Rebreathing and coaxial circuits: A comparison of the bain and mera F

This study compares the rebreathing characteristics of the Bain modification of the Mapleson ‘D’ type of T-piece circuit with those of the Mera F system which is used with the standard “circle” anaesthetic machine. Six healthy adults anaesthetized with halothane were studied breathing spontaneously. The volume of inspired carbon dioxide was measured on each breath as a measure of rebreathing. The tidal volume (Vt) frequency of respiration (f) and blood Pco₂ were also noted. These measurements were made initially with either the BAIN or the Mera F system and then changed to the alternate circuit for further studies. All measurements were made with a fresh gas flow rate (FGF) of 100ml · kg^-1· min^-1 which is recommended with the Bain system. The inspired volume of carbon dioxide (rebreathing) with the Bain system was significantly greater than when the mera F was used. Although the mean blood Pco₂ was not significantly lower when the mera F was used, some patients who cannot adequately compensate for this inspired carbon dioxide volume did become hypercapneic (maximum 8kPa [60torr]). This hypercapnia could be reduced by using a mera F system. The mera F is a co-axial system that combines the convenience of the tube-in-a-tube structure with the beneficial effects of controlled rebreathing during controlled ventilation. In these advantages it is no different from the Bain system. The mera F however, has the advantage of being adaptable to the commonly used “circle” anaesthetic machines for spontaneous respiration in adults. This eliminates the rebreathing of carbon dioxide at a fresh gas flow of 100ml · kg^-1· min^-1, which occurs in adults during spontaneous respiration. The only disadvantage of the mera F system that we used in adults was its length (90cm). However, from a functional viewpoint, it can be lengthened without altering the rebreathing characteristics of the system.     

Arterial Carbon Dioxide Tensions during Anaesthesia with Manual Ventilation A Descriptive Study of the Effects of Various Non-Polluting Circuits

In 660 supine, intubated and anaesthetized, healthy patients scheduled for various elective surgical procedures, the distribution of arterial carbon dioxide tension (PaCO2) was investigated during manual non-monitored ventilation. The study comprised six equal groups: group 1: ventilation with a circle circuit absorber system; group 2: ventilation with the Hafnia A circuit using a total fresh gas flow (FGF) of 100 ml . kg-1 . min-1; groups 3-6: ventilation with a Hafnia D circuit with fresh gas flows of 100, 80, 70 and 60 ml . kg-1 . min-1, respectively. The mean PaCO2's of the first three groups were situated in the lower range of normocapnia (the observations in the first group having the greatest total range), whereas the rebreathing (Hafnia A and D) circuits resulted in a clustering of observed data. Employing the rebreathing circuits, protection against hypocapnia can be achieved by lowering the fresh gas flow. The most satisfying result was obtained with the Hafnia D circuit with a fresh gas flow of 70 ml . kg-1 . min-1 resulting in normocapnia with a modest and limited spread towards hypo- and hypercapnia. FGF in excess of this level must be considered as wasted. The study indicates that corrections of fresh gas flows for age are superfluous. Use of relaxants and type of surgery had no influence on the observations.     

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.     

A comparison between the Bain and Magill anaesthetic systems during spontaneous breathing

The efficiency of the Bain system has been compared with that of the Magill system in ten conscious subjects breathing spontaneously. Air was supplied at fresh flow rates of 150 ml/kg and decreased stepwise at four-minute intervals until a flow of 50 ml/kg was attained. Expired minute volume and end-tidal carbon dioxide concentrations were measured. No rebreathing could be demonstrated with the Magill stystem at flow rates above approximately 70 ml/kg. In contrast, rebreathing was evident at all flow rates with the Bain system. It is concluded that acceptable carbon dioxide levels during spontaneous breathing with the Bain circuit can only be maintained by considerable active hyperventilation when using flow rates of 150 ml/kg and less.     

Effects of Minute Volume Increases on the Rebreathing Characteristics of the Bain Anaesthesia Circuit During Controlled Ventilation

In the course of a study on the carbon dioxide rebreathing characteristics of the Bain anaesthesia circuit, it was noted that raising the minute volume without changing the fresh gas inflow invariably led to increased rebreathing of expired gases. Altering tidal volume and rate in order to reproduce a given minute volume had the same effect on rebreathing. A nomogram constructed to quantitate increases in rebreathing in function of carbon dioxide production per minute, fresh gas flow from the anaesthesia machine, and minute volume is produced. It can be used to assess the amount of fresh gas flow necessary to mantain a steady and inspired gas composition.     

The Ruben circle anaesthesia System

The Ruben circle anaesthesia system was studied in the spontaneous breathing mode, and under certain conditions there was incompetence of the replaceable mushroom control valve. This causes a reverse flow of gas which results in rebreathing of expired gas when the system is used on spontaneously breathing patients. A patient simulator was used to investigate the way in which the reverse flow of gas depends on ventilatory parameters and fresh gas flow. The inspired concentration of carbon dioxide increased for increased fresh gas flow and for decreased tidal volume. These results were confirmed by observations on anaesthetised adult and paediatric patients during spontaneous breathing. We conclude that the system in its present form, is not suitable for use on spontaneously breathing paediatric patients.     

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.     

Controlled Ventilation Using the Lack Circuit with an Injector Device

A Lack breathing system with an injector device has been used for controlled ventilation. Oxygen was delivered through an injector device, which was positioned between the Lack circuit and the catheter mount, whilst nitrous oxide was delivered from the anaesthetic machine. The circuit was fully tested on two model lungs. Satisfactory tidal volumes were obtained and with fresh gas flow rates of about 4 l X min-1, rebreathing was not detected. The circuit was then studied on ten patients undergoing surgery and it was found that an end-tidal carbon dioxide concentration of 5-6% could be maintained. The use of this system during general anaesthesia is discussed.     

Respiratory waveform and rebreathing in T-piece circuits: a comparison of enflurane and halothane waveforms

The effects of respiratory waveform on rebreathing in a modified Mapleson D circuit were studied in 18 healthy adult patients anesthetized with either enflurane or halothane. At high fresh gas flow (FGF) rate, when no rebreathing of CO2 occurred, the duration of inspiration (Ti) with enflurane was 41 per cent greater than that with halothane. With enflurane there was a characteristic long end-expiratory pause, 0.69 s, whereas with halothane it was only 0.196 s. The mean inspiratory flow rate (Vt/Ti) was higher (224 ml/s) when halothane was used than with enflurane (187 ml/s). When the FGF rate was reduced to 100 ml/kg/min in the modified Mapleson D circuit, patients breathing halothane had increases in minute volumes (VE) in response to increases of 53-75 per cent in inspired volumes of CO2. The increases in VE resulted fro increases in Vt/Ti of 34-38 per cent. The volume of CO2 inspired when enflurane was used did not increase until FGF rate was as low as 70 ml/kg/min. The reduced rebreathing was related to the respiratory waveform. The advantage of reduced rebreathing with enflurane is counter-balanced by the more profound respiratory depression it causes. The FGF needed to abolish rebreathing of CO2 is highly variable, and is dependent on respiratory waveform.     

Evaluation of the Humphrey A.D.E. breathing system

A new breathing circuit (the Humphrey A.D.E., double lever model) was evaluated in adults to determine (1) the fresh gas flow (FGF) needed to achieve normocapnia during controlled ventilation and to just induce rebreathing during spontaneous ventilation, (2) end-expired CO2 (PECO2) at those FGF values, (3) the standard deviation of FGF requirements for controlled and spontaneous breathing (reliability of recommended FGF settings) and (4) the magnitude of change in PECO2 produced by varying FGF from the recommended values (sensitivity of the system). The FGFs that provided normocapnia with controlled ventilation and just induced rebreathing with spontaneous ventilation were 67 +/- 10 and 52 +/- 7 ml . kg-1 . min-1 (mean +/- SD), respectively. PECO2 values were 36.0 +/- 0.3 and 41.6 +/- 3.9 mmHg respectively. During controlled ventilation low reliability was offset by low sensitivity so that PECO2 changed little when FGF was raised or lowered from recommended values (0.2 mmHg/ml . kg-1 . min-1). In contrast, during spontaneous ventilation low reliability was additive with high sensitivity when using FGFs lower than the mean value that just induced rebreathing. A threshold was reached where lowering FGF from recommended values caused large changes in PECO2 (1.1 mmHg/ml . kg-1 . min-1). It is concluded that the FGF recommended by Humphrey for controlled ventilation is satisfactory. However, the FGF recommended by Humphrey for spontaneous ventilation may result in hypercapnia in some patients. This can be prevented either by using a higher FGF of 66 ml . kg-1 . min-1 routinely in all patients or by using lower flows with CO2 monitoring.     

Does fresh gas flow rate affect monitoring requirements?

Opinions vary on the monitoring requirements associated with low flow to closed circuit anesthesia. Fresh gas flow rate affects variables of anesthesia ventilation such as the time constant of the breathing system, the inspired concentrations of O2, N2O and anesthetic vapor and the potential for rebreathing. Furthermore, very low flow rates challenge the performance of rotameters and vaporizers. Consequently, the safe conduct of minimal flow or closed circuit anesthesia mandates oximetry, which should be redundant; the use of anesthetic agent monitors ("anesthetico-meters") is extremely helpful, and so is capnometry. However, none of these safety monitors is beyond the scope of the "essential requirements" proposed for anesthesia workstations by international standard-writing groups, such as CEN or ISO. It may hence be concluded that fresh gas flow rate does affect variables to be monitored, but it does not affect essential monitoring requirements.     

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.     

A method for producing normocarbia during general anasthesia for Caesarean section

Twenty-six patients were anaesthetised for Caesarean section using the Bain anaesthetic system for intermittent positive pressure ventilation. There was an inverse relationship between maximum end tidal carbon dioxide tension and the fresh gas flow (FGF) to the system. A significant difference existed between the patients receiving 80 ml/kg/min FGF and those receiving 120 ml/kg/min. Estimated carbon dioxide levels in the pregnant term patient were higher at each FGF rate than the levels reported in non-pregnant patients by other workers. In order to maintain maternal arterial carbon dioxide tension at or close to the normally quoted term value of 4.1-4.4 kPa, when using positive pressure ventilation with a Bain system, a fresh gas flow rate of at least 120 ml/kg body weight/minute is required.     

Hypercapnia due to rupture of the unidirectional valve in the inspiratory limb of the breathing system after induction of general anesthesia--a case report

Malfunction of either inspiratory or expiratory check valve in a breathing circuit system may allow carbon dioxide (CO2) rebreathing and result in hypercapnia. The subsequent increase of PaCO2 may entail increased sympathetic activity which in turn causes serious problems such as tachyarrhythmia and myocardial ischemia, particularly in patients who have history of coronary artery disease (CAD). Here, we report an incident of rupture of the inspiratory valve in the breathing circuit which happened to a patient during induction of general anesthesia and eventuated in markedly heightened end-tidal CO2 (EtCO2) of the patient. The recognition, related complications and management of the inspiratory valve malfunction are discussed.     

Flow requirements for the Bain breathing circuit during anaesthesia for caesarean section

We studied the relationship between arterial carbon dioxide tension (PaCO2) and fresh gas flow (FGF) during use of the Bain breathing circuit for Caesarean section anaesthesia. Thirty-one patients undergoing Caesarean section were anaesthetised using the Bain circuit with intermittent positive pressure ventilation. The PaCO2 were measured at FGF of 70 ml X kg-1 X min-1, 80 ml X kg-1 X min-1, and 100 ml X kg-1 X min-1. The FGF requirement to maintain a given PaCO2 during Caesarean section anaesthesia is the same as the requirements for nonpregnant subjects, despite the increase in carbon dioxide production associated with pregnancy. This is probably because the total FGF determined by body weight and given during Caesarean section anaesthesia is 15-20 per cent higher than nonpregnant levels, due to the weight gain associated with pregnancy. A FGF of 100 ml X kg-1 of pregnant weight/min maintains PaCO2 of 4.44 kPa predelivery, which is in the desirable range of PaCO2 during Caesarean section.     

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.     

An interchangeable Mapleson A-E breathing system is practical and cost effective

BACKGROUND: In locations where oxygen and anesthesia gas supplies are limited, and where circle systems are not practical, means to reduce fresh gas flow during maintenance of inhalational anesthesia are of potential value. We investigated whether a common transport breathing apparatus could be modified to allow interchange between Mapleson D (Map-D) and Mapleson A (Map A) configurations. METHODS: A common Map-D transport system was converted to a Map-A system by switching positions of the exhaust valve and the elbow connector where fresh gas is delivered; these two breathing systems were compared in this study. The key question was whether rebreathing of CO2 could be eliminated at a lower fresh gas flow rate (FGF) with the Map-A design. A structured protocol was followed. RESULTS: A mean decrease in FGF of 2.8 l/min was seen with the Map-A apparatus when compared with the Map-D (P=0.003). With no significant differences in physiologic or anesthetic variables, FGF/V(E) was significantly lower with the Mapleson A configuration than with the Mapleson D system design (1.1 vs. 1.8; P=0.007). The extent to which FGF could be lowered when switching between Mapleson D and A systems correlated strongly with the patients' respiratory rate while under anesthesia (r=0.45, P<0.01). CONCLUSIONS: Cost and resource savings can be realized through the use of a breathing system modification that achieves appropriate ventilation at lower fresh gas flows.     

Accuracy of prediction of fresh gas flow requirements during spontaneous breathing with the T-piece

In 40 spontaneously breathing children (7.3-47.9 kg) anaesthetized with halothane for minor surgical procedures the fresh gas flow (FGF) at onset of rebreathing (FGFr) was determined and end-tidal CO2 concentration (ETCO2), minute ventilation (VE), tidal volume (VT) and respiratory rates (f) were registered. The accuracy of predicting the FGFr from 2 X VE, 3 X VE and from two formulae (FGF = 15 X kg X f and FGF = 3 X (1000 + 100 X kg) was evaluated. FGFr ranged from 3.5 to 10 l min-1. FGF calculated from 2 X VE was inadequate. Calculations of FGF from 3 X VE and with the two formulae gave an adequate FGF in more than 80% of the children. No serious under-estimations were found. In a few cases FGF level was overestimated by more than 150%. It is suggested that when the theoretical calculation of FGF results in flow rates well over 10 l min-1 an upper flow rate limit of 10 l min-1 may be used in children weighing less than 30 kg since no child required a FGF over this rate.     

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.     

Duration of carbon dioxide absorption by soda lime at low rates of fresh gas flow

STUDY OBJECTIVE: To determine the impact of a low fresh gas flow rate on the duration of carbon dioxide (CO2) absorption by soda lime. DESIGN: Nonclinical, experimental. SETTING: Experimental laboratory. METHODS: In vitro test with Sodasorb and a semiclosed breathing circle ventilating a test lung with a CO2 inflow of 250 ml per minute. Fresh gas flow rates of 0.25, 0.5, 1, 2, and 4 L/min were studied. MEASUREMENTS AND MAIN RESULTS: CO2 was measured at the breathing circuit test lung interface with a mainstream capnometer. Duration of CO2 absorption was determined as the time for the inspired CO2 tension (PICO2) to increase from 0 mm to 7 mm of mercury. The times of this interval were recorded four times for each fresh gas flow rate and compared by analysis of variance; p less than 0.05 was considered significant. Time to soda lime failure was significantly longer at 2 L/min than at 1 L/min fresh gas flow and at 1 L/min than at 0.25 L/min fresh gas flow. CONCLUSION: Because soda lime color indicators are unreliable, when a semiclosed breathing circle is used at a low rate of fresh gas flow without CO2 monitoring, the CO2 absorbent must be replaced more frequently.