Effects of face mask ventilation in apneic patients with a resuscitation ventilator in comparison with a bag-valve-mask

The Journal of Emergency Medicine, Vol. 30, No. 1, pp. 63– 67, 2006 Copyright © 2006 Elsevier Inc. Printed in the USA. All rights reserved 0736-4679/06 $–see front matter

doi:10.1016/j.jemermed.2005.02.021

Selected Topics: Critical Care

EFFECTS OF FACE MASK VENTILATION IN APNEIC PATIENTS WITH A RESUSCITATION VENTILATOR IN COMPARISON WITH A BAG-VALVE-MASK Achim von Goedecke, MD, Volker Wenzel, MD, Christoph Hörmann, MD, Wolfgang G. Voelckel, Horst G. Wagner-Berger, MD, Angelika Zecha-Stallinger, MD, Thomas J. Luger, MD, and Christian Keller, MD

MD,

Department of Anesthesiology and Critical Care Medicine, Medical University of Innsbruck, Innsbruck, Austria Reprint Address: Achim von Goedecke, MD, Department of Anesthesiology and Critical Care Medicine, Medical University of Innsbruck, Anichstrasse 35, 6020 Innsbruck, Austria

e Abstract—Bag-valve-mask ventilation in an unprotected airway is often applied with a high flow rate or a short inflation time and, therefore, a high peak airway pressure, which may increase the risk of stomach inflation and subsequent pulmonary aspiration. Strategies to provide more patient safety may be a reduction in inspiratory flow and, therefore, peak airway pressure. The purpose of this study was to evaluate the effects of bag-valve-mask ventilation vs. a resuscitation ventilator on tidal volume, peak airway pressure, and peak inspiratory flow rate in apneic patients. In a crossover design, 40 adults were ventilated during induction of anesthesia with either a bag-valve-mask device with room air, or an oxygen-powered, flow-limited resuscitation ventilator. The study endpoints of expired tidal volume, minute volume, respiratory rate, peak airway pressure, delta airway pressure, peak inspiratory flow rate and inspiratory time fraction were measured using a pulmonary monitor. When compared with the resuscitation ventilator, the bag-valve-mask resulted in significantly higher (mean ⴞ SD) peak airway pressure (15.3 ⴞ 3 vs. 14.1 ⴞ 3 cm H2O, respectively; p ⴝ 0.001) and delta airway pressure (14 ⴞ 3 vs. 12 ⴞ 3 cm H2O, respectively; p < 0.001), but significantly lower oxygen saturation (95 ⴞ 3 vs. 98 ⴞ 1%, respectively; p < 0.001). No patient in either group had clinically detectable stomach inflation. We conclude that the resuscitation ventilator is at least as effective as traditional bag-valve-mask or face mask resuscitation in this population of very controlled elective surgery patients. © 2006 Elsevier Inc.

e Keywords—resuscitation ventilator; ventilation; bagvalve-mask; artificial respiration; tidal volume

INTRODUCTION The distribution of inspiratory gas volume during bagvalve-mask ventilation between lungs and stomach in a patient with an unprotected airway depends on variables such as upper airway pressure, inspiratory flow rate and time, airway resistance and compliance, and lower esophageal sphincter pressure (1). The latter is ⬃20 to 25 cm H2O in a healthy adult, but may be lower in patients during induction of anesthesia, and especially in patients with cardiac arrest (2). Bag-valve-mask ventilation is often applied with a high flow rate, short inflation time, and therefore, a high peak airway pressure. Accordingly, when peak airway pressure exceeds lower esophageal sphincter pressure during ventilation with an unprotected airway, the stomach is inflated. Thus, an important approach during ventilation of an unintubated patient is to keep peak airway pressure as low as possible. Several strategies are possible to reduce peak inspiratory flow rates, and therefore, peak airway pressure; for example, by employing pediatric instead of adult bagvalve-mask or a resuscitation ventilator (3,4). These

Selected Topics: Critical Care Medicine is coordinated by Joseph Varon, MD, of Baylor College of Medicine, Houston, Texas

RECEIVED: 26 February 2004; FINAL ACCEPTED: 15 February 2005

SUBMISSION RECEIVED:

15 October 2004; 63

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strategies may provide a built-in safety, when less experienced rescuers perform assisted ventilation. Unfortunately, clinical data about efficacy and safety of resuscitation ventilators in the past suggested pulmonary barotrauma due to excessive peak inspiratory flow rates; therefore, lower peak inspiratory flow rates were recommended (5). Interestingly, clinical data about resuscitation ventilators seem to be lacking. Accordingly, the purpose of the present study was to evaluate the effects of bag-valve-mask ventilation vs. a resuscitation ventilator on respiratory variables in apneic patients undergoing routine induction of anesthesia. Our hypothesis was that there would be no differences in study endpoints between groups.

MATERIALS AND METHODS Forty adults (ASA 1–2, aged 18 – 64 years) presenting for scheduled surgery were enrolled in this prospective, randomized, crossover study. Institutional review board approval and written informed consent were obtained before initiation of the investigation. Exclusion criteria were a known or predicted respiratory disease, oro-pharyngeal or facial pathology, a body mass index ⬎ 30 kg · m⫺2, or if patients were at risk of aspiration. Premedication was with oral midazolam 7.5 mg, 1 h preoperatively. Anesthesia was in the supine position with the patient’s head on a pillow 5 cm in height. A standard anesthesia protocol was followed and routine monitoring applied. Patients were preoxygenated for 3 min. Anesthesia was induced with fentanyl 2 ␮g · kg⫺1, propofol 2.5–3.5 mg · kg⫺1 given over 30 s and than 10 mg · kg⫺1 · h⫺1 for maintenance. Chin support and backward tilt of the head was performed. A well-fitting facemask (female #3, male #4; Rüsch, Kernen, Germany) was used to ventilate the lungs first with the circlesystem of a ventilator (Julian, Draeger, Lübeck, Germany). Airway function was assessed using gentle hand ventilation, observation of synchronized bilateral expansion of the chest, auscultation, and capnography. At this point, patients were randomly allocated by opening an opaque sealed envelope, to either a resuscitation ventilator (Oxylator FR-300, CPR Medical Devices, Toronto, Canada) ventilation followed by bag-valve-mask (Ambu, Glostrup, Denmark) ventilation, or to bag-valve-mask ventilation followed by resuscitation ventilator ventilation before insertion of an airway device, always done by the same board-certified anesthesiologist (⬎ 5000 facemask ventilations). The resuscitation ventilator (Figure 1) is an oxygen-powered, patient responsive resuscitator and is intended to provide emergency ventilatory support,

Figure 1. OXYLATOR FR-300.

primarily for use with a mask by the first responder in a manual or continuous cycling mode. It has an airway pressure limit of 20 cm H2O, a constant flow-rate of 30 l · min⫺1 and weighs 0.18 kg (0.4 lbs). In our study, it was used in the manual cycling mode. The adult bag-valve-mask bag was employed using room air. Ventilation was performed with a face mask with a respiratory rate of 15 · min⫺1, indicated by a metronome, and a target end-tidal carbon dioxide of ⬃35 mm Hg. After an equilibration phase of 5 min, cardiorespiratory variables were recorded over a 1-min period for each ventilatory mode. After finishing measurements (Figure 2), anesthesia was routinely

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ysis was performed with Student’s t-test. Unless otherwise noted, data are presented as mean (SD). Significance was taken as p ⬍ 0.05. RESULTS Forty patients were enrolled in this crossover study (Table 1). No patient in either group had clinically detectable stomach inflation. When compared with the resuscitation ventilator, the bag-valve-mask resulted in significantly (p ⫽ 0.001) higher peak airway pressure and delta airway pressure (p ⬍ 0.001), but significant lower oxygen saturation (p ⬍ 0.001; Table 2). DISCUSSION

Figure 2. (a, b) Representative flow (V˚), tidal volume (Vt), and airway pressure (Paw) tracings of bag-valve-mask (a) and resuscitation ventilator (b). Each box represents 1 second.

continued before laryngeal mask insertion for the surgical procedure. Respiratory variables were measured and analyzed using a pulmonary monitor (CP-100, Bicore Monitoring System, Irvine, CA) attached to a variable orifice pneumotachograph (Var flex, Allied Health Products, Riverside, CA) (6). The pneumotachograph was connected directly to the proximal end of the face-mask, measuring airway pressure and flow. The following data were recorded and the average reading taken: expired tidal volume, minute volume, respiratory rate, peak airway pressure, delta airway pressure, peak inspiratory flow rate, inspiratory time fraction, oxygen saturation, heart rate, non-invasive mean arterial pressure, and end-tidal carbon dioxide. The anesthesiologist was blinded to the pulmonary monitor. The carbon dioxide sampling port was sited between flow transducer and ventilation device. In addition, epigastric auscultation was performed during face mask ventilation by another investigator to detect any stomach inflation (7). Sample size was selected to detect a projected difference of 30% between groups with respect to peak airway pressure for a type I error of 0.05, and a power of 0.8. The power analysis was based on data from a pilot study of 7 patients. The distribution of data was determined using Kolmogorov-Smirnov analysis (8). Statistical anal-

Studying face mask ventilation with a resuscitation ventilator vs. bag-valve-mask ventilation revealed higher oxygen saturation, lower peak inspiratory flow rates, and therefore, lower peak airway pressures, but comparable tidal volumes. Pulmonary aspiration due to inadequate ventilation in an unprotected airway during routine induction of anesthesia is relatively low, because patients usually have an empty stomach, and anesthesiologists are experienced in bag-valve-mask ventilation. If problems occur during face-mask ventilation, preoxygenation before respiratory arrest provides about 5 to 10 min sufficient oxygenation. This scenario may be fundamentally different in emergency patients, when the stomach may be full, rescuers may be less experienced than anesthesiologists, and lower esophageal sphincter pressure may not be a significant barrier anymore to prevent stomach inflation. Accordingly, an alternative ventilation device with a special feature may be possibly lifesaving for critically injured or sick patients in the emergency medical service. Unfortunately, evaluating a given new ventilation strategy in emergency patients poses substantial logistical, and especially, ethical problems. Therefore, we have evaluated the aforementioned resuscitation ventilator in an operating-room setting of apneic patients undergoing routine induction of anesthesia.

Table 1. Patient Characteristics Variable n Age (years) Height (cm) Weight (kg) Sex; M:F (n) ASA; I/II (n) Data are given as mean ⫾ SD, or numbers.

40 38 ⫾ 13 174 ⫾ 10 76 ⫾ 14 27:13 17/23

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Table 2. Hemodynamic and Respiratory Characteristics During Bag-Valve-Mask and Resuscitation Ventilator Ventilation (n ⴝ 40)

Heart rate (min ⫺1) Mean blood pressure (mm Hg) Oxygen saturation (%) End-tidal carbon dioxide (mm Hg) Respiratory rate (min⫺1) Expiratory tidal volume (mL) Minute volume (l · min⫺1) Peak airway pressure (cm H2O) Delta airway pressure (cm H2O) Peak inspiratory flow rate (l · s⫺1) Inspiratory time fraction (%) Stomach inflation (n)

BVM

RV

p-value

68 ⫾ 10 77 ⫾ 7 95.2 ⫾ 2.9 33.3 ⫾ 3.8 15.8 ⫾ 1.3 640 ⫾ 110 11.4 ⫾ 1.7 15.3 ⫾ 2.7 14 ⫾ 2.7 1.32 ⫾ 0.32 28 ⫾ 6 0/40

67 ⫾ 10 76 ⫾ 7 98.2 ⫾ 0.7 33.4 ⫾ 3.5 15.4 ⫾ 1.3 673 ⫾ 78 11.4 ⫾ 1.6 14.1 ⫾ 2.9 12 ⫾ 2.8 1.17 ⫾ 0.31 28 ⫾ 3.4 0/40

NS NS ⬍ 0.001 NS NS NS NS 0.001 ⬍ 0.001 NS NS NS

Data are shown as mean ⫾ SD, or numbers. NS ⫽ non-significant differences; BVM ⫽ bag-valve-mask; RV ⫽ resuscitation ventilator.

Interestingly, when bag-valve-mask ventilation was performed during advanced cardiac life support, respiratory rates were up to ⬃40/min instead of 10 –15/min, reflecting sustained operator stress (9). This elevated respiratory rate may be an important, but unfortunately underrecognized issue. Pepe et al. recently demonstrated an impairing effect of elevated (⬃30/min) respiratory rates during hemorrhagic shock (10). Thus, nervous rescuers performing bag-valve-mask ventilation with too high respiratory rates may potentially cause harm without even knowing about it. Because many trauma protocols recommend a respiratory rate of 15 to 20/min, the bag-valve-mask device may be a strategy that does not necessarily prevent such problems. Although the respiratory rate was designed to be ⬃15/min in both groups in our study, ventilatory parameters can be more easily adjusted with a resuscitation ventilator; however, that hypothesis needs to be confirmed in future studies. Also, we have observed paradoxic reactions of experienced rescuers during simulated ventilation of a cardiac arrest patient; namely, when less air seemed to enter the patient’s lungs, rescuers tried to counterbalance this shortcoming with more forceful bag-valve-mask ventilation, reflecting higher peak flow rates (3). When extrapolating these observations, it may be beneficial to provide healthcare personnel with a specific strategy to ensure safe basic life support ventilation. Although bag-valvemask ventilation is the established maneuver to perform ventilation in an unprotected airway, some modern anesthesia machines offer a pressure-controlled ventilation feature, enabling anesthesiologists to provide ventilation in an unprotected airway with a new level of patient safety (11). Although pressure-controlled ventilation may be an alternative ventilation strategy in the hospital, a small and lightweight resuscitation ventilator may be an option for rescuers responding to emergencies within the hospital, or in the emergency medical service.

Any increase of peak inspiratory flow rate and therefore, peak airway pressure, may increase the risk of stomach inflation (12). For example, stomach inflation during bag-valve-mask ventilation occurred in more than two-thirds of the patients at an inflation pressure of ⬃27 cm H2O; accordingly, the authors recommended to limit peak airway pressure at 20 cm H2O (13). In our setting, bag-valve-mask ventilation resulted in peak airway pressures of only ⬃15 cm H2O, because an experienced anesthesiologist ventilated the patient‘s lungs. The reduction in peak airway pressure with the resuscitation ventilator to about 14 cm H2O seems relatively minor in this context, but may be much greater once bag-valvemask ventilation is performed by a rescuer who is not an experienced board-certified anesthesiologist. For example, when extrapolating previous inspiratory flow rates when paramedics ventilated a bench model of an unprotected airway, the difference between calculated peak airway pressure with a bag-valve-mask device vs. the resuscitation ventilator would not be ⬃1 cm H2O as in our study, but ⬃8 cm H2O (3,14). Accordingly, a resuscitation ventilator may represent a bigger built-in safety margin than bag-valve-mask ventilation to avoid stomach inflation when the airway is unprotected; especially, if non-anesthesiologist rescuers are responding to a respiratory arrest patient. This, in turn, may improve patient protection (15). In our study, all patients were preoxygenated; however, oxygen saturation during bag-valve-mask ventilation with room air decreased to ⬃95%, whereas an oxygen saturation of ⬃98% was maintained with the oxygen-powered resuscitation ventilator. In a previous clinical investigation of an unprotected airway, we showed that smaller tidal volumes of ⬃350 mL containing 100% oxygen, but not room air, were sufficient to ensure adequate ventilation and oxygenation (16). Thus, we could have probably cut our tidal volumes by

Resuscitation Ventilator vs. Bag-Valve-Mask

almost half, which would have decreased peak airway pressure even further, as in our study. Therefore, if a resuscitation ventilator is adjusted to small tidal volumes, non-professional rescuers may be able to administer even safer ventilation than we were able to demonstrate in the present study. As a limitation, only healthy ASA status 1–2 patients without underlying respiratory disease, oro-pharyngeal, facial pathology, or risk of aspiration were enrolled into the study. Second, arterial partial pressure of oxygen was not measured. Third, anesthesia was performed by only one experienced anesthesiologist, as demanded by the institutional review board. Fourth, this may also be the reason why peak airway pressures were relatively similar between bag-valvemask and resuscitation ventilator. Accordingly, although differences in peak airway pressure are statistically significantly different (probably due to the large sample size), they must be deemed clinically not relevant. Fifth, although our present setting of employing healthy patients undergoing routine surgical procedures is unable to simulate oxygenation conditions of a hypoxic or hypercarbic patient requiring immediate airway management, it may be a useful tool to assess respiratory mechanics of two different ventilation strategies in an unprotected airway. Thus, although the strategy of our study may not be necessary during routine induction of anesthesia, it may be extremely valuable in a difficult situation. Sixth, we used the resuscitation ventilator in the manual mode, which reduces peak airway pressure. In conclusion, the resuscitation ventilator is at least as effective as traditional bag-valve-mask or face mask resuscitation in this population of very controlled elective surgery patients.

Acknowledgment—This work was supported, in part, by the Science Foundation MFF of the Tyrolean State Hospitals (TILAK), Tyrol, Austria and the Austrian Science Foundation grant P14169-MED, Vienna, Austria.

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