## Sunday, October 4, 2009

### Calculation of Tidal Volume

Tidal volume is calculated by:

A. Inspiratory Capacity minus the Inspiratory Reserve Volume
B. Total Lung Capacity minus the Reserve Volume
C. Functional Residual Capacity minus residual volume
D. Vital Capacity minus Expiratory Reserve Volume

Answer is A. Inspiratory Capacity minus Inspiratory Reserve volume

Tidal volume is the air that moves into the lungs with each normal inspiration or the volume of air that moves out of the lungs with each expiration, which is 500 ml in a normal adult.

A brief summary of the various lung volumes:
• The tidal volume (TV), about 500 ml, is the amount of air inspired during normal, relaxed breathing.
• The inspiratory reserve volume (IRV), about 3,100 ml, is the additional air that can be forcibly inhaled after the inspiration of a normal tidal volume.
• The expiratory reserve volume (ERV), about 1,200 ml, is the additional air that can be forcibly exhaled after the expiration of a normal tidal volume.
• Residual volume (RV), about 1,200 ml, is the volume of air still remaining in the lungs after the expiratory reserve volume is exhaled.

Summing specific lung volumes produces the following lung capacities:

• The total lung capacity (TLC), about 6,000 ml, is the maximum amount of air that can fill the lungs (TLC = TV + IRV + ERV + RV).
• The vital capacity (VC), about 4,800 ml, is the total amount or air that can be expired after fully inhaling (VC = TV + IRV + ERV = approximately 80% TLC).
• The inspiratory capacity (IC), about 3,600 ml, is the maximum amount of air that can be inspired (IC = TV + IRV).
• The functional residual capacity (FRC), about 2,400 ml, is the amount of air remaining in the lungs after a normal expiration (FRC = RV + ERV).

Respiratory minute volume at rest - 6 L/min
Alveolar ventilation at rest - 4.2 L/min
Maximal voluntary ventilation - 125 to 170 L/min
Timed vital capacity - 83% of total in 1 second and 97% in 3 seconds
Work of quiet breating is 0.5 kg-m/min
Maximal work of breathing - 10 kg-m/breath

To answer the question, let us go through the formula for Inspiratory Capacity which contains Tidal Volume as one of the components.

Inspiratory Capacity is the maximum amount of air that can be inspired beyond a normal expiration.
Inspiratory Capacity (IC) = Tidal Volume + Inspiratory Reserve Volume

Hence, Tidal Volume = Inspiratory Capacity - Inspiratory Reserve Volume.

However, it should be noted that this is only theoretical and tidal volume should be measured.