The goal of this observational pilot study is to learn if sodium bicarbonate can be used safely and effectively as a contrast agent to map lung blood flow using electrical impedance tomography (EIT) in adults on mechanical ventilation. Electrical impedance tomography (EIT) is a bedside imaging method that uses a soft belt with small sensors around the chest to track changes in electrical signals related to breathing and blood flow.

The main questions are:

Does sodium bicarbonate create clear, readable lung blood flow images with EIT? Are these images similar in quality and pattern to images made with hypertonic saline (10% sodium chloride)? Is the short-term safety profile acceptable, including effects on blood pressure, heart rhythm, and blood tests?

Researchers will compare two contrast agents within the same participant to see if image quality and lung blood flow patterns match:

Hypertonic saline (10% sodium chloride) Sodium bicarbonate (5%)

Participants will:

Have an EIT belt placed around the chest during routine ICU care Pause the ventilator briefly during image capture to reduce motion Receive two small intravenous boluses through an existing central line, one of hypertonic saline and one of sodium bicarbonate, with time between doses Have routine monitoring of vital signs; blood gases and electrolytes may be checked per clinical care Be observed for any short-term side effects Findings from this study will show whether sodium bicarbonate is a practical and safe option for EIT-based lung blood flow assessment and will guide larger future studies.

This single-center, prospective, observational pilot study evaluates sodium bicarbonate as a conductivity contrast agent for electrical impedance tomography (EIT) to assess pulmonary perfusion in mechanically ventilated adults. EIT uses a belt with surface electrodes placed at a standard chest level to measure changes in electrical impedance. A small intravenous bolus of a conductive solution produces a short first-pass signal that can be analyzed to map relative lung blood flow.

Procedure:

Placement: An EIT belt is positioned around the thorax and signal quality is verified.

Acquisition: Perfusion-focused recordings are obtained during brief end-expiratory pauses on the ventilator to limit breathing-related signal changes.

Contrast sequence: Each participant receives two small contrast boluses through a central venous catheter in a prespecified order with adequate washout between injections:

10% sodium chloride (hypertonic saline) 5% sodium bicarbonate Monitoring: Continuous bedside monitoring (for example, heart rhythm and blood pressure) accompanies each acquisition. Blood gases and electrolytes may be obtained in temporal proximity to injections when clinically available.

Data handling and endpoints:

Image quality is assessed by predefined criteria for analyzable recordings and signal strength adequate for first-pass analysis.

Regional perfusion patterns from sodium bicarbonate are compared within participants to patterns from hypertonic saline to evaluate similarity in image quality and distribution.

Short-term safety is described by changes in vital signs and clinically available laboratory values, and by recording adverse events possibly related to the boluses.

This pilot uses a small convenience sample appropriate for feasibility work and is not designed to detect differences in long-term clinical outcomes. Results will inform parameter selection and sample size for subsequent studies that use EIT to assess lung blood flow.

Inclusion Criteria:

• Age ≥ 18 years
• Admitted to the intensive care unit (ICU)
• Receiving invasive mechanical ventilation with controlled or assisted-controlled mode
• Presence of a central venous catheter suitable for contrast bolus injection
• Hemodynamically stable (no ongoing cardiopulmonary resuscitation; no uncontrolled shock judged by treating physician)
• Expected to remain on mechanical ventilation for the duration of the EIT examination
• Written informed consent obtained from the patient or legally authorized representative

Exclusion Criteria:

• Known pregnancy
• Known allergy, intolerance, or contraindication to sodium bicarbonate or hypertonic saline
• Severe arrhythmia or unstable cardiovascular status in which small bolus injection is deemed unsafe by the treating physician
• Uncontrolled agitation or conditions that preclude correct EIT belt placement (e.g., extensive chest wounds, large chest dressings, chest wall deformity that prevents belt use)
• Implanted electronic devices or metallic implants in the thoracic area considered a contraindication to EIT by the manufacturer's instructions
• End-stage disease with expected survival of only a few hours, in whom participation is not appropriate as judged by the treating physician
• Participation in another interventional trial that, in the opinion of the investigator, could interfere with EIT measurements or safety assessment

Critically ill adult patients admitted to a mixed intensive care unit, receiving invasive mechanical ventilation and equipped with a central venous catheter. All participants undergo a single bedside electrical impedance tomography examination with sequential bolus injections of hypertonic saline and 5% sodium bicarbonate for pulmonary perfusion assessment.