How do echoes form inside intubated endotracheal tubes? The human airways are a network of bifurcating branches starting at the trachea and ending at the alveoli—the small sacs where the oxygen and carbon dioxide exchange takes place. An interesting property of the airways is that even though the cross sectional area of each individual segment decreases as the branching depth increases, the total cross sectional area (arrived at by adding up the cross sectional areas of all parallel segments) undergoes a rapid increase after several generations of airway branching. In other words, the airways behave acoustically like a horn with a bell at the end, and sound waves traveling down the airways will be reflected at the bell. The negative pressure wave from this bell-shaped region is used by the system as a reference to which changes in ETT tip position are tracked. The bell begins around the 6th branching generation (approximately 5 cm past the carina in adults).
We are often asked if there is an echo from the carina that can be tracked. Unfortunately, since the additive cross sectional area of the two main stem bronchi is approximately equal to the trachea’s cross sectional area, there are no echoes of significance that arise from the carina.
To illustrate the relationships between cross sectional area, amplitude, and time delay, the figure to the left shows an intubated ETT (top) with its corresponding echo signal, as recorded by the AirWave™ system (bottom). The pressure amplitude is represented on Y-axis and the time delay is represented on X-axis). For each deflection in the echo signal, an arrow denotes the corresponding region in the ETT and airways from which that echo arises.
Relationship of AirWaveTM adapter and intubated ETT (top) to the corresponding echo signal recorded by the AirWaveTM system (bottom).
Referring to the figure, the first echo is a positive deflection (positive pressure) indicating a cross sectional area decrease. This corresponds to the decrease in the nozzle’s diameter from 9 mm to 8 mm. The second echo is a positive deflection immediately followed by a negative deflection, indicating a cross sectional area decrease and then an increase. This echo could be from a small obstruction in the ETT, from a kink in the ETT, or from a patient biting on the ETT. If the echo amplitude were larger, this would correspond to a larger obstruction. The AirWave™ estimates the obstruction size from the echo amplitude and the obstruction location from the echo delay time.
The third echo is a negative deflection indicating a cross sectional area increase. This echo, referred to as the ETT tip echo, is analyzed by the AirWave™ to estimate the passageway size (or effective diameter) around the ETT. A negative deflection echo indicates that the ETT is located in a passageway that has a larger cross sectional area than the ETT. This would be the case for an ETT that is in the trachea. If this echo were to change to a positive deflection, it would indicate that the ETT is located in a passageway that has a smaller cross sectional area than the ETT. This may correspond to an ETT that is in the esophagus or bronchus or that it is clogged at the tip, for example, from mucus.
The last echo, referred to as the airway echo, arises from the bell shaped region in the lower airways. The AirWave™ tracks the time delay of this airway echo, estimating relative changes in the distance between the ETT tip and the airway echo region. For example, if the time delay between the ETT tip echo and the airway echo is decreasing (airway echo moving to the left), then this indicates that the ETT tip is getting closer to the airway echo region or that the ETT is migrating down the trachea.
AirWave™ sounds are audible
While a majority of medical devices that use acoustics operate in the ultrasonic frequency range, the AirWave™ operates in an audible range below 8 kHz because of the frequencies at which tubes behave as waveguides. Since the sounds typically found in the ventilator circuit—such as respiratory sounds, secretion sounds, or cuff leak sounds—can potentially interfere with the echo signals used by the system, a series of advanced data collection algorithms are used to obtain a clean echo signal during ventilation.
When connected to a patient, the AirWave™ collects a majority of its measurements during the quiet period of ventilation between end expiration and inspiration. As a result, the AirWave™ monitor provides updates to the ETT status approximately every patient breath, depending on the level of noise present between breaths. For cases where excessive noise interferes with acoustical measurements such that the ETT status is not updating, the AirWave™ gives the clinician the option to listen directly to the adapter microphones via the monitor speaker. This assists the clinician in determining the interfering noise source so they can rectify it if possible. Examples of interfering noise sources may include: a leaky ETT cuff, secretions in the airway and/or ETT, a high respiratory rate, a nebulizer, or patient coughing.
The expected financial benefits for hospitals using the AirWave™ are summarized in a white paper
. Sources of savings may include reduced mechanical ventilation time, reduced ICU length of stay, reduced chest x-rays, reduced procedures such as suctioning and bronchoscopy, and reduced therapeutic expenses.
SonarMed® is pleased to announce “Partners in Progress” (PIP), a new program designed to foster independent clinical research in the respiratory field.
The PIP program provides opportunities for researchers to conduct independent studies for publication using the SonarMed® AirWave™. Researchers may study the impact of the system on clinical practices and/or outcomes, or they may use the AirWave™ to conduct studies that were previously not possible or practical, but are now enabled by the unique capabilities of the AirWave™.
By increasing the availability of the AirWave™ for research, and by deferring some of the costs associated with acquiring and using the AirWave™ for research, SonarMed® hopes to enable independent research on a wide variety of respiratory topics that might not otherwise move forward. The program is open to any clinician interested in conducting research in the respiratory space.
Clinicians are encouraged to submit study ideas or abstracts to SonarMed® for consideration. If the proposal is accepted into the PIP program, SonarMed® will assist with study costs by providing study equipment on a loan, donation or discount basis. The investigator remains responsible for all other study costs.
the AirWave™ monitor
The AirWave™ monitor is a handheld, portable device with a color LCD display that provides information about the endotracheal tube position, obstruction, and movement. The monitor has sophisticated circuitry for sending, receiving and processing audio signals from the AirWave™ adapter.
SonarMed®’s proprietary algorithms interpret the signals received from the patient’s airway and provide feedback to the clinician through intuitive text and graphics.
The AirWave™ monitor is equipped with a strap for carrying or hanging the unit and a kickstand for propping the unit on a cart or table. The unit can also be accessorized with a mounting clamp to provide even more options for placing it at the patient bedside.
the AirWave™ adapter
The AirWave™ adapter is a small, latex-free, single-patient use disposable device that inserts into the breathing circuit between the ventilator hose and the endotracheal tube, replacing the standard airway adapter. The AirWave™ adapter contains an array of acoustic sensors used for interrogating the artificial airway and communicates with the AirWave™ monitor through a cable. The AirWave™ adapter can be connected to endotracheal tube sizes 6.5 to 9.0 mm ID.
the AirWave™ monitor mounting bracket
The AirWave™ monitor mounting bracket is an accessory to the AirWave™ monitor designed to provide flexibility for positioning the monitor near the patient bedside. The bracket is designed to quickly attach to the back of the AirWave™ monitor. It allows the monitor to be mounted to a round pole (such as an IV stand), or a rectangular rail (such as those found on many ventilators). The bracket also rotates easily between vertical and horizontal orientations, and allows the monitor to be positioned to an optimum viewing angle.
the Standard Adapter Removal Tool (StART)
The AirWave™ StART is a small plastic hand tool designed to assist
in removing a tightly attached adapter (standard or AirWave™) from an
endotracheal tube. This is especially useful when a patient is already
intubated and an attempt to manually remove the adapter could
inadvertently dislodge the ETT. The wedge-based design of the StART
allows the adapter to be removed in one smooth, linear motion rather
than requiring the caregiver to rock, pull, or twist the adapter.