Pressure and Flow

Medical evaluation and monitoring of the circulation’s performance is most commonly assessed through determination of the heart rate (or pulse) and blood pressure. However, these two parameters do not actually represent the key functions of the circulation, which are delivery of oxygen and nutrients to cells.

The physical concepts of pressure and flow are related through Ohm’s law, although Ohm wasn’t actually concerned with physiology or the circulation. Georg Simon Ohm was a German physicist and mathematician working on electrical circuits and currents in the early 1800s. The law named after him states “V = IR”. In that equation, “I” is the current of electrons moving through the conductor in units of amperes (or “amps”), “V” is the voltage measured across the conductor in units of volts, and “R” is the resistance of the conductor in units of ohms.

Similarly, in terms used for circulatory performance, blood flow (represented as a dotted Q) is analogous to current, blood pressure (BP) is analogous to voltage, and systemic vascular resistance (SVR) is the term applied for the resistance to circulatory flow.

However, the body is dependent upon the flow of oxygen and nutrients via the circulation so that tissue cells can survive. It is very important to understand that it is circulatory flow that delivers the cellular fuel. That flow, in turn, is produced by cardiac function continuously pumping oxygenated blood through the circulation. While blood pressure is commonly measured and monitored, it does not represent the degree of fuel delivery necessary for cell viability and organ function.

In fact, maintaining blood pressure is not the primary goal in circulatory shock resuscitation. Blood pressure is often managed in clinical practice with the administration of vasoconstrictive medications, also known as vasopressors. However, such treatment does nothing to improve the delivery of oxygen to cells and tissues. Rather, it is blood flow that determines how much oxygen the tissues receive.