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Many people are asking...


...should we consider helmet non-invasive ventilation in COVID-19?
This piece is courtesy of Dr. Camille Petri.
The FLARE Four:
  1. Small studies suggest a benefit of NIPPV in ARDS, particularly mild ARDS. For this reason, some have suggested more widespread use of NIPPV in COVID-19.  
  2. NIPPV may decrease need for sedation and/or intubation but may also lead to worse outcomes due to suboptimal tidal volumes, PEEP, and patient-ventilator asynchrony. The role of NIPPV in the treatment of ARDS has yet to be substantiated in large randomized clinical trials. 
  3. Some have advocated for the use of helmet NIPPV specifically given initial data demonstrating success in treating ARDS patients and the perception that it is associated with a lower risk of aerosolization.
  4. Currently available studies of NIPPV, including helmet NIPPV, indicate the risk of aerosolization cannot be completely dismissed and should be weighed when deciding to use NIPPV on a given patient during the COVID-19 pandemic.
During the SARS epidemic in 2003, nosocomial transmission and superspreading of SARS-CoV-1 to healthcare workers was attributed, in part, to the use of non-invasive positive pressure ventilation (NIPPV) (Tran et al. 2012; Yu et al. 2007). For these reasons, NIPPV has been avoided by many hospitals during the COVID-19 pandemic. However, in light of feared ventilator shortages, or to provide care aligned with a patient’s goals and preferences, some have advocated for the use of helmet ventilation (Cabrini, Landoni, and Zangrillo 2020; Lucchini et al. 2020). In tonight’s FLARE, we will explore the evidence behind NIPPV for ARDS, including the helmet device, and consider its utility in the COVID-19 pandemic.
What are considerations for using NIPPV in ARDS?
NIPPV has many patient-centered benefits in comparison to invasive mechanical ventilation. For those who can tolerate the mask interface, NIPPV allows patients to spontaneously breathe without sedation (the pitfalls of which have been reviewed in May 2nd FLARE). In non-ARDS patients with respiratory failure, NIPPV has also been associated with a decreased risk of nosocomial pneumonia (Nourdine et al. 1999; Carlucci et al. 2001; Hess 2005). Furthermore, NIPPV has the convenience of being provided in discrete sessions, allowing patients to “take breaks” intermittently, and if clinically stable, may provide the patient and clinical team an opportunity to discuss endotracheal intubation and its risks (i.e. acting as a “bridge”).
NIPPV is not without drawbacks. These include risks of aspiration and mucus plugging. Importantly, the measurement of delivered and exhaled tidal volumes during NIPPV is unreliable. Tidal volume varies based on patient effort, and the measurement of tidal volume may be inaccurate due to mask leak (and the volume of air in the mask itself). Therefore NIPPV is unlikely to achieve low tidal volume ventilation (Carteaux et al. 2016). A second limitation involves PEEP - although NIPPV can provide positive end-expiratory pressure, PEEP titration cannot be performed accurately, and the level of PEEP needed may be intolerable to the patient. By virtue of being mechanically ventilated, even non-invasively, patient-ventilator dyssynchrony is still a real concern (Gay 2009). For example, patients may not be able to effectively trigger the ventilator to provide pressure support during inspiration. Furthermore, the simultaneous negative pleural pressure generated by the patient and positive pressure from NIPPV may lead to large swings in transpulmonary pressure during the respiratory cycle (see figure) and may potentiate ventilator-induced lung injury. Large transpulmonary pressures (stress) may lead to higher tidal volumes (strain), thereby perpetuating lung injury. All of these limitations require the clinician to monitor a patient receiving NIPPV closely, with the constant threat of further respiratory decline from the underlying disease process, fatigue, or complications from NIPPV use.
Figure: Panel A showing normal transpulmonary pressures (Palv - Ppl) in a healthy, spontaneously breathing patient at end-expiration. Panel E showing elevated transpulmonary pressures in a patient on NIPPV (Slutsky and Ranieri 2013).
What is known about NIPPV in ARDS?
Early evidence suggested that NIPPV lowered the rate of intubation in ARDS and/or acute hypoxemic respiratory failure (Antonelli et al. 1998; Antonelli et al. 2007; Rocker et al. 1999; Ferrer et al. 2003). However, this early evidence consists largely of studies with small sample sizes, single institutional reports, and retrospective cohort designs. Furthermore, this literature spans the period during which major advances in ARDS treatment, including low tidal volume ventilation, conservative fluid management, and prone positioning, were being rolled out, making comparison to modern invasive mechanical ventilation strategies challenging. Importantly, there is no large scale, randomized controlled trial demonstrating non-inferior or improved outcomes for ARDS patients treated with NIPPV. 
We can glean some insight from the larger and more recent retrospective studies. Bellani and colleagues performed a sub-group analysis of patients who received NIPPV in the LUNG-SAFE trial, a worldwide study describing the epidemiology, respiratory characteristics, and outcomes of contemporary patients with ARDS (Bellani et al. 2017). In this retrospective study of 436 patients with ARDS who received NIPPV, the severity of ARDS was positively correlated with likelihood of NIPPV failure (22.2% of mild, 42.3% of moderate, and 47.1% of patients with severe ARDS). Furthermore, in a propensity-matched cohort of patients with moderate-severe ARDS (P/F < 150), patients receiving NIPPV had a significantly increased mortality. As one might expect, patients receiving NIPPV had significantly lower levels of PEEP and higher minute ventilation than those who were intubated. Patients receiving NIPPV often received tidal volumes > 6-8 mL/kg. The authors posit that these arguably less lung protective ventilation strategies may have been related to delayed recognition of ARDS in patients undergoing NIPPV. Taken together, these data suggest that if NIPPV has a role in ARDS, it is likely only appropriate in carefully selected patients.
In another large retrospective cohort study, Taha and colleagues used the National Inpatient Sample to identify 4,227 patients who received positive pressure ventilation during 2016 for ARDS (Taha et al. 2019). Patients were divided into two groups: (1) those who received invasive mechanical ventilation initially and (2) those who received NIPPV initially. Rates of NIPPV success i.e. (avoidance of intubation) and failure were examined. NIPPV was used prior to invasive mechanical ventilation in 32% of patients, of whom 21% eventually failed and required invasive mechanical ventilation. As shown in the figure below, patients for whom NIPPV failed carried a similar mortality rate than those who were intubated initially (26.9% vs. 25.1%). Furthermore, the hospital length of stay was similar between the NIPPV failure and invasive mechanical ventilation groups. Though the granularity of the data is limited (there is a lack information about the severity of hypoxemia), this study suggests that the initial use of NIPPV in patients with ARDS may identify a small percentage of patients who can avoid intubation, while not substantially impacting mortality or length of stay for those who do not.
Figure: Adjusted estimates with 95% CI for all-cause in-hospital mortality (left) and length of stay (right) based on study group. IMV: invasive mechanical ventilation; NIPPV: noninvasive positive pressure ventilation (Taha et al. 2019).
If we do use NIPPV in ARDS, is there a better way?
Based on the studies by Bellani and Taha reviewed above, it is clear that some proportion of ARDS patients receive ventilatory support with NIPPV. However, there is substantial heterogeneity in the specific ways in which NIPPV is provided - clinicians have access to different masks and interfaces, and ventilators have different properties and capabilities. Of interest to some is NIPPV delivered through a helmet apparatus.
For those unfamiliar with helmet ventilation
The earliest reports of helmet ventilation are from over 100 years ago (reviewed in Beitler, Owens, and Malhotra 2016). In more recent times, helmet ventilation has been proposed as an alternative to face mask non-invasive positive pressure ventilation that might mitigate issues with face mask use, such as breakdown across the bridge of the nose, poor seal across the contour of the face, and eye irritation, and therefore allowing for longer continuous use (Antonelli et al. 2002). Offsetting these potential advantages, expertise in helmet ventilation is limited, and no helmets are currently approved in the US.
The basic setup is as follows: the apparatus consists of a transparent helmet (typically round or cylindrical in shape) which is placed over the patient’s head and a collar fitted at the base of the patient’s neck (measured to fit them specifically). The device is held in place with straps extending underneath the axilla. An example is shown in the figure below.
Figure: Helmet ventilation set up. UptoDate, “Noninvasive ventilation adults with acute respiratory failure: Practical aspects of initiation”, April 15, 2020.
There are important consequences to the helmet being the receptacle for the pressurized flow of air, rather than air being directly delivered into the patient. To begin, a high flow rate of pressurized air is needed into the helmet space, so that pressurization of that space occurs in a timely fashion. Furthermore, because of the compliance of the helmet itself, changes in inspiratory flow or tidal volume occur less rapidly, leading to slower rate of change detected by the ventilator and possibly inefficient triggering. This may lead to dyssynchrony. Indeed, in patients with hypercarbic respiratory failure, helmet ventilation led to worse patient-ventilator synchrony (Navalesi et al. 2007). Furthermore, there is a heightened concern for increased dead space in the helmet and rebreathing of CO2 (Patroniti et al. 2003; Racca et al. 2005, 2008). Finally, being inside the helmet also exposes patients to noise and pressure on the tympanic membranes. Some of these concerns may be exacerbated by or ameliorated with different settings, models, and materials (Costa et al. 2008).
Is helmet ventilation helpful in ARDS?
The benefits of NIPPV via helmet interface have been demonstrated in prior studies with COPD (Antonelli et al. 2004; Antonaglia et al. 2011), acute cardiogenic pulmonary edema (Foti et al. 2009; Tonnelier et al. 2003) and post-extubation (Squadrone et al. 2005). Notably, these are all settings in which face mask NIPPV has also been shown to be beneficial. As above, however, the evidence for use of helmet NIPPV in ARDS is less robust. Patients with ARDS often require respiratory support for much longer. For this reason, the use of NIPPV and helmet ventilation in ARDS has been controversial.
To date, there have been a relatively small number of trials examining the use of helmet ventilation in ARDS (selected studies in table below). Study comparison also becomes challenging given the evolving criteria for ARDS over the past 15-20 years, and a variety of trial designs (e.g. no comparison, differing outcome measures, different helmet devices, etc). Importantly, for our purposes, a 2016 meta-analysis of helmet ventilation versus control strategies (face mask NIPPV or Venturi mask) evaluated 11 trials with a total of 621 patients and found that helmet ventilation had the largest effect on mortality (OR 0.38) for those presenting with hypoxemic respiratory failure (as compared to other etiologies) (Liu et al. 2016).
Table. Selected trials of helmet ventilation for acute hypoxemic respiratory failure.
Though the 2016 Patel trial was met with enthusiasm at the time of its publication, this was tempered by contemporaneous release of data by Frat and colleagues (Frat et al. 2015). This study randomized 310 patients with P/F < 300 without hypercapnia to HFNC, nasal cannula or face mask NIPPV. While there was no statistically significant difference in the rate of intubation across the groups, the number of ventilator free days was significantly higher in the HFNC group with a lower hazard ratio for death at 90-days.
Taken together, helmet ventilation has yet to be studied in a large, multi-center trial for patients with ARDS or compared to invasive mechanical ventilation. Currently, it is not clear that helmet ventilation provides any substantial benefit over other NIPPV methods for ARDS.
What about the aerosolization risk with NIPPV?
Face mask NIPPV
Two major studies have evaluated exhaled air dispersion during NIPPV delivered via face mask (Hui et al. 2006, 2009). These studies, both by the same research group, utilize a high-fidelity patient simulator with tunable mechanics, inspiratory and expiratory pressures. Airflow was visualized by infusing oil-based smoke particles into the patient simulator's lungs such that any particles found outside the mask were evidence of leak. Air leakage from the mask was visualized using a laser light-sheet. Though the specific face mask tested in each of these studies varies, all demonstrated substantial exposure of exhaled air ranging from 0.4m to a 1.0m radius from the patient (see figure below).
Figure: exhaled air dispersion in a patient receiving NIPPV via at inspiratory pressure of 10 cmH2O and expiratory pressure of 4 cm H2O (Hui et al. 2009).
Helmet NIPPV
In 2015, the same group (Hui and colleagues) examined the dispersion of exhaled air using two different helmet apparatuses (Hui et al. 2015). The authors simulated different degrees of lung injury, including normal, mild and severe. Helmet ventilation was tested with inspiratory positive airway pressure (IPAP) ranging from 12-20 cmH2O; expiratory positive airway pressure (EPAP) was held constant at 10 cmH2O. In this study, leakage of exhaled air was minimal only when there was an adequate seal at the neck-helmet interface (in addition to a double-limb circuit and air filter). Otherwise, air leakage was detected as far as 2.7cm away from the simulator, which occurred under “normal lung” settings. Other important factors not studied in this report include indoor air ventilation, negative pressure airflow in the patient’s room and provider personal protective equipment. 
Currently absent from the literature are studies evaluating aerosolization occurring during the application and removal of NIPPV apparatus itself.
Taken together, these studies do suggest a real likelihood of aerosolization during the use of NIPPV, though real-world data in SARS-CoV-2 infection are lacking. Fortunately, at this time, we are not aware of any reports of nosocomial transmission related to the use of NIPPV.
NIPPV has been utilized in ARDS for decades, and has shown benefits for some specific, albeit limited, patient populations. Studies of both face mask and helmet NIPPV have suggested that a trial of NIPPV in some selected patients with mild ARDS, and no other contraindications to NIPPV use, may be appropriate. However, major concerns exist about “masking” (pun intended) worsening ARDS or providing suboptimal ventilation strategies, precluding widespread application to all ARDS patients. 
Importantly, according to the Surviving Sepsis Campaign guidelines for the treatment of critically ill COVID-19 patients, a trial of NIPPV with close monitoring and frequent re-assessment is weakly recommended for those without urgent need for intubation. There is no recommendation for or against the use of helmet NIPPV (Alhazzani et al. 2020). As institutional practices regarding the use of NIPPV in COVID-19 patients are evolving, we await further data about the success of NIPPV in COVID-19 patients.
Based on the data presented above, the risk of aerosolization of SARS-CoV-2 with NIPPV should not be discounted. Importantly, availability of PPE for healthcare personnel should be considered prior to adoption of protocols involving NIPPV, particularly in light of increased patient monitoring needed during NIPPV. 
The use of NIPPV versus invasive mechanical ventilation is, of course, couched within a greater discussion including the risks and benefits of endotracheal intubation, sedation, and other advanced therapies for ARDS. We save this crucial question - when to intubate a patient with severe COVID-19 - for a separate FLARE.
FLARE is a collaborative effort within the Pulmonary and Critical Care Division and the Department of Medicine at Massachusetts General Hospital. Its mission is to appraise the rapidly evolving literature on SARS-CoV-2 with a focus on critical care issues.

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(c) MGH FLARE team, 2020.

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