Administration of oxygen before anesthetic induction and tracheal intubation (preoxygenation) is a widely accepted maneuver designed to increase oxygen reserves and thereby delay the onset of arterial hemoglobin desaturation of oxygen during apnea (1–3). This is particularly important when manually controlled ventilation is undesirable, as during rapid sequence induction of anesthesia or when difficult tracheal intubation and/or difficult ventilation are suspected (4).
Various techniques and regimens have been advocated to accomplish preoxygenation. Tidal volume breathing (TVB) of oxygen for 3–5 min has been commonly practiced (1,2). Gold et al. (5) challenged the need for 3 min of TVB (TVB/3 min) by demonstrating that four deep breaths (DB) within 0.5 min and TVB for 5 min (TVB/5 min) using a semiclosed circle absorber system were equally effective in increasing arterial oxygen tension (Pao2). Although some subsequent investigations corroborated their findings (6,7), others showed that TVB/3 min provided more effective preoxygenation, and longer protection against hypoxemia during apnea than the 4 DB/0.5 min method (8–11). Moreover, Baraka et al. (12) showed 8 DB/min at an oxygen flow of 10 L/min could produce Pao2 values comparable to that achieved with TVB/3 min and also delay the onset of apnea-induced hemoglobin desaturation of oxygen in comparison with TVB/3 min and 4 DB/0.5 min.
The inconsistent findings in the previous studies are likely attributable to methodological differences including those related to patient populations, various anesthetic systems (circle absorber (5–8), Mapleson A (11,13), Mapleson D (9–12), and nonrebreathing systems (2,10,14)) and widely varying fresh gas flows (FGFs) ranging from 4 L/min to 35 L/min (7,9). Because of the breathing characteristics of the circle absorber system (15), the minute ventilation during deep breathing may exceed the FGF and may result in rebreathing of exhaled gases at a smaller Fio2 than during TVB. Thus, the level of FGF may influence the effectiveness of preoxygenation, especially during deep breathing. There have been no studies comparing the effectiveness of the various techniques of preoxygenation and the influence of varying FGF in the same subjects using the circle absorber system. Accordingly, the current study was undertaken in adult healthy volunteers to compare the effectiveness of preoxygenation using TVB, 4 DB/0.5 min, and the newly described 8 DB/min methods, to assess the effect of extending the deep breathing technique to 1.5 and 2 min, and to investigate the influence of varying FGF on preoxygenation using the aforementioned techniques.
Materials and Methods
After IRB approval, 24 consenting healthy ASA physical status I volunteers (14 men, 10 women) with a mean age (± sd) of 36 ± 8 yrs and a mean weight of 69 ± 13 kg, without history of lung or cardiac disease, were studied. The volunteers were nonbearded and nonsmoking operating room personnel recruited without inducement. They were allowed to eat a light breakfast on the day of the studies. The preoxygenation techniques were explained to the subjects and ample time was allowed for them to become familiar with mask breathing while maintaining a tight seal.
A single anesthesia machine (Model 210 Excel; Datex-Ohmeda, Madison, WI) was used throughout the studies. The circle absorber system consisted of an Ohmeda GMS (Gas Management System) absorber with barium hydroxide lime, USP (United States Pharmacopia) (Baralyme; Chemetron Medical Division, St. Louis, MO) absorbent, disposable 60-in corrugated breathing tubes, and a 3-L capacity natural latex breathing bag (King Systems, Noblesville, IN). The reservoir bag was fully inflated using the oxygen flush and partially occluding the mask. Preoxygenation was performed with 100% oxygen and a tight-fitting mask, with the subjects in the supine position. The following techniques were tested in each subject: 1) normal TVB for a 5-min period (TVB/5 min) and 2) DB for a 2-min period (4 DB/0.5 min, 8 DB/min, 12 DB/1.5 min & 16 DB/2 min). During TVB, the subjects were periodically reminded to maintain a normal tidal volume and a normal respiratory rate (RR). During DB, the subjects were prompted to breathe deeply at 7.5-s intervals. Each preoxygenation technique was tested using 5 L/min, 7 L/min, and 10 L/min FGF in each subject. The tests were performed in a randomized order with respect to technique and FGF using a computer-generated random assignment table. Data were collected during room air TVB (baseline) and at 0.5-min intervals during oxygen administration. Fifteen-min periods of room air breathing were allowed between tests.
Measured variables included inspired oxygen concentration (InspO2) and end-tidal oxygen concentration (ETO2), end-tidal nitrogen (ETN2), end-tidal carbon dioxide (ETco2), and RR. Measurements were recorded using an Ohmeda Rascal II anesthetic and respiratory gas monitor (Datex-Ohmeda). Side-stream respiratory gases were sampled at a rate of 240 mL/min from a sampling port interposed between the filter and the Y-piece of the anesthetic circuit. Calibration with known gas mixtures was carried out according to the manufacturer’s specifications.
Survival statistics were applied to ETO2 versus time (washin) curves to determine differences between FGFs in reaching a target ETO2 ≥90%. The Wilcoxon (Gehan) statistic and two-tailed probabilities were calculated and used for overall and pairwise comparisons of the washin curves. A one-way analysis of variance for repeated measurements was used to detect differences in responses as functions of FGF (5, 7, and 10 L/min) and time. After determining the time point at which ETO2 reached a plateau, the Student-Newman-Keuls test was used as a post hoc test. Individual Student’s t-tests were used to identify statistical differences between techniques (TVB versus DB) using pooled mean values at various time intervals. Statistical significance was accepted when P < 0.05. Values are reported as mean ± sd.
During TVB, the InspO2 remained unchanged during the 5-min periods of observation at all FGFs tested. During TVB with an FGF of 5 L/min, InspO2 (aggregate mean, 95.8%) was less than that seen with 7 and 10 L/min FGF (aggregate means, 98.1% and 98.4% for 7 and 10 L/min, respectively) (Fig. 1). The InspO2 values during TVB using 7 and 10 L/min were not different from each other. During DB, InspO2 values increased significantly as FGF was increased from 5 to 7 and 10 L/min (aggregate means, 88.7%, 91.8%, and 95.1%, at 5, 7, and 10 L/min FGF, respectively. In addition, InspO2 increased significantly at 12 DB/1.5 min and 16 DB/2 min, as compared with 4 DB/0.5 min and 8 DB/min (Fig. 1).
During TVB, ETO2 increased rapidly between 0.5 and 2 min and plateaued by 2.5 min at 86.2 ± 4.5%, 88.1 ± 4.8%, and 87.7 ± 5.4% with 5, 7, and 10 L/min FGF, respectively (Fig. 2). Mean ETO2 ≥90% were reached between 3 and 4 min of TVB. The ETN2 decreased inversely (Fig. 3). There were no changes in ETO2 or ETN2 between 2.5 and 5 min. During TVB, mean ETco2 and RR did not vary (Fig. 4). There were no differences in ETO2, ETN2, ETco2 or RR between 5, 7, and 10 L/min FGF during TVB (Figs. 2–4). In addition, pairwise survival analysis (volunteers reached a target ETO2 ≥90% faster) revealed no differences among the three washin curves.
During DB, ETO2 increased steeply (as ETN2 decreased in a parallel fashion) to 75.2 ± 8.7%, 77.5 ± 5.6%, and 80.1 ± 4.6% at 4 DB/0.5 min using 5, 7, and 10 L/min FGF. These values did not differ from each other. With 5 L/min FGF, ETO2 did not change when DB was continued to 8 DB/min, 12 DB/1.5 min, or 16 DB/2 min (Fig. 2). With 7 and 10 L/min FGF, ETO2 continued to increase during extended DB, although much less steeply. At 8 DB/min, ETO2 increased with 7 and 10 L/min FGF to 82 ± 5% and 87 ± 3%, respectively. However, with 12 DB/1.5 min and 16 DB/2 min, ETO2 reached 90% or higher with 10 L/min FGF (Fig. 2). Pairwise survival analysis demonstrated that DB with 10 L/min FGF was superior (i.e., more volunteers reached a target ETO2 ≥90% faster) compared with 5 and 7 L/min FGF.
Regardless of FGF and duration, DB resulted in higher ETO2 (and lower ETN2) in comparison with TVB/0.5 min and TVB/min. The value for ETO2 at 16 DB/2 min using 10 L/min was equivalent to TVB/3 min and TVB/3.5 min, but smaller than that with TVB/4 min to TVB/5 min (Figs. 2, 3).
During DB, ETco2 decreased from a room air value of 40.4 ± 3.9 mm Hg to 36.1 ± 3.9, 33.8 ± 5.1, and 34.5 ± 5.2 mm Hg at 1.5 min using 5, 7, and 10 L/min FGF, respectively. At 16 DB/2 min, ETco2 decreased to 34.2 ± 5.1, 32.4 ± 5.0, and 33.8 ± 4.9 mm Hg at 5, 7 and 10 L/min FGF, respectively (Fig. 4).
Both TVB and DB were well tolerated and acceptable to all volunteers. However, on ambulation, five subjects complained of transient dizziness and one subject complained of transient nausea, all after completion of 16 DB/2 min. There were no other complications related to the study.
Previous studies on preoxygenation have focused on measurements of indices reflecting its efficacy and efficiency (16). Measurements of alveolar oxygenation (11,17,18), alveolar denitrogenation (19), or Pao2(5,12,19,20) reflect the efficacy of preoxygenation, whereas the decline of hemoglobin saturation of oxygen during apnea is indicative of its efficiency. Because many factors can cause faster arterial hemoglobin desaturation of oxygen during apnea (3,5,13,21), the need to maximally preoxygenate before anesthetic induction and tracheal intubation is essential, especially in patients with oxygen transport limitations and those in whom difficult intubation and/or difficult ventilation are anticipated (15,22,23). Maximal preoxygenation is achieved when the alveolar, vascular (arterial and venous), and tissue compartments are all saturated with oxygen (16,24). Failure to breathe a high InspO2(25), inadequate time for preoxygenation (16,17), and the presence of leak under the mask (3,11,26,27) can all result in submaximal alveolar oxygenation. In the current investigation, the volunteers received training in mask breathing beforehand, so as to avoid leaks, which influence InspO2(3,11,26,27). Although various anesthetic systems can be used for preoxygenation, we studied the circle absorber because it is the system most frequently used in the operating room and it is as effective as other systems specifically designed for preoxygenation (25).
The end points of alveolar preoxygenation and denitrogenation have been defined as an ETO2 of approximately 90% and an ETN2 of 5%(9,17,18). Applying this criterion to our findings, it is obvious that at least 3 min of TVB is needed to achieve maximal preoxygenation. Increasing the FGF from 5 L/min to 10 L/min did not result in any faster increase in ETO2 or decline in ETN2 during TVB. This implies that when a circle absorber system is used, the resulting minor differences in InspO2 and nitrogen rebreathing with increasing FGF have no discernible impact on preoxygenation during TVB. This finding does not imply that using high FGF has no merit during preoxygenation. In clinical practice, it may somewhat compensate for any existing leak around the face mask (3,11,26,27).
Although not all previous studies are in agreement, most have found the 4 DB/0.5 min method of preoxygenation to be less effective than TVB for 2 min or longer. For example, two studies in parturients yielded conflicting results; one showed both techniques to be equally effective (6), whereas in the other 4 DB/0.5 min provided suboptimal alveolar oxygenation compared with TVB/2 min (11). In healthy patients, the times for arterial hemoglobin saturation of oxygen to decrease from 99%–100% to 97%, 95%, and 90% during apnea were found to be shorter after 4 DB/0.5 min compared with TVB/3 min (8). Investigations in elderly patients revealed that preoxygenation with TVB/3 min conferred longer protection against hypoxemia during prolonged apnea than did the 4 DB/0.5 min technique (9,10).
Our findings confirm that 4 DB/0.5 min provides suboptimal preoxygenation (no subject achieved ETO2 values ≥90%), resulting in ETO2 values comparable to TVB/1.5–2 min, but less than that of TVB/2.5–5 min. Despite the increase in InspO2 concomitant with the increased FGF to 10 L/min, ETO2 was unaffected with the 4 DB/0.5 min method of preoxygenation. This finding may seem to be in conflict with that of a study (12) that suggested an exponential increase in Pao2 with 4 DB/0.5 min as FGF was increased from 5 L/min to 20 L/min using a Mapleson D system. However, such high FGFs are impractical because they exceed the limits of most anesthesia machines. Because 4 DB/0.5 min results in suboptimal preoxygenation with the commonly used FGFs in a circle absorber system, this technique should be used only in emergency situations where time is limited.
Our findings confirmed a previous observation (12) that extending DB to 8 DB/min improves preoxygenation, though ETO2 values did not reach 90%. They further suggest that extending the duration of DB to 12 DB/1.5 min or 16 DB/2 min enhances preoxygenation to ETO2 values >90%, especially when high FGF is used. Thus, it appears that increasing the FGF becomes effective in enhancing preoxygenation by minimizing nitrogen rebreathing only when the duration of DB is extended beyond 1 min.
Baraka et al. (12) found that the 8 DB/min method of preoxygenation delays the onset of apnea-induced hemoglobin desaturation of oxygen in comparison with TVB/3 min (12). Because a rapid sequence induction of anesthesia was performed in that study, oxygenation via a face mask was continued until apnea, a period of time described as 15–30 s during which an additional 3–4 breaths were administered (28). Although no arterial or ETco2 measurements were obtained in their study, our findings during 12 DB/1.5 min and 16 DB/2 min suggest that hypocapnia may have occurred. Thus, the delayed onset of desaturation in this previous study may have reflected the leftward shift of the oxyhemoglobin dissociation curve rather than any real protection to the patient. Hypocapnia may have some other undesirable effects (29), including an increase in oxygen consumption (30) and cerebral vasoconstriction. Transient side effects presumably related to hypocapnia, including dizziness and nausea, were noted in our study in five volunteers after extended DB. Our data suggest that a 2-min period is the practical upper limit for DB and that, if extended beyond 2 min, DB may be associated with some undesirable side effects.
In conclusion, with the use of the circle absorber system, TVB for 3 to 5 min is effective in achieving maximal preoxygenation before induction of anesthesia. The 4 DB/0.5 min method results in suboptimal preoxygenation, and therefore should be reserved only for emergency situations where time is limited. Neither TVB nor the 4 DB/0.5 min method of preoxygenation is influenced by increasing the FGF. Although the 8 DB/min technique improves preoxygenation at higher FGFs, ETO2 values of ≥90% are not attained. Extending the duration of DB to 1.5 or 2 min with a high FGF (10 L/min) further improves preoxygenation and yields ETO2 equivalent to TVB/3 min.
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