The organism has its way to counter the deleterious effects of certain health problems, such as low blood pressure and low levels of glucose in the blood. Trying to compensate hypoglycemia, the heart needs to create extra glucose, which does not happen fast enough. However, there’s a way to compensate hypotension, and it is by increasing your heart rate. That is why, in most cases, you will see these individuals with very low blood pressure and accelerated heart rate. If you can’t measure a patient’s blood pressure, you can still test how fast is his heart beating, and that would give you an estimate of whether or not his blood pressure might be low.
However, you must keep in mind that many different things may be increasing blood pressure, including anxiety. Also, in some types of patient, blood pressure might be quite low, and their heart does not react by increasing heart rate, or it may do so in a small degree. Thus, the ultimate way to know his blood pressure readings is through a consistent measure made by a doctor.