Benefits of secretion clearance with high frequency percussive ventilation in tracheostomized critically ill patients: a pilot study

Physiological processes involved in the clearance of the airways are compromised during mechanical ventilation (MV). On one hand, the endotracheal tube (ETT) itself alters the mucociliary function [1]; on the other hand, sedation inhibits the physiological cough reflex. Nonetheless, the cold and dry air-oxygen delivered during MV alters the bronchial epithelium function preventing the mucus transit to upper airways [2‐4]. As a result, therisk of ventilator associated pneumonia (VAP) and lung atelectasis is increased [2, 5, 6].

While that ETT suction alone results in a partial airways clearance [7], the combination of suctioning and chest physiotherapy leads to several physiological and clinical benefits [8] including re-expansion of atelectatic portions of the lung, improvement of the respiratory system compliance [9] and expiratory flow rates [10] as well as reduction of VAP [11].

Over the years, novel approaches have been proposed to ease the clearance of the airways. In this regard, high-frequency percussive ventilation (HFPV) demonstrated its ability to mobilise secretions in patients affected by cystic fibrosis [12, 13] and neuromuscular disease [14]. In line with these previous results, we designed a physiological study to investigate the benefits of HPFV when superimposed to conventional MV in tracheostomized patients with a high amount of secretions. Changes in lung aeration following the application of HFPV was the primary aim of the study. Secondary aims were the changes in distribution of ventilation within different regions of the parenchyma and the gas exchange variation.

This observational study was conducted in the Intensive Care Unit of the “Mater Domini”—University Hospital of Catanzaro (Italy) after local Ethical committee approval (Ethical Committee Approval number 314 dated 16th September 2021). Written consent for publication of individual details including images was obtained from the patients or the next of kin.

This trial was compliant with the consolidated standards of reporting trials (CONSORT) reporting guidelines and was prospectively registered (NCT05200507; www.​clinicaltrials.​gov, dated 6th January 2022).

Fifteen adult patients were enrolled consecutively from January 2022 to April 2022 and all of them completed the study protocol. Demographic, anthropometric and clinical features of the patients are reported in Table 1. The mean (Standard Deviation) PEEP was 8 (2) cmH₂O and the mean inspiratory PS was 10 (2) cmH₂O. Mean value of inspired Oxygen fraction (FiO₂) was 0.39 (0.03).

Figure 1 depicts EIT images from one representative patient (see figure legend for explanation). As shown in Table 2, the TIV did not change after the application of HFPV neither globally (p = 0.132), nor in dorsal (p = 0.114) and ventral (p = 0.125) lung regions. Compared to baseline (T0), ∆EELI improved significantly straight after HFPV cycle (T1). Similar results were also observed after one (T2) and 3 h (T3) from the baseline (T0) prior to HFPV cycle. To note, alveolar recruitment was achieved on both dorsal and ventral lung regions. The GI was not modified after the cycle (p = 0.114).

Figure 2 reports the arterial blood gases obtained at each step of the protocol, from T0 to T3. Detailed data are shown in Table E1 in the ESM. The pH (p = 0.226) and the PaCO₂ (p = 0.114) did not change after the HFPV cycle, while PaO₂/FiO₂ significantly improved along the protocol. Specifically, PaO₂/FiO₂ was 174 (171; 185)) at T0, 211 (198; 225), p < 0.001) at T1, 230 (213; 251), p < 0.001) at T2 and 225 (203; 251), p < 0.001) at T3.

Compared to baseline (T0), ∆PaO₂/FiO₂ was observed at each protocol step. Specifically, from T0 to T1 ∆PaO₂/FiO₂ was 28 [25; 45] mmHg, from T0 to T2 it was 45 [40; 68] mmHg and from T0 to T3 it was 44 [26; 76] mmHg. As shown in Fig. 3, the ∆PaO₂/FiO₂ correlated with the increase in ∆EELI (R_rm = 0.53, 95%CI (0.222, 0.747); p = 0.002).

No modifications of vital parameters (heart rate, mean arterial pressure and respiratory rate) were observed (see Table E1 in the ESM).

Our study demonstrated the application of the HFPV cycle during MV in hypersecretive tracheostomized patients promotes alveolar recruitment and improves oxygenation. Both benefits are remarkable and last up to 3 h after the treatment. Besides these physiological advantages, HFPV cycles also have a positive impact on reducing the manual work load of nursing staff and physiotherapists [7].

The management of airways secretions gained progressive attention in critically ill ventilated patients as it may lead to complications such as VAP and extubation delay [24, 25]. Despite several methods being suggested, the clearance of deep airways remains a challenge. Mechanically assisted cough devices mimicked the cough process through positive and negative air pressure which showed promising outcomes in the paediatric population and those patients with neuromuscular disorders. However, these devices can only reach upper airways [26]. Another alternative technique involved oscillating devices: inspiratory and expiratory chest oscillations at high rate promote the migration of mucus towards the bronchial tree where it was cleared by suction [7]. Despite the benefit, evidence in literature remains contradictory as the superior effectiveness of these devices to chest physiotherapy has not been demonstrated yet [27].

Among the group of high Frequency Ventilation, HFPV is mainly applied to patients with inhalation injury or Acute Respiratory Distress Syndrome. It improved oxygenation and enhanced mobilisation of secretions from lower to upper airways [28]. Given this, Clini et al. investigated the effects of HFPV in extubated and tracheostomized patients during weaning for the first time [29]. Specifically, forty-six patients were randomised to receive either conventional chest physiotherapy or the combination of HFPV together with physiotherapy. Results confirmed the superiority of HFPV and physiotherapy for secretion clearance and improvement of gas-exchanges [29]. Similarly, Tsuruta et al. explored the effects of HFPV superimposed to MV in ten obese patients. HFPV was continuously applied from study enrolment till the beginning of the weaning process [30]. Gas exchange and lung dynamic compliance was better after 3 h, assuming a potential recruitment effect of HFPV [30]. In a small sample of patients with moderate-to-severe Acute Respiratory Distress Syndrome, Godet et al. reported that HFPV improves lung aeration and gas exchange, by recruiting the dorsal lung regions while assuring a protective and gentle ventilation[31]. A previous study conducted by our research group assessed the effects of high-frequency chest wall oscillation (HFCWO) in ventilated patients with significant and minimal secretion load. Increase in lung aeration was noted only in patients with high secretion load, however there was no gas exchange improvement. An explanation for these results were addressed to the amelioration of the ventilation-perfusion mismatch rather than reduction of the intrapulmonary shunt [7]. To note, the benefit on lung aeration did not last beyond 3 h from the HFCWO cycle [7].

In addition, we did not record any difference of GI after the one HFPV cycle in our population. The GI is a parameter that assess the inhomogeneity of air distribution within the lung [32]. A higher GI implies more heterogeneous spatial ventilation in the lung (i.e. a “sicker” lung) [33]. Several studies have shown that the application of a personalized PEEP to avoid alveolar derecruitment guarantees lower GI values in patients with acute respiratory failure [34‐36]. However, when PEEP is set guided by GI, its best value may be overestimated [37]. Beside the relationship between GI and PEEP, we used GI to assess if the intra-tidal distribution and lung inhomogeneity was different before and after HFPV application. In fact, the recruitment induces by the application of HFPV may have also reduced the intra-tidal distribution and the lung inhomogeneity. However, this event did not occur in our patients, probably because they were not so acute and severe as in previous other studies [38‐40].

Divergence between our current results and previous ones could be partially explained by two aspects: the ventilation settings and the study population we had enrolled. Unlike previous trials where MV was delivered throughout ETT [7], our patients underwent MV via tracheostomy. Since patients with tracheostomy are generally those with longer stay in ICU and worse gas exchange, their weaning process is usually more challenging [7, 41]. As a result, the amount of recruitable lung can remarkably vary between tracheostomized and intubated patients.

Given the benefits of the HFPV cycle lasting up to 3 h, we extended its application beyond our research protocol. Specifically, HFPV was delivered for 10 min every 6 h till 48 h after the disconnection from the ventilator. Opposite to previous Clini’s study results [29], none of our patients required bronchoscopic procedures for secretion clearance and they all have been successfully weaned from MV. Discrepancy in results could be related to the time interval when the cycle was delivered. Specifically, Clini et al. applied the cycle every 12 h [29], whereas we halved the interval to 6 h. This may suggest benefits of HFPV when applied more frequently, however designed studies are required to prove this hypothesis. Our study also has limitations which must be addressed.

Firstly, the sample size population. Given the physiological design of this study, a formal sample size calculation has not been performed. Despite the total number of enrolled patients (15) being relatively small and the population is quite heterogeneous, several studies enrolled similar or even lower numbers of patients [20, 21, 36, 42]. Hence, we considered 15 patients as sufficient to address our hypothesis based on our previous trial [7]. In addition, we assumed all our data as non-parametric.

Secondly, the definition of hypersecretive patient could be arguable. We adopted the same definition mentioned in previous studies [7, 43, 44] as other criteria for definition of hypersecretive status have been considered in literature but they seem to be more operator dependent, hence open to criticism [45].

Thirdly, the lack of a standardised bronchoaspiration protocol. In other words, the clinical evaluation of the single operator (nurse) in deciding how often suctions were needed within 8 h could have generated a bias in the patient’s selection.

Last, we expressed variation in EIT volumes (TIV and ∆EELI) in mL, instead of arbitrary units. It should be mentioned that EIT device we used, measures an eclipse with a central diameter of 5–10 cm [20]. Therefore, the assessment is limited to a 5–10 cm slice of the lung, rather than the entire lung. Noteworthy, this method has been previously extensively used in the literature [7, 16, 17].

The use of HFPV superimposed to MV is promising, however additional evidence on less selected populations are needed.