Chapter 7. Manual Noninvasive Ventilation: Bag-Mask-Ventilation

Providing effective ventilation and oxygenation using a bag-mask is probably the single most important aspect of airway management. Bag-mask-ventilation (BMV) refers to the use of a bag-valve-mask (BVM) system/device to deliver gas rich in oxygen either passively or actively by manually ventilating the patient using a face-mask interface. Manual noninvasive ventilation also accurately describes the use of a BMV device to provide positive pressure ventilation (PPV). This should be differentiated from mechanical noninvasive ventilation which also uses a face-mask interface but provides respiratory effort assistance (PPV) delivered by specialized ventilator.

7.1.1 Is There Still a Role for Bag-Mask-Ventilation in This Advanced World of Difficult Airway Devices?

Definitive airway management has traditionally been defined as the placement of an endotracheal tube in the trachea. Although few would argue that there has been a philosophical and evidence-based shift away from defining airway management by the method of gas exchange to focus on the goals of resuscitation namely, maintaining patient oxygenation and ventilation while preserving hemodynamic status. In other words, endotracheal tubes do not save lives; providing adequate perfusion and gas exchange does. Oxygenation and ventilation may be provided using endotracheal tubes, extraglottic devices, BMV devices, and surgical methods. Which method is most appropriately employed will depend on patient characteristics, the clinical situation, and practitioner's skill.

Bag-mask-ventilation particularly in the prehospital setting has been shown to be no less effective than endotracheal intubation (ETI) or extraglottic device (EGD) use.1-3 With mounting evidence that prehospital ETI is of questionable benefit and in certain scenarios potentially harmful, alternative methods of maintaining oxygenation and ventilation, including BMV are being reaffirmed as an essential airway management skill.4-10

Bag-mask-ventilation has been compared to other ventilatory strategies in the prehospital, operating room, and simulation settings.11-22 Extraglottic devices, such as the laryngeal mask airway (LMA) and Combitube™, have become accepted intermediate alternatives to BMV and go to options in failed intubation and failed BMV (or "can't intubate, can't ventilate") scenarios.

There is a theoretical advantage in using EGDs in patients suffering a of cardiac arrest where chest compressions can continue uninterrupted. In general, however, ventilation has been deemphasized in the early phase of adult nonasphyxia-related resuscitation, where oxygen delivery is more dependent on blood flow than on arterial oxygen content.

This is in keeping with the recent American Heart Association recommendation in which ventilation is de-emphasized in the early phase of adult non-asphyxia related resuscitation because oxygen delivery has been shown to be more dependent on blood flow than arterial oxygen content. Ultimately, however, the American Heart Association guidelines recommend BMV as an equivalent option compared to other advanced life support (ALS) options (ETI, LMA, or Combitube) stating that "there is no evidence that advanced airway measures improve survival rates in the setting of prehospital cardiac arrest."11

Bag-mask-ventilation is a challenging skill to learn and perform effectively.23-25 Despite the advent of numerous alternative devices, just as direct laryngoscopy remains the current gold standard for endotracheal tube placement, BMV still remains the primary method of providing initial basic life support (BLS) oxygenation and ventilation in most resuscitation settings.24,26 Although this may change, there currently is no compelling evidence of superiority of extraglottic devices over BMV.11-22 Compared to BMV in the hands of inexperienced health-care providers, LMA insertion has been reported to be rapidly placed, easy to use, and effective as ventilation device.1,27,28 Other reports of LMA field use have, however, demonstrated low success rates (64%) despite self-reported ease of use.22 In the controlled setting of the operating room when compared to both LMA and Combitube use, it is reported that BMV performed favorably having no oxygenation failures and producing the smallest decline in oxygen saturation during placement.1 Success and complications of any airway device are more often related to training and experience than the device itself.1

Despite its effectiveness in skilled hands, BMV is facing growing competition from EGDs that even in unskilled hands, are relatively easy to teach, learn, and ultimately deliver as a primary method for oxygenation and ventilation. While BMV remains a vital skill to master, it may be relegated to a subsidiary position to EGDs in the foreseeable future.

7.1.2 What Are the Key Components of a Bag-Mask System?

Also referred to as manual resuscitators, these devices employ a bag and an integrated one-way valve to be connected to an endotracheal tube, an EGD, or a mask to manually provide positive-pressure ventilation. Despite there being various types of BMV systems, they share common features (Figure 7-1)29:

• A standard 22 mm connector which fits standard face masks, endotracheal tubes, EGDs, and surgical airway devices.
• A nonrebreathing valve.
• A self-inflating bag that is supplied in adult (1600 mL), child (500 mL), and infant (240 mL) sizes that when manually compressed deliver a tidal volume.
• An oxygen inlet valve that is used to provide unidirectional flow from the oxygen reservoir to the self-inflating bag.
• An oxygen reservoir bag that is inflated by receiving high oxygen flow through an adjacent connector.
• Other features of BMV systems may include a positive pressure relief or pop-off valve located on the patient end of the system with the intent of limiting peak inspiratory pressures to avoid barotrauma when the manual resuscitator is connected to an ETT.
• A flow-limiting valve that is located at the patient end of the self-inflating bag designed to limit inspiratory flow, decreasing the risk of both hyperventilation and excessive airway pressures (Smart Bag®).

The face masks used in conjunction with the manual resuscitator vary in material, size, seal type, and transparency. Traditionally, black/opaque rubber masks with an anatomically contoured seal were used in the operating room setting connected to an anesthetic circuit. These have been replaced to a large extent in most environments by transparent silicone, or plastic, latex-free, nondisposable, and single-use masks that provide the added benefit of being able to visualize the mouth/nose-mask interface and, therefore, react to the presence of vomitus and other secretions. Rather than anatomically conforming to the patient's face, the seals in these masks are either made of foam or an air-filled cushion that molds to the underlying facial anatomy.

7.2.1 How Do You Accurately Anticipate Difficult Mask-Ventilation?

Perhaps the simple answer is you can never ensure 100% accuracy when you are trying to predict anything. The safest approach is always anticipate the unexpected. In the anticipated difficult airway, proceeding depends on context. In an elective operating room setting, predicted difficulty will often mean a very different course (including canceling the case) than in an emergency situation in which a choice of not proceeding is usually not an option. In trying to anticipate difficulty, the core two questions that all practitioners should ask themselves prior to proceeding are:

• Will I be able to maintain oxygenation and ventilation by BMV if intubation attempts fail?
• If not, will I be able to oxygenate and ventilate rapidly using a rescue device or technique, such as an extraglottic device or surgical airway?

Early literature on predicting the difficult airway focused on laryngoscopy and intubation.30-32 Recognizing that maintenance of oxygenation and ventilation is the priority in airway management and that this is often best achieved rapidly and early by BMV, the identification of predictors of difficult mask-ventilation has been a focus of more recent research.33-36 Most airway management decision algorithms require a formal patient assessment focused on identifying predictors of difficulty. This assessment is critically important in deciding whether neuromuscular blockade can be safely used to facilitate intubation. While it is important to assess all aspects of the difficult airway, BMV is perhaps most important as this intervention is the primary go to technique when tracheal intubation attempts fail. Despite numerous alternative rescue options available to the practitioner, BMV is universally available, familiar to most and almost always effective.

Earlier data estimated that cannot intubate/cannot ventilate clinical situations occurred at a rate between 0.01 and 2 in 10,000 general anesthetic cases.30 More recent data have reported the incidence of difficult mask-ventilation (DMV) in the operating room setting as varying from a low of 0.9% to a high of 7.8%.18,33-36 This variation likely relates to differences in definition, outcome criteria, and sample size. In the largest study of over 50,000 patients, DMV occurred in 2.2% of cases while "impossible" BMV defined as an inability to establish BMV using two-hand technique and multiple airway adjuvants, occurred in 0.15% of the study population.34

Langeron et al prospectively evaluated 1502 patients requiring routine general anesthesia to determine both the incidence and factors associated with DMV.35 The reported incidence of DMV in this population was 5%. Five independent factors associated with DMV were identified: presence of a beard; age older than 55; body mass index greater than 26 kg·m−2; lack of teeth; and a history of snoring. The presence of two of these factors in a patient was 72% sensitive and 73% specific for DMV.35

Other studies involving large patient populations have validated the above findings and identified additional risk factors, including male gender, a history of neck radiation, high Mallampati grade (Grade III or IV), and limited jaw protrusion (Table 7-1).33-36 In the study by Kheterpal et al34 involving over 50,000 adult patients receiving a general anesthetic at a tertiary care hospital, there was a diverse group of practitioners (trainees, nurse, and physician anesthetists) however, being a junior anesthesia practitioner was not found to be an independent predictor for impossible mask-ventilation (IMV). The presence of three or more predictors (neck radiation, male, OSA, Mallampati Grade III or IV, beard) significantly increased the risk of IMV with an odds ratio of 8.9 compared to patients without these risk factors. In addition, this study also found that 25% of these IMV patients had difficult tracheal intubations.

These recent reports differentiating DMV from IMV.18 A numeric representation this DMV continuum has been proposed; however, it has not been consistently used or accepted to date in the literature.37 The difference between DMV and IMV is simply that difficult mask-ventilation is usually correctable (ie, two-hand and two-person technique) whereas impossible-mask ventilation represents a failure and the need to abandon BMV in favor of another intervention (DL if not attempted, EGD or a surgical airway).

It is important to appreciate the fact that these studies did not examine the incidence of DMV in patients requiring emergency airway management. Levitan et al examined the ability to assess for predictors of the difficult airway in emergency department patients requiring intubation and found that only 32% of this population would have been able to be assessed adequately for difficulty because of limitations, such as an inability to follow commands or being immobilized for cervical spine (C-spine) precautions.38

The incidence of DMV is not known in this population. However, emergency department cricothyrotomy (as a marker for cannot intubate, cannot ventilate) rates between 0.5% and 1.0% have been reported, which are much higher than that reported in the controlled operating room setting.39-41Table 7-2 summarizes the likely pathophysiology behind the various predictors of DMV.

7.2.2 What Anatomic Factors Need to Be Considered in Providing Safe and Effective BMV?

In the unconscious or anesthetized patient with normal anatomy, it has been traditionally thought that obstruction to the easy to-and-fro movement of gas with BMV was primarily related to the effect of a relaxed tongue falling back against the posterior pharyngeal wall. Data gathered during fluoroscopy in studies of obstructive sleep apnea (OSA) patients has improved our understanding of the pathophysiology of upper airway dynamics in the sleeping patient. In addition to obstruction caused by the tongue, there is also a loss of velopharyngeal and hypopharyngeal muscle tone.42,43 This results in soft tissue collapse leading to the posterior displacement of both the soft palate and epiglottis to oppose the posterior pharyngeal wall and contribute to obstruction (velopharyngeal and hypopharyngeal collapse).42 The hypopharyngeal site of obstruction is clinically supported by the observation that placement of an OPA without performing an adequate jaw thrust may not alleviate obstruction caused by normal upper airway soft tissues.

When a patient is placed in the sniffing position, there is flexion in the cervicothoracic region with extension in occiptocervical region. This position has been traditionally thought to facilitate alignment of the axes necessary to visualize the glottic inlet during direct laryngoscopy. Although there has been some question as to whether this position provides any advantage over simple head extension, more recent data support the combination of neck flexion with head elevation in enabling glottic exposure during laryngoscopy.44,45 It is less well understood, however, if this position improves upper airway patency for bag-mask-ventilation. There is some evidence that the retropalatal and retroglossal region is enlarged by placing anesthetized nonobese patients with OSA in the sniffing position.46 In addition, it is well known that obese patients desaturate early, a clinical phenomena which can be delayed by denitrogenating in the sitting (as opposed to supine) position.47 At this point, it is reasonable to state that head and neck repositioning from neutral to a position that involves a degree of neck flexion with head elevation may improve BMV and is appropriate in anticipation to perform direct laryngoscopy should it become necessary.

While head and neck positioning is considered vitally important for direct laryngoscopy, the key anatomic manipulation that facilitates BMV is performing a jaw thrust. This maneuver originally described over a century ago (Esmarch-Heiberg maneuver) has been demonstrated to be superior to chin lift and head tilt when performed alone, during observations made of anesthetized patients undergoing (preprocedure) head and neck fluoroscopy.48,49 Recognizing that obstruction in the unconscious patient is related to more than the tongue falling posteriorly, the "triple airway" maneuver (open mouth, head tilt, jaw thrust) was suggested to be the best approach.50,51 More recent evidence supports the jaw thrust alone as being equally effective to the triple airway maneuver in relieving obstruction.49

7.2.3 What Is the Role of BMV in Difficult Airway Algorithms?

Numerous algorithms have been published to guide practitioners in the management of the difficult airway.52-60 Most of these guidelines or recommendations are generated using available evidence and expert opinion by specialized working groups and/or anesthesiology societies. All algorithms have limitations, some are too complicated; and others impractical for application in emergency situations outside of the operating room, where awakening the patient or cancelling the case is not an option.

There are only a few difficult airway algorithms to guide the nonanesthesia practitioner responsible for airway management. ATLS guidelines have moved away from its early edition "can't intubate, cut the neck" approach to more recent versions defining the role of BMV and the use of extraglottic devices is more clearly defined.61 Emergency medicine has also published airway algorithms which are presented in airway education programs such as The Difficult Airway Courses in the United States and AIME (Airway Interventions and Management in Emergencies) in Canada.60,61

A difficult airway may be any or all of the following difficult BMV, difficult laryngoscopy, difficult intubation, difficult extraglottic device use, or difficult surgical airway placement. Most algorithms or approaches separate the anticipated from the unanticipated (or encountered) difficult airway. In the former scenario, predicted difficulty leads a defined, usually more controlled path, whereas in the latter, whether predicted or not, real time difficulty is being experienced and demands an immediate and specific course of action, depending on the type of difficulty encountered.

When approaching the difficult airway, the ability to successfully perform BMV is a critical management junction in all algorithms. The most prominent place for BMV as part of any difficult airway algorithm is between failed intubation attempts. However, it is important to appreciate the role of BMV even before a first attempt at laryngoscopy and intubation. Assuming the patient can generate sufficient tidal volumes with an adequate respiratory rate, the bag portion of the BMV device does not have to be squeezed to deliver close to 100% oxygen. It is not uncommon (and in some situations is potentially hazardous) that when switching from a nonrebreathing mask (or another mask type) to BMV, positive pressure is often instinctively applied. If assisted BMV is applied without synchrony, gastric inflation is much more likely to occur.

In the preintubation phase of airway management, the use of a BMV device to denitrogenate should be encouraged. This approach offers several advantages:

1. Passive delivery of high-concentration (approaching 100%) oxygen

2. Opportunity to size the mask properly

3. Provides hands-on feel for predicting DMV

4. Provides opportunity to improve gas exchange with assisted BMV

Various methods of denitrogenation have been suggested in an attempt to minimize desaturation during laryngoscopy and intubation. This is relatively easy to accomplish in healthy adults with normal pulmonary mechanics and oxygen consumption rates. Mort examined the effects of PPV denitrogenation on critically ill patients using a manual resuscitator for both 4 and 8 minutes prior to performing an RSI and found only marginal benefit.62,63 Traditionally, during RSI, it has been advised that BMV be halted after induction and neuromuscular blockade to avoid gastric insufflation. This teaching, however, was based more on theory than science. In fact, Sellick's original paper stated that manual PPV in combination with cricoid pressure could be done without gastric distention risk.64 Data have since supported active denitrogenation using a manual resuscitator with or without the application of cricoid pressure as long as good technique is used (avoiding high airway pressures).65 BMV is in fact clinically indicated during RSI in certain patients (obese, hypoxemic, or pediatric) who may have low baseline oxygen saturation, high oxygen consumption rates, and/or low functional residual capacity.66 Finally, the knowledge of adequate BMV soon after the drugs are given is reassuring, particularly in situations where difficult intubation may be encountered.

7.3.1 What Defines Optimal BMV Technique and How Do You Assess the Adequacy of Ventilation?

There are three important components to proper BMV technique: mask seal, airway opening, and ventilation.

Mask Seal: An appropriately sized face mask is attached to the bag-mask device and applied to the patient's face. The lower border of the mask's cuff is first applied to the groove between the lower lip and the chin and then the mask can be placed down across the nasal bridge. The thumb and index finger of the airway practitioner's hand applies sufficient pressure on the face mask to achieve a good seal (Figure 7-2). Note, however, that sealing pressure must be achieved without excessive downward pressure on the patient's mandible, as this may worsen functional obstruction—rather, the mandible is lifted to meet the mask. Small adjustments to the position of the mask on the patient's face (eg, with small movements to left or right) are made as needed to achieve a seal.

Airway Opening: The ring and long fingers of the nondominant hand grasp the bony ridge of the patient's mandible, and, if practical, the fifth finger hooks under the angle of the mandible to provide a jaw thrust (Figure 7-2). In the event the airway practitioner has a small hand, the long finger is hooked under the mentum to provide a jaw pull. These three digits provide counter-pressure to the digits applying the mask to the face, but also apply an upward lift to the mandible to help perform an airway opening jaw thrust. Note that these three fingers should not be placed directly under the patient's chin unless lifting it forward, as midline pressure under the chin can contribute to airway obstruction. This latter directive is particularly important in small children and infants. Concomitantly, the entire hand also attempts to keep the head extended (if no C-spine precautions).

Ventilation: The practitioner's dominant hand is free to gently squeeze the bag. Volumes should be delivered with attention to the inflating pressure as well as the patient's status: if apneic, the patient should be carefully ventilated (attached to high-flow oxygen) at a rate of 10 to 12 breaths per minute, at a tidal volume of 6 mL·kg−1, or 500 to 600 mL in the average adult.11 Smaller tidal volumes (eg, 300-400 mL in the adult) at increased rates (15-18 breaths per minute) may lead to less gastric insufflation. Although adult (1.6 L) manual resuscitators may deliver varied volumes, excessive and rapid compression of the bag must be avoided. The goal, as stated previously, is to produce visible chest rise.11 In the patient still demonstrating respiratory effort, assisted bag-mask-ventilation should be performed, synchronizing the positive pressure breath to the patient's inspiratory effort. If the patient is tachypneic, it will be appropriate to simply deliver an assisted ventilation with every third or fourth breath.

7.3.2 What Do You Do If BMV Is Difficult?

With optimal technique, significant difficulty with BMV is rarely encountered in the absence of airway pathology.33-36 In the acute setting, BMV is often delegated to non-physician health-care practitioner while the physician prepares for definitive airway management. While this may be appropriate, it is important to accept that BMV is a difficult skill for those who perform it infrequently, and vigilance rather than inattention is recommended. Abandoning BMV in the uncommon scenario of failing BMV should only occur after the most experienced set of hands have failed.

Difficult BMV (DMV) is often defined as the inability to maintain an acceptable oxygen saturation despite using good technique. Failure to maintain acceptable oxygen saturations or falling saturations demands a change in approach. Although one response to a DMV situation is to proceed to intubation, DMV may itself predict difficulty with laryngoscopy and/or intubation.67

In the setting of a failed airway in which one is not able to maintain acceptable saturations, immediate preparation for a cricothyrotomy is mandatory while one simultaneously attempts better BMV. Response to DMV requires a staged response that may include the following:

1. Reposition the head by performing an exaggerated head tilt/chin lift (if not contraindicated).

2. Open the mouth to permit anterior translation of the mandible and tongue in concert with an aggressive jaw thrust.

3. Insert an appropriate size oropharyngeal airway (Figure 7-3) and as many as two nasopharyngeal airways.

4. Perform two-person mask-ventilation technique.

5. If cricoid pressure is being applied, ease up on, or release it.

6. Consider a mask change (size or type) if seal is an issue.

7. Rule out foreign body in the airway.

8. Consider a rescue ventilation device, for example, an EGD, such as a laryngeal mask airway (LMA) or Combitube™.

9. Consider an early attempt at intubation.

Steps A, B, and C, as listed above, should occur almost simultaneously and very early in the DMV situation. DMV is often due simply to the failure to adequately open a functionally obstructed airway. Attempted ventilation against this obstruction results in a leak at the mask-face interface, often resulting in the practitioner's attempting to remedy the problem by pushing down harder on the mask to attain a seal.

This can worsen an already obstructed airway. Rather, what must occur is a more pronounced jaw lift or thrust, with resultant airway opening occurring as anterior movement of the mandible elevates the tongue, epiglottis, and soft palate away from the posterior pharyngeal wall. This is best performed with the aid of a second person. Two-person mask-ventilation is easy to perform and is often much more effective than one-person BMV. As shown in Figure 7-4, the two-person technique can be performed in a number of ways.68

Oropharyngeal airways (OPAs) help alleviate functional airway obstruction caused by relaxation of the tongue against the soft palate and to a lesser extent the posterior pharyngeal wall. They are most often used as an adjunct to bag-mask-ventilation of an obtunded or unconscious patient. Made of plastic, the component parts are a curved hollow lumen (in the Guedel version) (Figure 7-5) or side gutters (the Berman version), both with a proximal flange which abuts the patient's lips, and a proximal bite block which may also be used as a color-coded size indicator.

OPAs are sized by length in centimeters, and are available in sizes for all ages. Choosing the appropriate size is important. If the OPA is too long, it may precipitate laryngospasm; if too small it may be ineffective. Although never formally validated, many airway practitioners approximate correct OPA length by placing it alongside the patient's cheek69: from the corner of the mouth, the tip of the OPA should reach the angle of the mandible or the tragus of the ear (Figure 7-3). A typical adult female will take an 8-cm OPA, and an adult male, 9 or 10 cm.

The OPA should be inserted inverted, (ie, with its concave surface directed cephalad) and advanced until the distal tip will proceed no further in the inverted position. At that point, the OPA is rotated 180 degrees, so that the concavity faces caudad. Advancement continues around the curve of the tongue until fully inserted. This technique prevents the tip of the OPA from impinging the tongue pushing it backwards and making the obstruction worse. Alternatively, it can be inserted noninverted with a tongue depressor to manage the tongue: this is the preferred technique in infants and younger children, to help avoid trauma to delicate tissues.

OPAs are not well tolerated in the awake or semiconscious patient with intact airway reflexes, where insertion may stimulate gagging, laryngospasm, or vomiting and aspiration. In addition, care must be taken to rule out a foreign body in the oropharynx prior to OPA insertion.

A nasopharyngeal airway (NPA) may be a useful option where trismus precludes OPA insertion, and may be better tolerated than an OPA in the awake or semiconscious patient with intact airway reflexes. While effective at alleviating functional airway obstruction, disadvantages of the NPA include transient patient discomfort during insertion and the potential for causing epistaxis. While the application of a vasoconstrictor (eg, oxymetazoline or phenylephrine) is frequently performed, there is no evidence that it reduces the incidence of epistaxis, nor may it be practical in an urgent situation. NPAs, also known as nasal trumpets, are made from soft material, for example, latex or silicon, have a hollow lumen and a bevelled leading edge, and a proximal flange to abut the patient's nostril (Figure 7-6).

Adult NPAs are generally sized by their internal diameter (ID) in millimeter. Typical adult sizes for small, medium, and large NPAs are 6, 7, and 8 mm ID respectively. One commonly used (but nonvalidated) sizing method is to use an NPA of a length corresponding to the distance from nose tip to the tragus of the ear. Sizing based on patient height makes more anatomic sense, resulting in a recommendation for a 6-mm ID NPA for an average adult female and 7 mm for an average male.

The NPA is lubricated and advanced into the patient's nostril, perpendicular to the face, resulting in passage along the floor of the major nasal airway. Authorities vary in their recommendation whether the bevel of the NPA should face toward or away from the nasal septum. A slight twisting motion can be used during insertion. If significant resistance is encountered, insertion should be attempted through the other nostril. Insertion continues until the flange of the NPA abuts the nasal ala.

NPA use is relatively contraindicated in known bleeding diathesis, including heparinized, warfarinized, or recently thrombolyzed patients, and in suspected cribriform plate fracture. In the head-injured patient, common sense dictates balancing the substantial risk of hypoxemia with the benefits of obtaining a patent nasopharyngeal airway, if an oral airway cannot be used.

Cricoid pressure can cause difficulty with both BMV and laryngoscopy as previously discussed. Excessive cricoid pressure (as may be applied during an RSI) may distort the airway and result in a partial or even complete obstruction. If significant difficulty with BMV is encountered during application of cricoid pressure, the assistant should momentarily ease (initially by 50%) or release the applied pressure.

It may become apparent once BMV is underway that the chosen mask size is incorrect. This is often the case where initial sizing occurred with the patient's dentures in place. Especially with encountered difficulty, the improved seal allowed by an appropriately sized mask makes the change worthwhile.

The decision to move to a rescue EGD, such as a laryngeal mask airway or laryngeal tube (King LT), will depend on the patient's clinical status and whether direct laryngoscopy (DL) has yet been attempted. If there has been no initial attempt at DL, it may be appropriate to proceed to an intubation attempt. If, on the other hand, DMV is encountered in the setting of an already failed attempt at intubation, placement of a rescue ventilation device, such as an LMA should be considered. Direct laryngoscopy is also the method of choice to rule out obstructing lesions, including foreign bodies and lingual tonsillar hypertrophy.

7.3.3 Is Cricoid Pressure Appropriate to Use with BMV?

Aspiration incidence rises with the number of attempts at intubation to a high of 22% in the emergency setting.70 Mortality from aspiration has fallen but still remains relatively high at 4.6%.18 Does the application of cricoid pressure prevent regurgitation and more importantly does it reduce morbidity and mortality?

Since Sellick's description in 1961, cricoid pressure has been recommended as a safe and necessary airway maneuver meant to reduce the risk of aspiration during airway management.64 Supportive literature seemed to confirm its value during BMV in preventing gastric inflation, particularly in the pediatric population.67,71-74 More recently, however, based on accumulating evidence, this standard use of Sellick's maneuver has been questioned.71,72 In a review of the literature Ellis et al presented 10 studies reporting potential negative effects of cricoid pressure administered during BMV that include reduced tidal volumes, increased peak inspiratory pressure, and difficult ventilation.71 It was concluded that there is little evidence to support the widespread use of cricoid pressure to prevent aspiration and that there is evidence of potential harm in certain situations.71

Anatomically, as studied by CT and MRI, the value of cricoid pressure has also been questioned based on observations that the cricoid cartilage moves laterally with compression and causes incomplete luminal esophageal opposition.75 More recent literature has identified that the hypopharynx at the level of the glottis and not the esophagus is compressed by Sellick's maneuver because it moves in concert with the cricoid ring, making the position and degree of compression of the esophagus below it irrelevant.26

The question of whether cricoid pressure reduces morbidity and mortality by preventing aspiration has not, and likely will not be answered. Aspiration has been documented to occur in cases where cricoid pressure has been applied, however, it has been argued that this could have been from inadequate technique.71,72 The clinical risk/benefit debate over the use of cricoid pressure will likely continue. At this point, it can be said that when performed correctly, by occluding the hypopharynx and in preventing gastric inflation during BMV, cricoid pressure may reduce the risk associated with aspiration during airway management. In addition, during BMV, care should be taken to employ good technique by opening the airway, paying attention to inspiratory time, inflation pressure, and delivering appropriate tidal volumes to avoid gastric inflation. If cricoid pressure is being applied, it should be done by an experienced assistant and if BMV becomes difficult, cricoid pressure should be released to assess whether it may be impeding ventilation.

7.4.1 How Does One Maximize Gas Exchange While Minimizing the Risk of Gastric Inflation and Regurgitation BMV?

With an unprotected airway, the risks of gastric inflation and subsequent aspiration are a real risk during BMV. In autopsy patients of failed CPR attempts, the incidence of aspiration has been reported to be 29%.74 Delivering an intended tidal volume and avoiding gastric inflation during BMV depends on various factors, such as lung compliance, airway resistance, and lower esophageal sphincter pressure (LESP).76 In healthy adults, the LESP is 20 to 25 cm H2O.76 In the anesthetized patient, this pressure is likely lower and in cardiac arrest patients, it may be as low as 5 cm H2O.76 This information leads to the recommendation by the authors that airway practitioners performing BMV employ smaller tidal volumes at higher rates to minimize inflation pressures and the risk of gastric insufflation.

In order to minimize the risk of aspiration and its related potential morbidity, high airway pressure during BMV should be avoided. Underlying lung pathology and problems with pulmonary mechanics, such as obesity, will adversely affect lung compliance but are not always immediately modifiable. Airway resistance in the upper airway can be reduced during BMV by paying attention to technique, such as using an oral or nasal airway and applying a jaw thrust. Although most discussion around gastric inflation focuses around BMV use in the apneic patient, the breathing patient who is receiving assisted manual PPV using a BMV device is at particularly high risk. In this situation, if the airway practitioner is not paying close attention to timing, and delivery occurs during the patient's expiratory phase, higher airway pressures will more likely result in gastric inflation. This eventual gastric inflation cycle can result in increased intra-abdominal pressure causing reduced lung compliance, which in turn requires higher airway pressures and diverts more gas to the stomach.76 In extreme circumstances this can lead to decreased cardiac output from increased intrathoracic pressure and a can't ventilate scenario.

Stress and an excited state may contribute to the overzealous ventilation with large tidal volume and rapid respiratory rates that have been documented in the acute prehospital setting.27,77 This response often leads to further gastric inflation, breath stacking, and the cycle described above, resulting in worsening oxygenation and ventilation and a further decrease in cardiac output. The other consequence of increased minute ventilation is the development of respiratory alkalosis which is gaining recent attention as a contributor to poor patient outcomes in certain acute settings.78

Research related to the delivery of appropriate tidal volumes when using a BMV device has yielded different results depending on the clinical situation.27 The American Heart Association recommendations for BMV is to deliver sufficient tidal volume to produce a visible chest rise.11 This recommendation is based on evidence that in an unprotected airway, smaller tidal volumes (approximately 500 mL) result in less gastric inflation. Secondly, although in anesthetized patients with normal perfusion tidal volumes of 8 to 10 mL·kg−1 are used, during CPR perfusion is approximately 30% of normal with resultant delivery of oxygen and clearance of CO2. Therefore, lower tidal volumes and respiratory rates are needed.

In patients with a sudden dysrhythmic cardiac arrest (ventricular fibrillation), hypoxemia and acidosis develop over several minutes.27 In this very early phase of resuscitation, oxygen delivery is more dependent on tissue perfusion than arterial oxygen content.11 These findings are in part behind the recommendations that prioritize early CPR in advance of ventilation in this subgroup of patients.11 In the asphyxiated, respiratory arrest patient, maximal oxygen depletion has usually already developed (over time) in association with a lactic acidosis and CO2 accumulation. In this situation, delay in oxygenation and ventilation should not occur during resuscitation efforts.

In a manikin study using a standard adult manual resuscitator, one-hand BMV provides tidal volumes between 450 mL and 600 mL. Squeezing the bag completely delivered volumes between 888 mL and 1192 mL.79 When presented with the task of delivering 500-600 ml by 'half compression' of a 1.6 L bag with two hands subjects consistently delivered inadequate volumes.80 Use of a pediatric self-inflating bag instead of the larger adult one, despite resulting in less gastric inflation provided inadequate tidal volumes.16 Current recommendations are to deliver tidal volumes in the adult of 500 to 600 mL in both arrhythmia and asphyxia related cardiac arrest but emphasize that "the volume delivered should produce visible chest rise".11

Bag-mask-ventilation remains an important potentially life-saving airway management skill. However, it can be a difficult skill to teach, learn, and perform adequately unless one does so on a regular basis. The advent of extraglottic devices that are easy to teach, learn and perform are likely to supplant BMV as a first-line airway management technique in certain settings in the future.

Difficult mask-ventilation will usually respond to corrective measures and impossible mask-ventilation is uncommon in experienced hands. Predicting difficult or impossible BMV is never fool-proof but is fundamental to the practice of advanced airway management, influencing subsequent decision making on how to proceed.