Sentence:  The original electrocardiograph primarily indicates irregularities in the heartbeat , but today 's techniques allow exact measurements of the flow of blood through the aorta , dimensioning of the heart and its chambers , and a much more detailed study of each <WSD>heartbeat</WSD> . 
Meaning of the word marked with <WSD> tags:  the rhythmic contraction and expansion of the arteries with each beat of the heart