What would happen if the atria and ventricles of the heart contracted simultaneously?

Because of immense pressure from the ventricles, the atrioventricular valves (mitral valve and tricuspid valve) remain closed. The blood pressure of the right atrium is 0-8 mmHg and that of left atrium is 4-12 mmHg. The blood pressure of the right ventricle is 15-25 mmHg (systolic) over 0-8 mmHg (diastolic) and that of left ventricle is 110-130 mmHg (systolic) over 4-12 mmHg (diastolic). The blood from the right atrium will flow back to the vena cavae causing leg swelling. The blood from the left atrium will flow back to the lungs causing pulmonary edema.