The Impact of High-Frequency Chest-Wall Compression on Mechanical Ventilation Delivery and Flow Bias

Abstract

BACKGROUND:

It is unclear if high-frequency chest-wall compression (HFCWC) has a role to assist with secretion clearance in patients on mechanical ventilation. The effect of HFCWC on the delivery of mechanical ventilation is unknown. This study describes the effect of HFCWC on mechanical ventilation delivery and flow bias in an orally intubated and mechanically ventilated bench model.

METHODS:

An orally intubated mannequin was mechanically ventilated in 5 commonly used modes of ventilation at settings that reflect current practice. HFCWC was applied via a randomized combination of oscillation frequencies and pressure settings. Mechanical ventilator flow, flow bias, and breathing frequency were measured before and during the application of HFCWC.

RESULTS:

HFCWC led to 3- to 7-fold increases in ventilator-delivered breathing frequency during synchronized intermittent mandatory ventilation, bi-level (with pressure support), bi-level-assist, and pressure-regulated volume control modes of ventilation. Only in the bi-level mode without pressure support was the ventilator breathing frequency unaffected by HFCWC. During HFCWC, peak inspiratory flow to peak expiratory flow ratios toward an expiratory flow bias, particularly at higher HFCWC pressures, only in pressure-regulated volume control and synchronized intermittent mandatory ventilation modes were peak inspiratory flow to peak expiratory flow ratios of <0.9 generated that would facilitate secretion clearance.

CONCLUSIONS:

HFCWC led to 3- to 7-fold increases in ventilator breathing frequency delivered by mechanical ventilation except in the bi-level mode. The bi-level mode may be the optimal mode to use HFCWC to minimize disruption to the delivered ventilator breathing frequency. The peak inspiratory flow to peak expiratory flow ratios < 0.9, the optimal flow bias for secretion clearance, was only achieved in the pressure-regulated volume control and synchronized intermittent mandatory ventilation modes. However, the findings in this bench model with a fixed low compliance may not be generalizable to the patient in the ICU, and we recommend further investigation into the effects of HFCWC in the patient in the ICU.

Introduction

High-frequency chest-wall compression (HFCWC) has been shown to be an effective treatment technique for enhancing secretion clearance in patients with cystic fibrosis.¹ A portable air pulse generator supplies a background compression pressure to the chest wall (1–10, no units) via a wrap-around vest, and pulses of air (5-20 Hz) are superimposed on this background pressure to oscillate the chest wall and create oscillatory air flow in the airways. The oscillated air flow acts as a physical mucolytic, reduces mucus viscosity, and enhances tracheal mucus clearance at chest-wall oscillation frequencies of 13 Hz.²

The benefits of HFCWC are thought to be through enhancing expiratory flows.³ In vitro and animal studies have shown that both increased expiratory flow and expiratory air-flow bias are required for cephalad mucus movement⁴ and that a peak inspiratory flow (PIF) to peak expiratory flow (PEF) ratio (PIF/PEF) of <0.9 is considered to be most effective⁵ (note that some researchers may report on PEF/PIF or PEF-PIF differences,⁶ but, in this publication, we report PIF/PEF differences). The highest mucus clearance rates occur between HFCWC settings of 11–15 Hz with a peak at 13 Hz,^2,7,8 which suggests that increased clearance of mucus is related to reduced mucus viscosity and frequency. The Vest Airway Clearance System Model 105 (Hill Rom, Leicestershire, United Kingdom) is one device used to deliver HFCWC, which uses a soft wrap that is secured around the patient's chest with velcro straps (Appendix 1 and 2 [see the supplementary materials at http://www.rcjournal.com]). Although an arbitrary Vest pressure setting is chosen, the tightness of fastening the Vest initially may also influence the pressure delivered to the chest wall and is frequently not reported.^1,9,10

It is unclear how widely HFCWC is used in patients on mechanical ventilation, with one report that did not distinguish the differences in impact between patients who were intubated and those who were not intubated.¹¹ Hence, little is known about the effect of HFCWC in patients on mechanical ventilation. HFCWC may be potentially unsuitable for patients on mechanical ventilation because the chest-wall compression and oscillation may interfere with the breath delivery by the ventilator and triggering of additional ventilator breaths.¹² HFCWC may also cause changes in inspiratory-expiratory flows, flow bias, and/or airway pressures (both positive and negative). These changes may be falsely interpreted by the mechanical ventilator as an inspiratory trigger that leads to the delivery of additional ventilator-supported breaths and hence lead to an increased breathing frequency and minute volume. The aim of this study, therefore, was to describe the effect of HFCWC on the breathing frequency, inspiratory and expiratory flow, and expiratory-inspiratory flow bias in an intubated and ventilated bench model.

QUICK LOOK.

Current Knowledge

There is limited understanding of the impact of high-frequency chest-wall compression (HFCWC) on the delivery of mechanical ventilation. External chest compression and decompression could result in volume and pressure changes which result in ventilator triggering.

What This Paper Contributes to Our Knowledge

In the standard ventilator modes used in intensive care high-frequency chest-wall compression causes significant changes in the delivery of mechanical ventilation. Importantly, the expiratory flow bias was often not improved with the addition of high-frequency chest-wall compression unless high pressures were used, which makes it unclear if this therapy would be clinically indicated.

Methods

Ethics approval (Appendix 3 [see the supplementary materials at http://www.rcjournal.com]) was gained from the UCL Institute of Child Health. A prototype resuscitation mannequin (TruCorp, Belfast, Northern Ireland) with an anatomically correct rib cage (see online supplementary materials at http://www.rcjournal.com), inflatable synthetic lungs, and realistic thoracic movement was orally intubated (size 8 portex endotracheal tube, Smiths Medical, California) and mechanically ventilated (Draegar EvitaXL, Lübeck, Germany). The fixed respiratory mechanics of the mannequin include inspiratory airway resistance of 12.55 cm H₂O/L/s and dynamic compliance of 11.6 mL/cm H₂O (normal values are expected to be 64 mL/cm H₂O¹³). The airway pressure and tidal volume data were collected but not included in the analysis and reporting because these lung compliance values would be equivalent to patients with severe ARDS,¹³ with the high airway pressures generated at baseline, not reflective of the cohort of patients in whom HFCWC may be used clinically.

Five commonly used mechanical ventilator modes were used: synchronized intermittent mandatory ventilation (SIMV) (volume controlled), bi-level positive airway pressure with set ventilator frequency with no synchronization with patient effort (bi-level), bi-level with pressure support, bi-level assist with patient effort triggering synchronized triggering of additional ventilator breaths (bi-level assist), and pressure-regulated volume control (PRVC). The ventilator settings for each mode reflected those used in clinical practice internationally at the time of the study (Table 1).¹⁴ The Vest Airway Clearance System Model 105 and an appropriately sized Wrap Vest were secured around the mannequin (during a mechanical ventilator inspiratory hold maneuver) and left in place throughout the trial (see online supplementary materials at http://www.rcjournal.com). For each mode of mechanical ventilation, a randomized combination of HFCWC pressures (1–10 arbitrary units) and HFCWC oscillation frequencies (10, 13, and 15 Hz) were used. The oscillation frequency settings were chosen to reflect the maximum potential for mucus clearance (10–15 Hz, with a peak at 13 Hz).⁷ At each stage of breathing frequency, airway pressure and flow were allowed to stabilize for 1 min. Measurements were then taken over a 2- min period for each combination.

Table 1.

Ventilator Settings

A NICO₂ Respiratory Profile Monitor (Respironics United Kingdom, Chichester, United Kingdom) in the ventilator circuit (between the endotracheal tube and the ventilator circuit) recorded specific respiratory parameters (see online supplementary materials in Appendix 4 at http://www.rcjournal.com) with the Vest in situ during HFCWC operation at each combination of settings (mean breathing frequency, mean airway pressure [cm H₂O], PIF [L/min], PEF [L/min], and PIF/PEF). Changes in the PIF/PEF with the values decreasing (eg, PIF/PEF from 1.2 to 0.9) are termed “increasing expiratory flow bias”; if the PIF/PEF were < 0.9, then that may favor the clearance of airway secretions; whereas increases in the PIF/PEF (eg, PIF/PEF from 0.6 to 1.0) are termed “increasing inspiratory flow bias,” which may facilitate secretion retention if the PIF/PEF was > 0.9. Peak airway pressure (cm H₂O) and mean tidal volume (mL) were also explored in this study but are not reported because the baseline peak and mean airway pressures were high, with the bench model having poor lung compliance and not reflective of the general ICU patient.

The statistical packages R. R Foundation for Statistical Computing, Vienna, Austria) and SPSS version 27.0 (IBM, Armonk, NY) were used for data analysis. Data were tested for normality by using the Kolmogorov-Smirnov test. The Wilcoxon test was used to compare the outcomes between baseline and during HFCWC for each mode of ventilation. The difference in outcomes among the ventilation modes was tested by using the Kruskal-Wallis test. Regression analysis was used to explore the relationship among individual modes of mechanical ventilation, Vest oscillation frequencies, Vest pressure settings, and the outcomes. Statistical significance was set at P < .05.

Results

There were 180 combinations of ventilation modes, Vest oscillation frequencies, and pressure settings investigated in this trial. However, for the purposes of this publication, Vest pressure settings 1, 5, and 10 were selected for analysis and reporting because they represent low, medium, and high pressure settings and should reflect options used in clinical practice. Unfortunately, the data for bi-level pressure support with the Vest setting 13 Hz and pressure 1 were missing from the data set and hence unable to be reported.

HFCWC increased the ventilator-delivered breathing frequency from 12 breaths/min at baseline in SIMV to a mean difference of 57/min, 95% CI 52–63 (P =.008); bi-level with pressure support to a mean difference of 50/min, 95% CI 46–53 (P = .01); bi-level assist to a mean difference of 18/min, 95% CI 17–18 (P = .004); and, in PRVC, to a mean difference of 52/min, 95% CI 49–55 (P = .007). The HFCWC did not change the breathing frequency in the bi-level mode except at Vest oscillation of 13 Hz and pressure 1 when a breathing frequency of 60/min occurred (we believe this to have been an artifact because the breathing frequency did not change at any other Vest settings during the bi-level mode). This equated to a 3-fold increase in breathing frequency during the bi-level assist mode, a 5-fold increase during the bi-level mode with pressure support and PRVC, and a 5- to 7-fold increase during SIMV (Fig. 1).

Fig. 1.

A scatter plot to show the effect of high-frequency chest-wall compression (HFCWC) on the breathing frequency in synchronized intermittent mandatory ventilation (SIMV), bi-level positive airway pressure with set ventilator rate with no synchronization with patient effort (bi-level), bi-level with pressure support, bi-level assist, and pressure-regulated volume control (PRVC) modes. The baseline breathing frequency (blue dotted line) was 12/min and increased during HFCWC in all modes apart from in the bi-level mode. At the Vest oscillation of 13 Hz and pressure 1 in the bi-level mode, the breathing freqency was 60/min (We believe this to have been an artifact because the rate did not change at any other Vest settings during the bi-levelmode). (The data set for the bi-level with pressure support with the Vest settings of 13 Hz and pressure 1 was missing).

In SIMV, bi-level, bi-level assist, bi-level with pressure support, and PRVC modes for a given HFCWC oscillation frequency, the HFCWC pressure setting seemed to have no significant effect. In the bi-level assist mode, the 3-fold increase in breathing frequency compared with baseline was due to additional pressure-controlled breaths triggered by the ventilator during HFCWC. The ventilator flow waveform during bi-level assist at HFCWC settings 10 Hz and pressure 1 are displayed in Figure 2. The baseline breathing frequency was 12 breaths/min, and this is displayed between 0 and 20 s. The HFCWC was switched on at 20 s, and HFCWC resulted in the breathing frequency increase from baseline. The HFCWC had stabilized by 40 s, and the increased breathing frequency (30 breaths/min) can be seen between 40 and 60 s.

Fig. 2.

A flow waveform during bi-level assist with the Vest settings at 10 Hz and pressure 1, showing the effect of high-frequency chest-wall compression (HFCWC) on breathing freqeuncy. The baseline frequency is shown between 0 and 20 s. HFCWC caused the frequency to increase to 30 breaths/min as shown between 40 and 60 s. bi-level = bi-level positive airway pressure with set ventilator frequency with synchronization with patient effort.

During SIMV, bi-level with pressure support, and PRVC modes, however, the breathing frequency increased from baseline during HFCWC due to additionally triggered pressure-supported breaths. The flow waveform during SIMV at HFCWC settings 10 Hz and pressure 1 are displayed in Figure 3. The baseline frequency is shown to be between 0 and 20 s. HFCWC was switched on at 20 s and, as the system stabilized, the number of mandatory breaths remained constant but additional pressure-supported breaths were triggered. At 40 s, the HFCWC had stabilized, and periods of pressure-supported breaths can be seen between mandatory breaths from 40 and 60 s. Because the inspiratory time of pressure-supported breaths varied from breath to breath as determined by the flow changes sensed by the ventilator, the number of pressure-supported breaths delivered in a respiratory cycle varied. This is shown, in Figure 3, between 40 and 60 s, in which the number of pressure-supported breaths per cycle varied from 4 to 5.

Fig. 3.

A flow waveform during SIMV with the Vest settings at 10 Hz and pressure 1 to show the effect of high-frequency chest-wall compression (HFCWC) on breathing frequency. The baseline breathing frequency is shown between 0 and 20 s. HFCWC was started at 20 s and had stabilized at 40 s, and the number of mandatory breaths delivered remained constant (2 mandatory breaths are indicated by red arrows). The total breathing frequency increased to 60 breaths during HFCWC due to additional pressure-supported breaths being triggered. The number of pressure-supported breaths delivered between mandatory breaths was not constant at each respiratory cycle, and the green arrow indicates a series of 5 pressure-supported breaths, whereas the gray arrow indicates a series of 4 pressure-supported breaths.

HFCWC in all modes also caused flow oscillation that did not result in additional mechanical ventilator breath delivery. The frequency and amplitude of oscillation increased with higher HFCWC frequency and pressure settings. A typical 10-s interval during HFCWC in SIMV at settings of 10 Hz and pressure 1, and 15 Hz and pressure 10 is shown in Figure 4.

Fig. 4.

A typical 10-s flow waveform during high-frequency chest-wall compression (HFCWC) in synchronized intermittent mandatory ventilation (SIMV) at the Vest settings at 10 Hz and pressure 1 (A) and 15 Hz and pressure 10 (B) shows that oscillatory flow generated by HFCWC increased in frequency and amplitude at the higher HFCWC frequency setting. A mandatory breath is indicated by a red arrow, and 2 pressure-supported breaths are indicated by green arrows. At 10 Hz and pressure 1 peak oscillation flow was ∼10 L/min (marked by gray arrows), whereas, at 15 Hz and pressure 10, the peak oscillation flow was more frequent and ∼20 L/min (marked by gray arrows). This pattern of flow oscillation was similar in all modes of ventilation.

Regression analysis was used to explore whether there was a link between the ventilator mode and HFCWC oscillation frequency and the difference in breathing frequency during HFCWC. It revealed a strong relationship (adjusted R² = 0.87) (Table 2). Further analysis showed that only bi-level (β = –52.0, P ≤ .001, 95% CI –59.6 to –44.4) and bi-level assist (β = –39.66, P ≤ .001, 95% CI –47.28 to –32.05) had a significant negative effect on breathing frequency. There was also a small positive but non-significant relationship between the change in breathing frequency and oscillation at 13 Hz (β = 8.48, P = .07, 95% CI –2.4 to 14.5). PIF and PEF of mandatory and assisted breaths increased from baseline in all modes during HFCWC, and the change was significant in all the modes (P < .001) (Fig. 5). The higher the HFCWC pressure, the greater the PEF and, to a lesser extent, the PIF.

Table 2.

The coefficients revealed by regression analysis of the relationship between the difference in respiratory rate during HFCWC and the predictors of ventilation mode, Vest oscillation frequency and Vest pressure

Fig. 5.

A scatter plot to show the increase in peak inspiratory flow (PIF) and peak expiratory flow (PEF) of mandatory and assisted breaths during high-frequency chest-wall compression (HFCWC) in all modes of ventilation. The higher the Vest pressure, the greater the PEF and, to a lesser extent, the PIF.

The PEF increase during HFCWC was larger in SIMV than in the other modes. However, the PIF/PEF of mandatory and assisted breaths increased during HFCWC in SIMV but decreased in all other modes (Fig. 6). In SIMV, the baseline PIF/PEF was 0.62 and increased significantly during HFCWC to between 0.77 and 0.82 (mean difference 0.18, 95% CI 0.16-0.19; P = .008), which indicated that expiratory flow bias occurred at baseline but was increased during HFCWC but was still < 0.9 and hence still favored an expiratory flow bias. At baseline in all other modes, PIF/PEF was at or close to 1, which showed little or no expiratory flow bias: bi-level (1.00), bi-level with pressure support (0.99), bi-level assist (1.01), and PRVC (0.96).

Fig. 6.

A bar chart to show the peak inspiratory flow (PIF) to peak expiratory flow (PEF) ratio of mandatory and assisted breaths at baseline and during high-frequency chest-wall compression (HFCWC). PIF/PEF in synchronized intermittent mandatory ventilation (SIMV) was 0.62 at baseline and increased during HFCWC but remained < 0.9. In bi-level, bi-level with pressure support (PS), bi-level assist, and pressure-regulated volume control (PRVC), PIF/PEF were at or close to 1 at baseline and decreased during HFCWC to < 1 except in bi-level assist, in which a very small increase in PIF/PEF was seen at 10_1 (from 1.01 to 1.03) and at 15_1 (from 1.01 to 1.04). PIF/PEF seemed to be related to the Vest pressure setting in SIMV (10 Hz only) bi-level, bi-level with pressure support, and bi-level assist modes with lower PIF/PEF that would favor an expiratory flow bias at higher Vest pressures. bi-level = bi-level positive airway pressure with set ventilator frequency with no synchronization with patient effort.

During HFCWC, PIF/PEF decreased to < 1 except at bi-level assist, when very small increases occurred at 10 Hz and pressure, 1 (from 1.01 to 1.03) and at 15 Hz and pressure, 1 (from 1.01 to 1.04) (see online supplementary materials for full PIF/PEF data set [see the supplementary materials at http://www.rcjournal.com]). The difference in PIF/PEF from baseline to during HFCWC was statistically significant in the bi-level mode (mean difference 0.07, 95% CI 0–0.13; P = .008), bi-level with pressure support (mean difference 0.09, 95% CI 0.02-0.16; P = .01), bi-level assist (mean difference 0.04, 95% CI –0.02 to 0.11; P = .033), and PRVC (mean difference 0.09, 95% CI 0.03-0.16; P = .007). During HFCWC, the higher the Vest pressure setting, the lower the PIF/PEF in SIMV (10 Hz only) and all the Vest settings in bi-level, bi-level with pressure support, and bi-level assist modes.

In SIMV, bi-level with pressure support and PRVC, in which HFCWC triggered additional pressure-supported breaths, PIF/PEF of the pressure-supported breaths were analyzed and are shown in Figure 7. Only with settings of HFCWC 10 Hz and power of 1 were the PIF/PEF < 0.9, which favors an expiratory flow bias. In all 3 of these modes, PIF/PEF at 13 Hz was > 1.0, hence an inspiratory flow bias and would not facilitate secretion clearance. PIF/PEF of pressure-supported breaths did not seem to be related to the HFCWC pressure used.

Fig. 7.

A bar chart to show the peak inspiratory (PIF) to peak expiratory flow (PEF) ratio during high-frequency chest-wall compression (HFCWC) of pressure-supported breaths triggered in synchronized intermittent mandatory ventilation (SIMV), Bi-level pressure support_PS and pressure-regulated volume control (PRVC). During HFCWC at 13 Hz, PIF/PEF was > 1 but tended to be < 1 at 10 or 15 Hz. The PIF/PEF during HFCWC did not seem to be related to the Vest pressure settings (data from bi-level with pressure support 13_1 missing). bi-level = bi-level positive airway pressure with set ventilator frequency with no synchronization with patient effort.

Regression analysis revealed a relationship between the difference in PIF/PEF of mandatory and assisted breaths at baseline and during HFCWC and mode of ventilation, frequency of oscillation, and HFCWC pressure (adjusted R² = 0.38). There was a statistically significant and positive relationship for all the modes of ventilation for the change in PIF/PEF during HFCWC; bi-level (β = 0.24, 95% CI 0.21-0.28; P < .001), bi-level with pressure support (β = 0.27, 95% CI 0.23-0.31; P < .001), bi-level assist (β = 0.22, 95% CI 0.19-0.26; P < .001), and PRVC (β = 0.28, 95% CI 0.24-0.32; P < .001) (Table 3). The frequency of oscillation at 10 Hz was statistically significant, with a small positive relationship to the change in PIF/PEF (β = 0.03, 95% CI 0.01- 0.06; P = .02). HFCWC pressure at 1 and 5 was negatively related; pressure 1 (β = –0.08, 95% CI –0.11 to –0.05; P < .001) and pressure 5 (β = 0-.06, 95% CI –0.09 to –0.03; P < .001), which reflect the lower PIF/PEF seen at higher HFCWC pressures.

Table 3.

The coefficients revealed by regression analysis of the relationship between the difference in PIF/PEF of mandatory and assisted breaths at baseline and during HFCWC and the predictors of ventilation mode, HFCWC oscillation, and HFCWC pressure

Discussion

To our knowledge, this study is the first to report on the effect of HFCWC on the delivery of mechanical ventilation in an intubated and mechanically ventilated bench model. During HFCWC in SIMV, bi-level with pressure support, bi-level assist, and PRVC, the ventilator-delivered breathing frequency increased significantly from baseline, presumably due to the triggering of additional breaths from the oscillatory flow generated by HFCWC, with the ventilator set with a flow trigger of 2 L/min. The flow trigger setting of 2 L/min is within the range of reported flow trigger settings often used during mechanical ventilation (between 1 and 4 L/min)¹⁵ and hence reflective of real-life conditions in the ICU setting. In the bi-level assist mode, no pressure support is set but additional synchronized pressure-controlled breaths were delivered due to HFCWC. HFCWC had no effect on breathing frequency during the bi-level mode because this ventilator mode is not synchronized with any patient efforts.

The additional ventilator breaths (3- to 7-fold) generated due to HFCWC seen in this study during HFCWC and SIMV, bi-level with pressure support, bi-level assist, or PRVC modes may increase minute ventilation, cause patient-ventilator synchrony, and result in gas trapping. The work by Chuang et al¹² reported on the impact of HFCWC in mechanically ventilated subjects with pneumonia (who may be expected to have increased airway resistance and reduced lung-thorax compliance similar to the mannequin used in this study), and demonstrated significant short-term increases in breathing frequency and minute ventilation during HFCWC delivery (frequency set at 12 Hz), which resolved after the end of the HFCWC therapy.

The values of PIF and PEF recorded in this study demonstrated that higher HFCWC pressure settings increased the expiratory flow to a greater extent than the inspiratory flow, especially during the SIMV mode. However, previous work in healthy subjects has shown that, at higher HFCWC pressure settings, the end-expiratory lung volume is reduced.¹⁶ Higher HFCWC pressure settings have been shown to impair arterial oxygenation in patients with pneumonia who are acutely ill.¹² Of note, HFCWC in the patient who is intubated and ventilated, and required frequent suctioning may improve lung ventilation (as evidenced by electrical impedance tomography), presumably due to the increased secretion clearance. A PIF/PEF of < 0.9 is considered to be effective for mucus movement⁵ in in vitro tube models.

In this study, during HFCWC, effective PIF/PEF were generated by mandatory breaths only in SIMV and PRVC modes. Of note, even though the PIF/PEF was still < 0.9 with HFCWC during SIMV, there was a greater expiratory flow bias (at ∼0.6) during baseline SIMV settings than during HFCWC. The favorable expiratory flow bias during conventional SIMV settings presumably may be due to the low lung compliance settings of the bench model as reported by other investigators, in which low lung compliance leads to an expiratory flow bias, whereas normal-to-high lung compliance leads to an inspiratory flow bias during conventional ventilation.¹⁷ Effective PIF/PEF < 0.9 for mucus clearance were only generated in bi-level, bi-level with pressure support, and bi-level assist modes at higher HFCWC pressure settings. The main limitations in this investigation included the use of an inert mannequin with fixed characteristics for airway resistance (high) and low lung thorax compliance, which may not reproduce the findings in general ICU patients. Hence, these findings may not be transferable to patients on ventilation, especially in patients who are spontaneously triggering ventilator breaths with normal lung compliance.

It is also unclear when HFCWC may be indicated over and above standard chest physiotherapy for secretion clearance. However, recent studies in vivo in patients with pneumonia and on ventilation have replicated the findings in our study with similar increases in breathing frequency during the application of HFCWC (HFCWC frequency of 10 to 12 Hz and a pressure setting of 1 to 2 for 15 min). The ventilator trigger settings during HFCWC may need to be increased to prevent inappropriate triggering of ventilator breaths in ventilator modes such as SIMV, bi-level with pressure support, bi-level assist, or PRVC modes, and this should be investigated in future trials.

The findings of this study suggest that HFCWC may have a significant impact on ventilator breathing frequency, inspiratory-expiratory flows, and flow bias during mechanical ventilation in a bench model with low lung compliance. The findings in this bench model, however, may not be generalizable to all patients in the ICU and we recommend further investigation of the impact of HFCWC in the ICU patient on mechanical ventilation delivery, flow bias, gas exchange, and mucus clearance to determine if there are any clinical benefits and potential risks when using this device.

Conclusions

Ventilator mode, HFCWC oscillation frequency, and HFCWC pressure setting each affected the ventilator breathing frequency, inspiratory-expiratory flows, and flow bias. High ventilator breathing frequency were recorded during HFCWC in SIMV, bi-level with pressure support, and PRVC, with the HFCWC associated with the triggering of additional pressure-supported breaths, whereas in bi-level assist, additional pressure-control assisted breaths were seen. Ventilator breathing frequency was unaffected by HFCWC in the bi-level mode, and this may be the safest mode to use the HFCWC in patients on mechanical ventilation if it is indicated when standard chest physiotherapy proves ineffective. We hypothesize that removing pressure support in SIMV and PRVC modes during HFCWC may allow the HFCWC to be used without triggering additional ventilator breaths, but this requires clinical confirmation.

The PIF/PEF was enhanced by HFCWC, particularly at higher HFCWC pressure settings, although an expiratory flow bias was not consistently seen in this model except in the SIMV mode, hence, it is unclear when HFCWC may be indicated. In all the ventilator modes and HFCWC settings used in this ventilated model, significant changes were seen in the mechanical ventilation delivery. Potential modification of the ventilator mode or settings such as trigger setting, HFCWC oscillation, or pressure settings may be necessary to allow the safer use of HFCWC in mechanically ventilated patients. However, the findings in this bench model with fixed low compliance may not be generalizable to the ICU patient, and we recommend further investigations in bench models that more closely replicate the general ICU patient.