What is the role of Point-of-Care Ultrasound (POCUS) in assessing diaphragm function in Intensive Care Unit (ICU) patients?

POCUS Diaphragm Assessment in ICU

Point-of-Care Ultrasound (POCUS) is an essential bedside tool for assessing diaphragmatic function in ICU patients, particularly for evaluating weaning readiness from mechanical ventilation and monitoring respiratory workload. 1

Anatomical and Technical Considerations

• Diaphragmatic ultrasound assessment should be considered a basic skill for intensivists due to its non-invasive nature, rapid application, and bedside feasibility 1, 2
• Two primary measurement techniques are used for diaphragmatic assessment:
  • Diaphragmatic excursion (displacement): Measures the caudal movement of the diaphragm during inspiration 1, 3
  • Diaphragm thickness and thickening fraction: Measures the increase in diaphragm thickness during inspiration 2, 3

Clinical Applications

Weaning from Mechanical Ventilation

• Diaphragmatic dysfunction is a major cause of weaning failure and is present in approximately 80% of patients with ICU-acquired weakness 1, 4
• Optimal cutoff values for predicting successful extubation:
  • Diaphragmatic excursion: 10-14 mm 2
  • Thickening fraction: 30-36% 2
• A thickening fraction less than 20% is observed in 70% of patients with ICU-acquired weakness 4

Monitoring Respiratory Workload

• During assisted mechanical ventilation, diaphragmatic thickening serves as an accurate index of respiratory muscle workload 2, 5
• Targeting appropriate diaphragm activity during mechanical ventilation has potential to mitigate diaphragm dysfunction 5

Assessing Diaphragm Atrophy

• POCUS allows reliable monitoring of diaphragm atrophy in patients undergoing mechanical ventilation 2, 6
• Both insufficient and excessive diaphragmatic contractile effort can cause atrophy or injury 5

Advanced Assessment Techniques

• Transdiaphragmatic pressure (Pdi) measurements:
  • Requires simultaneous recordings of esophageal and gastric pressures 1
  • Negative Pga/Pdi ratio indicates severe diaphragmatic dysfunction or paralysis 1
• Diaphragmatic electromyography can quantify crural diaphragm activity and examine precise timing of activation 1
• Newer techniques include shear-wave elastography to evaluate diaphragm muscle stiffness 6

Pitfalls and Limitations

• Incorrect placement of measurement tools can mimic severe diaphragmatic dysfunction 1
• Recruitment of abdominal muscles during expiration followed by sudden relaxation at inspiration can mimic diaphragmatic dysfunction 1
• Diaphragm ultrasound measurements show high reproducibility but require proper technique and interpretation 2, 6
• No large-scale 'real-life' studies are currently available to standardize protocols 6

Clinical Outcomes

• Diaphragm dysfunction is both a marker of illness severity and a predictor of poor patient outcomes in the ICU 5
• In patients with ICU-acquired weakness and diaphragm dysfunction, approximately 50% can still be successfully extubated 4
• Diaphragm dysfunction is poorly correlated with overall ICU-acquired weakness scores, suggesting it requires specific assessment 4