Important pushback on the Anderson/Guyton model: is pressure really a driver, or just a consequence of volume and compliance? A quick thread to unpack the physiology (and clinical relevance) 🧵👇
@icmteaching @ICS_updates @Wilkinsonjonny @iceman_ex @ThinkingCC @khaycock2 @msiuba It is interesting to recognize that this is not the only model attempting to explain our major uncertainties regarding venous return. It is useful to begin by revisiting the fundamental relationship between flow and volume: V(t) = ∫Q(t) dt and Q(t)=dV(t) / dt However,
2
14
43
Replies
1/ Thanks Diego @DiegoEscarraman You're absolutely right: pressure isn't a primary “source” in energetic terms. It emerges from the interaction between volume, compliance, and flow — and real understanding must include that dynamic.
1
0
5
2/ The Anderson model actually aligns well with that view. It rejects the idea of the heart as a "suction pump" or flow initiator. Instead, it sees the circulation as a pre-pressurised system (via stressed volume), with the heart acting to transmit, not drive, flow.
1
0
6
3/ In that context, flow occurs down pressure gradients, but pressure itself is contingent on volume and compliance. So we might use pressure to describe the system or understand flow, but it’s not the fundamental origin.
1
0
5
4/ Guyton’s classic equation: Q = (Pms−Pra) / R is a useful steady-state approximation. But as you point out, it omits compliance, inertial terms, and energy input — so it’s incomplete in dynamic or thermodynamic terms.
1
0
5
5/ Your framing — dP/dt = Qin−Qout/C — nicely captures the time-dependent, compliant nature of the system. It shows pressure as the result of volume displacement into a compliant compartment over time.
1
0
3
6/ So perhaps we need both lenses: Pressure gradients to explain how flow moves, And compliance-volume-energy models to explain why pressure exists and how it changes. They're not rivals — just different views of the same system.
1
0
6
7/ Ultimately, we need models that help us treat patients. A vasopressor increases flow not by “pushing harder,” but by raising Pms — increasing the upstream pressure and restoring flow. That’s Anderson’s value: it translates mechanics into medicine.
2
1
9
original thrread:
1/ Most people think the heart drives circulation. But what if that’s backwards? Anderson’s model flips the whole idea of cardiac output on its head — and it changes how you think about fluid, flow, and failure. 🧵👇 #physiology #FOAMed #MedTwitter #criticalCare #cardiacOutput
0
0
4
