Department of Anaesthesiology, Intensive Care Medicine and Pain Therapy, Unfallkrankenhaus Berlin, Hospital of the Free University of Berlin, Warener Straße 7, D-12683 Berlin, Germany*Corresponding author
Accepted for publication: August 14, 2001
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Abstract |
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Br J Anaesth 2001; 87: 92831
Keywords: anaesthetic techniques, induction; equipment, masks anaesthesia
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Introduction |
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Materials and methods |
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The study was approved by the ethics committee of the Free University of Berlin. Thirty healthy, non-smoking volunteers whose age ranged from 22 to 70 yr (median 44.5 yr) were included after pulmonary function testing to exclude respiratory disease. All tests were performed using a standardized breathing system (Fig. 1). To supply 100% oxygen, the inhalation limb of a Sulla 808 respirator (Drägerwerk, Lübeck, Germany) was used. The concentrations of oxygen and carbon dioxide in every breath were measured using a gas monitor (M 1026 A; Hewlett- Packard, Palo Alto, CA, USA) that was connected to the respiration system via a sampling tube (HP 13901A; Hewlett-Packard). The gas monitor was calibrated before the experiments using a standardized gas sample (HP 13906B; Hewlett-Packard). The expiratory minute volume was measured by a calibrated volumeter (Datex-Ohmeda, Helsinki, Finland).
A standard respiratory mask (Rüsch Hospital Vertriebs, Vertriebs GmbH, Böblingen, Germany), a mouthpiece with a nose-clip (preOX-RS, Schwalbach, Germany) and a one-way NasOral-system (preOX-RS) were connected to the tubing system (Fig. 1). Because of the modifications, some mixing between the inspiratory and expiratory air occurred at the connector when the face mask and the mouthpiece were used. The connector consisted of a standard Y-piece and a small filter. The volume between the junctions of the Y-piece and the content of the filter was about 30 ml. Testing for leaks was done by connecting the gas outlet to the expiration branch of the respirator and testing for leakage according to the manufacturers instructions. Inspiratory oxygen flow was adjusted to about 8 litre min1 to keep the reservoir bag half-filled but not distended. The trials were performed in different sequences. For the purpose of the analysis, six volunteers were allocated to one of five age groups. The volunteers were studied supine with 20° head elevation. The studies were done by one examiner in a standard way. Every trial was started with room air to measure the baseline FE'O2. Hyperventilation (defined as a FE'CO2 of less than 32 mmHg) was ruled out by capnometry. After four tidal volume breaths, the system was switched to 100% oxygen. Recording was continued until there was no further change in the FE'O2 of three consecutive breaths, which was defined as the maximal FE'O2. Between the tests, the volunteers breathed room air for a minimum of 10 min until the FE'O2 had returned to the basal value. After completion of the test series, the volunteers were asked to score the devices for comfort, a feeling of adequate gas supply, and anxiety using, a visual analogue scale (VAS).
Statistical analysis was performed using SPSS 9.0 for Microsoft Windows. For comparisons, the Wilcoxon test, rank correlation analysis according to Spearman, the 2 test and Fishers exact test were used. Data are given as mean (SD).
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Results |
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FE'O2
Using the face mask, the maximal FE'O2 was significantly higher [91.0 (2.9)%; maximum 95.0%, minimum 82.4%] than with the modified NasOral system [87.3 (8.4)%; maximum 96.5%, minimum 54.5%] (P<0.05). Preoxy genation with the mouthpiece gave a greater maximal FE'O2 [92.1 (2.6)%; maximum 96.0%, minimum 84.0%] than the face mask (P<0.05) and modified NasOral system (P<0.01). A FE'O2 of 0.90 was reached by 73% (22 of 30) of the volunteers with the face mask or mouthpiece. Only 46% (14 of 30) of the volunteers were able to reach a FE'O2 of 0.90 using the modified NasOral system. This did not vary according to the age of the volunteers. There was no statistically significant correlation between the FE'O2 that any volunteer obtained with different preoxygenation devices.
Using the mouthpiece, the FE'O2 after 2 min was significantly greater than using the face mask [91.0 (3.0)%; maximum 96.0%, minimum 84.5% vs 89.0 (4.2)%; maximum 94.0%, minimum 76.0%; P<0.01] or the modified NasOral system [86.6 (8.6)%; maximum 95.5%, minimum 52.5%; P<0.01]. There was no statistically significant difference between face mask and modified NasOral system.
The time needed to reach the maximal FE'O2 was significantly less using the NasOral system [100 (30) s; maximum 171 s, minimum 34 s] than using the face mask [131 (31) s; maximum 180 s, minimum 67 s] (P<0.001) or the mouthpiece [132 (28) s; maximum 192 s, minimum 88 s] (P=0.0001). There was no significant difference between face mask and mouthpiece.
VAS ratings
There was no significant difference between the VAS ratings of face mask and mouthpiece [66 (23.0) vs 68 (23.5)%]. Both had significantly better ratings than the modified NasOral system [42 (25)%] (both P<0.01).
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Comment |
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Our results indicate that a mouthpiece with a nose-clip can be an alternative for preoxygenation. The disadvantage of preoxygenation using a mouthpiece, however, is the difficulty of ventilating the patient after induction of anaesthesia. Furthermore, infants and about 7% of adults are reported to have difficulty breathing by the oral route. This appears to be more common in the elderly.12
The time to reach the maximal FE'O2 was significantly less with the modified NasOral system, which is partly explained by the higher expiratory minute volume using this system. However, fewer volunteers reached an FE'O2 of 0.90 and the mean FE'O2 was less, which are significant disadvantages. These results are surprising, because preoxygenation with the NasOral system has the advantage in principle that no expiratory air re-enters the inspiration branch. We did not observe significant leakage of the nasal mask included with the NasOral system. The probable reason for the poor results is the special breathing technique necessary with the modified NasOral system. Difficulties with this technique were reported by many of the volunteers and could explain the limited acceptability of this system. The breathing technique with the modified system is not different from that with a standard NasOral system. The compliance of emotionally distressed patients would certainly be much worse than in the quiet setting of a controlled study. Therefore, our results do not support the introduction of this system into clinical practice.
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References |
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