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Duration of carbon dioxide absorption by soda lime at low rates of fresh gas flow
Duration of Carbon Dioxide Absorption by Soda Lime at Low Rates of Fresh Gas Flow Maria Ohm, MD,* Nikolaus Gravenstein, Michael L. Good, MD$ Departments
of Anesthesiology
of Medicine,
Gainesville,
and Neurosurgery,
Study Objective:
To determine
the impact
of a low fresh
duration
dioxide
absorption
by soda lime.
of carbon
(CO,)
u test lung with a CO, inflow 0.5,
interface
was determined
with a mainstream
each fresh considered
in Anesthesiology
*Assistant
in Anesthesiology
Professor
circle ventilating
Fresh gas,flow
Duration
rates of 0.25,
CO, tension
The times of this interual
min than at 1 Llmin fresh
gas flow and at 1 Llmin
circuit
of CO, absorption
(PICO,)
to increase from
were recorded four
gas flow rate and compared by analysis of variance; significant. Time to soda lime failure was significantly
and
Conclusion: breathing
Because
circle
the CO, absorbent
Address reprint requests to at the Editorial Office, Anesthesiology, Box J-254, Health Center, Gainesville, C’SA.
Dr. Gravenstein Department of J. Hillis Miller FL 32610-0254,
Received for publication March 28, 1990; revised manuscript accepted for publication September 28, 1990. 0 199 1 Butterworth-Heinemann
J. Clin.
breathing
rate on the
times for
p < 0.05 was longer at 2 Ll
than at 0.25 Llmin fresh
gas
flow.
tAssociate Professor Neurosurgery
104
cupnometer.
as the time for the inspired
0 mm to 7 mm of mercury.
of
and a semiclosed
of 250 ml per minute.
gas jlow
were studied. and Main Results: CO, was measured at the breathing
Measurements
Department
College
1, 2, und 4 Limin
test lung
Medicine,
of Florida
FL.
Design: Nonclinical, experimental. Setting: Experimental laboratory. Methods: In vitro test with Sodasorb
*Resident in Anesthesiology
University
MD,-!
Anesth.,
vol. 3, March/April
soda lime color indicators
is used at a low rate qf fresh must be replaced
Keywords: Anesthesia; equipment; supplies.
are unreliable,
when a semiclosed
gas flow without CO, monitoring,
more frequently.
blood gas monitoring;
carbon dioxide;
Introduction Economic considerations,l operating room (OR) pollution control,” heat and moisture conservation,“,’ and closed-circuit anesthesia have contributed to the increasing use of lower rates of fresh gas flow (i.e., <5 L/min). Effectiveness of the CO, absorbent in the Hafnia circle breathing system varies inversely with the fresh gas flow rate (2 Limin compared with 6 L/
1991
CO, absorption by soda
min).5 Incomplete absorption of CO, by soda lime may be assessed by measuring the concentration of PICO, or the temperature of the soda lime canister,‘j neither of which is standard procedure in many anesthesia practices. Using a dye indicator in the absorbent that changes color as CO, absorption decreases is unreliable.6,5 An informal survey by us has suggested that, in most practices, CO, absorbent is replaced according to a routine maintenance schedule (e.g., once a week). The purpose of the present in vitro study was to quantify the effect of lower rates of fresh gas flow on the effectiveness of a commercially available soda lime preparation (Sodasorb, W.R. Grace and Co, Lexington, MA) in a standard anesthesia circle system.
lime:
Ohm
et al.
corder. ‘The end point of each trial was a PICO, of 7 mmHg. Four separate cartridges were tested at each flow rate. The times to reach the end point of 7 mmHg at each fresh gas flow rate were compared by analysis of variance (p < 0.05 being significant).
Results The 20 absorbent canisters studied were similar in weight before testing: 1109.3 * 19.5 g (range, 1085 to 1145 g). Times to reach the critical PICO, of 7 mmHg differed significantly (p < 0.05) between fresh gas flow rates (Figure 1). The rate of increase of PICO, tension was linear, with very little scatter of data points.
Materials and Methods A mechanical
lung model (Vent Aid Training Test Lung, Michigan Instruments, Grand Rapids, MI) with a CO, inflow of 0.25 L/min regulated by a calibrated flow meter was mechanically ventilated via a semiclosed circle system by a standard anesthesia ventilator (Air Shields Ventimeter, Air Shields, Hatboro, PA; or Ohio Anesthesia Ventilator, Ohio Mechanical Products, Madison, WI). The inspiratory-to-expiratory ratio (1:E) of the ventilator was set at 1:3, respiratory rate was 10 breaths/minute, and tidal volume was 700 i- 50 ml (Wright Respirometer, BOC Medishield, Essex, UK). PICO,, and end-tidal CO, (PETCOJ were monitored continuously with a mainstream capnograph (Novametrix 7000, Novametrix Medical Systems, Wallingford, CT) at the connection between the endotracheal tube and breathing circuit. The ventilator and lung model parameters maintained a P&O, of 35 to 40 mmHg. A humidifier (Conchatherm III, Respiratory Care Inc., Arlington, IL) was inserted between the test lung and endotracheal tube to impart a temperature of 37°C and a humidity of 99% to the expired gases at the endotracheal tube; the humidity was monitored intermittently with a hygrometer. A nonconductive, disposable anesthesia breathing circuit and ventilator tubing (Intertech/Ohio DABC, Lincolnshire, IL) were used. A fresh gas flow of 100% oxygen (0,) was directed into the circle system at live flow rates: 0.25, 0.5, 1.0, 2.0, and 4.0 L/min. Prior to each trial, a fresh soda lime cartridge (Sodasorb Pre-Pak, USP-NF, 1.13 kg) was weighed. The single absorbent cartridge used for each trial was placed in the upper position in the absorber canister (Ohio Anesthesia Absorber, Ohio Medical Products, Madison, WI). The lower absorber cartridge position was left empty. The capnogram was recorded continuously on a calibrated, time-based strip chart re-
Discussion The upper limit of clinically acceptable PICO, has been recommended as 0.1% to 1.070.” Therefore, in the present study, a PICO, of 7 mmHg, roughly l%, was used as an end point. Variables that may affect the rate of CO, absorption include the patient’s CO, production and the absorbent’s packing, mesh size, hardness, size of particles, and weight. The absorbent’s manufacturing variables vary widely and may have contributed to a variation in the results (i.e., SD of the mean time in hours; Figure 1). The present study demonstrates the importance of an anesthesiologist-influenced variable: fresh gas flow rate. At
II’ I
I
I
L
3
5
3
PICOZ (mmHd
1. Time (means -C SD) to inspired CO, tension (PICO,) of 0.5 to 7.0 mmHg plotted in 0.5 mmHg increments at five rates of fresh gas flow (Limin). (‘“p < 0.05 compared with times for PICO, to reach 7 mmHg at other rates of fresh gas flow.)
Figure
J. Clin. Anesth., vol. 3, March/April 19911
105
Or@nal Contributions
Figure 2. Mechanism
by which the rate of fresh gas flow affects the CO, absorbent. (FCF = fresh gas flow; CO,-containing gases are represented by the shaded portions of the circuit.) (A) CO,-containing gases from the patient pass through the ventilator hose into the ventilator bellows during exhalation. (B) CO,-containing gases pass through the ventilator hose, into the ventilator bellows, and out of the spill valve during the respiratory pause. (C) Gases flow retrograde through the absorbent during inspiration.
of‘fresh gas flow (52 Limin), time to a PICO, of 7 mmHg decreased significantly (i.e., the rate of soda lime depletion increased). At a rate less than 2 Limin, less than 15 hours was required to reach a PICO, of 7 mmHg, while at a flow of 4 Limin, more than 30 hours was required. The mechanism by which fresh gas flow affects the duration of CO, absorption by soda lime is shown in Figure 2. During active exhalation (Figure 2A), CO,containing gas passes through the ventilator hose. Simultaneously, fresh gas, which dilutes the exhaled COP, flows retrograde through the absorber and into the ventilator hose. During the respiratory pause that follows exhalation (Figure 2B), fresh gas continues to flow retrograde through the absorber and pushes a portion of the exhaled COY-containing gases out of the ventilator hose and ventilator by way of the ventilator spill valve. The higher the rate of fresh gas flow and the longer the respiratory pause, the more CO, is removed by this nonabsorbent process. During the subsequent inspiratory phase, gases that remain in the ventilator hose or ventilator (the CO, content of which depends on the fresh gas flow) are pushed through the absorbent, where any CO, is removed and then sent to the patient via the inspiratory limb of the breathing circuit (Figure 2C). The design of the semiclosed circle system places the fresh gas flow inlet and pop-off valve in locations that allow for the observed conservation of absorbent with increased fresh gas flo~.~.” In summary, the amount of CO, that the absorbent must filter is inversely related to the rate of fresh gas flow. This finding is in keeping with the observation that at very high fresh gas flow rates, the circle system can be used without any absorbent at all. In the semilow rates
106
J. Clin. Anesth.,
vol. 3, March/April
1991
closed circle system, the clinically effective absorbent life span depends on the fresh gas flow in a predictable manner. By using the value manufacturers provide for liters of CO, absorbed by 100 g of the product, calculations can approximate the effect of varied fresh gas flows on the absorbent’s life span. The formula that predicts this life span is as follows: LCO,I 100 g absorbent
x absorber t = %CO, [ ri -
weight
(g)
I
(1 - i:E) (FGF) ] x 60
where t is time (hours), LCO, is the specification for the amount of CO, absorbed per 100 g, %CO, is the CO, concentration of the exhaled gas, p is minute ventilation (L/min), Z:E is the inspiratory-to-expiratory ratio, and FGF is the fresh gas flow (Limin). This formula, applied to predict the CO, absorbent’s function, tends to overestimate time to failure by assuming ideal absorption (i.e., no channeling). Therefore, predictions may deviate substantially from clinically observed values. The data presented indicate that when routine maintenance schedules for soda lime absorbent replacement are in effect, they should be modified when lower rates of fresh gas flow are used in the OR. If capnography is routinely performed, this procedure also serves as an effective monitor for incomplete absorption of CO, (i.e., increased PICO,).
Acknowledgments The authors thank Tim Newell for graphic Michelle Martin for editorial assistance.
assistance
and
CO, absorption
References Matjasko
impact
of low-flow
anesthesia.
1987;67:863-4.
Virtur RW, Escobar A, Model1 J: Nitrous oxide levels in operating room air with various gas flows. Can Anaesth Sot J
Ohrn et al.
5. Jorgensen
J: Economic
Anesthesiology
by soda lime:
6. 7.
1979;26:313-8.
Chalon J, Pate1 C, Ramanathan S, et al: Humidification of the circle absorber system. Anesthesiology 1978;48: 1426. Chase HF, Kilmore MA, Trotta R: Respiratory water Anestheloss via anesthesia systems: mask breathing. siology 1961;22:205-9.
8.
9.
B, Jorgensen S: The 600 gram CO, absorption canister: an experimental study. Acta Anaesthesiol &and 1977;21:437-44. Tsuchiya M, Ueda W: Heat generation as an index of exhaustion of soda lime. Anesth Analg 1989;68:783-7. Adriani J, Rovenstine EA: Experimental studies on carAnesthesiology bon dioxide absorbers for anesthesia. 1941;2:1-19. Dorsch JA, Dorsch SE: Understanding iinesthesia Equipment: Construction, Care, and Complications. Baltimore: Williams & Wilkins, 1984:210-46, 226-30. Rendell-Baker L: On the promise of economy denied.
Anesthesiology
1968;29:5-6.
J. Clin. Anesth.,
vol. 3, March/April
1991
107
of Anesthesiology
of Medicine,
Gainesville,
and Neurosurgery,
Study Objective:
To determine
the impact
of a low fresh
duration
dioxide
absorption
by soda lime.
of carbon
(CO,)
u test lung with a CO, inflow 0.5,
interface
was determined
with a mainstream
each fresh considered
in Anesthesiology
*Assistant
in Anesthesiology
Professor
circle ventilating
Fresh gas,flow
Duration
rates of 0.25,
CO, tension
The times of this interual
min than at 1 Llmin fresh
gas flow and at 1 Llmin
circuit
of CO, absorption
(PICO,)
to increase from
were recorded four
gas flow rate and compared by analysis of variance; significant. Time to soda lime failure was significantly
and
Conclusion: breathing
Because
circle
the CO, absorbent
Address reprint requests to at the Editorial Office, Anesthesiology, Box J-254, Health Center, Gainesville, C’SA.
Dr. Gravenstein Department of J. Hillis Miller FL 32610-0254,
Received for publication March 28, 1990; revised manuscript accepted for publication September 28, 1990. 0 199 1 Butterworth-Heinemann
J. Clin.
breathing
rate on the
times for
p < 0.05 was longer at 2 Ll
than at 0.25 Llmin fresh
gas
flow.
tAssociate Professor Neurosurgery
104
cupnometer.
as the time for the inspired
0 mm to 7 mm of mercury.
of
and a semiclosed
of 250 ml per minute.
gas jlow
were studied. and Main Results: CO, was measured at the breathing
Measurements
Department
College
1, 2, und 4 Limin
test lung
Medicine,
of Florida
FL.
Design: Nonclinical, experimental. Setting: Experimental laboratory. Methods: In vitro test with Sodasorb
*Resident in Anesthesiology
University
MD,-!
Anesth.,
vol. 3, March/April
soda lime color indicators
is used at a low rate qf fresh must be replaced
Keywords: Anesthesia; equipment; supplies.
are unreliable,
when a semiclosed
gas flow without CO, monitoring,
more frequently.
blood gas monitoring;
carbon dioxide;
Introduction Economic considerations,l operating room (OR) pollution control,” heat and moisture conservation,“,’ and closed-circuit anesthesia have contributed to the increasing use of lower rates of fresh gas flow (i.e., <5 L/min). Effectiveness of the CO, absorbent in the Hafnia circle breathing system varies inversely with the fresh gas flow rate (2 Limin compared with 6 L/
1991
CO, absorption by soda
min).5 Incomplete absorption of CO, by soda lime may be assessed by measuring the concentration of PICO, or the temperature of the soda lime canister,‘j neither of which is standard procedure in many anesthesia practices. Using a dye indicator in the absorbent that changes color as CO, absorption decreases is unreliable.6,5 An informal survey by us has suggested that, in most practices, CO, absorbent is replaced according to a routine maintenance schedule (e.g., once a week). The purpose of the present in vitro study was to quantify the effect of lower rates of fresh gas flow on the effectiveness of a commercially available soda lime preparation (Sodasorb, W.R. Grace and Co, Lexington, MA) in a standard anesthesia circle system.
lime:
Ohm
et al.
corder. ‘The end point of each trial was a PICO, of 7 mmHg. Four separate cartridges were tested at each flow rate. The times to reach the end point of 7 mmHg at each fresh gas flow rate were compared by analysis of variance (p < 0.05 being significant).
Results The 20 absorbent canisters studied were similar in weight before testing: 1109.3 * 19.5 g (range, 1085 to 1145 g). Times to reach the critical PICO, of 7 mmHg differed significantly (p < 0.05) between fresh gas flow rates (Figure 1). The rate of increase of PICO, tension was linear, with very little scatter of data points.
Materials and Methods A mechanical
lung model (Vent Aid Training Test Lung, Michigan Instruments, Grand Rapids, MI) with a CO, inflow of 0.25 L/min regulated by a calibrated flow meter was mechanically ventilated via a semiclosed circle system by a standard anesthesia ventilator (Air Shields Ventimeter, Air Shields, Hatboro, PA; or Ohio Anesthesia Ventilator, Ohio Mechanical Products, Madison, WI). The inspiratory-to-expiratory ratio (1:E) of the ventilator was set at 1:3, respiratory rate was 10 breaths/minute, and tidal volume was 700 i- 50 ml (Wright Respirometer, BOC Medishield, Essex, UK). PICO,, and end-tidal CO, (PETCOJ were monitored continuously with a mainstream capnograph (Novametrix 7000, Novametrix Medical Systems, Wallingford, CT) at the connection between the endotracheal tube and breathing circuit. The ventilator and lung model parameters maintained a P&O, of 35 to 40 mmHg. A humidifier (Conchatherm III, Respiratory Care Inc., Arlington, IL) was inserted between the test lung and endotracheal tube to impart a temperature of 37°C and a humidity of 99% to the expired gases at the endotracheal tube; the humidity was monitored intermittently with a hygrometer. A nonconductive, disposable anesthesia breathing circuit and ventilator tubing (Intertech/Ohio DABC, Lincolnshire, IL) were used. A fresh gas flow of 100% oxygen (0,) was directed into the circle system at live flow rates: 0.25, 0.5, 1.0, 2.0, and 4.0 L/min. Prior to each trial, a fresh soda lime cartridge (Sodasorb Pre-Pak, USP-NF, 1.13 kg) was weighed. The single absorbent cartridge used for each trial was placed in the upper position in the absorber canister (Ohio Anesthesia Absorber, Ohio Medical Products, Madison, WI). The lower absorber cartridge position was left empty. The capnogram was recorded continuously on a calibrated, time-based strip chart re-
Discussion The upper limit of clinically acceptable PICO, has been recommended as 0.1% to 1.070.” Therefore, in the present study, a PICO, of 7 mmHg, roughly l%, was used as an end point. Variables that may affect the rate of CO, absorption include the patient’s CO, production and the absorbent’s packing, mesh size, hardness, size of particles, and weight. The absorbent’s manufacturing variables vary widely and may have contributed to a variation in the results (i.e., SD of the mean time in hours; Figure 1). The present study demonstrates the importance of an anesthesiologist-influenced variable: fresh gas flow rate. At
II’ I
I
I
L
3
5
3
PICOZ (mmHd
1. Time (means -C SD) to inspired CO, tension (PICO,) of 0.5 to 7.0 mmHg plotted in 0.5 mmHg increments at five rates of fresh gas flow (Limin). (‘“p < 0.05 compared with times for PICO, to reach 7 mmHg at other rates of fresh gas flow.)
Figure
J. Clin. Anesth., vol. 3, March/April 19911
105
Or@nal Contributions
Figure 2. Mechanism
by which the rate of fresh gas flow affects the CO, absorbent. (FCF = fresh gas flow; CO,-containing gases are represented by the shaded portions of the circuit.) (A) CO,-containing gases from the patient pass through the ventilator hose into the ventilator bellows during exhalation. (B) CO,-containing gases pass through the ventilator hose, into the ventilator bellows, and out of the spill valve during the respiratory pause. (C) Gases flow retrograde through the absorbent during inspiration.
of‘fresh gas flow (52 Limin), time to a PICO, of 7 mmHg decreased significantly (i.e., the rate of soda lime depletion increased). At a rate less than 2 Limin, less than 15 hours was required to reach a PICO, of 7 mmHg, while at a flow of 4 Limin, more than 30 hours was required. The mechanism by which fresh gas flow affects the duration of CO, absorption by soda lime is shown in Figure 2. During active exhalation (Figure 2A), CO,containing gas passes through the ventilator hose. Simultaneously, fresh gas, which dilutes the exhaled COP, flows retrograde through the absorber and into the ventilator hose. During the respiratory pause that follows exhalation (Figure 2B), fresh gas continues to flow retrograde through the absorber and pushes a portion of the exhaled COY-containing gases out of the ventilator hose and ventilator by way of the ventilator spill valve. The higher the rate of fresh gas flow and the longer the respiratory pause, the more CO, is removed by this nonabsorbent process. During the subsequent inspiratory phase, gases that remain in the ventilator hose or ventilator (the CO, content of which depends on the fresh gas flow) are pushed through the absorbent, where any CO, is removed and then sent to the patient via the inspiratory limb of the breathing circuit (Figure 2C). The design of the semiclosed circle system places the fresh gas flow inlet and pop-off valve in locations that allow for the observed conservation of absorbent with increased fresh gas flo~.~.” In summary, the amount of CO, that the absorbent must filter is inversely related to the rate of fresh gas flow. This finding is in keeping with the observation that at very high fresh gas flow rates, the circle system can be used without any absorbent at all. In the semilow rates
106
J. Clin. Anesth.,
vol. 3, March/April
1991
closed circle system, the clinically effective absorbent life span depends on the fresh gas flow in a predictable manner. By using the value manufacturers provide for liters of CO, absorbed by 100 g of the product, calculations can approximate the effect of varied fresh gas flows on the absorbent’s life span. The formula that predicts this life span is as follows: LCO,I 100 g absorbent
x absorber t = %CO, [ ri -
weight
(g)
I
(1 - i:E) (FGF) ] x 60
where t is time (hours), LCO, is the specification for the amount of CO, absorbed per 100 g, %CO, is the CO, concentration of the exhaled gas, p is minute ventilation (L/min), Z:E is the inspiratory-to-expiratory ratio, and FGF is the fresh gas flow (Limin). This formula, applied to predict the CO, absorbent’s function, tends to overestimate time to failure by assuming ideal absorption (i.e., no channeling). Therefore, predictions may deviate substantially from clinically observed values. The data presented indicate that when routine maintenance schedules for soda lime absorbent replacement are in effect, they should be modified when lower rates of fresh gas flow are used in the OR. If capnography is routinely performed, this procedure also serves as an effective monitor for incomplete absorption of CO, (i.e., increased PICO,).
Acknowledgments The authors thank Tim Newell for graphic Michelle Martin for editorial assistance.
assistance
and
CO, absorption
References Matjasko
impact
of low-flow
anesthesia.
1987;67:863-4.
Virtur RW, Escobar A, Model1 J: Nitrous oxide levels in operating room air with various gas flows. Can Anaesth Sot J
Ohrn et al.
5. Jorgensen
J: Economic
Anesthesiology
by soda lime:
6. 7.
1979;26:313-8.
Chalon J, Pate1 C, Ramanathan S, et al: Humidification of the circle absorber system. Anesthesiology 1978;48: 1426. Chase HF, Kilmore MA, Trotta R: Respiratory water Anestheloss via anesthesia systems: mask breathing. siology 1961;22:205-9.
8.
9.
B, Jorgensen S: The 600 gram CO, absorption canister: an experimental study. Acta Anaesthesiol &and 1977;21:437-44. Tsuchiya M, Ueda W: Heat generation as an index of exhaustion of soda lime. Anesth Analg 1989;68:783-7. Adriani J, Rovenstine EA: Experimental studies on carAnesthesiology bon dioxide absorbers for anesthesia. 1941;2:1-19. Dorsch JA, Dorsch SE: Understanding iinesthesia Equipment: Construction, Care, and Complications. Baltimore: Williams & Wilkins, 1984:210-46, 226-30. Rendell-Baker L: On the promise of economy denied.
Anesthesiology
1968;29:5-6.
J. Clin. Anesth.,
vol. 3, March/April
1991
107