Physiologic Dead Space

The Physiologic Dead Space calculator computes the volume of dead space (V_D) based on the tidal volume (V_T) and the partial pressures of carbon dioxide in arterial blood, exhaled air and ambient air.

INSTRUCTIONS: Choose units and enter the following:

• (V_T)  This is the tidal volume
• (PaCO2)  This is the partial pressure of carbon dioxide in arterial blood
• (PeCO2)  This is the partial pressure of carbon dioxide in exhaled air
• (PiC02)  This is the partial pressure of carbon dioxide.

Volume of Dead Space (V_D): The calculator returns the volume in liters.  However, this can be automatically converted to compatible units via the pull-down menu.

Related Calculators

• Physiologic Dead Space (Bohr Method)

The Math / Science

The formula for the Physiologic Dead Space is:

     `Vd = Vt * ((P_a CO_2 - P_e CO_2)/ (P_a CO_2-P_i CO2))`

where:

• Vd is the dead space volume
• Vt is the tidal volume
• P_aCO₂ is the partial pressure of carbon dioxide in arterial blood
• P_eCO₂ is the partial pressure of carbon dioxide in exhaled air
• P_iC0₂ is the partial pressure of carbon dioxide.

In physiology, dead space is the volume of air which is inhaled that does not take part in the gas exchange, either because it (1) remains in the conducting airways, or (2) reaches alveoli that are not perfused or poorly perfused. In other words, not all the air in each breath is available for the exchange of oxygen and carbon dioxide. Mammals breathe in and out of their lungs, wasting that part of the inspiration which remains in the conducting airways where no gas exchange can occur.

Just as dead space wastes a fraction of the inhaled breath, dead space dilutes alveolar air during exhalation. By quantifying this dilution it is possible to measure anatomical and alveolar dead space, employing the concept of mass balance, as expressed by Bohr equation.