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Correspondence |
Duesseldorf, Germany
To the Editor:
In their recent paper,1 Baraka et al. compared in ten adult patients the effect of preoxygenation by a single vital capacity breath (SVCB) with preoxygenation by three minutes of tidal volume breathing on mean arterial oxygen partial pressure (PaO2). From the finding that PaO2 was not different between the two groups, the authors conclude in their abstract that the single vital capacity breath technique "can rapidly provide adequate preoxygenation within 30 sec".
We agree that the study contributes interesting data. However, we feel that a problem arises from using solely PaO2 as a marker of "adequate preoxygenation". In his recent paper in Anesthesiology, we can learn from the same author that "the time to desaturate is a more appropriate outcome measure for the efficiency of preoxygenation".2 And he further explains that "the only reason that we perform preoxygenation maneuvers is to attempt to increase the oxygen body store and to prevent hemoglobin desaturation, and this is obviously a function of more than acute changes in PaO2". Indeed, several studies have shown that the time to desaturation after a few vital capacity breaths is significantly shorter compared with a longer period of tidal volume breathing of 100% oxygen (for review see references 3 and 4).
A minor problem may arise from the study protocol with two different fresh gas flows of 5 and 10 L•min–1, respectively. The lower flow was used in the standard preoxygenation group with an adult Mapelson D circuit. Although this flow is comparable to the alveolar ventilation,5 it does not prevent nitrogen rebreathing2,4 and thus, may not provide optimal preoxygenation.
In conclusion, the data of the study of Baraka et al.1 do not show that a single vital capacity breath "can rapidly provide adequate preoxygenation within 30 sec". Although this method may improve oxygenation before a fast induction of inhalational anaesthesia as suggested by the authors, even in this setting, traditional preoxygenation methods remain a more efficient method to improve patient safety by prolonging the time to arterial desaturation.
References
1 Baraka A, Haroun-Bizri S, Khoury S, Chehab IR. Single vital capacity breath for preoxygenation. Can J Anaesth 2000; 47: 1144–6.[Abstract]
2 Baraka AS, Taha SK, Aouad MT, El-Khatib MF, Kawkabani NI. Preoxygenation: comparison of maximal breathing and tidal volume breathing techniques. Anesthesiology 1999; 91: 612–6.[Medline]
3
Campbell IT, Beatty PCW. Monitoring preoxygenation. Br J Anaesth 1994; 72: 3–4.
4 Anonymous. Preoxygenation: physiology and practice. Lancet 1992; 339: 31–2.[Medline]
5
Baraka A. PCO2 control by fresh gas flow during controlled ventilation with a semi-open circuit. Br J Anaesth 1969; 41: 527–30.
Beirut, Lebanon
Thank you for referring to me the constructive comments of Schlack and Heck concerning our report "Single vital capacity breath for preoxygenation".1 The report shows that the single vital capacity breath (SVCB) technique, which has been recommended for fast induction of inhalation anesthesia, can also result in rapid increase of arterial PO2.
The main oxygen store exists in the functional residual capacity of lung (FRC), and hence all techniques of preoxygenation are based on denitrogenation of FRC, using 100% oxygen. The arterial PO2 rapidly equilibrates with the alveolar PO2, and can be taken as a marker for the degree of alveolar denitrogenation.
The alveolar and arterial oxygen stores are supplemented by the oxygen tissue stores which need extra time of preoxygenation. Thus, the subsequent desaturation time may differ despite similar PaO2 values.2 It is expected that preoxygenation by the traditional tidal volume breathing for three minutes or by the eight deep breaths for 60 sec2 can delay desaturation more than preoxygenation by the four deep breaths for 30 sec3 or by the SVCB.1
The traditional or the eight deep breaths techniques are preferred whenever prolonged apnea is expected in patients with difficult airway, or whenever FRC is decreased in the pregnant or obese patients, and in patients with pulmonary dysfunction.
SVCB after forced exhalation can be used for rapid preoxygenation in patients with normal airway and FRC, particularly in those who cannot tolerate a tight-fitting face mask for a long period. Forced exhalation to residual volume may be used to enhance denitrogenation of FRC not only prior to the SVCB technique, but also before other techniques of preoxygenation.
References
1 Baraka A, Haroun-Bizri S, Khoury S, Chehab IR. Single vital capacity breath for preoxygenation. Can J Anesth 2000; 47: 1144–6.
2 Baraka AS, Taha SK, Aouad MT, El-Khatib MF, Kawkabani NI. Preoxygenation: Comparison of maximal breathing and tidal volume breathing techniques. Anesthesiology 1999; 91: 612–6.
3
Gold M, Duarte J, Muravchik S. Arterial oxygenation in conscious patients after 5 minutes and after 30 seconds of oxygen breathing. Anesth Analg 1981; 60: 313–5.
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