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. Since the vessel narrowing in is is so diffuse and widespread we don’t see the effects of vessel obstruction that accompanies localized occlusion such as stroke or myocardial infarction. Hypertension has a physical correlate which is vascular narrowing and turbulence globally. This is present at the time of the measurement. This is not just a risk factor for some disease that may be manifest in the future.
It is possible but quite difficult to measure systemic vascular resistance(SVR) rather than peripheral blood pressure. We measure the blood pressure because it is convenient. Nevertheless, the elevated SVR is the real problem. It does suffice to measure BP in most circumstances such as monitoring Essential Hypertension.* It would help our understanding if we could image the entire vascular system to demonstrated the generalized vascular disease that 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 but clearly this is not practical. 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, now not in the future.
With stress such as sepsis or drug overdose the resistance to the flow of blood will become evident and these illnesses will be worsened by the high resistance. It is not simply with the passage of time that hypertension is manifest in symptomatic illness. A present illness will demonstrate this the limitation to blood flow by high baseline resistance. Sepsis is a notable example of a stressful condition with worse outcome in persons with hypertension. Other types of stress including drug overdose and trauma with hemorrhage will fare worse. Exercise tolerance is also reduced.
It is reasonable and appropriate to use blood pressure for monitoring Essential Hypertension but we should assume and accept that resistance is actually high and this in not just that blood pressure is higher than it needs to be. One could suppose that the blood pressure is set too high for the resistance but this means that the system is in a state of hyperperfusion. To demonstrate this we would have to measure cardiac output. This is just as impractical as measuring SVR so given the choice we can either postulate hyperperfusion, or that the increased blood pressure is a response to increasing resistance as aging occurs. Increased resistance is the better assumption. This is better than concluding that blood pressure is set abnormally high amid normal resistance to flow. When we treat Essential Hypertension we are treating High Systemic Vascular Resistance*.
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, it is impractical but we should realize that if we did image the vascular system it would show disease. The disease, Essential Hypertension is a present illness, a vascular disease, not just a risk factor for disease in the future.
*Blood pressure and SVR are not linearly related to each other. This is important particularly in people that are significantly ill. It is tempting to measure blood pressure and equate this to blood flow. 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.