Heart-Lung interaction in spontaneous breathing subjects: the basics

Abstract

Heart-lung interactions occur primarily because of two components of lung inflation, changes in pleural pressure and changes in transpulmonary pressure. Of these, changes in pleural pressure dominate during spontaneous breathing. Because the heart is surrounded by pleural pressure, during inspiration the environment of the heart falls relative to the rest of the body. This alters inflow into the right heart and outflow from the left heart. Alterations in transpulmonary pressure can alter the outflow from the right heart and the inflow to the left heart. These interactions are modified by the cardiac and respiratory frequency, ventricular function and magnitude of the respiratory efforts.

Keywords: Pleural pressure; cardiac function; transpulmonary pressure; venous return.

Figure 1

Interaction of return and cardiac functions. Bottom left shows the venous reservoir (blue) draining through a resistance to the heart (red) which then pumps the blood back to the venous reservoir. The box indicates the thoracic compartment. The vertical double arrow indicates the pressure difference for venous return (dVR). The top left shows plot of cardiac and return functions. Pra = right atrial pressure and Q = cardiac output and venous return. Ptm-1 is the transmural filling pressure of the RV. The right side shows what happens with a decrease in Ppl. The heart is lowered relative to the venous reservoir (increase vertical double arrow). The cardiac function curve is shifted to the left (dotted line). Ptm increases and so does Q.

Figure 2

Effect of inspiratory effort on venous return and cardiac output. On the left, right atrial pressure (Pra) falls with inspiration (black boxes). The cardiac function curve shifts to the left and right heart filling increases. On the right, Pra does not fall with inspiration because the heart is functioning on the flat part of the cardiac function curve. On the left there could be a small increase in cardiac output with a volume infusion but not in the example on the right side.

Figure 3

Effect of a decrease in Ppl on the left ventricular pressure-volume relationship. The decrease in pleural pressure (Ppl) moves the pressure volume loop (P-V) down but does not affect the initial aortic valve opening pressure. This increases the isovolumetric part of ventricular contraction and decreases the volume ejected and increases left ventricular afterload. The end-systolic P-V relationship (Pes) is shifted to the right because of the change in the referencing of the intra-cardiac pressures relative to atmosphere.

Figure 4

Effect of increase in transpulmonary pressure (Ptm) on pulmonary flow. On the left, zone III conditions, lowering Ppl to -5 does not affect pulmonary flow. On the right, when Ppl is lowered to -10, left atrial pressure (Ppla) is less than alveolar pressure (Palv) and pulmonary veins collapse creating a vascular waterfall.

Figure 5

Transmural pressure changes during a Mueller Maneuver. Pw = pulmonary artery occlusion pressure. Mouth pressure was measured with a manometer. At the start of the Mueller, Mouth pressure fell by 40 mmHg, Pw fell below zero (note that zero for Pw is in the middle of the tracing), and there was a small fall in aortic pressure. At the end of the maneuver Pw was –5 mmHg and therefore the net change was –10 mmHg. Since Mouth pressure fell by –40 mmHg, transmural Pw rose by 30 mmHg and the net value of Pw was 35 mmHg.

Figure 6

Changes in blood pressure and heart rate during a Valsalva maneuver. See text for details.