Comparative evaluation of three interfaces for non-invasive ventilation: a randomized cross-over design physiologic study on healthy volunteers

The choice of the interface is one of the crucial factors determining the success of noninvasive ventilation (NIV) in both the acute [1] and chronic [2] setting. Different types of face mask (FM) that is, oral, nasal, oronasal, total full face of different size, design, and material are available to increase the patient’s comfort. Despite the broad availability of FMs, most NIV failures are still associated with mask-related side effects such as air leaks [3], skin breakdown [4, 5], and mask discomfort [1, 2, 6].

New interfaces have been introduced with the aim of improving patient comfort and overcoming these side effects. The helmet is a transparent latex-free polyvinylchloride hood; in its standard configuration it is joined by a rigid plastic ring to a soft collar and secured by two padded armpit braces at four hooks (two in the front and two in the back of the plastic ring). The standard helmet (H_S) has been shown to improve NIV comfort over time compared to FM and to reduce skin breakdown, gastric distension, and eye irritation [7, 8]. Despite these advantages, the helmet has specific drawbacks primarily related to its highly compliant soft collar [9]. Inspiratory pressure dissipation increases the time lag between inspiratory effort and ventilator assistance, worsening patient ventilator synchrony [10‐12]. Furthermore, compared to FM the helmet has been found to be associated with less efficient inspiratory-muscle unloading, both in healthy volunteers [11] and patients [10, 12]. In addition, the armpit braces that maintain the helmet in place may cause patients discomfort and skin lesions leading to NIV intolerance and failure [13].

To reduce these technical problems, a new helmet (H_N) (NIV-Castar R Next, Starmed, Mirandola, Italy) has been developed and introduced into clinical use, in which an opening ring placed underneath an inflatable cushion secures the helmet without the need for armpit braces. Moreover, compared to H_S, with H_N the pressure dissipated because of the downward displacement of the soft collar during ventilator insufflation is eliminated, or at least reduced to a large extent. Very recently a bench study comparing H_S and H_N suggested a better performance of the latter in terms of triggering performance, patient-ventilator synchrony, and rate of pressurization [14].

The rotational use of different interfaces has been proposed to improve patients’ tolerance and prolong NIV application both in hypoxemic [15] and hypercapnic [16] patients. On one hand, patients needing several daily hours of NIV may benefit from the rotational use of different interfaces to reduce the risk of discomfort and the side effects of one specific interface [15‐17]; on the other hand, the use of diverse interfaces with equal ventilator assistance may result in a different extent of muscle unloading and patient-ventilator synchrony [10, 12]. To overcome the decreased efficacy of the H_S in delivering ventilator assistance, compared to the FM, Vargas et al. recently proposed to increase the level of both positive end-expiratory pressure (PEEP) and inspiratory pressure support (PS) [12].

The objective of this physiologic study was to assess and compare the efficacy of FM, H_S, and H_N in delivering NIV in healthy volunteers. Interaction and synchrony between subject and ventilator, and the subject’s inspiratory effort and comfort were compared at baseline settings, that is, at PEEP 5 cmH₂O and PS 5 cmH₂O, and at increased settings, that is, PEEP 8 cmH₂O and PS 8 cmH₂O.

The protocol was approved by the institutional review board (Comité de Protection des Personnes Sud Méditéranée IV, Montpellier; approval number 2012-A00098-35), and written informed consent was obtained from each subject. Twelve healthy volunteers were studied in a semi-recumbent position. The subjects (three women and nine men, age 30 ± 9 years) were all nonsmokers and had a body mass index of 24 ± 4 kg/m². NIV was delivered trough an FM (Performatrak, Philips Respironics), H_S (NIV-Castar R), and H_N (NIV-Castar R Next), using an ICU ventilator equipped with software for air-leaks compensation (NIV module) (Servo-I, Maquet, Solna, Sweden) set in PS ventilation mode.

We found that in healthy volunteers FM, H_S and H_N at baseline settings, that is, PEEP 5 cmH₂O and PS 5 cmH₂O, were not significantly different with respect to inspiratory muscles unloading, WOB and comfort. With respect to patient-ventilator interaction, TI,synchrony and Delay,TR-insp were significantly improved by FM compared to both helmets, whereas Delay,TR-exp was shorter with FM, as opposed to H_S, but not H_N. At increased settings (that is, PEEP 8 cmH₂O and PS 8 cmH₂O), inspiratory effort was significantly lower, compared to SB_I and FM, with H_N, while not HS. Comfort was inversely related to the pressure applied to the airway opening by the ventilator, regardless of the interface. Patient-ventilator synchrony, as assessed by Delay,TR-insp and Delay,TR-exp, was equivalent between FM at baseline setting and H_N at increased settings, but remained inferior with H_S even at increased settings. TI synchrony remained inferior with both helmets at increased settings, compared to FM.

Before discussing our findings, some limitations of this study merit consideration. First, our investigation was conducted on healthy individuals, which makes our findings not entirely inferable to the clinical setting. We share this limit with several studies, some of which were performed to evaluate H_S at the time it was introduced into clinical use [9, 11, 21]. As a matter of fact, studies on H_N are lacking, which justifies this first in vivo evaluation in healthy volunteers. Second, of the seven trials performed on each subject, the two spontaneous breathing trials and four helmet trials had the same breathing apparatus with mouth-piece and nose-clip, whereas the FM trial did not. The use of breathing apparatus that is, mouth-piece and nose-clip, has been repeatedly shown to affect breathing pattern by increasing V_T and V_E with few changes in the RR [27, 28]. In keeping with these studies, during the FM trial V_T and V_E were smaller than during the other six trials, the reduction in V_E being statistically significant, whereas the RR was only slightly decreased. As V_T is one of the variables, in addition to Pes, computed in the measurement of WOB, the use of a breathing apparatus might have influenced it to a similar extent. Third, we limited the time of each trial to 5 minutes, which is indeed rather short, especially when compared to that utilized in other similar studies performed either in healthy volunteers [9, 21] or patients [10, 12]. It should be considered, however, that in contrast to these studies, in our experimental set up the breathing apparatus was connected to the subject through the mouthpiece, which produces discomfort, potentially affecting either the breathing pattern or the indexes of respiratory muscle effort. Although we cannot exclude that the relatively short period of evaluation could be not sufficient to fully achieve a steady state, we are convinced that the poor tolerance to the breathing apparatus would have represented more bias. Finally, potential bias related to the non-blinded setting of the study has to be taken into consideration, although we share this limitation with several studies [9, 11, 21].

In hypercapnic patients with severe chronic obstructive pulmonary disease, Navalesi et al. observed a significant decrease per minute in PTPdi of 65% and 43% with FM and H_S, respectively, as opposed to SB [10, 12]. In patients at high risk of developing post-extubation respiratory failure, Vargas et al. found that at baseline settings, FM and H_S significantly reduced PTPdi per minute compared to SB, at 69% and 60%, respectively [12]. In our healthy volunteers, we also observed a reduction in PTPdi per minute at baseline settings with respect to SB, but the extent of this decrease range was lower (15 to 20%) and not statistically significant, or significantly different between the three interfaces. On one hand, these discrepancies are likely a consequence of the lower inspiratory pressure applied and on the other, due to the fact that in contrast to the aforementioned studies [10, 12], we performed our investigation in normal subjects without underlying respiratory disorders. Indeed, other previous studies on healthy subjects at similar baseline settings also failed to demonstrate a difference in PTPdi between FM and H_S[11], and between H_S and SB [21]. In keeping with Vargas et al., we found that at increased settings H_S and H_N were both as effective as FM at the baseline setting, in reducing inspiratory effort compared to SB; of note, PTPdi expressed per breath, per minute or per liter, was significantly lower with H_N at increased settings than with FM at baseline settings.

Surprisingly, the differences in WOB between trials were rather small and not significant. The large V_T, that to some extent was a consequence of the breathing apparatus [27, 28] as already mentioned, may have influenced this variable, which is influenced by both Pes and V_T. This may also explain the lower, though not significantly lower values of WOB/breath and WOB/minute observed during the FM trial, in contrast to WOB/L, which, in fact was not influenced by the amount of V_T. A possible expiratory activation of the expiratory respiratory muscles may contribute to the better performance of both H_S and H_N compared to the FM.

In agreement with previous work [10, 12], at baseline settings both Delay,TR-insp and Delay,TR-exp were longer with H_S than with FM, whereas with H_N Delay,TR-insp, but not Delay,TR-exp, was significantly higher, with respect to FM. Whereas Vargas et al. found that Delay,TR-insp with H_S was significantly lower with the increased settings than with the baseline settings, in our study H_S at increased settings compared to H_S at baseline settings did not produce significant improvements in Delay,TR-insp, Delay,TR-exp or TI,synchrony, which all remained worse than the corresponding values obtained with FM at standard settings. Conversely, with H_N the increased settings improved all three variables, which were not significantly different compared to FM at baseline.

H_S was shown to be better tolerated than FM in studies comparing the two interfaces over the medium (hours) and long (days) period [7, 8]. In the present investigation, comfort was not influenced by the type of interface, as already reported in the previous short term physiologic investigations comparing FM and H_S over brief periods of time (20 to 30 minutes) [10, 12], which has been attributed to the longer time required for most of the major determinants of patient discomfort during NIV to take place [10, 12]. Consistent with previous work on healthy volunteers [21], however, we found that comfort was affected by the amount of applied pressure.

Very recently, studies conducted either in hypoxemic [15] and hypercapnic [16] patients, and a European survey [17] on the use of NIV, indicate the rotational use of different interfaces as a possible means to improve patients’ tolerance and prolong NIV application. Our study shows that different interfaces can be used to achieve comparable results with respect to inspiratory effort reduction. In addition, we found that in healthy volunteers, although FM performs better overall than helmets at baseline settings, at increased settings both helmets had improved performance and resulted in lower inspiratory effort lower than SB with both helmets, and even better than FM with H_N only. Furthermore, at increased settings H_N achieved better results for interaction and synchrony between subject and ventilator compared to H_S, which indicated that this new interface performed similarly to FM.

H_N might hold some advantages compared to the other interfaces with respect to interaction and synchrony but studies on patients are needed to confirm these findings in the clinical setting.