San Sebastian 2004 Session 2-2
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Low flow for smooth induction / Inducción suave con flujos bajos.

Claire L Knaggs¹ B. Sc. (Hons) and Gordon B Drummond² FRCA, Edinbourgh, UK.

 

¹ Honours Student

Clinical Neurosciences

Edinburgh University

1 George Square

Edinburgh

 

² Senior Lecturer

Department of Anaesthesia, Critical Care, and Pain Medicine

Royal Infirmary

51 Little France Crescent

Edinburgh

EH16 4SA

 

Correspondence to Dr Drummond

Department of Anaesthesia, Critical Care, and Pain Medicine

Royal Infirmary

51 Little France Crescent

Edinburgh

EH16 4SA

United Kingdom

Tel 44 (0)131 242 3134

Fax 44 (0) 131 242 3138

g.b.drummond@ed.ac.uk

 

No reprints will be provided

 

Presented in part at the Anaesthetic Research Society meeting held in Cardiff, Wales, July 2002 and published in abstract form in Br J Anaesth 2002; 89: 673P

 

Supported by funds from the Department of Neuroscience, Edinburgh University and Endowment Funds of the Lothian Health Board.

 

Short running head: Inhalation induction with rebreathing (37 characters)

Low flow induction of anaesthesia

Abstract

When a breathing system is used without carbon dioxide absorbtion, rebreathing will occur if the fresh gas flow is limited. Rebreathing can be used to overcome the hypocapnia often present at induction of anaesthesia, and thus sustain ventilation when consciousness is lost. If fresh gas flow is too small this could slow induction because insufficient anaesthetic is supplied. We compared induction using 8%sevoflurane, given by three different methods. We randomly allocated 65 patients to either a Mapleson A breathing system  with a fresh gas flow of 9 l.min^-1 (group A9), a coaxial Mapleson D system with a fresh flow of 6 l.min^-1 (group D6) or the same system with a fresh flow of  3 l.min^-1 (group D3)

The median (quartiles) induction times were 64(52, 92), 50(42, 65), and 58(45, 72) seconds in the groups D3, D6, and A9 respectively. Induction of anaesthesia took longer (P < 0.01) and was more variable in group D3. In this group, end tidal sevoflurane concentration was less (P<0.05). In group A9, end tidal carbon dioxide was less (P<0.05). We conclude that induction with sevoflurane is most rapid and reliable when patients breathe a fresh gas flow of 6 l.min^-1 from a Mapleson D circuit.

Key words. Anaesthesia, induction, inhalation. Breathing system, Mapleson. Carbon dioxide, rebreathing.

Induction of anaesthesia by inhalation of sevoflurane is popular for day surgery, since this agent is easy to breathe and has limited solubility. However loss of consciousness is often associated with transient reduction or cessation of breathing, which interferes with the process of uptake. To overcome this problem, several methods have been suggested which allow carbon dioxide accumulation during inhalation induction, and sustain breathing. These are mainly based on voluntary manoeuvres such as breath holding[1].[2], and some patients, often those who require a swift inhalation induction, find these manoeuvres difficult to perform.

Adding carbon dioxide to the inhaled gas facilitates induction of anaesthesia[3], but this is awkward and can be dangerous. Inhalation induction was more reliable, and did not need voluntary manoeuvres, when we used a breathing circuit with a reduced fresh gas flow rate. This allowed rebreathing, carbon dioxide values increased slightly, and breathing was better maintained[4].

However a reduced fresh gas flow rate could reduce the rate of delivery of anaesthetic to the patient and in theory limit anaesthetic uptake, and slow down induction of anaesthesia.

To examine these factors we compared induction of anaesthesia using three different procedures, a non-rebreathing method, mild rebreathing, and moderate rebreathing. We compared the time taken to induce anaesthesia and measured the respired anesthetic and carbon dioxide values. We used loss of arm tone to indicate anaesthesia, as this is a reliable and realistic measure of induction of anaesthesia[5].[6]

Methods

We obtained approval for the study from the Research Ethics Committee for our institution. Each patient gave written informed consent. We recruited women, ASA 1 or 2, about to have minor gynecological surgery. We recorded their age, height, and weight. We excluded patients who were taking opioid analgesics, sedatives, or antidepressants. No premedication was used. Routine monitors were applied (ECG, pulse oximeter, and non-invasive blood pressure) and an intravenous cannula placed in the hand after infiltration of local anesthetic (1% lidocaine).

We prepared opaque envelopes, blocked in groups of 15, containing equal allocations to the three groups. The envelope was opened just before induction of anaesthesia. The three treatments were with different breathing systems and fresh gas flows. We used coaxial disposable circuits (Intersurgical), and oxygen as the carrier gas. The treatments were using the Mapleson A with a gas flow of 9 l.min^-1 (group A9), a Mapleson D (a Bain circuit) supplied with a fresh gas flow of 6 l.min^-1 (group D6); and a Mapleson D system supplied with a fresh gas flow of 3 l.min^-1 (group D3). Gas was sampled from the angle piece of the breathing circuit and analyzed for carbon dioxide and sevoflurane (Datex)

The patient lay supine on a trolley with the head supported on a single pillow. After baseline measurements of pulse, blood pressure and oxygen saturation, the patient was asked to hold one arm straight out, at about 45° to the horizontal, and to try to keep it in that position as long as possible. The patient was then asked breathe in, and the mask was then applied to the patient’s face to obtain a gas-tight seal. As soon as the display of the gas analyzer indicated a good trace of carbon dioxide, the sevoflurane vaporizer was set to 0.5%, and the concentration was then increased every two breaths, in the sequence 1,2,4,and 8%. We took the time from setting the vaporizer to 8% to the time that the arm came down to the horizontal as the time for induction of anaesthesia.

We recorded gas composition and flow in some of the patients. A gas analyzer (Datex Capnomac II) sampled respired gas from the angle piece of the breathing circuit. Signals for carbon dioxide and sevoflurane concentration were recorded through an analogue to digital converter (Cambridge CED 104) and a desktop computer (PC type, running Windows 2000, using Spike software, type 2.03). Each breathing system was connected to the patient mask by two bacterial filters. The first, next to the mask, allowed separation of the patient from the circuit. We uised the other as a pneumotachograph, and measured the pressure across this filter with a transducer (Furness MC40) and recorded the signal via the AD converter.

The times and concentrations of end tidal values for sevoflurane and carbon dioxide were taken from the records and fitted to a best fit second order polynomial using a statistical software package (Prism, version 4.00 for Windows, GraphPad Software, San Diego California USA, www.graphpad.com). For comparative purposes we used the values measured between the time the vaporizer was set to 8% and 40 seconds later, so that data from all the patients were used, and censoring of data was minimized. Statistical comparisons between the groups were made with the Kruskal Wallis test, followed if necessary by Dunn’s test. Data are presented as median (quartile values) unless otherwise stated.

Results.

We asked 101 patients to participate in the study. Of these, 72 agreed, and we could only study 67 for operational reasons. In the first 37, we also set out to record carbon dioxide and sevoflurane concentrations and obtained satisfactory records in 35. The details of the patients in the two parts of the study are summarized in table 1. The groups were well matched for age, height, and weight. No problem occurred during induction of anaesthesia. Pulse oximeter values remained above 95% throughout the procedure in all patients.

The induction times for the groups 3, 6, and 9 were 64 (52, 92), 50 (42, 65), and 58 (45, 72) seconds respectively (P <0.01). The induction time for group D3 was not only significantly greater, but the scatter of induction times was greater. (fig 1)

The pattern of increase in end-tidal sevoflurane is shown in figure 2. There was highly significant difference between the fitted polynomial curves (P<0.001). The sevoflurane values were consistently less in group D3 (P<0.05), and the end-tidal sevoflurane concentration at the time of loss of consciousness was less in these patients. The values of end-tidal sevoflurane at induction were 2.8 (2.4,3.1), 3.5 (3.3,4.2), and 4.1 (3.5,4.5)% for the groups D3, D6, and A9 respectively (P<0.05).

 

 

Discussion.

As expected, a small fresh gas flow reduced the rate of increase of sevoflurane concentration, and increased the time for induction of anaesthesia. For most patients a fresh gas flow rate of 6 l.min^-1 gave prompt and reliable induction of anaesthesia. No specific instructions or breathing strategies, such as maximal expiration, inspiration, or breath holds, are needed. The only instruction needed was to warn the patient to take in a breath before breathing from the mask, which avoids the reservoir bag collapsing at the first inspiration. A good fit of the mask to the face is needed to prevent air entrainment, but this can be easily checked by the appearance of the capnograph tracing. An increase in carbon dioxide concentration should be seen during inspiration.

Comparisons of different studies of this topic are difficult. Many used premedication[7],[8] nitrous oxide[9],[10] or different agents[11]. Nitrous oxide contributes little to the process[12]. Some showed it is difficult to distinguish between agents[13] but others have been unequivocal[11]. In addition, the methods of assessing time to induction are disparate. Different results may be obtained in volunteers, who are more cooperative and perform the maneuvers better than patients, who are anxious, inattentive, and untrained. In our study, many patients, when asked to “breathe normally” breathed very slowly when the mask was applied. Consequently, the time over which the fresh gas concentration was increased differed between patients. To limit the effect of this variation we chose to measure the time taken to induce anaesthesia from the time when the fresh gas concentration was set at 8%, which was eight breaths from the start of the induction. This could be between 30 and 60 seconds from the time the mask was first applied. By using steps of doubling the concentration we could increase the concentration rapidly and without causing airway irritation. As an endpoint for anesthetic induction we used the descent of the arm. This is a simple and reliable index, which represents deeper anaesthesia than other indexes such as the cessation of finger tapping, the lash reflex, or dropping a weight[14].

Because we studied patients without premedication, many would have been nervous. We used no adjuncts such as fentanyl or nitrous oxide; and chose a measure of anaesthesia that takes longer to achieve than others. We asked the patients to breathe normally, and achieved times for induction of anaesthesia that were similar to those obtained after vital capacity maneuvers, and using 67% nitrous oxide[15]. This was achieved by exploiting the partial rebreathing offered by the Mapleson D circuit.

With the Magill system there is selective inspiration of fresh gas, and end-tidal carbon dioxide depends on the patient’s intrinsic respiratory drive. In contrast, with the Bain circuit, there is a degree of rebreathing. The carbon dioxide in the circuit gases is regulated by several factors. The rate of elimination of carbon dioxide from the breathing system is regulated by the fresh gas flow. The difference between the carbon dioxide in the breathing system and the alveolar gas depends upon the alveolar ventilation and also on the pattern of breathing. We found a significantly smaller end-tidal carbon dioxide value in the group who were more able to affect their carbon dioxide elimination by their own breathing. The relatively low values indicate the increased drive to breathe from wakefulness and probable anxiety in patients before surgery. Loss of consciousness causes loss of ventilatory drive[16]. Adding carbon dioxide to the fresh gas accelerates induction of anaesthesia with isoflurane and reduces the incidence of difficulty during the process, but modern anesthetic machines do not provide this option, which is intrinsically unsafe.[3] Using the patient’s own carbon dioxide is a simple and effective alternative.

In theory, it would be logical to adjust the fresh gas flow according to the carbon dioxide production of the patient, which can be predicted from body weight[17]. In this way the degree of hypercapnia might be regulated. However since our patients were relatively uniform, this would not contribute materially to variation in response. In patients with different characteristics, such as children, or wasted or muscular patients, the appropriate gas flows could be altered to meet their predicted carbon dioxide production or anaesthetic requirement, to about 100 ml.kg^-1. Since anaesthetic uptake is likely to be related to the same physical characteristics as carbon dioxide production (muscle mass and cardiac output) then this gas flow may well also deliver the appropriate quantity of anesthetic vapor as well.

We were previously unable to distinguish between the time to induction of anaesthesia using 3 and 6 litre.min^-1, and found induction with the Magill circuit was slower[14] because more patients became apnoeic. In theory, less delivery of anesthetic vapour to the patient would delay the increase in alveolar, arterial, and brain concentration and slow induction. This concern was substantiated by the present study, and the mechanism demonstrated. The rate of increase in end-tidal concentration was slower and the time to induction of anaesthesia was significantly longer in group D3.

In summary we have shown that rapid inhalation induction of anaesthesia can be obtained in unpremedicated patients, without having to use specific breathing strategies, if a Mapleson D circuit is used with a fresh gas flow of 6 litre.min^-1 (approximately 100 ml.kg^-1).

Acknowledgement

Funded in part by the University of Edinburgh (Neuroscience teaching) and partly by Lothian Health Board Endowment fund 70780.

The authors have no conflicts of interest.

References

   1.   Ruffle JM, Snider MT, Rosenberger JL, Latta WB. Rapid induction of halothane anaesthesia in man. Br J Anaesth 1985; 57:607-11.

   2.   Ruffle JM, Snider MT. Comparison of rapid and conventional inhalation inductions of halothane oxygen anaesthesia in healthy men and women. Anesthesiol 1987; 67:584-7.

   3.   Coleman SA, McCrory JW, Vallis CJ, Boys RJ. Inhalation induction of anaesthesia with isoflurane: effect of added carbon dioxide. Br J Anaesth 1991; 67:257-61.

   4.   Guracha Boru K, Drummond GB. Comparison of breathing methods for inhalation induction of anaesthesia. Br J Anaesth 1999; 83:650-3.

   5.   Strickland TL, Drummond GB. Comparison of signs of anaesthesia using propofol, methohexital and sevoflurane. Br J Anaesth 1999; 83:180P-1P.

   6.   Thompson S, Drummond GB. Loss of volition and pain response during induction of anaesthesia with propofol or sevoflurane. Br J Anaesth 2001; 87:283-6.

   7.   Loper K, Reitan J, Bennett H, Benthuysen J, Snook L. Comparison of halothane and isoflurane for rapid anaesthetic induction. Anesth Analg 1987; 66:766-8.

   8.   Muzi M, Colinco MD, Robinson BJ, Ebert TJ. The effects of premedication on inhaled induction of anaesthesia with sevoflurane. Anesth Analg 1997; 85:1143-8.

   9.   Wilton NCT, Thomas VL. Single breath induction of anaesthesia, using a vital capacity breath of halothane, nitrous oxide and oxygen. Anaesthesia 1986; 41:472-6.

  10.   Yurino M, Kimura H. A comparison of vital capacity breath and tidal breathing techniques for induction of anaesthesia with high sevoflurane concentrations in nitrous oxide and oxygen. Anaesthesia 1995; 50:308-11.

  11.   Hall JE, Oldham TA, Stewart JIM, Harmer M. Comparison between halothane and sevoflurane for adult vital capacity induction. Br J Anaesth 1997; 79:285-8.

  12.   O'Shea H, Moultrie S, Drummond GB. Influence of nitrous oxide on induction of anaesthesia with sevoflurane. Br J Anaesth 2001; 87:286-8.

  13.   Bacher A, Burton AW, Uchida T, Zornow MH. Sevoflurane or halothane anaesthesia: can we tell the difference? Anesth Analg 1997; 85:1203-6.

  14.   Strickland TL, Drummond GB. Comparison of pattern of breathing with other measures of induction of anaesthesia, using propofol, methohexital, and sevoflurane. Br J Anaesth 2001; 86:639-44.

  15.   Yurino M, Kimura H. Induction of anaesthesia with sevoflurane, nitrous oxide, and oxygen: a comparison of spontaneous ventilation and vital capacity rapid inhalation induction (VCRII) techniques. Anesth Analg 1993; 76:598-601.

  16.   Dempsey JA, Skatrud JB. A sleep-induced apneic threshold and its consequences. Am Rev Respir Dis 1986; 133:1163-70.

  17.   Bain JA, Spoerel WE. Flow requirements for a modified Mapleson-D system during controlled ventilation. Canad Anaesth Soc J 1973; 20:629-36.

 

Table 1

Characteristics of the patients studied. The patients in the kinetic study are a subset of the induction study patients.