Noninvasive ventilation with helmet versus control strategy in patients with acute respiratory failure: a systematic review and meta-analysis of controlled studies

Noninvasive ventilation (NIV) has proved to be a useful technique for breathing support that improved gas exchange and reduced the need for intubation and mortality in patients with exacerbation of chronic obstructive pulmonary disease, acute cardiogenic pulmonary edema, and blunt chest trauma [1‐4]. However, the overall failure of NIV occurred in 16–30 % of patients [5, 6], which depended on not only the underlying cause of acute respiratory failure (ARF) and the severity of the patients’ disease, but also the multiple technical causes including interface-associated complications [7, 8].

The mask turned mechanical ventilation without intubation into reality and was a key success factor in NIV. Despite great improvements in the material quality, shape, size, and fixing system of the mask, complications such as skin lesions, nasal pain, ulcerations of the nose bridge, and discomfort were very common [7, 9, 10]. NIV might fail in 18 % of cases attributable to mask discomfort [11], and failure of facial mask-delivered NIV was associated with a threefold increase in hospital mortality [12]. In an attempt to improve performance and reduce complications, the helmet was developed, which was a new-fashioned interface with the advantage of avoiding skin lesions and improving patient comfort independent of face morphology [13, 14]. Nevertheless, there has been no clear consensus on its effect until now. Whether NIV delivered by helmet could reduce the intubation rate and mortality was still causing controversy [15, 16], furthermore, the effect on gas exchange was also confusing. Some studies indicated NIV delivered by helmet could improve oxygenation further compared with a face mask [17, 18], while others showed there was no significant difference in oxygenation [19‐21], and some studies even demonstrated helmet-induced carbon dioxide rebreathing [22].

Therefore, we conducted a meta-analysis to investigate the effect of NIV with a helmet on hospital mortality, intubation rate, complications, and gas exchange versus the control strategy in patients with ARF.

This systematic review and meta-analysis was performed in adherence to the recently published Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement [23] and our prespecified protocol. Ethical approval and patient consent were not required since this was a meta-analysis of previously published studies. The eligible participants were adults (at least 18 years old) who suffered from ARF defined by the widely accepted consensus and needed breathing support. Interventions in the experiment group were NIV with a standard or new-generation helmet independent of ventilation mode; breathing support strategies in the control group included NIV or oxygen therapy with any kind of noninvasive interface. The primary outcomes were hospital mortality, intubation rate, and complications. The secondary outcomes included the length of stay in the intensive care unit (ICU), arterial blood gas exchange [including pH, the ratio of arterial oxygen partial pressure to fractional inspired oxygen (PaO₂/FiO₂), and partial pressure of carbon dioxide in the blood (PaCO₂)] and respiratory rate. Randomized and case-control trials that met the eligible criteria were included, while crossover studies were excluded.

To our knowledge, this is the first meta-analysis to investigate the effect of NIV with a helmet on ARF patients. We found that NIV with a helmet significantly reduced hospital mortality, tracheal intubation rate, and complications compared with the control techniques. And subgroup analyses found NIV with a helmet, especially in PSV mode, could decrease the PaCO₂ in hypercapnic patients, but did not increase PaCO₂ in the overall group. The helmet was as efficient as a mask on shortening the length of ICU stay, improving oxygenation and pH.

NIV delivered breathing support without the use of endotracheal or tracheostomy tube. The interface between the patients and NIV ventilators included nasal, oronasal, and facial mask. These noninvasive interfaces differentiated NIV from invasive mechanical ventilation, and maintained the airway protection mechanisms and the patient’s ability to swallow. Well- cooperated NIV decreased mortality in selected patients [1, 2, 35‐37]. However, patient discomfort and complications might limit continuous application of NIV [7], further decrease its efficacy, and even increase mortality. Some of these complications and discomfort were related to the mask [7‐9, 38]. In order to improve tolerability for patients, a transparent helmet made from latex-free polyvinyl chloride was developed [13].

This meta-analysis has shown NIV with a helmet reduced hospital mortality further compared with other masks in both RCTs and case-control trials, which meant the helmet itself had an effect on the patient’s survival. This effect might be attributed to the unique advantages that masks did not possess. First, the helmet allowed patients to freely drink, communicate and expectorate. It improved clearance of sputum and collaboration with caregivers without interference of the NIV. Second, the helmet was a better tolerated interface, patients need not try hard to cater to the interface as with a mask. The patient tolerance scale in the helmet group was remarkably higher than in the facial mask control group [14, 17, 29, 39]. Better tolerance allowed a longer continuous application [14, 31] and increased the success rate of NIV [40]. In fact, no patients failed NIV as a result of claustrophobia, discomfort, or pain related to a helmet while 38 % patients might have a failed NIV owing to a mask [14], which would increase hospital mortality. Third, the helmet could be applied to any patient regardless of their face contour. During the course of NIV with a mask, 24–64 % of patients had to change to another type of mask [12, 41] and the most common reason for non-adaptation to the mask was the shape of the face [41, 42]. Unlike the mask, the helmet could also be used in difficult anatomical situations such as in edentulous or facial trauma patients [13]. In short, good amenity, better tolerance, and universal application of the helmet ensured NIV was better used and reduced intubation (as shown in this analysis in Fig. 2b), which might contribute to preventing ventilator-associated pneumonia and decreasing hospital mortality [43].

An exciting finding was that the helmet could reduce the mortality of patients with hypoxemic ARF. Some studies aimed to explore applying NIV through a mask in this kind of patient, however, the results were disappointing, successful treatment was lower with a limited effect on the prognosis [43‐46]. This might be relevant to the different breathing patterns in hypoxemic ARF and hypercapnic ARF patients. In patients with hypoxemic ARF, mouth breathing was more common, with unavoidable air leaks although a facial or whole face mask was used [47]. The helmet improved tolerance and increased efficient ventilation, which was very important for patients with hypoxemic ARF. The continuous application of NIV was crucial in the onset phases of ARF, which otherwise might forebode NIV failure and life-threatening consequences [14, 48‐50]. It should be noted that this finding did not mean NIV with a helmet had no effect on the mortality of patients with hypercapnic ARF, since only two studies with small sample sizes had reported this variable in this meta-analysis [20, 30]; this question warrant further investigation.

The pooled effect indicated that NIV through a helmet decreased complications and intubation rate dramatically. The helmet was a transparent plastic hood, which did not come into contact with the patient’s face, especially the nose bridge and did not cause skin lesions [13]. With this device, air leaks were localized around the neck, which prevented eye irritation and conjunctivitis. The fixation system was not as complicated as the traditional mask. A simplified fixation system should carry a lower risk of cutaneous lesions. The new-generation helmet, characterized by an annular openable ring placed underneath an inflatable cushion, further reduced discomfort and axillary skin lesions caused by padded armpit braces in a standard helmet, so complications might only be confined to the neck [51]. Fewer complications aided the patients’ adherence to the NIV and increased the success rate although the effect on reducing complications was mainly observed in observational studies and larger RCTs are still needed to confirm the results. In addition, the use of specific ventilator settings with the fastest pressurization rate, higher inspiratory and expiratory pressures could improve patient ventilator interaction [52, 53]. All of these key issues prevented some patients from intubation.

Our analysis informed that NIV via a helmet was as effective as existing masks in gas exchange although respiratory rates, reflecting the degree of dyspnea to a certain extent, did not decrease noticeably. NIV with a helmet could improve oxygenation by delivering high oxygen concentration, unloading respiratory muscles, recruiting alveoli and increasing functional lung volumes, which was no different from NIV with other interfaces or invasive mechanical ventilation [54]. Moreover, the helmet further reduced leaks in contrast with masks. After all, the aim of introducing the helmet, still a noninvasive interface, was to reduce complications and increase patient comfort, to achieve better use of NIV and improve the prognosis, not just to improve gas exchange further. In theory, the helmet predisposed to CO₂ rebreathing, meanwhile the CO₂ concentration within the interface depended on the patient’s CO₂ production and could be minimized by specific settings including high fresh gas flow or higher inspiratory pressure [55‐57]. NIV through a helmet might take a longer time to achieve the target level of pressure support due to its larger inner volume. At a lower level of pressure support, the helmet might not decrease the work of breathing and has a restricted effect on respiratory rate.

In the past three decades, NIV played a critical role in the treatment of ARF, especially using CPAP and PSV. From a physiological rationale level, PSV could provide more benefit than CPAP. Regrettably, PSV has not been found to offer any advantage in terms of intubation or mortality [58]. As shown in the subgroup analysis by ventilation mode, both CPAP and PSV with a helmet could reduce the mortality and intubation rate, although PaCO₂ decreased mainly in the subgroup with PSV, and improve prognosis. This result was similar with the findings in patients with cardiogenic pulmonary edema [58].

Several limitations should be considered when interpreting our findings. First, because of the paucity of studies in this novel strategy, both RCTs and case-control trials were included. Fortunately, the results were substantially confirmed by pooled RCTs or case-control trials separately with low heterogeneity among the studies. Second, patients recruited in the included studies suffered from hypercapnic or hypoxemic ARF and were not highly consistent with each other. Third, the ventilation characteristics were not homogeneous, some were in CPAP mode, others in PSV, and the interface in the control group involved facial, nasal, oronasal and Venturi masks. Fourth, high heterogeneity existed across the studies for the variable PaCO₂ (I² equal to 72 %), this could be partly explained by the ARF type. In addition, publication bias might exist measuring by Begg’s and Egger’s test, but the final results were credible and steady tested by the trim and fill method, which tend to be more efficient when limited studies are included in a meta-analysis [59]. Finally, all of the included RCTs had high risk of performance bias attributed to the dramatic difference between a helmet and mask, which made it impossible for participants and personnel blinding to the interface and might influence the outcomes in a certain extent, we suggest that more attention should be paid to this key issue in future researches.

In conclusion, this meta-analysis indicated NIV with a helmet improved the prognosis, reduced the requirement of intubation and complications. The helmet was as effective as the mask in gas exchange despite no additional advantage. It should be noted that there is not sufficient scientific evidence to recommend it in designated patients due to the limited number of trials available. Large RCT studies are still needed to provide more robust evidence.