(Chapter 5) Footnote 42:

Presumably, the “beats” and the “dilations” were palpated pulses. For Vesalius, pulse was the contraction and dilation of the arterial walls brought about by the transmission of pneuma/anima from the heart via the arterial walls or coats. It was thus not a consequence of blood flow but a capacity to alter the shape and movement of the artery, and hence the amount of blood therein. The man described here could have had atrial fibrillation or electromechanical dissociation; this latter dysrhythmia is usually seen only shortly before death.