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EFFECT OF DIFFERENT FLOW PATTERNS ON THE WRIGHT RESPIROMETER
Br.J. Anaesth. (1979), 51, 895
EFFECT OF DIFFERENT FLOW PATTERNS ON THE WRIGHT RESPIROMETER J. A. BUSHMAN SUMMARY
The problems of using the Wright respirometer for measurements at small rates of flow associated with small volumes are discussed. Accuracy is very dependent on the wave form of the flow passing through it.
to the flowmeter outlet. Some of the respirometers were affected by the angle of their main axis to the perpendicular. All instruments A to E were tested, although E was a prototype. For the first reading each instrument was attached to the rotameter with its axis vertical and the dial horizontal face upwards. The flow rate was then increased smoothly and slowly until the vane could be seen rotating and then decreased until the vane just remained rotating without stalling for 1 min. The test was repeated with the instrument axis horizontal (the face in a vertical plane).
Test 2 Instruments A to D were tested on a rig consisting Five different types of Wright respirometer (A to E) of a Starling pump ventilating a 5-gallon drum. The were examined, all, apart from one, available on the respirometers were placed in the circuit, two at a market. The manufacturers are not named as the time, in series, at the junction of the Y-piece from history of each instrument before test varied and no the pump with the drum. In each set of measureconclusions can be made about the relative merits of ments the dial on one instrument was vertical and the the instruments from these results. other horizontal. One instrument was positioned to measure "inspiration", the other expiration. For each procedure the two instruments were changed Test I The flow rate required just to maintain rotation of round so that readings were obtained for each of two the vane was measured. To take account of the weight positions (vertical and horizontal) and for each of the hands, flow rates were recorded only after the direction of flow (inspiration and expiration). most directly geared hand had rotated by 360°. This The Starling pump and drum were chosen as a test "starting flow" was measured by passing an air flow rig as the combination produces an approximately generated by a variable speed fan through a flowmeter sinusoidal input from the pump and a high initial (calibrated for air at 20 °C between 0.6 and 5 litre flow rate on expiration from the drum because of its min""1 with an accuracy of 1% of full range = compliance and the valve arrangement on the Starling 0.05 litre min" 1 ), the respirometer being connected pump. Figure 1, which shows the difference in flow JOHN A. BUSHMAN, L.M.S.S.A., F.F.A.R.C.S., F.INST.M.C, patterns, was produced using a Fleisch flow transCOMPANION I.E.E., Research Department of Anaesthetics, ducer and a Greer defocusing manometer. A potentioRoyal College of Surgeons, Lincoln's Inn Fields, London metric-type recorder with a comparatively low slew WC2A 3PN. MATERIALS AND METHODS
0007-0912/79/090895-04 $01.00
© Macmillan Journals Ltd 1979
Downloaded from http://bja.oxfordjournals.org/ at Emory University on August 24, 2015
The fact that there are problems in measuring low flows with the Wright respirometer has been known for some time and this characteristic of the instrument was clearly defined by Nunn and Ezi-Ashi (1962). They studied continuous flow and showed that with less than 16 litre min" 1 the registered volume underestimated the absolute volume by an amount which increased very rapidly as flow was reduced. This report shows that the flow pattern of a given volume passing through the instrument in unit time has a large effect on the volume registered. The flow patterns used, similar to those found in clinical practice, were demonstrated artificially in a way which ensured accurately repeatable results.
896
BRITISH JOURNAL OF ANAESTHESIA TABLE I I . Minimwnflow requiredjust to keep the vane of each of the five respirometers rotating. The two readings given with the axis horizontal demonstrate the different flows required to move the more or most sensitive hand against or with the aid of gravity
Minimal running flows (litre min"1) Respirometer
Axis vertical
Axis horizontal
A B C D E
2.1 2.25 1.9 2.0 1.76
2.0/2.2 1.4/2.8 1.1/2.2 2.0/2.0 1.6/1.7
rotating when the more or most sensitive hand of the instrument was being rotated upwards compared with when it was running downwards assisted by its own weight.
rate was used and this underestimates the fast rising component of the expiratory flow. The actualflows(table I) were measured by displayRESULTS AND DISCUSSION ing the output from the Greer manometer on an oscilloscope screen. This shows a three- to four-fold Table II shows the minimum flows for the five difference between the inspiratory and expiratory flow respirometers tested. These are similar when the rates, a condition sometimes found in clinical practice. axis of the vane is vertical and the face of the dial is horizontal and uppermost. With the dial face vertical TABLE I. Peak inspiratory and expiratory flow rates the minimum flow to rotate the vanes changes accordat different volume settings of the Starling pump ing to the position of the hands on the dial (two Peak inspiratory Peak expiratory figures under the second heading). Volume set flow rate flow rate Instrument D had the most highly geared hand (ml) (litre min"1) (litre min"1) (x 10) while the most sensitive hand in it was small. As might be expected, this instrument was not 300 25 18 sensitive to its position, although it was difficult to 200 11 100 read from a distance with accuracy. Instrument B was easy to read several feet away, The stroke volume of the Starling pump was set in but showed the greatest sensitivity to dial face turn at one of three volumes—300 ml, 200 ml or orientation. 100 ml. The drive belt was adjusted to give 12.5 Instrument E is a prototype which appears to be strokes per min and each test was run for 2 min, the independent of orientation of the dial face. crank of the pump being started and stopped from These results accord with those of Nunn and Eziexactly the same position after 25 complete revolutions. Ashi (1962). The measurements recorded from the respirometers The respirometers were always started from zero. Each test was performed twice. There was no particu- on the Starling pump-drum rig are shown in table III. lar reason for choosing the combination of the The registered expired volume was always larger than respirometers (A with C and B with D) other than the inspired volume and the former was always each of the pair in any given test was of different closer to the calculated volume displaced. The manufacture and had a different arrangement of hands accuracy of the inspired volume measurement became progressively worse as the inspired volume decreased. on the dial. The two readings given in table II demonstrate the The accuracy of the expired volume remained high difference in the flow required to keep the vane even at tidal volumes as small as 100 ml.
Downloaded from http://bja.oxfordjournals.org/ at Emory University on August 24, 2015
FIG. 1. The "inspiratory" and "expiratory" flow patterns of the Starling pump and drum system. Note the slow sinusoidal shape of the inspiratory flow (upwards) and the fast initial component of the expiratory flow (downwards). The movement of the paper under the pen was from left to right. The magnitude of the fast rising component is an underestimate of the real flow because of the comparatively slow response of the potentiometric recorder used. The pump was set at 200 ml at 12.5 rev min" 1 for this recording, but the wave form was exactly the same at the high and low settings of the pump, 300 and 100 ml respectively.
STUDIES OF THE WRIGHT RESPIROMETER
897
TABLE I I I . The result of connecting pairs of the four respirometers in series, one measuring inspiration and one measuring expiration, in a system consisting of a Starling pump ventilating a 5-gallon drum. The instruments were arranged so that one of them had its dial vertical and the other horizontal. Two readings were taken for each position. The tests were run for exactly 25 revolutions of the crank of the pump at a speed of 12.5 revmin'1. The calculated total volume for this 2-min period is shown at the head of each set of results. The accuracy of the average of each pair of readings is expressed as a percentage of this calculated total volume
Accuracy
Accuracy
Inspiration
Expiration
= 7.5 litre) 5.53 5.84 5.5 6.6
73.7 77.8 73.3 88.0
7.25 8.82 7.55 7.46
7.15 8.75
200 ml at 12.5 rev min" 1 for 2 min (total B Face vertical 3.08 D Face horizontal 3.36 B Face horizontal 3.0 D Face vertical 3.25
= 5 litre) 3.08 3.36 3.03 3.25
61.6 67.2 60.3 65.0
4.83 5.26 5.05 5.08
4.85
100 ml at 12.5 rev min" 1 for 2 min (total B Face vertical 0.55* D Face horizontal 0.6 B Face horizontal 0.4 D Face vertical 0.3
= 2.5 litre) 0.59 0.56 0.4 0.3
22.8 23.2 16.0 12.0
300 ml at 12.5 rev min" for 2 min (total = 7.5 litre) A Face vertical 5.88 5.84 C Face horizontal 5.8 5.83 A Face horizontal 5.56 5.58 C Face vertical 5.79 5.83
78.1 77.5 74.3 77.5
7.2
7.28
7.75 7.52 7.42
7.8
65.1 63.2 61.3 64.5
4.84 5.23 5.06 5.01
29.8 21.0 29.0 22.4
2.5
7.5
7.45
5.3
4.92 4.92
2.35
2.35
2.5
2.5
2.45
2.35
2.6
2.5
96.0 117.1 100.3 99.4 96.8 105.6 99.7 100.0 94.0 100.0 96.0 102.0
1
1
200 ml at 12.5 rev min" for 2 min (total A Face vertical 3.26 C Face horizontal 3.16 A Face horizontal 3.08 C Face vertical 3.23
= 5 litre) 3.25 3.16 3.05 3.22
7.72 7.6
96.5 103.7 101.6 100.1
5.1
96.4 104.1 102.1 101.1
2.52 2.67 2.58 2.56
100.4 106.4 103.2 102.2
4.8
. 5.18 5.15
1
100 ml at 12.5 rev min- for 2 min (total = 2.5 litre) A Face vertical 0.77 0.72 C Face horizontal 0.55 0.5 A Face horizontal 0.74 0.71 C Face vertical 0.6 0.52 :
Stalled.
Although one instrument (B) stalled during one of the smallest volume tests, the measurements from all the instruments at the smallest inspired volumes were more than four times less than they should have been, while the measured expired volumes remained close to the calculated swept volume. Although the first test showed that the minimum flow rates from each instrument were different (table II), this difference between the instruments is not evident in table III and neither was the effect of the orientation of the instrument, except in the case of instrument B, which stalled once when the more 73
2.65 2.58 2.55
sensitive hand had to be rotated against the effect of gravity. These results suggest that it is not possible to rely directly on the reading from these instruments with spontaneously breathing patients with low tidal volumes, such as may occur in patients breathing high concentrations of halothane spontaneously. The effect is less likely to occur if the lungs are ventilated artificially, particularly with the respirometer in the expiratory line as the initial flow rate from the patient will be high at the moment of opening of the expiratory valve. •
Downloaded from http://bja.oxfordjournals.org/ at Emory University on August 24, 2015
300 ml at 12.5 rev min" 1 for 2 min (total B Face vertical 5.53 D Face horizontal 5.84 B Face horizontal 5.5 D Face vertical 6.6
BRITISH JOURNAL OF ANAESTHESIA
898 CONCLUSIONS
The Wright respirometer is generally accurate when presented with flow patterns with a high initial flow rate. The accuracy deteriorates if the initial flow rate is low and accelerates only slowly. The instrument is inherently safe, however, as it will underestimate tidal volume. REFERENCES
Nunn, J. F., and Ezi-Ashi, T. I. (1962). The accuracy of the respirometer and ventigrator. Br. J. Anaesth., 343 422.
DIE WIRKUNG VERSCHIEDENER STROMUNGSSTRUKTUREN AUF DAS WRIGHTATEMMESSGERAT ZUSAMMENFASSUNG
Probleme beim Einsatz des Wright-Atemmessgerates fur Messungen bei den mit kleinen Volumen verbundenen geringen Durchflussmengen. Genauigkeit hangt sehr von der Wellenform des durchfliessenden Stromes ab. EFECTO DE DISTINTOS COMPORTAMIENTOS DE FLUJO EN EL RESPIROMETRO DE WRIGHT SUMARIO
RESUME
Les problemes qui decoulent de l'usage du respirometre de Wright pour la mesure des petits debits associes a la precision des petits volumes ont ete discutees. L'exactitude depend beaucoup de la forme d'onde du debit qui le traverse.
Se hace hincapie en los problemas del uso del respirometro de Wright para mediciones a bajos ritmos de flujo asociados con pequeiios volumenes. La exactitud depende mucho de la forma de la onda del flujo pasa a traves del aparato.
Downloaded from http://bja.oxfordjournals.org/ at Emory University on August 24, 2015
EFFETS DES DIFFERENTS MODELES DE DEBIT SUR LE RESPIROMETRE DE WRIGHT
EFFECT OF DIFFERENT FLOW PATTERNS ON THE WRIGHT RESPIROMETER J. A. BUSHMAN SUMMARY
The problems of using the Wright respirometer for measurements at small rates of flow associated with small volumes are discussed. Accuracy is very dependent on the wave form of the flow passing through it.
to the flowmeter outlet. Some of the respirometers were affected by the angle of their main axis to the perpendicular. All instruments A to E were tested, although E was a prototype. For the first reading each instrument was attached to the rotameter with its axis vertical and the dial horizontal face upwards. The flow rate was then increased smoothly and slowly until the vane could be seen rotating and then decreased until the vane just remained rotating without stalling for 1 min. The test was repeated with the instrument axis horizontal (the face in a vertical plane).
Test 2 Instruments A to D were tested on a rig consisting Five different types of Wright respirometer (A to E) of a Starling pump ventilating a 5-gallon drum. The were examined, all, apart from one, available on the respirometers were placed in the circuit, two at a market. The manufacturers are not named as the time, in series, at the junction of the Y-piece from history of each instrument before test varied and no the pump with the drum. In each set of measureconclusions can be made about the relative merits of ments the dial on one instrument was vertical and the the instruments from these results. other horizontal. One instrument was positioned to measure "inspiration", the other expiration. For each procedure the two instruments were changed Test I The flow rate required just to maintain rotation of round so that readings were obtained for each of two the vane was measured. To take account of the weight positions (vertical and horizontal) and for each of the hands, flow rates were recorded only after the direction of flow (inspiration and expiration). most directly geared hand had rotated by 360°. This The Starling pump and drum were chosen as a test "starting flow" was measured by passing an air flow rig as the combination produces an approximately generated by a variable speed fan through a flowmeter sinusoidal input from the pump and a high initial (calibrated for air at 20 °C between 0.6 and 5 litre flow rate on expiration from the drum because of its min""1 with an accuracy of 1% of full range = compliance and the valve arrangement on the Starling 0.05 litre min" 1 ), the respirometer being connected pump. Figure 1, which shows the difference in flow JOHN A. BUSHMAN, L.M.S.S.A., F.F.A.R.C.S., F.INST.M.C, patterns, was produced using a Fleisch flow transCOMPANION I.E.E., Research Department of Anaesthetics, ducer and a Greer defocusing manometer. A potentioRoyal College of Surgeons, Lincoln's Inn Fields, London metric-type recorder with a comparatively low slew WC2A 3PN. MATERIALS AND METHODS
0007-0912/79/090895-04 $01.00
© Macmillan Journals Ltd 1979
Downloaded from http://bja.oxfordjournals.org/ at Emory University on August 24, 2015
The fact that there are problems in measuring low flows with the Wright respirometer has been known for some time and this characteristic of the instrument was clearly defined by Nunn and Ezi-Ashi (1962). They studied continuous flow and showed that with less than 16 litre min" 1 the registered volume underestimated the absolute volume by an amount which increased very rapidly as flow was reduced. This report shows that the flow pattern of a given volume passing through the instrument in unit time has a large effect on the volume registered. The flow patterns used, similar to those found in clinical practice, were demonstrated artificially in a way which ensured accurately repeatable results.
896
BRITISH JOURNAL OF ANAESTHESIA TABLE I I . Minimwnflow requiredjust to keep the vane of each of the five respirometers rotating. The two readings given with the axis horizontal demonstrate the different flows required to move the more or most sensitive hand against or with the aid of gravity
Minimal running flows (litre min"1) Respirometer
Axis vertical
Axis horizontal
A B C D E
2.1 2.25 1.9 2.0 1.76
2.0/2.2 1.4/2.8 1.1/2.2 2.0/2.0 1.6/1.7
rotating when the more or most sensitive hand of the instrument was being rotated upwards compared with when it was running downwards assisted by its own weight.
rate was used and this underestimates the fast rising component of the expiratory flow. The actualflows(table I) were measured by displayRESULTS AND DISCUSSION ing the output from the Greer manometer on an oscilloscope screen. This shows a three- to four-fold Table II shows the minimum flows for the five difference between the inspiratory and expiratory flow respirometers tested. These are similar when the rates, a condition sometimes found in clinical practice. axis of the vane is vertical and the face of the dial is horizontal and uppermost. With the dial face vertical TABLE I. Peak inspiratory and expiratory flow rates the minimum flow to rotate the vanes changes accordat different volume settings of the Starling pump ing to the position of the hands on the dial (two Peak inspiratory Peak expiratory figures under the second heading). Volume set flow rate flow rate Instrument D had the most highly geared hand (ml) (litre min"1) (litre min"1) (x 10) while the most sensitive hand in it was small. As might be expected, this instrument was not 300 25 18 sensitive to its position, although it was difficult to 200 11 100 read from a distance with accuracy. Instrument B was easy to read several feet away, The stroke volume of the Starling pump was set in but showed the greatest sensitivity to dial face turn at one of three volumes—300 ml, 200 ml or orientation. 100 ml. The drive belt was adjusted to give 12.5 Instrument E is a prototype which appears to be strokes per min and each test was run for 2 min, the independent of orientation of the dial face. crank of the pump being started and stopped from These results accord with those of Nunn and Eziexactly the same position after 25 complete revolutions. Ashi (1962). The measurements recorded from the respirometers The respirometers were always started from zero. Each test was performed twice. There was no particu- on the Starling pump-drum rig are shown in table III. lar reason for choosing the combination of the The registered expired volume was always larger than respirometers (A with C and B with D) other than the inspired volume and the former was always each of the pair in any given test was of different closer to the calculated volume displaced. The manufacture and had a different arrangement of hands accuracy of the inspired volume measurement became progressively worse as the inspired volume decreased. on the dial. The two readings given in table II demonstrate the The accuracy of the expired volume remained high difference in the flow required to keep the vane even at tidal volumes as small as 100 ml.
Downloaded from http://bja.oxfordjournals.org/ at Emory University on August 24, 2015
FIG. 1. The "inspiratory" and "expiratory" flow patterns of the Starling pump and drum system. Note the slow sinusoidal shape of the inspiratory flow (upwards) and the fast initial component of the expiratory flow (downwards). The movement of the paper under the pen was from left to right. The magnitude of the fast rising component is an underestimate of the real flow because of the comparatively slow response of the potentiometric recorder used. The pump was set at 200 ml at 12.5 rev min" 1 for this recording, but the wave form was exactly the same at the high and low settings of the pump, 300 and 100 ml respectively.
STUDIES OF THE WRIGHT RESPIROMETER
897
TABLE I I I . The result of connecting pairs of the four respirometers in series, one measuring inspiration and one measuring expiration, in a system consisting of a Starling pump ventilating a 5-gallon drum. The instruments were arranged so that one of them had its dial vertical and the other horizontal. Two readings were taken for each position. The tests were run for exactly 25 revolutions of the crank of the pump at a speed of 12.5 revmin'1. The calculated total volume for this 2-min period is shown at the head of each set of results. The accuracy of the average of each pair of readings is expressed as a percentage of this calculated total volume
Accuracy
Accuracy
Inspiration
Expiration
= 7.5 litre) 5.53 5.84 5.5 6.6
73.7 77.8 73.3 88.0
7.25 8.82 7.55 7.46
7.15 8.75
200 ml at 12.5 rev min" 1 for 2 min (total B Face vertical 3.08 D Face horizontal 3.36 B Face horizontal 3.0 D Face vertical 3.25
= 5 litre) 3.08 3.36 3.03 3.25
61.6 67.2 60.3 65.0
4.83 5.26 5.05 5.08
4.85
100 ml at 12.5 rev min" 1 for 2 min (total B Face vertical 0.55* D Face horizontal 0.6 B Face horizontal 0.4 D Face vertical 0.3
= 2.5 litre) 0.59 0.56 0.4 0.3
22.8 23.2 16.0 12.0
300 ml at 12.5 rev min" for 2 min (total = 7.5 litre) A Face vertical 5.88 5.84 C Face horizontal 5.8 5.83 A Face horizontal 5.56 5.58 C Face vertical 5.79 5.83
78.1 77.5 74.3 77.5
7.2
7.28
7.75 7.52 7.42
7.8
65.1 63.2 61.3 64.5
4.84 5.23 5.06 5.01
29.8 21.0 29.0 22.4
2.5
7.5
7.45
5.3
4.92 4.92
2.35
2.35
2.5
2.5
2.45
2.35
2.6
2.5
96.0 117.1 100.3 99.4 96.8 105.6 99.7 100.0 94.0 100.0 96.0 102.0
1
1
200 ml at 12.5 rev min" for 2 min (total A Face vertical 3.26 C Face horizontal 3.16 A Face horizontal 3.08 C Face vertical 3.23
= 5 litre) 3.25 3.16 3.05 3.22
7.72 7.6
96.5 103.7 101.6 100.1
5.1
96.4 104.1 102.1 101.1
2.52 2.67 2.58 2.56
100.4 106.4 103.2 102.2
4.8
. 5.18 5.15
1
100 ml at 12.5 rev min- for 2 min (total = 2.5 litre) A Face vertical 0.77 0.72 C Face horizontal 0.55 0.5 A Face horizontal 0.74 0.71 C Face vertical 0.6 0.52 :
Stalled.
Although one instrument (B) stalled during one of the smallest volume tests, the measurements from all the instruments at the smallest inspired volumes were more than four times less than they should have been, while the measured expired volumes remained close to the calculated swept volume. Although the first test showed that the minimum flow rates from each instrument were different (table II), this difference between the instruments is not evident in table III and neither was the effect of the orientation of the instrument, except in the case of instrument B, which stalled once when the more 73
2.65 2.58 2.55
sensitive hand had to be rotated against the effect of gravity. These results suggest that it is not possible to rely directly on the reading from these instruments with spontaneously breathing patients with low tidal volumes, such as may occur in patients breathing high concentrations of halothane spontaneously. The effect is less likely to occur if the lungs are ventilated artificially, particularly with the respirometer in the expiratory line as the initial flow rate from the patient will be high at the moment of opening of the expiratory valve. •
Downloaded from http://bja.oxfordjournals.org/ at Emory University on August 24, 2015
300 ml at 12.5 rev min" 1 for 2 min (total B Face vertical 5.53 D Face horizontal 5.84 B Face horizontal 5.5 D Face vertical 6.6
BRITISH JOURNAL OF ANAESTHESIA
898 CONCLUSIONS
The Wright respirometer is generally accurate when presented with flow patterns with a high initial flow rate. The accuracy deteriorates if the initial flow rate is low and accelerates only slowly. The instrument is inherently safe, however, as it will underestimate tidal volume. REFERENCES
Nunn, J. F., and Ezi-Ashi, T. I. (1962). The accuracy of the respirometer and ventigrator. Br. J. Anaesth., 343 422.
DIE WIRKUNG VERSCHIEDENER STROMUNGSSTRUKTUREN AUF DAS WRIGHTATEMMESSGERAT ZUSAMMENFASSUNG
Probleme beim Einsatz des Wright-Atemmessgerates fur Messungen bei den mit kleinen Volumen verbundenen geringen Durchflussmengen. Genauigkeit hangt sehr von der Wellenform des durchfliessenden Stromes ab. EFECTO DE DISTINTOS COMPORTAMIENTOS DE FLUJO EN EL RESPIROMETRO DE WRIGHT SUMARIO
RESUME
Les problemes qui decoulent de l'usage du respirometre de Wright pour la mesure des petits debits associes a la precision des petits volumes ont ete discutees. L'exactitude depend beaucoup de la forme d'onde du debit qui le traverse.
Se hace hincapie en los problemas del uso del respirometro de Wright para mediciones a bajos ritmos de flujo asociados con pequeiios volumenes. La exactitud depende mucho de la forma de la onda del flujo pasa a traves del aparato.
Downloaded from http://bja.oxfordjournals.org/ at Emory University on August 24, 2015
EFFETS DES DIFFERENTS MODELES DE DEBIT SUR LE RESPIROMETRE DE WRIGHT