Mechanical Versus Manual Ventilation via a Face Mask During the Induction of Anesthesia: A Prospective, Randomized, Crossover Study

Abstract

One approach to make ventilation safer in an unprotected airway has been to limit tidal volumes; another one might be to limit peak airway pressure, although it is unknown whether adequate tidal volumes can be delivered. Accordingly, the purpose of this study was to evaluate the quality of automatic pressure-controlled ventilation versus manual circle system face-mask ventilation regarding ventilatory variables in an unprotected airway. We studied 41 adults (ASA status I–II) in a prospective, randomized, crossover design with both devices during the induction of anesthesia. Respiratory variables were measured with a pulmonary monitor (CP-100). Pressure-controlled mask ventilation versus circle system ventilation resulted in lower (mean ± sd) peak airway pressures (10.6 ± 1.5 cm H₂O versus 14.4 ± 2.4 cm H₂O; P < 0.001), δ airway pressures (8.5 ± 1.5 cm H₂O versus 11.9 ± 2.3 cm H₂O; P < 0.001), expiratory tidal volume (650 ± 100 mL versus 680 ± 100 mL; P = 0.001), minute ventilation (10.4 ± 1.8 L/min versus 11.6 ± 1.8 L/min; P < 0.001), and peak inspiratory flow rates (0.81 ± 0.06 L/s versus 1.06 ± 0.26 L/s; P < 0.001) but higher inspiratory time fraction (48% ± 0.8% versus 33% ± 7.7%; P < 0.001) and end-tidal carbon dioxide (34 ± 3 mm Hg versus 33 ± 4 mm Hg; not significant). We conclude that in this model of apneic patients with an unprotected airway, pressure-controlled ventilation resulted in reduced inspiratory peak flow rates and peak airway pressures when compared with circle system ventilation, thus providing an additional patient safety effect during mask ventilation.

In Brief

IMPLICATIONS: In this model of apneic patients with an unprotected airway, pressure-controlled ventilation resulted in reduced inspiratory peak flow rates and lower peak airway pressures when compared with circle system ventilation, thus providing an additional patient safety effect during face-mask ventilation.

Anesthesia closed-claims analysis from the United States reveals that approximately 38% of the adverse outcomes with respiratory events were related to inadequate ventilation; 94% of them resulted in death or permanent brain damage, with a median payment of $240,000 ⁽¹⁾. Thus, it is of utmost importance to ensure both ventilation and oxygenation when the airway is protected and, especially, when the airway is unprotected. During manual ventilation, the distribution of gas flow between lungs and stomach depends on patient components, such as airway resistance, compliance, and lower esophageal sphincter pressure, and variables applied by the operator of controlled ventilation, such as inspiratory flow rate and time ⁽²⁾. The combination of both components determines peak airway pressure; in turn, the relation between lower esophageal sphincter pressure and peak airway pressure determines whether gas flow enters the lungs or, at least partially, the stomach, resulting in regurgitation and subsequent pulmonary aspiration.

One approach to make ventilation safer in an unprotected airway has been to limit tidal volumes by using a pediatric instead of an adult self-inflatable bag ⁽³⁾ and by using bag-valve-mask ventilation devices with a built-in feature that limits inspiratory peak flow rate ^(4,5). Another option especially for anesthesia induction may be pressure-controlled ventilation, because it can be applied by modern anesthesia machines and enables the operator to seal the face mask better ⁽⁶⁾. However, it is unknown whether pressure-controlled ventilation can deliver adequate tidal volumes while decreasing peak airway pressure in comparison with circle system ventilation.

Accordingly, the purpose of this study was to evaluate the effects of automatic pressure-controlled versus manual circle system mask ventilation in apneic patients regarding peak inspiratory flow rate, peak airway pressure, and tidal volume. Our hypothesis was that there would be no differences in study end-points between groups.

Methods

Forty-one adults (ASA status I–II; aged 18–64 yr) presenting for elective peripheral musculoskeletal surgery were enrolled in this prospective crossover study. Ethical committee approval and written, informed consent were obtained before the beginning of the investigation. Exclusion criteria were a known or predicted respiratory disease, oropharyngeal or facial pathology, a body mass index >30 kg/m², or risk of aspiration (fasted <6 h or gastroesophageal reflux more than once weekly).

Premedication was with oral midazolam 7.5 mg 1 h before surgery. 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. Patients breathed oxygen for 3 min, and anesthesia was induced with fentanyl 2 μg/kg; propofol 2.5–3.5 mg/kg was given over 30 s followed by propofol 10 mg · kg⁻¹ · h⁻¹ for maintenance. Chin support and backward tilt of the head was performed. A well-fitting face mask (female #3, male #4; Rüsch, Kernen, Germany) was used to ventilate the lungs via a Julian ventilator (Draeger, Lübeck, Germany). Airway function was assessed by using gentle hand ventilation, observation of synchronized bilateral expansion of the chest, auscultation, and capnography. At this point, patients were randomized, allocated by opening an opaque, sealed envelope, to either automatic pressure-controlled ventilation followed by circle system bag ventilation or to circle system bag ventilation followed by pressure-controlled ventilation before insertion of an airway device, which was always performed by the same consultant anesthesiologist (>5000 face-mask ventilations). He was blinded to the anesthesia machine and the pulmonary monitor and fixed the face mask in both ventilation strategies with one hand only.

Respiratory variables were measured and analyzed by using a pulmonary monitor (CP-100; Bicore Monitoring System, Irvine, CA) attached to a variable-orifice pneumotachograph (Var flex; Allied Health Products, Riverside, CA) ^(3,7). 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 was taken: inspired and expired tidal volume, minute ventilation, respiratory rate, peak airway pressure, δ airway pressure, peak inspiratory flow rate, peak expiratory flow rate, inspiratory time fraction, oxygen saturation, heart rate, noninvasive mean blood arterial pressure, and end-tidal carbon dioxide. The carbon dioxide sampling port was sited above the flow transducer. In addition, epigastric auscultation was performed during face-mask ventilation by another blinded investigator to detect any stomach inflation ⁽⁸⁾.

Automatic pressure-controlled ventilation was performed with a respiratory rate of 15 breaths/min, a fresh gas flow of oxygen 3 L/min, an anesthesia machine flow of 30 L/min, an inspiratory/expiratory ratio of 1:1, and a positive end-expiratory pressure of 0 cm H₂O, and the peak airway pressure was set to get a tidal volume of 8–10 mL/kg. During manual bag-valve-mask ventilation, the respiratory rate was 15 breaths/min as well, as indicated by a metronome providing an inspiratory/expiratory ratio of 1:1. The pop-off valve was set at 20 cm H₂O ⁽⁹⁾ at a fixed gas flow of 0.5 L/min of oxygen to compensate for oxygen consumption and unavoidable small leakage and to prevent the development of positive end-expiratory pressure. The anesthesiologist should ventilate using his experience until the chest clearly rises. After an equilibration phase of 5 min, the cardiorespiratory variables were recorded over a 1-min period for each ventilatory mode. After measurements were finished, anesthesia was routinely continued before laryngeal mask insertion for the surgical procedure.

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. The distribution of data was determined by using Kolmogorov-Smirnov analysis ⁽¹⁰⁾. Statistical analysis 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-one patients were enrolled in this crossover study (Table 1). There were no significant differences in hemodynamic variables, end-tidal carbon dioxide, or oxygen saturation between groups (Table 2). No patient in either group had clinically detectable stomach inflation. When compared with circle system ventilation, pressure-controlled ventilation resulted in significantly (P < 0.001) smaller peak airway pressures, δ airway pressures, peak inspiratory flow rates, minute ventilation, expired tidal volumes (P = 0.001), and respiratory rates (P = 0.015) but longer inspiratory time fractions (P < 0.001;Table 2).

Table 1: Patient Characteristics

Table 2: Hemodynamic and Respiratory Characteristics in 41 Patients During Pressure-Controlled Ventilation (PCV) and Circle System Ventilation (CSV)

Discussion

Ventilating apneic patients during the induction of anesthesia with automatic pressure-controlled instead of manual circle system ventilation with comparable tidal volumes resulted in an approximately 25% reduction in peak airway pressures. Pulmonary aspiration due to complications of ventilation in an unprotected airway during routine induction of anesthesia is relatively infrequent, because patients usually have an empty stomach and anesthesiologists are experienced in using manual circle system ventilation. In the operating room, peak airway pressures of approximately 27 cm H₂O resulted in stomach ventilation in approximately 71% of the patients during mask ventilation with a Magill system ⁽¹¹⁾. In that study, in only one patient was tidal volume resulting in gastric inflation less than 1000 mL, but peak airway pressure was approximately 32 cm H₂O; accordingly, the authors suggested limiting inspiratory pressures to 20 cm H₂O to prevent stomach inflation.

In an effort to decrease peak inspiratory pressure and, therefore, stomach inflation, we reduced the peak inspiratory flow rate by using pediatric instead of adult self-inflatable bags from approximately 2.1 L/s to approximately 1.8 L/s in a bench model, indicating a moderate reduction of 15% in peak inspiratory flow rate by decreasing tidal volume ⁽³⁾. In another study, peak inspiratory flow rate was reduced by a transport ventilator from approximately 1.9 L/s to approximately 0.8 L/s by increasing inspiratory time from approximately 1.1 seconds to approximately 1.6 seconds ⁽⁴⁾. When inspiratory flow rate was reduced in a flow-limited bag-valve-mask device, peak inspiratory flow rate could be further decreased to 0.6 L/s ⁽⁵⁾. Accordingly, several strategies are possible to keep peak airway pressure limited by limiting peak inspiratory flow rate, inspiratory time, or both. Pressure-controlled ventilation may be a simple strategy that enables control of ventilation variables better than a handheld device. During circle system ventilation in our study, a self-inflatable bag with a maximum volume of 2000 mL was squeezed over approximately 1.3 seconds, whereas pressure-controlled ventilation reflected an inspiratory time of approximately 2 seconds, as recommended in the resuscitation guidelines ^(12,13). Therefore, circle system ventilation resulted in more rapid inspiratory peak flow rates than pressure-controlled ventilation (1.1 versus 0.8 L/s) and, subsequently, larger peak airway pressures (approximately 14 cm H₂O versus approximately 11 cm H₂O). These differences could have been even larger had an operator with larger hands squeezed the self-inflatable bag, resulting in larger tidal volumes, as shown in a previous study ⁽¹⁴⁾.

Whereas circle system ventilation is the established maneuver for performing ventilation in an unprotected airway, some modern anesthesia machines offer a pressure-controlled ventilation feature, which enables anesthesiologists to provide ventilation in an unprotected airway with a new level of patient safety. Moreover, maintenance of a tight mask seal may be more easily provided during pressure-controlled ventilation, because both hands can be used to ensure a mask seal instead of one hand in circle system ventilation while squeezing the self-inflatable bag. During volume-controlled ventilation, the most frequently used mode of an anesthesia machine, ventilation gas losses due to mask leakage are inadequately compensated for, and ventilation volume may be insufficient in face-mask ventilation. Although some of these issues may be of moderate value during the routine induction of anesthesia, they may even add critical benefit during an emergency. For example, when bag-valve-mask ventilation was performed during advanced cardiac life support, respiratory rates were up to approximately 40 breaths/min instead of 10–15 breaths/min, reflecting operator stress ⁽¹⁵⁾. Thus, pressure-controlled ventilation may provide both peak airway pressure control, especially in an unprotected airway, and respiratory rate control.

In our investigation, pressure-controlled ventilation enabled us to decrease peak airway pressure compared with manual circle system ventilation while maintaining almost identical tidal volumes of approximately 650 mL. Although these tidal volumes may be desired during the induction of anesthesia to supply the patient with rapid inflow of inhaled anesthetics or oxygen, they may not always be necessary. We have shown in non-preoxygenated respiratory arrest patients that smaller tidal volumes of approximately 500 mL with room air were barely sufficient to provide adequate oxygenation and ventilation ⁽¹⁶⁾. Thus, when 100% oxygen is used, tidal volumes may be further reduced to at least 350 mL ⁽¹⁷⁾; when extrapolating to pressure-controlled ventilation, peak airway pressure could possibly be further reduced from 11 cm H₂O, as in our setting, to 8 cm H₂O, indicating a further improvement in patient safety.

Some limitations of this study should be noted. First, only healthy ASA status I–II patients without underlying respiratory disease, obesity, oropharyngeal or facial pathology, or risk of aspiration were enrolled in the study. Second, we did not measure arterial partial pressure of oxygen. Third, anesthesia was performed by only one experienced consultant anesthesiologist. Fourth, although our setting of healthy patients undergoing routine surgical procedures was unable to simulate the 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 be not necessary during routine induction of anesthesia, it may be extremely valuable in a difficult situation. Fifth, to ensure the greatest level of patient safety, the pop-off valve of the anesthesia machine was set at 20 cm H₂O; therefore, stomach inflation was unlikely and may not be determinable with our model.

In conclusion, in this model of apneic patients with an unprotected airway, pressure-controlled ventilation resulted in significantly reduced inspiratory peak flow rates and lower peak airway pressures when compared with circle system ventilation, thus providing an additional patient safety effect during face-mask ventilation.