The Pre-brief
A 48 year old woman presents to your ICU with hypoxemia, diagnosed with COVID-19 pneumonia. Chest X-ray is notable for bilateral infiltrates.
Mental status is normal. Blood pressure is 144/70, HR 105, RR 34, with SpO2 88% on a 50% ventimask. Work of breathing is somewhat elevated. ABG done in the emergency department: pH 7.48, PaCO2 28, PaO2 60.
You are considering a non-invasive initial strategy and have heard about the helmet interface for non-invasive ventilation. Should you use it?
Introduction: What is it?
Figure 1: Simplified diagram of helmet NIV. The main difference from facemask NIV aside from the interface is the two-limb system as shown.
The helmet interface non-invasive ventilation (NIV) utilizes a two-limb system (inspiratory and expiratory), and thus often requires a traditional ventilator, rather than a typical machine. Importantly, you must set the machine to NIV mode (most, if not all, ventilators have this mode) so that leak compensation is provided.
Potential benefits of this interface over a traditional NIV facemask include a tighter seal, allowing delivery of higher pressures (inspiratory and PEEP), and avoidance of skin breakdown. The ventilatory assistance provided (Pvent) could potentially offload the effort required by the patient (Pmusc) to meet the required minute ventilation. In fact, offloading of Pmusc may be an important predictor of NIV success. It also may be associated with less particle dispersion, a key factor in viral transmission risk.
The use of NIV in ARDS remains controversial given a mixed evidence base. What can be said about Helmet NIV?
Evidence Base
This brief review will focus on the two major trials in the last 5 years, including HENIVOT, just released at the end of March.
Putting Things in Context
What have we learned?
These trials suggest that helmet non-invasive ventilation is safe and potentially effective for reducing intubation rate in ARDS when compared to HFNC or facemask NIV.
There are several important considerations when trying to generalize these findings. Helmet NIV requires additional expertise when choosing settings. Both trials aimed to achieve a minimal rise time and peak inspiratory flow of 100L/min to mitigate flow starvation and prevent CO2 rebreathing. Cycle dyssynchronies were avoided by adjustment of the cycle variable (% of peak flow, as in pressure support ventilation).
Perhaps most importantly, intubation criteria were carefully pre-defined and followed. If we’ve learned anything during the pandemic, timing of intubation varies greatly across different centers and clinicians.
The helmet interface allowed delivery of high PEEP compared to facemask NIV, and higher PEEP may mitigate some of the harmful transpulmonary pressure swings associated with spontaneous breathing in ARDS, as well as promoting alveolar recruitment to reduce lung stress. There is no clear way, however, to measure delivered tidal volume, a cornerstone of invasive mechanical ventilation management in ARDS.
NIV in ARDS: what’s the (potential) harm?
The use of NIV in ARDS remains controversial, especially when standard facemask interfaces are considered. Failure rates approach 50% in patients with moderate to severe ARDS and are associated with a high mortality rate.
There is an association of higher risk of NIV failure in the following scenarios:
- P:F ratio less than 150
- Tidal volumes greater than 9.5 ml/kg predicted body weight
- Presence of other organ failures
In addition, secondary analysis of patients treated with NIV in the LUNG-SAFE study showed increased mortality in patients with P:F ratio < 150 when compared to invasive ventilation. One wonders whether the delay of evidence-based therapies such as prone positioning and lung-protective ventilation are playing a role here.
The Debrief
Where do we go from here?
High flow nasal cannula has established itself as the standard of care in de novo hypoxemic respiratory failure since the FLORALI trial. It provides some degree of dead space washout, theoretically decreasing minute ventilation requirements and potential lung stress and strain. A nominal amount of PEEP is provided as well. In HENIVOT, however, the intubation rate was higher in the HFNC group compared to helmet NIV. Interestingly, there was more barotrauma in the HFNC group, though it did not achieve statistical significance. This may be due to excessively negative pleural pressures with vigorous spontaneous efforts.
When possible, safely avoiding invasive ventilation provides a huge benefit to both patients and health systems. In expert hands, helmet NIV appears to reduce the intubation rate of patients with moderate to severe ARDS. Further studies are needed to define the safety and effectiveness of helmet NIV with pragmatic use.
Tune in next week for a post which will delve into the essential physiology on positive pressure ventilation, negative pressure ventilation, and the potential for patient self-inflicted lung injury.
References
- Tonelli R, Fantini R, Tabbì L, et al. Early Inspiratory Effort Assessment by Esophageal Manometry Predicts Noninvasive Ventilation Outcome in De Novo Respiratory Failure. A Pilot Study. Am J Respir Crit Care Med. 2020;202(4):558-567. doi:10.1164/rccm.201912-2512OC
- Grieco DL, Menga LS, Cesarano M, et al. Effect of Helmet Noninvasive Ventilation vs High-Flow Nasal Oxygen on Days Free of Respiratory Support in Patients With COVID-19 and Moderate to Severe Hypoxemic Respiratory Failure: The HENIVOT Randomized Clinical Trial [published online ahead of print, 2021 Mar 25]. JAMA. 2021;10.1001/jama.2021.4682. doi:10.1001/jama.2021.4682
- Patel BK, Wolfe KS, Pohlman AS, Hall JB, Kress JP. Effect of Noninvasive Ventilation Delivered by Helmet vs Face Mask on the Rate of Endotracheal Intubation in Patients With Acute Respiratory Distress Syndrome: A Randomized Clinical Trial. JAMA. 2016;315(22):2435-2441. doi:10.1001/jama.2016.6338
- Hui DS, Chow BK, Lo T, Ng SS, Ko FW, Gin T, Chan MTV. Exhaled air dispersion during noninvasive ventilation via helmets and a total facemask. Chest. 2015 May;147(5):1336-1343. doi: 10.1378/chest.14-1934. PMID: 25392954; PMCID: PMC7094250.
- Munshi L, Hall JB. Respiratory Support During the COVID-19 Pandemic: Is It Time to Consider Using a Helmet? JAMA. 2021 Mar 25. doi: 10.1001/jama.2021.4975. Epub ahead of print. PMID: 33764370.
- Beitler JR, Owens RL, Malhotra A. Unmasking a Role for Noninvasive Ventilation in Early Acute Respiratory Distress Syndrome. JAMA. 2016;315(22):2401-2403. doi:10.1001/jama.2016.5987
- Morais CCA, Koyama Y, Yoshida T, et al. High Positive End-Expiratory Pressure Renders Spontaneous Effort Noninjurious. Am J Respir Crit Care Med. 2018;197(10):1285-1296. doi:10.1164/rccm.201706-1244OC
- Carteaux G, Millán-Guilarte T, De Prost N, et al. Failure of Noninvasive Ventilation for De Novo Acute Hypoxemic Respiratory Failure: Role of Tidal Volume. Crit Care Med. 2016;44(2):282-290. doi:10.1097/CCM.0000000000001379
- Bellani G, Laffey JG, Pham T, et al. Noninvasive Ventilation of Patients with Acute Respiratory Distress Syndrome. Insights from the LUNG SAFE Study. Am J Respir Crit Care Med. 2017;195(1):67-77. doi:10.1164/rccm.201606-1306OC
- Frat JP, Thille AW, Mercat A, et al. High-flow oxygen through nasal cannula in acute hypoxemic respiratory failure. N Engl J Med. 2015;372(23):2185-2196. doi:10.1056/NEJMoa1503326
