This study is designed as a prospective, observational, single-center cohort study conducted in the intensive care unit (ICU). Adult patients (≥18 years) admitted to the ICU with pneumonia and receiving invasive mechanical ventilation via endotracheal intubation will be eligible for inclusion. Patients with known neuromuscular diseases, high cervical spinal cord injury, prior major neck, thoracic, or diaphragmatic surgery, acute thoracic trauma, pneumothorax, central nervous system pathology, or those receiving continuous neuromuscular blockade will be excluded.
Patients will be followed from ICU admission until one of the following outcomes occurs: successful extubation, tracheostomy, or ICU mortality. The first 24 hours after ICU admission will be defined as ICU Day 1. Disease severity and baseline characteristics will be assessed on ICU Day 1 using the APACHE II, SOFA, frailty index, and Charlson Comorbidity Index scores. Sedation level will be evaluated daily using the Richmond Agitation-Sedation Scale (RASS).
The first day on which the patient demonstrates meaningful spontaneous breathing effort, defined as a RASS score of -3 or higher, will be designated as Measurement Day 0. Ultrasonographic assessments of respiratory muscles will be performed daily for the first three days starting from Measurement Day 0 and every 48 hours thereafter until extubation or another study endpoint is reached. Days with deep sedation (RASS -4 or -5) will be recorded, and only structural muscle thickness measurements will be obtained on those days.
All ultrasound measurements will be performed at the bedside in the supine position under standardized conditions. Ventilator settings will be kept unchanged for at least 30 minutes prior to measurements, and no airway suctioning or position changes will occur within 15 minutes before data acquisition. For each parameter, measurements will be averaged over three consecutive respiratory cycles obtained during a stable ventilation period.
Diaphragm assessment will be performed on the right hemidiaphragm using a low-frequency convex probe at the zone of apposition between the 9th and 10th intercostal spaces along the anterior axillary line. End-expiratory diaphragm thickness will be recorded on all measurement days. On days with RASS ≥ -3, end-inspiratory thickness will also be measured, and diaphragm thickening fraction will be calculated.
Parasternal intercostal muscles will be evaluated using a high-frequency linear probe at the right second and third intercostal spaces along the parasternal line. End-expiratory thickness will be measured on all days, and thickening fraction will be calculated on days with adequate spontaneous breathing effort.
Anterior scalene muscle assessment will be performed using a linear probe at the C6 level, positioned deep to the sternocleidomastoid muscle and lateral to the carotid artery. End-expiratory thickness will be measured daily, and thickening fraction will be calculated on days with RASS ≥ -3.
During each ultrasound session, ventilator mode, pressure support (if applicable), positive end-expiratory pressure (PEEP), fraction of inspired oxygen (FiO₂), respiratory rate, tidal volume, minute ventilation, and peripheral oxygen saturation will be recorded simultaneously. When available, indices reflecting respiratory drive and inspiratory muscle strength, including P0.1, negative inspiratory force (NIF), rapid shallow breathing index (RSBI), and the compliance-rate-oxygenation-pressure (CROP) index, will also be documented. Intra-abdominal pressure will be measured using the intravesical technique at predefined time points.
Ultrasonographic respiratory muscle parameters and their temporal changes will be analyzed in relation to weaning outcomes, with the aim of identifying markers associated with weaning failure.
