What is the approach to diaphragmatic assessment and management in the Intensive Care Unit (ICU)?

Diaphragmatic Assessment in the ICU

Diaphragmatic ultrasound assessment should be a core skill for intensivists, with estimation of diaphragmatic excursion being particularly valuable for evaluating diaphragmatic function in mechanically ventilated patients and during weaning. 1

Importance of Diaphragmatic Assessment

The diaphragm is vulnerable to injury during mechanical ventilation, and diaphragm dysfunction is both a marker of illness severity and a predictor of poor outcomes in ICU patients 2. Diaphragmatic dysfunction:

• Has high incidence in critically ill patients
• Is an under-recognized cause of respiratory failure
• Contributes significantly to prolonged weaning from mechanical ventilation 3
• Can develop rapidly during mechanical ventilation (ventilator-induced diaphragmatic dysfunction)

Ultrasound Assessment Techniques

1. Diaphragmatic Excursion Measurement

• Technique: Using M-mode ultrasound to measure the caudal displacement of the diaphragm during inspiration
• Normal values: 10-14 mm during normal breathing
• Clinical significance: Excursion <10 mm suggests diaphragmatic dysfunction 3
• Application: Particularly useful for weaning assessment

2. Diaphragm Thickness (Tdi) Measurement

• Technique: Measuring diaphragm thickness at end-expiration using B-mode ultrasound
• Location: Zone of apposition at the mid-axillary line between 8th-10th intercostal spaces
• Clinical significance: Progressive reduction in thickness suggests atrophy

3. Thickening Fraction (TF) Assessment

• Formula: TF = [(End-inspiratory thickness - End-expiratory thickness)/End-expiratory thickness] × 100%
• Clinical significance: TF <30-36% predicts weaning failure 3
• Note: While valuable, the European Society of Intensive Care Medicine (ESICM) notes that consensus is lacking on whether TF should be considered a basic skill for intensivists 1

Pressure-Based Measurements

Transdiaphragmatic Pressure (Pdi)

• Technique: Requires simultaneous recordings of esophageal (Pes) and gastric (Pga) pressures
• Formula: Pdi = Pga - Pes
• Normal values: Pdi,max varies between 60-240 cmH2O 1
• Clinical applications:
  • Absolute Pdi swings reflect mechanical coupling between respiratory muscles and respiratory system
  • Pdi,max estimates diaphragm strength
  • Pdi during sniff maneuver is simpler but not well validated in intubated patients 1

Pga/Pdi Ratio

• Purpose: Assesses diaphragmatic contribution to tidal breathing
• Interpretation: Higher ratio indicates greater diaphragmatic contribution
• Typical values: Often around 20% in ICU patients, indicating accessory muscle recruitment 1
• Warning sign: Negative Pga/Pdi ratio indicates severe diaphragmatic dysfunction or phrenic nerve injury 1

Clinical Applications in ICU

1. Weaning from Mechanical Ventilation

• Diaphragmatic excursion of 10-14 mm and TF >30-36% predict successful extubation 3
• Monitoring diaphragm function helps identify patients at risk for weaning failure
• Serial measurements can track recovery of diaphragm function

2. Monitoring Ventilator-Induced Diaphragm Dysfunction

• Regular ultrasound assessment can detect early diaphragmatic atrophy
• Helps guide ventilator adjustments to prevent further dysfunction
• Both insufficient and excessive diaphragmatic contractile effort can cause atrophy or injury 2

3. Patient-Ventilator Interaction Assessment

• Helps detect ineffective triggering efforts
• Identifies asynchronies that may contribute to diaphragm dysfunction
• Can guide ventilator adjustments to improve synchrony 1

Implementation in ICU Practice

• Diaphragmatic ultrasound should be included in the basic ultrasound skills curriculum for intensivists 1, 4
• Recommended as part of the first 4 weeks of critical care ultrasound training 4
• Serial assessments provide more valuable information than single measurements
• Documentation should include both quantitative measurements and qualitative observations

Pitfalls and Limitations

1. Technical challenges:

   • Incorrect placement of gastric balloon in the lower esophagus can mimic severe diaphragmatic dysfunction 1
   • Abdominal muscle recruitment during expiration followed by sudden relaxation can falsely suggest diaphragmatic dysfunction 1

2. Interpretation issues:

   • Pga is influenced by abdominal compliance, which varies with position and abdominal muscle tone 1
   • Wide range of normal values for Pdi,max (60-240 cmH2O) complicates interpretation 1

3. Patient factors:

   • Obesity and high PEEP can limit visualization
   • Patient cooperation is required for some assessments

By incorporating diaphragmatic assessment into routine ICU practice, clinicians can better identify patients with diaphragm dysfunction, optimize ventilator settings, improve weaning strategies, and potentially reduce ICU length of stay and mortality.