London 2003
Page 7 of 7	<Back to Abstract Home Page>   <Previous Page>      <Program>

 

 

INCREASING ANESTHETIC CONCENTRATIONS WITHOUT MODIFYING FGF BY SERIAL CONNECTION OF VAPORIZERS

 

Otero, P.E.¹, DVM; Rebuelto, M¹, DVM, PhD; Hallu, R.E.¹, DVM; Aldrete J.A.², MD, MS.

¹Faculty of Veterinary Science, University of Buenos Aires, Chorroarin 280 (1427) Buenos Aires, Argentina.

²Professor, Department of Anesthesiology, University of South Florida, School of Medicine, Tampa, FL. President of Arachnoiditis Foundation, Inc.

 

Introduction

When working with a vaporizer located outside the breathing circuit the anesthetic vapour delivered into the breathing system is dependent on fresh gas flow (FGF). For a constant FGF rate the maximum amount of agent delivered into the breathing system depends on the maximum output of the vaporizer employed. With a 300 ml/min basal FGF, a maximum of 15 or 24 ml/min of anesthetic agent may be provided, if working with a vaporizer maximal setting of 5 or 8%, respectively. As the requirements of an adult patient (70 kg BW) during the first minutes of anesthesia are above the mentioned, decreasing FGF from the beginning of the anesthesia will result on a prolonged induction phase due to the low levels of delivered anesthetic. Other disadvantage of using low flow anesthesia is the difficulty in producing rapid changes in the anesthetic effect without FGF adjustments, due to the long time constant of the low flow circuits. Flow-independent delivery of volatile agents  is an excellent alternative for this purpose. There are two possible approaches to making the volatile agents delivery independent of FGF i.e., placing the vaporizer inside the circuit (VIC) or injecting a calculated volume of the liquid agent directly into the circuit. Lastly, even though the use of vaporizers with a maximum output of volatile agent higher (3- or 4-fold) than the currently used vaporizers seems reasonable, safety recommendations may delay the use of such systems.

The purpose of this study was to design a vaporization system capable of increasing the anesthetic concentrations delivered to the breathing circuit without modifying the FGF maintaining de vaporizer outside de circuit.

Materials and methods

Agent-specific, variable-bypass, temperature- and flow-compensated  sevoflurane (Sevo-Wick, Muraco Medical Co., Ltd, , Tokyo, Japan, n = 4) and isoflurane (Vapor 19.1 Vaporizer, North American Drager, Telford, Pennsylvania, n = 4) vaporizers were used in this study.  Vaporizers were calibrated to deliver 5% maximum sevoflurane and isoflurane.

The same oxygen flowmeter calibrated in 100 mL/min increments for flows from 0 to 1000 mL/min was used all over this experience. Oxygen flow was started 30 minutes before measurements were done in order to avoid temperature changes.

Anesthetic concentrations (vol%) were determined by placing the sampling head from an anesthetic gas analyzer (Oxyanga Eku, Leiningen, Germany) previously calibrated for accuracy  following manufacturer's instructions into a corrugated tube (22 mm internal diameter, 60 cm length). Corrugated tubing was used as reservoir when FGF was less than the necessary volume for gas detection (150 mL/min). Each vaporizer was tested at 100, 200, 300, 500 and 1000 mL/min at its maximum tap setting for defining the actual output of anesthetic agent. The study was divided in two phases. Sevoflurane- (S1, S2, S3 and S4) and isoflurane- (I1, I2, I3 and I4) vaporizers were studied on phase 1 and 2, respectively. Vaporizers of each phase were connected to each other, increasing in one each time. Thus, the first vaporizer (S1 or I1, respectively) receives FGF from the oxygen flowmeter, the second vaporizer (S2 or I2, respectively) receives as FGF the output of the first vaporizer, the third vaporizer (S3 or I3) receives the output of the second vaporizer, which results from the delivery of first and second vaporizer (S1+S2 or I1+I2), and the fourth (S4 or I4) the addition of the first, second and third vaporizers (S1+S2+S3 or I1+I2+I3) as delivered by the third vaporizer. Thus, four series were described on each phase: S1, S1+S2, S1+S2+S3, and S1+S2+S3+S4 for phase 1; I1, I1+I2, I1+I2+I3, and I1+I2+I3+I4 for phase 2.

Anesthetic agent concentrations measurements for each series and each phase were measured at maximal vaporized settings (5%) and oxygen flow rates of 100, 200, 300, 500 and 1000 mL/min.  Each measurement was repeated 6 times and between measurements the vaporizers were turned off and the oxygen flow rate increased until no anesthetic agent concentration was detected in the circuit. Differences between expected (defined as the simple addition of the maximum values previously determined for each vaporizer) and measured values were calculated for each series of all phases. The ambient temperature in the working box was constant throughout the entire study. Box waste gas was eliminated by a gas-scavenging system.

Results are reported as mean ± SD. An analysis of variance was used to determine whether differences existed among means of anesthetic concentrations at each series of each phase at different FGF rates. When differences were found, a Tukey’s Multiple Comparison test was done for determining which means were different. Significance was set at a 5% level.

 

Results

Anesthetic agent concentration measurements for each series of each phase are shown on Table 1 and 2. Statistically significant differences were detected for series of both phases for 100 mL/min FGF and in phase 2 for 200 mL/min (P < 0.001).

The differences between expected and measured concentrations are listed on table 3.

 

Table 1. Anesthetic agent concentration measurements for each series of phase 1.

Phase 1 (Sevoflurane)
FGF (ml/min)	S1 (n = 24) (vol%)	S1+S2 (n = 6) (vol%)	S1+S2+S3 (n = 6) (vol%)	S1+S2+S3+S4 (n = 6) (vol%)
100	4.3 ± 0.40*	7.3 ± 0.09*	10.8 ± 0.05*	13.2 ± 0.08*
200	4.6 ± 0.38	8.2 ± 0.08	11.4 ± 0.10	13.9 ± 0.29
300	4.7 ± 0.29	8.3 ± 0.05	11.6 ± 0.16	14.1 ± 0.23
500	4.8 ± 0.31	8.3 ± 0.05	11.6 ± 0.18	14.1 ± 0.08
1000	4.7 ± 0.26	8.3 ±  0.11	11.3 ± 0.24	14.1 ± 0.16

* statistically significant differences (P<0,001) between FGF rates

 

Table 2. Anesthetic agent concentration measurements for each series of phase 2.

Phase 2 (Isoflurane)
FGF (ml/min)	I1(n =24) (vol%)	I1+I2 (n = 6) (vol%)	I1+I2+I3 (n= 6) (vol%)	I1+I2+I3+I4(n= 6) (vol%)
100	4.2 ± 1.32*	7.4 ± 0.20*	10.3 ± 0.10*	13.1± 0.08*
200	4.2 ± 0.91*	7.5 ± 0.20*	10.4 ± 0.15*	13.0 ± 0.07*
300	4.8 ± 0.52	8.3 ± 0.05	12.4 ± 0.08	15.5 ± 0.08
500	4.9 ± 0.32	8.5 ± 0.05	12.5 ± 0.18	16.1 ± 0.08
1000	4.9 ± 0.27	8.5 ± 0.06	12.5 ± 0.16	16.1 ± 0.08

* statistically significant differences (P<0,001) between FGF rates

 

Table 3. Differences between expected and measured concentrations for both series.

Values	S1+2	S1+2+3	S1+2+3+4	I1+2	I1+2+3	I1+2+3+4
Expected	9.2	13.8	18.4	9.2	13.8	18.4
Measured	8.1	11.3	13.9	8.4	11.6	14.8
Expected-Measured	1.1	2.5	4.5	0.8	2.2	3.6

 

Discussion

Our results show that the delivered final anesthetic agent concentration after connecting serially 2, 3 and 4 vaporizers increased with each vaporizer addition, however, the final output was lower than the expected one. This difference may be due to the fact that carrier gas composition for vaporizers 2, 3 and 4 contains partly the anesthetic agent. Thus, anesthetic vapour of the carrier gas flow mixes with anesthetic present in the vaporization chamber, were saturation pressure exists. This results in the impairment of  the carrier gas capability of vaporization. The decrease on final concentration is repeated along vaporizers and the influence is higher as the number of connected vaporizers increases. This behavior was similar for both sevoflurane and isoflurane, as they share the same vaporization method and they have similar physical properties. The difference found for 100 mL/min FGF for sevoflurane and 100 and 200 mL/min FGF for isoflurane were expected, as those flows are outside the range of calibration of the used vaporizers.

Approximation to final anesthetic concentrations may be calculated by means of equation 1:

 

[1] % vap _{1+2+3+4  =}      (Fl vap S 1-4) – (FL vap Ex S 2-4) x 100   

                                      FGF + (Fl vap S 1-4) – (FL vap Ex S 2-4)

where:

·        FL _{vapS 1-4} = final amount of the vapour delivered by each vaporizer.

·        FL _{vap Ex S 2-4,} = final amount of the vapour extracted by each vaporizer

 

Equation 1 is resolved as follows:

[2] FL _vap1 =   FGF x %V1

                       100- %V1

 

[3] V_carrier2 =       ((FGF + FL _vap1) x %V2)   x   100

             (100- %V2)                  %CVC

 

[4] FL _{vap Ex2} =       V_carrier2 x %V1

                 100

 

[5] FL _vap2 =      (V_carrier2 – Fl _vapEx2) x %CVC

                                             100

 

[6] % vap₁₊₂ =         (Fl _vap1 + Fl _vap2 – Fl _vapEx2) x 100

                                FGF + (Fl _vap1 + Fl _vap2 – Fl _vapEx2)

 

[7] V_carrier3 =          ((FGF + FL _vap1 + Fl _vap2 – Fl _vapEx2) x %V3)       x   100

                            (100- %V3)                                        %CVC

 

[8] FL _{vap Ex3} =         V_carrier3 x %vap₁₊₂

                    100

 

[9] FL _vap3 =     (V_carrier3 – Fl _vapEx3) x %CVC

                                          100

 

[10] % vap₁₊₂₊₃ =     (Fl _vap1 + Fl _vap2 +Fl _vap3 – Fl _vapEx2 - Fl _vapEx2) x 100

                                   FGF + (Fl _vap1 + Fl _vap2 +Fl _vap3– Fl _vapEx2 - Fl _vapEx2)

 

[11] V_carrier4 =     ((FGF + FL _vap1 + Fl _vap2+ Fl _vap3 – Fl _vapEx2 - Fl _vapEx2) x %V4)  x  100

                                                  (100-%V4)                                      %CVC

 

[12] FL _{vap Ex4  =}         V_carrier4 x %vap₁₊₂₊₃

                          100

 

[13] FL _vap4 =      (V_carrier4 – Fl _vapEx4) x %CVC

                                               100

 

[14] FL _{vapS 1-4} = [2] + [5] + [9] + [13]

 

[15] FL _{vap Ex S 2-4} = [4] + [8] + [12].

 

Where:

FGF (mL/min)= initial fresh gas flow

V_{carrier n} (mL/min)= Volume of carrier gas flow and _n = series location

%V_n (%)= tap setting of vaporizer and  _n = series location

%CVC (%)=  vapour concentration in the vaporization chamber

FL vap_n (mL)= vapour flow added by each vaporizer and _n = series location

FL vapEx_n (mL)= vapour flow extracted on each vaporizer and _n = series location

 

Figure 1 shows expected, measured and calculated (as equation 1) values.

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 1. Expected, measured and calculated (as equation 1) values.

 

 

 

Conclusions

In this study we demonstrated that anesthetic agent concentrations may be increased by serially connecting agent-specific vaporizers without modifying FGF and keeping the vaporizers outside the breathing system, and we propose an equation for predicting the approximate delivered anesthetic concentrations. This technique may be an alternative in low flow anesthesia, as provides constant and predictable final anesthetic concentrations.

 

References:

1.        Aldrete JA, Lowe HJ, eds. Low flow and closed system anesthesia. Grune & Stratton, New York, 1979.

2.        Baum Jan. Low Flow Anesthesia: the theory and practice of low flow, minimal flow and closed system anesthesia. 2^nd ed. 2001. Butterworth-Heinemann

3.        Lowe HJ, Ernst EA. The quantitative practice of anaesthesia. Use 0f closed circuit. Willians and Wilkins eds. Baltimore 1981

4.        Nunn G. Flow independent delivery of volatile agents. Lecture. ALFA 2002, Pisa, 26-27 April 2002

 

 

---

 

 

How long should be high flow phase in children for equilibration of sevoflurane prior to low flow anaesthesia when oxygen used as carrier gas?

 

P . Bozkurt, M Bakan, E  Tomatir*,  Ö Sen, N Kurt, M Posta, G Kaya

 

İstanbul University, Cerrahpasa Medical Faculty and Pamukkale University Medical Faculty*, Departments of Anaesthesiology, Turkey

 

Avoiding use of nitrous oxide had several advantages as being ecological and ease of teaching low flow anaesthesia to juniors (1). The literature about performance of low or minimal flow anaesthesia in children is limited (2). The anatomical and physiological differences of children bring up special management techniques also for low flow anaesthesia.

 

The aim of this study was to find out whether the high flow initial phase could be shortened in children when oxygen used as carrier gas with sevoflurane prior to low flow anaesthesia. 

 

Material and Method: Following institutional ethics committee approval 49 children who were free of respiratory diseases, anemia, low cardiac output or difficult intubation risk, ages ranging from 1day to 8 years included in this study. Children who had an intravenous line in were given thiopentone 5mg/kg or propofol 2mg/kg and atracurium or cis-atracurium for induction and remifentanil infusion at a rate of 0.5 mg/kg/min started. Mask ventilation with a tight fitting face mask was  started immediately using a semiclosed circle absorber system.  Sevoflurane 2-4vol % and 100% O₂ initiated at a flow of minute ventilation of the child and ventilated manually until the neuromuscular blocker drug effect achieved. Remifentanil infusion rate lowered to 0.25mg/kg/min 4 min later The children were intubated with endotracheal  tube which is appropriate for their age  by a single attempt. The tube secured immediately without disconnecting the tube and the Y piece and mechanical ventilation started with the settings tidal volume 8mL/kg and respiratory rate according to age.  A single anaesthesia machine (Cicero EM Drager-Germany) used during the study.  The flow rate of O₂, respiratory rate, presence of air leakage were recorded. Starting from the placement of the mask to the patients face the FiSevo ( inspired fraction of sevoflurane), FiO₂ ( inspired fraction of oxygen) FetSevo (endtidal fraction of sevoflurane)and ), FetO₂ ( endtidal fraction of O₂) were recorded every minute and  until intubation and for 10 minutes long after the intubation from the monitor of Cicero EM anaesthesia machine. The initial sevoflurane ratio and flow of O₂ were kept constant throughout the study period.   The time to equilibration between inspired and end tidal concentrations of sevoflurane and the concentrations at equilibrium were recorded. Equilibration is defined as reaching the FetSevo level reaching to  80% of FiSevo level. Later the composition of the gases changed to 50% oxygen in air for the rest of the anaesthesia.  The patient records were grouped according to their ages as children less than 2 years old and 2-8 years old groups.

 

The results were given as mean ± standard deviation. Between groups comparisons were performed with Student’s t test. P value less than 0.05 considered significant.      

 

Results: Twenty-one children were less than 2 years old and 28 were older than 2 years of age. The demographics and ventilatory settings of the groups are given in Table 1.

 

	Age  ( years)	Weight ( kg)	Resp. Rate (breaths/min)	O2 flow( L)	Duration of mask ventilation (min)
<2 years (min-max)	1±0.8 1day-2years	9.2±4.2 2.5-15	22.4±5.9 17-30	1.6±0.5 0.8-2.5	2.8±0.7 2-4
2-8 years (min-max)	5.5±2.5 2-8	19.4±7.7 7-24	17.6±1.7 20-14	2.8±0.9 1.4-3.7	2.9±0.8 1.5-4

 

None of the patients were hypoxic during the manual ventilation by mask ( SpO₂ >94%). SpO₂ ranged between 99% and 100% following intubation until the end of the study. In children less than 2 years old the equilibration between inspired and end tidal sevoflurane concentrations reached at 2.8± 2.5 min after induction. In the older children this period was 7.1±2.5 min. The difference between age groups was statistically significant ( p=0.0001). At this point inspired sevoflurane concentration was 2.3± 0.8vol % for both groups. The end tidal sevoflurane concentration was 2±0 .8vol % and 1.9± 0.6vol % for patients less than 2 years old and the older group respectively.

 

Discussion and Conclusion: The applicability of low flow and minimal flow techniques are increasing with the advent of modern anaesthesia machines also in children. Because the children are more prone to hypoxia during induction pure oxygen used. The tendency of using oxygen as carrier gas encouraged us for setting up this study.  Before decreasing the flow the uptake of inhalation agent required to reach to an equilibration for avoiding superficial anaesthesia. In children less than 2 years old duration is fairly short when compared to older children. This was probably dueto high alveolar ventilation and distribution of cardiac output and less blood-gas solubility ( by the influence of hematocrit, hemoglobin type and plasma protein fraction) (3). Baum had stated an interval of 10 minutes for high flow required for adults when N₂O use omitted (1).   The duration of high flow (in our setting high flow = minute ventilation) before the start of low flow could be decreased in small children. Low flow techniques could be started in a few minutes without risk of development of hypoxia or awakening when oxygen used as carrier gas for sevoflurane. This method also provides ease of teaching of low flow anaesthesia and could be applied safely in shorter cases while switching to low flow is more rapid.  

 

References: 

1. Baum JA. Low Flow Anaesthesia. Butterworth Heinemann .2nd ed. Oxford. 2001, p 269-280.
2. Meakin GH. Low-flow anaesthesia in infants and children. Br J Anaesth 1999;83, 50-57. 
3. Cook R, Davis PJ, Lerman J: Pharmacology of pediatric anesthesia. In : Motoyama EK, Davis PJ eds. Smith’s Anesthesia for Infants and Children. 6^th ed Mosby, St. Louis, 1996, 158-209.

 

 

Presenting author:

Pervin Bozkurt, M.D., Associate Professor of Anaesthesiology

İstanbul University Cerrahpaşa Medical Faculty

Department of Anaesthesiology

34303 İstanbul- Turkey

 

Tel: 90 212 560 81 87

Fax: 90 212 529 56 00

E-mail: apbs@istanbul.edu.tr or apbs@turk.net

 

 

---

 

 

 

This paper has been removed at the author's request.