What is the formula for calculating cardiac output in a patient with dengue shock?

Cardiac Output Formula

Cardiac output is calculated as heart rate multiplied by stroke volume (CO = HR × SV), and this fundamental relationship guides hemodynamic management in shock states including dengue shock. 1

Basic Formula and Measurement Methods

The core equation for cardiac output is straightforward:

• CO = HR × SV where cardiac output equals heart rate multiplied by stroke volume 1
• Cardiac index (CI) is calculated by dividing cardiac output by body surface area (BSA), reported in L/min/m² 1

Clinical Measurement Approaches

Direct Measurement Methods

Pulmonary artery catheterization with thermodilution remains the gold standard for direct cardiac output measurement in critically ill patients with hemodynamic instability. 2

• Thermodilution involves injecting a known volume and temperature of fluid into the right atrium, measuring temperature change downstream in the pulmonary artery 2
• Modern PA catheters provide semi-continuous cardiac output monitoring without repeated manual injections 2

Echocardiographic Methods

Two distinct approaches exist for calculating cardiac output via echocardiography:

Volumetric method: CO = (EDV - ESV) × HR, where end-diastolic volume minus end-systolic volume equals stroke volume 1

Doppler method: CO = (LVOT cross-sectional area × LVOT velocity time integral) × HR 1, 3

• The Doppler approach measures forward flow through the aortic valve but excludes backward flow from mitral regurgitation 1
• The volumetric method includes total flow (effective flow plus aortic and mitral regurgitation) 1

Target Values in Shock States

Pediatric and Neonatal Septic Shock

A cardiac index between 3.3 and 6.0 L/min/m² is associated with best outcomes in septic shock patients, compared to CI above 2.0 L/min/m² for non-septic shock patients. 1

• Low cardiac output is directly associated with increased mortality in pediatric septic shock 1
• Goal-directed therapy targeting CI >3.3 and <6 L/min/m² improves outcomes in fluid-refractory, dopamine-resistant shock 1

Cardiogenic Shock

Hemodynamic criteria for cardiogenic shock include CI <1.8 L/min/m² without vasopressors/inotropes or CI <2.2 L/min/m² with support. 1

• Cardiac power output (CPO) <0.6 watts indicates refractory shock, calculated as: (CO × MAP) ÷ 451 1, 2

Physiologic Relationships

Understanding the Components

Since CO = HR × SV, therapeutic interventions must address both components strategically:

• When stroke volume is low: Therapies directed at increasing SV often reflexively reduce HR and improve overall CO 1
• When heart rate is inadequate: If HR is too low, CO will be insufficient regardless of stroke volume 1
• When heart rate is excessive: Insufficient diastolic filling time reduces coronary perfusion, decreasing contractility and CO 1

Oxygen Consumption Relationship

The Fick principle relates cardiac output to oxygen consumption:

• V̇O₂ = CO × [C(a-v)O₂] where oxygen consumption equals cardiac output multiplied by arteriovenous oxygen content difference 4
• Rearranged: CO = V̇O₂ ÷ [C(a-v)O₂] for indirect Fick calculations 4
• Mixed venous oxygen saturation >70% suggests adequate cardiac output when hemoglobin is 10 g/dL and arterial saturation is 100% 1

Application to Dengue Shock

In dengue shock specifically, the same fundamental formula applies (CO = HR × SV), but management must account for:

• Capillary leak causing hypovolemia reduces preload and stroke volume, requiring aggressive fluid resuscitation to restore end-diastolic volume 1
• Myocardial dysfunction may occur, necessitating inotropic support when contractility is impaired despite adequate preload 1
• Target cardiac index should be 3.3-6.0 L/min/m² as with other forms of distributive shock 1

Common Pitfalls

• Blood pressure does not equal cardiac output: Normal blood pressure with high vascular resistance indicates reduced CO 1
• Tachycardia alone is insufficient: Children have limited HR reserve; a neonate cannot double heart rate from 140 to 280 bpm like an adult can from 70 to 140 bpm 1
• Measurement timing matters: All pressure measurements should be obtained at end-expiration for accuracy 4