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Department of Anesthesiology, American University Medical Center, Beirut, Lebanon
Address correspondence and reprint requests to Anis Baraka, MD, FRCA, American University of Beirut, Department of Anesthesiology, PO Box 11 0236, Beirut, Lebanon. Address e-mail to abaraka{at}aub.edu.lb
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Abstract |
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IMPLICATIONS: Oxygenation by using maximal exhalation before tidal volume breathing produced a significantly faster increase in end-expiratory oxygen concentration than oxygenation with tidal volume breathing alone.
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Introduction |
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This study was performed in healthy volunteers to compare the rate of increase of the end-expiratory oxygen concentration (EEO2) when maximal exhalation is used before oxygenation by traditional TVB with that achieved by traditional TVB without prior maximal exhalation. EEO2 has been previously shown to reflect the alveolar oxygen concentration and, hence, can be used clinically as a noninvasive tool to construct an oxygen washin curve in awake patients before the induction of anesthesia (7).
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Methods |
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A standard anesthesia machine (Datex ADU AS/3 anesthesia monitor; Helsinki, Finland) was used throughout the study with an absorber system and a 2-L reservoir bag. The reservoir bag was fully inflated by using the oxygen flush, and the mask was partially occluded with the palm of the hand. Oxygenation was performed while the volunteers were in the supine position by using a properly sized, tightly fitting face mask that ensured no leak. The traditional oxygenation technique consisted of 3 min of TVB with an oxygen flow of 10 L/min. In the second technique, the volunteers exhaled maximally to room air before TVB with the same oxygen flow. Oxygen saturation was continuously monitored throughout the investigation by pulse oximetry and ranged from 98% to 100%. The two oxygenation techniques were performed in a randomized order on each volunteer. Randomization was performed by the toss of a coin. Between the two techniques, volunteers breathed room air for approximately 5 min until EEO2 returned to its baseline value. Each individual served as his or her own control.
Measured variables included inspired oxygen concentrations, EEO2, and ETCO2. Measurements were recorded with a gas monitor (Datex ADU AS/3). Sidestream respiratory gases were sampled from a sampling port interposed between the face mask and the Y-piece of the anesthetic circuit. Calibration with known gas mixtures was performed according to the manufacturer’s specifications before each patient’s experiment. EEO2 values during the 2 oxygenation techniques were determined at the beginning of the experiment and every 15 s thereafter for 3 min. The washin curve of each oxygenation technique was constructed by plotting the mean ± SD of the EEO2 values at different time intervals. The time constant (
) of each oxygenation technique was estimated from fitting an exponential model to both washin curves.
A power analysis was performed to determine the number of subjects needed for the study. For that analysis, we considered that a 10% change in EEO2 was clinically significant. We also considered Type I and Type II errors of 5% and 10%, respectively. In a previous pilot study, we found that the SD of EEO2 is approximately 15%. On this basis, the power analysis indicated that at least 23 subject were needed for the study. All data were reported as mean ± SD. Student’s t-test and analysis of variance were used for statistical analysis. Significance was considered at P < 0.05.
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Results |
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The mean ± SD values of EEO2 with both oxygenation techniques are plotted in Figure 1. Both curves increased exponentially with values of 58 s during TVB without maximal exhalation and 35 s during TVB after maximal exhalation. The plateau level of the oxygen washin curve, defined as the first EEO2 statistically not different from the EEO2 obtained at 3 min, was achieved within 120 s during TVB without prior maximal exhalation, as compared with 105 s during TVB after maximal exhalation.
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Discussion |
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Denitrogenation during spontaneous breathing is 95% complete within two to three minutes when a subject is breathing a normal tidal volume from a circle anesthesia system with an oxygen flow of 5 L/min (3). With normal lung function, the oxygen washin and the nitrogen washout are exponential functions (9), and, thus, the rate of preoxygenation (denitrogenation) is governed by the of the curves. This is the same for both the washin and washout exponential functions and is proportional to the ratio of the FRC divided by the alveolar ventilation. As such, any increase in the alveolar ventilation by deep breathing will shorten the of the washin (washout) curve and increase the rate of preoxygenation (denitrogenation). This effect of deep breathing has been previously reported by Gold et al. (5), Baraka et al. (6), and Nimmagadda et al. (10). Similarly, any reduction in the FRC will also shorten and increase the rate of preoxygenation (denitrogenation). Combining the effects of a decreased FRC and an increased alveolar ventilation as achieved by maximal exhalation followed by deep breathing should result in the most rapid preoxygenation maneuver. A previous report by McCrory and Matthews (11) showed that maximal exhalation before application of the face mask improved preoxygenation when four vital capacity breaths were used. Also, Baraka et al. (12) demonstrated that forced exhalation even before a single vital capacity breath provided, within 30 seconds, a mean arterial oxygen tension value comparable to that achieved by TVB for 2–3 minutes.
However, deep breathing may result in rebreathing of expired nitrogen (13) because the minute ventilation volume may markedly exceed the fresh gas flow (14,15). This rebreathing reduces the inspired oxygen concentration and consequently decreases the rate of nitrogen washout/oxygen washin. A high fresh gas flow is required not only to prevent rebreathing, but also to match the high peak flow rates during deep-breathing maneuvers. Furthermore, some patients may be unable to perform or to maintain deep breathing because of preexisting medical illness, sedation, or fatigue.
The maximal exhalation to room air before breathing oxygen by the traditional TVB decreases the FRC to approximately the residual volume (11). In a healthy subject with an approximate FRC of 3 L, forced exhalation will reduce the lung volume to almost 1.5 L. This represents approximately 50% reduction in the FRC. The FRC, which is the main store of O2, reverts back to its original volume for the remainder of the oxygenation scheme. However, where the forced exhalation breath is 80% nitrogen and 20% oxygen, the initial breath after maximal exhalation is 100% oxygen, resulting in a subsequent 50% reduction in the FRC nitrogen volume. The 50% reduction of the FRC nitrogen volume will lead to a 50% reduction in the of the oxygen washin (nitrogen washout) curve. These postulations are confirmed by this report, in which a short ( = 35 seconds) of the oxygen washin curve was achieved with maximal exhalation before oxygenation with TVB. This represents almost a 50% reduction from the ( = 58 seconds) that was achieved with TVB oxygenation without prior maximal exhalation. Thus, the forced expiratory maneuver before tidal volume oxygenation decreases the of the oxygen washin exponential curve, with a consequent increase of the EEO2 to approximately 70% after 30 seconds and to approximately 80% after 60 seconds. As such, this technique may provide not only the fastest but also the most efficient TVB mode of preoxygenation. This technique may be particularly advantageous in emergency situations when rapid preoxygenation is required or in patients who cannot tolerate the face mask for a prolonged period.
In conclusion, this study used the EEO2 as a simple, noninvasive, clinical surrogate marker for evaluating the degree of oxygenation in awake patients breathing spontaneously. The results show that maximal exhalation before oxygenation by TVB can hasten oxygenation. This may be secondary to an initial decrease of the FRC to approximately the residual volume, which decreases the lung nitrogen volume and the subsequent dilution of the incoming O2.
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  D. T. Neilipovitz and E. T. Crosby No evidence for decreased incidence of aspiration after rapid sequence induction: [Aucune donnee probante concernant l'incidence reduite d'inhalation apres l'induction en sequence rapide] Can J Anesth, September 1, 2007; 54(9): 748 - 764. [Abstract] [Full Text] [PDF]
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U. Nimmagadda, M. R. Salem, N. J. Joseph, and I. Miko Efficacy of preoxygenation using tidal volume and deep breathing techniques with and without prior maximal exhalation: [Efficacite de la preoxygenation utilisant les techniques de respiration en volume courant et de respiration profonde avec et sans expiration maximale prealable] Can J Anesth, June 1, 2007; 54(6): 448 - 452. [Abstract] [Full Text] [PDF]
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S. Taha, M. El-Khatib, S. Siddik-Sayyid, C. Dagher, J.-M. Chehade, and A. Baraka Preoxygenation with the Mapleson D system requires higher oxygen flows than Mapleson A or circle systems: [La preoxygenation avec le systeme Mapleson D requiert un debit d'oxygene plus eleve que les systemes Mapleson A ou en cercle] Can J Anesth, February 1, 2007; 54(2): 141 - 145. [Abstract] [Full Text] [PDF]
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) versus tidal volume breathing alone (•). *P < 0.05.
