Cardiac Output and Electrocardiogram (ECG)

Cardiac output is the standard quantitative measure of how effectively the heart is actually pumping blood, and it is calculated from two simpler quantities that are each easy to measure or estimate on their own. Stroke volume is the volume of blood ejected by one ventricle in a single beat — roughly seventy millilitres in a resting adult — and heart rate is simply the number of heartbeats occurring per minute, roughly seventy-two at rest. Cardiac output is the product of the two: stroke volume multiplied by heart rate, which at these typical resting values works out to about seventy millilitres multiplied by seventy-two beats, or approximately five thousand millilitres, that is, about five litres, of blood pumped by each ventricle every minute. Because the total volume of blood in an adult body is itself only around five litres, this figure means that, remarkably, the heart pumps a volume equivalent to the entire blood supply of the body around the circulation roughly once every minute at rest — and cardiac output rises considerably above this resting figure during exercise, when both stroke volume and heart rate increase to meet the tissues' greater demand for oxygen.

While cardiac output measures the mechanical, pumping side of heart function, the electrocardiogram, or ECG, is a completely different kind of record: it is a graphical trace of the heart's own electrical activity, picked up non-invasively by electrodes placed on the surface of the body and connected to a recording instrument. Every mechanical contraction of cardiac muscle is triggered by a preceding wave of electrical depolarisation sweeping through that muscle, and it is this electrical signal, not the muscle contraction itself, that the ECG actually detects and records. …