Diaphragm Ultrasound: Excursion, Thickening Fraction, and Weaning Prediction

Diaphragm dysfunction is common in critically ill and ventilated patients, and it is one of the more overlooked reasons a weaning trial fails. Ultrasound assesses the diaphragm two complementary ways — how far it moves, and how much it thickens — and together they give a genuinely useful, repeatable, radiation-free window into respiratory muscle function.

---

Two Different Questions, Two Different Techniques

Excursion answers: how much does the diaphragm move with each breath? Measured with M-mode from a subcostal window.

Thickening fraction answers: how much contractile effort is the muscle actually generating? Measured with B-mode over the zone of apposition, on the chest wall.

They are not interchangeable — a diaphragm can move well passively (pulled by other respiratory muscles or positive pressure) while contracting poorly, and thickening fraction is the more specific measure of active muscle effort. Most complete assessments use both.

---

Diaphragm Excursion

Technique

Probe: Curvilinear, 2–5 MHz for most adults; a phased array works if a curvilinear isn't available.

Position: Supine or semi-recumbent, whichever the patient tolerates and matches your unit's protocol — position affects the numbers, so stay consistent for serial comparisons.

Right side (preferred — better acoustic window):

Subcostal, anterior axillary to mid-clavicular line, probe angled cephalad and slightly medial. Use the liver as an acoustic window and identify the diaphragm as an echogenic curved line at the liver's superior margin.

Left side: Same approach using the spleen as the window — smaller, and often the harder side to obtain a clean trace on.

The M-Mode Trace

1. Get a clear B-mode image with the diaphragm's brightest, most perpendicular segment in view.
2. Place the M-mode cursor perpendicular to the diaphragm, along its expected excursion path.
3. Have the patient breathe normally for a quiet breathing trace, then repeat with a maximal (deep/sniff) breath if assessing full capacity.
4. Measure the vertical excursion of the diaphragm's caudal movement during inspiration, peak to trough.

Angle matters. An oblique cursor angle produces a foreshortened, unreliable trace — the M-mode line has to run along the true direction of diaphragm movement, not simply straight down.

Normal Values

Measure	Approximate normal
Quiet breathing excursion	1.0–2.5 cm
Deep breathing excursion	4–9 cm (roughly 2–3× quiet breathing)
Sniff maneuver	Should show sharp, brisk downward deflection

Paradoxical movement — the diaphragm moving upward during inspiration instead of down — is a specific sign of paralysis, most reliably demonstrated with the sniff test: a short sharp inhale through the nose. A paralyzed hemidiaphragm typically shows little or no movement, or moves paradoxically upward, while the normal side moves briskly.

---

Diaphragm Thickening Fraction

Technique

Probe: High-frequency linear, 10–15 MHz — this is a superficial measurement.

Position: Zone of apposition — where the diaphragm lies directly against the rib cage rather than against lung. Typically the 8th–10th intercostal space, anterior to the mid-axillary line on the right.

Image: In B-mode, identify the diaphragm as two parallel echogenic lines (pleura and peritoneum) with a hypoechoic muscle layer between them. Measure thickness at end-expiration and end-inspiration.

Calculation

Thickening Fraction (%) = (Thickness at end-inspiration − Thickness at end-expiration) / Thickness at end-expiration × 100

Interpretation

Thickening fraction	Interpretation
< 20%	Suggests diaphragm dysfunction
20–30%	Borderline
> 30%	Generally adequate contractile function

Values vary somewhat across published studies — know which cutoff your unit has adopted, and prioritize the trend over any single measurement.

---

Clinical Applications

Predicting Extubation Success

This is the most established use. Diaphragm ultrasound performed during a spontaneous breathing trial helps predict whether a patient will tolerate extubation:

• Excursion < 1.0–1.1 cm during the trial is associated with higher extubation failure risk in several studies
• Thickening fraction < 20–30% during the trial similarly flags increased risk

Neither measurement alone is a standalone decision tool — both are used alongside standard weaning parameters (rapid shallow breathing index, oxygenation, mental status, secretion burden), not as a replacement for them.

Diagnosing Unilateral Diaphragm Paralysis

Compare excursion side to side. A hemidiaphragm with minimal or paradoxical movement on the sniff test, with a normal contralateral side, supports a diagnosis of phrenic nerve injury or paralysis — common after cardiac surgery, cervical trauma, or as a complication of central line placement or brachial plexus block.

Ventilator-Induced Diaphragm Dysfunction

Diaphragm thickness and thickening fraction can be trended over the course of mechanical ventilation. A declining thickness over days is consistent with disuse atrophy — a recognized complication of prolonged controlled ventilation, and a reason some units use serial diaphragm ultrasound to guide the timing of spontaneous breathing modes.

Diaphragm Dysfunction in Neuromuscular Disease

Excursion and thickening fraction both drop in conditions affecting respiratory muscle strength (myasthenia gravis, Guillain-Barré, ALS), and serial measurement can track disease progression or the effect of treatment.

---

Pitfalls

Positioning inconsistency. Excursion changes with body position — compare serial measurements taken in the same position, or note the difference explicitly.

PEEP and ventilator settings. Positive pressure alters diaphragm position and passive movement — document ventilator settings alongside the measurement, and be cautious comparing excursion values across different support levels.

Obesity and poor acoustic windows, particularly on the left, can make the spleen-window view genuinely difficult — report when a reliable trace could not be obtained rather than forcing an unreliable measurement.

Zone of apposition location varies with lung volume — at very high lung volumes the zone can shift and thin, so extreme hyperinflation can distort the thickening fraction measurement.

---

Key Images to Capture

1. Right hemidiaphragm — B-mode subcostal view showing the diaphragm against the liver
2. M-mode trace — quiet breathing, right side
3. M-mode trace — deep breath or sniff maneuver, right side
4. Left hemidiaphragm — same sequence
5. Thickening fraction — B-mode at end-expiration and end-inspiration, zone of apposition
6. Documentation of patient position and ventilator settings if applicable

---

Reporting Language

"Right hemidiaphragm excursion 1.8 cm with quiet breathing, 6.2 cm with deep inspiration — normal brisk caudal movement on sniff testing. Left hemidiaphragm excursion 0.3 cm with quiet breathing and paradoxical cephalad movement on sniff testing, consistent with left hemidiaphragm paralysis. Thickening fraction on the right measured 34% at the zone of apposition, within the normal range. Findings support unilateral left phrenic nerve dysfunction with preserved right-sided contractile function."

---

SonoBuddy is a reference tool for sonographers. Weaning and extubation decisions belong with the treating clinical team, using ultrasound findings alongside standard parameters.