The possible reasons for the symptoms of fatigue when the heart rate is almost back to normal include relative decrease in cardiac output, hypokalemia, anemia and other related diseases. 1. Relative reduction of cardiac output: cardiac output is the product of heart rate and stroke volume, when the heart rate increases within a certain range, it can cause cardiac output to increase. When the heart rate increases and returns to the normal state, the cardiac output decreases relative to the increase in heart rate, which can cause temporary weakness after the heart rate returns to normal. 2. Combined with hypokalemia: The main clinical manifestations of hypokalemia include numbness of limbs, fatigue and muscle weakness. When combined with hypokalemia, the heart rate will feel weak and uncomfortable when it returns to normal. 3. Combined with anemia: the main clinical manifestations of anemia include fatigue, dizziness, tinnitus, headache and so on. When combined with anemia, the heart rate increases back to normal, you will also feel weak and uncomfortable. Heart rate increase, fatigue and other symptoms may also be other reasons, it is recommended to consult a doctor in a timely manner, under the guidance of the physician to improve the relevant examination, according to the specific causes of standardized treatment, do not delay the condition, to avoid adverse consequences.