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compression” and “thoracic pump” describe the current mechanisms that explain blood flow during CPR ( Fig 9.11 ) In the “direct compression” model, the heart is squeezed between the sternum and the posterior vertebrae During compression (systole), blood moves through the AV valves and the aorta During relaxation (diastole), blood fills the myocardium in preparation for the next systole In the “thoracic pump” model, the heart is viewed as a conduit During compression, venous valves at the thoracic inlet close, preventing retrograde flow, the venous side of the circulation is compressed, and blood moves forward through the AV valves and the aorta During relaxation, negative intrathoracic pressures suck blood into the pulmonary bed and heart in preparation for the next systole In practice, both methods likely contribute to blood flow Because of the compliance and elasticity of the chest wall and the intrathoracic structures, direct compression may play a larger role in the pediatric patient FIGURE 9.11 Blood flow during external cardiac compression RV, right ventricle; LV, left ventricle Based on the these mechanisms, investigators have explored different techniques to increase blood flow through the aorta and the coronary arteries These have included high-frequency compression rates (more than 100