What is a flow void in a carotid artery?

Mostly because it has no valves, like veins do, so the flow in carotid arteries is a result of left ventricular contraction.

Sometimes a flow in the carotid artery is due to a blockage and may be called a 'flow void' and this is a serious condition which can dislodge a thrombosis (Blood clot)and allow it to flow in the cardiovascular system as an embolism, which is a freely moving blood clot. It can get stuck in the lungs (pneumothorax) or in the brain (stroke).