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REBREATHING CHARACTERISTICS OF THE BAIN CIRCUIT An Experimental and Theoretical Study
Br. J. Anaesth. (1984), 56, 303
REBREATHING CHARACTERISTICS OF THE BAIN CIRCUIT An Experimental and Theoretical Study O. STENQVIST AND H. SONANDER SUMMARY
The Bain circuit is a modification of the Mapleson type D system (Mapleson, 1954) and, in this theoretical study, Mapleson stated that rebreathing could be avoided if the fresh gas flow was somewhat greater than twice the total ventilation. However, if the pattern of inspiratory flow was variable, a further increase in fresh gas flow was necessary to avoid rebreathing. In addition, he suggested than an endexpiratory pause caused less rebreathing as fresh gas may accumulate at the patient end of the expiratory limb. Conway, Seeley and Barnes (1977), in a study on awake volunteers, confirmed that rebreathing in the Bain circuit occurred at fresh gas flows below two-and-a-half times the total ventilation. In a clinical study, Byrick and Jansson (1980) found differences in rebreathing between halothane- and enflurane-anae8thetized spontaneously breathing patients with the Bain circuit. They ascribed these differences to differences in the respiratory waveform—a long end-expiratory pause causing less rebreathing in the enflurane group compared with the halothane group. These conclusions were disputed by Keenan and Boyan (1981) who suggested that the differences were caused by different ratios between fresh gas flow rate and total ventilation. Thus, both low fresh gas flows and high minute ventilation would increase rebreathing. Bain and Spoerel (1972), and Spoerel, Aitken and Bain (1978), recommended a fresh gas flow below or equal to the total ventilation during spontaneous OLA STENQVIST, M.D., PH.D.; HANS SONANDER, M.D.; Depart-
ments of Anaesthesia and Intensive Care, Sahlgren's Hospital, University of Gothenburg, S-413 45 Gothenburg, Sweden.
ventilation, accepting a certain degree of rebreathing since, in their studies, the resulting alveolar concentrations of carbon dioxide were found to be within physiological limits. The aim of the present study was to evaluate the influence of respiratory time fractions, fresh gas flow and total ventilation on rebreathing in the Bain circuit during simulated spontaneous ventilation. Symbols are explained in the Appendix. MATERIALS AND METHODS
The investigation was undertaken using the model lung shown in figure 1. This consisted of a metal container with a volume of 2.5 litre and a Blease lung connected in series. Carbon dioxide was delivered to the container via a precision rotameter calibrated by the bubble burette method. Mixing of gases in the container was achieved with an electric fan. The Blease lung was set at a compliance of 0.5 litre kPa"1. A pneumotachograph head and a gas sampling port were placed between the model lung and the Bain circuit. Five minutes after a steady state was achieved gas was sampled at aflowof 0.1 litre min"1 over periods of 10 s for carbon dioxide analysis (infra-red carbon dioxide analyser: Beckman LB2). The total geometric deadspace of the model lung was 130 ml. Ventilatory volume and flow were recorded on a Mingograph 81 (Elema-Schonander) recorder together with the carbon dioxide concentration. Mean inspired carbon dioxide concentration was calculated by numerical integration (planimetry) from the recordings since this form of integration was valid because the inspiratoryflowwas constant. Thus, recordings of carbon dioxide concentration v. © The Macmillan Press Ltd 1984
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The Bain circuit was studied in a model lung on the assumption that, in addition to the ratio of fresh gas flow to total ventilation ( VFG/ VE), different time fractions of the respiratory cycle might influence rebreathing. We found that the time fraction for active expiration (,FE[) governed rebreathing for each VFG/VE value. With F E , as an independent variable, a theoretical formula was derived for rebreathing. Rearranging this formula made it possible to calculate the necessary increase in ventilation to keep end-tidal carbon dioxide constant for each VFG/PE. Thus, at a fresh gas flow of 70 ml kg" 1 min" 1 , Vi has to be increased 2.6 times. For spontaneously breathing patients inhalation anaesthetics that do not depress carbon dioxide sensitivity seem to be better suited to use in the Bain circuit. The FECO2 c a n t n e n ^ '^P 1 c o n s t a n t through increased ventilation in spite of the concomitant increase in rebreathing.
BRITISH JOURNAL OF ANAESTHESIA
304
>ry
_PTQ|
RECORDER
BLEASE LUNG
FIG. 1. Diagram of experimental apparatus. PTG = pneumotachograph, CO2-carbon dioxide analyser, FG™ fresh gas inlet.
time equalled carbon dioxide concentration against volume. A constant flow, time-cycled ventilator, of the bag in bottle type (AGA UV 705), was used to provide breathing. The spill valve of the system was controlled by the pressure in the bottle so that excess gas left the system during expiration, as would be the case in spontaneous ventilation. However, the respiratory wave form differed from that obtained during spontaneous breathing—the inspiratory flow being constant and the expiratory flow exponential. The inspiratory time of the ventilator could be set at 10, 20, 25, 33 and 50% of the respiratory cycle. The duration of active expiration could be varied by changing the resistance of the Blease lung. (A) With a constant V"E of 9 litre min -1 and a frequency of 20 b.p.m. (giving VA = 6.4 litre min"1, t/FG was set at 3.5,4.9, 7.0, 8.75,10.5,12.25,14.0 and 15.75 litre min"1 (corresponding to 50, 70,100, 125, 150, 175, 200and225mlkg" 1 min- 1 ina70-kg man). Each fresh gas flow was tested with an inspiratory time of 10, 20, 25, 33 and 50% of the respiratory cycle. The expiration was 50% of the cycle and the end-expiratory pause was 40, 30, 25, 17 and 0% respectively. (B) With VE 91itremin-\ frequency 20b.p.m. and V F G = 100 ml kg"1 min"1, inspiratory time was set at 10% and expiration prolonged to 90% of the respiratory cycle. (C) With a frequency of 20 b.p.m. and inspiratory time of 50%, VE and V'FG were set at 8 and 16 litre min"1, respectively, to assess the FECO2 ^
r
^
*
^
^
value during non-rebreathing conditions and check that .FICO2 was zero. Then V'FG was decreased to 8 litre min"1 and carbon dioxide measurements were made at VE of 8,12 and 16 litre min~' corresponding to VA= 5.4, 9.4 and 13.4 litre min"1. RESULTS In experiment A, end-tidal and mean inspired carbon dioxide concentrations were independent of inspiratory time fraction (fig. 2) and end-expiratory pause time fraction. Both concentrations increased as VFG decreased (fig. 3). In experiment B, prolonging the expiration from 50 to 90% of the breathing cycle caused an increase in FECO2 from 0.050 to 0.056 and FicCh fromO.021 to 0.026. In experiment C, VFG 16 litre min"1 and VE Slitremin- 1 caused no rebreathing; mean inspired carbon dioxide was zero. -FECO2 was 0.027. When V'FG was decreased to 8 litre min"[ and VE was set at 8, 12 and 16 litre min"1, FEcOj were 0.047, 0.038 and 0.037, respectively. The corresponding Flco? were 0.012,0.018 and 0.020 (fig. 4). DISCUSSION During inspiration, with the Bain circuit gas delivered to the patient from the fresh gas limb amounts —
^ ^
— ^ -
j
-
-
— -•
-—
—•
—
f - j
• • "
^
to:
FFG 'Flt
(1)
The highest possible value of equation (1) is Vl, thus:
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VENTILATOR
REBREATHING IN THE BAIN CIRCUIT
305
Mean insp.
End-tidal
• End-tidal 'Mean insp. 0.04-
-•70
0.06-
0.02-
0.00
J
8
12 16 S/i. (litre mirf 1 ) Downloaded from http://bja.oxfordjournals.org/ at Harvard University on July 11, 2015
FIG. 4. The effect of increasing VB at a constant PFG of 8 litre min ~' on FECO2 " ^ ^CO2 • The carbon dioxide recordings are drawn schematically.
0.02-
0.00-J
-Flt 120^33 25
50
10 20 33
50
dx>
Inspiratory time (X) FIG. 2. End-tidal and mean inspired carbon dioxide concentrations at different fresh gas flows (ml kg~' min~'), and inspiratory times. Body weight — 70 kg.
0.06-
0.04-
0.02-
0.00 J 50 70
100 125 150 175 200 225 t'FG(mlkg-'min-1)
FIG. 3. End-tidal and mean inspired carbon dioxide concentrations when varying the t'FG at constant ventilation. • • = FicO2- Mean of values from figure 2.
(2)
Vl
is the fraction of Vl that consists of fresh gas delivered from the fresh gas limb during inspiration. The rest of Vl is rebreathed from the expiratory limb, so the volume fraction rebreathed is: 1 -
FFG >F\t Vl
(3)
The composition of this volume as regards carbon dioxide depends on the degree of mixing in the expiratory limb. Gas mixing is far from complete, even in corrugated tubing, so plug flow can be anticipated. This incomplete mixing has been confirmed in studies where carbon dioxide was sampled at different locations in the expiratory limb (Conwayj Seeley and Barnes, 1977). Fluctuations in carbon dioxide concentrations were found in the expiratory limb 100 cm distal from the mouth of the patient, and in the vented gas. It is evident that the rebreathed wlume depends on inspiratory time fraction, the shorter the fraction the larger the rebreathed volume. If this volume is mixed completely, a shorter inspiratory time fraction would lead to an increase in the volume of carbon dioxide giving a greater i'lco?»but this was not the case (fig. 2). Plug flow in the expiratory limb can explain the fact that .FiCO? is independent of inspiratory time fraction but dependent upon the expiratory time fraction. In figure 5 an anempt has been made to show that the total amount of fresh gas in each breath is independent of the inspiratory time fraction when the expiratory time fraction is constant (fig. 5A and B).
306
BRITISH JOURNAL OF ANAESTHESIA FG-limb
EXP-hmb EfQ
/FG
£AG
£DG
SL*~ pat
RB
RB
(6)
The mean inspired FCO2 is derived by dividing equation (6) by Vl: RB FIG. 5. Schematic graph of the effect of plug flow on rebreathing in the Bain circuit. FG-limb =- fresh gas limb; EXPlimb — expiratory limb; /FG •* fresh gas during inspiration delivered direct from FG-limb; EFG = fresh gas in the expiratory limb delivered during the previous expiration; i:AG —expired alveolar gas; ifDG = expired gas from deads pace; RB — gas volume to be rebreathed from expiratory limb; pat = patient; Pro - VE. A: insp - 50%, active exp - 50%; B: insp «• 10%, active exp-'50%, end-exp. pause 40%; C: insp-30%, active exp-70%.
FECO2 can then be derived by adding the amount of carbon dioxide produced divided by VA (giving mean produced alveolar carbon dioxide concentration): -co 2 ~
FFG(1 - F E f ) \ / KCO2 \ FCO2 However, an increase in the expiratory time fraction Vl I IFFG + KCO2 I VA with a constant inspiratory time fraction caused an (8) increase in the volume of rebreathed carbon dioxide calculated by equation (8) shows good (fig. 5C). Thus, inspiration consists of a part VFG • Fl, of agreement with measured values (fig. 6). When comparing equation (8) with the equation fresh gas from the fresh gas limb and a part VFG • Fv, of fresh gas from the expiratory limb. The proposed by Keenan and Boyan (1978) for the Bain circuit, the main difference lies in the part expresrest of the inspired volume: sing the amount of gas that is rebreathed. In the Vl - FFG -(Flt+FPt) (4) Keenan-Boyan formula this volume is supposed to consists of a mixture of exhaled gas and fresh gas be (VE - VFG)/ VE. This expression has the disaddelivered during expiration, but Fl,+ F?,= 1 - FE, vantage of giving negative values when VFG is larger and by dividing by Vl equation (4) can be changed than VE. AS .FECO2 is the sum of -Flco? and VCO2/VA, a negative FlcOj will result in FECO2 to: being less than VCO2/VA when the VFG is larger (5) than VE. The value of FECO2 may even be less than 1 zero with high fresh gas flows. This is, of course, not V\
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which is the volume fraction rebreathed. This is a fraction of what otherwise would leave the system and has the carbon dioxide concentration of what leaves the system irrespective of the type of flow in the system, plug or mixed. The carbon dioxide concentration of what leaves the system is: KCO2 volume leaving the system Thus, the amount of carbon dioxide rebreathed becomes the volume fraction rebreathed multiplied by FcCh/volume leaving the system, multiplied by the total ventilation. In our model lung, the volume leaving the system is VFG + VCO2. Therefore the amount of carbon dioxide rebreathed becomes:
REBREATHING IN THE BAIN CIRCUIT
307
Derived Measured • End-tidal • Mean insp • Mean vented
0.06-
2.6 2.4 2.2
<5"O.O4-
o 2.0 S
0.02-
1.8 1.6
1.4J
1.2 50 70
100 125 150 175 200 225 1/E (lit
1
)
FIG. 6. Comparison between measured values of FECO2 and mean inspired carbon dioxide (from fig. 2) and values calculated from equations (6), (7) and (8). VCO2- 0.2 litre min" 1 , V l - 9 litre min" 1 , VA = 6.4litremin~ l ,FB,= 0.5.
1.0 0.7
1.0
1.25
1.5
1.75
2.0
FIG. 7. Diagram showing the necessary increase in ventilation to keep FECO2 unchanged and normal at 0.05. Note that when t'FG is twice the Vb there is no rebreathing, and therefore no need for
alterations in Vl. F E , - 0.5; V D / V T - 0 . 5 . a satisfactory equation, as was noted by Keenan and Boyan (1981). Equation (8) above, where the volume fraction to be rebreathed is derived from equation (5), takes into account that the true volume to ventilation during non-rebreathing) with F E , = 0.5 be rebreathed is dependant not only on VFG and Vl, and VD/VT = 0.5. but also on the fractional length of the expiration. At When, as in figure 7, the units on the axes are normal spontaneous breathing with I: E = 1:1, equa- multiples of ventilation during non-rebreathing, Vl tion (5) results in non-rebreathing conditions at VFG becomes independent of FCQj. A change in FE,will double that of VE, which is in accordance with move the curve along the ordinate, but a low VD/ V"T Mapleson's statement about the fresh gas flow re- during non-rebreathing will increase the curvature, quirements to achieve non-rebreathing conditions making the demand for increased V/ Vo higher. in a D-type circuit. Radford's nomogram (Radford, 1955) for calculaIf the volume leaving the system is assumed to be tion of VE 1gives values of approximately (1972) recomVFG (in clinical practice it would be lOOmlkg-'min" . Bain and Spoerel's 1 1 VFG + VcCh - VOz, however, the VCO2 and the VCh.mendation for VFG of 70 ml kg" min" results in a are almost equal at R = 0.8 and are small compared fresh gas flow of 0.7 VE. From figure 8 it is obvious with VFG and, therefore, omitted) and that this will lead to a very high demand on the Vl = VA + VD, equation (8) can be rearranged and capacity to increase VE in the Bain circuit to achieve non-rebreathing values of i^EcOj^FG solved for Vl: 125 ml kg"1 min"1 seems to be more adequate as this KFG-FCO2 only requires VE 1.25 times the non-rebreathing value. " KFG - VD/VT) (9) The validity of equation (9) can be tested in where VD/VT is the fractional deadspace during comparison with results from unanaesthetized volnon-rebreathing conditions. Equation (9) can be unteers. Willis, Pender and Mapleson (1975) used to calculate, at different VFG, the necessary fa studied rebreathing in a T-piece and derived forto keep FECO2 unchanged from non-rebreathing mulae that took I:E ratios and VD/ VT into account. values. Figure 7 is a plot of V\VO (total ventilation They plotted their values as V/Vo against VFG/V0 divided by total ventilation during non-rebreathing) and found good agreement between theoretical and against VVG\Va (fresh gas flow divided by total experimental values. They assumed a constant •
_
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0.00
BRITISH JOURNAL OF ANAESTHESIA
308
Derived values From Willis (volunteers)
2.01.81.61.41.21.01.2
1-4
1.6
1.8
2.0
FIG. 8. Comparison of data from figure 6 in Willis, Pendcr and Mapleson (1975) of VlVovrith VPG/Voand calculated values using equation (9) (for explanation see text).
VD/VT and an I:E ratio of 1:1.2. They remarked that there was evidence that anatomical deadspace increased during hyperventilation, but this was only about 22 ml for each 1-litre increase in tidal volume (Gray, Grodins and Carter, 1956), so a constant deadspace seemed reasonable. The l:E ratio was chosen as a mean value, the ratio being lower during normoventilation and near 1:1 during hyperventilation. If E stands for FE,, as in our formula, an I:E of 1:1 seems reasonable as the longer total expiratory time during normoventilation also includes end-expiratory pause. They only mention apparatus deadspace, so we have assumed a VV/ VT of 0.5 and put it together with FE, = 0.5 into equation (9). In figure 8 we have plotted data from their figure 6 together with our calculated values. The agreement is good. In Byrick and Janssen's paper (1980), rebreathing was greater in halothane-anaesthetized patients than in patients anaesthetized with enflurane. However, the patients who received halothane were more normocapnic. This must depend on their greater ventilation which is explained by our experiment C (fig. 4). For a given VCO? and VFG, increasing VE leads to both a decrease in FEQCh an<^ !kD increase in -FICQ2- If the ventilation was infinite, FECO2 an£ l -FicOj would be close to the mean vented carbon dioxide fraction. Halothane-anaesthetized patients have an almost normal response to carbon dioxide (Knill and Gelb, 1978) so when they begin to rebreathe, for instance because of a low VFG, they
increase VE to keep .FECO2 constant. This leads in increased rebreathing (an increase in Ficoi) resulting from the increased ventilation (fig. 9). Thus, the increased rebreathing in halothaneanaesthetized, spontaneously breathing patients is both the cause and effect of the increase in VE. Patients anaesthetized with enflurane respond poorly to carbon dioxide (Knill, Manninen and Clement, 1979) and do not increase VE when rebreathing occurs so their rebreathing will be low but their Derived — Measured • End-tidal • Mean insp. • Mean vented 0.04-
I 0.02-
0.00
J
8
12 16 V'E (litre min"1)
FIG. 9. The effect of increasing ^E at constant VFG. Note that the mean vented carbon dioxide is independent of ventilation. Increased V'E increases rebreathing, that is, increased FICO21 but decreased FECOI- Experimental values as in figure 4. Calculated values assuming FE, — 0.5.
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1.0
REBREATHING IN T H E BAIN CIRCUIT
309 KniU, R. L., Manninen, P. M., and Clement, J. L. (1979). Ventilation and chemoreflexes during enflurane sedation and anaesthesia in man. Can. Anaath. Soc. J., 24, 353. Mapleson, W. W. (1954). The elimination of rebreathing in various semklosed systems. Br. J. Anaath., V>, 323. Radford, E. P. (1955). Ventilation standards for use in artificial respiration. / . Appl. Phytiol., 7,451. Spoerel, W. E., Aitken, R. R., and Bain, J. A. (1978). Spontaneous respiration with the Bain breathing circuit. Can. Anaath. Soc./., 25, 30. Willis, B. A., Pender, J. W., and Mapleson, W. W. (1975). Rebreathing in a T-piece: Volunteer and theoretical studies of the Jackson—Rees modification of Ayre's T-piece during spontaneous respiration. Br. J. Anaath., 47,1239.
APPENDIX
Le circuit de Bain a etc etudie sur un modele de poumon en faisant lTiypothese que, outre le rapport entre le debit de gaz frais et la ventilation totale ( VGF/VE), des modifications de duree relative des differentes fractions du cycle respiratoire pouvaient influencer le rebreathing. Nous avons retrouve que la fraction de temps reservee a l'expiration active (FE,) etait un facteur determinant du rebreathing pour chaque valeur de VGF/VE. En considerant F E , comme une variable independante, nous en avons deduit une formule theorique pour le calcul du rebreathing. En rearrangeant cette formule, nous avons pu calculer qudle etait raugmentation de ventilation necessaire pour garder la PCO2 de fin d'expiration constante pour chaque valeur de VGF/ VE. Ainsi, pour un debit de gaz frais de 70 ml kg~' min~', la Vl doit ctre augmentee 2,6 fois. Lorsque les sujets sont en ventilation spontanee, il vaut mieux, avec lc circuit de Bain, utiliser des anesthesiqucs par inhalation qui ne depriment pas la sensibilite au CO2. La FE0O2 peut etre maintenue constante grace a une augmentation de la ventilation malgre une augmentation concomittante du rebreathing.
SYMBOLS FECO2 FICO2 VCO2 VFG Vl V'A VE VD VT Vo Fl,
End-tidal carbon dioxide fraction Mean inspired carbon dioxide fraction Carbon dioxide production Fresh gas flow Inspired total ventilation Alveolar ventilation Expired total ventilation Deadspace Tidal volume Total ventilation during non-rebreathing conditions Inspiratory time fraction
FE, FP,
Expiratory time fraction End-expiratory pause time fraction
CARACTERISTIQUES DU REBREATHING AVEC LE CIRCUIT DE BAIN line etude ihtoriqiu el experimentale
ACKNOWLEDGEMENTS
The work was supported by grants from the University of Gothenburg and Gdteborgs liikarsallskap. REFERENCES
RUCKATMUNGSCHARAKTERISTIKA DES BAIN CIRCUIT Eine exptrimentelle und thtorttische Untersuchung
Bain, J. A., and Spoerel, W. E. (1972). A streamlined anaesthetic system. Can. Anaath. Soc. J., 19,426 Byrick, R. J., and Janssen, E. G. (1980). Respiratory waveform and rebreathing in T-piece circuits. Anathaiology, 53, 371. Conway, C. M., Seeley, H. F., and Barnes, P. K. (1977). Spontaneous ventilation with the Bain anaesthetic system. Br. J. Anaath., 49,1245. Gray, J. S., Grodins, F. S., and Carter, E. T. (1956). Alveolar and total ventilation and the dead space problem. / . Appl. Physiol., 9, 307. Keenan, R. L., and Boyan, P. C. (1978). How rebreathing anaesthetic systems control Pacoj: Studies with a mechanical and a mathematical model. Can. Anaath. Soc. J., 25,117. (1981). Facton affecting rebreathing in T-piece circuits. Anathaiology, 55, 84. KnOl, R. L., and Gelb, A. W. (1978). VentOatory responses to hypoxia and hypercapnia during halothane sedation and anesthesia in man. Anathaiology, 49, 244.
Unter der Annahme, dafi aufier dem Verhaltnis Frischgasflow zu Gesamtventilation (VFG/VE) verschiedenc Zeitabschnitte des Atmungszyklus die Ruckatmung becinflussen kdnnten, wurde der Bain-Kreislauf an einer Modellunge untersucht. Wir fanden, das der Zeitabschnitt aktiver Exspiration (-FE,) die Ruckatmung bei iedem 1^FG/VE-Wert bestimmt. Mit F E , als unabhangjger Variablen wurde eine theoretische Formel fur Ruckatmung abgeleitet. Durch Umformung wares mit dieser Formel moglich, fur jedes VFG/ VE den fur ein konstantes endexspiratorisches CO2 notwendigen Vcntilationsanstieg zu errechnen. So mufi bei einem Frischgasflow von 70 mlkg~ l min" 1 Vl um das 2,6fache gesteigert werden. Bei Spontanarmung erscheinen volatile Anasthetika, die die CO2-Sensirivitat nicht herbsetzen, fur den BainKreislauf besser geeignet. FECO2 k a n n ( l a l u l durch gesteigerte Ventilation konstant gehahen werden trotz gleichzeitig grofter werdender Ruckatmung.
ZUSAMMENFASSUNG
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FECO2 will increase compared with non-rebreathing conditions. Thus, rebreathing in the Bain circuit is governed by FE, and v'FG/V'E ratio. If, for any reason, a patient has a prolonged active expiration (for example, in chronic obstructive lung disease), VFG should be increased. When using low fresh gas flows, halothane seems to be a better choice than enflurane for spontaneously breathing patients, since halothane causes less depression of the breathing centre, making it impossible for the patient to increase his VE and lower his FECO2—which has been increased by rebreathing. During controlled ventilation, VFG can be kept lower if the ventilation is increased sufficiently to keep .FECO2 within acceptable limits.
310
BRITISH JOURNAL OF ANAESTHESIA CARACTERISTICAS DE RE-RESPIRACION DEL CIRCUITO DE BAIN Un anidio experimental y teorico SUMARIO
Se estudio el circuito de Bain en un pulmon modelo en base a la hipotesis de que, ademas de la relacion de la corriente de gai fresco para con la ventilacidn total ( VFG/ VE), las fracciones de tiempo distintas del ciclo respiiatorio pueden influenciar la rerespiracion. Observamos que la fraction de tiempo para la expiracion activa (FE,) regia la re-respiracion por cada valor de FFG/ VE. Con la F E , coma variable independiente, se obtuvo una formula
teorica para la re-respiracion. Al arrcglar de nuevo esta formula, se hizo posible el calculo del aumento necesario de la ventilacifin para mantener constante el anhidrido carbonico respiratorioterminal para cada VFG/VE. Entonces, con una corriente de gas fresco de 70mlkg~ 1 min~ 1 , el" Vi debe ser aumentado en 2,6 veces. Para los pacientes que respiran espontaneamente, los ancstcticos por inhalacion que no deprimen la sensibilidad del anhidrido carbonico parecen adecuarse mejor al uso en el circuito de Bain. El FECO2 puede entonces mantenerse constante mediante una mayor ventilacion a pesar del aumento concomitante de la re-respiracion.
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REBREATHING CHARACTERISTICS OF THE BAIN CIRCUIT An Experimental and Theoretical Study O. STENQVIST AND H. SONANDER SUMMARY
The Bain circuit is a modification of the Mapleson type D system (Mapleson, 1954) and, in this theoretical study, Mapleson stated that rebreathing could be avoided if the fresh gas flow was somewhat greater than twice the total ventilation. However, if the pattern of inspiratory flow was variable, a further increase in fresh gas flow was necessary to avoid rebreathing. In addition, he suggested than an endexpiratory pause caused less rebreathing as fresh gas may accumulate at the patient end of the expiratory limb. Conway, Seeley and Barnes (1977), in a study on awake volunteers, confirmed that rebreathing in the Bain circuit occurred at fresh gas flows below two-and-a-half times the total ventilation. In a clinical study, Byrick and Jansson (1980) found differences in rebreathing between halothane- and enflurane-anae8thetized spontaneously breathing patients with the Bain circuit. They ascribed these differences to differences in the respiratory waveform—a long end-expiratory pause causing less rebreathing in the enflurane group compared with the halothane group. These conclusions were disputed by Keenan and Boyan (1981) who suggested that the differences were caused by different ratios between fresh gas flow rate and total ventilation. Thus, both low fresh gas flows and high minute ventilation would increase rebreathing. Bain and Spoerel (1972), and Spoerel, Aitken and Bain (1978), recommended a fresh gas flow below or equal to the total ventilation during spontaneous OLA STENQVIST, M.D., PH.D.; HANS SONANDER, M.D.; Depart-
ments of Anaesthesia and Intensive Care, Sahlgren's Hospital, University of Gothenburg, S-413 45 Gothenburg, Sweden.
ventilation, accepting a certain degree of rebreathing since, in their studies, the resulting alveolar concentrations of carbon dioxide were found to be within physiological limits. The aim of the present study was to evaluate the influence of respiratory time fractions, fresh gas flow and total ventilation on rebreathing in the Bain circuit during simulated spontaneous ventilation. Symbols are explained in the Appendix. MATERIALS AND METHODS
The investigation was undertaken using the model lung shown in figure 1. This consisted of a metal container with a volume of 2.5 litre and a Blease lung connected in series. Carbon dioxide was delivered to the container via a precision rotameter calibrated by the bubble burette method. Mixing of gases in the container was achieved with an electric fan. The Blease lung was set at a compliance of 0.5 litre kPa"1. A pneumotachograph head and a gas sampling port were placed between the model lung and the Bain circuit. Five minutes after a steady state was achieved gas was sampled at aflowof 0.1 litre min"1 over periods of 10 s for carbon dioxide analysis (infra-red carbon dioxide analyser: Beckman LB2). The total geometric deadspace of the model lung was 130 ml. Ventilatory volume and flow were recorded on a Mingograph 81 (Elema-Schonander) recorder together with the carbon dioxide concentration. Mean inspired carbon dioxide concentration was calculated by numerical integration (planimetry) from the recordings since this form of integration was valid because the inspiratoryflowwas constant. Thus, recordings of carbon dioxide concentration v. © The Macmillan Press Ltd 1984
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The Bain circuit was studied in a model lung on the assumption that, in addition to the ratio of fresh gas flow to total ventilation ( VFG/ VE), different time fractions of the respiratory cycle might influence rebreathing. We found that the time fraction for active expiration (,FE[) governed rebreathing for each VFG/VE value. With F E , as an independent variable, a theoretical formula was derived for rebreathing. Rearranging this formula made it possible to calculate the necessary increase in ventilation to keep end-tidal carbon dioxide constant for each VFG/PE. Thus, at a fresh gas flow of 70 ml kg" 1 min" 1 , Vi has to be increased 2.6 times. For spontaneously breathing patients inhalation anaesthetics that do not depress carbon dioxide sensitivity seem to be better suited to use in the Bain circuit. The FECO2 c a n t n e n ^ '^P 1 c o n s t a n t through increased ventilation in spite of the concomitant increase in rebreathing.
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304
>ry
_PTQ|
RECORDER
BLEASE LUNG
FIG. 1. Diagram of experimental apparatus. PTG = pneumotachograph, CO2-carbon dioxide analyser, FG™ fresh gas inlet.
time equalled carbon dioxide concentration against volume. A constant flow, time-cycled ventilator, of the bag in bottle type (AGA UV 705), was used to provide breathing. The spill valve of the system was controlled by the pressure in the bottle so that excess gas left the system during expiration, as would be the case in spontaneous ventilation. However, the respiratory wave form differed from that obtained during spontaneous breathing—the inspiratory flow being constant and the expiratory flow exponential. The inspiratory time of the ventilator could be set at 10, 20, 25, 33 and 50% of the respiratory cycle. The duration of active expiration could be varied by changing the resistance of the Blease lung. (A) With a constant V"E of 9 litre min -1 and a frequency of 20 b.p.m. (giving VA = 6.4 litre min"1, t/FG was set at 3.5,4.9, 7.0, 8.75,10.5,12.25,14.0 and 15.75 litre min"1 (corresponding to 50, 70,100, 125, 150, 175, 200and225mlkg" 1 min- 1 ina70-kg man). Each fresh gas flow was tested with an inspiratory time of 10, 20, 25, 33 and 50% of the respiratory cycle. The expiration was 50% of the cycle and the end-expiratory pause was 40, 30, 25, 17 and 0% respectively. (B) With VE 91itremin-\ frequency 20b.p.m. and V F G = 100 ml kg"1 min"1, inspiratory time was set at 10% and expiration prolonged to 90% of the respiratory cycle. (C) With a frequency of 20 b.p.m. and inspiratory time of 50%, VE and V'FG were set at 8 and 16 litre min"1, respectively, to assess the FECO2 ^
r
^
*
^
^
value during non-rebreathing conditions and check that .FICO2 was zero. Then V'FG was decreased to 8 litre min"1 and carbon dioxide measurements were made at VE of 8,12 and 16 litre min~' corresponding to VA= 5.4, 9.4 and 13.4 litre min"1. RESULTS In experiment A, end-tidal and mean inspired carbon dioxide concentrations were independent of inspiratory time fraction (fig. 2) and end-expiratory pause time fraction. Both concentrations increased as VFG decreased (fig. 3). In experiment B, prolonging the expiration from 50 to 90% of the breathing cycle caused an increase in FECO2 from 0.050 to 0.056 and FicCh fromO.021 to 0.026. In experiment C, VFG 16 litre min"1 and VE Slitremin- 1 caused no rebreathing; mean inspired carbon dioxide was zero. -FECO2 was 0.027. When V'FG was decreased to 8 litre min"[ and VE was set at 8, 12 and 16 litre min"1, FEcOj were 0.047, 0.038 and 0.037, respectively. The corresponding Flco? were 0.012,0.018 and 0.020 (fig. 4). DISCUSSION During inspiration, with the Bain circuit gas delivered to the patient from the fresh gas limb amounts —
^ ^
— ^ -
j
-
-
— -•
-—
—•
—
f - j
• • "
^
to:
FFG 'Flt
(1)
The highest possible value of equation (1) is Vl, thus:
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VENTILATOR
REBREATHING IN THE BAIN CIRCUIT
305
Mean insp.
End-tidal
• End-tidal 'Mean insp. 0.04-
-•70
0.06-
0.02-
0.00
J
8
12 16 S/i. (litre mirf 1 ) Downloaded from http://bja.oxfordjournals.org/ at Harvard University on July 11, 2015
FIG. 4. The effect of increasing VB at a constant PFG of 8 litre min ~' on FECO2 " ^ ^CO2 • The carbon dioxide recordings are drawn schematically.
0.02-
0.00-J
-Flt 120^33 25
50
10 20 33
50
dx>
Inspiratory time (X) FIG. 2. End-tidal and mean inspired carbon dioxide concentrations at different fresh gas flows (ml kg~' min~'), and inspiratory times. Body weight — 70 kg.
0.06-
0.04-
0.02-
0.00 J 50 70
100 125 150 175 200 225 t'FG(mlkg-'min-1)
FIG. 3. End-tidal and mean inspired carbon dioxide concentrations when varying the t'FG at constant ventilation. • • = FicO2- Mean of values from figure 2.
(2)
Vl
is the fraction of Vl that consists of fresh gas delivered from the fresh gas limb during inspiration. The rest of Vl is rebreathed from the expiratory limb, so the volume fraction rebreathed is: 1 -
FFG >F\t Vl
(3)
The composition of this volume as regards carbon dioxide depends on the degree of mixing in the expiratory limb. Gas mixing is far from complete, even in corrugated tubing, so plug flow can be anticipated. This incomplete mixing has been confirmed in studies where carbon dioxide was sampled at different locations in the expiratory limb (Conwayj Seeley and Barnes, 1977). Fluctuations in carbon dioxide concentrations were found in the expiratory limb 100 cm distal from the mouth of the patient, and in the vented gas. It is evident that the rebreathed wlume depends on inspiratory time fraction, the shorter the fraction the larger the rebreathed volume. If this volume is mixed completely, a shorter inspiratory time fraction would lead to an increase in the volume of carbon dioxide giving a greater i'lco?»but this was not the case (fig. 2). Plug flow in the expiratory limb can explain the fact that .FiCO? is independent of inspiratory time fraction but dependent upon the expiratory time fraction. In figure 5 an anempt has been made to show that the total amount of fresh gas in each breath is independent of the inspiratory time fraction when the expiratory time fraction is constant (fig. 5A and B).
306
BRITISH JOURNAL OF ANAESTHESIA FG-limb
EXP-hmb EfQ
/FG
£AG
£DG
SL*~ pat
RB
RB
(6)
The mean inspired FCO2 is derived by dividing equation (6) by Vl: RB FIG. 5. Schematic graph of the effect of plug flow on rebreathing in the Bain circuit. FG-limb =- fresh gas limb; EXPlimb — expiratory limb; /FG •* fresh gas during inspiration delivered direct from FG-limb; EFG = fresh gas in the expiratory limb delivered during the previous expiration; i:AG —expired alveolar gas; ifDG = expired gas from deads pace; RB — gas volume to be rebreathed from expiratory limb; pat = patient; Pro - VE. A: insp - 50%, active exp - 50%; B: insp «• 10%, active exp-'50%, end-exp. pause 40%; C: insp-30%, active exp-70%.
FECO2 can then be derived by adding the amount of carbon dioxide produced divided by VA (giving mean produced alveolar carbon dioxide concentration): -co 2 ~
FFG(1 - F E f ) \ / KCO2 \ FCO2 However, an increase in the expiratory time fraction Vl I IFFG + KCO2 I VA with a constant inspiratory time fraction caused an (8) increase in the volume of rebreathed carbon dioxide calculated by equation (8) shows good (fig. 5C). Thus, inspiration consists of a part VFG • Fl, of agreement with measured values (fig. 6). When comparing equation (8) with the equation fresh gas from the fresh gas limb and a part VFG • Fv, of fresh gas from the expiratory limb. The proposed by Keenan and Boyan (1978) for the Bain circuit, the main difference lies in the part expresrest of the inspired volume: sing the amount of gas that is rebreathed. In the Vl - FFG -(Flt+FPt) (4) Keenan-Boyan formula this volume is supposed to consists of a mixture of exhaled gas and fresh gas be (VE - VFG)/ VE. This expression has the disaddelivered during expiration, but Fl,+ F?,= 1 - FE, vantage of giving negative values when VFG is larger and by dividing by Vl equation (4) can be changed than VE. AS .FECO2 is the sum of -Flco? and VCO2/VA, a negative FlcOj will result in FECO2 to: being less than VCO2/VA when the VFG is larger (5) than VE. The value of FECO2 may even be less than 1 zero with high fresh gas flows. This is, of course, not V\
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which is the volume fraction rebreathed. This is a fraction of what otherwise would leave the system and has the carbon dioxide concentration of what leaves the system irrespective of the type of flow in the system, plug or mixed. The carbon dioxide concentration of what leaves the system is: KCO2 volume leaving the system Thus, the amount of carbon dioxide rebreathed becomes the volume fraction rebreathed multiplied by FcCh/volume leaving the system, multiplied by the total ventilation. In our model lung, the volume leaving the system is VFG + VCO2. Therefore the amount of carbon dioxide rebreathed becomes:
REBREATHING IN THE BAIN CIRCUIT
307
Derived Measured • End-tidal • Mean insp • Mean vented
0.06-
2.6 2.4 2.2
<5"O.O4-
o 2.0 S
0.02-
1.8 1.6
1.4J
1.2 50 70
100 125 150 175 200 225 1/E (lit
1
)
FIG. 6. Comparison between measured values of FECO2 and mean inspired carbon dioxide (from fig. 2) and values calculated from equations (6), (7) and (8). VCO2- 0.2 litre min" 1 , V l - 9 litre min" 1 , VA = 6.4litremin~ l ,FB,= 0.5.
1.0 0.7
1.0
1.25
1.5
1.75
2.0
FIG. 7. Diagram showing the necessary increase in ventilation to keep FECO2 unchanged and normal at 0.05. Note that when t'FG is twice the Vb there is no rebreathing, and therefore no need for
alterations in Vl. F E , - 0.5; V D / V T - 0 . 5 . a satisfactory equation, as was noted by Keenan and Boyan (1981). Equation (8) above, where the volume fraction to be rebreathed is derived from equation (5), takes into account that the true volume to ventilation during non-rebreathing) with F E , = 0.5 be rebreathed is dependant not only on VFG and Vl, and VD/VT = 0.5. but also on the fractional length of the expiration. At When, as in figure 7, the units on the axes are normal spontaneous breathing with I: E = 1:1, equa- multiples of ventilation during non-rebreathing, Vl tion (5) results in non-rebreathing conditions at VFG becomes independent of FCQj. A change in FE,will double that of VE, which is in accordance with move the curve along the ordinate, but a low VD/ V"T Mapleson's statement about the fresh gas flow re- during non-rebreathing will increase the curvature, quirements to achieve non-rebreathing conditions making the demand for increased V/ Vo higher. in a D-type circuit. Radford's nomogram (Radford, 1955) for calculaIf the volume leaving the system is assumed to be tion of VE 1gives values of approximately (1972) recomVFG (in clinical practice it would be lOOmlkg-'min" . Bain and Spoerel's 1 1 VFG + VcCh - VOz, however, the VCO2 and the VCh.mendation for VFG of 70 ml kg" min" results in a are almost equal at R = 0.8 and are small compared fresh gas flow of 0.7 VE. From figure 8 it is obvious with VFG and, therefore, omitted) and that this will lead to a very high demand on the Vl = VA + VD, equation (8) can be rearranged and capacity to increase VE in the Bain circuit to achieve non-rebreathing values of i^EcOj^FG solved for Vl: 125 ml kg"1 min"1 seems to be more adequate as this KFG-FCO2 only requires VE 1.25 times the non-rebreathing value. " KFG - VD/VT) (9) The validity of equation (9) can be tested in where VD/VT is the fractional deadspace during comparison with results from unanaesthetized volnon-rebreathing conditions. Equation (9) can be unteers. Willis, Pender and Mapleson (1975) used to calculate, at different VFG, the necessary fa studied rebreathing in a T-piece and derived forto keep FECO2 unchanged from non-rebreathing mulae that took I:E ratios and VD/ VT into account. values. Figure 7 is a plot of V\VO (total ventilation They plotted their values as V/Vo against VFG/V0 divided by total ventilation during non-rebreathing) and found good agreement between theoretical and against VVG\Va (fresh gas flow divided by total experimental values. They assumed a constant •
_
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0.00
BRITISH JOURNAL OF ANAESTHESIA
308
Derived values From Willis (volunteers)
2.01.81.61.41.21.01.2
1-4
1.6
1.8
2.0
FIG. 8. Comparison of data from figure 6 in Willis, Pendcr and Mapleson (1975) of VlVovrith VPG/Voand calculated values using equation (9) (for explanation see text).
VD/VT and an I:E ratio of 1:1.2. They remarked that there was evidence that anatomical deadspace increased during hyperventilation, but this was only about 22 ml for each 1-litre increase in tidal volume (Gray, Grodins and Carter, 1956), so a constant deadspace seemed reasonable. The l:E ratio was chosen as a mean value, the ratio being lower during normoventilation and near 1:1 during hyperventilation. If E stands for FE,, as in our formula, an I:E of 1:1 seems reasonable as the longer total expiratory time during normoventilation also includes end-expiratory pause. They only mention apparatus deadspace, so we have assumed a VV/ VT of 0.5 and put it together with FE, = 0.5 into equation (9). In figure 8 we have plotted data from their figure 6 together with our calculated values. The agreement is good. In Byrick and Janssen's paper (1980), rebreathing was greater in halothane-anaesthetized patients than in patients anaesthetized with enflurane. However, the patients who received halothane were more normocapnic. This must depend on their greater ventilation which is explained by our experiment C (fig. 4). For a given VCO? and VFG, increasing VE leads to both a decrease in FEQCh an<^ !kD increase in -FICQ2- If the ventilation was infinite, FECO2 an£ l -FicOj would be close to the mean vented carbon dioxide fraction. Halothane-anaesthetized patients have an almost normal response to carbon dioxide (Knill and Gelb, 1978) so when they begin to rebreathe, for instance because of a low VFG, they
increase VE to keep .FECO2 constant. This leads in increased rebreathing (an increase in Ficoi) resulting from the increased ventilation (fig. 9). Thus, the increased rebreathing in halothaneanaesthetized, spontaneously breathing patients is both the cause and effect of the increase in VE. Patients anaesthetized with enflurane respond poorly to carbon dioxide (Knill, Manninen and Clement, 1979) and do not increase VE when rebreathing occurs so their rebreathing will be low but their Derived — Measured • End-tidal • Mean insp. • Mean vented 0.04-
I 0.02-
0.00
J
8
12 16 V'E (litre min"1)
FIG. 9. The effect of increasing ^E at constant VFG. Note that the mean vented carbon dioxide is independent of ventilation. Increased V'E increases rebreathing, that is, increased FICO21 but decreased FECOI- Experimental values as in figure 4. Calculated values assuming FE, — 0.5.
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1.0
REBREATHING IN T H E BAIN CIRCUIT
309 KniU, R. L., Manninen, P. M., and Clement, J. L. (1979). Ventilation and chemoreflexes during enflurane sedation and anaesthesia in man. Can. Anaath. Soc. J., 24, 353. Mapleson, W. W. (1954). The elimination of rebreathing in various semklosed systems. Br. J. Anaath., V>, 323. Radford, E. P. (1955). Ventilation standards for use in artificial respiration. / . Appl. Phytiol., 7,451. Spoerel, W. E., Aitken, R. R., and Bain, J. A. (1978). Spontaneous respiration with the Bain breathing circuit. Can. Anaath. Soc./., 25, 30. Willis, B. A., Pender, J. W., and Mapleson, W. W. (1975). Rebreathing in a T-piece: Volunteer and theoretical studies of the Jackson—Rees modification of Ayre's T-piece during spontaneous respiration. Br. J. Anaath., 47,1239.
APPENDIX
Le circuit de Bain a etc etudie sur un modele de poumon en faisant lTiypothese que, outre le rapport entre le debit de gaz frais et la ventilation totale ( VGF/VE), des modifications de duree relative des differentes fractions du cycle respiratoire pouvaient influencer le rebreathing. Nous avons retrouve que la fraction de temps reservee a l'expiration active (FE,) etait un facteur determinant du rebreathing pour chaque valeur de VGF/VE. En considerant F E , comme une variable independante, nous en avons deduit une formule theorique pour le calcul du rebreathing. En rearrangeant cette formule, nous avons pu calculer qudle etait raugmentation de ventilation necessaire pour garder la PCO2 de fin d'expiration constante pour chaque valeur de VGF/ VE. Ainsi, pour un debit de gaz frais de 70 ml kg~' min~', la Vl doit ctre augmentee 2,6 fois. Lorsque les sujets sont en ventilation spontanee, il vaut mieux, avec lc circuit de Bain, utiliser des anesthesiqucs par inhalation qui ne depriment pas la sensibilite au CO2. La FE0O2 peut etre maintenue constante grace a une augmentation de la ventilation malgre une augmentation concomittante du rebreathing.
SYMBOLS FECO2 FICO2 VCO2 VFG Vl V'A VE VD VT Vo Fl,
End-tidal carbon dioxide fraction Mean inspired carbon dioxide fraction Carbon dioxide production Fresh gas flow Inspired total ventilation Alveolar ventilation Expired total ventilation Deadspace Tidal volume Total ventilation during non-rebreathing conditions Inspiratory time fraction
FE, FP,
Expiratory time fraction End-expiratory pause time fraction
CARACTERISTIQUES DU REBREATHING AVEC LE CIRCUIT DE BAIN line etude ihtoriqiu el experimentale
ACKNOWLEDGEMENTS
The work was supported by grants from the University of Gothenburg and Gdteborgs liikarsallskap. REFERENCES
RUCKATMUNGSCHARAKTERISTIKA DES BAIN CIRCUIT Eine exptrimentelle und thtorttische Untersuchung
Bain, J. A., and Spoerel, W. E. (1972). A streamlined anaesthetic system. Can. Anaath. Soc. J., 19,426 Byrick, R. J., and Janssen, E. G. (1980). Respiratory waveform and rebreathing in T-piece circuits. Anathaiology, 53, 371. Conway, C. M., Seeley, H. F., and Barnes, P. K. (1977). Spontaneous ventilation with the Bain anaesthetic system. Br. J. Anaath., 49,1245. Gray, J. S., Grodins, F. S., and Carter, E. T. (1956). Alveolar and total ventilation and the dead space problem. / . Appl. Physiol., 9, 307. Keenan, R. L., and Boyan, P. C. (1978). How rebreathing anaesthetic systems control Pacoj: Studies with a mechanical and a mathematical model. Can. Anaath. Soc. J., 25,117. (1981). Facton affecting rebreathing in T-piece circuits. Anathaiology, 55, 84. KnOl, R. L., and Gelb, A. W. (1978). VentOatory responses to hypoxia and hypercapnia during halothane sedation and anesthesia in man. Anathaiology, 49, 244.
Unter der Annahme, dafi aufier dem Verhaltnis Frischgasflow zu Gesamtventilation (VFG/VE) verschiedenc Zeitabschnitte des Atmungszyklus die Ruckatmung becinflussen kdnnten, wurde der Bain-Kreislauf an einer Modellunge untersucht. Wir fanden, das der Zeitabschnitt aktiver Exspiration (-FE,) die Ruckatmung bei iedem 1^FG/VE-Wert bestimmt. Mit F E , als unabhangjger Variablen wurde eine theoretische Formel fur Ruckatmung abgeleitet. Durch Umformung wares mit dieser Formel moglich, fur jedes VFG/ VE den fur ein konstantes endexspiratorisches CO2 notwendigen Vcntilationsanstieg zu errechnen. So mufi bei einem Frischgasflow von 70 mlkg~ l min" 1 Vl um das 2,6fache gesteigert werden. Bei Spontanarmung erscheinen volatile Anasthetika, die die CO2-Sensirivitat nicht herbsetzen, fur den BainKreislauf besser geeignet. FECO2 k a n n ( l a l u l durch gesteigerte Ventilation konstant gehahen werden trotz gleichzeitig grofter werdender Ruckatmung.
ZUSAMMENFASSUNG
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FECO2 will increase compared with non-rebreathing conditions. Thus, rebreathing in the Bain circuit is governed by FE, and v'FG/V'E ratio. If, for any reason, a patient has a prolonged active expiration (for example, in chronic obstructive lung disease), VFG should be increased. When using low fresh gas flows, halothane seems to be a better choice than enflurane for spontaneously breathing patients, since halothane causes less depression of the breathing centre, making it impossible for the patient to increase his VE and lower his FECO2—which has been increased by rebreathing. During controlled ventilation, VFG can be kept lower if the ventilation is increased sufficiently to keep .FECO2 within acceptable limits.
310
BRITISH JOURNAL OF ANAESTHESIA CARACTERISTICAS DE RE-RESPIRACION DEL CIRCUITO DE BAIN Un anidio experimental y teorico SUMARIO
Se estudio el circuito de Bain en un pulmon modelo en base a la hipotesis de que, ademas de la relacion de la corriente de gai fresco para con la ventilacidn total ( VFG/ VE), las fracciones de tiempo distintas del ciclo respiiatorio pueden influenciar la rerespiracion. Observamos que la fraction de tiempo para la expiracion activa (FE,) regia la re-respiracion por cada valor de FFG/ VE. Con la F E , coma variable independiente, se obtuvo una formula
teorica para la re-respiracion. Al arrcglar de nuevo esta formula, se hizo posible el calculo del aumento necesario de la ventilacifin para mantener constante el anhidrido carbonico respiratorioterminal para cada VFG/VE. Entonces, con una corriente de gas fresco de 70mlkg~ 1 min~ 1 , el" Vi debe ser aumentado en 2,6 veces. Para los pacientes que respiran espontaneamente, los ancstcticos por inhalacion que no deprimen la sensibilidad del anhidrido carbonico parecen adecuarse mejor al uso en el circuito de Bain. El FECO2 puede entonces mantenerse constante mediante una mayor ventilacion a pesar del aumento concomitante de la re-respiracion.
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