Blood Pressure is commonly assumed to be a good indicator of blood flow or tissue perfusion. The higher the pressure the better the flow, generally it is assumed. Adequate blood pressure means adequate blood flow, mostly it does at least that is the general assumption. Low blood pressure is assumed to be for convenience more than anything else, to represent low blood flow. These assumptions work so well most of the time, we forget that they may not always.
The relationship between BP and blood flow would be linear and directly related to it if Systemic Vascular Resistance (SVR) were a constant value. SVR is however, variable. The relationship of BP to blood flow is described by equation of Olm’s law of blood flow. Blood Flow is equal to BP divided by the resistance to blood flow, SVR. There are regulatory mechanisms in humans and animals that moderate this relationship. The relationship is still a fact of science or thermodynamics. Resistance in the case of blood flow is usually unknown. It is difficult to measure so much so that it is usually ignored as a factor in the assessment of shock.
Shock should be considered to be low blood flow. However, as soon as the admonition is made that “shock is not low blood pressure”, blood pressure is for convenience used to define shock. Practice and science usually diverge in the management of shock in as much as the use of blood pressure is used to assess blood flow or perfusion.
Blood flow and the resistance to blood flow is much more difficult to measure than blood pressure. This is a fact that common practice and practicality often chooses to ignore. The use of blood pressure as an indicator of perfusion works most of the time. Understanding when blood pressure may not accurately indicate the state of blood flow is often a matter of educated guesswork rather than measuring flow or resistance. For instance, persons known to have long standing hypertension may be assumed to have higher than normal resistance to blood flow. We may suspect the resistance to blood flow to be higher even if we can’t measure it. This is in many situations the best we can do.