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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