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“Upward, not northward.”
-E. A. Abbott
A pressure chamber within a pressure chamber; the heart within the thorax. These are two pumps beating in-and-out of time, varying in physiology and pathophysiology between patients and within any one patient during the arc of an illness. As such, when we inspect the size and function of the heart, we must respect its surroundings, for it is the bony thorax, mediastinum and spongy lungs which fashion the stage upon which the heart sets its performance.
Yet cardiovascular physiology and thoracic mechanics are typically taught separately, ripped from each other’s intimate grasp; should this be so? Divorcing hemodynamics from respiratory mechanics has resulted in folly. Consider that the acquisition and interpretation error of the pulmonary artery occlusion pressure stems from ignorance of thoracic characteristics and ambient pressures [1, 2]. Will we repeat the same mistakes with less invasive means of hemodynamic assessment?
To consider the intimate linkage of the cardiovascular and respiro-thoracic systems, it may be fruitful to think about them simultaneously . Indeed, the entire thesis of heart-lung.org is to marry two classic diagrams onto a single graphic. While these musings may appear excessively complicated on first blush, I believe that mere appreciation for these interactions is imperative. The model proposed is not meant to completely explain heart-lung interaction, but instead awaken within or remind the learner of their existence.
I would like to thank the editorial board of the free, open access, on-line journal Critical Care Horizons for publishing an illustrative case as point-of-departure for this hybrid diagram. Please download, read and share this article found here. The case considers an obese patient with severe ARDS who is flipped into the prone position. Upon pronation, there are acute changes in chest wall and pulmonary compliances which will affect cardiac loading as well as venous return.
As a supplement to the journal article, I have created an illustrated and animated vodcast on linking these two diagrams – presented below. I hope that this helps clarify the meaning and interpretation of this diagram. As well, I hope that it triggers reflection upon the important physiological connections between the heart and lungs; accurate acquisition and interpretation of hemodynamic data – be it invasive or non-invasive – depend upon these principles.
For more background, or additional readings on this topic, please consider these previous posts:
Lastly, there is a new learning module up that covers the topic of hemodynamics and pronation [module 7].
Please check out the rest of the 1000 Word Series & Happy Boxing Day,
- Gnaegi, A., F. Feihl, and C. Perret, Intensive care physicians' insufficient knowledge of right-heart catheterization at the bedside: Time to act? Critical Care Medicine, 1997. 25(2): p. 213-220.
- Pinsky, M.R. and J.L. Vincent, Let us use the pulmonary artery catheter correctly and only when we need it. Critical Care Medicine, 2005. 33(5): p. 1119-1122.
- Kenny, J.-E., Predicting the Haemodynamic Response to Prone Positioning: A Novel and Simultaneous Analysis of the Guyton and Rahn Diagrams. Critical Care Horizons, 2017. 1(1): p. 1-7.