The anesthesia workstation is the primary tool that an anesthesia provider uses to deliver anesthetic gases and oxygen in controlled concentrations and to ventilate the patient’s lungs. The anesthesia breathing system consists of the inspiratory and expiratory unidirectional valves, the fresh gas inlet, the mechanical ventilator, the scavenging system connections (such as the ventilator relief valve), the absorbent canister and the disposable components of the breathing circuit (corrugated hoses, Y-piece, elbow, and reservoir bag).
There is a wide variety of anesthesia workstation models. Breathing systems differ among models in the configuration of their components. For example, the fresh gas inlet may be placed proximal or distal to the inspiratory unidirectional valve. Breathing systems also differ in volume.
Changes in breathing system gas concentration can more rapidly be achieved with a higher fresh gas flow (FGF). However, the placement of components within the circle breathing system also affects the equilibration time for oxygen and other anesthetic gases.1–7 Thus, breathing system configuration affects the rapidity with which the anesthesia provider can denitrogenate the patient, or alter the concentration of anesthetizing gases administered to the patient. This raises the question of whether different modern anesthesia breathing systems are equally efficient in attaining these objectives. The purpose of this study was to determine the speed of changes in gas concentration between different anesthesia workstation models, each with a type of circle breathing system. We hypothesized that equilibrium would be reached most quickly in systems with smaller volume and at higher FGF.
Three anesthesia workstations were studied: the Aestiva (Datex-Ohmeda, now a division of GE Medical, Madison, WI; Fig. 1), the ADU (Datex-Ohmeda, now a division of GE Medical, Madison, WI; Fig. 2), and the Fabius GS with a COSY-1 breathing system (Draeger Medical, Telford, PA; Fig. 3). Each machine was prepared using the manufacturer’s preuse checklist,8–11 and the breathing system was fitted with an adult breathing circuit (disposable corrugated tubing, Y-piece, elbow, and 3 L reservoir bag). The ADU was fitted with a Medisorb Compact Absorber containing 0.6 kg of absorbent granules (Part number 427002100) and an adult D-Lite sensor. A lung simulator was connected to the breathing circuit, and the rate of change of oxygen concentration was measured in the lung simulator.
The lung simulator was a “water lung,” which is a two-compartment Plexiglas box partially filled with water12 (Fig. 4). A fan continuously mixed the gas within the main chamber. This lung simulator has a functional residual capacity of 2000 mL, and a compliance of approximately 50–75 mL/cm water.13,14
The same galvanic-cell oxygen analyzer was used to measure oxygen concentration in all trials (Critcare POET IQ Model 602–3B, Criticare Systems, Waukesha, WI). This device has a rise time of 600 ms, and a stated accuracy of 3% in the range of 0%–90% oxygen and 4% in the range of 90%–100% oxygen. Gas was sampled within the main chamber of the lung model and was returned to the expiratory limb of the breathing circuit in all cases. The output of the oxygen analyzer was captured on a computer and recorded every 1 s.
The model lung was ventilated using volume-control mode (tidal volume of 1000 mL, respiratory rate of 10 breaths per minute, and an inspiratory: expiratory ratio of 1:2). FGF rates of 1, 2, 4, 6, and 8 L/min were studied. Ten trials were performed for each FGF setting with each machine, for a total of 150 trials. Each trial was conducted in a similar fashion. The model lung was ventilated with air at high FGF until a stable oxygen concentration of 21% was achieved within it. The FGF was then changed to oxygen, and the oxygen concentration was monitored until it reached a stable concentration near 100% oxygen (either the set concentration or a stable measured reading for 2 min). Times to 50%, 63%, 66%, 75%, and 90% change in oxygen concentration were recorded.
The dependent variable was time to 90% change in oxygen concentration in the model lung. The experiment was modeled with an analysis of variance. Independent variables included FGF (1, 2, 4, 6, 8 L/min), trial (1–10), gas machine model (ADU, Aestiva, Fabius GS), and finally the model-FGF interaction. This is a split-plot analysis of variance model, with repeated measures on the variable “trial.”15,16Post hoc tests with a Bonferroni correction were performed to identify any statistically significant differences between flows, between models, and between cells (if a statistically significant value was found for the F test overall). It was assumed that the individual breathing system and gas machine studied would perform identically to any other member of their class. If the particular gas machine chosen for study did not differ in its performance from any other example of that model, the results could be generalized to all similar models. SPSS 15.0 (SPSS, Chicago, IL) was used as an aid to statistical computation. A two-tailed P value <0.05 was accepted as evidence of statistical significance throughout.
Table 1 shows the results for time to a 90% change in oxygen concentration. Figure 5 shows the means and standard deviation of time to 50%, 63%, 75%, and 90% equilibration for all machines, at all FGF. The breathing systems did not differ in performance at a FGF of 6 or 8 L/min. At a FGF of 4 L/min, time to 90% change was similar in ADU and Fabius, but relatively prolonged in the Aestiva as compared to ADU (P < 0.001) or Fabius (P = 0.002). At FGF of 1 and 2 L/min, the ADU took less time to 90% change than either the Fabius (P < 0.001) or Aestiva (P < 0.001). Fabius took more time than Aestiva at a FGF of 1 L/min (P < 0.001), but there was no difference between them at 2 L/min.
A system attains 63% of equilibrium to a new inflow in 1 time constant.17 The apparent volume of each breathing system was calculated using the formula for time constant (τ) as follows
This formula applies to a single, well-mixed compartment, not a complicated anesthesia breathing system, with its absorber, valves, corrugated hoses, and test lung. Therefore the volumes calculated are only apparent volumes. The apparent breathing system volume was calculated for each trial (e.g., at 4 L/min it took 120 s to reach 63% equilibration during 1 trial, which yielded an apparent breathing system volume of 120/60 × 4 = 8 L). The apparent breathing system volumes were (mean ± sd) 6.5 ± 0.5 L for the ADU, 7.2 ± 1.0 L for the Fabius, and 7.6 ± 1.6 L for the Aestiva. These volumes included the volume of the disposable hoses and also the functional residual capacity of 2 L within the test lung. Subtracting the volume within the test lung, the apparent breathing system volume for ADU was 4.5 L, for Fabius 5.2 L and for Aestiva 5.6 L.
The purpose of this study was to determine the speed of changing gas concentration between different modern anesthesia breathing systems. To determine this, we measured the time it takes for three modern anesthesia breathing systems filled with air to attain equilibrium at various oxygen inflow rates. Results for early (50%), moderate (63% and 75%), and late (90%) equilibration were presented.
There are a number of important differences among the breathing systems that were tested. All of the systems have a carbon dioxide absorbent canister, with inspiratory and expiratory valves located near the absorber. The Aestiva (Fig. 1) is a traditional circle breathing system (i.e., similar to most Drager and Ohmeda breathing systems available in the United States during the 1980s and 1990s) with the fresh gas inlet located between the absorber and inspiratory valve, and the ventilator bellows and ventilator relief valve located between the expiratory valve and absorber. The ADU breathing system (Fig. 2) is similar to the Aestiva, with two important exceptions: the breathing system volume is much lower, and the fresh gas inflow is positioned between the inspiratory valve and the patient. The Fabius breathing system (Fig. 3) is different from the Aestiva in that the ventilator piston is positioned between the absorber and the inspiratory valve, and the reservoir bag remains in the breathing system during mechanical ventilation due to the fresh gas decoupled design. We tested the COSY-1 breathing system, which has the reservoir bag positioned on the inspiratory side of the breathing system, between the carbon dioxide absorbent canister and the inspiratory valve. Newer Fabius anesthesia workstations have a COSY-2 breathing system (Fig. 3), with the reservoir bag positioned on the expiratory side of the breathing system, between the expiratory valve and the carbon dioxide absorbent canister. The original COSY breathing system (COSY-1) was modified (COSY-2) to prevent leakage of gas to the scavenger before the reservoir bag is filled, which occurred in situations of rapid exhalation flow from the patient.
As expected, gas concentration changed more quickly at higher FGFs with each breathing system. There were no significant differences between breathing systems at FGFs of 6 or 8 L/min. This is surprising, since one would expect a faster equilibration time with the ADU breathing system at every fresh gas inflow rate due to its significantly smaller volume. The result can be explained by the position of the fresh gas inlet downstream of the inspiratory valve. With this position of the fresh gas inlet, fresh gas traverses the disposable tubing of the circuit during exhalation and is vented from the ventilator relief (exhaust) valve, especially at high FGF rates.2 In other circle breathing systems, in which the fresh gas inlet is positioned upstream of the inspiratory valve, FGF travels retrograde through the carbon dioxide absorbent canister during exhalation, which flushes the larger volume between the inspiratory valve and ventilator relief valve.
At FGF rates of 4 L/min and below, gas concentrations change more quickly in the ADU than in the Fabius and Aestiva breathing systems (except between ADU and Fabius at 4 L/min). This can be explained by the smaller breathing system volume in the ADU (2 L [all figures from manufacturer’s specifications]) than Fabius (2.8 L, plus the 3-L reservoir bag) or Aestiva (5.5 L).
At a FGF rate of 1 L/min, the Fabius breathing system takes significantly longer than the Aestiva to reach 90% of equilibration, and the variability between experiments is greater. This may be explained by the presence of the reservoir bag in the system. The reservoir bag is located on a cul-de-sac and may contain poorly mixed gases; thus, it could release trapped air into the breathing system at a variable rate. This effect would be most noticeable at low fresh gas rates and at a time when the rest of the breathing system approaches equilibrium.
With the ADU and Aestiva, the apparent volume of the breathing system was larger than the volume as determined from the manufacturer’s literature. Apparent breathing system volumes determined in the present study, not including the residual volume within the test lung, were 4.5 L for ADU, 5.2 L for Fabius, and 5.6 L for Aestiva. The manufacturer’s specifications state the following volumes: ADU 2 L, Fabius 2.8 L plus bag, and Aestiva 5.5 L (these do not include the volume of the disposable breathing circuit tubing, which was 0.5 L for the ADU and Aestiva, and 1 L for the Fabius).
The discrepancy between apparent breathing system volume and real breathing system volume (i.e., manufacturer’s specification plus disposable components) was more than zero for the ADU (4 L – (2 L + 0.5 L) = 2.0 L). This result suggests that the gas vented from the ventilator relief valve in this breathing system contains a higher than average percentage of fresh gas. As stated above, in the ADU, the fresh gas inlet is positioned downstream of the inspiratory valve, which would increase the proportion of fresh gas vented from the exhaust valve that opens during exhalation, especially at high flows. The discrepancies between apparent breathing system volume and real breathing system volume (i.e., manufacturer’s specification plus disposable components) were less than zero for the Fabius and Aestiva breathing systems (5.2 L−[2.8 L + 3 L + 1 L] = −1.6 L) and (5.6 L – [5.5 L + 0.5 L] = −0.4 L), respectively. This suggests that the gas vented from the ventilator relief valves in these breathing systems contains a lower than average percentage of fresh gas. In these breathing systems, FGFs retrograde into the reservoir bag or absorber canister during exhalation, and exhaled gas is preferentially vented from the exhaust valve.
A limitation of this study is that the model lung did not take up oxygen or produce carbon dioxide. Since oxygen uptake is small and constant, our results could still be applied to the rate of change in oxygen concentration when converting a patient between breathing air and oxygen. Greater care should be used in applying these results to changing levels of nitrous oxide or volatile inhaled anesthetics, especially those with high blood-gas solubility. In those situations, there are potentially large and variable differences between inhaled and exhaled concentrations, which would alter the response of these rebreathing systems, especially at low FGF rates.
In conclusion, gas concentration changes occurred more quickly with higher FGFs in all breathing systems, as compared to lower FGFs. At high flows (>4 L/min), there were no differences in the speed of gas concentration changes among the different breathing systems. At low to moderate flows (1–4 L/min), the gas concentration changed more quickly with the ADU breathing system than with the Fabius or Aestiva. Practitioners should note that differences in breathing system efficiency are most prominent at low FGFs (1–2 L/min), and that high flows (above 6 L/min) should be used during denitrogenation of the lungs and whenever rapid changes in gas concentrations are required.
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© 2009 International Anesthesia Research Society
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