Prior work linking RDW to echocardiographic findings in PE has largely focused on isolated parameters such as TAPSE or PASP and has rarely incorporated modern measures of RV-PA coupling. Whether admission RDW reflects the integrated RV-PA interaction - and not merely contractility or pressure in isolation - has not been adequately addressed. Establishing this link would support RDW as a simple, universally available marker of RV vulnerability at first presentation, and would lay the groundwork for future prognostic and mechanistic studies.

Consecutive adults admitted via the emergency department or transferred to the participating center with confirmed acute PE will be screened by the study team.

1. Demographic and clinical

   • Age, sex, body mass index and Smoking status.
   • Comorbidities: hypertension, diabetes mellitus, coronary artery disease, heart failure, Chronic Obstructive Pulmonary Disease, chronic kidney disease, prior venous thromboembolism, active cancer, recent surgery, recent immobilization, history of COVID -19 infection and vaccination.
   • Current medications relevant to erythropoiesis or hemodynamics (ACE inhibitors/ARBs, beta-blockers, diuretics, iron supplementation, erythropoiesis-stimulating agents, anticoagulants on admission).

2. Laboratory

   • Complete blood count: hemoglobin, MCV, MCH, RDW-CV, RDW, white cell count and differential, platelet count, mean platelet volume.
   • Renal and hepatic function: creatinine, eGFR, AST, ALT, total bilirubin.
   • Cardiac biomarkers: high-sensitivity troponin. 3 Imaging
   • CTPA: anatomic location of thrombus (main, lobar, segmental, subsegmental), presence of saddle embolus, RV/LV diameter ratio on axial imaging.

4.Echocardiography

All echocardiograms will follow a standardized acquisition protocol based on the American Society of Echocardiography and European Association of Cardiovascular Imaging recommendations for RV assessment. Key elements:

• Left lateral decubitus position when tolerated; semi-recumbent if dyspneic.
• ECG-gated digital loops of at least three consecutive cardiac cycles for each view.
• Mandatory views: parasternal long-axis and short-axis; apical four-chamber, RV-focused four-chamber, two-chamber, and three-chamber; subcostal four-chamber and IVC.
• M-mode through the lateral tricuspid annulus for TAPSE.
• Continuous-wave Doppler across the tricuspid valve for peak TR velocity (multiple windows attempted; agitated saline contrast may be used to improve TR signal at operator discretion).
• Tissue Doppler at the lateral tricuspid annulus for S'.
• Speckle-tracking acquisition optimized for RV strain (frame rate 50-80 fps). Studies will be acquired on a single ultrasound platform (or ≤2 platforms with documented inter-platform agreement) and analyzed offline using vendor-neutral software where possible. Studies failing image-quality criteria for the primary outcome will be flagged and the participant excluded from the primary analysis

Inclusion Criteria:

Age ≥ 18 years. Acute PE confirmed by computed tomography pulmonary angiogram (CTPA) performed within 24 h of presentation.

CBC obtained within 24 h of admission and before any blood transfusion. Transthoracic echocardiography feasible within 24 h of admission. Written informed consent.

Exclusion Criteria:

• Active hematologic malignancy or recent chemotherapy with myelosuppressive intent.
• Red blood cell transfusion within the preceding 90 days.
• Known hemoglobinopathy (e.g., sickle cell disease, thalassemia major).
• Chronic dialysis or eGFR < 15 mL/min/1.73 m².
• Pre-existing severe pulmonary hypertension (resting mean PAP ≥ 35 mmHg by prior right heart catheterization or echocardiographic PASP ≥ 60 mmHg before this admission), severe left-sided valvular disease, or known severe biventricular dysfunction.
• Hemodynamic instability requiring vasopressors at the time of echocardiography, as their hemodynamics may distort RV-PA coupling estimates.
• Inadequate echocardiographic image quality precluding measurement of TAPSE and a quantifiable TR jet for PASP estimation.
• Inability to provide informed consent.

Adults, hemodynamically stable with acute pulmonary embolism