What is the technique for assessing diaphragm excursion using ultrasound (USG) in ICU patients on ventilator support with weaning failure?

Ultrasound Assessment of Diaphragm Excursion in ICU Patients with Weaning Failure

Technique Overview

Diaphragmatic excursion measurement using ultrasound is a recommended basic skill for intensivists evaluating patients with weaning failure, performed using M-mode ultrasound through a subcostal approach during spontaneous breathing trials. 1

Patient Preparation and Positioning

• Position the patient semi-recumbent (30-45 degrees) or supine during the spontaneous breathing trial (SBT), ideally after 30-60 minutes of spontaneous breathing 2, 3
• Ensure the patient is breathing spontaneously, either on T-piece or low-level pressure support (5-8 cmH₂O) 4
• Do not attempt assessment if PaO₂ <55 mmHg on FiO₂ ≥0.40, as weaning should not be contemplated in such patients 1

Probe Selection and Placement

• Use a low-frequency (2-5 MHz) curvilinear or phased-array transducer 2
• Place the probe in the subcostal region, immediately below the costal margin in the mid-clavicular or anterior axillary line 2, 3
• Direct the ultrasound beam medially, cephalad, and dorsally to obtain a longitudinal view of the diaphragm 2

Image Acquisition Steps

B-Mode Identification

• First identify the diaphragm in B-mode as a thick echogenic line between the liver (right) or spleen (left) and the lung 2
• Visualize the diaphragm as it moves cranially during expiration and caudally during inspiration 2

M-Mode Measurement

• Once the diaphragm is clearly visualized in B-mode, switch to M-mode 2, 3
• Position the M-mode cursor perpendicular to the diaphragm at the point of maximal excursion 2
• Record several respiratory cycles (at least 3-5 breaths) to ensure reproducibility 2, 3
• Measure the vertical displacement of the diaphragm from end-expiration to end-inspiration 2, 3

Bilateral Assessment

• Measure both right and left hemidiaphragms separately, as unilateral dysfunction may be missed if only one side is assessed 3, 5
• The right hemidiaphragm is typically easier to visualize due to the acoustic window provided by the liver 2
• Calculate the mean value of bilateral measurements for optimal predictive accuracy 5

Interpretation of Results

Diagnostic Thresholds

• Diaphragmatic excursion <10-14 mm indicates diaphragmatic dysfunction and predicts weaning failure 2, 6
• The most validated cutoff value is 11-12.5 mm, with sensitivity of 89-97% and specificity of 75-82% for predicting successful extubation 3, 5
• Normal diaphragmatic excursion during spontaneous breathing is typically >15-18 mm 2

Clinical Context Integration

• Integrate ultrasound findings with clinical assessment, as recommended by the European Society of Intensive Care Medicine 1
• Consider correlation with other weaning parameters: respiratory rate/tidal volume ratio (RSBI), airway occlusion pressure (P0.1), and cough effectiveness 1
• Ineffective cough strength is independently associated with extubation failure and should be assessed alongside diaphragm function 7

Technical Pitfalls and Solutions

Common Errors to Avoid

• Avoid measuring during assisted ventilation modes with high pressure support, as this artificially reduces diaphragmatic excursion and may underestimate true function 8
• Do not rely on a single measurement; obtain multiple respiratory cycles to account for breath-to-breath variability 2, 3
• Ensure the M-mode cursor is perpendicular to the diaphragm; oblique angles will underestimate true excursion 2

Image Quality Optimization

• If visualization is poor, adjust patient position or try alternative acoustic windows (more lateral positioning) 2
• Obesity, subcutaneous emphysema, or abdominal distension may limit image quality 2
• In patients with severe hyperinflation (COPD), the diaphragm may be flattened and more difficult to visualize 2

Alternative Measurement: Thickening Fraction

While the European Society of Intensive Care Medicine could not provide a recommendation on thickening fraction as a basic skill 1, research suggests:

• Thickening fraction is measured using a high-frequency linear probe (10-15 MHz) in the zone of apposition between the mid-axillary and anterior axillary lines 2, 6
• Calculate as: (end-inspiratory thickness - end-expiratory thickness) / end-expiratory thickness × 100% 2, 3
• Thickening fraction <30-36% indicates dysfunction, with sensitivity of 91% and specificity of 61% 3, 6
• This technique is more technically challenging and requires more advanced training 1

Clinical Application in Weaning Decisions

• Perform diaphragm ultrasound during the spontaneous breathing trial, not while on full ventilatory support 2, 3
• Reduced excursion (<11-12.5 mm) should prompt consideration of delayed extubation and further optimization 3, 5
• Normal excursion (>14 mm) supports proceeding with extubation in patients who pass other weaning criteria 3, 5
• However, one multicenter study found that diaphragmatic dysfunction alone did not predict extubation failure in high-risk patients, emphasizing the need for comprehensive assessment including cough strength 7

Monitoring During Mechanical Ventilation

• Serial measurements can detect diaphragm atrophy in patients undergoing prolonged mechanical ventilation 2, 6
• Diaphragmatic thickening during assisted ventilation correlates with respiratory workload and can guide pressure support titration 8, 2
• Minimizing excessive diaphragmatic loading (over-assistance) and under-loading (under-assistance) may prevent ventilator-induced diaphragmatic dysfunction 8