High-frequency chest-wall oscillation (HFCWO) and high-frequency chest-wall compression (HFCWC) are two methods used for airway clearance. Airway clearance techniques fall into two broad categories; unassisted and assisted.1 The word assist in this context means that the respiratory device (eg, ventilator) does work on the respiratory system. One can tell if a device is doing work in the patient by examining the transrespiratory pressure (pressure at the airway opening relative to pressure on the body surface2). Work on the patient is indicated by an increase in transrespiratory pressure associated with flow in the inspiratory direction or a decrease in transrespiratory pressure associated with flow in the expiratory direction.3

Unassisted methods rely on the energy from passive exhalation to generate chest-wall oscillations. In contrast, active devices such as the intrapulmonary percussive ventilator, the various “vest” devices, and external high-frequency external oscillators create either a positive or negative transrespiratory pressure change to generate high- frequency, small-volume oscillations in the airways.

Intrapulmonary percussive ventilation creates positive changes in transrespiratory difference using short, rapid inspiratory flow pulses at the airway opening and relies on chest-wall elastic recoil for passive exhalation. In contrast, HFCWC generates negative changes in pressure, thereby compressing the chest externally to cause short, rapid expiratory flow pulses and relying on chest-wall elastic recoil to return the lungs to functional residual capacity. High-frequency chest-wall oscillation uses both positive and negative pressure changes using a chest cuirass. For any method, the general idea is to augment mucus movement toward the airway opening using a variety of mechanisms.

Oscillation and Compression

HFCWO is applied with a rigid chest cuirass connected to a compressor that can deliver both positive and negative pressures to the chest wall. This design allows the most control over inspiratory and expiratory flow ratios, which, in theory (discussed below), may help to optimize mucus clearance. The earliest HFCWO device operated at frequencies from about 1 to 17 Hz. It offered control of I:E ratio (1:6 to 6:1) and inspiratory pressure (-70 to 70 cm H2O).

Treatment parameters with HFCWO are based on anecdotal evidence. Fink and Mahlmeister4 give the example of a two-phase treatment with one at high frequency (up to 17 Hz) with an I:E ratio of 1:1 followed by a low-frequency (1 Hz) phase with an I:E ratio of 5:1.

HFCWC is implemented by wrapping the chest in an inflatable vest. A compressor rapidly inflates and deflates the vest. During inflation of the vest, pressure is applied to the body surface at approximately 5 to 20 cm H2O, compressing the chest wall slightly and generating a short burst of expiratory flow. These pressure pulses are superimposed on a small positive pressure baseline of about 12 cm H2O. During deflation of the vest, the chest wall recoils to its resting position, causing flow in the inspiratory direction. Vest devices generally operate at 2 to 25 Hz. Manufacturers’ literature suggests that HFCWC can generate volume changes from 17 mL to 57 mL and flows up to 1.6 L per second; these constitute mini coughs to mobilize secretions. There are no clear guidelines on how to use HFCWC, but a typical treatment may last 20 to 30 minutes and consists of short segments at different frequencies separated by huff coughs.

High-frequency chest-wall compression was originally delivered using a square pressure waveform, but this was replaced with a sine waveform.5 There may be significant differences between square, sine, and triangular waveforms in terms of patient volumes and flows, and it may be best to tune each patient/vest combination for optimal secretion clearance.6

HFCWC may cause a decrease in end-expiratory lung volume, but its consequences are not clear.7

Mechanisms of Action

The mucus transport effects of assisted airway clearance techniques have been attributed to numerous mechanisms.8 The most intuitively obvious explanation is that mucus secretion is enhanced by air-liquid shear forces when expiratory flow is higher than inspiratory flow (just as with a normal cough), and HFCWC and HFCWO devices simply stack many mini coughs into one normal-sized passive exhalation. This hypothesis is supported by data from in vitro and in vivo experiments.9,10 King et al10 found that HFCWO at 13 Hz enhanced tracheal mucus clearance in dogs to more than twice the normal rate. The clearance rate was greater when oscillation produced higher expiratory than inspiratory flows. Scherer et al11 developed a mathematical model based on theoretical considerations from previous studies to identify optimal settings for mucus transport:

OCI = [fx(FE,max/TE)/(FI,max/TI)]-f

where: OCI is the oscillatory clearance index, f is oscillatory frequency (in Hz), Fi,max is maximum inspiratory flow, Fe,max is maximum expiratory flow, Ti is the duration of inspiratory airway wall displacement, and Te is the duration of expiratory airway wall displacement.

Note that the final term in the equation, -f, is used to ensure that in the condition where inspiratory and expiratory flows are equal and inspiratory and expiratory times are equal, the OCI will equal zero, indicating no net mucus transport in either direction.

Using this model, we can predict that the higher the expiratory flow, the lower the inspiratory flow, the faster the inward displacement of the airway wall during expiration and the slower its outward displacement during inspiration, the higher the index (and the faster the rate of mucus transport) will be. Equal inspiratory and expiratory flows and wall displacements will cause OCI to be zero.

This model remains untested, but pulsatile expiratory flow exceeds pulsatile inspiratory flow in all forms of high-frequency assisted airway clearance while the patient is exhaling.1 From this observation, we might surmise that the patient should be instructed to prolong exhalation as long as possible to maximize the mucus clearance effect of differential flow rates.

Displacement of the airway wall radially may, itself, help disengage secretions and enhance the effect of air-liquid interaction on mucus movement.12 This is a concept popularized by Forrest Bird, MD, PhD, ScD, in his explanations of IPV.

High-frequency oscillations may, itself, change the rheology of bronchial secretions. One study13 found a frequency-dependent reduction in viscosity with oscillations from 3 to 16 Hz. Another study,14 however, found an increase in viscosity with oscillations from 1 to 8 Hz. A third study15 observed that viscosity decreased after 30 minutes of oscillation at a frequency of 22 Hz.

High-frequency chest-wall compression may enhance ciliary beating,16 but this idea is unsupported by any data. The strength of the ciliary beat may be increased by mechanical resonance (possibly in the range of 11-15 Hz).

Evidence Supporting Use

External high-frequency oscillation was first described in 1993,17 and little has been published concerning this device since then. The precursor to today’s vest devices was introduced in 1983.

Indications for HFCWC

  • Documented need for airway clearance as defined by the AARC clinical practice guidelines.24
    • Evidence of difficulty with secretion clearance.
    • Presence of atelectasis caused by or suspected of being caused by mucus plugging.
    • Diagnosis of disease such as cystic fibrosis, bronchiectasis, or cavitating lung disease.
  • Need for sputum sample for diagnostic evaluation.

One study of HFCWC13 showed that the tracheal mucus rate in dogs could be increased as much as 340% at a HFCWC frequency of 13 Hz. Warwick and Hansen first reported the use of HFCWC in patients with cystic fibrosis (CF).18 These authors suggested that therapy could be tuned using the relation among frequency, volume, and flow and selecting the frequencies that provide the highest flows and the largest volumes. Each of six frequencies was prescribed for 5 minutes for a total treatment time of 30 minutes. The authors reported that treatments resulted in positive effects on forced vital capacity (FVC) and forced expiratory volume in 1 second (FEV1).

Studies evaluating the effects of HFCWO or HFCWC on sputum clearance or pulmonary function are inconclusive, with some showing improved outcomes and some showing no improvement. Some authors believe that HFCWC increases mucolysis, mucus transport, and pulmonary function in patients with CF, while improving their quality of life.7 A more conservative opinion is expressed in the current ACCP evidence-based clinical practice guidelines19:

“In patients with CF, devices designed to oscillate gas in the airway, either directly or by compressing the chest wall, can be considered as an alternative to chest physiotherapy. Level of evidence, low; benefit, conflicting; grade of recommendation, I.”

An argument, however, can be made on the basis of considerations other than airway clearance. Cost is a consideration as suggested in work by Ohnsorg20 that showed a 49% reduction in the total direct expenditures for 23 CF patients after initiation of vest-type therapy. Staff opinion may also be relevant as 80% of respiratory therapists using the vest devices believe they saved time.21 There is some evidence that patients prefer vest therapy over manual CPT,21,22 although the opposite can also be demonstrated. In patients without CF, a randomized controlled trial in patients with amyotrophic lateral sclerosis showed a decrease in symptoms of breathlessness, decreased fatigue, and a trend toward slowing the decline of FVC.23


As far as I can tell, true HFCWO has disappeared due to the unavailability of the oscillator described by Hayek and Sohar.17 High-frequency chest-wall compression, on the other hand, is alive and well.

I believe these indications are reasonable, given the ambiguity of airway clearance treatment in general. In 2001, Hess stated in his landmark review of the airway clearance procedures: “I conclude that there is insufficient evidence to support the use of any secretion clearance technique … At best, the literature on this topic is disappointing.”25

Little has changed in the intervening years. Hess’ opinions are consistent with the previously mentioned ACCP guidelines and a recent Cochrane review,26 which stated:

“This review demonstrated no advantage of conventional chest physiotherapy techniques over other airway clearance techniques in terms of respiratory function. There was a trend for participants to prefer self-administered airway clearance techniques. Limitations of this review included a paucity of well-designed, adequately-powered, long-term trials.”

Given the scarcity of evidence for airway clearance techniques in general, we are forced to rely on theory (first principles) and practical considerations. It seems unlikely that acceptable clinical evidence for assisted (or unassisted) airway clearance techniques will be produced in the future. Apart from cost-effectiveness studies, which are difficult to conduct, there seems to be no other way to distinguish among airway clearance techniques (beyond patient preference). Self-administered airway clearance techniques may give patients a greater sense of independence, and these techniques offer advantages to those who lack the neuromuscular function to perform unassisted airway clearance techniques.27 As well, given the shortage of respiratory therapists in the United States, we need to select the technique that consumes the least amount of caregiver time.

Robert L. Chatburn, BS, RRT-NPS, FAARC, is clinical research manager, Section of Respiratory Care, Department of Pulmonary, Allergy, and Critical Care Medicine, Cleveland Clinic, and associate professor, Department of Medicine, Lerner College of Medicine of Case Western Reserve University, Cleveland.

*Adapted with permission from Chatburn RL. High-frequency asssisted airway clearance. Respir Care. 2007;52:1224-35.


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