You sit down and put your arm in the cradle. You press a button. The machine takes it from there. A near-infrared light sweeps your inner elbow, hunting for a vein. A puff of alcohol hits your skin. An ultrasound probe glides across your arm, mapping how deep the vessel runs and which way it bends. Doppler captures the direction of blood flow to rule out the artery. The cuff tightens around your…

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