Vd/Vt

lungs representing deadspace ventilation

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.

Vd/Vt = PaCO2-PECO2/PaCO2

Peco2 is obtained from the end tidal co2 monitor

What is deadspace?

Deadspace is wasted ventilation. There are three types of deadspace:

  1. Anatomic deadspace is 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. 
  2. Alveolar deadspace occurs 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.
  3. Mechanical deadspace is 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.

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Respiratory Therapist