Optimal Ventilator Strategies in Acute Respiratory Distress Syndrome

Michael C. Sklar, MD; Bhakti K. Patel, MD; Jeremy R. Beitler, MD, MPH; Thomas Piraino, RRT; Ewan C. Goligher, MD, PhD

Disclosures

Semin Respir Crit Care Med. 2019;40(1):81-93. 

In This Article

Noninvasive Ventilation for ARDS

The role of noninvasive ventilation (NIV) in the management of ARDS is controversial and evolving. Although the benefits of NIV for acute exacerbations of chronic obstructive pulmonary disease[81] and cardiogenic pulmonary edema are substantial,[82] evidence supporting the use of NIV in ARDS is limited. The potential to avoid invasive mechanical ventilation in patients with ARDS is intriguing given the known complications linked to invasive mechanical ventilation, deep sedation, neuromuscular blockade, and immobility.[83] Despite the paucity of evidence, NIV in ARDS is commonly used as an initial supportive therapy for ARDS[84,85] and was formally recognized as a therapeutic approach in the Berlin Criteria[86] defining ARDS.

Physiological Considerations

NIV has specific physiological effects in acute lung injury. PEEP can recruit lung to improve oxygenation but improves work of breathing to only a minor degree. Applying additional inspiratory pressure support further reduces work of breathing and dyspnea.[87] Given the high respiratory drive of patients with ARDS,[88] the use of pressure support to unload the respiratory muscles can lead to excessive tidal volumes, which are associated with NIV failure[89] and may exacerbate lung injury.[90] The concern for excessive tidal volume and inability to support the high respiratory drive of patients with ARDS was confirmed in the LUNG SAFE study.[91] This prospective multicenter observational study demonstrated that 15.5% of patients with ARDS are managed with NIV as an initial approach. Interestingly, NIV was applied at similar rates across all ARDS severity categories. Patients managed with NIV had lower levels of PEEP and higher respiratory rates and tidal volumes in comparison to invasively mechanically ventilated patients, highlighting the challenge of controlling respiratory drive with a noninvasive approach.[91] Furthermore, the inspiratory pressures required to improve the work of breathing may worsen mask leaks, gastric distension, and patient tolerance.[87]

Evidence Base

Notwithstanding the technical challenges of NIV titration in ARDS, there are data indicating some success with this approach. Antonelli and colleagues[84] investigated NIV as a first-line approach to patients with ARDS and found that endotracheal intubations were prevented 54% of the time. Correspondingly, avoiding endotracheal intubation was associated with less ventilator-associated pneumonia and decreased intensive care unit (ICU) mortality. A meta-analysis of randomized and observational studies which included 540 patients similarly indicated that NIV is successful in ARDS approximately 50% of the time.[92] However, the heterogeneity of the studies prevented interpretation on the effect of NIV on other outcomes such as mortality.

Thus, as clinicians grapple with applying these limited data for clinical care, there are a few points to consider. First, the success of NIV for ARDS varies based on severity of hypoxemia.[93] The rates of NIV failure double as severity of ARDS increases from mild to moderate or severe, increasing from 22.2 to 42.3 and 47.1%, respectively.[91] This has led some to recommend that NIV in ARDS be restricted to patients with mild–moderate severity, defined as PaO2/FiO2 ≥150 mm Hg.[93] Second, NIV failure is associated with worse outcomes,[94] which may reflect a higher severity of illness or possibly suggest that delays in intubation can be deleterious. Therefore, risk factors associated with NIV failure such as lack of improvement of the PaO2/FiO2ratio and shock[84,95] should be considered to optimize patient selection. In parallel, close observation for early signs of failure is needed to prevent delays in endotracheal intubation. A new bedside tool, the HACOR score (heart rate, acidosis, consciousness, oxygenation, and respiratory rate) has been developed and validated to predict NIV failure in hypoxemic patients with high accuracy at 1 hour of NIV to prevent intubation delays.[96] Finally, although there are data indicating possible harm with NIV in ARDS, it is important to consider the clinical context. A randomized clinical trial comparing the efficacy of standard oxygen, high-flow nasal cannula (HFNC), and NIV in patients with hypoxemic respiratory failure suggested increased mortality in the NIV group.[97] However, patients in the NIV group received NIV only for 8 hours a day for 2 days. Given that the physiological effects of NIV dissipate quickly after discontinuation, it is unclear if the NIV protocol mediated these findings as opposed to NIV per se.

The capacity to apply prolonged NIV at higher pressures may mediate success. One small single-center clinical trial examined the effect of the NIV interface on its success in ARDS.[98] The authors postulated that the helmet, a transparent hood that encompasses the entire head of the patient and has a seal at the neck, would enhance titration of positive pressure and be better tolerated in comparison to the traditional facemask. Endotracheal intubation rates were reduced from 62% in the facemask group to 18% in the helmet group. In addition, 90-day mortality was reduced by 22% with helmet NIV. In a 1-year follow-up study, the authors also demonstrated that the avoidance of endotracheal intubation was associated with improved long-term neuromuscular and functional outcomes in ARDS survivors.[99] Early termination of the clinical trial may have exaggerated the magnitude of the findings and these data need to be replicated in larger multicenter trials prior to widespread adoption.

There are no definitive recommendations for or against the use of NIV for ARDS due to the paucity of high-level evidence.[100] Invasive mechanical ventilation remains the mainstay of supportive care for ARDS. However, NIV may obviate the need for endotracheal intubation in carefully selected and closely monitored patients. Clinicians must weigh the risk of complications with invasive mechanical ventilation against the potential harm of NIV due to delays in endotracheal intubation or exacerbation of lung injury. As the debate between controlled invasive mechanical ventilation versus spontaneous breathing in ARDS continues, more research is warranted to identify subgroups of patients who may benefit from NIV, understand the effect of NIV interface on the physiology and outcomes of lung injury, and compare NIV to other respiratory support devices such as HFNC.

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