From numbers to stages: the relationship between shock index and SCAI classification in acute pulmonary embolism

Abstract Background Acute pulmonary embolism (PE) remains a leading cause of cardiovascular morbidity and mortality. Despite advances in diagnosis and management, early identification of patients at risk of hemodynamic deterioration continues to be a clinical challenge. Risk stratification tools such as the simplified PESI score and imaging parameters provide valuable information but may not always reflect real-time hemodynamic compromise. The Shock Index (SI), defined as the ratio between heart rate and systolic blood pressure, is a simple bedside parameter that has shown prognostic value in several critical conditions. On the other hand, the SCAI classification provides a standardized staging system for cardiogenic shock, ranging from patients at risk (stage A) to those in extremis risk (stage E). However, its application in the setting of acute PE has been scarcely explored. Objectives This study aimed to evaluate the relationship between SI and SCAI classification in patients with intermediate-high or high-risk PE, to assess whether SI could serve as a surrogate marker of hemodynamic severity. Methods A retrospective observational study was conducted including patients admitted to our Coronary Care Unit with a diagnosis of intermediate-high or high-risk PE between January 2019 and December 2024. Demographic, clinical, echocardiographic, and follow-up variables were collected. SI was calculated as the ratio of heart rate to systolic blood pressure. The SCAI classification for cardiogenic shock was stratified into five stages: A (at risk), B (beginning), C (classic), D (deteriorating), and E (extremis). Results A total of 115 patients were included. 56 patients were classified as SCAI A, 38 as SCAI B, 12 as SCAI C, 2 as SCAI D, and 7 as SCAI E. A significant positive correlation was observed between SI and SCAI classification (r = 0.66). The mean SI increased progressively across SCAI stages: A (0.75), B (1.02), C (1.11), D (1.58), and E (1.62). Graphically, the relationship between SI and SCAI showed a clear upward linear trend (p < 0.001). Conclusions In this cohort, the Shock Index demonstrated a strong and significant association with the SCAI classification, highlighting its potential as a rapid and accessible marker of hemodynamic compromise in acute PE. These results support the integration of SI into early risk assessment protocols, particularly in emergency and critical care settings. Given its simplicity and immediate availability, SI may complement established clinical and imaging parameters for stratifying severity and guiding therapeutic decisions.SI across SCAI Classification groups  SI according to SCAI classification