LABORATORY INVESTIGATION PHYSIOLOGY Sequence of mitral valve motion and transmitral blood flow during manual cardiopulmonary resuscitation in dogs

According to the thoracic pump model of cardiopulmonary resuscitation (CPR), the heart serves as a passive conduit for blood flow from the pulmonary to the systemic vasculature, necessitating an open mitral valve and anterograde transmitral blood flow during chest compression. To assess the applicability of this model to manual CPR techniques, two-dimensional echocardiograms were recorded from the right chest wall and/or the esophagus in nine dogs (18 to 26 kg) during manual CPR. The aortic valve opened with chest compression and closed with release, while the pulmonary and tricuspid valve leaflets closed with compression and opened during release. The mitral valve remained open during ventilation alone and during abdominal compressions. With the onset of brief, high-velocity (high-impulse) chest compressions, the mitral valve closed rapidly and the left ventricle was deformed, whether compressions were applied to the sternum or the left mid-chest wall. The mitral valve reopened with release of each compression. Left atrial echocardiographic contrast injections confirmed the absence of anterograde transmitral blood flow during high-impulse compression and its presence during release. Failure of mitral leaflet approximation during chest compression was observed only when a very low-velocity, prolonged (low-impulse) compression technique was used, or when regions that did not directly overlie the heart were compressed. Consistent with these observations, simultaneous recordings of the left ventricular and left atrial pressures during high-impulse sternal compressions in five dogs (19 to 25 kg) demonstrated peak and mean left ventriculoatrial pressure gradients of 38.5 ± 4.0 and 13.5 ± 2.9 mm Hg, respectively, and these pressure gradients declined with less impulsive compressions. The observations made during all but low-impulse chest compressions are inconsistent with the thoracic pump model, and support direct cardiac compression as the primary mechanism of forward blood flow with more impulsive manual chest compression techniques. Circulation 76, No. 2, 363-375, 1987. THE MECHANISMS that sustain the circulation during cardiopulmonary resuscitation (CPR) continue to be debated more than 25 years after Kouwenhoven et al.1 proposed that blood flow results from direct cardiac compression. An alternative theory, the thoracic pump theory,2'3 attributes blood flow during CPR to a generalized increase in intrathoracic vascular pressures that is transmitted to the extrathoracic arteries. CompeFrom the Departments of Surgery, Physiology, and Medicine, Duke University Medical Center, Durham, NC. Supported in part by NHLBI grants HL09315 and HL29436, and SCOR grant HL17670. Dr. Feneley is the recipient of a Fulbright Postdoctoral Scholarship, a Neil Hamilton Fairley Fellowship from the National Health and Medical Research Council of Australia, and a Telectronics Overseas Fellowship from the Royal Australasian College of Physicians. Address for correspondence: J. Scott Rankin, M.D., Associate Professor, Department of Surgery, Box 3851, Duke University Medical Center, Durham, NC 27710. Received July 7, 1986; revision accepted April 23, 1987. Vol. 76, No. 2, August 1987 tent venous valves4' 5 and the high systemic venous compliance are thought to prevent full retrograde pressure transmission to the extrathoracic veins, thereby creating the arteriovenous pressure gradient necessary for blood flow. Despite the development of alternative CPR techniques,4 manual chest compression remains the centerpiece of CPR protocols that are widely and rapidly applicable to the management of both in-hospital and out-of-hospital cardiac arrest.9'-1 Much of the controversy concerning CPR mechanisms may reflect the different mechanical and hemodynamic effects of manual and nonmanual compression techniques.' 1-4 Most of the divergent data concerning the mechanism of blood flow have been derived from hemodynamic studies in canine preparations with a variety of compression techniques.2 46 12-16 In support of a direct car363 by gest on A ril 7, 2017 http://ciajournals.org/ D ow nladed from