Electrical impedance tomography (EIT) has enabled the bedside monitoring of lung perfusion measurement through the indicator-based contrast method. Currently, hypertonic sodium chloride is the most commonly used contrast agent for lung perfusion evaluation by EIT. However, concerns may be raised regarding the potential risks of hyperchloremia and possible acute kidney injury associated with the use of hypertonic NaCl. Recently, two experimental studies found sodium bicarbonate (NaHCO3) might be an alternative to hypertonic sodium chloride in lung perfusion assessment by EIT. However, whether NaHCO3 results in acceptable bias is unknown in critically ill patients. This clinical study aimed to further investigate the correlation and agreement of lung perfusion and Ventilation/perfusion (V/Q) match by EIT contrast method between two contrast indicators (5%NaHCO3 vs. 10% NaCl) in critically ill patients with respiratory failure.

Inclusion Criteria:

• In need of lung perfusion assessment due to clinical condition
• With central venous catheter

Exclusion Criteria:

• contraindications to the use of EIT (automatic implantable cardioverter defibrillator, chest wounds limiting electrode belt placement, implantable pumps, etc.)
• severe hyperchloremia (>155 mmol/L)
• unable to tolerate breath-hold maneuver
• pregnant or breastfeeding females