Speaker's Highlight

• Don Sin University of British Columbia, St. Paul Hospital (Canada)
  Kenneth R. Chapman Toronto General Hospital Research Institute (Canada)
• Parameswaran Nair McMaster University (Canada)
  Carolyn Calfee UCSF (U.S.A.)
• Gregory P. Downey University of Colorado School of Medicine (U.S.A.)
  David A. Schwartz University of Colorado School of Medicine (U.S.A.)
• Neil Schluger Tuberculosis Control Branch, California Department of Public Health (U.S.A.)
  Nick Kim Critical Care & Sleep Medicine, University of California San Diego (U.S.A.)
• Nicola Hananiah Baylor College of Medicine (U.S.A.)
  Jae-Joon Yim Seoul National University College of Medicine (Republic of Korea)
• Koichiro Asano Tokai University School of Medicine (Japan)
  Diahn-Warng Perng Taipei Veterans General Hospital (Taiwan)
• Konstantinos Kostikas University of Ioannina (Greece)
  Karin Klooster University Medical Center Groningen (Kingdom of the Netherlands)

• Lung Ultrasound for Diaphragmatic Function Assessment in the ICU

  The diaphragm is the principal muscle of ventilation, and its dysfunction in critically ill patients is increasingly recognised as a driver of prolonged mechanical ventilation, failed extubation, and difficult weaning. Bedside ultrasound has emerged as the most practical window into this otherwise hidden structure, allowing intensivists to visualise muscle movement, measure thickness changes, and detect atrophy in real time.

  In Australian intensive care units, where case mix ranges from major trauma in tertiary centres in Sydney and Melbourne to complex sepsis management in regional Queensland hospitals, lung and diaphragm sonography now sits alongside echocardiography as a core point-of-care skill. Growing local training pathways, supported by the College of Intensive Care Medicine and ANZICS-affiliated ultrasound courses, mean that junior clinicians are often the first to acquire and interpret these images at the bedside.

  The Burden of Diaphragm Dysfunction in Critical Care

  Diaphragm weakness develops in more than half of mechanically ventilated patients within the first week of intubation, a phenomenon called ventilator-induced diaphragmatic dysfunction. Contributors include controlled ventilation modes, sepsis-related ICU-acquired weakness, malnutrition, sedation load, and systemic inflammation. The clinical consequences are tangible: extended ICU stays, higher rates of tracheostomy, increased ventilator-associated pneumonia, and elevated mortality.

  Australian data mirror international findings, with single-centre studies from Royal Melbourne and Royal Adelaide hospitals highlighting diaphragm thinning in patients ventilated beyond five days. Recognising dysfunction early matters because interventions such as inspiratory muscle training, careful sedation titration, and protocolised weaning are most effective before atrophy becomes established.

  How Ultrasound Visualises the Diaphragm

  Two complementary windows allow comprehensive assessment. A subcostal or lower intercostal approach on the right, using the liver as an acoustic window, captures the hemidiaphragm in motion and is useful for excursion amplitude and sniff tests. A zone of apposition view, typically at the eighth or ninth rib in the mid-axillary line, visualises the muscle as a three-layered structure, two echogenic fascial lines surrounding the hypoechoic muscle belly, and permits thickness measurements.

  A standard curvilinear or phased array probe with frequencies between 2 and 8 MHz suits both approaches. B-mode is used for anatomy and thickness, while M-mode traces motion over time and quantifies excursion during quiet breathing, deep breathing, and voluntary sniffs. Doppler can occasionally document tissue velocity, though its role remains adjunctive.

  Key Sonographic Parameters

  Several quantitative indices guide interpretation of diaphragmatic function in the ICU setting:

  • Diaphragm excursion in centimetres during quiet breathing, deep breathing, and a sniff manoeuvre
  • Diaphragmatic thickening fraction, calculated as end-inspiratory thickness minus end-expiratory thickness divided by end-expiratory thickness, expressed as a percentage
  • End-expiratory thickness as a surrogate for muscle bulk and atrophy progression
  • Tissue Doppler-derived contraction velocity, used in selected research protocols rather than routine care

  Thickening fraction below roughly 20 to 30 percent and excursion under 1 cm during deep breathing in supine patients have been associated with weaning failure in several adult cohorts. Normal thickness at end-expiration in healthy adults sits around 1.7 to 2.2 mm, with lower values signalling established atrophy.

  Role in Weaning from Mechanical Ventilation

  The most clinically established application is predicting extubation success. Patients who generate reliable excursion, preserve thickening fraction, and tolerate a spontaneous breathing trial with sustained diaphragmatic work are more likely to avoid reintubation within 48 hours. Ultrasound complements traditional weaning indices such as the rapid shallow breathing index by adding a direct muscle assessment, particularly valuable when airway protection or neurological recovery is uncertain.

  Multidisciplinary ICU rounds in Australian tertiary centres increasingly incorporate a brief diaphragm scan into the daily bundle, alongside venous thromboembolism prophylaxis review and sedation holidays. This routine has been associated with shorter ventilation duration in retrospective audits, although large multicentre Australian trials are still awaited. Related recent long-term asthma outcomes research from the regional respiratory community has emphasised the value of longitudinal functional assessment, a principle that translates directly to diaphragmatic monitoring.

  Integration with Broader Point-of-Care Imaging

  Lung ultrasound and diaphragm assessment pair naturally with cardiac, pleural, and venous studies. The same probe switch allows clinicians to evaluate pleural sliding, B-lines, consolidation, and effusion during the same bedside encounter, supporting differential diagnosis of dyspnoea in the post-extubation period or in spontaneously breathing ICU admissions. Protocols such as the BLUE protocol and its derivatives integrate these findings into structured decision algorithms, increasingly taught on Australian simulation courses run through institutions like the Alfred Hospital and Westmead.

  Practical indications for a focused diaphragm study include suspected phrenic nerve injury after cardiac surgery, thoracic trauma, or central line placement, as well as monitoring of neuromuscular disorders including Guillain-Barré syndrome and myasthenic crisis:

  • Suspected phrenic nerve injury following cardiac surgery, thoracic trauma, or central venous access
  • Unexplained weaning failure despite adequate gas exchange and mental status
  • Monitoring of neuromuscular disorders including Guillain-Barré and myasthenic crisis
  • Assessment before and after interventions such as phrenic nerve stimulators or lung volume reduction procedures

  For clinicians building broader point-of-care portfolios, additional ultrasound training resources offer useful cross-disciplinary perspectives on imaging interpretation, although specialty-specific supervision remains essential.

  Limitations and Research Horizons

  Diaphragm ultrasound is operator-dependent, and image quality degrades in obese patients, those with substantial subcutaneous emphysema, and after abdominal surgery where dressings obscure acoustic windows. Reference values vary across populations, ethnicity, and posture, and most published cut-offs derive from small single-centre studies. Australian researchers are contributing to multicentre efforts, including work coordinated through the Australian and New Zealand Intensive Care Society Clinical Trials Group, to standardise protocols and validate thresholds in diverse patient cohorts.

  Emerging applications include contrast-enhanced assessment of perfusion, speckle tracking of diaphragmatic deformation, and integration of ultrasound-derived metrics into machine-learning weaning models. Whether these technologies translate into tangible improvements in ventilation-free days, ICU length of stay, or survival within the Australian healthcare context remains an active area of investigation.

  Embedding diaphragm ultrasound into routine critical care practice moves the discipline beyond lung parenchymal assessment toward true respiratory muscle medicine. For Australian intensivists, the next practical step is consistent documentation, structured registrar training, and participation in collaborative research so that bedside imaging continues to shorten ventilation, reduce failed extubations, and improve outcomes across metropolitan and regional settings alike.

  Richard Russell Nuffield Department of Clinical Medicine, University of Oxford (United Kingdom)
• Mona Bafadhel King’s College London (United Kingdom)
  David Jackson Guy’s and St Thomas’ Hospital, King’s College London (United Kingdom)
• James Chalmers University of Dundee (United Kingdom)
  David Price University of Aberdeen (United Kingdom)