Fresh Gas Inlet Research Articles

The analysis of fresh gas flow in a circle anaesthetic breathing system, and its influence on CO2 absorbent desiccation

Background: If at the end of anaesthesia, the gases on the anaesthetic machine are not turned off, they continue to flow through the breathing circuit and may dry out the carbon dioxide absorbent in the absorber. At the beginning of the next anaesthetic procedure, the desiccated absorbent decomposes the volatile anaesthetic, resulting in the formation of toxic gas mixtures, which are dangerous for the patient. In such circumstances, the measured level of the volatile anaesthesic in the breathing circuit differs from the selected value. In the paper, we analyse potentially dangerous possible directions of continuous flow of fresh gas mixture in different anaesthetic breathing circuits. Methods: In the first part of the study, we simulated the continuous flow of gases in a Sulla anaesthesia machine using an experimental method in a simulation setting. In the second part, we analysed whether a retrograde flow through the absorber was possible in a convential anaesthetic breathing circuit and in the Fabius GS, Cato, Julian and Primus circuits with the Y-piece connector open or closed. Information on users’ requests for maintenance services due to discrepancies between the desired and measured levels of the volatile anaesthetic in the breathing circuit were obtained from the manufacturer’s local servicing agent. Results: In a Drager ISO 8 circle system (Sulla anaesthetic machine) with a standard gas supply connector, a retrograde flow is not possible. If the Y-piece connector is left open, the gases are vented to the outside via the absorber and the inspiratory tube. If the Y-piece connector is closed or open, the absorbent in the canister dries because of the continuous flow of gas. A retrograde flow is possible only in those Sulla anaesthetic machines where the fresh gas inlet is located below the inspiratory valve. A retrograde flow of gases causing absorbent desiccation can occur in a Cato, Julian, Fabius GS, or Primus anaesthetic machines with the Y-piece connector closed. Maintenance engineers point out that problems are reported mostly on Monday mornings before the first operation. Conclusions: After completed anaesthesia, the fresh gas inlets on the anaesthetic machine must always be closed. Especially in older machines, the carbon dioxide absorber must be checked before the first operation on a Monday morning. In new-generation machines, we must make sure that the emergency ventilation valve is closed after the end of anaesthesia.

Time to a 90% Change in Gas Concentration: A Comparison of Three Semi-Closed Anesthesia Breathing Systems

The speed with which gas concentration can be changed in the anesthesia breathing system affects the rate of denitrogenation, anesthesia induction, and emergence. Breathing system design also affects the speed at which gas concentration can be changed during maintenance. In this study, we sought to determine the speed of changes in gas concentration in modern semi-closed breathing systems. We hypothesized that equilibrium would be reached most quickly in breathing systems with smaller volume, and at high fresh gas flows. Three anesthesia workstations were studied in vitro: the ADU (Datex-Ohmeda, now a division of GE Medical, Madison, WI), the Fabius GS with a COSY-1 breathing system (Draeger Medical, Telford, PA), and the Aestiva (Datex-Ohmeda, now a division of GE Medical, Madison, WI). The breathing systems were connected to a test lung and ventilated with air. The fresh gas flow was then changed to oxygen at rates of 1, 2, 4, 6, or 8 L/min, and times to 50%, 63%, 66%, 75%, and 90% change in oxygen concentration within the test lung were recorded. Ten trials were performed for each breathing system, at each fresh gas flow. The results were analyzed with a split-plot analysis of variance followed by post hoc tests with a Bonferroni correction. At flows of 6 or 8 L/min, times to equilibration did not differ among the three breathing systems. At flows of 1 to 2 L/min, the gas concentration changed faster with the ADU than with the Aestiva or Fabius (P < 0.001). At 4 L/min, the ADU was faster than Aestiva (P < 0.001), but not Fabius. We concluded that, other than fresh gas flow rate, breathing system volume has the biggest effect on time to equilibrium when the composition of the fresh gas inflow is changed. The position of components (e.g., valves, carbon dioxide absorber, fresh gas inlet, ventilator bellows or piston) within the breathing system has a less pronounced effect.

Flame structure studies of rich ethylene–oxygen–argon mixtures doped with CO2, or with NH3, or with H2O

Structures of several premixed ethylene–oxygen–argon rich flat flames burning at 50mbar have been established by using molecular beam mass spectrometry in order to investigate the effect of CO2, or NH3, or H2O addition on species concentration profiles. The aim of this study is to examine the eventual changes of profiles of detected hydrocarbon intermediates which could be considered as soot precursors (C2H2, C4H2, C5H4, C5H6, C6H2, C6H4, C6H6, C7H8, C6H6O, C8H6, C8H8, C9H8 and C10H8). The comparative study has been achieved on four flames with an equivalence ratio (f) of 2.50: one without any additive (F2.50), one with 15% of CO2 replacing the same quantity of argon (F2.50C), one with 3.3% of NH3 in partial replacement of argon (F2.50N) and one with 13% of H2O in replacement of the same quantity of argon (F2.50H). The four flat flames have similar final flame temperatures (1800K).CO2, or NH3, or H2O addition to the fresh gas inlet causes a shift downstream of the flame front and thus flame inhibition. Endothermic processes CO2+H=CO+OH and H2O+H=H2+OH are responsible of the reduction of the hydrocarbon intermediates in the CO2 and H2O added flames through the supplementary formation of hydroxyl radicals. It has been demonstrated that such processes begin to play at the end of the flame front and becomes more efficient in the burnt gases region.The replacement of some Ar by NH3 is responsible only for a slight decrease of the maximum mole fraction of C2H2, but NH3 becomes much more efficient for C4H2 and C5 to C10 species. Moreover, the efficiency of NH3 as a reducing agent of C5 to C10 intermediates is larger than that of CO2 and H2O for equal quantities added.

Site of fresh gas inlet and ratios of the delivered fraction and inspired fraction of inhaled isoflurane and sevoflurane in low-flow anesthesia

The use of low-flow anesthesia causes a discrepancy between the delivered fraction (FD) and the inspired fraction (FI) of inhaled gases. We compared the FI/FD ratios of a new circle (fresh gas inlet located between the inspiratory valve and the patient) to those of the conventional circle (fresh gas inlet located between the inspiratory valve and the CO2 absorber) in low-flow isoflurane and sevoflurane anesthesia, using three anesthetic machines (Dräger NM-GS, Dräger Fabius-GS, and ACOMA KMA-1300-III). Eighty-two patients were randomly assigned to three experimental groups. For experiment 1, 32 patients were allocated to the NM-GS conventional/new, NM-GS new/conventional, ACOMA conventional/new, and ACOMA new/conventional groups. For experiment 2, 14 patients were allocated to ACOMA conventional/conventional and ACOMA new/new groups to measure isoflurane FI/FD ratios. For experiment 3, 36 patients were allocated to ACOMA conventional/conventional, ACOMA new/new, Fabius conventional/conventional, and Fabius new/new to measure sevoflurane FI/FD ratios. In experiment 1, the NM-GS showed no significant changes in the FI/FD ratios. However, in the ACOMA, the new circle improved the FI/FD ratio. In experiment 2, the isoflurane FI/FD ratios in the new circle of the ACOMA were significantly higher than those in the conventional circle. In experiment 3, the sevoflurane FI/FD ratios in the new circle of both the ACOMA and the Fabius were significantly higher than those in the conventional circles. The positioning of the fresh gas inlet between the inspiratory valve and the patient improved the FI/FD ratios of both isoflurane and sevoflurane during low-flow anesthesia in two decoupling-style anesthetic machines (ACOMA and Fabius).

Effect of dimethoxymethane addition on the experimental structure of a rich ethylene/oxygen/argon flame

Structures of premixed ethylene/oxygen/argon-rich flat flames burning at 50 mbar have been established by using molecular beam mass spectrometry to investigate the effect of methylal (dimethoxymethane) addition on species concentration profiles. The aim of this experimental study is to examine eventual changes of the concentration profiles of detected hydrocarbon intermediates which could be considered as soot precursors (C2H2, C4H2, C5H4, C5H8, C6H8, C6H4, C6H6, C6H8, C7H8, C6H6O, C8H6, C8H8, C9H8, and C10H8). The comparative study has been achieved on three flames with equivalence ratios (φ) of 2.25 and 2.50: two without any additive (F2.25 and F2.50) and one with 4.3% methylal in partial replacement of C2H4 (F2.50M). The three flat flames have similar final flame temperatures (≊1800 K). An increase of the flame equivalence ratio (φ) from 2.25 to 2.50 leads to an increase of maximum mole fractions of most hydrocarbon intermediates, much larger than the initial fuel content difference. Methylal addition to the fresh gas inlet causes a slight shift downstream of the flame front. The replacement of 5.7% C2H4 by 4.3% C3H8O2, keeping the equivalence ratio equal to 2.50, is responsible for a decrease of the maximum mole fractions of most of the detected intermediate species. If this phenomenon is barely noticeable for C2 to C4 intermediates, it becomes more efficient for C5 to C10 species. Although the equivalence ratio is quite different in flames F2.25 and F2.50M, most of the maximum mole fractions of C2 to C10 intermediates are very similar but lower than those in the F2.50 flame. It seems to indicate that similar initial C/O ratio (F2.25, 0.75: F2.50M, 0.76) in the fresh gases mixtures better encapsulates the influence on maximum concentrations of hydrocarbon intermediates and soot precursors in rich flames.

Reduction of fresh gas flow requirements by a circle-modified Bain breathing circuit

We modified a Bain circuit by placing the circuit into the Y piece of a standard carbon dioxide absorber circle, connecting the fresh gas hose on the anesthetic machine to the Bain's fresh gas inlet, and occluding the circle's fresh gas inlet. This circle-modified Bain breathing circuit was studied to evaluate whether it reduces fresh gas flow requirements. The Bain and modified Bain steady states were analyzed by mechanical and computer modeling. The mechanical model consisted of an artificial lung ventilated to steady state. Carbon dioxide was measured with capnography. Computer modeling was by compartmental analysis calculated with spread-sheet software. Steady-state solutions were obtained by numeric analysis. The circle-modified Bain greatly reduced retention of carbon dioxide. For example, with 1-liter tidal volumes, 10-liter minute volumes (10 breaths per minute), and a 2.1 L/min fresh gas flow, the steady-state end-tidal carbon dioxide values of the Bain and modified Bain were 9.3 and 4.6%, respectively, in the physical model (carbon dioxide inflow of 230 ml/min). Results from the mechanical model helped validate the computer model.

A new site for the adjustable pressure limiting valve on a circle absorber.

A new site for the adjustable pressure limiting valve has been suggested which will result in the most efficient utilisation of soda lime using typical flows with a circle absorber system. It will permit economy of fresh gases using old models of circle absorbers in which the adjustable pressure limiting valve and fresh gas inlet are both situated upstream of the inspiratory unidirectional valve. The cost of the new attachment is small.

FDA issues pediatric respiratory device alert

An attempt to facilitate life in critically ill newborns and small infants can lead to death if the components of the respiratory assistance devices used are not compatible, warned the Food and Drug Administration's National Center for Devices and Radiological Health (NCDRH) recently. In an alert letter sent to all US hospitals, NCDRH Office of Health Affairs director Marlene E. Haffner, MD, warned that this hazard can occur if certain breathing system connectors and adapters intended to reduce deadspace are used together. Ideally, there should be a slight gap between the lumen of the tracheal tube connector and that of the fresh gas (oxygen) inlet tube to serve as an exhalation pathway. If mismatched—that is, abutting—adapters and connectors are used during anesthesia or respiratory therapy, obliteration of this gap could either produce high resistance to exhalation or prevent it completely, thereby increasing intrapulmonary pressure and almost certainly causing pneumothorax. The

HALOTHANE ADSORPTION COMPLICATING THE USE OF SODA-LIME TO HUMIDIFY ANAESTHETIC GASES

If the anaesthetic circle system is arranged to increase the humidity of fresh anaesthetic gases by placing the carbon dioxide absorbent canister between the fresh gas inlet and the patient, drying of the soda-lime can occur. Very dry soda-lime adsorbs significant quantities of halothane. Using fresh soda-lime, effluent halothane concentration reached 50% of the input concentration in 35s, but this time increased to 500 s when dry soda-lime was used. The use of dry soda-lime can result in a slow inhalation induction or in the release of absorbed halothane during a subsequent anaesthetic.

Oxygen and carbon dioxide concentrations in anaesthetic circuits ventilated with an AGA UV 705 in an oxygen consuming model lung.

An oxygen consuming an carbon dioxide producing model lung was used for evaluation of anaesthetic circle systems. Two different circles, the AGA metal circle and the AGA Monosorb circle ventilated by an AGA UV-705 were tested with reference to inspired oxygen concentrations and end-tidal CO2 levels. Carbon dioxide is dependent on alveolar ventilation and CO2 production and thus was stable throughout the experiments. The oxygen concentration of inspired gas decreased progressively with decreasing fresh gas flow. When using the fresh gas inlet on the ventilator outlet block, the inspired oxygen concentration fell 8% as compared to using the fresh gas inlet in the circle. Fresh gas flows under 3 1/min should only be used with oxygen monitoring in the circle. The fresh gas inlet must be directly into the circle, and not in the ventilator outlet block, to avoid hypoxia.

FACTORS AFFECTING CARBON DIOXIDE HOMEOSTASIS DURING CONTROLLED VENTILATION WITH CIRCLE SYSTEMS

An experimental lung model was used, with controlled ventilation, to determine the effect of different circle arrangements and varying ventilatory frequencies on the efficiency of carbon dioxide removal from a circle system without carbon dioxide absorption. Greater efficiency was found when fresh gas entered the system between the unidirectional inspiratory valve and the subject that when the fresh gas inlet was on the ventilator side of this valve. At any fresh gas flow and minute volume, efficiency was greater at low respiratory frequencies. Good correlations existed between carbon dioxide concentration in the model lung, fresh gas flow and minute ventilation when respiratory frequency was constant. Paradoxical results were obtained when minute volume was varied by changes in frequency at a constant tidal volume. The major cause of the various differences in performance has been ascribed to variations in the degree of mixing of fresh and expired gas within the system.

Inaccurate ventimetry by fresh gas inlet position.

A new position is described for the fresh gas inlet on Boyle Anaesthesia Machines (Model M 222) with CO2 absorber Mark III. The location of this inlet between the valves directly in the breathing circuit leads to inaccurate measurement of the expiratory volume when using a Wright ventimeter on the expiratory limb of the circuit. Correction is made by the manufacturer on request.