Is the Guytonian framework justified in explaining heart-lung interactions?

In their recent article devoted to venous return and heart-lung interactions (1), Berger and Takala address well-known points of Guyton’s model of the systemic circulation, such as:

• The dependency of cardiac output (CO) on “venous return” (VR);
• The quantitative description of VR (and CO) as the pressure gradient between “mean systemic pressure” (Pms) and right atrial pressure (Pra) over the “venous resistance” (Rv) (as in their Equation 1);
• Pms as the driving force (or, “pressure head”) for VR;
• Pms physically persistent in the venous segments of the peripheral vasculature;
• Pra as a “back-pressure” limiting VR;
• Total blood volume comprised of “stressed” and “unstressed” components with the former responsible of Pms and VR.

While the authenticity of Guyton’s model as presented by the authors, and summarized above, has been refuted at many opportunities (2-10), here to me remains the question of whether it effectively explains the physiology of heart-lung interactions, i.e., how the ventilatory swing of intrathoracic pressures affects cardiac loading conditions, and the pulmonary circulation as well; since, the idea of Pra as a back-pressure is especially appealing in this particular scenario.

The answer is that “it does not”; since it is, as mentioned, cardiac loading what is affected by airway pressures and, hence, CO. With reduced atrial and ventricular distending pressures (defined as the difference between inside and outside pressures) with positive-pressure ventilation, their volumes are proportionally reduced.

This fundamental principle of CO regulation must not be confused with the pattern of cyclic variation of ventricular venous inflow, also induced by positive-pressure inspiration, which is about cyclic transients of regional, intrathoracic blood flow, not about steady-state dynamics. It would be analogous to confusing the regional pulsatility of arterial flow with the source of mechanical energy for steady-state CO.

Acknowledgements

To Evangelina Dalmau, MD, for discussions in flow:volume interaction in the peripheral vasculature.

Footnote

Conflicts of Interest: The author has no conflicts of interest to declare.

References

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