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CRITICAL CARE AND TRAUMA

Minimizing Stomach Inflation Versus Optimizing Chest Compressions

From the Department of Anesthesiology and Critical Care Medicine, Innsbruck Medical University, Innsbruck, Austria.

Address correspondence and reprint requests to Dr. Holger Herff, Department of Anesthesiology and Critical Care Medicine, Innsbruck Medical University, Anichstrasse 35, 6020 Innsbruck, Austria. Address e-mail to holger.herff{at}i-med.ac.at.

	Abstract

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In a bench model, we evaluated a bag-valve device (Smart Bag® MO) with limited maximum inspiratory gas flow developed to reduce the risk of stomach inflation in an unprotected airway. During simulated cardiopulmonary resuscitation with uninterrupted chest compressions, ventilation with the "disabled" Smart Bag® MO or an adult self-inflating bag-valve device provided only adequate tidal volumes if inspiratory time was 0.5 s. Ventilation with the "enabled" Smart Bag® MO, even in ventilation windows of 0.5 s, provided inadequate tidal volumes during simulated cardiopulmonary resuscitation and would result in hypoventilation in a patient.

	Introduction

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During cardiopulmonary resuscitation (CPR), ventilation of patients with an unprotected airway is often performed too aggressively, with excessive airway pressures resulting in subsequent stomach inflation, regurgitation, and aspiration of stomach contents.1–3 To reduce the risk of stomach inflation in an unprotected airway, an inspiratory flow-limiting, bag-valve device was developed that decreases peak airway pressure, in turn decreasing the likelihood of stomach inflation.4

During CPR with 100 chest compressions over 60 s with a 50% duty cycle,5 the secure time window for ventilation during the chest decompression phase is only 0.3 s. Although the inspiratory flow-limiting, bag-valve device is valuable to minimize stomach inflation in an unprotected airway, in a protected airway high flow ventilation may be needed if chest compressions are not interrupted for ventilation.

The purpose of this study was to assess effects of the inspiratory flow-limiting, bag-valve device and an adult self-inflating, bag-valve device during simulated CPR with uninterrupted chest compressions after intubation. Inspiratory times were 0.25, 0.3, and 0.5 s. Our null hypothesis was that the different bag-valve device settings would have comparable effects on the study end points tidal volume and respiratory mechanics.

	METHODS

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The inspiratory flow-limiting, bag-valve device (Smart Bag® MO, O-Two Medical Technologies, Mississauga, Ontario; Fig. 1) is an adult bag-valve device that reduces inspiratory gas flow and in consequence peak airway pressures during ventilation of an unprotected airway. When a lever is rotated on the side of the gas flow reducer, the piston of this element is fixed in its original position, which turns off the flow-limiting feature, converting the Smart Bag® MO to a standard adult bag-valve device (manual override mode).

View larger version (9K): [in this window] [in a new window]   Figure 1. Smart Bag® MO limits inspiratory gas flow. (A) Medium manual pressure on the Smart Bag® MO1 leaves the flow limiting element open2 and results in a medium inspiratory gas flow.3 (B) Forceful compression of the Smart Bag® MO1 closes the flow-limiting element2 and reduces inspiratory gas flow.3

To simulate ventilation of an intubated adult patient in a bench model, the Smart Bag® MO in its enabled flow-limiting state, the Smart Bag® MO in its disabled state, and an adult bag-valve device were directly attached to the air inlet of an adult test lung (MI Instruments, Grand Rapids, MI; Fig. 2). The test lung was connected to a personal computer using Pneuview standard software (MI Instruments, Grand Rapids, MI).

View larger version (12K): [in this window] [in a new window]   Figure 2. Experimental bench model. The tested bag-valve device (BVD) was attached to the air inlet of the test lungs. The proximal airway pressure, lung tidal volume, and respiratory mechanics were measured and recorded on a personal computer (PC). Airway resistance was adjusted with two modules simulating upper and lower airway resistance to a total of 5 cm H2O · mL–1 · s–1.

To simulate respiratory system conditions during prolonged continuing CPR, respiratory system compliance was set to 20 mL/cm H₂O.6 In a second approach simulating ventilation of a supine anesthetized adult, respiratory system compliance was set to 60 mL/cm H₂O, which simulates respiratory system mechanics shortly after onset of cardiac arrest.7 With both respiratory compliance settings, upper and lower airway resistance were adjusted to values resulting in a total airway resistance of 5 cm H₂O · L^–1 · s^–1, which is a physiological value for an intubated patient with no airway obstruction.7 In each respiratory compliance setting, the test lung was randomly ventilated with an enabled versus disabled Smart Bag® MO versus adult bag-valve device. The enabled Smart Bag® MO, the disabled Smart Bag® MO, and the adult bag-valve device were squeezed to produce inspiratory times of 0.25, 0.3, or 0.5 s. Only results within a margin of 0.01 s above or below the target times were accepted. In both respiratory system compliance settings for each inspiratory time, 20 successful ventilation attempts were recorded for each bag valve device. The rescuer was blinded to all monitor tracings except inspiratory time.

Distribution of data was determined using Kolmogorov-Smirnov analysis. For statistical analysis between self-inflating, bag-valve devices in each setting, Student’s t-test for unpaired data was used. Further, for each self-inflating, bag-valve device in a given setting, results of 0.25 s were compared with those of 0.3 s, and those of 0.3 s were compared with those of 0.5 s using the paired Student’s t-test. Data are presented as mean ± sd; statistical significance was considered as P < 0.05.

	RESULTS

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With a respiratory system compliance of 20 mL/cm H₂O, tidal volume provided with an enabled Smart Bag® MO increased from 83 ± 3 mL (mean ± sd) in 0.25 s to 96 ± 2 mL in 0.3 s (P < 0.05) and to 161 ± 7 mL in 0.5 s (P < 0.05). With the disabled Smart Bag® MO, tidal volume increased from 212 ± 13 mL to 271 ± 16 mL (P < 0.05) and then to 473 ± 28 mL (P < 0.05), and with an adult bag-valve device from 277 ± 28 mL to 283 ± 22 mL (P < 0.05) and then to 503 ± 26 mL (P < 0.05) in 0.25, 0.3, and 0.5 s, respectively.

With a respiratory system compliance of 60 mL/cm H₂O, tidal volume provided with the enabled Smart Bag® MO increased from 120 ± 3 mL in 0.25 s to 140 ± 4 mL in 0.3 s (P < 0.05) and to 190 ± 7 mL in 0.5 s (P < 0.05). With the disabled Smart Bag® MO tidal volume increased from 221 ± 16 mL to 285 ± 16 mL (P < 0.05) and then to 452 ± 30 mL (P < 0.05). With an adult bag-valve device, tidal volume increased from 244 ± 32 mL to 337 ± 21 mL (P < 0.05) and to 515 ± 29 mL (P < 0.05) in 0.25, 0.3, and 0.5 s, respectively (Fig. 3, A and B). (Further data on respiratory mechanics are available in the Table in the online supplement available at www.anesthesia-analgesia.org.)

View larger version (14K): [in this window] [in a new window]   Figure 3. Tidal volumes achieved with the different ventilation devices. *P < 0.05 compared with an adult bag-valve device; P < 0.05 for enabled Smart Bag® MO versus disabled Smart Bag® MO.

	DISCUSSION

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In this bench model, tidal volumes applied with the enabled Smart Bag® MO were <200 mL even with an inspiratory time of 0.5 s, and would mainly result in dead-space ventilation, leading to hypoventilation, hypoxia and, in consequence, probably to death.8,9 Thus, while the Smart Bag® MO has the advantage of preventing stomach inflation with an unprotected airway, a sufficient inspiratory time of >0.5 s is required, which may simply not be available when unsynchronized chest compressions are performed at the recommended rate of 100 per minute.

Therefore, we suggest that the enabled Smart Bag® MO not be used in asynchronous ventilation settings during CPR after intubation. The solution may be to switch a Smart Bag® MO to the manual override mode or even to use a standard adult bag-valve device. This ensures adequate tidal volumes of about 500 mL if an inspiratory time of 0.5 s can be secured.

Study limitations include the use of a test lung model and the fact that only one experienced rescuer provided ventilation. However, while test lungs cannot exactly replicate respiratory anatomy and physiology of a cardiac or respiratory arrest patient, they are established to study simulated ventilation scenarios.4

In conclusion, ventilation with the disabled Smart Bag® MO or an adult self-inflating bag-valve device provided only adequate tidal volumes in short ventilation windows during simulated CPR if inspiratory time was 0.5 s. Ventilation with the enabled Smart Bag® MO, even in ventilation windows of 0.5 s, provided inadequate tidal volumes during simulated CPR and would result in hypoventilation in a patient.

	Footnotes

 
Accepted for publication October 8, 2007.

Supported, in part, by the Science Foundation of the Austrian National Bank grant 11448, Vienna, Austria.

	REFERENCES

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1. Keul W, Bernhard M, Volkl A, Gust R, Gries A. Methods of airway management in prehospital emergency medicine. Anaesthesist 2004;53:978–92[Web of Science][Medline]
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3. Wenzel V, Idris AH, Banner MJ, Kubilis PS, Williams JL Jr. Influence of tidal volume on the distribution of gas between the lungs and stomach in the nonintubated patient receiving positive-pressure ventilation. Crit Care Med 1998;26:364–8[Web of Science][Medline]
4. Von Goedecke A, Paal P, Keller C, Voelckel WG, Herff H, Lindner KH, Wenzel V. Ventilation of an unprotected airway: evaluation of a new peak-inspiratory-flow and airway-pressure-limiting bag-valve-mask. Anaesthesist 2006;55:629–34[Web of Science][Medline]
5. Nolan J, Baskett P. Eur resuscitation council guidelines for resuscitation 2005. Resuscitation 2005;67:1–189
6. Wenzel V, Idris AH, Nolan JP, Parr MJ, Gabrielli A, Stallinger A, Lindner KH, Baskett PJ. The respiratory system during resuscitation: a review of the history, risk of infection during assisted ventilation, respiratory mechanics, and ventilation strategies for patients with an unprotected airway. Resuscitation 2001;49:123–34[Web of Science][Medline]
7. Ornato JP, Bryson BL, Donovan PJ, Farquharson RR, Jaeger C. Measurement of ventilation during cardiopulmonary resuscitation. Crit Care Med 1983;11:79–82[Web of Science][Medline]
8. Markstaller K, Karmrodt J, Doebrich M, Wolcke B, Gervais H, Weiler N, Thelen M, Dick W, Kauczor HU, Eberle B. Dynamic computed tomography: a novel technique to study lung aeration and atelectasis formation during experimental CPR. Resuscitation 2002;53:307–13[Web of Science][Medline]
9. Baskett JP, Nolan J, Parr M. Tidal volumes which are perceived to be adequate for resuscitation. Resuscitation 1996;31:231–41[Web of Science][Medline]

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