Air movement

In normal ventilation, the lungs are stretched out by the ribs and diaphragm. This creates a vacuum resulting in air filling up the pharynx and filling the lungs with air. In positive pressure ventilation, air is pushed into the patient’s pharynx with great pressure. The air fills the dead space of the airway and eventually the lungs

Blood movement

In normal ventilation, blood returns to the heart from the body by a vacuum created by the constant expansion and contraction of the lungs. In positive pressure ventilation, blood return to the heart is decreased when the lungs are inflated under higher pressure. Since positive pressure ventilation decreases blood flow to the heart from the body, it will also result in decreased cardiac output from the heart to the body (less coming in means less going out)

Esophageal opening pressure

In normal ventilation, the esophagus remains closed during normal breathing resulting in no air entering the stomach. In positive pressure ventilation (be with a bag valve mask or pocket face mask) air is pushed into the stomach due to the higher than normal airway pressure. Excess air in the stomach leads to vomiting.

Excessive rate or depth of ventilation using positive pressure ventilation can harm the patient as ventilating too fast or deep may cause low blood pressure (hypotension) or decreased blood flow when the chest is compressed during CPR.