Minute Volume, Tidal Volume and Frequency

The total volume of air exhaled per minute is the minute volume (VE). Minute volume (VE) equals the product of frequency of breathing (f) times the expired tidal volume (VT):

{short description of image}

For example, for a healthy adult breathing at a rate of 12 breaths/minute and a VT of 500 mL:
VE = 12 x 500 mL = 6000 mL/min (6.0 L/min)

Minute ventilation depends on body size and metabolic rate. Normal VE values range from about 5 to 10 L/min in resting adults.

{short description of image}

From Kacmarek RM, Foley K, Cheever P, Romagnoli D. Determination of ventilatory reserve in mechanically ventilated patients: a comparison of techniques. Respir Care. 1991;36:1085-92.

Minute volume is usually measured at the bedside using either a mechanical or electronic respirometer (see figure to the left). A nonrebreathing valve is used to separate out the expired volume. The tidal volume is then derived by dividing the minute volume by the rate of breathing during the measured minute: VE ÷ f = VT. Normal tidal volumes are about 5-7 mL/kg ideal body weight.

Deadspace Volume

A portion of each tidal volume occupies the conducting airways, which do not participate in gas exchange. Since the volume occupying the airways is rebreathed and 'wasted,' we call it anatomic deadspace. Normally, this anatomic deadspace is proportional to body sizeand estimated using the following formula:

Anatomic deadspace (ml) = ideal body weight (lbs) x 1

In certain disease states, some alveoli may be ventilated but not participate in gas exchange (e.g., pulmonary embolism). Since any gas that ventilates unperfused alveoli is also wasted ventilation, we call this type alveolar deadspace. The sum of the anatomic and alveolar deadspace is the physiologic deadspace.

The VD/VT ratio

The volume of fresh gas reaching the alveoli per breath equals the difference between the tidal volume and the physiologic deadspace. For example, if a healthy 150 lb male has a tidal volume of 500 mL, only about 350 mL of "fresh" gas reaches his alveoli per breath (500-150 = 350).

The ratio of physiologic deadspace to tidal volume (VDS/VT) provides an index of the efficiency of ventilation. Using the data from the prior example, the VDS/VT ratio is:

150/500 = 0.30 or 30%

If the same person were to suffer a pulmonary embolism and not increase his tidal volume, the volume of gas ventilating perfused alveoli would decrease, and the VDS/VT ratio would increase. Patients with VDS/VT ratios > 0.60 have difficulty maintaining spontaneous ventilation without either CO2 accumulation or increased work of breathing.

Alveolar Minute Ventilation

To compute alveolar volume per minute, we simply multiply the rate of breathing (f) times the difference between the tidal volume and the physiologic deadspace:

{short description of image}

For example, in a healthy adult breathing at a rate of 12/min with a VT of 500 mL
and an anatomic deadspace of 150 mL:

VA = 12 x (500 - 150) = 4200 mL

Clinical Significance

The table below shows the effect of changes in various parameters on alveolar ventilation. In healthy subjects, alveolar ventilation depends on the rate of breathing and VT. High rates and low tidal volumes result in a high proportion of wasted ventilation per minute (i.e., decreased alveolar ventilation). The most efficient breathing pattern is slow, deep breathing.

In pulmonary disease, increased deadspace causes a decrease in alveolar ventilation, unless compensation occurs. An increased breathing rate by itself worsens the problem. Effective compensation for increased deadspace requires an increased tidal volume. Elevating VT increases the elastic work of breathing. This increases oxygen consumption by the respiratory muscles. In some patients these increased demands cannot be met. In such cases, alveolar ventilation may not be adequate to meet body needs.

Effect of Rate, Tidal Volume, and Deadspace (VDphy) on Alveolar Ventilation

 

Rate of Breathing

Tidal Vol (mL)

Min Vol (mL)

VDphy
(mL)

Alveolar Vent (mL)

Normal

12

500

6000

150

4200

High rate,
low volume

24

250

6000

150

2400

Low rate,
high volume

6

1000

6000

150

5100

Increased deadspace

12

500

6000

300

2400

Compensation
for increased deadspace

12

650

7800

300

4200


Source: adapted from Ruppel, GL. Ventilation. In Scanlan, CL, Wilkins, RL & Stoller, JK (Eds.). Egan’s Fundamentals of Respiratory Care (7th Ed.). St. Louis, MO: Mosby, 1999.