Mechanisms of Blood Flow During Cardiopulmonary Resuscitation

Abstract

The report by Higano and associates in the November 1990 issue of the Proceedings (pages 1432 to 1440), entitled “The Mechanism of Blood Flow During Closed Chest Cardiac Massage in Humans: Transesophageal Echocardiographic Observations,” adds interesting and important new insights into the mechanisms of cardiopulmonary resuscitation in humans. In the analysis of their results, the authors propose a dichotomy that either (1) circulation occurs because the heart is squeezed between the sternum and spine and is constrained from slipping into the left side of the chest by the pericardium or (2) blood flows from the thorax because intrathoracic pressure exceeds extrathoracic vascular pressure and flow is restricted to the venous to arterial direction because of venous valves that prevent retrograde flow at the thoracic inlets. The authors further suggest that the two mechanisms can be distinguished by closure of the mitral valve because in the thoracic pump scenario the heart should act like a “passive conduit.” Without reference to this dichotomy, the findings of this report can be explained without requiring that the heart be compressed by the sternum and spine. Suppose that external compression of the chest caused a uniform increase in pleural pressure, which was equally applied to the outer surface of the heart and intrathoracic extrapulmonary arteries and veins, and retrograde systemic venous flow was prevented by venous valves at the thoracic inlet. No net gradient would be present for forward flow from the systemic veins through the right to the left side of the heart. The increase in pleural pressure, however, would not be completely transmitted to the intrapulmonary vessels. The pressure gradient thus created would produce flow into the lungs from both sides of the heart. The lungs represent the only opportunity for flow from the right side of the heart if the venous valve system is completely competent. Flow from the left side of the heart will be distributed between retrograde flow and forward flow, depending on the relative impedances of the two pathways. Closure of the mitral valve, however, would prevent retrograde flow from the left ventricle. Collapse of the left atrium in the presence of a closed mitral valve demonstrates retrograde flow from the atrium. Whether the mitral valve would close would depend on compliances of the right and left ventricles and the intrapulmonary vasculature, impedance to flow in the pulmonary venous and pulmonary and systemic arterial circuits, and lung mechanics that determine the magnitude of the difference between pleural and intraparenchymal perivascular pressures. Thus, the patency of the airway and the time profile of the imposed pleural pressure wave would be important factors. In the two cases presented, the left side of the heart was not a passive conduit in that the left ventricle provided a valve to prevent retrograde flow into the lower impedance pulmonary circuit. The left ventricle served as a reservoir of blood to be ejected from the thorax by the imposed pressure gradient. This mechanism, however, does not require that the heart be physically compressed between the sternum and the spine. Mechanisms of Blood Flow During Cardiopulmonary Resuscitation: The authors replyMayo Clinic ProceedingsVol. 66Issue 4PreviewWe appreciate Dr. Rodarte's letter, which provides additional insights into possible mechanisms of blood flow during closed chest cardiac compression. In our article, we reviewed the two mechanisms of forward blood flow during closed chest cardiac compression that had previously been proposed: the cardiac pump theory and the thoracic pump theory. Prior authors had stated that the movement of the mitral valve could distinguish between these two mechanisms because the mitral valve must close in the cardiac pump model and must remain open in the thoracic pump model. Full-Text PDF