A Holter monitor is a battery-operated portable device that measures and records your heart’s activity (ECG) continuously for 24 to 48 hours. The device is the size of a cell phone. It has wires with electrodes that attach to your skin. Holter monitor helps in detection of asymptomatic and symptomatic arrhythmia at a wide variety of rates for a short duration (typically 1 to 2 days). This test is completely painless.