What is cardiac output?

Cardiac Output Definition and Measurement

Cardiac output is the volume of blood pumped by the heart per unit time, expressed in liters per minute (L/min), and is calculated as the product of stroke volume and heart rate. 1

Definition and Components

• Cardiac output represents the volume of blood ejected from the heart each minute and is a fundamental parameter in assessing cardiac function 1
• It is calculated using the formula: Cardiac Output = Stroke Volume × Heart Rate 1
• Normal resting cardiac output typically ranges from 4-6 L/min, depending on body size 1
• Stroke volume, a key component of cardiac output, is the amount of blood ejected from the heart with each contraction (normally 80-90 mL at rest in a 70-kg individual) 1

Physiological Determinants

• Cardiac output is dependent on four primary factors: heart rate, contractility, preload, and afterload 2
• Heart rate directly affects cardiac output by determining how many times per minute the stroke volume is ejected 1
• Preload (ventricular filling) influences stroke volume through the Frank-Starling mechanism 2
• Contractility represents the intrinsic strength of cardiac muscle contraction independent of loading conditions 2
• Afterload (resistance against which the heart pumps) affects the volume of blood ejected with each contraction 2

Measurement Methods

• The thermodilution method using a pulmonary artery catheter has traditionally been considered the gold standard for cardiac output measurement 3
• Modern non-invasive or minimally invasive methods include:
  • Doppler echocardiography (measuring left ventricular outflow tract velocity and diameter) 1, 4
  • Fick principle (using oxygen consumption or carbon dioxide production) 5, 6
  • Pulse contour analysis (analyzing arterial pressure waveforms) 3, 6
  • Thoracic bioimpedance/bioreactance (measuring changes in thoracic electrical conductivity) 5, 3
  • Rebreathing of inert gases 5

Clinical Significance

• Cardiac output measurement is fundamental to the diagnosis, treatment, and prognostic evaluation of all heart diseases 5
• It provides critical information about tissue perfusion and oxygen delivery 3
• Cardiac output can be indexed to body surface area (cardiac index) to normalize values across different body sizes 1
• Measurement of cardiac output is particularly important in critically ill patients and high-risk surgical patients 6

Calculation Methods in Clinical Practice

• Volumetric method: Cardiac output = (End-diastolic volume - End-systolic volume) × Heart rate 1
• Doppler method: Cardiac output = LVOT cross-sectional area × LVOT velocity time integral × Heart rate 1, 4
• The cardiac power (measured in watts) can be calculated as: Cardiac output × Mean arterial pressure ÷ 451 1

Clinical Applications

• Assessment of overall cardiac function in patients with suspected cardiovascular disease 7
• Evaluation of hemodynamic status in critically ill patients 3, 6
• Guiding fluid management and vasopressor/inotropic therapy 2, 3
• Monitoring response to therapeutic interventions 2
• Cardiopulmonary exercise testing to assess functional capacity 1, 5