It depends because..
• CARDIAC LAG -> when exercise intensity suddenly increases it takes awhile for your heart rate to climb and match the high outputs you are training at, so it not an accurate measurement of your exertion during short high intensity intervals
INSTEAD, use another metric like watts/speed to measure your perceived exertion when performing high intensity intervals that last less than 60 seconds
UNLESS, you are using the monitor to track your recovery capacity, by measuring how it takes for your heart rate to drop
Head Strength & Conditioning Coach at Shakopee High School