Rosh Emergency Medicine Practice Test

In the context of trauma, what mechanism leads to decreased preload in a patient with elevated intrathoracic pressure?

1. Decrease in blood volume

2. Decrease in cardiac output

3. Decrease in venous return

4. Elevated intrathoracic pressure

The correct answer is: Elevated intrathoracic pressure

When considering the effects of elevated intrathoracic pressure in trauma, it's important to understand how this impacts preload, which refers to the volume of blood in the ventricles at the end of diastole. Elevated intrathoracic pressure can result from mechanisms such as tension pneumothorax, resulting in a decrease in venous return. In a healthy physiological state, intrathoracic pressure decreases during inspiration, facilitating venous return to the heart as the negative pressure helps draw blood into the thoracic cavity. However, when pressure within the thorax becomes elevated, as can occur in traumatic scenarios, this inhibits the return of blood to the heart. The high pressure effectively compresses the great vessels and decreases the gradient necessary for blood to flow back to the heart, leading to reduced preload. Therefore, stating that elevated intrathoracic pressure is the mechanism leading to decreased preload accurately identifies the root cause of this physiological change in trauma patients.