Which addition to a mechanical ventilation circuit would most significantly affect arterial carbon dioxide levels?

The addition of a heat-moisture exchanger (HME) and tubing to a mechanical ventilation circuit can significantly impact arterial carbon dioxide (CO2) levels. An HME functions by capturing exhaled moisture and heat from the patient's expired air and using it to condition the incoming inspired air. This process is essential for maintaining adequate humidity levels within the airways, especially during mechanical ventilation.

When an HME is added to the circuit, it can increase dead space in the respiratory circuit, especially if it is not optimally designed for the specific ventilation parameters being used. Increased dead space means that a portion of the ventilated air does not participate in gas exchange, leading to a rise in arterial CO2 levels. As more CO2 is retained in the lungs due to this ineffective ventilation caused by increased dead space, the PaCO2 in the arterial blood will increase.

In addition, the tubing that accompanies the HME can contribute to further dead space by adding additional volume to the circuit. Together, these factors can create a situation where ventilation-perfusion mismatches are more pronounced, leading to elevated CO2 levels. It is critical to monitor carbon dioxide levels when incorporating devices that can change the dynamics of ventilation, especially in patients who may already have compromised