According to thermodynamic principles energy flows from high energy states to low energy states. This can be seen as heat moving from hot to cold, electric current from high to low voltage and fluids moving from high to low pressure. This always occurs through some type of resistance. Blood flows from an area of high energy that is high pressure to an area of low energy, low pressure. In the case of blood, the resistance to flow is determined by the physical properties of the blood vessels. The general equation describing blood flow is Flow = Pressure / Resistance which for the entire body is Cardiac Output, CO = Blood Pressure / Systemic Vascular Resistance written CO = BP/SVR*. Flow such as cardiac output is what carries oxygen. This flow is related to pressure, increasing with increased pressure. and also, and just as important, inversely related to resistance. Increasing resistance reduces flow and the most important effect of that is that increasing resistance reduces the delivery of oxygen.
Thermodynamic principals are the same in humans, animals, vertebrates and invertebrates, on earth and also in the rest of the universe. We don’t look for experiments to verify the relationship of CO = BP/SVR*. This relationship exists as a matter of universal law.
Biologically or physiologically blood flow is modulated by various mechanisms such as feedback loops and auto regulatory processes. In practice, or in an intact body deviations in measured values to CO = BP/SVR occur as physiologic processes work to preserve blood flow and oxygen delivery. At a single point in time this equation CO = BP/SVR will always hold true. As one of these parameters is modified by an intervention such as using a medication to affect blood pressure the resulting change in the other parameters will be affected by autoregulation. Blood flow or Cardiac Output is still directly related to Blood Pressure and inversely related to resistance. Neurological regulation of blood flow is overlaid on top of this relationship but does not replace it.
Generally is assumed that an intervention that raises blood pressure will increase blood flow and an intervention that reduces blood pressure will decrease blood flow. This is not true where vasoconstriction is used to increase blood pressure. These will have the opposite effect because it is the increasing resistance that increases blood pressure at that intervention will always reduce flow.
Blood pressure is easily measured and in a number of simple ways and can be easily followed continuously. Unfortunately we frequently assume the infallibility of a perception that is almost instantaneous. There is the generally accepted notion that when blood pressure goes up then perfusion also goes up. This is not true when vasoconstriction is used to increase blood pressure because vasoconstriction increases the blood pressure by restricting the flow of blood. The perception is wrong in this case when the blood pressure is improving because blood flow is actually decreasing. This is not apparent at the bedside. We simply have to know that vasoconstriction causes a restriction to blood flow. Measurements of blood flow are not available under the usual clinical setting such as a clinic or the emergency setting. This is especially true when we consider flow or perfusion into internal organs such as the liver, kidneys, muscles. The illusion will continue in the short term.
It will eventually become apparent that vasoconstriction is reducing flow when acidosis combines with clinical deterioration. The short term illusion has been misleading clinicians for decades. A drug known as Levophed is colloquially known as “Leave um Dead” because this counter intuitive impression. The short term results, immediate and easily seen of increased blood pressure results in long term negative consequences, deteriorating physical condition and sometimes death.
The brain is sensitive to short term reductions in blood flow but autoregulation preserves perfusion to the brain. This is good but it also helps to creates the illusion of stability when overall cardiac output may be declining.
One of the few early or sudden indications that blood flow is impaired are mental status changes. Fainting when blood flow crashes is an immediate indication that perfusion is inadequate. Other organs such as liver, kidney, muscle tissue may become affected but the effect is delayed. The blood pressure goes up but flow goes down and there is little immediate indication of the illusion except on the brain. In the overall, the grave consequences of decreasing cardiac output on the rest of the body are obscured when flow is preserved into brain tissue.
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 diverge in this situation. Blood pressure and blood flow are not the same but is common practice is to consider them as such.
A drug such as a vasoconstrictor that increases the resistance to blood flow will reduce blood flow. The blood pressure may increase which is immediately apparent but overall flow will decrease. The misguided clinical impression is that increasing blood pressure no matter how it is achieved benefits blood flow. Most clinicians always equate increased blood pressure to increased blood flow. Skepticism is warranted. Sometimes blood flow might be redistributed from one vascular bed to another but it is easy to use this argument to reject a universal law of physics, asking for trouble. The practice of using blood pressure as a substitute for flow is unfortunately encoded into many inflexible protocols and procedures so this situation will not be rectified anytime soon.
1 https://en.wikipedia.org/wiki/Heresy “In other contexts the term does not necessarily have pejorative overtones and may even be complimentary when used, in areas where innovation is welcome, of ideas that are in fundamental disagreement with the status quo in any practice and branch of knowledge.”
* Technically the flow is related to the pressure gradient Arterial Pressure minus Central Venous pressure but generally the Central Venous Pressure is low enough to ignore and we can relate flow directly to arterial blood pressure