USMLE (United States Medical Licensing Examination) Step 1 Practice Exam

What occurs in mechanically ventilated patients regarding the amount of anatomic dead space?

1. It decreases

2. It remains constant

3. It is not affected

4. It increases

The correct answer is: It increases

In mechanically ventilated patients, the amount of anatomic dead space typically increases. Anatomic dead space refers to the portion of the respiratory system where gas exchange does not occur, predominantly consisting of the conducting airways such as the trachea and bronchi. During mechanical ventilation, particularly with certain ventilator settings, tidal volumes can be increased, leading to ventilation of the lungs without corresponding perfusion. Additionally, the ventilatory strategy can alter the distribution of airflow within the lungs, potentially causing some alveoli to become less perfused or not perfused at all. This can effectively increase the amount of dead space in the respiratory system because more of the air ventilated during mechanical breaths does not participate in gas exchange. Furthermore, factors such as positive pressure ventilation can also contribute to changes in lung physiology, resulting in a redistribution of lung perfusion and consequently impacting the effective ventilation/perfusion ratio. Thus, while the anatomical dead space remains relatively constant in a healthy individual, the dynamics of mechanical ventilation can lead to an increase in the functionally relevant dead space in the setting where some areas of the lung are ventilated but poorly perfused.