Anaesthetic breathing systems

A breathing system is a series of components that allows the delivery of oxygen and other anaesthetic gases to the patient as well as aiding in the removal of carbon dioxide. There are key elements that feature in all anaesthetic breathing systems with numerous classification systems used. The layout of individual breathing systems determines their clinical application and use. All of the above will be discussed further in this article as well as a brief summary of the use of carbon dioxide absorbers and their function.     

Anaesthetic breathing systems

A breathing system is a series of components assembled to allow delivery of oxygen and other anaesthetic gases to the patient as well as assisting the removal of carbon dioxide. There are elements that feature in all anaesthetic breathing systems regardless of classification and the layout of individual breathing systems determines their clinical application and use. These will be discussed further in this article together with a brief summary of the use of carbon dioxide absorbers and their function.     

Accessory anesthetic breathing systems: verification before use

Accessory or ancillary anaesthesia breathing systems can be defined as all those connected to the fresh gas outlet of the anaesthetic apparatus and used instead of the circle system associated with the ventilator, which is the main circuit. They include: the Mapleson systems, the systems with a nonrebreathing valve and the disposable systems with a carbon dioxide absorber. They can be a cause of major accidents when not checked before and monitored during use. This technical note describes techniques of preanaesthetic checking and monitoring during anaesthesia.     

A comparison of anaesthetic breathing systems during spontaneous ventilation

Five anaesthetic breathing systems (Magill, Lack, Humphrey ADE, enclosed Magill and Bain) were compared using spontaneous ventilation in a simple lung model. The fresh gas flow at which rebreathing occurred was determined for each system by the application of four modified definitions of rebreathing. Two were based on the measurement of minimum inspired and two on end-expired carbon dioxide. The four A systems performed similarly with each individual definition. The rebreathing points found for each individual breathing system differed markedly between definitions, with those determined by the minimum inspired CO2 occurring at low, and probably misleading, FGF/VE ratio. The Bain system demonstrated rebreathing at considerably higher fresh gas flows whichever definition was used.     

The mechanisms of carbon monoxide production by inhalational agents

Carbon monoxide can be formed when volatile anaesthetic agents such as desflurane and sevoflurane are used with anaesthetic breathing systems containing carbon dioxide absorbents. This review describes the possible chemical processes involved and summarises the experimental and clinical evidence for the generation of carbon monoxide. We emphasise the different conditions that were used in the experimental work, and explain some of the features of the clinical reports. Finally, we provide guidelines for the prevention and detection of this complication.     

Respiratory depression in children at different end tidal halothane concentrations

Respiratory motor function and timing were investigated at end tidal halothane concentrations of 1.5%, 1.0% and 0.5% before and during 4% carbon dioxide stimulation in 10 spontaneously breathing children who weighed between 10.2 and 25.2 kg, during hypospadias repair under halothane anaesthesia. Their tracheas were intubated and all received a caudal block to eliminate surgical stimulation. Pneumotachography and capnography were used and in three cases movements of ribcage and abdomen were also studied by magnetometers. Respiratory drive was evaluated by occlusion tests. Ventilation was depressed at an end tidal halothane concentration of 1.5%, with smaller tidal volumes, higher respiratory rates, higher end tidal carbon dioxide tensions and a weaker respiratory drive compared with 1.0% and 0.5% halothane. Paradoxical breathing was noted at 1.5% as well as at 1.0% but not at 0.5% halothane anaesthesia; the ribcage moved inwards during inspiration. Respiratory compensation during periods of 4% carbon dioxide stimulation was inadequate at 1.5% halothane, as indicated by higher end tidal carbon dioxide tensions, less negative occlusion pressures and movements of ribcage and abdomen that were unresponsive to 4% carbon dioxide, when compared with 1.0% and 0.5% halothane. Respiratory rates were higher and duration of inspiration longer at 1.5% than at 1.0% and 0.5% halothane. Respiratory timing was unaltered by carbon dioxide stimulation. It is concluded that the ventilatory motor response to carbon dioxide is dose dependent and improves at more superficial anaesthetic levels, while respiratory timing is unresponsive to carbon dioxide stimulation irrespective of the halothane concentration used. Paradoxical breathing existed at end tidal halothane concentrations higher than 1%.     

COMPARISON OF THE MAGILL AND LACK ANAESTHETIC BREATHING SYSTEMS IN ANAESTHETIZED PATIENTS

The Magill and Lack anaesthetic breathing systems were comparedby measuring inspired and expired carbon dioxide concentrationsand expired minute volumes in lightly anaesthetized, unstimulatedsubjects. There were no significant differences between thetwo breathing systems at fresh gas flow rates of approximately50 and 70 ml kg^–1 min^–1. Inspired carbon dioxideconcentrations increased in one of six subjects at the higherfresh gas flow rate using the Magill system and in two usingthe Lack system. Inspired carbon dioxide concentration did notincrease in only one of six subjects at the lower fresh gasflow rate with both systems. Expired carbon dioxide concentrationsand expired minute volume increased in the majority of subjectsat both fresh gas flow rates using each system. We concludethat a fresh gas flow rate greater than 70 ml kg^–1 min^–1(which approximated to alveolar minute volume in our subjects)should be supplied to the Magill and Lack breathing systems. ^*Present address: Burton General Hospital, New Street, Burtonupon Trent, Staffordshire DE14 3QH.     

Simulated Spontaneous Breathing. A New Model for Testing Anaesthetic Circuits

A carbon-dioxide-producing lung model capable of simulating spontaneous breathing is presented. It consists of a piston in a cylinder, a mixing chamber and a dead space volume. The piston is driven by a direct-current motor controlled by a micro-processor and a servo unit. Respiratory waveform and rate, tidal volume, carbon dioxide production and dead space are easily adjustable within a wide range. The model is easy to handle and accurately mimics a given breathing pattern. It seems suitable for investigations of rebreathing and carbon dioxide elimination in different anaesthetic circuits.     

Interference of volatile anaesthetics with infrared analysis of carbon dioxide and nitrous oxide tested in the Drager Cicero EM using sevoflurane

In theory, setting an infrared multi-gas analyser to measure a volatile anaesthetic different from that in the sampled gas mixture may cause interference with carbon dioxide and nitrous oxide readings. The theory was investigated during evaluation of the Drager Cicero EM anaesthetic workstation for the Medical Devices Agency. Interference occurred as predicted, and was most pronounced when the vapour analyser of the Cicero EM was deliberately and erroneously set to measure isoflurane, but with sevoflurane present in the gas mixture. With 6% sevoflurane in the gas mixture, the carbon dioxide reading decreased from 5% to 3.6%, and the nitrous oxide reading increased from 0% to 8% although, as the apparent isoflurane reading was 9%, the Cicero EM would alert the operator to the problem. However, operators are encouraged to ensure that, when using gas analysers such as that incorporated into the Cicero EM, the analyser is set to measure the correct volatile anaesthetic (the Cicero EM does this automatically when a Vapor vaporizer is attached) and the breathing system does not contain any other volatile anaesthetic agents.      

Minimum fresh gas flow requirements of anaesthetic breathing systems during spontaneous ventilation: a graphical approach

A general solution is presented to the problem of finding the minimum fresh gas flow requirements, during spontaneous ventilation, of anaesthetic breathing systems in the Mapleson classification. The solution is applicable to any pattern of breathing, dead space volume and tidal volume. The method is graphical and its use and understanding require no mathematical skills. However, if an analytical form of the respiratory waveform is known, the method is easily extended by use of calculus to obtain a precise analytical solution.     

The Bain anaesthetic circuit.

The Bain co-axial circuit is a recent and versatile addition to the semiclosed anaesthetic breathing systems. The relationship between the patient's arterial carbon dioxide tension (PaCO2) and fresh gas flow during intermittent positive pressure ventilation (IPPV) using this circuit has been reassessed. A mean PaCO2 of 33,4 mmHg for 64 patients was recorded using a fresh gas flow of 100 ml/kg/min and a mean PaCO2 of 37,3 mmHg for 55 patients using a fresh gas flow off 70 ml/kg/min.     

A low flow open circle system for anaesthesia Part 2: Clinical evaluation

A prototype valveless ventilator was attached by open deadspace tubing to a circle system and used to ventilate the lungs of 12 patients with low flows of anaesthetic gases for periods between 60 and 120 minutes during intra-abdominal surgery. Anaesthesia was induced with thiopentone and maintained with nitrous oxide 50% in oxygen and enflurane. This was reduced to 2 litres/minute after a 10-minute period of nitrogen wash out and stabilisation of anaesthetic gas concentration, with an initial anaesthetic gas flow of 6 litres/minute. The concentration of oxygen, carbon dioxide, nitrous oxide and enflurane were measured in the outflow from both the anaesthetic machine and the inspiratory limb of the circle system. The measured mean inspired oxygen and nitrous oxide concentrations showed no significant variation throughout the low flow period of the study. This new low flow open circle ventilation system appears to offer some advantages in terms of safety and versatility over other systems which are discussed.     

Calculation of end–tidal carbon dioxide fractions in the Bain system

The validity of the Stenqvist-Sonander formula for calculating the end-expiratory fraction of carbon dioxide (FACO2) in the coaxial Mapleson D (Bain) systems was evaluated using a lung model for simulated spontaneous breathing with an optional respiratory wave form. Two different respiratory flow patterns were used, one representing relaxed breathing in a volunteer and one resembling the respiration found in halothane anaesthesia. Each pattern was used with five different fresh gas flows and three different respiratory rates. The formula was found to be quite accurate when the flow pattern of an awake volunteer was simulated, but it underestimated the observed FETCO2 value by about 10% in halothane breathing. It is concluded that the formula can be recommended for use in theoretical and educational situations but that it is too complicated for application in clinical practice.     

Modified parallel ''Lack'' breathing system for use in dental anaesthesia

A twin-tube breathing system for inhalational anaesthesia in dental surgery is described. The system is a modification of a parallel Mapleson 'A' breathing system and is suitable for use with continuous flow anaesthetic machines. Resistance to airflow has been evaluated and is within the recommended ranges at all flows likely to be experienced in normal clinical conditions. The system is suitable for children and adults, easy to use and efficient. The expiratory valve is located remote from the face and the system is suitable for scavenging by active, assisted or passive systems.     

Breathing systems

Breathing systems are the fundamental components that couple the patient's respiratory system to the anaesthetic machine, and enable the intermittent respiratory pump to be fed by a continuous flow of gas. although often consisting of only a few simple components, the correct functioning of these components is vital to the safe conduct of anaesthesia. Valve-based breathing systems have been the mainstay of adult anaesthetic practice for many years, whilst, historically, non-valved systems have been preferred in paediatric practice because of their lower imposed respiratory load. Various classifications of breathing systems are discussed, although that proposed more than 50 years ago by Mapleson remains the preferred choice. Whilst 'rebreathing' is often seen as abad thing, some breathing systems preferentially allow the recycling of alveolar dead-space gas that has already been warmed and humidified and can hardly be considered to be undesirable. the use of 'circle-type' breathing systems is increasingly supplanting the use of more traditional breathing systems for the maintenance of anaesthesia because of their reduced environmental pollution and much greater economy.     

Funktionsweise des „Anaesthetic Conserving Device“

The new anaesthetic conserving device (ACD) allows the use of isoflurane and sevoflurane without classical anaesthesia workstations. Volatile anaesthetic exhaled by the patient is absorbed by a reflector and released to the patient during the next inspiration. Liquid anaesthetic is delivered via a syringe pump. Currently the use of the ACD is spreading among European intensive care units (ICU). This article focuses on the functioning of the device and on particularities which are important to consider. The ACD constantly reflects 90% of the exhaled anaesthetic back to the patient, but if one exhaled breath contains more than 10 ml of anaesthetic vapour (e.g. >1 vol% in 1,000 ml), the capacity of the reflector will be exceeded and relatively more anaesthetic will be lost to the patient. This spill over decreases efficiency but it also contributes to safety as very high concentrations are averted. Compared to classical anaesthesia systems the ACD used in conjunction with ICU ventilators offers advantages in the ICU setting: investment costs are low, carbon dioxide absorbent is not needed, breathing comfort is higher, anaesthetic consumption is low (equal to an anaesthesia circuit with a fresh gas flow of approximately 1 l/min) and anaesthetic concentrations can be controlled very quickly (increased by small boluses and decreased by removal of the ACD). On the other hand, case costs are higher (single patient use) and a dead space of 100 ml is added. There are pitfalls: by a process called auto-pumping, expansion of bubbles inside the syringe may lead to uncontrolled anaesthetic delivery. Auto-pumping is provoked by high positioning of the syringe pump, heat and prior cooling of the liquid anaesthetic. Inherent to the device is an early inspiratory concentration peak and an end-inspiratory dip which may mislead commonly used gas monitors. Workplace concentrations can be minimized by proper handling, a sufficient turnover of room air is important and gas from the expiration port of the ventilator should be scavenged. Inhalational compared to intravenous ICU sedation offers the advantages of better control of the sedation level, online drug monitoring, no accumulation in patients with renal or hepatic insufficiency and bronchodilation. With a lowered opioid dose spontaneous breathing and intestinal motility are well preserved. A clinical algorithm for the care of patients with respiratory insufficiency including inhalational sedation is proposed. Inhalational sedation with isoflurane has been widely used for more than 20 years in many countries and even for periods of up to several weeks. In the German S3 guidelines for the management of analgesia, sedation and delirium in intensive care (Martin et al. 2010), inhalational sedation is mentioned as an alternative sedation method for patients ventilated via an endotracheal tube or a tracheal cannula. Nevertheless, isoflurane is not officially licensed for ICU sedation and its use is under the responsibility of the prescribing physician.     

The Bain, ADE, and Enclosed Magill breathing systems A comparative study during controlled ventilation

The Enclosed Magill, Humphrey ADE and the Bain breathing systems are all used for controlled ventilation of the lungs. This study compares the three systems in vitro with a lung model and in clinical practice. No difference was observed, with ventilatory variables commonly used in clinical practice, between the Bain and the ADE, while significantly lower end-tidal carbon dioxide values were observed with the Enclosed Magill (about 7%). Lower fresh gas flows can be used under these circumstances to maintain normocapnia with the Enclosed Magill than either the Bain or the Humphrey ADE.     

Sedierung mit volatilen Anästhetika auf der Intensivstation

The use of volatile anaesthetics in intensive care medicine has so far been limited by the lack of equipment suitable for daily routine use and the need for an anaesthetic machine. The new Anaesthetic Conserving Device (AnaConDa) enables the routine use of volatile anaesthetics for long-term sedation via intensive care ventilators. The Anaesthetic Conserving Device replaces the common heat and moisture exchanger in the ventilation circuit. The volatile anaesthetic is continuously applied in liquid status via a syringe pump to a form of mini-vaporiser where the anaesthetic agent is vaporised. The expired anaesthetic gas is stored in the carbon filter and approximately 90% of the gas is resupplied into the breathing cycle. The current experiences suggest that volatile anaesthetics present an alternative for long-term sedation in intensive care units, providing optimised pathways, from a medical as well as from an economical point of view. It must, however, be emphasized that the use of volatile anaesthetics for longer periods of time is an off-label use and should only undertaken by medical professionals at their own risk.     

Carbon dioxide—a survey of its use in anaesthesia in the UK

A random postal survey of 1528 anaesthetists in the UK was performed to assess their use of carbon dioxide in anaesthesia and opinion on its safety. Of 1100 replies (72% response rate), 60.9% used it daily and 77.1% would object to its exclusion from future anaesthetic machines. There was no consistent age-related trend with regard to its use or opinion on its removal. 62.9% of anaesthetists did not regard its presence on the anaesthetic machine as hazardous, but 81.8% agreed that a limit to the maximum flow of carbon dioxide delivered to one litre/minute would improve safety.     

Effects of a heat and moisture exchanger on carbon dioxide equilibrium during mechanical ventilation with the Bain circuit

The introduction of a heat and moisture exchanger (HME) into the anaesthetic circuit may cause a rise in carbon dioxide (CO2) tension through an increase in dead space. We studied the effects of the Ultipor Pall BB50 filter included 'in series' in the Bain circuit on CO2 equilibrium. Arterial carbon dioxide tension (PaCO2) was measured in 81 patients scheduled for elective surgery before and after the insertion of the filter. Results showed that: females were always more hyperventilated than males when fresh gas flow was set at 70 ml kg-1 ideal body weight; the inclusion of the filter increased the PaCO2 in the group as a whole (the difference was statistically, but not clinically, significant); PaCO2 increased after the application of the filter only in females; the effects of the filter were completely independent of the patient's age. It is concluded that the use of the Ultipor Pall BB50 filter is a safe procedure during mechanical ventilation with the Bain breathing system and there is no need to modify ventilation.