Sometimes patient may experience palpitations, dizziness, chest pain, breathlessness or other symptoms which may not be ruled out in routine EGC. 24 hr Holter monitor testing is ordered when you require more information about the functioning of heart than a routine ECG can give. A Holter monitor is a small of portable, battery-powered medical device that measures the heart’s activity (such as rate and rhythm). This device has electrodes and electrical leads exactly like a regular electrocardiogram. A patient wears the Holter monitor for 12 to 48 hours as they go about their normal daily routines.