[Why does the symptom of venous air embolism appear suddenly?]

A glass model of the human heart with the vena cava was made to study the behavior of air entering into a vein by accident. The capacities of the right atrium and ventricle of this model were 50 ml and 130 ml respectively. The superior and inferior venae cavae were irrigated with the running water. Direction of the vena cava, relative posture of the right heart against the vena, flow rate of the water, volume and frequency of air infusion were changed in turn. Migration of the air bubbles was observed. Most of the air injected into the vena cava formed multiple bubbles of various sizes and kept floating in the running water in the vena. Some air bubbles were transferred to the atrium and ventricle and trapped there until they were suddenly washed away. This happened after the air bubbles grew larger to impede the water flow, when the flow rate of water was increased, or when the vena cava was laid horizontal and the atrium and ventricle were positioned higher than the vena cava. Although this hard model is quite different from the contracting human heart, the behavior of the air in the flowing fluid affords us some indication of an explanation of the sudden onset of the symptom of venous air embolism during a surgical operation.