Title: Validating the Accuracy of Lung Blood Flow Monitoring Without Breath-Holding Under Spontaneous Breathing Support

Purpose: The purpose of this study is to evaluate the accuracy of a new Electrical Impedance Tomography (EIT) technique that measures lung blood flow (perfusion) without requiring patients to hold their breath. We aim to determine if this "non-apnea" method provides results consistent with the current clinical gold standard (the "pause" method).

Background: EIT is a non-invasive bedside tool used to monitor lung function. The traditional method for measuring lung blood flow requires patients to hold their breath for 5-8 seconds during a saline injection to avoid interference from breathing. However, many patients-especially those using a nasal cannula, high-flow nasal oxygen (HFNO), or pressure support ventilation (PSV)-must maintain continuous spontaneous breathing and find it difficult or unsafe to hold their breath. If a "non-apnea" method is proven accurate, it will make monitoring safer and easier for these patients.

What the Study Involves: The study includes stable adult patients who are breathing spontaneously. Each patient undergoes two measurements in a randomized order:

Standard Reference (Pause Method): Measurement taken during a brief, guided breath-hold.

Test Method (Non-Apnea Method): Measurement taken during normal, uninterrupted breathing while receiving respiratory support (Nasal Cannula, HFNO, or PSV).

Expected Significance: By comparing the lung images and ventilation/perfusion (V/Q) data from both methods, this study seeks to validate the reliability and precision of the non-apnea approach. This would allow clinicians to assess lung health in spontaneously breathing patients without interrupting their natural breathing rhythm.

Study Design

This is a prospective, randomized, cross-over, within-subject validation study of accuracy.

The primary objective is to quantitatively assess the agreement between lung perfusion data acquired during uninterrupted spontaneous breathing (non-apnea method) and data acquired during the standard end-expiratory pause (pause method).

Participant Population and Respiratory Support

The study enrolls stable adult patients with active spontaneous breathing efforts.

Participants are categorized by the level of respiratory support they receive: Conventional Oxygen Therapy (Nasal Cannula), High-Flow Nasal Oxygen (HFNO), or Pressure Support Ventilation (PSV).

Conventional Oxygen Therapy includes patients using a nasal cannula or simple mask without pressure support.

HFNO and PSV groups include patients receiving constant high-flow oxygen or patient-triggered mechanical ventilation.

Procedures and Interventions

Each participant receives two 10 mL boluses of 10% NaCl (hypertonic saline) via a central venous catheter.

A 10-minute washout period is maintained between injections.

In the Pause Method, injection occurs during a 5-8 second end-expiratory hold (coached for nasal cannula/HFNO; ventilator-controlled for PSV).

In the Non-Apnea Method, injection occurs during continuous, ongoing breathing without any breathing commands or ventilator setting changes.

Technical Approach to Ensure Accuracy

Specialized algorithms are used to extract saline-contrast signals from the "noise" of spontaneous tidal breaths.

Advanced frequency filtering is applied to separate ventilation-related and perfusion-related impedance changes.

In non-apnea mode, the software identifies the perfusion window using a pixel-triggered approach.

This method detects the impedance drop relative to the End-Expiratory Lung Impedance (EELI) baseline to maintain precision despite moving baselines.

Outcome Measures

The primary outcome is the pixel-wise correlation of perfusion maps between the two methods, assessed by the Spearman correlation coefficient (r).

Secondary outcomes include the agreement of V/Q match%, dead space%, and shunt% using Bland-Altman analysis.

The study also assesses whether the type of support (Nasal Cannula vs. HFNO vs. PSV) affects the accuracy of the non-apnea method.

Feasibility is documented by the success rate of analyzable acquisitions and categorized failure reasons.

Inclusion Criteria:

• Patients aged ≥ 18 years.

Requiring respiratory support including Nasal Cannula, High-Flow Nasal Oxygen (HFNO), or Pressure Support Ventilation (PSV).

Hemodynamically stable.

Able to tolerate a brief respiratory pause (for the reference group).

Signed informed consent.

Exclusion Criteria:

• Pregnancy or breastfeeding.

Contraindications to electrical impedance tomography (e.g., cardiac pacemaker, skin integrity issues on the chest).

Severe circulatory failure or cardiogenic shock.

Known allergy to hypertonic saline.