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Narrative Review: Ventilator-Induced Respiratory Muscle Weakness

Martin J. Tobin, MD; Franco Laghi, MD; and Amal Jubran, MD
[+] Article, Author, and Disclosure Information

From the Edward Hines Jr. Veterans Affairs Hospital and Loyola University of Chicago Stritch School of Medicine, Hines, Illinois.

Grant Support: By a Merit Review grant from the Veterans Affairs Health Services Research and Development Service and by a grant from the National Institutes of Health (RO1 NR008782).

Potential Conflicts of Interest: Disclosures can be viewed at www.acponline.org/authors/icmje/ConflictOfInterestForms.do?msNum=M10-0956.

Corresponding Author: Martin J. Tobin, MD, Edward Hines, Jr. Veterans Affairs Hospital, 111 N 5th Avenue and Roosevelt Road, Hines, IL 60141; e-mail, mtobin2@lumc.edu.

Current Author Addresses: Drs. Tobin, Laghi, and Jubran: Edward Hines, Jr. Veterans Affairs Hospital, 111 N 5th Avenue and Roosevelt Road, Hines, IL 60141.

Author Contributions: Conception and design: M.J. Tobin.

Analysis and interpretation of the data: M.J. Tobin, A. Jubran, F. Laghi.

Drafting of the article: M.J. Tobin, A. Jubran, F. Laghi.

Critical revision of the article for important intellectual content: M.J. Tobin, A. Jubran, F. Laghi.

Final approval of the article: M.J. Tobin, F. Laghi.

Obtaining of funding: F. Laghi.

Ann Intern Med. 2010;153(4):240-245. doi:10.7326/0003-4819-153-4-201008170-00006
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Clinicians have long been aware that substantial lung injury results when mechanical ventilation imposes too much stress on the pulmonary parenchyma. Evidence is accruing that substantial injury may also result when the ventilator imposes too little stress on the respiratory muscles. Through adjustment of ventilator settings and administration of pharmacotherapy, the respiratory muscles may be rendered almost (or completely) inactive. Research in animals has shown that diaphragmatic inactivity produces severe injury and atrophy of muscle fibers. Human data have recently revealed that 18 to 69 hours of complete diaphragmatic inactivity associated with mechanical ventilation decreased the cross-sectional areas of diaphragmatic fibers by half or more. The atrophic injury seems to result from increased oxidative stress leading to activation of protein-degradation pathways. Scientific understanding of ventilator-induced respiratory muscle injury has not reached the stage where meaningful controlled trials can be done, and thus, it is not possible to give concrete recommendations for patient management. In the meantime, clinicians are advised to select ventilator settings that avoid both excessive patient effort and excessive respiratory muscle rest. The contour of the airway pressure waveform on a ventilator screen provides the most practical indication of patient effort, and clinicians are advised to pay close attention to the waveform as they titrate ventilator settings. Research on ventilator-induced respiratory muscle injury is in its infancy and portends to be an exciting area to follow.


Grahic Jump Location
Figure 1.
Measurement of transdiaphragmatic pressure in ventilated patients.

Pdi = transdiaphragmatic pressure; Pes = esophageal pressure; Pga = gastric pressure. A. Instrumentation for measurement of Pdi in response to stimulation of the phrenic nerves. Balloon catheters are passed through the nose to record Pes and Pga; Pdi is calculated by subtracting Pes from Pga. A special magnet is used to stimulate the phrenic nerves. B. Decrease in Pes and increase in Pga and Pdi in response to phrenic nerve stimulation (arrows). C. Values of “twitch” transdiaphragmatic pressure in response to stimulation of the phrenic nerves in patients who require mechanical ventilation. The boxed area represents the 95% CI of values obtained in healthy participants. Data represented by open and closed circles are taken respectively from references (2930).

Grahic Jump Location
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Figure 2.
Relationship between respiratory drive and patient respiratory effort.

Patient effort during the time that the ventilator is delivering a breath (measured as inspiratory pressure-time product per breath in cm H2O · s) is closely related to a patient's respiratory drive (measured as dP/dt in cm H2O/s) at the moment that a patient triggers the ventilator (r = 0.78). The inspiratory muscles of a patient who has a low respiratory drive at the time of triggering the ventilator will do very little work during the remainder of inspiration when the ventilator provides assistance. Conversely, the inspiratory muscles of a patient who has a high respiratory drive will expend considerable effort throughout the inspiration, even though the mechanical ventilator is providing assistance. Data are from reference (31).

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Grahic Jump Location
Figure 3.
Waveforms of airway pressure on a ventilator screen.

Airway-pressure waveforms recorded in a patient shortly after the initiation of mechanical ventilation, in a patient making no respiratory effort (controlled mechanical ventilation), and in a patient receiving an appropriate level of assist-control mechanical ventilation. The dotted lines on the left and right waveforms reproduce the tracing achieved by passive, controlled mechanical ventilation as it occurs in a patient receiving neuromuscular-blocking agents. Left. Waveform depicting a patient in respiratory distress who has an excessive work of breathing; this can be inferred from the initial concavity, which results from vigorous inspiratory effort and the spike at the end of ventilator assistance, which is the result of expiratory muscle recruitment. Middle. Waveform depicting a patient making no respiratory effort and, thus, is at risk for ventilator-induced respiratory muscle weakness. Right. Waveform depicting a patient performing an appropriate amount of respiratory work. The small downward dip at the start of the breath indicates the small inspiratory effort required to trigger the ventilator, and the distance between the solid line (actual airway pressure) and the dotted line (expected tracing during controlled ventilation, as in the middle waveform) is proportional to the amount of work done by the patient's inspiratory muscles while the ventilator is providing assistance. The patient in the right waveform is doing much more respiratory work than the patient in the middle waveform and much less work than the patient in the left waveform.

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