Preoxygenation: a comparison of three different breathing systems

J. Hirsch, I. Führer, P. Kuhly and W. Schaffartzik

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


    Abstract
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Comment
 References
 
An end-tidal expiratory oxygen concentration (FE'O2) greater than 0.90 is considered to be adequate for preoxygenation. This is generally achieved using a face mask, but this can be unsatisfactory in some patients. We compared preoxygenation in 30 healthy volunteers using a face mask, the NasOral system, which is a novel preoxygenation device, and a mouthpiece with a nose-clip. We measured the maximal FE'O2, the FE'O2 after 2 min and the time to reach maximal FE'O2 and recorded the subjective judgement of the volunteers. The maximal FE'O2 with face mask and mouthpiece was significantly greater than with the modified NasOral system (P<0.05 and P<0.01). With the former devices, a FE'O2 of 0.90 was achieved in 73% of the volunteers vs 46% with the modified NasOral system. Using the mouthpiece, the FE'O2 after 2 min was significantly higher than using the face mask (P<0.01) or the modified NasOral system (P<0.01). The time to maximal FE'O2 was significantly shorter using the modified NasOral system than with the face mask or mouthpiece (P<0.001 and P=0.0001). The volunteers gave more positive ratings to the face mask and mouthpiece than to the modified NasOral system (P<0.001 and P<0.01). We conclude that the use of a mouthpiece can improve preoxygenation in some patients. The results obtained with the modified NasOral system do not justify its introduction into clinical practice.

Br J Anaesth 2001; 87: 928–31

Keywords: anaesthetic techniques, induction; equipment, masks anaesthesia


    Introduction
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Comment
 References
 
Preoxygenation is a standard procedure to prevent arterial desaturation during the induction of general anaesthesia.1 It is considered to be adequate if end-tidal expiratory oxygen concentration FE'O2 is greater than 0.90.24 This is most often achieved by 1.5–3 min of tidal volume breathing with 100% oxygen via a face mask.4 5 The use of a mouthpiece with a nose-clip has been shown to provide a similar FE'O2.3 6 Recently, a novel device for preoxygenation, the NasOral system, has been introduced.7 8 Inspiration with this system is from a nasal mask equipped with a one-way valve and a reservoir bag that ensures a steady, unidirectional gas flow. The patient exhales via a sealed mouthpiece, which is also fitted with a one-way valve. 9 The expense of this system and the need for different tubing has prevented further evaluation. Therefore, we fitted the valves in the inspiratory and expiratory branches to allow re-use of the whole tubing system, including the reservoir bag (Fig. 1).



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Fig 1 On the left, the tubing system fitted with a 3.5 litre reservoir bag is shown. The valves in the inspiratory and expiratory branches create a unidirectional gas flow. Preoxygenation with the NasOral system was performed as shown with the inspiratory branch in position A. Using the face mask or mouthpiece, the tubing was connected as indicated in position B. The connector consisted of a standard Y-piece and a filter and had a volume of about 30 ml. By connecting a mouthpiece with a nose-clip, a face mask and a NasOral system (as shown on the right) to this tubing system, the efficacy of the three devices was compared.

 

    Materials and methods
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Comment
 References
 
We compared preoxygenation with tidal volume breathing using a standard face mask, a mouthpiece with a nose-clip and the modified NasOral system using the following variables: (1) the maximal FE'O2 and the ability to obtain an FE'O2 of 0.90; (2) the FE'O2 after preoxygenation for 2 min; (3) the subjective judgement of the volunteers on the devices.

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 manufacturer’s instructions. Inspiratory oxygen flow was adjusted to about 8 litre min–1 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 {chi}2 test and Fisher’s exact test were used. Data are given as mean (SD).


    Results
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Comment
 References
 
Expiratory minute volume
The expiratory minute volume of the volunteers was significantly less with the face mask [9.6 (1.1) litres] and the mouthpiece [9.5 (1.2) litres] than with the NasOral system [10.8 (1.6) litres] (both P<0.001). There was no significant difference between face mask and mouthpiece.

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).


    Comment
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Comment
 References
 
Our finding that, using a face mask, a FE'O2 of 0.90 can only be reached in about 75% of persons confirms previous studies.10 Poor patient cooperation, anxiety, the shape of the patient’s face, a beard, facial burns, trauma and obesity can impede the face mask seal. A good face mask seal is necessary for adequate preoxygenation.11 Because many of the factors above are also risk factors for difficult tracheal intubation, good preoxygenation is important for safety during induction of anaesthesia.1 FE'O2 measurement has become common practice in most centres. Therefore, patients who cannot be preoxygenated sufficiently with a face mask can be identified before anaesthesia induction. In our study, the results of preoxygenation with one breathing system was not significantly correlated to the results with the other devices. A patient who cannot be adequately preoxygenated with a face mask may therefore be preoxygenated sufficiently with another breathing system.

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.


    References
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Comment
 References
 
1 Kung MC, Hung CT, Ng KP, et al. Arterial desaturation during induction in healthy adults: should preoxygenation be a routine? Anaesth Intens Care 1991; 19: 192–6[ISI][Medline]

2 Bhatia PK, Bhandari SC, Tulsiani KL, Kumar Y. End-tidal oxygraphy and safe duration of apnoea in young adults and elderly patients. Anaesthesia 1997; 52: 175–8[ISI][Medline]

3 Winship S, Skinner A. Vital capacity and tidal volume preoxygenation with a mouthpiece. Br J Anaesth 1998; 81: 787–9[Abstract/Free Full Text]

4 Braun U, Hudjetz W. [Duration of preoxygenation in patients with normal and impaired lung function.] Anaesthesist 1980; 29: 125–31[ISI][Medline]

5 Rooney MJ. Pre-oxygenation: a comparison of two techniques using a Bain system. Anaesthesia 1994; 49: 629–32[ISI][Medline]

6 Everatt J, Ng WS. Pre-oxygenation using face mask or mouthpiece with and without nose clip: patient preferences and efficacy. Anaesthesia 1998; 53: 387–9[ISI][Medline]

7 Mertzlufft F, Zander R. Optimal pre-oxygenation: the NasOral system. Adv Exp Med Biol 1994; 345: 45–50[Medline]

8 Mertzlufft F, Zander R. A new device for the oxygenation of patients: the NasOral system. Adv Exp Med Biol 1992; 317: 421–7[Medline]

9 Mertzlufft F, Zander R. [Intrapulmonary O2 storage with the NasOral system.] Anästhesiol Intensivmed Notfallmed Schmerzther 1994; 29: 235–7[Medline]

10 Berry CB, Myles PS. Preoxygenation in healthy volunteers: a graph of oxygen ‘washin’ using end-tidal oxygraphy. Br J Anaesth 1994; 72: 116–8[Abstract]

11 McGowan P, Skinner A. Preoxygenation—the importance of a good face mask seal. Br J Anaesth 1995; 75: 777–8[Abstract/Free Full Text]

12 Ward KA, Nicholls DP, Stanford CF. The prevalence of preferential nasal breathing in adults. Respir Med 1993; 87: 295–7[ISI][Medline]