= Pressure-flow distinction in cardiovascular assessment
\b[Pressure-flow distinction in cardiovascular assessment] is the separation of <blood pressure>, a mechanical loading quantity, from <blood flow>, a volume transported per unit time. <Cardiac output> is a flow rate. A pressure reading alone does not specify the volume of <blood> delivered by the <heart>.
In a simplified steady model,
$$
\Delta p=RQ,
$$
where $\Delta p$ is the pressure difference across the represented vascular region, $Q$ is its <blood flow> and $R$ its effective <vascular resistance>. The difference between upstream and downstream <blood pressure> matters, rather than one pressure considered in isolation. The pressure-gradient and resistance account is developed in the primary physiology text.<Anatomy and Physiology 2e by J. Gordon Betts and others>[§20.2, Blood Flow, Blood Pressure, and Resistance]{ref}
An original counterexample holds the pressure difference fixed at 90 mmHg. With $R=18$ mmHg/(L/min), flow is 5 L/min; with $R=30$ mmHg/(L/min), flow is 3 L/min. The same driving pressure can accompany different delivery rates. Conversely, at fixed flow a higher measured pressure difference can represent a higher effective resistance.
The quantities also have different dimensions. The product $\Delta pQ$ has units of energy per time, describing a hydraulic rate of mechanical energy transfer in this model. A pressure alone has neither flow units nor power units.
A primary coronary <thermodilution> apparatus combined a pressure/temperature-sensing guidewire with a controlled infusion <catheter>; its flow estimate used infusion rate and temperature measurements.<Direct volumetric blood flow measurement in coronary arteries by thermodilution by Wilbert Aarnoudse and others>[Abstract, Methods]{ref} The availability of pressure and thermal signals in one instrument does not make them the same observable. Finally, pulsatile circulation includes storage and changing vessel geometry, so the steady resistance relation is an explanatory approximation. Matching time averages, anatomical sites and model assumptions is part of interpreting pressure and flow together.
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