volume
Dry-gas meter (L/min)
Mean respirometer reading (L/min)
Mean difference as % of respirometer
SE % of mean
SD % of observations
Small Large
10.25 10.23
10.02 10.65
-2.4
1.99 1.97
6.47 6.19
CONTINUOUS FLOW
INTERMITTENT FLOW
For intermittent flows the small systematic error of this technique of measurement is given by the mean difference. The 95 per cent confidence limits of the mean, expressed as percentage of the mean, are ±4.5 per cent (9 d.f.; *=2.26), for both stroke volumes indicating the degree of uncertainty about the mean difference. The 95 per cent confidence limits of the population are ± 14.6 per cent and ± 14.0 per cent for small and large stroke volumes respectively. This means that only 1 in 20 of all respirometers would give results which differed from the mean in table III by more than ± 15 per cent (i.e. - 11 per cent to +19 per cent for the large stroke volumes and — 17 per cent to
J-3.9
than that when assessed by a continuous or intermittent flow (Ng and Mapleson, personal communication). So despite the apparent inadequacy of continuous, intermittent or sinewave flow the actual performance of the respirometer when used in association with automatic ventilators is unlikely to be very different from that obtained in the response to these test-flow patterns. CONCLUSIONS
Respirometers can be satisfactorily repaired by the technical staff of a clinical department. This practice results in prompt return to use though the work is time-consuming. Some
Downloaded from http://bja.oxfordjournals.org/ at University of Iowa Libraries/Serials Acquisitions on May 24, 2015
umes, approximately 150 ml and 500 ml, were used in order to simulate the effect of different minute-volume ventilations (approximately 3 l./min and 10 L/min). For each flow rate the total gas volume delivered was 10 litres (i.e. one complete revolution of the drygas-meter). Two experiments at each flow rate were made with each respirometer. The arithmetic mean of the two results was used in the statistical calculation.
1129
BRITISH JOURNAL OF ANAESTHESIA
1130 check of accuracy after each repair should be made, and if unsatisfactory results are obtained the respirometer should be re-examined. These arrangements seem to work well in this hospital and respirometers are therefore always available when required. Useful clinical information can be obtained without resort to more expensive apparatus although this would clearly be necessary for more precise measurements.
L'EFFET DE REPARATIONS SUR LA PERFORMANCE DU RESPIROMETRE DE WRIGHT
ACKNOWLEDGEMENTS
DIE WIRKUNG VON REPARATUREN AUF DIE LEISTUNG DES RESPIROMETERS NACH WRIGHT
REFERENCE
Nunn, J. F., and Ezi-Ashi, T. I. (1962). The accuracy of the respirometer and ventigrator. Brit. J. Anaesth., 34, 422.
ZUSAMMENFASSUNG
Reparaturen an Respirometern werden an unserer Abtcilung selbst durchgefuhrt. Nicht immer bleibt dabei die vom Hersteller angegebenc Leistung erhalten, die Gerate sind jedoch weiterhin in der Klinik brauchbar. Eine zufaUige Auswahl von Geraten im klinischen Gebrauch hat gezeigt, dafi die Anzeige im Durchschnitt bei niedrigem Atemvolumen 2 % zu tief und bei grofiem Atemvolumen 4 % zu hoch liegt Einzelne Mefigerate wichen vom Mittelwert bis zu ±15 % ab. Beschadigungen btim Fallenlassen der Gerate sind die haufigste Ursache technischer Defekte.
Downloaded from http://bja.oxfordjournals.org/ at University of Iowa Libraries/Serials Acquisitions on May 24, 2015
It is a pleasure to acknowledge the encouragement and helpful criticism of Professor W. W. Mushin and Dr W. W. Mapleson. Dr B. M. Wright has also kindly commented on the manuscript. We are grateful to Mr R. Saunders, departmental technician, for his careful work and record-keeping without which this article could not have been written.
SOMMAIRE
Les respirometres sont repares dans ce department. Ils ne conservent pas toujours les specifications de performance du fabriquant, mais demeurent cliniquement utilisables. Une selection au hasard des appareils en usage dinique a montre que les respirometres indiquent en moyenne un taux trop bas de 2 pourcent pour les volumes courants peu eleves et un taux trop grand de 4 pourcent pour les volumes courants eleves. IndividueUement les metres different dc la moyenne jusqtfa ±15 pourcent. L'endommagement par chute est responsable de la majorite des defauts techniques.