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.     

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.     

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.     

A new concept in controlled ventilation of children with the Bain anesthetic circuit

The Bain anesthesia circuit was studied as a semi-open or partial rebreathing system during controlled ventilation in 16 children weighing from 7.5 to 48 kg. During anesthesia the lungs were ventilated with a volume ventilator set at three times the calculated alveolar ventilation to provide optimum mixing in the exhalation tube of the Bain circuit. Fresh gas inflow rates initially were set equal to the calculated alveolar ventilation, and after 30 to 45 min, PCO2, PO2 and pH values were measured. At the same time, the fractional concentration of mixed expired carbon dioxide (FECO2) was recorded from a capnograph inserted between the ventilator and the Bain circuit. After initial readings, the fresh gas inflow was varied over a range of 1,400-3,000 ml/m2/min at 20-min intervals, with the arterial blood-gas values and FECO2 recorded at each setting. The results indicate that a lower fresh gas inflow than previously recommended can be used safely in children. When the minute ventilation is three times the fresh gas inflow, values for FECO2 correlate closely with PaCO2 values; with a fresh gas inflow of 2,500 ml/m2/min,PaCO2 values can be maintained near 40 torr.     

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.     

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.     

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.     

How rebreathing anaesthetic systems control PaCO2: studies with a mechanical and a mathematical model

Results from a proposed equation for rebreathing systems, (formula: see text), were compared with results from a mechanical model "lung" ventilated either with a Bain Breathing Circuit, or a circle system (Eger-Ethans type A) without soda lime. When values for carbon dioxide excretion (VCO2), dead spacetidal volume ratio (VD/VT), minute volume ventilation (VE), and fresh gas flow (VF) were varied over a wide range, the model and the equation yielded identical values of PaCO2. When VCO2 = 2.25 ml/kg, VD/VT = 0.5, and VE = 140 ml/kg, then fresh gas flows (VF) with both the equation and the model produced values of PaCO2 which were very close to those found by Bain and Spoerel in anaesthetized, artificially ventilated humans. It is concluded that the equation is an accurate mathematical representation of how rebreathing anaesthetic systems control PaCO2. It is expected that the equation will be useful in the clinical application of rebreathing anaesthetic systems, allowing the selection of minute volumes and fresh gas flows which will yield predictable PaCO2 values.     

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.     

AN EVALUATION OF REBREATH1NG IN A MODIFIED T-PIECE SYSTEM DURING CONTROLLED VENTILATION OF ANAESTHETIZED CHILDREN

Measurements of minute volumc, airway pressure, and inspiredand end-expired carbon dioxide concentrations were made on anaesthetizedchildren during the use of controlled ventilation with a modifiedT-piece system. The data obtained indicate that a fresh gasinflow as low as 100 ml/lb. (220 ml/kg) body weight per minute,with a minimum total flow of 3 1./min, is sufficient to preventcarbon dioxide retention. The influence of the rate of ventilationupon end-expired carbon dioxide concentration also was assessed. ^* Present address: The London Hospital London, E.1.     

The sevoflurane saving capacity of a new anaesthetic agent conserving device compared with a low flow circle system

BACKGROUND: An anaesthetic agent conserving device (ACD) has been added to a Bain system to approach the agent-saving capacity of a low flow circle system. METHODS: Randomly selected ASA physical status I patients received a standardized anaesthetic with sevoflurane in air/O2 through either a circle system with 1.5 l/min fresh gas flow (n = 8), or through a Bain system with an added ACD with fresh gas flow 4.4-6.4 l/min (n = 8). A target controlled infusion of remifentanil was used for analgesia. RESULTS: The median sevoflurane consumption was 19.7 and 22.0 ml/MAC/h with the low flow circle system and the Bain system + ACD, respectively (P=0.10, Mann-Whitney U-test), or when adjusted for weight 0.25 and 0.28 ml/MAC/h/kg (P=0.26, Mann-Whitney U-test). CONCLUSION: The expenditure of sevoflurane with a Bain system + ACD was close to that in a circle system with 1.5 l/min fresh gas flow. It is thereby possible to use sevoflurane to all its potential, performing for example rapid alterations in end-tidal concentration using high fresh gas flows by combining a Bain system with an ACD. Although the price is not decided for this not yet commercially available device, a potential for a lower cost exists. Additionally, there will be no concerns of toxic compounds produced in the absorber.     

A non-rebreathing coaxial anaesthesia system: dependence of end-tidal gas concentrations on fresh gas flow and tidal volume

A non-rebreathing adaptation of the Bain coaxial anaesthesia circuit was developed in Nepal as a simple and economical anaesthetic system for underdeveloped countries. It was made by inserting a coaxial (Bain) tubing between an Ambu-E valve and an Ambu self-inflating bag. The present study examined the dependence of end-tidal gas concentrations on fresh gas flow and tidal volume during halothane/oxygen/air inhalation anaesthesia. Four levels of fresh gas flow with normocapnia (0.2–3 l.min⁻¹) and three levels of tidal volume at a constant respiratory rate of 15 breath.min⁻¹ (to achieve end-tidal carbon dioxide values of 4 ± 0.5%, 5 ± 0.5% and 6 ± 0.5%) were introduced in random order. Twelve ASA class 1 and 2 adult patients having intra-abdominal or pelvic surgery were studied. With increasing fresh gas flow rates, there were proportionate increases in the end-tidal concentrations of oxygen and halothane; with decreasing tidal volume and therefore less air dilution, there were proportionate increases in the end-tidal concentrations of carbon dioxide, oxygen and halothane. Both effects were statistically and clinically significant. Thus, when this system is used as described, the end-tidal concentrations of oxygen and halothane are highly dependent upon both the fresh gas flow and the tidal volume.     

Re-evaluation of the Farman entrainer in a low-pressure system for field anaesthesia

The aim of this project was to develop a portable anaesthesia system that was compatible with modern anaesthesia practice under field conditions, when compressed gas supplies are limited. We assembled and evaluated a low-pressure plenum system, based upon the Farman entrainer, which was adaptable to spontaneous, assisted or intermittent positive pressure ventilation (IPPV). The entrainer was tested using a low flow of compressed gas, O2 at 1-3 L.min-1. We measured the fresh gas flow (FGF) and O2 concentrations (F1O2) delivered at various source gas flow rates (O2 flow), and with various breathing circuits. Entrainment ratio, FGF, and F1O2 were highly dependent upon resistance to flow in the different breathing circuits. With a wide bore T-piece the air/O2 entrainment ratio was 6:1, and the F1O2 was 0.3. When circuit resistance was higher, e.g., with the Bain circuit, air entrainment and FGF were reduced, but F1O2 was higher. Because it offered the lowest resistance, the T-piece circuit was selected for a clinical trial.     

Brain tissue pH, oxygen tension, and carbon dioxide tension in profoundly hypothermic cardiopulmonary bypass. Pulsatile assistance for circulatory arrest, low-flow perfusion, and moderate-flow perfusion

The brain tissue pH, oxygen tension, and carbon dioxide tension were experimentally examined during profoundly hypothermic cardiopulmonary bypass with core cooling and core rewarming. Sixty-minute circulatory arrests (n = 28, group I), 120-minute low-flow perfusions (25 ml/kg/min; n = 16, group II), and 120-minute moderate-flow perfusions (50 ml/kg/min; n = 16, group III) were accomplished with and without pulsatile flow. In group I, progressive brain tissue acidosis and hypercapnia were recovered with pulsatile assistance. In group II, brain tissue acidosis and hypercapnia were recovered completely with pulsatile assistance but incompletely without it. In group III mild acidosis was eliminated with pulsatile assistance where the pH was significantly higher than in groups I and II, and brain tissue carbon dioxide pressure was significantly lower than in groups I and II with and without pulsatile assistance. Brain tissue hypoxia was severe in group I, slight in group II, but not found in group III. We concluded that a perfusion flow rate will decide the safe period, and a pulsatile assistance will promote brain protection at any flow rate in profoundly hypothermic cardiopulmonary bypass.     

Carbon dioxide embolism during laparoscopy: effect of insufflation pressure in pigs.

Carbon dioxide embolism is a rare but potentially devastating complication of laparoscopy. To determine the effects of insufflation pressure on the mortality from carbon dioxide embolism, six swine had intravascular insufflation with carbon dioxide for 30 seconds using a Karl Storz insufflator at a flow rate of 35 mL/kg/min. The initial insufflation pressure was 15 mm Hg. Following recovery from the first embolism, intravascular insufflation using a pressure of 20 mm Hg at the same flow rate was performed in the surviving animals. Significantly less carbon dioxide (8.3 +/- 2.7 versus 16.7 +/- 3.9 mL/kg; p < 0.02) was insufflated intravascularly at 15 mm Hg than at 20 mm Hg pressure. All of the pigs insufflated at 15 mm Hg pressure with a flow rate of 35 mL/kg/min survived. In contrast, 4 of the 5 pigs insufflated at 20 mm Hg pressure died. The surviving pig died when insufflated with 25 mm Hg pressure following an embolism of 15.7 mL/kg. Intravascular injection was often associated with an initial rise in end-tidal carbon dioxide tension, followed by a rapid fall in all cases where the embolism proved fatal. Insufflation should be begun with a low pressure and a slow flow rate to limit the volume of gas embolized in the event of inadvertent venous cannulation. Insufflation should immediately be stopped if a sudden change in end-tidal carbon dioxide tension occurs.     

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.     

Predictable normocapnia [correction of normocapnoea] in controlled ventilation of infants with Jackson Rees or Bain system

We have devised a formula for ventilator settings which provide normal minute ventilation without rebreathing during controlled ventilation using a Jackson Rees or Bain system. As VT = VS + VF- VL, where VT = delivered tidal volume, VS = set tidal volume, VF = the volume of fresh gas entering during the inspiratory phase and VL = the lost volume due to the compliance of the system, VS was derived: VS = VL + VT x [1-b/(1 + a)] where a = expiratory-to-inspiratory ratio and b = the ratio of fresh gas flow to the minute ventilation. It was evaluated in 62 infants. Arterial partial pressure of carbon dioxide (mean (SD)) was 4.6 (0.5) kPa (35 (4) mmHg) with a range of 3.42-5.78 kPa (26-44 mmHg). The 90th percentile was 5.1 kPa (39 mmHg). It is concluded that predictable normocapnia [corrected] can be conveniently achieved in infants in controlled ventilation with Jackson Rees or Bain system if our formula is applied.     

Regional cerebrovascular reactivity to carbon dioxide during cardiopulmonary bypass in patients with cerebrovascular disease

In patients with cerebrovascular disease, hypercarbia may cause redistribution of regional cerebral blood flow from marginally perfused to well-perfused regions (intracerebral steal), as evidenced by regional cerebral blood flow studies during carotid endarterectomy. During hypothermic cardiopulmonary bypass, the pH-stat method of acid-base management produces relative hypercarbia. To determine whether pH-stat management produces relative hypercarbia. To determine whether pH-stat management induces intracerebral steals, we investigated nine patients with cerebrovascular disease undergoing coronary artery bypass grafting. During hypothermic cardiopulmonary bypass, arterial carbon dioxide tension was varied in random order between 40 mm Hg and 60 mm Hg (uncorrected for body temperature). Regional cerebral blood flow was measured by clearance of 133 xenon injected into the arterial inflow cannula. Nasopharyngeal temperature (26.8 degrees-28.0 degrees +/- 2.2 degrees-3.0 degrees C), perfusion flow rate (2.14-2.18 +/- 0.70-0.73 L/min/m2), mean arterial pressure (67-68 +/- 6-9 mm Hg), arterial carbon dioxide tension (302-308 +/- 109-113 mm Hg), and hematocrit (23% +/- 4%) were maintained within narrow limits in each patient during arterial carbon dioxide tension manipulation. Global mean cerebral blood flow values were similar to previously reported values in patients free of cerebrovascular disease; patients in this study averaged 15.2 +/- 2.5 ml/100 gm/min at an arterial carbon dioxide tension of 46.1 +/- 8.4 mm Hg and 25.3 +/- 6.1 ml/100 gm/min at an arterial carbon dioxide tension of 71.1 +/- 11.8 mm Hg. Carbon dioxide reactivity, defined as mean global cerebral blood flow (in ml/100 gm/min) divided by arterial carbon dioxide tension (in mm Hg), was similar in the region having the lowest regional cerebral blood flow and in the brain as a whole. No patient developed evidence of an intracerebral steal at the higher arterial carbon dioxide tension. During hypothermic cardiopulmonary bypass, higher levels of arterial carbon dioxide tension, such as those associated with the pH-stat management technique, are apparently not associated with potentially harmful redistribution of cerebral blood flow in patients with cerebrovascular disease.     

High frequency jet ventilation: use of the Bain system for entrainment

The use of a Bain system to convey anaesthetic gases for entrainment during high frequency jet ventilation (HFJV) was evaluated by examining the effect of varying the fresh gas flow (Vf) on the end-tidal carbon dioxide (PECO2) in 46 ASA physical status I and II patients undergoing extracorporeal shock-wave lithotripsy (ESWL). Anaesthesia was induced with methohexitone (1-2 mg.kg-1), fentanyl (1-1.5 micrograms.kg-1) and vecuronium (0.1 mg.kg-1). After endotracheal intubation with a Mallinckrodt Hi-Lo Jet cuffed endotracheal tube, the patient was immersed in a water bath and HFJV at 150 breaths per minute was instituted with an Acutronic AMS 1000 jet ventilator attached to the side channel of the Hi-Lo tube. A Bain system was attached to the proximal end of the endotracheal tube to provide gases for entrainment. Anaesthesia was maintained with an intravenous infusion of methohexitone (5 mg.kg-1.h-1) and 50% nitrous oxide in oxygen for both the jetted and entrained gases. PECO2 was determined at 5-min intervals by a single-breath technique using a calibrated Engstrom Eliza capnograph. Thirty patients were randomly allocated to receive Vf's of 50 (Group 1), 75 (Group 2) and 100 (Group 3) ml.kg-1.min-1, respectively. A further eight patients (Group 4) received a Vf of 100 ml.kg-1.min-1 for 15 min, 75 ml.kg-1.min-1 for the next 15 min and 50 ml.kg-1.min-1 thereafter. In a further group of eight patients (Group 5), Vf was initially 25 ml.kg-1.min-1 for 10 min and was then switched off for the remainder of the procedure.(ABSTRACT TRUNCATED AT 250 WORDS)