Diaphragm ultrasound
Diaphragm ultrasound has become one of the most useful bedside tools in the ICU. It provides a real-time assessment of diaphragmatic structure and function, helps predict weaning success, detects ventilator-induced diaphragmatic dysfunction (VIDD), identifies phrenic nerve injury, and follows recovery over time.
Two parameters are routinely measured:
- Diaphragm Excursion (DE) → movement of the diaphragm
- Diaphragm Thickening Fraction (DTF or TFdi) → contractility of the diaphragm
Although related, they measure different physiological properties.
Parameter | Measures | Ultrasound Mode | Window |
Diaphragm Excursion | Cranio-caudal movement | M-mode | Subcostal |
Diaphragm Thickness | Muscle thickness | B-mode | Zone of apposition |
Thickening Fraction | Active contraction | B-mode | Zone of apposition |
Anatomy Relevant to Ultrasound
The diaphragm consists of
- Central tendon
- Costal portion
- Lumbar portion
- Crural portion
The portion examined for DTF is the Zone of Apposition (ZOA)
This is where the diaphragm lies against the rib cage.
It is ideal because
- muscle fibers are parallel
- diaphragm is superficial
- thickening can be measured accurately
Diaphragm Excursion (DE)
Diaphragm excursion is The vertical displacement of the diaphragmatic dome during inspiration.
It reflects
- diaphragmatic contraction
- phrenic nerve function
- lung expansion
- respiratory effort
Unlike DTF, excursion mainly measures movement, not muscle contraction.
Technique
Patient Position
Usually Supine (0–30°),Can also perform Semi-recumbent
Probe-Low-frequency,2–5 MHz curvilinear or Phased array
Probe Position
Right Hemidiaphragm
| Left Hemidiaphragm
|
Subcostal | Subcostal
|
Mid-clavicular line | Anterior axillary line
|
Liver acts as acoustic window.
| Spleen acts as window
|
Probe marker Toward head.
|
|
Image Acquisition
- Use B-mode to Locate Bright diaphragmatic line Then switch to M-mode
- Cursor placed perpendicular to diaphragm movement.
M-mode Appearance
- Produces Sinusoidal wave
- During inspiration Upward deflection
- During expiration Downward return
- Excursion measured From end-expiration baseline to peak inspiration
Normal Excursion in Quiet Breathing
Sex | Normal |
Men | 1.8–2.5 cm |
Women | 1.6–2.3 cm |
Average ≈2 cm
Deep Breathing-Normal 3.5–7 cm
Some healthy individuals Up to 9–10 cm
Sniff Test
The sniff test is a dynamic assessment of diaphragmatic function in which the patient performs a short, sharp, forceful inspiration (“sniff”) through the nose while diaphragmatic motion is observed. It is primarily used to diagnose unilateral diaphragmatic paralysis, although it can also identify severe bilateral dysfunction.
Interpretation
Diaphragm Excursion | Interpretation |
Normal | ≈1 cm during quiet (tidal) breathing. Indicates preserved diaphragmatic excursion. (During deep inspiration, normal excursion is substantially greater, typically several centimeters.) |
Mild Dysfunction | 0.8–1.0 cm during tidal breathing. Suggests mildly reduced diaphragmatic contractile function. |
Severe Dysfunction | <0.8 cm during tidal breathing. Indicates markedly impaired diaphragmatic excursion and significant dysfunction. |
Paralysis | Minimal or absent diaphragmatic movement during inspiration. Consistent with diaphragmatic paralysis or profound weakness. |
Paradoxical Motion | Cranial (upward) movement during inspiration instead of the normal caudal descent. Highly suggestive of diaphragmatic paralysis, most commonly due to phrenic nerve injury or complete diaphragmatic dysfunction. |
Causes of Reduced Excursion
Cause of Reduced Diaphragm Excursion | Examples |
Neurological Disorders | Examples:Phrenic nerve injury, Amyotrophic lateral sclerosis (ALS), Myasthenia gravis, Guillain–Barré syndrome (GBS), high cervical spinal cord injury (C3–C5). |
Mechanical Causes | Examples:Pleural effusion, pneumothorax, lung hyperinflation (COPD), ARDS, massive ascites, abdominal compartment syndrome, obesity, pregnancy. |
ICU-Related Causes | Diaphragm weakness due to critical illness or prolonged mechanical ventilation. Examples:Ventilator-induced diaphragmatic dysfunction (VIDD), sepsis, critical illness polyneuropathy (CIP), and critical illness myopathy (CIM). |
Limitations of Excursion
Excursion depends on
- respiratory drive
- accessory muscles
- ventilator support
- chest wall compliance
- abdominal pressure
Therefore Normal excursion ≠ normal diaphragm strength
Diaphragm Thickness
Definition
Distance between Pleural line and Peritoneal line representing diaphragm muscle.
Probe
High-frequency ,7–15 MHz linear probe
Probe Position
Zone of apposition,Usually 8th–10th intercostal space ,Anterior axillary or Mid-axillary line
Appearance
- Three layers
- Pleural line—Hypoechoic muscle—Peritoneal line
- Thickness measured Between Pleural and Peritoneal membranes
Normal Thickness
- End expiration 0.15–0.30 cm(1.5–3 mm)
- Most adults ≈2 mm
- Thickness <1.5 mm Suggests Atrophy
Diaphragm Thickening Fraction (DTF)
Definition
DTF measures
Percentage increase in diaphragm thickness during inspiration.
It reflects Active muscle contraction.
Unlike excursion DTF is less affected by chest wall movement.
Formula
DTF (%) = [(Thickness at end inspiration − Thickness at end expiration) ÷ Thickness at end expiration] × 100
Normal Values
Healthy adults 20–100%
Typical 30–80%
Average ≈40%
Interpretation
Thickening Fraction | Interpretation |
>30–36% | Normal contractility |
20–30% | Borderline |
<20% | Significant dysfunction |
<15% | Severe dysfunction |
0% | Paralysis |
Different studies use 20% or 30% or 36% as cutoffs.
Why Thickening Occurs
Diaphragm contracts—Muscle fibers shorten—Muscle becomes thicker
DTF in Weaning
DTF | Interpretation |
>30% | Good |
>36% | Excellent predictor |
<20% | High failure risk |
Sensitivity Approximately 80–90%
Specificity Approximately 70–90%
Depends on study.
Excursion vs Thickening
Feature | Excursion | Thickening Fraction |
Measures | Movement | Contraction |
Mode | M-mode | B-mode |
Probe | Curvilinear | Linear |
Window | Subcostal | Zone of apposition |
Influenced by ventilator | Yes | Less |
Influenced by respiratory drive | Yes | Less |
Predicts weaning | Good | Better |
Detects VIDD | Limited | Better |
Detects paralysis | Excellent | Excellent |
Which is Better?
Neither replaces the other.
They provide complementary information.
A patient may have:
- Normal excursion but low DTF if ventilator assistance or accessory muscles contribute significantly to diaphragmatic movement.
- Reduced excursion with preserved DTF when diaphragm contraction is relatively intact but movement is mechanically restricted (e.g., obesity, pleural effusion, abdominal distension).
Combining Both Measurements
Excursion | DTF | Interpretation |
Normal | Normal | Normal diaphragm |
Low | Low | Severe dysfunction or paralysis |
Low | Normal | Mechanical restriction (obesity, abdominal hypertension, pleural disease) |
Normal | Low | Ventilator-assisted motion, early VIDD, or accessory muscle contribution |
Role During Spontaneous Breathing Trial (SBT)
Measure during an SBT because it reflects the patient’s true diaphragmatic effort.
Patients with:
- DTF ≥30–36%
- Excursion ≥1–1.2 cm during quiet breathing
generally have a higher likelihood of successful extubation, although ultrasound findings should always be integrated with clinical assessment, gas exchange, cough strength, secretion burden, mental status, and overall readiness to wean.
Pitfalls
- Measure at end-expiration and end-inspiration consistently.
- Use the right hemidiaphragm whenever possible because the liver provides a superior acoustic window.
- Excessive ventilator support can falsely increase excursion without reflecting true diaphragmatic strength.
- Tachypnea and irregular breathing can make end-inspiratory measurements difficult.
- Obesity, subcutaneous emphysema, dressings, chest tubes, and poor acoustic windows reduce image quality.
- Avoid excessive probe pressure over the zone of apposition, which can alter thickness measurements.
- Serial measurements should be performed using the same patient position, probe location, and ventilator settings whenever possible.