Spontaneous respiration with the Bain breathing circuit

Impact of gas delivery systems on imaging studies of human cerebral blood flow

Purpose. To compare a semiopen breathing circuit with a non-rebreathing (Hudson mask) for MRI experiments involving gas delivery. Methods and Materials. Cerebral blood flow (CBF) was measured by quantitative phase contrast angiography of the internal carotid and basilar arteries in 18 volunteers (20–31 years). In 8 subjects, gases were delivered via a standard non-rebreathing (Hudson mask). In 10 subjects, gases were delivered using a modified “Mapleson A” semiopen anesthetic gas circuit and mouthpiece. All subjects were given 100% O₂, medical air, and carbogen gas (95% O₂ and 5% CO₂) delivered at 15 L/min in a random order. Results. The Hudson mask group showed significant increases in CBF in response to increased FiCO₂ compared to air (+9.8%). A small nonsignificant reduction in CBF (−2.4%) was seen in response to increased inspired concentrations of oxygen (FiO₂). The Mapleson A group showed significantly larger changes in CBF in response to both increased inspired concentrations of carbon dioxide (FiCO₂) (+32.2%, P < 0.05) and FiO₂ (−14.6%, P < 0.01). Conclusions. The use of an anaesthetic gas delivery circuit avoids entrainment of room air and rebreathing effects that may otherwise adversely affect the experimental results.

Comparison of the Bain system and Uniflow universal anaesthetic breathing systems in spontaneously breathing young pigs

OBJECTIVE

To compare minimum fresh gas flow (V(min)) requirements and respiratory resistance in the Uniflow and Bain anaesthetic breathing systems used in the Mapleson D mode. Animals Seven pigs, aged 8-12 weeks, anaesthetized for ophthalmic surgery.

MATERIALS AND METHODS

Anaesthesia was maintained with halothane delivered in oxygen using a (Mapleson D) Bain breathing system. The V(min) that prevented re-breathing was found, and peak inspiratory (PIP) and peak expiratory (PEP) pressures measured. The fresh gas flow (V(f)) was then increased to V(min) + 50%, then V(min) + 100%, and respiratory pressures re-measured. A heat and moisture exchanger (HME) was inserted at the endotracheal tube and the procedure repeated. The breathing system was then exchanged for a Uniflow and the protocol repeated. After final disconnection from the breathing system, the animals' peak inspiratory and expiratory flows, tidal, and minute volumes (Vm) were measured over five respiratory cycles.

RESULTS

The V(min) (L minute(-1); mL kg(-1) minute(-1)) required to prevent rebreathing in the Uniflow system [8.1(mean) +/-1.7 (SD); 332 +/- 94] was significantly greater than the Bain system (6.5 +/- 1.1; 256 +/- 64). At V(min), PEP with the Uniflow (3.5 +/- 0.1 cm H(2)O) was significantly higher than the Bain system (2 +/- 0.7 cm H(2)O), but PIP values did not differ (Uniflow -0.6 +/- 2.1 cm H(2)O; Bain system -0.2 +/- 0.6 cm H(2)O). With both systems, PEP increased significantly (p < 0.001) with each increase in V(f): Uniflow system 4.2 +/- 0.4 (V(min) + 50%) and 5.5 +/- 0.5 cm H(2)O (V(min) + 100%); Bain system 2.8 +/- 0.7 (V(min) + 50%) and 3.5 +/- 0.7 cm H(2)O (V(min) + 100%). Insertion of the HME did not alter pressures. The mean tidal volume was 6.4 +/- 1.6 mL kg(-1); mean Vm was 184.9 +/- 69.8 mL kg(-1) and mean respiratory rate was 28 +/- 5 breaths minute(-1). In one pig breathing with the Uniflow system PEP rose sharply; respiratory and heart rates increased, and ventricular dysrhythmias occurred. When the system was changed and V(f) reduced, physiological variables became normal.

CONCLUSION

The study discredited the hypothesis that the two breathing systems behave similarly. Values for V(min) and PEP were higher with the Uniflow system. Increasing V(f) increased PEP with both systems. Insertion of an HME did not affect respiratory pressures.

CLINICAL RELEVANCE

The Uniflow used in Mapleson D mode is not suitable for anaesthesia in young spontaneously breathing pigs.

The importance of the expiratory pause. Comparison of the Mapleson A, C and D breathing systems using a lung model

A physical lung model simulating spontaneous respiration was used to investigate the influence of the respiratory pattern on the efficiency of the Mapleson A, C and D breathing systems. It is shown that the Mapleson A system is always the most efficient breathing system and that its performance is relatively independent of the respiratory pattern. When the expiratory pause is minimal, the Mapleson C system is almost as efficient as the Mapleson A, but becomes ever less efficient as the expiratory pause increases. The Mapleson D system is very inefficient when the expiratory pause is short. With a longer expiratory pause, this system's efficiency approaches that of the Mapleson A. The experimental results are compared with predictions generated by a mathematical model. There is good agreement between the two, validating the mathematics used.

A mass balance model for the Mapleson D anaesthesia breathing system

A mathematical model is described which calculates the alveolar concentration of CO2(FACO2) in a patient breathing through a Mapleson D anaesthesia system. The model is derived using a series of mass balances for CO2 in the alveolar space, dead space, breathing system limb volume and reservoir. The variables included in the model are tidal volume (VT), respiratory rate, fresh gas flow rate (Vf), dead space volume, I:E ratio, and expiratory limb volume (Vl) time constant of lung expiration, and carbon dioxide production rate. The model predictions are compared with measurements made using a mechanical lung simulator in both spontaneous and controlled ventilation. Both the model and the experimental data predict that at high fresh gas flow rates and low respiratory rates, FACO2 is independent of Vf; at low fresh gas flow rates and high respiratory rates, FACO2 is independent of respiratory rate. The model and the data show that the VT influences FACO2, independent of minute ventilation alone, during both partial re-breathing and non-rebreathing operation. Therefore, describing the operation in terms of minute ventilation is ambiguous. It is also shown that Vl influences FACO2 such that, for any combination of patient and breathing-system variables, there is a Vl that minimizes the Vf required to maintain FACO2. In addition, expiratory resistance can increase the fresh gas flow rate required to maintain a given FACO2. The respiratory patterns observed with spontaneous and controlled ventilation are responsible for the difference in Vf required with each mode of ventilation.

A comparison of anaesthetic breathing systems during spontaneous ventilation. An in-vitro study using a lung model

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.

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.

Rebreathing and ventilatory response to different fresh gas flows in the Bain and Lack systems. A clinical study

Thirty-four adults were studied during halothane anaesthesia with spontaneous breathing, while undergoing orthopaedic surgery. They were randomly divided into two groups according to whether the Bain (n = 18) or the Lack (n = 16) system was used. Respiratory flows were recorded and arterial blood gases drawn at different fresh gas flows (VF). The values obtained were compared with those recorded under non-rebreathing conditions (NRC). In the Bain system the proportion of rebreathers was 0.22, 0.25, 0.55 and 0.83 when the VF was 175, 150, 125 and 100 ml X min-1 X kg-1 body weight (b.w.), respectively. In the Lack system these proportions were 0.43, 0.55 and 0.92 at VF of 85, 70 and 55 ml X min-1 kg-1 b.w., respectively. The ventilatory response to rebreathing was an increase in minute ventilation (VE), keeping the partial pressure of arterial carbon dioxide (PACO2) almost unaltered. In the Bain system the VE X kg-1 X b.w. thus increased by 18% and 38% at VF of 125 and 100 ml X min-1 X kg-1 b.w., respectively, when compared to NRC (P less than 0.05). The corresponding increases in the Lack system were 15% and 37% at VF of 70 and 55 ml X min-1 X kg-1 b.w., respectively (P less than 0.01). In the Lack group also the PACO2 increased by 6% when a VF of 55 ml X min-1 X kg-1 b.w. was used compared to the value obtained under NRC (P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Single lever Humphrey A.D.E. low flow universal anaesthetic breathing system. Part I: Comparison with dual lever A.D.E., Magill and Bain systems in anaesthetized spontaneously breathing adults

The single lever Humphrey A.D.E. anaesthetic system, in both coaxial and parallel (non-coaxial) forms, has recently been introduced. In principle the system offers efficient "universal" function by combining the advantages of Mapleson A, D and E systems. A within-patient comparison of its function in the Mapleson A mode (lever up) in spontaneously-breathing anaesthetized subjects was made to that of the original two lever A.D.E., the Magill (Mapleson A) and the Bain (Mapleson D) systems. The coaxial and parallel single lever A.D.E. systems functioned identically to each other and to the original two lever A.D.E. system, a mean fresh gas flow (FGF) of 51 ml X kg-1 X min-1 causing minimal rebreathing. Under identical conditions, the mean FGF required to just cause rebreathing increased to a mean of 71 ml X kg-1 X min-1 and 150 ml X kg-1 X min-1 with the Magill and the Bain systems respectively. With the single lever system, the switch to its Mapleson E mode for controlled ventilation involves the selection of the only alternative lever position (lever down) without further adjustment. The function and practical advantages in this E mode are presented in Part II.

Fresh gas flow in coaxial Mapleson A and D circuits during spontaneous breathing

In a lung model simulating spontaneously breathing halothane anaesthesia, the rebreathing characteristics of the coaxial Mapleson A (Lack circuit) and D (Bain circuit) systems were tested. Using decreasing fresh gas flows (VF), the end-tidal carbon dioxide fraction (FACO2) was monitored and the point of rebreathing (R.P.) detected. The effects of changes in minute volume (VE), dead-space to tidal volume ratio (VD/VT) and carbon dioxide elimination (VCO2) were studied. The effect of increased tidal volumes (VT) on FACO2 was investigated for some different fresh gas flows (VF). The VF/VE ratio for R.P. in the Bain circuit was approximately 2 and in the Lack circuit 0.88. In both circuits an increase in VE and a decrease in the VD/VT ratio resulted in higher demands on VF if rebreathing was to be avoided. The latter effect was much more pronounced in the Lack circuit. In neither system did any changes in VCO2 affect the rebreathing characteristics. The conclusion was drawn that the Lack system is a much better choice concerning the fresh gas flows for anaesthesia with spontaneous breathing than the Bain system. It was also concluded that the fresh gas flows recommended by Humphrey for the Lack system (i.e. 51 ml X min-1 X kg b.w.-1) and by the manufacturers for the Bain system (i.e. 100 ml X min-1 X kg b.w.-1) are inadequate and should be increased if a considerable degree of rebreathing is to be avoided.

No moving parts. A modification of a Bain system and a Penlon Nuffield 200 ventilator

Interchange between ventilatory and spontaneously breathing modes using a Bain system and a Penlon Nuffield 200 ventilator may be achieved by the single action of switching the ventilator on or off. No alteration is made to the expiratory valve, reservoir bag, ventilator tubing or scavenging system and there are no levers.

Evaluation of rebreathing in various modifications of the Mapleson-D system

Several modifications of the Bain system for nitrous oxide analgesia or general anesthesia were tested for rebreathing and retention of CO2. The classic Bain circuit causes the most rebreathing as measured by the fractional concentration of CO2 in a sample of inspired gas (FICO2), but stimulation of the respiratory system seems to allow near normal end tidal CO2 and arterial CO2 tensions. The most predictable systems that have the least amount of rebreathing are those that have the least dead space.

The MultiCircuit System. 2. A study during spontaneous ventilation in awake volunteers using the Mapleson A mode

Mapleson's A and D are the most efficient semiclosed nonabsorbing circuits used for spontaneous and controlled ventilation. A new device, the MultiCircuit System, allows single-lever selection of Mapleson A or D functions, when used with the conventional two hoses or Mera F coaxial circuit attached. In the A mode, in a study performed on awake volunteers, the MultiCircuit System with two hoses performed comparably to the Magill attachment. When coupled with the Mera F circuit as a coaxial system, the MultiCircuit System was less efficient than the Magill, and had a resistance to expiration exceeding 2cm H2O pressure at a gas flow of 30 l/minute.

The Lack and Bain systems in spontaneous respiration

In conscious subjects breathing spontaneously the performance of the Lack semi-closed system resembled the Magill attachment in that rebreathing was only detected at fresh gas flows of less than resting minute volume. With the Bain semi-closed system rebreathing was detectable at fresh gas flows of the order of 2.5 times the minute volume. The two co-axial circuits behaved similarly for anaesthetised patients. In adults breathing spontaneously the Lack system is efficient and more economical than the Bain.

The Lack, Magill and Bain Anaesthetic Breathing Systems: A Direct Comparison in Spontaneously-Breathing Anaesthetized Adults

The performances of the Lack (Mapleson A), Magill (Mapleson A) and Bain (Mapleson D) anaesthetic breathing systems were compared in each of 20 anaesthetized adult patients breathing spontaneously with fresh gas flows of 70 ml kg−1 min−1. In every patient the Lack system caused the least rebreathing, as seen by the lowest inspired and end-expired CO2 tensions using capnography. The Magill caused more rebreathing than the Lack though less than the Bain.

Comparative fresh gas flows for each system at the point where rebreathing started to occur were examined in 10 further patients. The Lack system required approximately 51 ml kg−1 min−1, the Magill 72 ml kg−1 min−1 while the Bain required 153 ml kg−1 min−1.

Of the three systems the Lack is concluded to be the most efficient and economical system for spontaneous respiration in adults, with the additional advantages of convenient access to the exhaust valve, easy scavenging and low expiratory resistance. In addition, it also offers many useful advantages over the circle absorber system.

Halothane anaesthesia with spontaneous respiration for tonsillectomy in children

Bains's anaesthetic circuit was used in 22 children undergoing tonsillectomy under halothane anaesthesia with spontaneous respiration. End-tidal CO2 was monitored by capnography. The median maximum end-tidal CO2 was 7%, and during surgery nine patients had an end-tidal CO2 higher than 7%, corresponding to a Paco2 close to 8 kPa when the arterial to end-tidal CO2 difference is taken into consideration. Increase in fresh gas flow or change to a non-rebreathing system had virtually no effect on end-tidal CO2. However, following discontinuation of halothane or during controlled respiration, acceptable values of end-tidal CO2 were reached, leading to the conclusion that respiratory depression was responsible for the high values of end-tidal CO2 rather than properties of Bain's circuit or too low gas flow rates.

Respiratory compensation during spontaneous ventilation with the Bain circuit

The volume of carbon dioxide rebreathed by spontaneously breathing patients under halothane anaesthesia at various fresh gas flow rates (FGF) with the Bain modification of the Mapleson "D" breathing circuit is measured. The effect of rebreathing on a heterogeneous patient population is shown to be unpredictable hypercapnia in those patients who cannot respond adequately to this carbon dioxide challenge. All adults rebreathe significant volumes of carbon dioxide at a FGF rate of 100 ml . kg-1 . min-1. This carbon dioxide load is a potential risk to every patient and this hypercapnia is preventable by using high FGF rates. Rebreathing occurs because the inspired carbon dioxide load is unpredictable in a given patient and the patient's response is uncontrolled. Patients respond to this carbon dioxide challenge by increasing inspiratory flow rate (Vt/Ti), which results in increased rebreathing of carbon dioxide from the expiratory limb of the circuit. To prevent potentially dangerous rebreathing of carbon dioxide in all patients the fresh gas flow rate must be much higher than presently recommended.

The "D circle": closed-circuit operation of the Bain circuit

A method of converting a Mapleson D (Bain) circuit to closed-circuit operation is presented, utilizing a laboratory air pump and a Waters carbon dioxide absorber canister to recirculate exhaled gas. The elimination of carbon dioxide from the circuit was studied and found to be adequate. The circuit would allow the use of low fresh gas flows for the maintenance of anaesthesia without the danger of carbon dioxide rebreathing. We suggest that such a circuit could provide appropriate conditions of gas humidity and temperature for endotracheal anaesthesia, while realizing the advantage of a circulator in mask anaesthesia is possible. Further design considerations for a "D circle" breathing system for clinical use are discussed.