It’s About Time

“Intubating is fun, but you can save a life if you can mask ventilate.” This concept, with variant verbiage, is often uttered by sage anesthesiology faculty to their residents, medical students, and ancillary clinical staff in training. Unfortunately, this wise statement has historically failed to translate into an emphasis on the study of the epidemiology, pathophysiology, and management of difficult face mask ventilation. In this issue of Anesthesiology, Sato et al.¹  significantly advance our understanding of the pathophysiology of difficult mask ventilation—a crucial first step toward identifying strategies to prevent or manage the challenging event.

The study of mask ventilation has already evolved from a basic understanding of its incidence to a clear understanding of its clinical predictors. Beginning in 2000, the seminal paper by Langeron et al.²  identified that as many as 5% of patients require airway manipulation such as an oral airway or two-hand ventilation. In addition, the data established that patient risk factors such as a history of obstructive sleep apnea, obesity, advanced age, lack or dentition, or beard increased the likelihood of difficult mask ventilation. With this inspiration, others have more critically defined difficult mask ventilation and determined its relationship with impossible mask ventilation and difficult intubation.^3,4  The strong association between obstructive sleep apnea, which is a subset of the spectrum of diseases known as sleep disordered breathing, and difficult mask ventilation, has been reaffirmed multiple times. All of this literature may help inform the management of patients presenting to the operating room for anesthesia. Providers can use an understanding of difficult mask ventilation and difficult intubation epidemiology to be optimally prepared for each patient they encounter. However, the low incidence of difficult mask ventilation naturally limits the value of any predictive algorithm. As a result, it is reasonable to question whether additional research into mask ventilation is a worthy pursuit.

As Sato et al.¹  have demonstrated, understanding the pathophysiology of airway closure during the induction of anesthesia and muscle paralysis is an important next step to improving patient management. In their current article, the authors have demonstrated or confirmed several important observations: (1) expiratory failure may be the root cause of many episodes of difficult mask ventilation in patients with sleep-disordered breathing; (2) mild sleep-disordered breathing may have limited, if any, impact on mask ventilation; and (3) difficult mask ventilation can be objectively defined. While others have theorized that inspiratory obstruction is not the underlying issue during difficult mask ventilation, the current study’s elegant and rigorous measurement of inspired versus expired tidal volumes increases the focus on expiratory obstruction among patients with sleep-disordered breathing demonstrating difficult mask ventilation. Next, the detailed sleep-disordered breathing assessment performed on each patient before mask ventilation assessment allowed the authors to scientifically prove the intuitive: mild sleep-disordered breathing (defined as an apnea-hypopnea index less than 5) presents fundamentally different mask ventilation challenges than severe sleep-disordered breathing (apnea-hypopnea index greater than 20). Finally, the categorization of expired carbon dioxide waveforms and the use of tidal volume comparison offer an objective definition of difficult mask ventilation far more reliable than the previous literature using treatment proxies (use of oral airway, two-hand ventilation). It is certainly more precise than my colloquial clinical definition of difficult mask ventilation: “something you don’t want to do for three minutes while the vecuronium kicks in.”

These important observations can serve as the springboard for additional research that may shed new light on the current airway management standard of care. For example, previous guidance recommending withholding neuromuscular blockade until evaluation of mask ventilation has been called into question.^5–8  Next, although hundreds of iterations of intubating devices have flooded the marketplace in the past 20 yr, the standard combined oronasal facemask remains the standard of care despite preliminary data suggesting that some patient populations may benefit from alternative techniques of mask ventilation.^9,10  Nasal-only mask ventilation may be the preferred technique for selected patients based upon these data. In the case of critical care patients without a history of difficult mask ventilation, a novel helmet-based positive pressure mask ventilation technique, when compared to standard face mask ventilation, markedly reduced intubation rates and 30-day mortality, a surprising and welcome result for a study focused on noninvasive ventilation.¹¹  Given that many patients demonstrating difficult mask ventilation are also difficult to intubate, identifying more effective mask ventilation techniques is a necessary focus for the field of anesthesiology. The absence of such efforts stands in stark contrast to our widely held belief that mask ventilation is “essential for the anesthesiologist and may be lifesaving for the patient.”¹² 

In an ideal world, the study by Sato et al.¹  in this issue will spur other researchers and clinicians to consider the next phase of mask ventilation research: pathophysiology and management. With the science of prediction coming asymptotically close to its practical peak, we must shift to understanding what makes a wide variety of patients difficult to mask ventilate. Just as difficult intubation is not a singular clinical phenomenon, but a spectrum of pathologic anatomic and physiologic entities, so too is difficult mask ventilation. The current study compared a limited number of sleep-disordered breathing patients with nonsleep-disordered breathing controls. Other specific abnormalities (lack of dentition, morbid obesity, limited jaw protrusion, etc.) must also be studied ethically and rigorously. Only by increasing our current focus on pathophysiology can we take the final step of identifying the novel devices and techniques necessary to improve the safety and efficacy of mask ventilation for all patients. More than 80 yr since the original description of a “nontraumatic pharyngeal airway” by Guedel,¹³  we are still using the device as our primary, if not sole, mask ventilation management technique. This stagnation in approach is caused by a lack of data to guide clinicians and researchers seeking new therapeutic options. Although the laryngeal mask airway will always play a vital role in the management of the difficult airway, avoiding additional instrumentation of the airway by effective mask ventilation remains a clinical priority.

In summary, Sato et al.¹  should be commended for leading by example. They are complementing essential advances in endotracheal tube and supraglottic airway management with unique work shedding light on the pathophysiology of difficult mask ventilation. With continued efforts to advance our knowledge of the causes underlying the spectrum of difficult mask ventilation, we may be able to offer our patients a safer anesthetic induction tailored to specific airway challenges.