What Causes Numbness And Pins And Needle In Arm After Vigorous Exercise?

Following periods of vigorous exercise (on my road-bike, exercise bike, or cross-trainer) I get numbness and pins-and-needles in my left hand. This goes when I shake my arm about and let my arm hang. Why does this affect just my left hand? Should I be concerned about my cardiac health? Thanks in advance for your comments / feedback