How to Calculate Cardiac Output: Step-by-Step Guide

Calculating cardiac output (CO) is a cornerstone of hemodynamic assessment in various medical fields. Understanding how to calculate cardiac output involves knowing the basic cardiac output formula (CO = HR × SV) and appreciating the more complex methods used in clinical practice like the Fick principle, thermodilution, and Doppler echocardiography. This guide provides step-by-step tutorials for these key methods and directs you to our specialized cardiac output calculators.

1. Basic Cardiac Output Calculation (HR x SV)

The simplest way to conceptualize and often estimate CO is using the primary determinants: Heart Rate (HR) and Stroke Volume (SV).

Formula: CO (L/min) = HR (beats/min) × SV (L/beat)

Steps:

1. Determine Heart Rate (HR): Measure the patient’s pulse for one minute or use an EKG monitor. Example: HR = 75 bpm.
2. Determine Stroke Volume (SV): This is the volume of blood ejected by the ventricle per beat. It’s harder to measure directly without specific tools.
   • Echocardiography: SV can be calculated from measurements like Left Ventricular Outflow Tract (LVOT) diameter and Velocity Time Integral (VTI). See our Doppler Echo CO Calculator.
   • Estimation: For a rough estimate, average SV in adults is 60-100 mL/beat. Example: SV = 70 mL/beat (0.070 L/beat).
3. Calculate CO: Multiply HR by SV. Example: CO = 75 bpm × 0.070 L/beat = 5.25 L/min.

While conceptually simple, obtaining an accurate SV is the challenge with this direct approach. Our Stroke Volume Calculator can assist with this.

2. Fick Method for Cardiac Output Calculation

The Fick principle is a highly accurate method, often considered a gold standard, though invasive. It relates oxygen consumption to arteriovenous oxygen difference. You can use our dedicated Fick Method Cardiac Output Calculator.

   

   

(Note: The factor of 10 converts dL to L if CaO2 and CvO2 are in mL O2/dL. If they are in mL O2/L, then CO = VO2 / (CaO2 – CvO2). Our calculator handles units.)

Steps:

1. Measure Oxygen Consumption (VO2): This is the total body oxygen uptake per minute. It can be measured directly with a metabolic cart or estimated (e.g., 125 mL O2/min/m² × BSA or ~3.5 mL O2/min/kg × weight).
2. Measure Arterial Oxygen Content (CaO2): Requires an arterial blood sample. CaO2 = (Hb × 1.34 × SaO2) + (PaO2 × 0.003).
3. Measure Mixed Venous Oxygen Content (CvO2): Requires a mixed venous blood sample (from pulmonary artery). CvO2 = (Hb × 1.34 × SvO2) + (PvO2 × 0.003).
4. Calculate CO: Input these values into the Fick formula.

3. Thermodilution Method for Cardiac Output Calculation

Thermodilution involves injecting a cold saline bolus into a proximal port of a pulmonary artery catheter (PAC) and measuring temperature change downstream. It’s common in ICUs. Our Thermodilution CO Calculator explains this further.

Principle: Based on the Stewart-Hamilton equation, which relates the amount of indicator (cold) injected to the change in blood temperature over time.

Steps (Simplified):

1. Catheter Placement: A PAC is inserted, with ports in the right atrium/vena cava (for injection) and pulmonary artery (for temperature sensing).
2. Injection: A known volume of cold saline at a known temperature is injected rapidly.
3. Temperature Curve: A thermistor at the PAC tip records the temperature change in pulmonary artery blood. This generates a “thermodilution curve.”
4. Calculation: A dedicated cardiac output monitor computes CO from the area under this curve and other factors (injectate volume/temperature, blood temperature, computation constant).

This method provides intermittent CO measurements. Continuous methods (CCO) also exist. For more on advanced techniques, see advanced CO monitoring.

4. Doppler Echocardiography for Cardiac Output Calculation

A non-invasive method using ultrasound to measure blood flow velocity and dimensions of cardiac structures. Our Doppler Echo CO Calculator is designed for this.

Key Measurements:

• LVOT Diameter (D): Measured from parasternal long-axis view. Used to calculate LVOT cross-sectional area (CSA = π × (D/2)²).
• LVOT Velocity Time Integral (VTI): Measured using pulsed-wave Doppler in apical 5-chamber or 3-chamber view. Represents distance blood travels per beat.

Steps:

1. Obtain LVOT Diameter: Measure accurately during mid-systole.
2. Obtain LVOT VTI: Trace the spectral Doppler envelope of LVOT flow.
3. Calculate Stroke Volume (SV): SV (mL) = CSA (cm²) × VTI (cm).
4. Measure Heart Rate (HR).
5. Calculate CO: CO (L/min) = (SV (mL) × HR (bpm)) / 1000.

Choosing the Right Method

The choice of method depends on availability, patient condition, need for continuous vs. intermittent data, and invasiveness tolerance. Each has pros and cons regarding accuracy and application. Our article on Doppler vs. Fick and the general calculator comparison can provide more insight.

For a deeper dive into all cardiac output formulas, please visit our Cardiac Output Formulas Explained page. For further reading from external medical resources, the PubMed Central database is an excellent source of peer-reviewed articles on hemodynamic monitoring.

Remember, all calculations should be interpreted within the patient’s full clinical context. Consult reputable medical sources like Mayo Clinic for general information on heart conditions.