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Print version ISSN 0034-7094
Rev. Bras. Anestesiol. vol.55 no.5 Campinas Sept./Oct. 2005
Oxygenation: the impact of face mask coupling*
Impacto del acoplamiento de la máscara facial sobre la oxigenación
Waldemar Montoya de Gregori, M.D.I; Lígia Andrade da Silva Telles Mathias, TSA, M.D.II; Luiz Piccinini Filho, M.D.III; Ernesto Leonardo de Carpio Pena, M.D.I; Aníbal Heberto Mora Vicuna, M.D.I; Joaquim Edson Vieira, TSA, M.D.IV
IMédico Assistente, Hospital
Central, Irmandade da Santa Casa de São Paulo
IIDiretora do Serviço e Disciplina de Anestesiologia, Irmandade Santa Casa de Misericórdia de São Paulo e Faculdade de Ciências Médicas da Santa Casa de São Paulo; Responsável pelo Centro de Ensino e Treinamento, CET/SBA, ISCMSP
IIIMédico Assistente, Hospital Central, Irmandade da Santa Casa de São Paulo; Diretor do Serviço de Anestesiologia do Hospital Santa Isabel
IVCoordenador do Centro para Desenvolvimento da Educação Médica, CEDEM da Faculdade de Medicina da Universidade de São Paulo
BACKGROUND AND OBJECTIVES: Different oxygenation
techniques aim at promoting denitrogenation before apnea during induction. The
main reason why CIO2 = 100% cannot be reached is the lack of adequate
face mask coupling, allowing the entry of room air. Although anesthesiologists
know this principle, not all of them apply it correctly, facilitating the entry
of air in fresh gases flow and consequently diluting CIO2. This prospective
study was performed to comparatively evaluate, through the variation of oxygen
expired concentration (CEO2), the efficacy of the oxygenation technique
via face mask in the conditions routinely used by anesthesiologists, simulating
situations of progressive leaks.
METHODS: Oxygen end-tidal concentrations of 15 volunteers, physical status ASA I, were studied with 8 deep breaths (vital capacity) in 60 s with fresh gas flow of 10 L.min-1. The face mask was: tightly fitted with 100% CIO2 (Tf100) or varying from 50% to 90%, (Tf50, Tf60, Tf70, Tf80, Tf90); gravity-coupled to face and 100% CIO2 (Grav) and moved 1 cm away from face with 100% CIO2 (Aw). CEO2 was recorded at 10 s intervals. P < 0.05 was considered statistically significant.
RESULTS: CEO2 has increased for all groups (p < 0.001), but only Tf100 reached values close to ideal (82.20 - 87). Comparing mean CEO2 of Grav and Tf100 at the end of 60s, (82.20 and 65.87) there was a difference of approximately 20% between both techniques, since gravity-coupled mask only did not provide adequate oxygenation. There were no significant differences between groups Tf70 and Grav (65.87 and 62.67) in all studied moments, suggesting that the latter simulates a 70% CIO2 at 60 s. Mean Aw group CEO2 increased to 47.20 at 60s showing that this technique may be associated to unacceptable risk of hypoxemia.
CONCLUSIONS: All situations of face mask coupling gradually increased CEO2, although with decreased oxygenation efficacy due to situations of face mask malposition. This study has shown the need for attention during oxygenation, using well coupled face mask and eliminating normal practices of moved away or gravity-coupled masks.
Key Words: EQUIPMENTS: face mask; GASES, oxygen; VENTILATION: oxygenation
JUSTIFICATIVA Y OBJETIVOS: Las diferentes
técnicas de oxigenación existentes tienen por objetivo producir desnitrogenización
previa al período de apnea durante la inducción. La principal razón
en que la concentración inspirada de oxígeno (CIO2) no
alcanza 100% y la falta de acoplamiento adecuado de la máscara facial,
permitiendo la entrada de aire ambiente. Aunque los anestesiólogos conozcan
este principio, ni todos lo aplican correctamente, facilitando la entrada de
aire en el flujo de gases frescos (FGF) y, consecuentemente, diluyendo la CIO2.
Este estudio buscó evaluar comparativamente, a través de la variación
de la CEO2, (concentración expirada de O2), la eficacia
de la técnica de oxigenación con máscara facial, en las condiciones
habitualmente empleadas por los anestesiólogos, simulando situaciones de
MÉTODO: Fueron estudiadas las CEO2 de 15 voluntarios, estado físico ASA I, sometidos a la técnica de oxigenación con ocho respiraciones profundas (capacidad vital) en 60s con flujo de gas fresco de 10 L.min-1. La máscara facial fue bien acoplada con CIO2 del 100% (Ac100), variando del 50% a 90% (Ac50; Ac60; Ac70; Ac80; Ac90) y máscara acoplada por la gravedad y CIO2 100% (Grav); máscara a 1 cm de la faz y CIO2 a 100% (Afast). La CEO2 fue registrada en intervalos de 10s hasta 60s. En los tests estadísticos p < 0,05 fue considerado significativo.
RESULTADOS: La CEO2 aumentó en todos los grupos, sin embargo, solamente el grupo Ac100 alcanzó valores próximos del ideal (82,20 - 87), (p < 0,001). Comparándose las CEO2 alcanzadas al final de 60s, se observó diferencia estadística significativa entre las técnicas Ac100 y Grav (82,20 y 65,87), mostrando que la utilización de la máscara acoplada apenas por la gravedad no produjo oxigenación adecuada. No hubo diferencia significativa entre los grupos Grav y Ac70 (65,87 y 62,67) en todos los momentos estudiados, sugiriendo que la técnica del acoplamiento por la gravedad simula a los 60s, una CIO2 del 70%. La CEO2 media del grupo Afast alcanzó valor de 47,20, valor que muestra que esa técnica puede ser asociada a un riesgo inaceptable de hipoxemia.
CONCLUSIONES: Hubo un aumento progresivo de la CEO2 en todos los grupos estudiados, aunque con reducción de la eficacia de la técnica de oxigenación debida a las varias situaciones de acoplamiento no adecuado. Este estudio mostró, así, la importancia de la atención al momento de la oxigenación, utilizándose la máscara bien acoplada, eliminándose las conductas habituales de máscara apartada o acoplada por la gravedad.
Different anesthetic techniques aim at promoting denitrogenation before apnea during induction, by replacing alveolar nitrogen with oxygen. So, the process denitrogenizes functional residual capacity (FRC) increasing oxygen reserves and delaying hypoxemia and arterial desaturation during apnea after anesthetic induction 1-3.
The main reason for not reaching 100% oxygen inspired concentration (CIO2) and 87 oxygen alveolar concentration (CAO2) is inadequate face mask adaptation, which allows the entry of room air. Inadequate face mask coupling may not be compensated by increased oxygenation time and minor leaks may be difficult to detect 4,5.
Although all anesthesiologists know this principle, not all of them apply it correctly, allowing the entry of room air in the mixture of fresh gases and the consequent dilution of the CIO2. A justification for not forcing the coupling of the mask to the face is increased anxiety and/or patient's refusal 6-8.
We were unable to find in the literature studies determining, within the clinical context, how leaks produced by inadequate face mask coupling may change oxygen expired concentration (CEO2) when using the circle system.
This study aimed at comparatively evaluating the efficacy of the 8 deep breaths oxygenation technique with face mask with different coupling methods and simulating leak situations.
After the Institution's Research Ethics Committee approval, 15 male volunteers were selected.
After obtaining their informed consent, volunteers were submitted to preanesthesia evaluation being included only those aged < 45 years, physical status ASA I, with no chronic or acute use of medications or morbid background. They were informed that the night before the experiment they should have a normal sleep (at least 8 hours) and should not take alcoholic beverages or medications.
All volunteers were placed on operating table in the supine position and were asked to slowly and deeply take 8 breaths (8DB = 8 deep breaths), and tests were performed to assure adequacy to the proposed method. When volunteers were trained, they were tested by breathing via a transparent latex face mask (previously checked and with perfect adaptation assured), in the following conditions: mask tightly-fitted to face; mask juxtaposed to face by gravity without any additional pressure; mask moved 1 cm away from the face.
The experiment was started after this training period and volunteers were asked to perform 8DB in the following situations, which constituted the study groups:
Tf100 = tightly-fitted face mask with 100% CIO2;
Tf90 = tightly-fitted face mask with 90% CIO2;
Tf80 = tightly-fitted face mask with 80% CIO2;
Tf70 = tightly-fitted face mask with 70% CIO2;
Tf60 = tightly-fitted face mask with 60% CIO2;
Tf50 = tightly-fitted face mask with 50% CIO2;
Grav = gravity-fitted mask without any additional pressure and 100% CIO2;
Aw = mask moved 1 cm away from face and 100% CIO2.
For the latter condition (Aw) small pieces of hard foam were glued to the mask to position it 1 cm away (measure with ruler) from nasal border and chin.
Each oxygenation period took 60 seconds separated by 10-minute intervals during which volunteers breathed room air. Circle system and total fresh gas flow of 10 L.min-1 was used for all volunteers. Before face mask coupling, the anesthesia machine and the reservoir bag were filled with 100% oxygen.
Oxygen expired concentration (CEO2) was recorded at 10-second intervals (10, 20, 30, 40, 50 and 60 s), using Datex Instrumentarium Corporation's gases analyzer. Gases analyzer sensor was placed between the face mask and the Y piece of the circuit, thus minimizing the dead space and allowing better approximation between expired gas measure alveolar gas.
Volunteers submitted to oxygenation with the mask 1 cm away from face and 100% oxygen expired concentration were instructed to inspire by the nasal route and expire by the mouth through a small Guedel cannula previously coupled to the gases analyzer sensor. This mechanism allowed an accurate measurement without any loss of expired volume, since mask was not perfectly coupled to volunteer's face. Data were presented in descriptive tables (mean ± standard deviation). Analysis of Variance (ANOVA) for repeated samples was used and when ANOVA showed statistically significant differences, Tukey's post hoc test was applied significance level was set to p < 0.05. Polynomial regression lines were plotted. Tests were performed on Sigma Stat for Windows, release 2.03, SPSS Inc.
All volunteers were male, aged 28.8 ± 2.5 years, 75.5 ± 4.5 kg weight and 173.8 ± 9.4 cm height (mean ± SD). Oxygen expired concentration values increased with exposure time and also with oxygen inspired fraction (Table I) in all groups.
Statistically significant differences at p values lesser than < 0.001 were found between groups. Significant between-group differences occurred among 10s and 30s; 10s and 40s; 10s and 50s; 10s and 60s; 20s and 40s; 20s and 50s; 20s and 60s; 30s and 60s.
CEO2 values in the different moments (10s; 20s; 30s; 40s; 50s; 60s) were significantly different (p < 0.001) among study groups. Significant differences between close moments were detected from 40 second measures ahead groups Tf100, Tf90 and Tf80. At 30 second measures, there were significant differences among groups Tf80, Tf70 and Tf60. There were no differences between groups Tf70 and Grav in all studied moments. Group Aw differed from all other groups from the 50% expired concentration (Tf50) ahead.
Mean CEO2 x time curves were plotted according to polynomial regression. Trend lines may be observed in figure 1. Curves have the following equations: Tf100 CEO2 = -0.2286xt2 + 5.6095xt + 56.8, Tf90 CEO2 = -0.4369xt2 + 6.9688xt + 50.513, Tf80 CEO2 = -0.5012xt2 + 6.9369xt + 45.833, Tf70 CEO2 = -0.4714 xt2 + 6.4581xt + 40.68, Tf60 CEO2 = -0.2893xt2 + 4.6117xt + 39.047, Tf50 CEO2 = -0.3286xt2 + 4.4352xt + 35.627, GRAV CEO2 = -0.0226xt2 + 4.9526xt + 36.953 and AWAY CEO2 = -0.1202xt2 + 4.0398xt + 27.34, where t is time in seconds.
Different techniques have been suggested as the most adequate for oxygenation before anesthetic induction: 3 to 5 minutes breathing with tidal volume in 100% CIO2 9-12; four deep breaths during 30 seconds in 100% CIO2 11,13,14; and 8 deep breaths during 60 seconds in 100% CIO2 15.
Eight deep breaths during 60 s is better than 4 breaths, but similar to the 3-minute period. So, 8 deep breaths under 100% oxygen concentration could be used as alternative to traditional oxygenation before anesthetic induction in situations of difficult intubation and ventilation or during rapid sequence induction 15.
With such technique, oxygen expired concentration (CEO2) reaches 82 and 87, respectively, when flow is changed to 7 or 10 L.min-1. Increased oxygenation time to 1.5 to 2 minutes and increased flow (10 L.min-1) have increased CEO2 to values above 90 12.
Our study has shown that oxygenation using the 8 deep breaths technique under different conditions was effective in time, promoting significant CEO2 increase in all groups. However, considering CEO2 between 87 and 90 as ideal oxygenation criteria, it could be observed that no technique was able to reach such values at the end of the established period. Group Tf100 (tightly-fitted mask with 100% CIO2) reached the maximal mean CEO2 value of 82.20.
There are three situations in which patients may not reach adequate alveolar oxygenation represented by CEO2 = 90 4,5: impossibility of breathing high oxygen concentrations18; inadequate oxygenation time 4,19 and leaks due to poor face mask fitting. This latter situation allows the admixture of room air with 100% oxygen with consequent decrease in oxygen inspired concentration (CIO2). These situations may lead to hypoxemia and early desaturation after anesthetic induction 20-23.
All three situations could be consequence of poor face mask fitting. So, adequate face mask adaptation is one of the major and determining factors for adequate oxygenation.
Decreasing CIO2 in groups Tf90, Tf80, Tf70, Tf60 and Tf50 (tightly-fitted mask and 90%, 80%, 70%, 60% and 50% CIO2, respectively), has adequately simulated situations of mask mispositioning with increasing leaks. So, inadequate mask adaptation implies in progressively poorer denitrogenation as the mask fitting deteriorates.
It was not possible to find data in the literature to adequate correlate whether 82.20 CEO2 would or not be as effective as 90. Most studies have evaluated CEO2 at the end of each oxygenation period and the few that evaluated CEO2 along time have not determined nitrogen alveolar concentration 5,15-17,25.
So, it was also not possible to infer that close values although below 82.20 could be considered at least reasonable in terms of denitrogenation. What these values may indicate is that certainly the level of denitrogenation is lower than ideal and that hemoglobin desaturation time during apnea would be shorter, resulting in high risk of hypoxemia.
According to the polynomial regression equation derived from data of the Tf100 group tightly fitted mask and 100% CIO2), we might infer that a CEO2 = 89 would be attained after 100 seconds.
There were significant differences between mean final CEO2 values of the Grav group (mean = 65.87) and the Tf100 group (mean = 82.20) showing that gravity-coupled mask in these conditions (10 L.min-1 flow and 60 seconds) has not promoted adequate oxygenation. This suggests that the technique should only be used when absolutely impossible to adequately couple the face mask, and knowing that the result is unsatisfactory and that hypoxemia may ensue during periods of apnea.
It is noteworthy that at al study moments there were no statistically significant differences in CEO2 between groups Tf70 and Grav, suggesting that this latter technique would be equivalent to a situation of well adapted mask with CIO2 = 70%. This suggests that in the presence of leaks equivalent to 70% CIO2 (gravity-adapted mask) oxygenation time may be important to determine the efficacy of the technique.
It may also be interesting to notice that trend lines plotted for groups Grav and Tf100 allow us to infer that CEO2 for both techniques would be similar at approximately 100 s. Although inadequate in terms of CEO2, in some circumstances and for patients not tolerating the mask such as children, maxillofacial trauma and burned patients, an option would be gravity-coupled mask, however with longer time (at least 110 s) to reach adequate oxygenation.
Mean Aw group CEO2 was 47.20, considered a low value. Analyzing its trend line, maximum value would be 58 after 120 s. So, one may conclude that the mask 1 cm away from face and 100% CIO2 is a technique in which, even increasing time to 120 s, there is no CEO2 increase to, if not ideal, at least acceptable values. This approach may be associated to unacceptable risk of hypoxemia, especially for patients with decreased functional residual capacity or with suspected difficult airways, such as obese, pregnant and pediatric patients.
It is necessary to remind that the denitrogenation process is not dependent only on oxygen inspired fraction, whatever it may be, but also on oxygen time and adequate mask fitting. So, these comparisons are statistically valid, but in fact we are only analyzing CEO2. There are no CEN2 (nitrogen expired concentration) values or apnea time for the same groups and moments, so there is no way to assure that similar CEO2 found in different times and mask coupling situations imply also similar levels of denitrogenation, which is very unlikely, since this is a time-dependent process. Our results (similar CEO2 found in different times and mask coupling situations) should be cautiously interpreted and are not recommended for daily practice until further studies are performed to include or exclude such considerations.
It is important to remind that all reported observations are valid within the limitations of this study involving young individuals, physical status ASA I. In case of patients with different clinical conditions, certainly results might be different, probably with lower CEO2 values.
A weakness of this study was the observation time established as 60 seconds. It is also necessary to include other methods to evaluate denitrogenation. There is a paucity of studies in the literature with serial CEN2 measurements alongside with CEO2, SaO2 and PaO2, which suggests the continuation of this study with this method.
This study has shown the importance of attention to oxygenation before anesthetic induction using, whenever patients' conditions permit, tightly-fitted facial masks and eliminating non-effective routine approaches such as using moved away or gravity-adapted masks.
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Dra. Lígia Andrade da Silva Telles Mathias
Address: Alameda Campinas, 139/41
ZIP: 01404-000 City: São Paulo, Brazil
Submitted for publication February 15, 2005
Accepted for publication June 9, 2005
* Received from CET/SBA do Serviço de Anestesiologia da Irmandade Santa Casa de Misericórdia de São Paulo (ISCMSP), SP