This study will enroll adult patients (>18 years) with clinically suspected or confirmed acute pulmonary embolism (PE) presenting with acute respiratory failure, who either undergo computed tomography pulmonary angiography (CTPA) within 24 hours or are scheduled for CTPA within the next 24 hours. Eligible participants need to have an indwelling central venous catheter (internal jugular or subclavian vein) or a peripheral forearm venous line, with no restrictions on gender.

This study will enroll patients with clinically suspected or confirmed acute pulmonary embolism (PE) presenting with acute respiratory failure, who undergo electrical impedance tomography (EIT) with saline contrast within 24 hours before or after computed tomography pulmonary angiography (CTPA). The EIT procedure is performed as follows:

1. Breath-Hold Maneuver: (1)Spontaneous breathing patients: Supine position, instructed to hold expiration for ≥8 seconds. (2)Mechanically ventilated patients: Expiratory hold (>8 sec) achieved via ventilator; deeper sedation will be applied when feasible to prolong breath-hold.
2. Venous Access: (1)Preferred: Central venous catheter (internal jugular or subclavian vein).(2)Alternative: Peripheral forearm vein (preferably the median cubital vein).
3. Contrast Administration:(1)Central venous route: 10 mL of 10% NaCl.(2)Peripheral venous route: 15 mL of 5% NaHCO₃.

The participant undergoes a saline contrast EIT examination, after which the distribution of pulmonary ventilation and perfusion will be obtained, respectively. A V-Q map will be subsequently generated. Using offline EIT data analysis tool, the proportion of lung regions with ventilation but without perfusion is calculated, representing the proportion of dead space ventilation (expressed as a percentage). Area under the receiver operating characteristic curve (AUROC), sensitivity, specificity, Youden's index, accuracy, Kappa statistic, positive predictive value (PPV), negative predictive value (NPV), positive likelihood ratio (PLR) and negative likelihood ratio (NLR) will be used to evaluate the ability of proportion of dead space ventilation to diagnose acute pulmonary embolism (PE).

Following a saline contrast EIT examination, the participant's pulmonary ventilation and perfusion distributions will be obtained, allowing for the generation of a V-Q map. Using offline EIT data analysis tool, the proportion of lung regions demonstrating both ventilation and perfusion will be calculated, representing the proportion of V/Q matching (%). Area under the receiver operating characteristic curve (AUROC), sensitivity, specificity, Youden's index, accuracy, Kappa statistic, positive predictive value (PPV), negative predictive value (NPV), positive likelihood ratio (PLR) and negative likelihood ratio (NLR) will be used to evaluate the ability of proportion of V/Q matching to diagnose acute pulmonary embolism (PE).