The impedance threshold device may improve circulation during CPR by:

• A. Maximizing the amount of air in the lungs following chest recoil, which hyperinflates the lungs and forces more blood from the ventricle during each compression.
• B. Maintaining increased intrathoracic pressure during the downward stroke of each chest compression, which forces more blood from both of the ventricles.
• C. Drawing all of the air out of the lungs in between chest compressions, which causes positive intrathoracic pressure and a reduction of blood return to the right side of the heart.
• D. Limiting the amount of air that enters the lungs during the recoil phase between chest compressions, which results in negative intrathoracic pressure and improved cardiac filling.

Answer: (D)

The impedance threshold device improves circulation during CPR by manipulating intrathoracic pressure during chest recoil. By limiting the amount of air that re-enters the lungs during recoil, the device creates negative intrathoracic pressure. This negative pressure acts like a vacuum, enhancing venous return to the right side of the heart and improving filling (preload) between compressions, which supports better overall blood flow during CPR.

The other ideas would not aid CPR this way: increasing air in the lungs after recoil would raise intrathoracic pressure and hinder return; keeping higher pressure during the downward chest compression would also impede venous return; and trying to expel air to create positive pressure would reduce, not improve, venous return.