Single lever Humphrey A.D.E.lowflow universal anaesthetic breathing system

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.kg^-1.min^-1 causing minimal rebreathing. Under identical conditions, the mean FGF required to just cause rebreathing increased to a mean of 71 ml.kg^-1.min^-1 and 150 ml.kg^-1.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.     

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.     

Influence of the respiratory flow pattern on rebreathing in Mapleson A and D circuits

In a lung model the rebreathing effects of different respiratory flow patterns (RFP) were studied in the coaxial Mapleson A (Lack) and D (Bain, Coax-II) systems during spontaneous breathing. In the Mapleson A system RFP was not found to have any impact. In the D systems FACO2 was higher with an RFP typical of halothane-anaesthetized patients than with an RFP with an exponentially decreasing expiratory flow and an end-expiratory flow pause (FTEP). The difference in FACO2 was 26% with a VF corresponding to 100 ml X min-1 X kg-1 body weight. The RFP in a non-anaesthetized volunteer was intermediate between these two patterns. Rebreathing decreased in the D systems with prolongation of FTEP and when a decelerating expiratory flow was used.     

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.     

Rebreathing, resistance and external work of breathing in three different coaxial Mapleson D systems

Using a lung model, rebreathing characteristics, resistance against gas flow and the external work of breathing were tested in three different coaxial Mapleson D systems: the Medicvent D system, the Bain original system and the Coax-II system. The rebreathing characteristics were found to be similar in all systems in both spontaneous and controlled ventilation. The Bain system was found to have the lowest resistance and work of breathing and the Coax-II system the highest. The differences were small and clinically insignificant. Both the resistance and the work of breathing increased with fresh gas flow. The resistance against expiration was found to be in the range 135-160 Pa at a total gas flow of 31 1.min-1, which is well within the acceptable level. The resulting end-expiratory pressure was never above 100 Pa (1 cmH2O) in any system. We concluded that there was no clinically significant difference among the three systems despite differences in design. The coaxial Mapleson D systems can also be used safely with high fresh gas flows with regard to resistance and end-expiratory pressures.     

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.     

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.     

Predictable Paco2, with two different flow settings using the Mapleson D system

Two different settings of fresh gas flow (VFG) and minute ventilation (VE) used with the coaxial Mapleson D system (Bain), were evaluated in 59 adults (ASA I-III) during controlled ventilation and different types of surgical procedures. The two flow settings (alternatives A and B) were VFG of 75 and 110 ml.min-1.kg-1 and VE of 150 and 175 ml.min-1.kg-1, aiming to generate normocapnea and mild hypocapnea, respectively. The PaCO2 obtained with alternative A was 5.5 +/- 0.5 kPa (mean +/- s.d.), with 92% of the patients within the range 4.7-6.1 kPa. With alternative B, the PaCO2 was 4.4 +/- 0.5 kPa, with 82% of the patients within the range 3.5-4.9 kPa. It is concluded that these two flow regimes are suitable for clinical use when either normocapnea or mild hypocapnea is desired.     

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.     

An assessment of the Humphrey ADE anaesthetic system in the Mapleson A mode during spontaneous ventilation

The Humphrey ADE anaesthetic breathing system in the Mapleson A mode has been compared with the Magill system in spontaneously breathing conscious volunteers and anaesthetised patients. In the latter, rebreathing occurred at a significantly lower fresh gas flow with the ADE system than when the Magill system was used (mean 45.6 ml/kg/minute and 56.5 ml/kg/minute respectively). There was no significant difference between the fresh gas flow at which rebreathing occurred in conscious volunteers.     

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)     

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.     

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 new co-axial breathing system

A new, simple, versatile co-axial breathing system combining the features of Mapleson A, D and E type systems is described. The change from an A system to a D/E system is effected by a single switch and without reversal of the gas flow. Fresh gas flows in the order of 70 ml/kg/min are required for both spontaneous ventilation in the Mapleson A mode and controlled ventilation in the Mapleson D mode. The co-axial configuration offers the advantages of a single, lightweight breathing system with easy scavenging of anaesthetic gases, while the ability to switch between the A and D or E configurations offers the economic advantages of low fresh gas flows and the need for a single anaesthetic breathing system for all situations.     

Rebreathing in the Mapleson A, C and D breathing systems with sinusoidal and exponential flow waveforms

The degree of rebreathing in Mapleson A, C and D breathing systems for sinusoidal and exponential flow waveforms is analysed mathematically. The effects of altering the I:E ratio and of introducing an expiratory pause are investigated. The results for sinusoidal waveforms closely resemble those for a square wave. Exponential flow waveforms produce results similar to triangular flow waveforms. The Mapleson A system is always the most efficient. The Mapleson C system is efficient when the I:E ratio is 1:1, becoming less efficient with longer expiration and very inefficient with an expiratory pause. The Mapleson D system becomes efficient when the expiratory pause is long.     

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.     

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.     

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.     

Preoxygenation with the Mapleson D system requires higher oxygen flows than Mapleson A or circle systems

PURPOSE: This study investigates the efficacy of preoxygenation with Mapleson A and Mapleson D breathing systems vs the circle system with CO2 absorber.Methods: Thirteen healthy volunteers underwent tidal volume breathing for three minutes via facemask using Mapleson A, Mapleson D breathing systems or the circle system with CO2 absorber while breathing 100% O2 at flow rates of 5 L.min-1 and 10 L.min-1. Each volunteer acted as his/her own control by going through each of six preoxygenation protocols in random order. Fractional end-tidal O2 concentration (FETO2) was measured at 30-sec intervals. The results were compared among the three anesthesia systems at the two fresh gas flow rates. RESULTS: At a fresh gas flow rate of 5 L.min-1, the Mapleson A and circle systems achieved F(ETO2) values of 90.8+/-1.4% and 90.0+/-1.1%, respectively, compared with the lower F(ETO2) (81.5+/-6.3%, P<0.05), achieved with the Mapleson D system. When breathing O2 at 10 L.min-1, the F(ETO2) values after three minutes were similar with the Mapleson A, circle, and Mapleson D breathing systems (91.8+/-2.3%, 91.2+/-1.7%, 90.6+/-2.7%, respectively). CONCLUSION: When using the Mapleson A and the circle systems for preoxygenation, an oxygen flow rate of 5 L.min-1 can adequately preoxygenate the patient within three minutes, while an oxygen flow of 10 L.min-1 is required to achieve a similar fractional end-tidal O2 concentration with the Mapleson D system.     

Control of Carbon Dioxide in Modified Mapleson A and D (Hafnia) Anaesthetic Systems. An Experimental Model

The effects of varying ventilations (V_e) and fresh gas flows (FGF) on end-expiratory CO₂ (F eco ₂) levels were investigated in an experimental model lung, employing the Hafnia modification of the Mapleson A and D anaesthetic systems during CO₂-absorption and CO₂-wash-out (rebreathing). Identical results were found in both systems: F eco ₂ was constant and independent of FGF with CO₂-absorption and constant V_e, whereas rebreathing resulted in increasing F eco ₂ levels as FGF was decreased. As control of F eco ₂ in the rebreathing systems by regulating FGF could only take place within F eco ₂ levels higher than that determined by V_e at complete CO₂-absorption, e. g. for the Hafnia A and D rebreathing systems, control of FGF necessitates relative hyperventilation. F eco ₂ with constant FGF decreased with increasing V_e during CO₂-absorption, as well as during rebreathing, although this decrease was less in the rebreathing systems. Thus a decrease in F eco ₂ with rising V_e can be avoided and hypocapnia prevented. The results agree with those obtained in clinical studies.