What happens mechanically during the QRS complex?

• A. The ventricles contract, pushing blood to systemic and pulmonary circulations
• B. The atria contract
• C. The atria relax
• D. The ventricles contract

Answer: (A)

The QRS complex reflects ventricular depolarization, and that electrical event triggers the ventricles to contract (ventricular systole). As the ventricles contract, blood is ejected into the systemic and pulmonary circulations—left ventricle pushing blood into the aorta and the right ventricle pushing blood into the pulmonary artery. This combination of depolarization and the resulting strong ventricular squeeze is what makes the mechanical action during the QRS: the ventricles contract and push blood out to the body and the lungs. The atria, which contract with the P wave, are not the primary mechanical event during the QRS, and atrial relaxation is not the focus of this complex.