Circle Breathing System Research Articles

Evaluation of a CO2 partial rebreathing functional residual capacity measurement method for use during mechanical ventilation

There is a need for an automated bedside functional residual capacity (FRC) measurement method that does not require a step change in inspired oxygen fraction. Such a method can be used for patients who require a high inspired oxygen fraction to maintain arterial oxygenation and for patients ventilated using a circle breathing system commonly found in operating rooms, which is not capable of step changes in oxygen. We developed a CO(2) rebreathing method for FRC measurement that is based on the change in partial pressure of end-tidal carbon dioxide and volume of CO(2) eliminated at the end of a partial rebreathing period. This study was designed to assess the accuracy and precision of the proposed FRC measurement system compared to body plethysmography and nitrogen washout FRC. Accuracy and precision of measurements were assessed by comparing the CO(2) rebreathing FRC values to the gold standard, body plethysmography FRC, in twenty spontaneously breathing volunteers. The CO(2) rebreathing FRC measurements were then compared to nitrogen washout FRC in twenty intensive care patients whose lungs were mechanically ventilated. For each subject, an average value of CO(2) rebreathing FRC was compared to the average gold standard method. Measurements were accepted for statistical analysis if they had been recorded from periods of stable tidal ventilation, defined as a coefficient of variation of tidal volume of <0.13. Compared to body plethysmography, the accuracy (average error) for the CO(2) rebreathing method during stable ventilation (n=8) was 0.03L and precision (1 standard deviation of the error) was 0.29L (0.8 ± 7.6% of body plethysmography). During stable mechanical ventilation (n=9), the accuracy was -0.02L and precision was 0.26L (-1.1 ± 12.6% of nitrogen washout). The CO(2) rebreathing method for FRC measurement provides acceptable accuracy and precision during stable ventilation compared to the gold standards of body plethysmography and nitrogen washout. The results based on periods of stable ventilation best approximate the performance of the system in the likely areas of application during controlled mechanical ventilation. Further study of the CO(2) rebreathing method is needed to evaluate accuracy in a larger group of controlled mechanical ventilation patients, including patients with respiratory insufficiency and significant lung injury.

Evaluation of the isoflurane-sparing effects of lidocaine infusion during umbilical surgery in calves

ObjectiveTo evaluate the isoflurane-sparing effects of lidocaine administered by constant rate infusion (CRI) during umbilical surgery in calves. Study designRandomized ‘blinded’ prospective clinical study. AnimalsThirty calves (mean 4.7 ± SD 2.5 weeks old) undergoing umbilical surgery. MethodsAfter premedication with xylazine (0.1 mg kg−1, IM), anaesthesia was induced with ketamine (4 mg kg−1, IV) and maintained with isoflurane in O2 administered through a circle breathing system. The calves were assigned randomly to receive a bolus of 2 mg kg−1 lidocaine IV after induction of anaesthesia, followed by CRI of 50 μg kg−1 minute−1 (group L, n = 15) or a bolus and CRI of 0.9% sodium chloride (NaCl, group S, n = 15). End-tidal isoflurane was adjusted to achieve adequate depth of anaesthesia. Heart rate, direct arterial blood pressure and body temperature were measured intraoperatively. Groups were compared by t- tests, anova or Mann–Whitney rank sum test as appropriate. ResultsThe end-tidal concentration of isoflurane (median, IQR) was significantly lower in group L [1.0% (0.94–1.1)] compared to group S [1.2% (1.1–1.5)], indicating a 16.7% reduction in anaesthetic requirement during lidocaine CRI. Cardiopulmonary parameters and recovery times did not differ significantly between groups. Conclusion and clinical relevanceLidocaine CRI may be used as a supplement to inhalation anaesthesia during umbilical surgery in calves in countries where such a protocol would be within the legal requirements for veterinary use in food animals. This study did not show any measurable benefit to the calves other than a reduction in isoflurane requirement.

The volatile consumption: uptake ratio as a measure of the efficiency of semi‐closed circle breathing system usage

The volatile consumption: uptake ratio as a measure of the efficiency of semi‐closed circle breathing system usage

Clinical evaluation of an end-tidal target-controlled infusion closed-loop system for isoflurane administration in horses undergoing surgical procedures

A new volatile anaesthetic agent delivery system was tested in 15 horses undergoing scheduled surgical procedures. The delivery system consisted of a laptop computer (with dedicated software), a computer-controlled syringe driver (loaded with liquid isoflurane) connected to the inspiratory arm of a large-animal circle breathing system and a respiratory gas monitor, providing isoflurane end-tidal concentrations (ET(measured)) every 20 s to the computer. Following induction and connection to the breathing system, mechanical ventilation was started. The bodyweight (BW), fresh gas flow, breathing system and ventilator volume, and end-tidal isoflurane target (ET(target)) were entered into the computer. Using Lowe's equation, the software calculated the prime dose to be delivered by the syringe driver over 2 min. After this, the system delivered each minute the amount of isoflurane as determined by the following equation: Isoflurane (mL) = {2 × λ(B/G) × (200 × BW(0.75)) × (ET(target) - ET(measured)) + (fresh gas flow - (BW(0.75) × 0.07)) × (ET(measured))}/206. A fresh gas flow of 4 L oxygen min(-1) was administered until the inspired fraction of oxygen reached 0.7, and was then decreased. A target of 1.5% end-tidal isoflurane was initially used and subsequently adjusted to the clinical requirements. The system performance was evaluated using the median prediction error (MDPE) and the median absolute performance error (MDAPE), which were -3.6% and 5.29%, respectively. It was concluded that this system was useful to achieve end-tidal target-controlled infusion of isoflurane during equine anaesthesia.

Functional characteristics of anesthesia machines with circle breathing system

Presently, the majority of anesthesia machines or workstations are ventilators that incorporate a classic circle system/circuit with a CO 2 absorber that allows rebreathing of exhaled gases. The performance characteristics of these machines are related to the physical structure of the breathing circuit. Limitations related to the classical circle breathing circuit are progressively disappearing with the arrival on the market of modern anesthesia machines with modified circular circuits, which offer a series of advantages in relation to the classic circular circuit. The present review describes the basic functional characteristics common to any anesthesia machine and that determine their clinical performance. The knowledge of these characteristics is essential for the clinical use of anesthesia machines and for analyzing their evolution.

Tube-in-Tube Rescue Airway Management for a Perforated Endotracheal Tube by Patient Biting

I read with great interest the recent case report by Duma et al. 1regarding tube-in-tube emergency airway management after a bitten endotracheal tube (ETT) caused by repetitive transcranial electrical stimulation during spinal cord surgery. This is indeed a favorable rescue airway measure for the ETT perforation by patient biting or electrical surgical instrument,2–4especially when the ETT exchange is difficult or impossible because of patient position, space limit, device deficiency, and others. The authors stated that this method was a new strategy to manage the ETT rupture during surgery. In fact, Peskin and Sachs2used a tube-in-tube technique to manage a partially severed nasotracheal tube during orthognathic surgery in 1986. Moreover, in this article, the tube-in-tube study in table 1 gives the best sizes for the inner tubes that correspond to the sizes for the outer tubes. It has been shown that the outer diameter of the ETT for a given internal diameter from different manufacturers varies because of differences in wall thickness.5Therefore, these results may be only suitable for the ETTs tested by the authors.Just as the authors have found, a main disadvantage of the tube-in-tube technique is increased resistance to ventilation because of small internal diameter of an inner tuber. We had used a modified method to establish emergency airway after a size 7.5-wire-reinforced ETT was perforated by biting in a patient undergoing face and neck scar resection under the lateral position. The destructed wire-reinforced ETT and a size 5.0 cuffed polyvinyl chloride ETT (used as an inner tube) are cut at 0.5–1.0 cm distal to the site of attachment of the external inflation tube of the cuff (figs. 1A and B). The shortened inner tube is adequately lubricated and inserted into the destructed wire-reinforced ETT until the midpoint of the cuff of the inner tube is located at the damage site of the wire-reinforced ETT (fig. 1C). Subsequently, the cuff of the inner tube is inflated, and the inner tube is connected to the circle breathing system by the standard connector. During the intermittent positive pressure ventilation with a tidal volume of 10 ml/kg, a ventilatory frequency of 10 breaths/min, an inspiratory time-respiratory cycle time ratio of 1:2, and a fresh gas flow of 2.5 l/min, the mean and peak airway pressures observed were 9 cm H2O and 21 cm H2O in this patient.As compared with methods reported by Duma et al. 1and Peskin and Sachs,2we consider that this modified technique has some possible advantages. First, removing the tube connector and the proximal part of destructed wire-reinforced ETT may facilitate insertion of an inner tube. Second, except for the radius of the ETT and density of gas, the length of the ETT is another important contributing factor to airflow resistance.6Therefore, shortening the length of the inner tube can significantly decrease ventilation resistance. Third, inflating the cuff of the inner tube can firmly secure stabilization of the two tubes together and avoid airway leakage without requiring any auxiliary maneuver or tool.Finally, it must be emphasized that when ETT perforation or rupture accidentally occurs during surgery, the tube-in-tube technique can only be used as a temporary rescue airway measure, especially for the patients with a large body weight. If the tear in the ETT is small and it may be possible to approximate the damage site of the ETT, moreover, sealing the leak with gauze,7nylon ties, adhesive tape, or finger4are also alternatives to emergency airway management to obtain a functional ETT. However, once this condition is allowed, the ETT exchange should be performed as soon as possible.4*Plastic Surgery Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing, People's Republic of China. fruitxue@yahoo.com.cn

Unusual Cause of Leak in Datex Aisys

To the Editor: We wish to report a serious incident encountered while using the Aisys Carestation (Datex-Ohmeda, GE Health Care, Finland, through GE India). While delivering anesthesia to a young male undergoing craniotomy, we noticed that the soda lime exhausted. The Aisys Reusable Multi Absorber canister was removed, refilled with fresh soda lime, and then replaced on the Carestation. The water trap was also drained at this time. After replacement of the canister on the Carestation, a large leak was noted in the circle breathing system. The canister was removed and refitted, but the leak persisted and it was impossible to ventilate the patient’s lungs manually or in ventilator mode, even with a high fresh gas flow. Changing the breathing system to a Bain coaxial circuit connected to the alternate common gas outlet of the Aisys eliminated the leak and allowed ventilation of the patient’s lungs. A subsequent thorough examination of the Carestation revealed a soda lime granule trapped in the flap valve of the water trap, rendering that valve incompetent (Fig. 1). Removal of the granule eliminated the leak when the Carestation was checked out with a circle system. It is unclear how the soda lime granule reached the drain valve of the water trap. One possible explanation is that some brands of soda lime have a high content of dust and flowing gas may have carried that dust through the breathing system, leading to deposits caking in the condenser reservoir. Despite following the manufacturer’s instructions for refilling absorber canisters and in particular replacing the filters, we have noticed clogged dust collecting in the condenser reservoir. Choosing a brand of soda lime with low dust content may prevent this mishap.Figure 1.: (a) Water trap of Aisys with granule stuck in rubber flap valve; (b) canister with water trap.As the anesthesia workstation evolves, it becomes more complicated and less intuitive for the user attempting to troubleshoot a malfunction. The circle breathing systems on many of the new anesthesia workstations have become more complex, and some of the components may be hidden or at least not obvious. The condenser and water trap are integral components of the Aisys circle system. Education and in service training when using a new anesthesia workstation will enable early recognition of system failure. Prashant Kummar, MD Grace Korula, MD, DA Sathish Kumar, MD, DA P. A. Saravanan, MD, DA Christian Medical College and Hospital Vellore, Tamil Nadu, India [email protected]

Preoxygenation in Pregnancy: The Effect of Fresh Gas Flow Rates Within A Circle Breathing System

Obstetric Anesthesia Digest: June 2009 - Volume 29 - Issue 2 - p 77 doi: 10.1097/01.aoa.0000350616.06863.ae

The Relation Between Oxygen Consumption and the Equilibrated Inspired Oxygen Fraction in an Anesthetic Circle Breathing System: A Mathematic Formulation &amp; Laboratory Simulations

Measuring a patients' oxygen consumption (VO2) is valuable in critical care and during anesthesia. Up to now, there has been no satisfactory equation describing the relation between the VO2, the fresh gas, and F(I)O(2) in a semi-closed circle breathing system. By adopting a "volume-weighted average concentration" approach and stepwise calculations, we have proposed an equation. We constructed a model with known simulated O(2) consumption ((SIM)VO2) to test our equation and two other previous methods (Biro's and Azami's). After 32 different laboratory scenarios, the %-error of the calculated VO2 ((CAL)VO2) from our method is -4.0 +/- 2.9%, which is significantly better than those from Azami's method (-8.8 +/- 6.2%, p < 0.01) and from Biro's method (-27.4 +/- 5.1%, p < 0.01). We also produce a Bland-Altman analysis of our (CAL)VO2 and (SIM)VO2. The 95% limits of agreement are -18.6-3.3 mL/min with a mean bias of -7.7 mL/min, which shows a good agreement. Our equation also explains the difference between F(I)O(2) and the oxygen concentration of the fresh gas in a semi-closed circle breathing system.

A Method to Improve Use of a Preformed Nasal Tube for Fiberoptic Nasal Intubation

To the Editor: Preformed nasal tubes (PNT) are commonly selected for nasal-tracheal intubation in patients undergoing head and neck surgery because they are easy to secure, provide convenient surgical access, and help reduce pressure on the nares.1 However, there are some limitations with such tubes. First, passing a fiberoptic bronchoscope (FOB) through the PNT is often difficult because of its preformed distal acute curve, particularly when an adult version FOB, such as Olympus LF-TP FOB with an outer diameter of 5.1 mm, is used. Second, when the PNT is threaded over a FOB, the FOB can be deformed (Fig. 1A) and rotational movement applied to the body of the FOB is not easily transmitted to its anterior flexible tip, reducing maneuverability of the FOB.2 Third, after the FOB is inserted into the trachea, the deformation described may cause difficulty in advancing the PNT over the FOB. Fourth, the PNT is longer than the conventional endotracheal tube and the distal end of the PNT will be impacted in the nares before the FOB’ tip is inserted into the trachea. To overcome these limitations, we have adapted a simple technique to improve use of the PNT in fiberoptic nasal intubation.Figure 1.: (A) The distal insertion tube of the fiberoptic bronchoscope (FOB) is deformed (1) because of the distal curve of the Portex® cuffed preformed nasal tubes (PNT). When a size 7.5 mm Portex® cuffed PNT and a FOB with the insertion tube of 60 cm are used, moreover, the distance between the FOB’s tip and the distal end of the PNT is only 14 cm. (B) The PNT is cut at the midpoint of the distal curve (2) to become the two portions: the internal portion (4) and outer portion (4). A short metallic tube is preformed to a curvature similar to the distal curve of the PNT (3). (C) The precurved metallic tube is inserted into the cut end of the outer portion (6). The internal portion is loaded on the FOB (7). (D) After intubation, the internal and outer portions are rejoined by the precurved metallic tube (8).The PNT is cut at the midpoint of the distal curve dividing into the internal and outer portions. The cut site is located approximately 2 cm distal to the site of attachment of the external inflation tube of the cuff. A 4-cm long metal tube (with walls 1 mm thick) bent to a curvature similar to the distal curve of the PNT (Fig. 1B) is inserted into the cut end of the outer portion. When a size 7.5 mm Portex® cuffed PNT and a FOB with the insertion tube of 60 cm are used, the distance between the FOB’s tip and the distal end of the internal portion is 22 cm (Fig. 1C). This allows the FOB to be inserted into the trachea sufficiently far before the distal end of the internal portion is impacted on the nares.3 After the FOB is inserted into the trachea, the internal portion is advanced over the FOB into the trachea. After the FOB is removed, the cut end of the internal portion is connected to the precurved metallic tube (Fig. 1D). The reconstituted preformed tube is connected to the circle breathing system by the standard endotracheal tube connector. After local ethics committee approval and written informed consent, we have used this technique in more than 500 adult patients undergoing head and neck surgery requiring nasal-tracheal intubation. No kinking, partial obstruction, increased airway resistance, or separation of the reconstituted tube occurred and the original profile of the PNT is preserved allowing it to be readily positioned over the patient’s forehead to facilitate surgical access. Fu S. Xue, MD Ya C. Xu, MD Xu. Liao, MD Department of Anesthesiology Plastic Surgery Hospital Chinese Academy of Medical Sciences and Peking Union Medical College Beijing, People’s Republic of China [email protected]

Detection of inner tube defects in co‐axial circle and Bain breathing systems: a comparison of occlusion and Pethick tests

The performance of the occlusion and Pethick tests in detecting faulty inner tubes in co-axial circle and Bain systems was compared. Twelve co-axial circle and 12 Bain anaesthetic breathing systems were tested using the occlusion and the Pethick tests. For each system, three tubes were intact, and the remaining nine had a defect deliberately created in the inner tube (three proximal, three middle and three distal). The investigators were blinded to which of the tubes were defective, and to each other's results. The results showed 100% specificity for both tests. The sensitivity of the occlusion test for detecting faulty breathing systems was found to be good (98%). Our results suggest that the occlusion test should be performed in preference to the Pethick test when testing co-axial circle and Bain systems.

Pre‐oxygenation in pregnancy: the effect of fresh gas flow rates within a circle breathing system*

We have investigated the effect of oxygen flow rate on pre-oxygenation in pregnant patients at term using a circle system. Twenty patients presenting for elective Caesarean section maintained tidal volume breathing through a standard circle system for 3 min. Subjects were pre-oxygenated using oxygen flow rates of 5 l.min(-1), 10 l.min(-1) and 15 l.min(-1) presented in random order. The mean (SD) fractional end-tidal oxygen at the end of 3 min was 0.86 (0.07) for 5 l.min(-1), 0.92 (0.05) for 10 l.min(-1)and 0.90 (0.09) for 15 l.min(-1) (p < 0.001). Entrainment of air occurred in 22% of pre-oxygenation sessions. Oxygen flow rates of 10 l.min(-1) or above provide optimal pre-oxygenation using a circle system in term parturients. In our study, entrainment of air occurred in a surprisingly high percentage of cases.

Do New Anesthesia Ventilators Deliver Small Tidal Volumes Accurately During Volume-Controlled Ventilation?

During mechanical ventilation of infants and neonates, small changes in tidal volume may lead to hypo- or hyperventilation, barotrauma, or volutrauma. Partly because breathing circuit compliance and fresh gas flow affect tidal volume delivery by traditional anesthesia ventilators in volume-controlled ventilation (VCV) mode, pressure-controlled ventilation (PCV) using a circle breathing system has become a common approach to minimizing the risk of mechanical ventilation for small patients, although delivered tidal volume is not assured during PCV. A new generation of anesthesia machine ventilators addresses the problems of VCV by adjusting for fresh gas flow and for the compliance of the breathing circuit. In this study, we evaluated the accuracy of new anesthesia ventilators to deliver small tidal volumes. Four anesthesia ventilator systems were evaluated to determine the accuracy of volume delivery to the airway during VCV at tidal volume settings of 100, 200, and 500 mL under different conditions of breathing circuit compliance (fully extended and fully contracted circuits) and lung compliance. A mechanical test lung (adult and infant) was used to simulate lung compliances ranging from 0.0025 to 0.03 L/cm H(2)O. Volumes and pressures were measured using a calibrated screen pneumotachograph and custom software. We tested the Smartvent 7900, Avance, and Aisys anesthesia ventilator systems (GE Healthcare, Madison, WI) and the Apollo anesthesia ventilator (Draeger Medical, Telford, PA). The Smartvent 7900 and Avance ventilators use inspiratory flow sensors to control the volume delivered, whereas the Aisys and Apollo ventilators compensate for the compliance of the circuit. We found that the anesthesia ventilators that use compliance compensation (Aisys and Apollo) accurately delivered both large and small tidal volumes to the airway of the test lung under conditions of normal and low lung compliance during VCV (ranging from 95.5% to 106.2% of the set tidal volume). However, the anesthesia ventilators without compliance compensation were less accurate in delivering the set tidal volume during VCV, particularly at lower volumes and lower lung compliances (ranging from 45.6% to 100.3% of the set tidal volume). Newer generation anesthesia machine ventilators that compensate for breathing circuit compliance and for fresh gas flow are able to deliver small tidal volumes accurately to the airway under conditions of normal and low lung compliance during volume-controlled ventilation. Accurate VCV may be a useful alternative to PCV, as volume is guaranteed when lung compliance changes, and new strategies such as small volume/lung protective ventilation become possible in the operating room.

The ideal tool for decorators: a novel use for disposable laryngoscope blades

The International Climate Change Conference in Montreal in December 2005 raised awareness of the potential for catastrophic climate change as a result of human actions. It behoves all of us...

Cardiopulmonary effects of desflurane in horses

ObjectiveTo determine the cardiopulmonary effects of desflurane (DES) in horses. AnimalsSix healthy adult horses, three males and three females, aged 9 ± 4 (mean ± SD) years and weighing 370 ± 36 kg. Materials and methodsAnaesthesia was induced with an O2 (10 L minute−1) and DES mixture (vaporizer setting 18%). After oro-tracheal intubation, horses were positioned in right lateral recumbency. Anaesthesia was maintained with DES in O2 (20 mL kg−1 minute−1) delivered through a large animal circle breathing system. The minimum alveolar concentration of DES (MACDES) that prevented purposeful movement in response to 60 seconds of electrical stimulation of the oral mucous membranes was determined for each horse. The delivered concentration of DES was then increased to achieve end-tidal concentrations corresponding to 1.5 × MACDES, 1.75 × MACDES, and 2.0 × MACDES. Heart rate (HR), mean arterial blood pressure (MAP), respiratory rate (fr), tidal volume (Vt), minute volume (Vm) and core temperature were determined, and blood samples for arterial blood gas analysis taken at each DES concentration. All data were analysed by two-way anova for repeated measures and Fisher's test for multiple comparisons. A probability level of p < 0.05 was applied. ResultsDesflurane concentrations of 2.0 × MACDES increased HR whereas lower concentrations did not. Mean arterial pressure was not affected by 1.0 × MACDES 1.5 × MACDES or 1.75 × MACDES, whereas it decreased at 2.0 × MACDES. All concentrations of DES examined significantly depressed fr, Vt and Vm. Conclusions and clinical relevanceDesflurane concentrations between 1.0 and 1.75 × MACDES reduces fr and Vm but does not affect HR or MAP in horses.

A prospective, randomised controlled trial comparing the efficacy of pre‐oxygenation in the 20° head‐up vs supine position*

We investigated whether positioning patients undergoing general anaesthesia for cholecystectomy in a 20 degrees head-up position, as opposed to supine, improved the efficacy of 3 min of standard pre-oxygenation via a circle breathing system. Following pre-oxygenation, patients received a standard induction of anaesthesia and the apnoea time (from administration of rocuronium to the arterial oxygen saturation to fall to 95%) was recorded. Mean (95% CI) apnoea time was 386 (343-429) s in the 20 degrees head-up position (n = 17) vs 283 (243-322) s in the supine position (n = 18; p = 0.002). Pre-oxygenation is significantly more efficacious and by inference more efficient in the 20 degrees head-up position than in the supine position.

Vital capacity and patient-controlled sevoflurane induction

To the Editor: We read with interest the article by Yogendran et al.1 comparing patient controlled inhalation induction (PCI) with the most commonly used vital capacity induction (VCI). The authors concluded that PCI was comparable to VCI in sevoflurane induction with respect to the speed of induction, side effects during induction and patient satisfaction. We partially disagree with this conclusion because we believe that there was a problem with the VCI in the study. Considering that the volume of gas inhaled in a vital capacity inspiration is 70 mL·kg–1 (5.6 L for an 80-kg patient) and requires a very high peak flow that current vaporizers cannot accommodate, and considering that the authors used a circle breathing system connected to a Datex-Ohmeda® AS/3 unit with a canister volume of 900 mL and a 2-L reservoir (information obtained from authors), and assuming the average patient’s weight is 80 kg, the primed circuit afforded half of the volume needed for a VCI induction. Consequently, patients in the VCI group didn’t breath sevoflurane 6 to 8%, but instead received diluted sevoflurane 3 to 4% during the first minute of induction. This might explain the 1.5-min necessary to achieve loss of eyelash reflex in the VCI group in the present study, in contrast to the induction times (32 to 54 sec) generally reported by other authors,2–4 even without N2O. 5 Five-litre reservoir bags should be used, and fresh gas flow can be reduced to 2 L·min–1 after loss of conciousness to diminish sevoflurane consumption in VCI.

Anesthesia machine basics

The anesthesia machine is that component of the anesthesia delivery system that receives medical gases (oxygen, nitrous oxide, air, heliox) under pressure and controls the flow of each gas individually. It creates a gas mixture of known composition at a known flow rate and delivers it to the common gas outlet of the machine. From here, the fresh gas flow is conducted to the anesthesia circle breathing system. This review will discuss the storage of compressed oxygen and nitrous oxide and how these gases arrive to a generic anesthesia machine. The paths taken by these gases as they flow through the generic machine will be described. The important components used to create the precisely controlled fresh gas mixture, as well as the safety features of the machine will also be described. The reader should gain an understanding of the machine basics that he/she can use to understand his/her particular model of machine.

Minimum inspired oxygen concentration alarm

EDITOR: The use of anaesthetic machine and patient monitor alarms, in addition to clinical observation, can enhance patient safety, but it is very important that these limits are accurately set. The importance of these settings is highlighted by a critical incident which occurred recently in one of our theatres. Case report: An 8-yr-old, American Society of Anesthesiologists (ASA) I patient was scheduled for an elective ear, nose and throat procedure. Induction was uneventful and general anaesthesia was maintained with spontaneous ventilation via a laryngeal mask. The patient's head was draped and surgery commenced. An unobserved disconnection occurred at the junction of the circle-system breathing circuit tubing and the filter to which the gas analyser sampling line was attached (Fig. 1). The reduced movement of the reservoir bag was not initially noticed and the capnography trace continued as per normal. The inspired oxygen concentration decreased but the alarm did not sound as its setting had not been changed from the factory default of 18% for oxygen. Had it been set to above 21% it would have been activated. Fortunately the disconnection was discovered and the circuit reconnected before the patient sustained any harm.Figure 1.: Site of disconnection between circle breathing system and filter.The case prompted an audit at two hospitals in our region which demonstrated that the minimum inspired oxygen concentration alarm was changed from the factory reset by less than 10% of anaesthetists. Personal communication with a colleague from a neighbouring region revealed that two identical incidents had recently occurred in their hospital, and the same audit is at present being conducted in their hospital. Fortunately no patient suffered any harm as a result of the disconnections but the potential for patient awareness, movement, adverse airway events and hypoxia is evident. The use of an oxygen analyser is mandatory in accordance with the checklist for anaesthetic apparatus as set out by the Association of Anaesthetists of Great Britain and Ireland and is recommended in many other countries [1]. The Association guidelines however give no advice regarding the values to which the analyser's alarm limits should be set [2]. Oxygen concentrations are usually measured at either the fresh gas outlet or via the end-expired gas sampling line attached to, or near to, the filter. In addition to guarding against the delivery of hypoxic mixtures, the analyser (sampling end-tidal gases) can also help to detect problems with breathing circuit and ventilator integrity, and with alveolar hypoventilation [3,4]. A study by Knaack-Steinegger showed that disconnections were evident within 30 s using expired oxygen concentration alarms [5]. Disconnections are evident within a few breaths when using end-tidal gas monitoring with oxygen concentration alarms set to above 21%. Capnography alarms and the waveform are used by anaesthetists as a monitor for early detection of problems with patients' ventilation, including disconnections in the breathing system. In spontaneously breathing patients, where there is a disconnection proximal to the capnography sampling point, the capnography trace may well continue as per normal and hence this alarm would not detect the disconnection. An anaesthetic agent analyser is also useful to detect disconnections in a breathing system and guard against patient awareness, but factory default settings of 0% would need to be reset to minimum alveolar concentration (MAC) awake or above. The audit performed in our hospital also investigated the alarm settings of the agent analyser and found that in no theatres were the limits changed to above 0%. The oxygen alarms are often not set above 21% as doing so results in the alarm sounding when there is no (enriched) oxygen flowing through the circuit. This can be overcome by leaving the sampling line in continuity with the circuit and allowing a small quantity of oxygen to flow (most modern anaesthetic machines have a minimum oxygen flow preset). An alternate solution is to cap the circuit off at the end of the case. This also helps to prevent wastage of anaesthetic gases and stops the anaesthetic agent analyser alarm sounding (if set above 0%). Ventilator pressure alarms can detect leaks during mechanical ventilation, but they are not foolproof [6]. The minimum inspired oxygen alarm is a useful monitor but should be set above 21%, especially for spontaneously breathing patients and when the circuit is covered. Setting the anaesthetic agent alarm to above 0% further adds to patient safety. M. T. Dale D. Jadhav *Department of Anaesthesia, Critical Care and Pain Management, Leicester Royal Infirmary, Leicester, England

The moisture-conserving performance of breathing system filters in use with simulated circle anaesthesia breathing systems.

Breathing system filters can be used to humidify gases delivered to patients. Performance can be determined by measuring the net moisture loss (the difference between expired and inspired levels of humidity) from a patient model. The net moisture loss should be decreased by increasing the level of humidity in the breathing system by, for example, using a circle breathing system. The effect of four different filters, three different levels of humidity in the breathing system (7, 13 and 19 g.m-3) and two tidal volumes (0.5 l and 1.0 l) on the net moisture loss from a patient model was measured. The net moisture loss decreased as the humidity in the breathing system increased and was less for the lower tidal volume. Adequate levels of humidity (>/= 20 g.m-3) will be delivered to patients by most filters provided they are used in conjunction with circle breathing systems and low fresh gas flows.