Excerpt
Noninvasive pressure support ventilation (NPSV) provides safe and effective assistance to patients with acute respiratory failure due to various causes [
1,
2]. However patient tolerance to the technique is a critical factor determining its success in avoiding endotracheal intubation [
3]. One of the key factors determining tolerance to NPSV is optimal synchrony between the patient’s spontaneous breathing activity and the ventilator’s settings, known as patient ventilator interaction [
4,
5]. Optimal patient ventilator synchrony during NPSV can prove very difficult to achieve due to the presence of leaks, the type of interface which can interfere with various aspects of ventilator function [
6‐
8]. This interference can increase the risk of asynchrony which in turn leads to an increase in the work of breathing and patient discomfort, thereby further worsening the situation [
4,
5]. NPSV is usually delivered via an oronasal facemask. Air leaks, patient discomfort, and pressure sores are common problems with facemasks [
8‐
10] that limit the duration of NPSV use and explain a large proportion of failures [
11]. A helmet has been developed that has no contact with the head. The helmet may be more comfortable, as there is no need for a seal around the nose and mouth. In recent non randomized studies, oxygenation improvements were similar with the facemask and helmet for NPSV delivery, but tolerance was better with the helmet [
12,
13]. However, other studies suggested greater patient ventilator asynchrony [
14,
15]. Compared to the facemask, the helmet was associated with less inspiratory-muscle unloading. Patient ventilator synchrony during NPSV with the helmet device is often compromised when using conventional pneumatic triggering, with the ventilator-delivered inspiratory support starting after the patient’s inspiratory effort [
16,
17]. A substantial portion of the inspiratory flow serves to expand the helmet, whose large inner volume and high compliance lead to dissipation of the inspiratory pressure delivered by the ventilator [
16,
17]. Greater asynchrony with the helmet has been reported previously in healthy volunteers and in patients with COPD [
16,
17]. In 10 patients with COPD, the inspiratory trigger delay was significantly longer with a helmet than with a facemask, and the inspiratory effort needed to trigger the ventilator was nearly three times greater with the helmet [
17]. The helmet was also associated with significantly greater asynchrony at the end of inspiration, with the ventilator cycling off either too early or too late compared to the end of the patient’s inspiratory effort. Overlap between ventilator-delivered support and the patient’s neural expiration is significantly longer with the helmet than with the facemask [
16,
17]. With a helmet, cycling off seems to occur in response to flow changes caused by the mechanical characteristics of the helmet rather than by the patient’s effort and mechanical characteristics [
16]. In COPD patients, when NPSV is delivered by helmet the ineffective efforts account for almost one-third of the overall pressure time product by minute, indicating that the inspiratory effort is affected by the worsened patient ventilator interaction. [
17]. Given the mechanical characteristics of the helmet and the algorithms governing inspiratory and inspiratory to expiratory NPSV triggers, the combination of the helmet with this latter mode of assisted ventilation can induce significant patient ventilator asynchrony and can impair efficacy of NPSV to unload the respiratory muscles [
16]. …
