Elevated Blood Pressure or BP as in Essential Hypertension is considered a disease. The disease, is in reality, a vascular disease that produces an elevated resistance to blood flow known as systemic vascular resistance or SVR. This is the total resistance that the flow of cardiac output pushes against.
The increase in systemic vascular resistance in aging and with other causes is due to vascular changes including narrowing of the vessels and irregularities of the lumen that produce turbulence. Generally as people age these changes become more significant and the resistance to blood flow increases. Unfortunately, SVR is is a particularly difficult cardiovascular parameter to measure. In most clinical situations, BP can be measured but systemic vascular resistance or SVR cannot. SVR is as difficult to measure as it generally requires invasive catheters in the central circulation of the heart. More about this later.
Elevated Blood Pressure to preserve flow is not a Disease
Elevated blood pressure is a response to the real disease, the rise in SVR. This response is a compensation to preserve flow. If the blood pressure did not rise then flow would be limited and the downstream tissue would suffer the lack of blood flow. The most critical problem is the delivery of oxygen and the support of metabolism. The rise in blood pressure in this sense is a good thing. It allows individuals to continue to live with the vascular disease that would otherwise cause illness or be fatal.
Flow or Cardiac Output CO equals BP divided by SVR. This equation CO = BP/SVR can be rearranged algebraically to derive any of the parameters if two of the others are known. Only the BP is easily measured. Unfortunately CO is as difficult to know as SVR. Both generally require invasive catheters in the central circulation of the heart. Unfortunately, BP and SVP are not related by a simple conversion factor. If BP were related in a linear way to SVR with a conversion factor the relationship would be a simple one and we could use BP to track SVR.
It is possible to measure, track and monitor blood pressure. In other words, BP is observable, Systemic Vascular Resistance is not. Essentially all research on Essential Hypertension for practical reasons is done using observations of BP not SVR. BP is what is measured, and indexed against outcomes. There are no doubt important considerations that would be different, if we could monitor SVR to track the changes in the vascular system.
It has been demonstrated that diuretics while lowering blood pressure do not reduce the risk of vascular disease. Since diuretics do not act directly on the vascular system, this is not surprising. There are some studies that show that diuretics do reduce the risk of vascular disease. It is likely that a study of the effect of diuretics on systemic vascular resistance would be helpful.