What is the best explanation for low blood pressure in a patient with a very high heart rate?

The best explanation for low blood pressure in a patient with a very high heart rate is that the increased heart rate leads to decreased ventricular filling time. When the heart beats rapidly, there is not enough time for the ventricles to fill with blood between contractions, which reduces the volume of blood pumped out during each heartbeat (stroke volume).

Low stroke volume directly impacts blood pressure; if less blood is being ejected with each heartbeat, overall blood pressure can drop. This situation is often seen in cases of tachycardia where compensatory mechanisms may be overwhelmed, leading to insufficient cardiac output even though the heart is beating quickly. Therefore, the combination of a high heart rate and low blood pressure can typically be attributed to inadequate filling of the heart, which results in diminished stroke volume and thus lower blood pressure.