This prospective, single-center interventional study aims to evaluate the effect of two commonly used intravenous antihypertensive agents - nicardipine and nitroglycerin - on lung ventilation-perfusion (V/Q) distribution in patients with acute respiratory failure complicated by hypertension. Electrical Impedance Tomography (EIT) will be used for noninvasive monitoring of pulmonary ventilation and perfusion distribution before and after drug administration.
The study will compare the changes in V/Q ratio, oxygenation index, and hemodynamic variables after administration of the two drugs. The findings are expected to provide evidence for the optimal antihypertensive strategy in critically ill patients with respiratory failure and to clarify whether specific vasodilators exacerbate or improve ventilation-perfusion mismatch.
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Single group
Nitroglycerin
DrugContinuous intravenous infusion of nitroglycerin for blood pressure reduction in acute respiratory failure. The drug is administered at an initial rate of 5 µg/min, titrated up to 100 µg/min as needed to maintain target mean arterial pressure (MAP 65-85 mmHg). Each infusion period lasts 30 minutes under stable mechanical ventilation. Hemodynamic parameters, arterial blood gases, and electrical impedance tomography (EIT) data are recorded at Baseline, 30 min, and 60 min. A washout phase is observed before crossover to the second intervention.
Nicardipine
DrugContinuous intravenous infusion of nicardipine for blood pressure control during mechanical ventilation. The infusion begins at 1 mg/h and is titrated up to 5 mg/h according to real-time blood pressure monitoring to maintain target MAP 65-85 mmHg. Each infusion period lasts 30 minutes. Electrical impedance tomography (EIT) is used to record ventilation-perfusion matching, dead-space, and shunt fraction at Baseline, 30 min, and 60 min. After completion, a 30-minute washout period or until baseline hemodynamics are restored precedes the next crossover phase.
Change in Ventilation-Perfusion Matching Index
The ventilation-perfusion matching index quantifies the correlation between regional ventilation and perfusion signals derived from EIT imaging. The index ranges from 0 to 1; higher values indicate better matching.
Change in Physiological Dead Space Fraction (Vd/Vt)
Physiological dead space is calculated as (PaCO₂ - PeCO₂)/PaCO₂, supplemented by EIT-based regional ventilation analysis. The expected range is 0 to 0.8; higher values indicate greater alveolar wastage and poorer gas exchange.
Change in Intrapulmonary Shunt Fraction (%)
The shunt fraction represents the percentage of non-oxygenated blood flowing through the lungs without gas exchange. It is computed as Qs/Qt = (CCO₂ - CaO₂) / (CCO₂ - CvO₂), approximated by EIT regional perfusion measurement. Typical range 0-40 %; higher values indicate worse oxygenation efficiency.
Change in Pulmonary Perfusion Distribution
Regional perfusion ratio analyzed via EIT contrast impedance variation after saline bolus indicator; expressed as percentage of dependent to non-dependent lung regions. Higher ratios reflect more homogeneous perfusion.
Change in Pulmonary Ventilation Distribution
Regional ventilation heterogeneity index calculated from EIT ventilation waves; value ranges 0-1, where higher values represent less homogeneous ventilation.
Change in Oxygenation Index (PaO₂/FiO₂ Ratio)
Ratio of arterial oxygen partial pressure to fraction of inspired oxygen, unit mmHg; range ≤ 300 mmHg inclusion criterion; higher values indicate better oxygen exchange.
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