- Email: [email protected]
100% Inspired oxygen from a Hudson mask—a realistic goal?
Resuscitation 57 (2003) 69 /72 www.elsevier.com/locate/resuscitation
100% Inspired oxygen from a Hudson mask* a realistic goal? /
S.M. Boumphrey a, E.A.J. Morris b, S.M. Kinsella c,* a
Department of Anaesthesia, Derriford Hospital, Plymouth PL6 8DH, UK Department of Anaesthesia, Southmead Hospital, Bristol BS10 5NB, UK c Department of Anaesthesia, St. Michael’s Hospital, Bristol BS2 8EG, UK b
Received 14 October 2002; received in revised form 15 November 2002; accepted 4 December 2002
Abstract Background: The administration of oxygen at a high-inspired concentration is often required in medicine, particularly in resuscitation of critically ill patients. However, there is a lack of evidence-based guidance on how to achieve this using currently available apparatus. The aim of this study was to assess how maximum inspired oxygen concentrations can be delivered using existing equipment. Methods: Ten healthy female volunteers breathed oxygen through two types of Hudson non-rebreathing mask with reservoir bag, one with a safety vent in the mask body and the other with a valve replacing this safety vent (3-valve mask). Oxygen flow was adjusted to either 10 or 15 l min 1 and the masks were fitted to the face either loosely or tightly. The expired oxygen concentration was measured using an oxygen analyzer. Findings: By using the Hudson non-rebreathing mask with three valves, increasing the oxygen flow to 15 l min 1, and fitting the mask tightly to the face the average expired oxygen fraction could be raised to 0.85. This equates to an average inspired oxygen fraction of 0.97 in these subjects. Interpretation: The three simple measures mentioned above result in a significant improvement in the performance of the Hudson non-rebreathing mask. Together they allow the delivery of an inspired oxygen concentration close to maximum. # 2003 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Breathing; Emergency treatment; Oxygen; Resuscitation
Resumo Contexto: A administrac¸a˜o de oxige´nio numa concentrac¸a˜o inspirada elevada e´ frequentemente necessa´ria em medicina, particularmente na reanimac¸a˜o de doentes criticamente doentes. Contudo na˜o existem guias orientadores baseadas na evideˆncia de como obter este objectivo usando os dispositivos correntemente disponı´veis. O objectivo deste estudo foi avaliar como e´ que concentrac¸o˜es inspiradas ma´ximas de oxige´nio podem ser fornecidas usando o equipamento existente. Me´todo: Dez volunta´rios sauda´veis do sexo feminino respiraram oxige´nio atrave´s de 2 tipos de ma´scara de Hudson sem reinalac¸a˜o, com saco reservato´rio e uma abertura de seguranc¸a no corpo da ma´scara e a outra com uma va´lvula a substituir esta abertura de seguranc¸a (ma´scara com 3 va´lvulas). O fluxo de oxige´nio foi ajustado para 10 ou 15 l min 1 e as ma´scaras foram adaptadas a` face quer levemente ou apertadas. A concentrac¸a˜o de oxige´nio expirado foi medida usando um analisador de oxige´nio. Resultados: Usando a ma´scara de Hudson sem reinalac¸a˜o com 3 va´lvulas, aumentando o fluxo de oxige´nio para 15 l min 1, e adaptando a ma´scara de forma apertada a` face, a fracc¸a˜o me´dia de oxige´nio expirado pode ser aumentado para 0.85. Isto equivale a uma fracc¸a˜o inspirada de oxige´nio me´dia de 0.97 nestes sujeitos. Interpretac¸a˜o: As 3 medidas simples acima mencionadas resultam numa melhoria significativa no desempenho da ma´scara de Hudson sem reinalac¸a˜o. Em conjunto permitem o aporte de uma concentrac¸a˜o inspirada de oxige´nio muito pro´xima do ma´ximo. # 2003 Elsevier Science Ireland Ltd. All rights reserved. Palavras chave: Respirac¸a˜o; Tratamento emergente; Oxige´nio; Reanimac¸a˜o
* Corresponding author. Tel.: /44-0117-928-5203; fax: /44-0117-928-5209. E-mail address: [email protected] (S.M. Kinsella). 0300-9572/03/$ - see front matter # 2003 Elsevier Science Ireland Ltd. All rights reserved. doi:10.1016/S0300-9572(02)00436-7
70
S.M. Boumphrey et al. / Resuscitation 57 (2003) 69 /72
Resumen Antecedentes : La administracio´n de oxı´geno inspirado en alta concentracio´n es frecuentemente requerido en medicina, particularmente en resucitacio´n y en pacientes crı´ticamente enfermos. Sin embargo, no existe una guı´a basada en evidencia acerca de como alcanzar esto usando los dispositivos actualmente disponibles. El objetivo de este estudio fue evaluar de que´ manera pueden entregarse ma´ximas concentraciones inspiradas de oxı´geno usando los equipos existentes. Me´todos : 10 mujeres sanas voluntarias respiraron oxı´geno por dos tipos de ma´scaras de Hudson, sin reinspiracio´n y con reservorio, una con una ventilacio´n de seguridad en el cuerpo de una ma´scara y la otra con una va´lvula reemplazando esta ventilacio´n (ma´scara de tres va´lvulas). Se ajusto´ el flujo de oxı´geno a 10 o 15 litros por minuto 1 y las ma´scaras fueron fijadas suave o fuertemente a la cara de las voluntarias. Se midio´ el oxı´geno espirado usando un analizador de oxı´geno. Hallazgos : al usar la ma´scara de Hudson de tres va´lvulas sin reinspiracio´n, aumentando el flujo a 15 litros por min 1 y con la ma´scara estrechamente ajustada la fraccio´n espirada de oxı´geno promedio fue elevada a 0.85. Esto equivale a una fraccio´n inspirada de 0.97 en estos sujetos. Interpretacio´n : estas tres medidas previamente mencionadas llevan a una mejorı´a significativa en el desempen˜o de la ma´scara de Hudson sin reinspiracio´n. Estas medidas juntas permiten la entrega de una fraccio´n inspirada cercana al ma´ximo. # 2003 Elsevier Science Ireland Ltd. All rights reserved. Palabras clave: Respiracio´n; Tratamiento de emergencia; Oxı´geno; Resucitacio´n
1. Introduction Oxygen therapy is used in many fields of medicine. It is sometimes necessary to provide an inspired oxygen fraction (F IO2) as close as possible to 1.0 (i.e. 100% oxygen), for example in resuscitation and in the management of some types of severe respiratory failure. It has been demonstrated that patients with severe ventilation /perfusion mismatch who have a ‘virtual shunt’ of 0.3 or greater will become hypoxaemic if the F IO2 is below 1.0 [1]. Furthermore, an F IO2 of 1.0 administered to pregnant women has been shown to increase oxygen transfer to the fetus [2,3] which may be of importance in the intrauterine treatment of acute fetal hypoxaemia. The national ‘Use of Electronic Fetal Monitoring’ guidelines call for research on maternal oxygen delivery apparatus used for this purpose [4]. Oxygen masks may be classified as fixed or variable performance devices, depending on the effect of the patient’s peak inspiratory flow rate (PIFR) on the concentration of oxygen that they deliver. Fixed performance devices that provide a specified F IO2 regardless of PIFR, such as the Ventimask and other devices which rely on the Venturi principle, are not available for an F IO2 requirement above 0.6. Variable performance masks provide an F IO2 that depends on patient factors and how the mask is used; those that incorporate a reservoir bag are often used when a high F IO2 is required with the aim of providing extra oxygen during periods when the subject’s PIFR is greater than the supplied oxygen flow. We have previously studied the effects of mask type and oxygen flow, as well as the influence of pregnancy, on expired oxygen fraction (F EO2) [5]. The effective F IO2 provided by variable performance masks may be derived from the F EO2 using the alveolar gas equation (see below). The highest mean F EO2 achieved in nonpregnant women in this earlier study was 0.72, using a
Hudson mask with reservoir bag and safety vent at an oxygen flow of 15 l min 1, equivalent to an F IO2 of 0.83. This fell well below the expectation of an F IO2 up to 0.98 at 10 /12 l min 1 claimed by the manufacturer [6]. The aim of the present study was to determine the factors that might have led to the failure of that mask to achieve its specification, as well as to explore the feasability of attaining an F IO2 approaching 1.0 using the variable performance type of mask. One factor might be mask design, as there are two patterns of reservoir mask produced by one manufacturer, which differ only in the vents on the body of the mask. A second variable might be the fit of the mask to the face. Milross et al. noted that the oxygenation achieved using a simple Hudson mask was increased by applying the mask tightly [7]. Finally, increased flow rate increases oxygen delivery with the reservoir mask [5]. However, use of high flow rates for prolonged periods may not be practical when stores are limited, for example during ambulance transfers. We therefore investigated a flow rate of 10 l min 1, as described in the product information, as well as 15 l min 1, which is the maximum available from most oxygen flow meters.
2. Material and methods 2.1. Study population After obtaining approval from the local research ethics committee, 10 healthy non-smoking female volunteers were recruited. They were all between 20 and 40 years old, greater than 1.5 m in height and less than 100 kg. Female volunteers were chosen because our original study was aimed at assessing the potential benefits of high concentration oxygen in the treatment of fetal hypoxia in labour, and the use of a similar group of non-
S.M. Boumphrey et al. / Resuscitation 57 (2003) 69 /72
pregnant subjects allowed for comparison between the results. The number of subjects was based on these data [5].
2.2. Design and procedures Two masks were compared, the Hudson non-rebreathing reservoir mask with safety vent (Hudson RCI, Ashby De La Zouch, UK, Cat No. 41059 */Fig. 1) and the Hudson non-rebreathing reservoir mask without safety vent (same supplier, Cat No. 41060). The former has one disk valve that prevents rebreathing into the reservoir bag and a second on the body of the mask. There is also a perforated ‘safety’ opening on the body of the mask to allow air entrainment. The latter, 3valve mask, has a second valve on the mask body in place of this opening. The other variables that were studied were oxygen flows of 10 versus 15 l min 1, and a tight fit of the mask to the face versus a loose fit. A loose fit was achieved by placing the mask over the nose and mouth and pulling on the elastic strap just enough to keep the mask in that position. A tight fit was achieved by moulding the metal nose bridge to the nose and
71
tightening the elastic strap to minimise any gap between the edges of the mask and the face. The methodology has been described in detail previously [5]. Briefly, the subjects were studied while reclining comfortably in an armchair. Dry oxygen was supplied via a calibrated flowmeter. Expired oxygen fraction was sampled at the nostril and measured using the magnetoacoustic method (Hewlett Packard M1025 B). The analogue signal was recorded onto a paper trace for storage and analysis (Multitrace 4, Lectromed, Jersey, UK). The subjects breathed room air between periods of breathing oxygen, allowing F EO2 to return to baseline before each measurement. Statistical analysis was performed using repeated multivariate ANOVA using SPSS v 6.14. (SPSS Inc., Chicago, USA).
3. Results The results are shown in Table 1. The presence of the extra valve increased F EO2 by 0.13 (SE9/0.01, P B/ 0.0005), the higher oxygen flow rate increased F EO2 by 0.17 (SE9/0.019, P B/0.0005), and tight fit increased F EO2 by 0.11 (SE9/0.018, P /0.0002).
4. Discussion The alveolar oxygen fraction (F AO2) and inspired oxygen fraction (F IO2) are related by the alveolar gas equation: F AO2 :F IO2 F ACO2 =R
Fig. 1. The non-rebreathing oxygen mask with safety vent. The mask without safety vent has a second flap valve on the mask body (left side of the photograph).
where F ACO2 is the alveolar fraction of carbon dioxide and R is the respiratory quotient. In resting adults, F ACO2 is approximately 0.05 and R is 0.8, giving a difference between the saturated F IO2 in the airways and F AO2 of 0.06. Because F EO2 approximates to F AO2, it can therefore be assumed that the difference between expired and inspired oxygen fraction is 0.06. To correct for humidification of the inhaled gases by the upper airway, the derived value of F IO2 must be multiplied by 1.066 to give a final value for the F IO2 of the dry gas that was supplied. All three factors studied were found to increase the F EO2 delivered by the mask. The mask with safety vent is supposed to reduce the degree of rebreathing of expired air if the oxygen supply is accidentally discontinued. However, the use of a tight fit on either mask will reduce air entrainment and increase rebreathing, and therefore attention must be paid to maintaining the oxygen supply at all times. Increasing oxygen flow had the largest effect on F EO2, and the use of an oxygen flow of 15 l min 1 can be recommended in the hospital setting. The use of
S.M. Boumphrey et al. / Resuscitation 57 (2003) 69 /72
72
Table 1 Mean F EO2 with range and standard deviation (SD), achieved using each type of mask, different oxygen flow rates and altered fitting of the mask Mask type
Safety vent
3-Valve
O2 flow rate
10 l min 1
Mask fitting
Loose
Tight
Loose
Tight
Loose
Tight
Loose
Tight
Mean F EO2 SD Range
0.574 0.071 0.45 /0.72
0.688 0.042 0.64 /0.76
0.648 0.09 0.44 /0.77
0.808 0.022 0.77 /0.84
0.669 0.028 0.62 /0.70
0.761 0.051 0.70 /0.86
0.74 0.048 0.66 /0.80
0.855 0.037 0.79 /0.91
15 l min1
10 l min 1
10 l min 1 may be more appropriate when oxygen supplies are limited, for example during ambulance transfers. Our results demonstrate that the use of a tight fitting 3-valve mask at a flow rate of 10 l min 1 performs more effectively than either mask fitting loosely at 15 l min 1. In our initial study, we asked our subjects to adjust the mask until they felt comfortable but in this study we paid specific attention to mask fit. Because of the impact that a tight fit has on F EO2, we suggest that the importance of such a fit is explained to staff and patients who use reservoir masks. Caution must be exercised when applying these results to critically ill patients. The mask is likely to perform less well than on the healthy population we examined if they are hyperventilating, since this may result in a PIFR exceeding the oxygen supply provided by the fresh gas supply and reservoir. Nevertheless, in any situation, use of the methods discussed would be expected to result in the maximum oxygen delivery possible when using a variable performance mask. It would be unsafe to explore interactions between the three variable factors in a small study such as this. However, we have demonstrated the most appropriate combination of mask fit, mask type and oxygen flow to achieve the highest oxygen delivery. In this group of subjects, a tight fitting, 3-valve mask connected to oxygen at a flow of 15 l min 1 produced a mean F EO2 of 0.85, equivalent to an F IO2 of 0.97.
15 l min1
Acknowledgements We are grateful to Dr Tim Lovell for his statistical advice, and to the Charitable Trusts for the United Bristol Hospitals for their financial support.
References [1] Lawler PGP, Nunn JF. Assessment of the validity of the isoshunt graph. Br J Anaesth 1984;56:1325 /35. [2] Ramanathan S, Gandhi S, Arismendy J, Chalon J, Turndorf H. Oxygen transfer from mother to fetus during cesarean section under epidural anaesthesia. Anesth Analg 1982;61:576 /81. [3] McNamara H, Johnson N, Lilford R. The effect on fetal arteriolar oxygen saturation resulting from giving oxygen to the mother measured by pulse oximetry. Br J Obstet Gynaecol 1993;100:446 / 9. [4] The use of electronic fetal monitoring. Evidence-based guideline Number 8, London Royal College of Obstetrics & Gynaecology Press, 2001. [http://www.rcog.org.uk] [5] Morris EAJ, Smith AJ, Kinsella SM. Performance of standard and reservoir-type Hudson masks in pregnant and non-pregnant subjects. Int J Obstet Anesth 2001;10:284 /8. [6] Manufacturer’s Data. Henley’s Medical Supplies Ltd., Welwyn Garden City, Herts AL7 1AN, UK. [7] Milross J, Young IH, Donnelly P. The oxygen delivery characteristics of the Hudson Oxy-one face mask. Anaesth Intensive Care 1989;17:180 /4.
100% Inspired oxygen from a Hudson mask* a realistic goal? /
S.M. Boumphrey a, E.A.J. Morris b, S.M. Kinsella c,* a
Department of Anaesthesia, Derriford Hospital, Plymouth PL6 8DH, UK Department of Anaesthesia, Southmead Hospital, Bristol BS10 5NB, UK c Department of Anaesthesia, St. Michael’s Hospital, Bristol BS2 8EG, UK b
Received 14 October 2002; received in revised form 15 November 2002; accepted 4 December 2002
Abstract Background: The administration of oxygen at a high-inspired concentration is often required in medicine, particularly in resuscitation of critically ill patients. However, there is a lack of evidence-based guidance on how to achieve this using currently available apparatus. The aim of this study was to assess how maximum inspired oxygen concentrations can be delivered using existing equipment. Methods: Ten healthy female volunteers breathed oxygen through two types of Hudson non-rebreathing mask with reservoir bag, one with a safety vent in the mask body and the other with a valve replacing this safety vent (3-valve mask). Oxygen flow was adjusted to either 10 or 15 l min 1 and the masks were fitted to the face either loosely or tightly. The expired oxygen concentration was measured using an oxygen analyzer. Findings: By using the Hudson non-rebreathing mask with three valves, increasing the oxygen flow to 15 l min 1, and fitting the mask tightly to the face the average expired oxygen fraction could be raised to 0.85. This equates to an average inspired oxygen fraction of 0.97 in these subjects. Interpretation: The three simple measures mentioned above result in a significant improvement in the performance of the Hudson non-rebreathing mask. Together they allow the delivery of an inspired oxygen concentration close to maximum. # 2003 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Breathing; Emergency treatment; Oxygen; Resuscitation
Resumo Contexto: A administrac¸a˜o de oxige´nio numa concentrac¸a˜o inspirada elevada e´ frequentemente necessa´ria em medicina, particularmente na reanimac¸a˜o de doentes criticamente doentes. Contudo na˜o existem guias orientadores baseadas na evideˆncia de como obter este objectivo usando os dispositivos correntemente disponı´veis. O objectivo deste estudo foi avaliar como e´ que concentrac¸o˜es inspiradas ma´ximas de oxige´nio podem ser fornecidas usando o equipamento existente. Me´todo: Dez volunta´rios sauda´veis do sexo feminino respiraram oxige´nio atrave´s de 2 tipos de ma´scara de Hudson sem reinalac¸a˜o, com saco reservato´rio e uma abertura de seguranc¸a no corpo da ma´scara e a outra com uma va´lvula a substituir esta abertura de seguranc¸a (ma´scara com 3 va´lvulas). O fluxo de oxige´nio foi ajustado para 10 ou 15 l min 1 e as ma´scaras foram adaptadas a` face quer levemente ou apertadas. A concentrac¸a˜o de oxige´nio expirado foi medida usando um analisador de oxige´nio. Resultados: Usando a ma´scara de Hudson sem reinalac¸a˜o com 3 va´lvulas, aumentando o fluxo de oxige´nio para 15 l min 1, e adaptando a ma´scara de forma apertada a` face, a fracc¸a˜o me´dia de oxige´nio expirado pode ser aumentado para 0.85. Isto equivale a uma fracc¸a˜o inspirada de oxige´nio me´dia de 0.97 nestes sujeitos. Interpretac¸a˜o: As 3 medidas simples acima mencionadas resultam numa melhoria significativa no desempenho da ma´scara de Hudson sem reinalac¸a˜o. Em conjunto permitem o aporte de uma concentrac¸a˜o inspirada de oxige´nio muito pro´xima do ma´ximo. # 2003 Elsevier Science Ireland Ltd. All rights reserved. Palavras chave: Respirac¸a˜o; Tratamento emergente; Oxige´nio; Reanimac¸a˜o
* Corresponding author. Tel.: /44-0117-928-5203; fax: /44-0117-928-5209. E-mail address: [email protected] (S.M. Kinsella). 0300-9572/03/$ - see front matter # 2003 Elsevier Science Ireland Ltd. All rights reserved. doi:10.1016/S0300-9572(02)00436-7
70
S.M. Boumphrey et al. / Resuscitation 57 (2003) 69 /72
Resumen Antecedentes : La administracio´n de oxı´geno inspirado en alta concentracio´n es frecuentemente requerido en medicina, particularmente en resucitacio´n y en pacientes crı´ticamente enfermos. Sin embargo, no existe una guı´a basada en evidencia acerca de como alcanzar esto usando los dispositivos actualmente disponibles. El objetivo de este estudio fue evaluar de que´ manera pueden entregarse ma´ximas concentraciones inspiradas de oxı´geno usando los equipos existentes. Me´todos : 10 mujeres sanas voluntarias respiraron oxı´geno por dos tipos de ma´scaras de Hudson, sin reinspiracio´n y con reservorio, una con una ventilacio´n de seguridad en el cuerpo de una ma´scara y la otra con una va´lvula reemplazando esta ventilacio´n (ma´scara de tres va´lvulas). Se ajusto´ el flujo de oxı´geno a 10 o 15 litros por minuto 1 y las ma´scaras fueron fijadas suave o fuertemente a la cara de las voluntarias. Se midio´ el oxı´geno espirado usando un analizador de oxı´geno. Hallazgos : al usar la ma´scara de Hudson de tres va´lvulas sin reinspiracio´n, aumentando el flujo a 15 litros por min 1 y con la ma´scara estrechamente ajustada la fraccio´n espirada de oxı´geno promedio fue elevada a 0.85. Esto equivale a una fraccio´n inspirada de 0.97 en estos sujetos. Interpretacio´n : estas tres medidas previamente mencionadas llevan a una mejorı´a significativa en el desempen˜o de la ma´scara de Hudson sin reinspiracio´n. Estas medidas juntas permiten la entrega de una fraccio´n inspirada cercana al ma´ximo. # 2003 Elsevier Science Ireland Ltd. All rights reserved. Palabras clave: Respiracio´n; Tratamiento de emergencia; Oxı´geno; Resucitacio´n
1. Introduction Oxygen therapy is used in many fields of medicine. It is sometimes necessary to provide an inspired oxygen fraction (F IO2) as close as possible to 1.0 (i.e. 100% oxygen), for example in resuscitation and in the management of some types of severe respiratory failure. It has been demonstrated that patients with severe ventilation /perfusion mismatch who have a ‘virtual shunt’ of 0.3 or greater will become hypoxaemic if the F IO2 is below 1.0 [1]. Furthermore, an F IO2 of 1.0 administered to pregnant women has been shown to increase oxygen transfer to the fetus [2,3] which may be of importance in the intrauterine treatment of acute fetal hypoxaemia. The national ‘Use of Electronic Fetal Monitoring’ guidelines call for research on maternal oxygen delivery apparatus used for this purpose [4]. Oxygen masks may be classified as fixed or variable performance devices, depending on the effect of the patient’s peak inspiratory flow rate (PIFR) on the concentration of oxygen that they deliver. Fixed performance devices that provide a specified F IO2 regardless of PIFR, such as the Ventimask and other devices which rely on the Venturi principle, are not available for an F IO2 requirement above 0.6. Variable performance masks provide an F IO2 that depends on patient factors and how the mask is used; those that incorporate a reservoir bag are often used when a high F IO2 is required with the aim of providing extra oxygen during periods when the subject’s PIFR is greater than the supplied oxygen flow. We have previously studied the effects of mask type and oxygen flow, as well as the influence of pregnancy, on expired oxygen fraction (F EO2) [5]. The effective F IO2 provided by variable performance masks may be derived from the F EO2 using the alveolar gas equation (see below). The highest mean F EO2 achieved in nonpregnant women in this earlier study was 0.72, using a
Hudson mask with reservoir bag and safety vent at an oxygen flow of 15 l min 1, equivalent to an F IO2 of 0.83. This fell well below the expectation of an F IO2 up to 0.98 at 10 /12 l min 1 claimed by the manufacturer [6]. The aim of the present study was to determine the factors that might have led to the failure of that mask to achieve its specification, as well as to explore the feasability of attaining an F IO2 approaching 1.0 using the variable performance type of mask. One factor might be mask design, as there are two patterns of reservoir mask produced by one manufacturer, which differ only in the vents on the body of the mask. A second variable might be the fit of the mask to the face. Milross et al. noted that the oxygenation achieved using a simple Hudson mask was increased by applying the mask tightly [7]. Finally, increased flow rate increases oxygen delivery with the reservoir mask [5]. However, use of high flow rates for prolonged periods may not be practical when stores are limited, for example during ambulance transfers. We therefore investigated a flow rate of 10 l min 1, as described in the product information, as well as 15 l min 1, which is the maximum available from most oxygen flow meters.
2. Material and methods 2.1. Study population After obtaining approval from the local research ethics committee, 10 healthy non-smoking female volunteers were recruited. They were all between 20 and 40 years old, greater than 1.5 m in height and less than 100 kg. Female volunteers were chosen because our original study was aimed at assessing the potential benefits of high concentration oxygen in the treatment of fetal hypoxia in labour, and the use of a similar group of non-
S.M. Boumphrey et al. / Resuscitation 57 (2003) 69 /72
pregnant subjects allowed for comparison between the results. The number of subjects was based on these data [5].
2.2. Design and procedures Two masks were compared, the Hudson non-rebreathing reservoir mask with safety vent (Hudson RCI, Ashby De La Zouch, UK, Cat No. 41059 */Fig. 1) and the Hudson non-rebreathing reservoir mask without safety vent (same supplier, Cat No. 41060). The former has one disk valve that prevents rebreathing into the reservoir bag and a second on the body of the mask. There is also a perforated ‘safety’ opening on the body of the mask to allow air entrainment. The latter, 3valve mask, has a second valve on the mask body in place of this opening. The other variables that were studied were oxygen flows of 10 versus 15 l min 1, and a tight fit of the mask to the face versus a loose fit. A loose fit was achieved by placing the mask over the nose and mouth and pulling on the elastic strap just enough to keep the mask in that position. A tight fit was achieved by moulding the metal nose bridge to the nose and
71
tightening the elastic strap to minimise any gap between the edges of the mask and the face. The methodology has been described in detail previously [5]. Briefly, the subjects were studied while reclining comfortably in an armchair. Dry oxygen was supplied via a calibrated flowmeter. Expired oxygen fraction was sampled at the nostril and measured using the magnetoacoustic method (Hewlett Packard M1025 B). The analogue signal was recorded onto a paper trace for storage and analysis (Multitrace 4, Lectromed, Jersey, UK). The subjects breathed room air between periods of breathing oxygen, allowing F EO2 to return to baseline before each measurement. Statistical analysis was performed using repeated multivariate ANOVA using SPSS v 6.14. (SPSS Inc., Chicago, USA).
3. Results The results are shown in Table 1. The presence of the extra valve increased F EO2 by 0.13 (SE9/0.01, P B/ 0.0005), the higher oxygen flow rate increased F EO2 by 0.17 (SE9/0.019, P B/0.0005), and tight fit increased F EO2 by 0.11 (SE9/0.018, P /0.0002).
4. Discussion The alveolar oxygen fraction (F AO2) and inspired oxygen fraction (F IO2) are related by the alveolar gas equation: F AO2 :F IO2 F ACO2 =R
Fig. 1. The non-rebreathing oxygen mask with safety vent. The mask without safety vent has a second flap valve on the mask body (left side of the photograph).
where F ACO2 is the alveolar fraction of carbon dioxide and R is the respiratory quotient. In resting adults, F ACO2 is approximately 0.05 and R is 0.8, giving a difference between the saturated F IO2 in the airways and F AO2 of 0.06. Because F EO2 approximates to F AO2, it can therefore be assumed that the difference between expired and inspired oxygen fraction is 0.06. To correct for humidification of the inhaled gases by the upper airway, the derived value of F IO2 must be multiplied by 1.066 to give a final value for the F IO2 of the dry gas that was supplied. All three factors studied were found to increase the F EO2 delivered by the mask. The mask with safety vent is supposed to reduce the degree of rebreathing of expired air if the oxygen supply is accidentally discontinued. However, the use of a tight fit on either mask will reduce air entrainment and increase rebreathing, and therefore attention must be paid to maintaining the oxygen supply at all times. Increasing oxygen flow had the largest effect on F EO2, and the use of an oxygen flow of 15 l min 1 can be recommended in the hospital setting. The use of
S.M. Boumphrey et al. / Resuscitation 57 (2003) 69 /72
72
Table 1 Mean F EO2 with range and standard deviation (SD), achieved using each type of mask, different oxygen flow rates and altered fitting of the mask Mask type
Safety vent
3-Valve
O2 flow rate
10 l min 1
Mask fitting
Loose
Tight
Loose
Tight
Loose
Tight
Loose
Tight
Mean F EO2 SD Range
0.574 0.071 0.45 /0.72
0.688 0.042 0.64 /0.76
0.648 0.09 0.44 /0.77
0.808 0.022 0.77 /0.84
0.669 0.028 0.62 /0.70
0.761 0.051 0.70 /0.86
0.74 0.048 0.66 /0.80
0.855 0.037 0.79 /0.91
15 l min1
10 l min 1
10 l min 1 may be more appropriate when oxygen supplies are limited, for example during ambulance transfers. Our results demonstrate that the use of a tight fitting 3-valve mask at a flow rate of 10 l min 1 performs more effectively than either mask fitting loosely at 15 l min 1. In our initial study, we asked our subjects to adjust the mask until they felt comfortable but in this study we paid specific attention to mask fit. Because of the impact that a tight fit has on F EO2, we suggest that the importance of such a fit is explained to staff and patients who use reservoir masks. Caution must be exercised when applying these results to critically ill patients. The mask is likely to perform less well than on the healthy population we examined if they are hyperventilating, since this may result in a PIFR exceeding the oxygen supply provided by the fresh gas supply and reservoir. Nevertheless, in any situation, use of the methods discussed would be expected to result in the maximum oxygen delivery possible when using a variable performance mask. It would be unsafe to explore interactions between the three variable factors in a small study such as this. However, we have demonstrated the most appropriate combination of mask fit, mask type and oxygen flow to achieve the highest oxygen delivery. In this group of subjects, a tight fitting, 3-valve mask connected to oxygen at a flow of 15 l min 1 produced a mean F EO2 of 0.85, equivalent to an F IO2 of 0.97.
15 l min1
Acknowledgements We are grateful to Dr Tim Lovell for his statistical advice, and to the Charitable Trusts for the United Bristol Hospitals for their financial support.
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