Ambulatory ECG pnn50 or PNN50. normal reference value is 16.7 plus or minus 12.3, i.e. between 4.4 and 29. The PNN50 of the ambulatory ECG is the percentage of the number of sinus beats in which the difference between adjacent NNs is greater than 50 milliseconds, and is normally between 4.4 and 29. It is one of the indicators for evaluating heart rate variability. Generally speaking, when the human body encounters tension, emotional excitement, and exertion, sympathetic excitation will reflexively cause an increase in the number of heartbeats, and the number of heartbeats can slowly return to normal after rest. Therefore, PNN50 can reflect the reserve function of the heart to a certain extent. If PNN50 is abnormal, further examination of cardiac enzymes and cardiac ultrasound is needed to find out the cause, and then treatment will be carried out under the guidance of doctors.