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

Diaphragmatic Assessment in the Intensive Care Unit

Diaphragmatic ultrasound assessment is a useful and feasible bedside tool that should be considered a basic skill for intensivists, particularly for evaluating diaphragmatic function in patients being weaned from mechanical ventilation. 1

Importance of Diaphragmatic Assessment

• Diaphragmatic dysfunction has a high incidence in critically ill patients and is an under-recognized cause of respiratory failure and prolonged weaning from mechanical ventilation 2
• Diaphragm weakness, resulting from dysfunction and atrophy, is associated with serious clinical outcomes including increased duration of mechanical ventilation, ICU stay, and mortality 3
• Conventional assessment of diaphragm function lacks specific, noninvasive tools that can be easily performed at the bedside 4

Ultrasound Assessment Techniques

Diaphragmatic Excursion (DE)

• Recommended as a basic skill for intensivists to assess diaphragmatic function, particularly in patients being weaned from mechanical ventilation 1
• Measures the caudal displacement of the diaphragm during inspiration 5
• Optimal cutoffs for predicting successful extubation range from 10 to 14 mm 2
• Moderately correlated with diaphragm strength as measured by phrenic nerve stimulation (r=0.45) 3

Diaphragmatic Thickening Fraction (TF)

• Calculated as (thickness at end-inspiration - thickness at end-expiration)/thickness at end-expiration 2
• Strongly correlated with diaphragm strength (r=0.87) under pressure support ventilation 3
• A TF <29% reliably identifies diaphragm dysfunction with sensitivity and specificity of 85% and 88% 3
• Associated with increased duration of ICU stay, mechanical ventilation, and mortality 3
• No consensus recommendation exists on using TF as a basic skill due to its technical challenges 1

Transdiaphragmatic Pressure Measurements

• Transdiaphragmatic pressure (Pdi) requires simultaneous recordings of esophageal pressure (Pes) and gastric pressure (Pga) 1
• Pdi is obtained by measuring the differential pressure between these two signals 1
• The ratio of inspiratory Pga swings to Pdi helps assess diaphragmatic contribution to tidal effort 1
• A negative Pga/Pdi ratio indicates severe diaphragmatic dysfunction or paralysis 1
• Maximum Pdi (Pdi,max) can estimate diaphragm strength but has a wide normal range (60-240 cm H2O) 1

Clinical Applications in ICU

Weaning from Mechanical Ventilation

• Diaphragmatic dysfunction is a major cause of weaning failure 1
• Ultrasound measurements of diaphragm excursion and thickening fraction can predict extubation success or failure 2
• Maximum relaxation rate of respiratory muscles slows during weaning failure, suggesting respiratory muscle fatigue 1
• Phrenic nerve stimulation with twitch measurements is an appealing method to monitor diaphragmatic contractility, independent of patient effort 1

Monitoring Respiratory Workload

• During assisted mechanical ventilation, diaphragmatic thickening accurately indexes respiratory muscle workload 2
• Minimizing tidal Pdi has been proposed as a target for titrating pressure support ventilation 1
• Diaphragmatic electromyography via esophageal electrode can quantify crural diaphragm activity and examine precise timing of activation 1

Detecting Diaphragm Atrophy

• Ultrasound is a reliable method to assess diaphragm atrophy in mechanically ventilated patients 2
• Monitoring diaphragm thickness changes over time can help guide ventilator strategies to prevent ventilator-induced diaphragm dysfunction 5

Pitfalls and Caveats

• Incorrect placement of the gastric balloon in the lower esophagus can mimic severe diaphragmatic dysfunction when measuring Pga/Pdi ratio 1
• Recruitment of abdominal muscles during expiration followed by sudden relaxation at inspiration can also mimic diaphragmatic dysfunction 1
• Diaphragm ultrasound measurements under assist-control ventilation correlate poorly with diaphragm strength, limiting their utility in this setting 3
• Patient positioning, probe placement, and respiratory effort can significantly affect ultrasound measurements, requiring standardized protocols 6

Emerging Techniques

• Shear-wave elastography can evaluate diaphragm muscle stiffness, which correlates with diaphragm function in healthy individuals 5
• Speckle tracking can assess diaphragm strain, also correlated with function 5
• Magnetic stimulation of phrenic nerves has advantages over electrical stimulation and may increase the feasibility of twitch stimulation in ICU settings 1