Bain System Research Articles

ENCLOSED AFFERENT RESERVOIR BREATHING SYSTEMS: Description and Clinical Evaluation

A new group of breathing systems, namely the Enclosed Afferent Reservoir (EAR) systems, is described. They allow for the selective elimination of alveolar gas in association with both spontaneous and controlled ventilation. A comparison with the Bain system in controlled ventilation demonstrates greater efficiency in eliminating carbon dioxide. A fresh gas flow (VF) of 70 ml kg-1 min-1 using an EAR system gave mild hypocarbia which equated to a VF of 100 ml kg-1 min-1 using the Bain system. Smaller minute volumes of ventilation are required for optimal performance than with the Bain system. The minimum recommended minute volume of ventilation (VI) should equal VF plus anatomical deadspace ventilation (VDanat). The pattern of ventilation appears to have little influence upon the efficiency of carbon dioxide elimination when using an EAR system, whereas the Bain system does appear to be affected.

Echanges gazeux per- et postopératoires : hyperventilation isocapnique versus normoventilation

The effects of isocapnic hyperventilation (A) and normoventilation (B) on Pa co 2, Pa o 2 and a-aD o 2 were compared in 102 patients undergoing elective surgery, randomized into two comparable groups A and B. Cases for thoracic, high abdominal and intracranial surgery were excluded, as well as patients with clinically evident pulmonary pathology. A volumetric ventilator was used in association with three different breathing systems (A : Bain system and circle system without CO 2 absorption; B : circle system with CO 2 absorption). The groups were comparable, except for percentage of overweight : 75 % in group A and 56 % in group B. Overweight was defined as weight above the mean ideal weight of 100 %, and obesity as a weight above 120 % the mean ideal weight. Blood gases were sampled 1) preoperatively, 2) 15 min and 3) 60 min after the beginning of mechanical ventilation, 4) postoperatively, 90 min after extubation, without supplemental oxygen. The preoperative mean Pa o 2 values were 79±11.4 mmHg (A) and 82.5±13.2 mmHg (B); the Pa co 2 were 37.2±3.7 mmHg (A) and 37.2±3.8 mmHg (B). During surgery, Pa o 2 was distinctly higher (p <0.01) in group A than in group B (on average 15–20 mmHg higher), indicating the favourable effect of great tidal volumes on gas exchange. Correspondingly, the a-aD o 2 was less increased in group A than in group B (p <0.01). At 15 min, 33 % of the patients were hypocapnic (Pa co 2 <35 mmHg) : one case in group A and three in group B could be classed as severe hypocapnia (Pa co 2 = 25–30 mmHg). Hypocapnia tended to become more pronounced with time in group A but not in group B (p <0.05); this is explained by the higher proportion of overweights in group A. The postoperative Pa co 2 and Pa o 2 were comparable in both groups, and showed no significant difference with the preoperative values. The larger tidal volumes did not seem to have any residual protective effect on the postoperative Pa o 2. There were 25 cases of postoperative hypoxaemia (Pa o 2 <65 mmHg), without a significant difference between the two groups; they were significantly (p <0.05) related to overweight, even of moderate importance. There was no relationship with age greater than 60 years or with smoking. No patient suffered pulmonary complications during the four days following surgery. The effects of isocapnic hyperventilation on the Pa o 2 and the a-aD o 2, obvious during the operative period, did not persist in the early postoperative period (90 min after extubation). In view of certain disadvantages of this ventilation technique (high costs, environmental pollution, risks of barotrauma and of cardiovascular instability), it is recommended that controlled ventilation with physiological tidal volumes using a system with CO 2 absorption be used in patients undergoing elective surgery who do not have any pulmonary pathology associated with an increased a-aD o 2. Another relative indication for the Bain circuit is surgery, where the limited bulk of this system is useful (mainly head and neck surgery).

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 CO 2. The classic Bain circuit causes the most rebreathing as measured by the fractional concentration of CO 2 in a sample of inspired gas (Fi CO 2 ), but stimulation of the respiratory system seems to allow near normal end tidal CO 2 and arterial CO 2 tensions. The most predictable systems that have the least amount of rebreathing are those that have the least dead space.

Capnography and the Bain circuit I: A computer model.

The Mapleson D anesthesia breathing system has no valves and allows rebreathing of carbon dioxide. Its coaxial version is known as the Bain system. The interpretation of capnograms obtained during its use requires an understanding of the interrelationships of patient and system variables. Toward that end, a systematic description of mechanical ventilation with the Bain circuit was undertaken based on the physical laws of gas transport. The mathematical formulation of the model contains the relations between pressure, flow, and volume in the tube, alveolar space, and ventilator. The flows, calculated from these relations, are used to determine the CO2 concentrations in the different parts of the model. Two sets of data are used--patient and system. The patient data, used to solve the equations numerically, are lung-thorax compliance, CO2 inflow into alveolar space (CO2 production), functional residual capacity, dead space volume, airway resistance, and respiratory quotient. The ventilation system data comprise the dimensions and volumes of the Bain circuit, ventilator, connectors, and tubes; spill valve pressure; resistances to flow in the individual tube parts; ventilator settings; and fresh-gas flow rates. After incorporation of a volunteer's respiratory variables into the model, capnograms obtained from the model compared well with those obtained from the volunteer. The structure of the model is such that it permits easy introduction or changes of patient and system variables to obtain individual results or model specific circumstances. This flexibility makes it a useful tool for understanding the properties of the Bain circuit under a variety of clinical circumstances. The results may be displayed in a number of different ways.

A Method for Producing Normocarbia During General Anaesthesia for Caesarean Section

Summary Twenty-six patients were anasthetised for Caesarean section using the Bain anasthetic 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 4.1–44 kPa, when using positive pressure ventilation with a Bain system, afresh gas flow rate of at least 120 ml/kg body weight/minute is required.

A method for producing normocarbia during general anaesthesia 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.

Another use for the Bain system

Another use for the Bain system

AN EVALUATION OF REBREATHING WITH THE BAIN SYSTEM DURING ANAESTHESIA WITH SPONTANEOUS VENTILATION

Data from 12 anaesthetized patients breathing spontaneously from the Bain system were used to calculate the degree of rebreathing occurring when the fresh gas flowrate (VF) was equal to 2, 1 and 0.7 times the estimated normal minute ventilation (Vtot). Measurements of the expired minute volume (VE) and end-tidal carbon dioxide tension (PE'CO2) were made to determine the effects of this rebreathing. No rebreathing occurred when VF was equal to twice Vtot. When VF was equal to Vtot rebreathing was usually small in amount and produced no changes in VE or PE'CO2. Changes attributable to rebreathing occurred in only two patients when VF was reduced to 0.7 Vtot. These results are explained by the presence of anaesthesia-induced ventilatory depression and favourable changes occurring in the respiratory wave forms in the majority of patients studied. In some patients, greater values of VE and rebreathing occurred in response to strong surgical stimulation. The net result of increased ventilation in these patients was a decrease in PE'CO2. It is concluded that during anaesthesia, when the Bain system is used with VF equal to Vtot, any increase in PE'CO2 which may result from rebreathing is likely to be small and seldom of clinical importance.

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.

CLINICAL COMPARISON OF THE BAIN AND MAGILL ANAESTHETIC SYSTEMS DURING SPONTANEOUS RESPIRATION

The Bain and Magill anaesthetic breathing systems were compared for spontaneous breathing during nitrous oxide in oxygen and halothane anaesthesia. A mean fresh gas flow (VF) of 150 ml kg-1 min-1 (SD +/- 30, range 106-250) was required with the Bain system to prevent rebreathing sufficient to cause respiratory stimulation; mean fresh gas flow/expired minute volume (VF/VE) was 1.49 (SD +/- 0.32, range 0.86-2.17). Equivalent figures for the Magill attachment were a mean VF of 82 ml kg-1 min-1 (SD +/- 19, range 43-125), while mean VF/VE was 0.76, (SD +/- 0.19, range 0.38-1.23, P less than 0.001). The results attest the efficiency of the Magill attachment in terms of gas economics, and indicate the very high flows required to avoid respiratory stimulation in some subjects when the Bain system is used.

The Bain and Lack breathing systems

The Bain and Lack breathing systems

Low frequency ventilation. The Bain breathing circuit with the Automatic-Vent and the Minivent.

SummaryTwo miniature ventilators, the ‘Automatic‐Vent’ and the ‘Minivent’, were used for controlled ventilation by intermittent occlusion of the outflow from the Bain breathing system. They were attached in a reverse position to that of their original use, so that the fresh gas inflow inflated the lungs up to the critical pressure of the magnetic valve. This resulted in low frequency ventilation with a long gradual inflation, a much shorter deflation, and no rebreathing.The combination of the Bain system and the miniature ventilators was examined in a simple lung model. The minute ventilation had an upper limit of about 5·5 litres/minute which could no longer be increased with fresh gas flows above 8·0 litres/minute due to the design of the ventilators. Changes in compliance or inflation pressure altered the frequency or the I:E ratio, but not the minute ventilation.Twenty‐eight patients were ventilated from 30 to 90 minutes with this system during routine anaesthesia, using fresh gas volumes usually recommended for the Bain system and inflation pressures between 15 and 20 cmH2O. A mean (± SD) fresh gas volume of 74 ± 4 ml/kg/minute resulted in a frequency of 6 ± litres/minute at an I:E ratio between 3:1 and 4:1. The mean PaCO2 was 4·92·0·67 kPa. The PaO2 ranged between 14·7 and 17·9 kPa with an inspired O2 of about 28%. No adverse cardiovascular effects were observed. Low frequency ventilation with an I: E ratio of up to 4:1 can provide adequate CO2 elimination and oxygenation. The combination of Bain circuit and magnetic vents could be used with caution as an economical ventilating system during anaesthesia.

Effect of inspiration-expiration ratio on rebreathing with the Mapleson D system (Bain's modification; coaxial system).

Rebreathing in the Mapleson D system with a varying fresh gas flow (FGF) was measured during an experimental set-up, which consisted of a compliance lung delivering the exhaled gases with the same, fixed concentration throughout the experiment, a commercially available Bain's system and a respirator. While changes in compliance, airway resistance an dead space hardly influenced the degree of rebreathing, changes in ventilatory pattern did so when the length of the expiratory pause was changed. Thus, with the Vt used which was not essentially greater than the capacity of the corrugated tube, a sufficiently long-lasting expiratory pause made it possible to establish non-rebreathing conditions with a FGF only slightly greater than the respiratory minute volume. Obviously, a short or no expiratory pause requires that the FGF rate be high enough to compensate for the lack of a fresh gas reserve in the corrugated tube at the start of inspiration. In such cases the length of the corrugated tube becomes of minor or no importance.