Lung Volumes

The lung can be divided into various volumes. These are identified either by measurements made during lung function testing, or according to the function of the lung in gas exchange.

Lung Volumes Derived from Spirometry

During quiet breathing a small volume of gas is moved in and out of the lungs repeatedly. If a maximal inspiration is taken, followed by a maximal expiration, the volume changes occurring can be recorded using a spirometer. Figure RR.2 shows a typical spirometer trace of these changes.

Lung volumes vary with age, sex and body size (more related to height rather than weight). The lung volumes are:

■ropocity

Rts'duol vijIurTifl

Rts'duol vijIurTifl

Figure RR.2 Spirometer trace of lung volumes

Figure RR.2 Spirometer trace of lung volumes

• Total lung capacity (TLC) - volume of gas present in the lungs at the end of maximal inspiration

• Tidal volume (VT) - amount of gas inspired and expired during normal quiet breathing

• Inspiratory reserve volume (IRV) - extra volume of gas that can be inspired over and beyond the normal VT

• Expiratory reserve volume (ERV) - amount of gas that can be forcefully expired at the end of normal tidal expiration

• Residual volume (RV) - amount of gas remaining in the lungs at the end of a maximum forced expiration

• Vital capacity (VC) - maximal volume of gas which can be expelled after a maximal inspiration

• Functional residual capacity (FRC) - lung volume following expiration during quiet breathing Some typical values for the above volumes are given in Figure RR.3.

Vital Capacity

Apart from body size, the major factors that determine VC are the strength of the respiratory muscles, and chest and lung compliance. It is an important clinical measure of respiratory sufficiency particularly in patients with restrictive diseases. VC < 10 ml/kg is indicative of impending respiratory failure.

VALUES FOR LUNG VOLUMES (ml)

Lung volume

Male

Female

TLC

6000

4200

VT

500

500

IRV

3300

1900

ERV

1000

700

RV

1200

Figure RR.3

Functional Residual Capacity

From the spirometry trace in Figure RR.2 it can be seen that FRC is the lung volume at the end of normal quiet expiration, and is also equal to the sum of ERV + RV. The thoracic cage normally has a resting volume > FRC, while the normal lung has a volume < FRC. Thus, FRC represents the equilibrium point between the tendency of the lungs to collapse and of the thoracic cage to expand. It is not a fixed volume and varies with normal respiration as well as depending on gravity and other factors. FRC is decreased by 20-25% in the supine position and is further decreased by the head down posture and induction of anaesthesia. Some of the factors affecting FRC are shown in Figure RR.4.

FACTORS AFFECTING FRC

Fatfor

Change in FRC

A&e

Posture

Anowthefio

Sufflery

Pulm^nory fibrosa

Pulmonary oedema

p Decreased

Obesity

Abdominal swelling

Thoracic wati distortion

Reduced muscle Ion«

Ptwilive introlhorgcii prejiyra

Emphysemo

» Increased

Aslhma

Was this article helpful?

0 0
Peripheral Neuropathy Natural Treatment Options

Peripheral Neuropathy Natural Treatment Options

This guide will help millions of people understand this condition so that they can take control of their lives and make informed decisions. The ebook covers information on a vast number of different types of neuropathy. In addition, it will be a useful resource for their families, caregivers, and health care providers.

Get My Free Ebook


Post a comment