During suspended respiration, what is expected to happen to the inferior vena cava (IVC)?

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When suspended respiration occurs, a notable physiological response is the modification in pressure within the thoracic cavity. During inhalation, the diaphragm descends, which creates a negative pressure in the thorax, allowing venous return to increase and facilitating the filling of the heart from the inferior vena cava (IVC). However, during suspended respiration (holding breath), the thoracic cavity does not maintain this negative pressure effectively. As a result, the IVC experiences a decrease in pressure, leading to dilation.

Dilation of the IVC is primarily due to the pooling of blood as the return to the heart diminishes in the context of the increased intrathoracic pressure that occurs when breath is held. This pooling can cause the IVC to widen temporarily because the volume of blood trying to return to the heart exceeds the capacity of the existing pressure dynamics to accommodate it. Hence, dilation represents the body's immediate physiological adjustment to the ongoing changes in intrathoracic pressure during this state of suspended respiration.