3 Cardiac Output and Cardiac Index

Cardiac output and stroke volume

Cardiac output (CO) is the volume of blood pumped by a ventricle each minute and is usually reported in $L/min$. In a stable circulation, right- and left-ventricular outputs are approximately equal.

Stroke volume (SV) is the volume ejected with each heartbeat. Cardiac output depends on heart rate and stroke volume:

If stroke volume is measured in $mL/beat$ and heart rate in $beats/min$, the product is in $mL/min$. Divide by $1{,}000$ to convert it to $L/min$. For example, $70\ mL/beat$ multiplied by $75\ beats/min$ gives $5{,}250\ mL/min$, or $5.25\ L/min$.

Stroke volume can also be found from ventricular volumes:

Here, $EDV$ is end-diastolic volume and $ESV$ is end-systolic volume.

Adjusting for body size

Cardiac index (CI) adjusts cardiac output for body size by dividing cardiac output by body surface area (BSA):

Cardiac index is reported in $L/min/m^2$. A commonly cited adult resting reference range is approximately $2.5–4.0\ L/min/m^2$, while cardiac output is often approximately $4–8\ L/min$. These values are guides, not stand-alone diagnostic cutoffs: reference ranges vary among sources and patients.

For example, if cardiac output is $5.4\ L/min$ and body surface area is $1.8\ m^2$, then:

Indexing helps compare flow across people of different body sizes, but it does not replace assessment of the patient’s condition and perfusion.

Methods for determining cardiac output

When heart rate and stroke volume are known or estimated, calculate cardiac output using $CO = HR \times SV$. Other methods estimate output using oxygen measurements, temperature change, or echocardiographic flow measurements.

Fick method

The Fick method calculates flow from oxygen consumption and the difference between arterial and mixed-venous oxygen content:

Here, $VO_2$ is oxygen consumption, and the denominator represents arterial oxygen content minus mixed-venous oxygen content. For the result to be in $L/min$, use $VO_2$ in $mL\ O_2/min$ and the oxygen-content difference in $mL\ O_2/L\ blood$. Oxygen content matters; oxygen saturation alone is not the same as oxygen content. Accurate oxygen measurements and a sufficiently steady state are needed, and rapid changes in the patient can reduce accuracy.

Thermodilution

With a pulmonary artery catheter, a known amount of cooler injectate is introduced into the right atrium. A downstream temperature sensor records the resulting change in blood temperature, and the monitor estimates flow from the temperature–time curve. Faster flow produces a shorter, smaller temperature change; slower flow produces a larger, more prolonged change. Technique, rhythm, and conditions such as significant tricuspid regurgitation can affect the result.

Doppler echocardiography

Doppler echocardiography can estimate forward stroke volume through the left ventricular outflow tract (LVOT):

Cardiac output is then calculated as:

The LVOT area is calculated from its diameter, so diameter measurement error can affect the calculated stroke volume. Rhythm irregularity and image quality also matter.

Factors that influence output

Cardiac output depends on heart rate and stroke volume. Stroke volume is influenced mainly by preload, contractility, and afterload, and these effects interact. A change in one factor does not always produce a predictable change in overall output.

• Heart rate and rhythm: A faster heart rate can raise output when the heart can fill and eject effectively. Very rapid rates may shorten filling time and lower stroke volume; bradycardia may also reduce output. Irregular rhythms can make both output and its measurement variable.

• Preload: This describes ventricular filling and stretch at the end of diastole. Within limits, greater filling can increase stroke volume through the Frank–Starling mechanism. Too little filling, such as with blood or fluid loss, may reduce stroke volume. More preload is not always beneficial, especially when the heart is failing or congested.

• Contractility: This is the strength of contraction at a given loading condition. Reduced contractility, as in myocardial injury or heart failure, can lower stroke volume; increased sympathetic stimulation or inotropic support may raise it.

• Afterload: This is the load or resistance the ventricle must overcome to eject blood. Increased afterload can reduce stroke volume, particularly when ventricular function is impaired. Reduced afterload may allow greater forward ejection.

Venous return, vascular tone, temperature, metabolic demand, medications, oxygen delivery, and underlying heart or lung disease can also alter cardiac output, directly or through these determinants.

Interpreting values in context

A cardiac-output or cardiac-index value is one part of assessment, not a complete measure of organ perfusion. Interpret it alongside the measurement method, trends, heart rate and rhythm, blood pressure, mental status, skin temperature, urine output, and other clinical findings.

A changing value or signs of worsening perfusion warrant prompt clinical assessment. Do not infer adequate perfusion from blood pressure or from a single cardiac-output or cardiac-index value alone.

For example, if stroke volume is $65\ mL/beat$ and heart rate is $80\ beats/min$, cardiac output is $5{,}200\ mL/min$, or $5.2\ L/min$. If heart rate rises substantially but stroke volume falls because filling time is reduced, the higher heart rate may not compensate; cardiac output may stay the same or decrease.