Every measurement considered abnormal has a physical correlate, a physical feature that can be demonstrated to be the reason for the abnormal measurement. Some physical correlates are obvious, body weight for instance is not just a number it accompanies the physical features of obesity. In some cases the correlate is less obvious. For instance, cardiac ejection fraction is the fraction of blood in the heart pumped out with each heartbeat. This can be expressed as a number, a measurement. An abnormal number would correlate to a disease of the cardiac valves or cardiac muscle, the physical correlates behind the abnormal measurement. There is a physical correlate for hypertension but it is difficult to demonstrate or even to visualize. If the heart were diseased and functioned poorly the blood pressure would if anything be low. Hypertension is not heart disease. Hypertension is a vascular disease. There is narrowing and turbulence in the vessels that increases the resistance to blood flow.
The physical correlate of high blood pressure is generalized vascular disease that increases the resistance to blood flow. Blood flow is preserved by the increased pressure. We don’t measure the increased resistance only the increased pressure. The increased resistance does exist but it is very difficult to measure so we never do but we know that it is elevated because the physics of blood flow. This is significant because hypertension is not just a risk factor for disease in the future, the disease is now. If one could visualize the vascular system in its entirety one could demonstrate the reason for the increased resistance. Vessel narrowing or turbulence that restricts the flow of blood is present in a widespread way. This is disease that could be demonstrated but never is. Nevertheless the disease is present, now, even if we don’t visualize it. Since is is so diffuse and widespread we don’t see the effects of vessel obstruction that comes from localized narrowing or occlusion. Hypertension has a physical correlate, vascular narrowing and turbulence globally, but we don’t visualize it out of practicality. This causes increased resistance to blood flow which we don’t measure. We measure blood pressure. Hypertension is caused by vascular disease, the physical correlate of the abnormal measurement.
It is possible but quite difficult to measure systemic vascular resistance(SVR) rather that peripheral blood pressure in the brachial artery. Nevertheless the elevated SVR is the real problem. It does work to measure BP in most circumstances such as monitoring Essential Hypertenson.* It would help our understanding if we could image the entire vascular system to demonstrated the generalized vascular disease is present. As difficult as measuring SVR is, this would be done with even greater difficulty. Imagine injecting a radioactive tracer and imaging with a special camera and then reconstructing the images with artificial intelligence. Now imagine seeing the doctor for a clinic visit and instead of using a blood pressure cuff we do this procedure. We might get better compliance with medication. Generally we don’t need to provide experimental evidence to show that generalized vascular disease is present. Science based physiology demonstrates that is present in in people with Essential Hypertension. It one tests it and obtains the proverbial evidence, one could get this wrong. If we had images of the vascular system it would show abnormalities and we would call it disease.
It addition to this eventually producing vessel obstruction causing stroke, myocardial infarction etc., this generalized vascular disease will increase the mortality in times of stress. It is not simply for the passage of time that hypertension is manifest. A present illness or stress will demonstrate this. Sepsis is a notable example of a stressful condition with worse outcome in persons with hypertension. Exercise tolerance is also reduced. The disease, Essential Hypertension is a present illness, a vascular disease, not just a risk factor for disease in the future.
In some cases, one could suppose that the blood pressure is set too high for the resistance. To demonstrate this one would have to measure resistance. This is impractical. It is fine to use blood pressure for monitoring Essential Hypertension but we should assume and accept that resistance is actually high and this in not just an increase in the set point. To consider that the blood pressure is set too high for the vascular system we declare that this is a state of hyperperfusion and we are back to the need to measure SVR. This is impractical. If the blood pressure is high we should accept that the resistance is high and move on.
What we call Essential Hypertension is not just a risk factor for future disease. This is easier to understand if we realize and accept that Essential Hypertension is actually increased total peripheral resistance and widespread vascular disease. We don’t visualize or image this disease the way other diseases are imaged but we should realize that if we did image the vascular system it would show disease.
*Blood pressure and SVR are not precisely linearly related to each other. This is important in people that are significantly ill. This is similar to the concept that blood pressure is not blood flow. The use of blood pressure to gauge the level of systemic vascular resistance in asymptomatic individuals is acceptable given the difficulty of measuring SVR.