British Journal of Anaesthesia, 2002, Vol. 89, No. 5 797-798
© 2002 The Board of Management and Trustees of the British Journal of Anaesthesia
Correspondence |
The output of two sevoflurane vaporizers in the presence of helium
London, UK
Editor—We read with interest the short communication by Carvalho and Sanders1 that highlights the issue of output from vaporizers for new inhalational anaesthetic agents.
A variation in performance is unsurprising when using a gas other than that for which the vaporizer has been specifically calibrated. However, a large variation in performance when using the carrier for which the vaporizer is specifically calibrated is unexpected.
We recently experienced a problem with the use of sevoflurane for the rapid induction of anaesthesia in a group of paediatric patients with congenital heart disease, undergoing diagnostic and interventional cardiological procedures. Patients were induced with sevoflurane at 8% dialled concentrations from one of two Blease Datum vaporizers with oxygen or air/oxygen as the carrier gas. Fresh gas flow rates between 5 and 10 litres min–1 were used with a Jackson-Rees modification of Ayre’s T-piece and standard anaesthetic flowmeters and backbar.
After a prolonged time to loss of consciousness (several minutes), and prolonged patient struggling,2 it was noted that maximum inspired concentration of sevoflurane achieved was 5% (measured at the patient end of the breathing system).
The backbars and vaporizers involved were tested by the hospital’s department of biomedical engineering, using the Ohmeda AGM 5330 and Agilent Viridia M0126A anaesthetic gas analysers. These tests were carried out under usual operating conditions (i.e. appropriate to the clinical situation). The output from the vaporizers at a fresh gas flow of 5 litres min–1 and a dialled concentration of 8% with air as the carrier gas (Table 1) was measured.
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The vaporizer performance was within the range specified by the manufacturer. Interestingly, however, the specified range is very wide (±20% of setting at 5 litres min–1 oxygen). Nielsen and colleagues3 have documented that fewer than 30% of anaesthetists would accept a variance beyond ±10% of the dial setting.
The delivery of the vaporizer decreased further at higher flows. We tested the effect of increasing flows incrementally to 12 litres min–1 on the output of the two vaporizers, with a time of 1 min between changes and measurements. This was based on advice from the manufacturer that the time to equilibrate is less than 1 min (Table 2). The results confirm our clinical observation of decreasing output with increasing flow rate.
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At a fresh gas flow of 10 litres min–1 (concentration setting of 8%) the specified output decreases by 1% (volume) compared with the base line at 5 litres min–1. Sevoflurane has a low saturated vapour pressure of 21.3 kPa at 20°C, requiring greater flow through the vaporizing chamber compared with vaporizers for other volatile agents. Previous studies have noted the lower than expected output of sevoflurane from a Sevotec 5 vaporizer at high concentrations, high flow and low fill states.4
The temperature compensation is known to be imprecise. A 5°C temperature drop reduces the output by approximately 0.25%, as specified in the supplier’s manual (greater reduction at higher fresh gas flow rates). The combination of these errors will result in an output that may be as low as 6.5% after a few minutes of vaporizer use. There may also be a small measurement error in the analysis equipment of 0.2%.
Previous studies demonstrate that rapid induction of anaesthesia with sevoflurane at maximal initial inspired concentrations of 8% can be achieved. Mean time to loss of consciousness of 1 min 12 s was observed.3
Underperformance of vaporizers will lead to prolonged inhalational induction. In paediatric patients, effects such as excessive salivation, arrhythmias and laryngospasm may be observed. We feel this is an important clinical issue. Anaesthetists need to be alerted to poor performance of equipment, particularly when the variation in performance is outside the limits most would find acceptable. Manufacturers need to address such performance problems, which may be related to poor temperature compensation with a change in flow rate. Our supplier has kindly agreed to replace all the sevoflurane vaporizers in the institution with those of a more acceptable performance characteristic.
N. A. McIlree
S. Sireau
S. Jaggar
London, UK
References
1 Carvalho B, Sanders D. The output of two sevoflurane vaporizers in the presence of helium. Br J Anaesth 2002; 88: 711–13
2 Sigston PE, Jenkins AM, Jackson EA, Sury MR, Mackersie AM, Hatch DJ. Rapid inhalation induction in children: 8% sevoflurane compared with 5% halothane. Br J Anaesth 1997; 78: 362–5
3 Nielsen J, Pedersen FM, Knudsen F, Jensen MB, Ibsen M. Accuracy of 94 anaesthetic agent vaporizers in clinical use. Br J Anaesth 1993; 71: 453–7
4 Seropian MA, Robins B. Smaller-than-expected sevoflurane concentrations using the Sevotec 5 vaporizer at low fill states and high fresh gas flows. Anesth Analg 2000; 91: 834–6
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