Quantifying respiratory effort is a key step in physiological and clinical research on exercise tolerance, mechanisms of fatigability, and ventilatory impairment. To date, reliable assessment relies on an invasive method: esophageal pressure (Pes) measurement using a balloon catheter inserted into the esophagus. Although Pes provides an indirect estimate of pleural and intrathoracic pressure, it is uncomfortable for patients, requires dedicated equipment, and may limit study participation.

In critically ill patients, a noninvasive approach to quantify respiratory effort would have major clinical and scientific value. In acute respiratory distress, accurately tracking the intensity and evolution of respiratory effort could support earlier therapeutic decision-making.

Shear Wave Elastography (SWE) is an ultrasound technology enabling very high acquisition rates and estimating tissue stiffness from shear-wave propagation induced by an acoustic impulse. Because muscle stiffness is strongly related to force produced during contraction, we hypothesize that intercostal muscle stiffness measured by SWE correlates with respiratory work and increases with rising inspiratory load.

Inclusion Criteria:

Adult patient with chronic obstructive pulmonary disease (COPD), stage 3-4.

Healthy adult volunteer.

Exclusion Criteria:

Refusal to participate or withdrawal of consent.

Age < 18 years.

BMI > 35 kg/m².

Pregnancy or breastfeeding.

Inability to understand instructions or cooperate (cognitive impairment, language barrier).

Neurological condition affecting breathing (e.g., recent stroke, ALS, myopathy).

Thoracic condition limiting access to the intercostal muscles (e.g., major chest wall deformity, surgical scar).

Recent thoracic or abdominal surgery (< 3 months).

Known esophageal disease (stricture, achalasia, severe esophagitis, diverticulum, history of perforation).

ENT or tracheal abnormality preventing probe insertion (e.g., tumor, malformation, tracheostomy).

Swallowing disorders or risk of aspiration.

Recent exacerbation (< 6 weeks).

Oxygen therapy > 2 L/min during exertion.

Hemodynamic instability.

Not affiliated with a social security scheme.

Legally protected persons (as defined by law).

Adults (≥18 years) will be enrolled in two cohorts: (1) healthy adult volunteers and (2) adult patients with chronic obstructive pulmonary disease (COPD), stage 3-4. A total of 20 participants will be recruited (10 COPD patients and 10 healthy volunteers). Each participant will complete a single study visit (~45 minutes) involving standardized breathing/inspiratory loading while intercostal muscle ultrasound shear wave elastography is recorded, with concurrent physiological reference measurements to quantify respiratory effort.

Individual participant data (IPD) will be made available upon reasonable request. Requests will be reviewed on a case-by-case basis and, if approved, de-identified data and relevant documentation (e.g., protocol and data dictionary) will be shared under an appropriate data use agreement.