Arterial Excess; a different angle on haemodynamic measurements.

Last week I posted on venous excess and was honoured with a Twitter comment from an Intensive Care expert. He wrote “are they not looking at the same physiology from different angles? Venous excess = venous congestion in Guyton’s?” There is much merit in that observation, and it led me to reflect that exploring a phenomenon “from a different angle” is often a worthwhile exercise. It may confirm your original concept, or it may shock you to find that what you have taken for granted is flawed. Some call this approach Thinking Outside The Box, and I think my hero Ludwig Wittgenstein would approve. In this post I invite you to join me in looking at arterial pressure regulation from a different angle; will it bring surprising insights for you?

First premise; baroreceptors measure arterial pressure (AP) and send a nervous signal to the vasomotor centre so  AP can be autoregulated. That is right, isn’t it? That’s what they teach us! Well actually, no. We like to call them pressure (baro) receptors because in clinical practice we measure AP and prescribe medications to treat it. But what the “baro”receptor is doing is responding to distension of the arterial wall it is in. The primarily regulated variable is not pressure, but the distending arterial-side volume that causes the pressure that we measure and treat. So what are the command variables that maintain arterial wall distending volume? At steady state the Big Two are the rate at which blood enters the arterial side of the circulation (CO) and the resistance to it leaving the arterial side (PR). Changes in arterial volume are due to imbalance in the rate at which volume enters (a function of cardiac output) and leaves the arterial space (a function of peripheral resistance), and the changes in arterial volume are reflected in changes in AP. The arterial pressures we find it easy and convenient to measure and treat are actually just indicators of the arterial distending volume. If you like the idea that the regulated variable is not actually pressure, but rather volume distension you will want to agree a name we can all call it. How about the Arterial Excess?

Further Reading: DOI: 10.1152/japplphysiol.00535.2004

 

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