Developing a Strategy to Improve Ventilation in an Unprotected Airway with a Modified Mouth-to-Bag Resuscitator in Apneic Patients

Abstract

The strategies to ensure safety during ventilation of an unprotected airway are limiting airway pressure and/or inspiratory flow. In this prospective, randomized study we assessed the effect of face mask ventilation with small tidal volumes in the modified mouth-to-bag resuscitator (maximal volume, 500 mL) versus a pediatric self-inflatable bag versus automatic pressure-controlled ventilation in 40 adult apneic patients during induction of anesthesia. The mouth-to-bag resuscitator requires the rescuer to blow up a balloon inside the self-inflating bag that subsequently displaces air which then flows into the patient’s airway. Respiratory variables were measured with a pulmonary monitor (CP-100). Mouth-to-bag resuscitator and pressure-controlled ventilation resulted in significantly lower (mean ± sd) peak airway pressure (8 ± 2 and 8 ± 1 cm H₂O), peak inspiratory flow rate (0.7 ± 0.1 and 0.7 ± 0.1 L/s), and larger inspiratory time fraction (33% ± 5% and 47% ± 2%) in comparison to pediatric self-inflating bag ventilation (12 ± 3 cm H₂O; 1 ± 0.2 L/s; 27% ± 4%; all P < 0.001). The tidal volumes were similar between groups. No stomach inflation occurred in either group. We conclude that using a modified mouth-to-bag resuscitator or automatic pressure-controlled ventilation with similar small tidal volumes during face mask ventilation resulted in an approximately 25% reduction in peak airway pressure when compared with a standard pediatric self-inflating bag.

In Brief

IMPLICATIONS: Using a modified mouth-to-bag resuscitator or automatic pressure-controlled ventilation during ventilation of an unprotected airway resulted in an approximately 25% reduction in peak flow rate and peak airway pressure when compared with a pediatric self-inflating bag.

During ventilation of an unprotected airway, gas follows the path of least resistance; therefore, low peak airway pressure favors lung inflation, whereas high peak airway pressure will also open the esophagus, making stomach inflation likely. Accordingly, keeping peak airway pressure low in unintubated patients ensures adequate oxygenation and carbon dioxide elimination, prevents stomach inflation, and therefore, enhances patient safety.

Pressure-controlled ventilation may be the best possible strategy to ensure low peak airway pressure when ventilating a patient with an unprotected airway, because inspiratory flow is generated by an anesthesia machine, and not manually by a rescuer who may be squeezing a self-inflatable bag with greater force. This may result in high peak flow rates and, therefore, high peak airway pressures (^1,2). One approach to limit peak inspiratory flow rates during ventilation of an unprotected airway is to use a newly developed self-inflating bag with a special valve that limits inspiratory gas flow from up to approximately 120 L/min of a standard self-inflating bag to approximately 30 L/min (³). Another approach to limit peak inspiratory flow rates is to decrease tidal volumes by switching from an adult to a pediatric self-inflatable bag (^4,5). However, the decrease in tidal volume may not result in an exactly proportionate decrease in peak inspiratory flow rates, because the rescuer’s hands may be simply able to compress a pediatric self-inflating bag more than an adult device. Thus, a possible further beneficial development is both a combination of small tidal volumes with a decreased inspiratory flow rate. The mouth-to-bag resuscitator ensures decreased inspiratory flow rates, but advantages could not be shown, because advantages were masked by too high tidal volumes (⁶). A subsequent reduction in tidal volumes from approximately 1050 to 500 mL applied by this novel device further revealed that this strategy may be useful (⁷); however, clinical confirmation is lacking.

The purpose of the present study was to assess the effect of automatic pressure-controlled ventilation versus a pediatric self-inflatable bag versus the mouth-to-bag resuscitator with decreased tidal volumes in supine apneic patients in order to simulate a respiratory arrest patient. Primary study end-points were respiratory variables such as peak inspiratory flow rate, peak airway pressure, and inspiratory tidal volume. Our hypothesis was that there would be no differences with either ventilation device in regard to study end-points.

Methods

Forty adults presenting for scheduled surgery were enrolled in this prospective study. Ethical committee approval and written informed consent were obtained before the beginning of the investigation. Exclusion criteria were respiratory disease, oropharyngeal or facial pathology, obesity, or 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 standard pillow. A standard anesthesia protocol was followed and routine monitoring applied. Patients breathed oxygen for 3 min and anesthesia was induced with fentanyl 2 μg/kg, propofol 2.5–3.5 mg/kg given over 30 s followed by propofol 10 mg · kg⁻¹ · h⁻¹ for maintenance. A well-fitting face mask (Rüsch, Kernen, Germany) was used to ventilate the lungs with a Julian ventilator (Draeger, Lübeck, Germany). Each patient was ventilated with each of three ventilation strategies in a random order. Patients were randomized allocated by opening an opaque sealed envelope to provide the order in which they received either automatic pressure-controlled ventilation, pediatric bag-valve-mask ventilation, or mouth-to-bag resuscitator ventilation. Ventilation was performed by 4 different anesthesiologists (10 patients each; 5–10 yr clinical experience); they were blinded to the anesthesia machine and the pulmonary monitor in order to avoid any feedback of the quality of ventilation, and subsequently adjust ventilation.

Respiratory variables were measured and analyzed using a pulmonary monitor (CP-100; Bicore Monitoring System, Irvine, CA) attached to a variable orifice pneumotachography (Varflex; Allied Health Products, Riverside, CA) (⁸). The pneumotachograph was connected directly to the proximal end of the face mask. Study end-points were respiratory mechanics. Epigastric auscultation was achieved during face mask ventilation by another blinded investigator to detect any stomach inflation (⁹).

Automatic pressure-controlled ventilation was performed with a respiratory rate of 20 breaths/min, a fresh gas flow of 3 L/min oxygen, an anesthesia machine flow of 30 L/min, an inspiratory/expiratory ratio of 1:1, a positive end-expiratory pressure of 0 cm H₂O, and the peak airway pressure was set to get a tidal volume of approximately 5 mL/kg. Bag-valve-mask ventilation was performed with oxygen (oxygen flow rate 5 L/min) by using a pediatric self-inflating bag (total volume, 410 mL) and the modified mouth-to-bag resuscitator (both from Ambu, Glostrup, Denmark). The modified mouth-to-bag resuscitator consists of a single-use balloon inside the self-inflating bag. The balloon, which was reduced to a volume of 500 mL, requires inflating by the rescuer’s expired air as driving gas only, thus displacing air which then flows into the patient’s airway (Fig. 1). The respiratory rate was 20 breaths/min with an inspiratory/expiratory ratio of 1:1. The anesthesiologists ventilated based on their clinical experience, until the chest clearly rose but they were guided by a metronome in order to assure constant frequency and inspiratory/expiratory ratio. After an equilibration phase of 5 min, cardiorespiratory variables were recorded over a 100-s period for each ventilatory mode before a laryngeal mask was inserted (Fig. 2).

Figure 1.: Modified mouth-to-bag resuscitator consisting of a single-use balloon with a reduced volume of 500 mL inside. During inspiration (A), the balloon requires inflating by the rescuer’s expired air as the driving gas, thus displacing air in the bag, which then flows into the patient’s airway. During expiration (B), the expired air flows through a valve in the surrounding along the way of least resistance and the bag is filled by air out of the oxygen reservoir bag.

Figure 2.: Representative flow (˙V), tidal volume (V_T), and airway pressure (Paw) tracings of pressure-controlled ventilation (A), pediatric self-inflating bag (B), and mouth-to-bag resuscitator (C). The hatched lines represent zero and each box represents 1 s.

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 0.9. The power analysis was based on data from a pilot study of seven patients. One-way analysis of variance with post hoc tests (Bonferroni) was used to compare the data. Unless otherwise noted, data were presented as mean ± sd. Significance was taken as P < 0.05.

Results

Forty patients were enrolled (Table 1); hemodynamic variables and oxygen saturation were comparable (Table 2), but end-tidal carbon dioxide was significantly (P < 0.05) less with pressure-controlled ventilation compared with either the pediatric self-inflating bag, or the mouth-to-bag resuscitator. The mouth-to-bag resuscitator versus pediatric self-inflating bag resulted in significantly lower peak airway pressure and peak inspiratory flow rate, but a higher inspiratory time fraction. In comparison with the pressure-controlled ventilation mode, there were significantly smaller inspiratory tidal volumes and shorter inspiratory time fractions with both devices. The pediatric self-inflating bag had significantly higher peak airway pressure and peak inspiratory flow rate and the mouth-to-bag resuscitator had a significantly lower minute ventilation compared with pressure-controlled ventilation. No stomach inflation occurred in any group.

Table 1: Patient Characteristics

Table 2: Hemodynamic and Respiratory Characteristics in 40 Patients During Pressure-Controlled Ventilation (PCV), Pediatric Self-Inflating Bag Ventilation (PSIB), and Mouth-To-Bag Resuscitator Ventilation (MTBR)

Discussion

Measuring respiration variables during basic life-support ventilation is extremely difficult, because a delay of life-saving interventions in patients with respiratory or cardiac arrest until study equipment is ready to use is both unacceptable and unethical. Although our setting does not entirely reflect the respiratory status of an untreated apneic patient with hypoxia and/or hypercarbia, we suggest that the present study using anesthetized, paralyzed supine adults may be the best possible model to simulate basic life-support ventilation with a minimum of confounding variables (^10,11).

When using an adult self-inflating bag, even experienced anesthesiologists inflated the stomach in approximately 13% of patients undergoing scheduled surgical procedures (¹²). This is surprising, because this setting usually does not favor stomach inflation. Observing professional health care workers administering bag-valve-mask ventilation during cardiopulmonary resuscitation in an emergency room revealed a respiratory rate that was twice as much as recommended (¹³). Also, experienced professional rescuers performing bag-valve-mask ventilation used an inspiratory time of approximately 0.5 seconds, resulting in high peak flow rates, and therefore, high peak airway pressures (⁴). Accordingly, it may be prudent that ventilation devices have a built-in safety feature with limited peak airway pressure and peak inspiratory flow. For example, inadvertent hyperventilation caused by excessive ventilation rates with a self-inflating bag resulted in increased intrathoracic pressure, decreased venous return, and therefore, decreased arterial blood pressure (¹⁴). Thus, the mouth-to-bag resuscitator may ensure that inexperienced or extremely stressed operators do not cause inadvertent complications.

We achieved an approximately 25% reduction in peak flow rate and peak airway pressure when compared with a pediatric self-inflating bag, indicating the potential for improved level of patient safety during ventilation of adults with an unprotected airway. It is remarkable that peak airway pressure with the mouth-to-bag resuscitator was as good as pressure-controlled ventilation with an anesthesia machine (¹), which may represent the least-possible peak airway pressure for a given tidal volume. Because lower esophageal sphincter pressure in anesthetized adults is approximately 20 cm H₂O (¹⁵) and the peak airway pressure in our study was approximately 8–12 cm H₂O, it was not surprising that there was no stomach inflation in any patient. Arguably, stomach inflation was extremely unlikely in our patients; however, when stomach inflation is to be expected because of altered respiratory mechanics such as in shock or cardiac arrest patients, this additional level of safety may be of clinical importance. It is possible that lesser clinically experienced rescuers, such as firefighters or emergency medical technicians, would have caused higher peak airway pressures than our experienced anesthesiologists. In that case, differences in study end-points would have been most likely even greater. Moreover, when applying mouth-to-bag resuscitator ventilation, the operator can use two instead of one hand to ensure a proper mask seal, thus decreasing mask leak (¹⁶).

As a limitation, only healthy ASA physical status I–II patients without underlying respiratory disease, oropharyngeal, facial pathology, or risk of aspiration were enrolled in the study. For example, ventilation of a patient having a respiratory arrest secondary to pulmonary edema may be severely limited by such a system. Second, arterial partial pressure of oxygen was not measured. Third, ventilation was performed by experienced anesthesiologists. Fourth, we simulated basic life-support ventilation of a patient with respiratory arrest only, and not a cardiac arrest patient.

In conclusion, using a modified mouth-to-bag resuscitator or automatic pressure-controlled ventilation with similar small tidal volumes during face mask ventilation resulted in an approximately 25% reduction in peak airway pressure when compared with a standard pediatric self-inflating bag.