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Acoustic monitoring? ... Sounds good to me!

Quantitative respiratory monitoring traditionally has concentrated on static and summary measures of mechanics and gas exchange. Resistance and compliance, plateau pressure and tidal volume, ventilation dead space and oxygenation indices provide useful information and treatment targets, but all characterize general performance characteristics, provide few clues to the nature of what is causing respiratory compromise, and do not inform us directly of regional distortions of topography (e.g., caused by atelectasis, pneumothorax, pleural effusion, or compressive intrathoracic lesions.) Although the humble stethoscope can help us to some extent in sorting out the character and distribution of such localized abnormalities, we generally rely on static images to provide definitive inputs. (I'll return to that point later.)


Computed tomography (CT) has raised awareness of the regionalized nature of acute lung pathology and of the potential for response to therapeutic intervention. While its precision for such purposes far exceeds that of the routine chest film, CT is not the ultimate answer for tracking the course of rapidly evolving events in critically ill patients,. The price for its valuable help comes steep; CT is costly, exposes the patient to ionizing radiation, and requires transport off the unit. CT cannot be considered a true monitoring technique, but rather an episodic anatomical probe whose acquisition is prompted by a triggering clinical question or event. The image it provides is anatomically detailed but static--in other words, CT possesses superb spatial resolution but poor temporal resolution. Single slice dynamic CT imaging can be accomplished for the breathing cycle, but the image is degraded, incomplete, and not entirely suitable for repeated aquisition.

The clinical need for bedside dynamic imaging that offers both regional detail and temporal resolution has generated considerable enthusiasm for electrical impedance tomography (EIT)--a technology adapted from non-medical fields that takes advantage of the differences in electrical conductivity of gas and fluid. EIT images are lacking the exquisite detail of CT, but they are acquired on a ongoing basis and can yield insights into regional differences in pathology, response to treatment, and the nature of threatening clinical events that involve the airways, parenchyma, and even central pulmonary vessels. The medical literature provides clear examples of EIT's promise in the imaged responses to a recruiting maneuver, pneumothorax, and a variety of other applications. Although considerable development needs to be done before EIT is widely deployed for clinical purposes, there can be little question of its potential.

Ultrasonography, another dynamic imaging modality, has given us bedside imaging of cardiac performance as well as structure. Unlike EIT, ultrasonic methods seem better suited to probing fluid-filled organs and compartments, rather than air-spaces. Although this limitation is generally true, pioneering work by a handful of investigators has shown the diagnostic value of thoracic ultrasonography for imaging diseased lung, as well. Patterns characteristic of pulmonary edema and consolidation have been given such colorful descriptive names as "lung rocket" and "stratosphere sign". Injecting sonic energy in this way has no known iatrogenic consequences and is perfectly well suited to bedside applications. Thoracic ultrasound helps provide the dynamic regional information we seek, but unfortunately, its depth range is limited and does not appear to offer the potential to track any but the most obvious consequences of airspace events. Ultrasonic imaging in true surveillance applications (monitoring, not diagnostics) has not been described.

Could the humble stethoscope--an instrument that passively acquires rather than injects sonic energy--complement these other emerging technologies to address our needs for dynamic regional imaging and surveillance of the diseased lung? I believe that the answer should be a qualified but enthusiastic 'yes'. In a sense, the relatively primitive ability of the stethoscope to localize has been exploited by clinicians listening for the tell-tale signs of pneumonia since the days of its inventor, Rene Laennec. Although the highly portable stethoscope has served medicine admirably with relatively little conceptual modification for the better part of two centuries, our modern ability to detect, filter, classify and record such sonic information has progressed immeasurably since then. Rather than discard the stethoscope in favor of other advanced imaging technologies, it may, at last, be time to re-visit and upgrade our old companion.

The stethoscope's single bell or diaphragm is coupled mechanically to the chest surface, periodically moved to a new locale. The resulting acoustic information depends on the observer's ears and brain for interpretation. What we identify as a wheeze, rhonchus, or crackle--signs that often prompt us to initiate or modify treatments--possess sound wave frequency "signatures" amenable to categorization and quantitation. To a lesser extent the same can be said of bronchial breath sounds and the relative acoustic silence of non-ventilated or poorly aerated zones. What's more, these audible sonic bursts, generated by dynamic events within the breathing cycle, are sharply attenuated by distance from their points of generation. Such geographical distribution of sonic energy allows for the creation of a continuously changing two dimensional acoustic chart that is continually drawn and re-drawn by airflow. With this supercharged alternative to the stethoscope a topographically dispersed array of miniaturized microphone pickups is linked to electronics that quantitate the amplitude, classify the nature of the sounds--or absence of them--and display an acoustic map of the structural or functional abnormalities that gave rise to that distribution of sonic energy. (One such multi-plexed system--vibration response imaging--appears to be nearing the point of commercial entry into the clinical arena. As a candidate for future monitoring technology it seems to offer the advantages of being noninvasive, well tolerated, and capable of ongoing, near-continuous data recording.)

Apart from the sound mapping function, four acoustic characteristics of the recorded signal are especially important: 1) nature; 2) amplitude; 3) timing; and 4) distribution of the detected sounds. A profusion of crackles that begins late in the inspiratory cycle can suggest the need for additional PEEP (either for improving gas exchange or preventing ventilator induced lung injury). Acoustic profiling might indeed be a logical--and relatively efficient-alternative or complement to monitoring oxygenation in setting "best PEEP". On the other hand, sudden change in the geographic distribution of acoustic information in the context of deteriorating gas exchange may suggest main-stem bronchus intubation, pneumothorax or mucus plugging of a major airway. A widely dispersed change in the nature of the acoustic signature suggests the emergence of hypopnea, pulmonary edema or bronchospasm. Rhonchi newly audible throughout the chest, or heard with disproportionate loudness over the trachea or major bronchi warn of secretion retention and the need for airway suctioning. In fact, acoustic monitoring of the airway sounds at the level of the endotracheal tube holds potential as a simple means for prompting suctioning of the intubated airway.

Conversely, the absence of rhonchi may restrain the well-intended caregiver from needless suctioning, with its attendant risks of patient discomfort, mucosal trauma, deteriorated gas exchange, bronchospasm or arrhythmia.induction.

Although simplified acoustic monitoring (such as the 'single pick-up' mounted adjacent to the ET tube) is now ready for immediate deployment in the clinical setting, numerous practical questions and technical details will need to be worked out to unleash the full potential of acoustic monitoring. Some relate to the physics of the listening method. A moment's reflection suggests that the depth of the breath, the pattern of breathing, the inspiratory flow amplitude and the flow profile influence the intensity and nature of the breath sounds generated. PEEP has been shown to alter lung sound distribution and likewise, position influences not only lung volume but also the distribution of tidal airflow. Crackles and wheezes that may be faint or inaudible in the sitting position are sometimes heard in profusion in the supine position. Idiotypical features of lung anatomy and airspace filling influence the projection of sonic energy. The presence of central airway secretions may generate such a profusion of sounds as to mask other noises of more interest or significance. Even when interrogated with equipment capable of sophisticated amplification and filtering, patients with thickened chest walls and those having a need for frequent access to the skin surface present genuine challenges to acoustic monitoring. Although quite feasible in a research environment, maintaining a stable and appropriately positioned array of microphonic pick-ups may prove cumbersome or unachievable in routine clinical practice. Displacement of the mounting bands is an ongoing worry in the frequently manipulated patient. We simply need more experience to be sure.

Acoustic monitoring is a dramatically upgraded but conceptually ancient means of assessing respiratory system performance. Despite numerous technical hurdles, sonic evaluation, once perfected, should offer an innovative--even standard settin-decision support tool. Together with thoracic ultrasound, this emerging clinical option complements our traditional "static' monitoring data with dynamic regional information long needed for optimal care of the ventilated patient.

by John Marini MD

Dr. Marini, Professor of Medicine at the Univ. of Minnesota, is a clinician-scientist whose investigative work has concentrated in the cardiopulmonary physiology and management of acute respiratory failure.
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Author:Marini, John
Publication:FOCUS: Journal for Respiratory Care & Sleep Medicine
Article Type:Clinical report
Geographic Code:1USA
Date:Jul 1, 2009
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