What is the significance of cardiac index in an adult patient with a history of cardiovascular disease?

Cardiac Index: Definition and Clinical Significance

Cardiac index (CI) is cardiac output normalized to body surface area (L/min/m²), representing the volume of blood pumped by the heart per minute per square meter of body surface area, and serves as a critical hemodynamic parameter for assessing cardiac pump function and tissue perfusion adequacy in cardiovascular disease. 1, 2

Definition and Calculation

• Cardiac index is calculated as cardiac output (CO) divided by body surface area (BSA), with normal values ranging from 2.5-4.0 L/min/m² 3, 4
• The measurement can be obtained invasively through right heart catheterization or non-invasively via echocardiography by measuring left ventricular outflow tract (LVOT) diameter and velocity time integral (VTI) 4
• CI provides a more accurate assessment than cardiac output alone because it accounts for body size differences between patients 1

Clinical Significance in Cardiovascular Disease

Prognostic Value

• In cardiogenic shock, cardiac power index (CPI)—calculated as mean arterial pressure × cardiac output/451—is the strongest independent hemodynamic correlate of in-hospital mortality (odds ratio 0.60 per 0.20 W increase, p=0.002) 2
• In ambulatory patients with advanced heart failure, diminished CPI (<0.44 W/m²) is associated with a 2.4-fold increased risk of death, cardiac transplantation, or ventricular assist device placement (95% CI 1.8-3.1, p<0.0001) 5
• In septic shock patients, a U-shaped relationship exists between CI and mortality, with optimal survival at CI levels of 1.85-2.8 L/min/m²; both low CI (HR=1.87) and high CI (HR=1.93) significantly increase mortality risk 3

Complementary Role with Ejection Fraction

• The European Society of Cardiology mandates that both ejection fraction (EF) and stroke volume index (SVI) be measured together for comprehensive cardiac assessment, as EF can be preserved while stroke volume is severely reduced 1
• EF is not an index of contractility and depends on volumes, preload, afterload, heart rate, and valvular function—it does not equal stroke volume 1
• Small hypertrophied ventricles can have EF >60% with SVI <30 mL/m², resulting in inadequate tissue perfusion despite "normal" EF 1
• Conversely, dilated cardiomyopathy can maintain adequate stroke volume through increased end-diastolic volume despite EF <40% 1

Clinical Applications

Risk Stratification

• Normal flow is defined as SVI ≥35 mL/m², while low flow is SVI <35 mL/m² 1
• In severe aortic stenosis, patients with SVI <30 mL/m² have significantly worse 1-year and 3-year survival compared to those with SVI >35 mL/m² 1
• The combination of SVI with diagnostic scores shows the highest accuracy for diagnosing heart failure with preserved ejection fraction (HFpEF) 1

Hemodynamic Monitoring

• In septic shock, the combination of heart rate <93.63 bpm and intermediate CI (1.85-2.8 L/min/m²) is associated with the highest survival rates 3
• Patients with heart rate >93.63 bpm and high CI have a 5.77-fold increased risk of 28-day ICU mortality (95% CI 2.98-11.28) 3
• Emergency department physicians can accurately measure CI using bedside focused cardiac ultrasound with 90% optimal LVOT measurements and 78.4% optimal VTI measurements 4

Critical Pitfalls to Avoid

• Do not assume high EF means adequate cardiac output—small hypertrophied ventricles can have EF >60% with critically low SVI <30 mL/m², causing inadequate tissue perfusion 1
• Do not assume low EF means low cardiac output—dilated cardiomyopathy can maintain adequate stroke volume despite EF <40% 1
• Do not rely on a single metric—the 2017 European Heart Journal guidelines emphasize that diagnosis requires integration of multiple parameters including both EF and SVI 1
• Do not ignore age and gender effects—increasing age correlates inversely with CPI (r=-0.334, p<0.001), and women have lower CPI than men (0.29 vs 0.35 W/m², p=0.005) even after age adjustment 2

Practical Implementation

• Calculate and report both cardiac index and stroke volume index routinely in patients with cardiovascular disease, particularly those with heart failure, valvular disease, or shock states 1
• Use CI to guide hemodynamic management in critically ill patients, targeting the optimal range of 1.85-2.8 L/min/m² in septic shock 3
• In cardiogenic shock, monitor cardiac power index as it provides superior prognostic information compared to cardiac output or blood pressure alone 2
• Serial measurements of CI offer clinicians critical clues to declining ventricular performance even without well-established cutoff points for mild versus moderate dysfunction 6