How I Teach: Heart–Lung Interactions during Mechanical Ventilation. Positive Pressure and the Right Ventricle

Figure 1.. Water column analogy of right heart filling. We illustrate flow between the venous reservoir and the right atrium (RA) as two water filled containers connected by a tube. The learner is able to identify that the higher the water column, the greater the pressure at the bottom of the container. (A) In this illustration, Container 1 represents the mean systemic filling pressure (MSFP) and Container 2 represents the RA. Connecting tubing allows water to flow from the higher pressure system to the lower pressure system. In this example, learners are able to identify that water will flow from Container 1 to Container 2, given the pressure difference. (B) Increasing the height of the water column in Container 1 and/or decreasing the height of the water column in Container 2 raises the pressure gradient, therefore increasing the rate of flow. We refer back to the physiologic model explaining that flow to the right heart will increase if MSFP is raised or if right atrial pressure (PRA) is lowered. The latter point may be less intuitive, as many assume that a higher PRA implies improved flow. Therefore, our diagram assists in dispelling this commonly held belief. (C) The tube connecting…

Figure 2.. Venous return curves. (A) Venous return is plotted in relation to right atrial pressure (PRA). At low values of PRA, the venous reservoir readily fills the right heart. With elevated right atrial pressures, the gradient between the mean systemic filling pressure (MSFP) and the PRA is abolished, leading to the cessation of flow (X-intercept). The plateau of the venous return curve occurs when the right atrial pressure decreases below zero. Here, venous return does not increase because of vascular collapse. (B) Volume loading leads to a rightward shift of the curve (light blue line) where, for a given PRA, venous return is higher. As a more nuanced point, crystalloid infusion, through hemodilution, also decreases resistance and, therefore, rotates the curve to the left while simultaneously shifting the MSFP as described. The administration of whole blood, by contrast, does not alter vascular resistance and shifts the curve purely as demonstrated. For simplicity, we described the movement of the venous return curve with a change in MSFP in isolation. (C) With vasoconstriction (an increase in resistance to venous return), the MSFP is unaltered; therefore, the X-intercept is identical…

Figure 3.. Transmural pressure. To explain the concept of transmural pressure (PTM), we model the heart within the thorax as a balloon within a box. (A) At end-expiration, the pleural pressure (PPL) is slightly negative (−3 mm Hg). A catheter within the right atrium (RA) (alternatively, a central venous catheter) reads 6 mm Hg. In this example, the RA PTM is equivalent to the right atrial pressure (PRA) minus the PPL. Specifically, the PTM with inspiration is 9 mm Hg, or PRA (6 mm Hg) − PPL (−3 mm Hg). (B) As the patient inspires, the PPL becomes more negative, stretching the RA allowing for increased filling. Accordingly, the PRA as measured by a catheter is lower (3 mm Hg in this example), given the drop in PPL. The lower PRA may lead one to erroneously conclude a decrease in right heart preload. However, the PTM, or the pressure “experienced” by the right heart chamber walls, increases as the fall in PPL is greater than the fall in PRA. In this example, the PTM with inspiration is 10 mm Hg, or PRA (3 mm Hg) − PPL (−7 mm Hg). Using this model, the learner can appreciate why a drop in the PRA with spontaneous inspiration increases right heart filling and stretch/preload of the myocardium…

Figure 4.. Pulmonary vascular resistance and lung volume. (A) Intra-alveolar vessels collapse with increasing lung volumes, leading to an increase in their resistance. (B) Extra-alveolar vessels have the opposite behavior where atelectasis/low lung volumes lead to their collapse and higher lung volumes increase their diameter, therefore lowering vascular resistance. (C) The superposition of the two graphs explains the combined effect of lung volumes on pulmonary vascular resistance. Accordingly, the lowest vascular resistance exists at intermediate lung volumes. Both atelectasis and alveolar overdistension are detrimental to right ventricular afterload. Created with BioRender.com.