Alveolar dead space is defined as the sum of the volumes of those alveoli which have little or no blood flowing through their adjacent pulmonary capillaries, i.e., alveoli that are ventilated but not perfused, and where, as a result, no gas exchange can occur.

Measurement of alveolar dead space may facilitate estimation of disease progression, increase the efficacy of some interventions (particularly ventilatory), and improve perioperative outcome

Alveolar dead space is measured by Bohr's method.

The Bohr equation is used to quantify the ratio of physiological dead space to the total tidal volume, and gives an indication of the extent of wasted ventilation.

The equation is:

Vd = Vt * (PaCO2 - PeCO2)/PaCO2

• Vd = Dead space volume
• Vt = Tidal volume
• PaCO2 = Pressure of arterial CO2
• PeCO2 = Pressure of expired CO2 (End tidal CO2 on capnography)

Reference:

1. Hardman JG, Aitkenhead AR. Estimating Alveolar Dead Space from the Arterial to End-Tidal CO2 Gradient: A Modeling Analysis Anesth Analg. 2003 Dec;97(6):1846-51.
2. Kline JA, Kubin AK, Patel MM, et al. Alveolar Dead Space as a Predictor of Severity of Pulmonary Embolism Acad Emerg Med. 2000 Jun;7(6):611-7.
3. Frankenfield DC, Alam S, Bekteshi E et al. Predicting dead space ventilation in critically ill patients using clinically available data. Crit Care Med. 2010 Jan;38(1):288-91. doi: 10.1097/CCM.0b013e3181b42e13.