The main reasons for a blood pressure differential of less than 30 mmHg include increased peripheral vascular resistance, increased heart rate, and pericardial effusion. 1. Increased peripheral vascular resistance, the blood flows from the aorta to the periphery during diastole, if the peripheral vascular resistance increases, more blood will stay in the aorta, which will cause the diastolic blood pressure to increase, and if the systolic blood pressure doesn’t change much, then it will lead to a smaller differential pressure. 2. Increased heart rate, the direct result of increased heart rate is that the average systolic time of the heart is prolonged and the average diastolic time is relatively shortened, so that the heart is easy to shoot more blood to the aorta in the same period of time, and the amount of blood in the aorta is increased, which will cause the diastolic blood pressure to rise and the pressure difference to become smaller. 3. Large amount of pericardial effusion. When there is a large amount of pericardial effusion, the volume of blood ejected from the heart to the aorta is reduced due to the limitation of the heart’s filling volume, thus causing the systolic blood pressure to decrease and the diastolic blood pressure not to change much, resulting in the blood pressure differential becoming smaller.