The Vd/Vt indicates how much of the tidal volume is composed of deadspace. This is important to monitor because increases in deadspace reduce gas exchange and increase the patient’s work of breathing.
What is deadspace?
Deadspace is wasted ventilation. There are three types of deadspace:
- Anatomic deadspaceis estimated at approximately 1 ml per pound of predicted body weight. This is the portion of the tidal volume that fills the nose, mouth, and conducting airways of the lung. We cannot change the amount of anatomic deadspace in our patients.
- Alveolar deadspaceoccurs when perfectly functional alveoli are not perfused with blood. Therefore, no gas exchange can occur between the alveoli and the pulmonary capillary membrane. Alveolar deadspace is a frequent cause of hypoxemia.
- Mechanical deadspaceis the portion of the tidal volume that fills the ventilator tubing from the Y adapter of the ventilator circuit to the patient. Mechanical deadspace can be added or removed to cause small changes in the patient’s CO2 levels. Adding mechanical deadspace increases the patient’s work of breathing. So, be careful adding deadspace to patients who may already have an increased work of breathing, such as COPD patients.
Bedside Assessment Tip
Removing mechanical deadspace can reduce the patient’s carbon dioxide level. Adding mechanical deadspace will increase the carbon dioxide level.