Blood Pressure is a Side Effect

Much of our clinical practice regarding the circulation involves evaluating and treating a patient’s blood pressure, usually measured in millimeters of mercury.  Blood pressure is the side-wall pressure measured in large arteries in the circulation, usually in vessels in the arms or legs.   We continue to use blood pressure as a stand-in assessment of cardiac output, which is the volume of blood flowing through the circulation over a period of time, typically in liters per minute. 

The pressure inside a pipeline is directly proportional to the amount of flow going through it: the higher the flow, the higher the pressure.  However, a pipeline has rigid pipes.  Human “pipes”, our arteries, are flexible.  They can stretch and contract, making the lumen larger or smaller, respectively.  So if our vessels change their size and the flow rate is no different, we will observe a lower blood pressure with a larger (vasodilated) set of vessels and a higher blood vessel with a smaller (vasoconstricted) set of vessels.  Thus, the use of blood pressure to reflect flow is flawed because of the inherent assumption that the blood vessels are rigid.

The problem is that those assumptions are not considered in standard clinical practice.  At best, a low blood pressure should be seen as a screening test.  When the blood pressure drops, an investigation should be undertaken to determine which of the various forms of shock (hypovolemic, cardiogenic, septic) is at work so that the problem can be fixed appropriately.  It makes little sense to provide increased afterload to someone who is in cardiogenic shock or to vasoconstrict the already vasoconstricted vessels that develop spontaneously in hypovolemic shock.  Yet, because we’re focused on fixing the low blood pressure, that is exactly what we do.  In fact, a recent and noteworthy clinical study continue to treat them as one.

A high blood pressure actually indicates that the heart is working too hard and the vasculature can become damaged.  Our current management is reasonable, as we seek to reduce global blood pressure and the risk of hypertensive heart disease.  However, there is a risk that a widespread reduction in blood pressure could impair the flow to some vital organs.  The paradigm shift in medicine will be to identify the abnormal mechanisms that drive the heart to pump so vigorously and treat those specifically.