Patients with pneumonia who require invasive mechanical ventilation in the intensive care unit (ICU) often experience difficulties during the process of being separated from the breathing machine (weaning). Failure of weaning is associated with longer ventilation duration, prolonged ICU stay, and increased risk of complications and death. Therefore, simple and reliable bedside tools are needed to better understand respiratory muscle function and to help predict weaning outcomes.

This prospective, observational, single-center study aims to evaluate respiratory muscle function using bedside ultrasound in adult ICU patients with pneumonia receiving invasive mechanical ventilation. The diaphragm, parasternal intercostal muscles, and anterior scalene muscles will be assessed using ultrasound during the weaning process. Measurements will be performed serially, starting from the first day when patients demonstrate meaningful spontaneous breathing effort and continuing until successful extubation, tracheostomy, or ICU mortality.

Ultrasound measurements of muscle thickness and thickening fraction will be analyzed in relation to weaning outcomes. In addition, ventilator parameters and commonly used weaning indices will be recorded at the time of each ultrasound assessment. The findings of this study are expected to improve understanding of respiratory muscle involvement during weaning and may contribute to earlier identification of patients at risk of weaning failure.

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.

Inclusion Criteria:

• Age ≥ 18 years
• Admission to the intensive care unit with a diagnosis of pneumonia
• Receiving invasive mechanical ventilation via endotracheal intubation
• Expected to undergo a weaning process from mechanical ventilation
• Written informed consent obtained from the patient or legally authorized representative

Exclusion Criteria:

• Known neuromuscular disease affecting respiratory muscles
• High cervical spinal cord injury
• History of major neck, thoracic, or diaphragmatic surgery
• Acute thoracic trauma or pneumothorax
• Central nervous system pathology affecting respiratory drive
• Continuous neuromuscular blocking agent use
• Pregnancy

The study population consists of adult patients aged 18 years and older who are admitted to the intensive care unit with pneumonia and require invasive mechanical ventilation via endotracheal intubation. Patients are followed prospectively during the weaning process from mechanical ventilation and undergo serial bedside ultrasonographic assessment of respiratory muscles as part of observational data collection. All patients receive standard intensive care management as determined by the treating clinical team.

Individual participant data will not be shared due to patient privacy considerations and the absence of a predefined data-sharing agreement for this single-center observational study.