Electro-Mechanical-Acoustical Airway Clearance
The EMAAC concept consists of a number of individual operational assumptions that must be considered together, but sequentially, as they apply to the task of promoting airway clearance acoustically.
Let us construct the case for EMAAC on a logical, sequential, step-by-step basis:
Acoustic sound energy is applied to the column of gas in the patient’s airway through a simple mouthpiece. The applied sound energy vibrates the column of gas in the airways during both the inspiratory and expiratory phases of the breathing cycle.
As the patient breathes normally through the mouthpiece attached to the transducer, the acoustic sound waveforms are superimposed over the normal respiratory waveforms and travel throughout the lungs via the conducting airway system.
A portion of the acoustical energy applied to the airway opening, which is vibrating the column of gas in the airways, is also transferred to the airway surfaces and/or the secretions in the airways, using the physical principle of sympathetic resonance.
Sympathetic resonance may or may not vibrate a specific airway itself and/or the airway secretions, depending upon two factors:
the amplitude of applied acoustic energy, and
the frequency of the applied acoustic energy.
The specific amplitude and frequency necessary to vibrate any portion of the airway, or the secretions in the airway, is unknown.
Dogma: The lung is reported to have a Resonant Frequency (RF) of between 5 and 40 Hz. Consequently, it is widely believed that vibrating the lungs and airways at or near their resonant frequency will generate the greatest shear stress at the interface between mucus and the airway surface, and will therefore be most effective.
Dogma is incorrect. The lung has multiple resonant frequencies because it has multiple airways of many different lengths and diameters! Treating the lung with a single frequency is not going to effectively create resonance in all the airways.
For effective airway clearance the lung must be exposed to a wide range of frequencies, starting with very low frequencies and progressing gradually to higher frequencies.
This is the basis of EMAAC and the reason why the Vibralung Acoustical Percussor applies such a wide range of frequencies (5 to 1,200 Hz) in two different kinds of acoustic patterns (random noise and sequential tone pulses).
To make the case for requiring a wide range of frequencies, according to well-accepted physiological treatises that describe the lungs in terms of their corresponding electrical circuitry, the airways of the lungs are essentially a series of impedances (multiple impedances in series). We know that there are 23 divisions in the normal adult human tracheobronchial tract, so there are 23 different “stages,” and countless hundreds of actual bifurcations, at which flow dynamics can change, as gas passes from one division to another.
This anatomical and physiological fact has been summed up nicely in this classic diagram:
Essentially, this chart describes a series of airways that are becoming ever-smaller over 23 generations. Representative values are assigned for generational airway length and diameter, as well as quantity and the resulting cross-sectional area they would cover. From this data, the resistance (R) and compliance or conductance (C), as well as the impedance (L) of each bronchopulmonary division level can be approximated.
In electrical nomenclature, an airway segment is similar to an LCR circuit, where L=impedance, C=conductance (or compliance), and R=resistance. Those familiar with electronics will recognize that the simple LCR circuit shown here is fundamentally an oscillator, or tone generator. An LCR circuit has the following schematic representation (with V being voltage, or pressure change):
The complete tracheobronchial tract, from trachea to alveolus, can be modeled as a series of LCR circuits.
Because LCR circuits can resonate under certain conditions, their RF can be predicted when other parameters are known. When the RFs are measured from such LCR circuit modeling, a progressively higher frequency is rendered as shown on the oscilloscope screen shot below.
Frequency is on the horizontal axis, ranging from 100 Hz to 100 KHz.
And so, the final assumption is that treating the lung with a single vibratory frequency is probably insufficient. Therefore, the Vibralung Acoustical Percussor is designed to treat the lung with a multitude of frequencies that incrementally advance over one of three ranges, from low to high, in a span of 7 minutes per treatment. The use of “random noise” spanning the frequency range of 5 to 1,200 Hz is also utilized at the beginning and end of the incremental advancing frequency sequence, or by itself.
McPeck M. Vibralung Acoustical Percussor: A New Paradigm in Airway Clearance Therapy. Respir Ther 2014; 9(Oct-Nov): 45-47. Scroll to beginning of article on page 45 of the journal.