Anesth Analg 2003;97:1686-1689
© 2003 International Anesthesia Research Society
TECHNOLOGY, COMPUTING, AND SIMULATION
A Pilot Study to Evaluate the SMART BAG®: A New Pressure-Responsive, Gas-Flow Limiting Bag-Valve-Mask Device
Horst G. Wagner-Berger, MD,
Volker Wenzel, MD,
Wolfgang G. Voelckel, MD,
Klaus Rheinberger, MSc,
Karl H. Stadlbauer, MD,
Tilko Müller, BS,
Sven Augenstein, MD,
Achim von Goedecke, MD,
Karl H. Lindner, MD, and
Christian Keller, MD
From the Department of Anesthesiology and Critical Care Medicine, Leopold-Franzens-University, Innsbruck, Austria
Address correspondence and reprint requests to Dr. Horst Wagner-Berger, Department of Anesthesiology and Critical Care Medicine, Leopold-Franzens-University, Anichstrasse 35, 6020 Innsbruck, Austria. Address email to Horst.Wagner-Berger{at}uibk.ac.at
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Abstract
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Reducing inspiratory flow rate and peak airway pressure may be important to minimize the risk of stomach inflation when ventilating an unprotected airway with positive pressure ventilation. In this study, we assessed the effects of a standard self-inflating bag compared with a new pressure-responsive, inspiratory gas flow-limiting device (SMART BAG®) on respiratory mechanics in 60 adult patients undergoing routine induction of anesthesia. Respiratory variables were measured using a pulmonary monitor. The SMART BAG® resulted in significantly decreased inspiratory flow rate and peak airway pressure while providing adequate tidal volume delivery.
IMPLICATIONS: The SMART BAG®, a new pressure-responsive, peak inspiratory gas flow-limiting bag-valve mask device, limits inspiratory gas flow from up to 120 L/min in a standard self-inflating bag to 
40 L/min. It is designed for use by all levels of health care professionals and has been proven in a clinical pilot study to effectively ventilate patients in respiratory arrest.
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Introduction
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The self-inflating bag was developed in 1955 by Henning Ruben, and it has been the primary method of ventilating a patient in respiratory and/or cardiac arrest for >45 yr (1). Previous studies have shown that bag-valve-mask ventilation with an unprotected airway is often poorly performed with high flow rates and unnecessarily large tidal volumes creating excessive peak airway pressures, leading to rapid rates of stomach inflation (2,3). An approach ensuring both adequate ventilation in an unprotected airway and patient safety may be to design a ventilation device with an incorporated feature ensuring a margin of protection for the patient’s airway, especially when used by less experienced rescuers. We have previously shown that decreasing peak airway pressure during bag-valve-mask ventilation by decreasing tidal volume from 1000 to 500 mL is one strategy in improving bag-valve-mask ventilation (4). Another option may be to limit inspiratory flow rate to reduce peak airway pressure. Based on this concept, a new pressure-responsive, inspiratory gas-flow-limiting bag-valve mask device has been developed (SMART BAG®; O-Two-Systems, Mississauga, Canada) that limits peak inspiratory gas flow from up to 120 L/min in a standard self-inflating bag to 40 L/min (Fig. 1).
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Figure 1. A, Incorporated special pressure-responsive valve limiting inspiratory gas flow based on the applied squeeze of the bag by the operator, and the patient’s airway resistance and compliance. Flow rate is controlled by the force applied to the SMART BAG® by the rescuer. Airway pressure is maintained below lower esophageal sphincter pressure in a patient with normal compliance and resistance. B, Flow rate is restricted by the SMART BAG® to maintain a low airway pressure. The visual ‘red pressure actuation indicator’ will move forward into the patient’s valve reminding the rescuer to reduce the force being applied to the bag. The response by the SMART BAG® is proportional to the rescuer’s squeeze. The harder the squeeze, the greater the restriction to flow. This alerts the rescuer to ease up on the squeeze, reducing the effort required, resulting in the airway pressure being kept to the minimum required to achieve adequate ventilation.
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The purpose of this study was to assess the effects of a standard self-inflating bag compared with the new pressure-responsive, inspiratory gas flow-limiting device on respiratory mechanics in adult supine patients undergoing routine induction of anesthesia. Our hypothesis was that there would be no differences in study end-points between groups.
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Methods
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With ethics committee approval and written informed consent, we studied 60 healthy adult ASA physical status I and II patients with no respiratory disease (age, 18–76 yr; mean body mass index, 25 kg/m2) who were scheduled for peripheral musculoskeletal surgery. Patients were excluded if they had a predicted respiratory disease, oropharyngeal or facial pathology, a body mass index >30 kg/m2, or if they had more than one symptom of gastroesophageal reflux per week.
Premedication was with oral midazolam 7.5 mg 1 h preoperatively. Anesthesia was in the supine position with the patient’s head on a standard pillow 5 cm in height. A standard anesthesia protocol was followed and routine monitoring was applied. Fentanyl 1.5 µg/kg was administered. Patients were preoxygenated for 3 min. Anesthesia was induced with propofol 2.5 mg/kg given over 30 s. Maintenance was with propofol 8–10 mg/kg/h in 30% O2 and air. When apnea was confirmed, patients were randomized for ventilation with either a standard self-inflating bag or with the new pressure-responsive, inspiratory gas-flow-limiting device by three anesthesiologists (experience, 3–6 yr) who were blinded as to all monitors measuring respiratory variables.
The following data were recorded: oxygen saturation, respiratory rate, noninvasive mean arterial blood pressure, heart rate, and end-tidal CO2. Each patient was ventilated with one bag-valve-mask device for 2 min with a respiratory rate of 15/min while respiratory mechanics were being measured with a respiratory monitor (CP-100; Bicore, Irvine, CA) and an oxygen saturation monitor (AS 3; Datex, Helsinki, Finland), respectively. Epigastric auscultation was performed to detect air entering the stomach.
The SMART (Synchronized Manual Actuation Response Technology) BAG® has been designed to allow the provision of consistent ventilation while almost reducing the risks of bag-valve-mask ventilation, such as stomach insufflation, regurgitation, and pulmonary aspiration. An incorporated special pressure-responsive valve limits inspiratory gas flow based on the applied squeeze of the bag by the operator and the patient’s airway resistance and compliance. The activated valve of the SMART BAG® has two warning characteristics for the rescuer. First, the increasing resistance of the bag being squeezed too hard advises the rescuer that he is performing too forced a ventilation, and second, the colored indicator of the valve, which appears in the neck of the bag if gas flow is too rapid, may be an additional visual help. These two warning mechanisms may render the SMART BAG® a safe alternative airway device for rescuers who have less experience in emergency ventilation of an unprotected airway and for health care professionals who infrequently use a Bag-Valve Mask resuscitator.
Sample size was selected for a type I error of 0.05 and a power of 0.9 and was based on a pilot study of 5 patients with a measured difference in the peak airway pressure of 30% between the two groups. Statistical analysis was performed with Wilcoxon’s ranked sum test. Unless otherwise noted, data are presented as mean ± SD. Significance was taken as P < 0.05.
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Results
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There were no significant differences in age, weight, and height in patients between groups (Table 1). When compared with the standard self-inflating bag, the new pressure-responsive, inspiratory gas flow-limiting device resulted in significantly (P < 0.01) decreased peak airway pressure, peak inspiratory flow, and inspiratory tidal volume (Table 2, Fig. 2).
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Table 2. Effects of a New Pressure-Responsive, Inspiratory Gas Flow-Limiting Bag-Valve-Mask Device (SMART BAG®) on Respiratory Mechanics in Supine Patients Undergoing Routine Induction of Anesthesia
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Figure 2. A, Representative inspiratory flow and B, peak airway pressure tracings of a standard self-inflating bag and the new pressure-responsive, inspiratory gas-flow-limiting SMART BAG®, respectively. These data were generated by a special data recording system (Dewetron port 2000, Graz, Austria; and Datalogger, custom-made software) and analyzed with MatLab (MathWorks Inc, Natick, MA).
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Discussion
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In the emergency medical services and in the hospital setting, bag-valve-mask devices are readily available and frequently used; unfortunately, rescuers may underestimate the level of training and retraining that is needed to retain adequate skills (2–4). For example, previous studies documented that experienced paramedics caused very high peak airway pressures when using a bag-valve-mask device (5–7). Also, when professional health care workers performed bag-valve-mask ventilation on cardiac arrest patients in the emergency room, ventilation rates were >40/min, which may indicate substantial stress (8). Accordingly, if even experienced rescuers have problems performing bag-valve-mask ventilation properly, less-experienced rescuers may not be able to provide adequate ventilation and oxygenation and may simply produce ventilation-related problems such as stomach inflation.
Distribution of gas during ventilation with an unprotected airway depends on the rescuer applying assisted ventilation and on patient components, such as lung compliance, airway resistance, and lower esophageal sphincter pressure. For example, if peak airway pressure exceeds lower esophageal sphincter pressure, bag-valve mask ventilation gas volume will not be directed completely towards the lungs but may induce a certain degree of stomach inflation. Although lower esophageal sphincter pressure in awake volunteers was shown to be 16 cm H2O (9), it may be less during induction of anesthesia and it may even decrease to 5 cm H2O during cardiac arrest (10). Thus, if peak airway pressure during ventilation in an unprotected airway is kept as low as possible, stomach inflation may be less likely. Although the SMART BAG® resulted in smaller tidal volumes than a standard self-inflating bag (539 versus 637 mL), its unique mode of operation decreased inspiratory gas flow (64 versus 42 L/min) and decreased peak airway pressure from 17 to 13 cm H2O. As the experienced anesthesiologists in our study applied a peak inspiratory flow of only 45 L/min, the differences between both ventilation devices would have been possibly even greater had less-experienced rescuers volunteered for the study. Accordingly, the SMART BAG® may provide a greater degree of safety in terms of peak airway pressure and possibly stomach inflation when compared with the standard bag-valve-mask device. This is even more remarkable considering that relatively experienced anesthesiologists used the ventilation devices during routine induction of anesthesia rather than, for example, nonanesthesia health care professionals using the devices in an emergency.
Some limitations of the study need to be noted. First, only healthy ASA physical status I–II patients without underlying respiratory disease, oropharyngeal or facial pathology, or a risk of aspiration were enrolled into the study. Second, we did not measure arterial blood gases. Third, our patients were preoxygenated; therefore, we were unable to determine whether our results could be extrapolated to emergency patients with respiratory, or even cardiac, arrest. Fourth, our data were derived from an optimized setting with experienced anesthesiologists within a standardized anesthesia induction situation. Further studies may have to validate the possible effects in an in-hospital or prehospital emergency situation with less experienced users, and to ensure the manufacturer’s claims that the valve in the device will "pressure balance" against low lung compliance and/or high airway resistance to provide adequate ventilation for patients with chronic obstructive pulmonary disease.
In conclusion, as compared with a standard self-inflating bag, using the SMART BAG® during induction of anesthesia in patients with respiratory arrest resulted in significantly decreased inspiratory flow rate and peak airway pressure while providing adequate tidal volume delivery.
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Acknowledgments
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Supported, in part, by the Austrian Science Foundation Grant P14169-MED, Vienna, Austria; O-Two Systems International, Mississauga, Ontario, Canada; and the Department of Anesthesiology and Critical Care Medicine, Leopold-Franzens-University, Innsbruck, Austria.
We are indebted to Kevin Bowden for his technical advice and assistance and to the enrolled patients for their trust.
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References
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- Wenzel V, Dörges V, Lindner KH, Idris AH. Mouth-to-mouth ventilation during cardiopulmonary resuscitation: word of mouth in the street versus science. Anesth Analg 2001; 93: 128–33.[Abstract/Free Full Text]
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- Milander MM, Hiscok PS, Sanders AB, et al. Chest compression and ventilation rates during cardiopulmonary resuscitation: the effects of audible tone guidance. Acad Emerg Med 1995; 2: 708–13.[ISI][Medline]
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Accepted for publication June 26, 2003.
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Abstract
Introduction
