Question

1. List the respiratory muscles and outline the basic process of inspiration and expiration.

2. Describe lung compliance and the factors that impact compliance.

3. List and describe the 8 different respiratory volumes we heard about.

Answer 1

1. The muscles of respiration are those muscles that contribute to inhalation and exhalation, by aiding in the expansion and contraction of the thoracic cavity. The diaphragm and, to a lesser extent, the intercostal muscles drive respiration during quiet breathing.

Accessory muscles of respiration :

The term "accessory muscles" refers to those that assist, but do not play a primary role, in breathing. Use of these while at rest is often interpreted as a sign of respiratory distress.There is no definitive list of accessory muscles, but the sternocleidomastoid and the scalenes (anterior, middle, and posterior) are typically included, as they assist in elevating the rib cage.The involvement of these muscles seems to depend on the degree of respiratory effort. During quiet breathing, the scalenes are consistently physically active, while the sternocleidomastoids are quiet. With an increase in the respiratory volume, sternocleidomastoids also become active.Both muscles are simultaneously activated when one breathes in at the maximal flow rate.

Apart from the above neck muscles, the following muscles have also been observed contributing to respiration: serratus anterior, pectoralis major and pectoralis minor, trapezius, latissimus dorsi, erector spinae, iliocostalis, quadratus lumborum, serratus posterior superior, serratus posterior inferior, levatores costarum, transversus thoracis, subclavius. The levator labii superioris alaeque nasi muscle lifts the sides of the nostrils.

Processes of inspiration and expiration:

When the intercostal muscles contract, they move up and away from the thoracic cavity. When the diaphragm contracts, it moves down towards the abdomen. This movement of the muscles causes the lungs to expand and fill with air, like a bellows (inhalation). Conversely, when the muscles relax, the thoracic cavity gets smaller, the volume of the lungs decreases, and air is expelled (exhalation).

2.

Lung compliance, or pulmonary compliance, is a measure of the lung's ability to stretch and expand (distensibility of elastic tissue). In clinical practice it is separated into two different measurements, static compliance and dynamic compliance. Static lung compliance is the change in volume for any given applied pressure.Dynamic lung compliance is the compliance of the lung at any given time during actual movement of air.

Low compliance indicates a stiff lung (one with high elastic recoil) and can be thought of as a thick balloon – this is the case often seen in fibrosis. High compliance indicates a pliable lung (one with low elastic recoil) and can be thought of as a grocery bag – this is the case often seen in emphysema. Compliance is highest at moderate lung volumes, and much lower at volumes which are very low or very high. The compliance of the lungs demonstrate lung hysteresis; that is, the compliance is different on inspiration and expiration for identical volumes.

Factors impact on compliance:

Factors identified to be in this group include patient's age, ethnicity, gender, education, and marital status. A summary of the impact of these factors on therapeutic compliance is presented

3. Four standard lung volumes, namely, tidal (TV), inspiratory reserve (IRV), expiratory reserve (ERV), and residual volumes (RV) are described in the literature. Alternatively, the standard lung capacities are inspiratory (IC), functional residual (FRC), vital (VC) and total lung capacities (TLC).

Lung Volumes:

• Tidal Volume(TV)

It is the amount of air that can be inhaled or exhaled during one respiratory cycle. This depicts the functions of the respiratory centres, respiratory muscles and the mechanics of the lung and chest wall.

The normal adult value is 10% of vital capacity (VC), approximately 300-500ml (6‐8 ml/kg); but can increase up to 50% of VC on exercise.

• Inspiratory Reserve Volume(IRV)

It is the amount of air that can be forcibly inhaled after a normal tidal volume.IRV is usually kept in reserve, but is used during deep breathing. The normal adult value is 1900-3300ml.

• Expiratory Reserve Volume(ERV)

It is the volume of air that can be exhaled forcibly after exhalation of normal tidal volume. The normal adult value is 700-1200ml. ERV is reduced with obesity, ascites or after upper abdominal surgery.

• Residual Volume(RV)

It is the volume of air remaining in the lungs after maximal exhalation. Normal adult value is averaged at 1200ml(20‐25 ml/kg) .It is indirectly measured from summation of FRC and ERV and cannot be measured by spirometry.

In obstructive lung diseases with features of incomplete emptying of the lungs and air trapping, RV may be significantly high. The RV can also be expressed as a percentage of total lung capacity and values in excess of 140% significantly increase the risks of barotrauma, pneumothorax, infection and reduced venous return due to high intra thoracic pressures as noticed in patients with high RV who require surgery and mechanical ventilation thus needs high peri-operative inflation pressures.

Lung capacities

• Inspiratory capacity(IC)

It is the maximum volume of air that can be inhaled following a resting state. It is calculated from the sum of inspiratory reserve volume and tidal volume. IC = IRV+TV

• Total Lung Capacity(TLC)

It is the maximum volume of air the lungs can accommodate or sum of all volume compartments or volume of air in lungs after maximum inspiration. The normal value is about 6,000mL(4‐6 L). TLC is calculated by summation of the four primary lung volumes (TV, IRV, ERV, RV).

TLC may be increased in patients with obstructive defects such as emphysema and decreased in patients with restrictive abnormalities including chest wall abnormalities and kyphoscoliosis.

• Vital Capacity(VC)

It is the total amount of air exhaled after maximal inhalation. The value is about 4800mL and it varies according to age and body size. It is calculated by summing tidal volume, inspiratory reserve volume, and expiratory reserve volume. VC = TV+IRV+ERV.

VC indicates ability to breathe deeply and cough, reflecting inspiratory and expiratory muscle strength.VC should be 3 times greater than TV for effective cough. VC is sometimes reduced in obstructive disorders and always in restrictive disorders.

• Function Residual Capacity(FRC)

It is the amount of air remaining in the lungs at the end of a normal exhalation. It is calculated by adding together residual and expiratory reserve volumes. The normal value is about 1800 – 2200 mL. FRC = RV+ERV.

FRC does not rely on effort and highlights the resting position when inner and outer elastic recoils are balanced. FRC is reduced in restrictive disorders. The ratio of FRC to TLC is an index of hyperinflation. In COPD, FRC is upto 80% of TLC.