It is useful to divide the total space within the lungs into volumes and capacities. These can be measured to aid in the definitive diagnosis, quantification and monitoring of disease. They allow an assessment of the mechanical condition of the lungs, its musculature, airway resistance and the effectiveness of gas exchange at the alveolar membrane while being, for the most part, cheap, non-invasive and simple to measure.
In this article we will look at the volumes and capacities within the lungs, how they can be measured and how they are affected by pathology.
|Tidal volume||Volume that enters and leaves with each breath, from a normal quiet inspiration to a normal quiet expiration||0.5L||
Changes with pattern of breathing e.g. shallow breaths vs deep breaths
Increased in pregnancy
|Inspiratory reserve volume||Extra volume that can be breathed in above tidal volume, from normal quiet inspiration to maximum inspiration||2.5L||Relies on muscle strength, lung compliance (elastic recoil) and a normal starting point (end of tidal volume)|
|Expiratory reserve volume||Extra volume that can be breathed out below tidal volume, from normal quiet expiration to maximum expiration||1.5L||
Relies on muscle strength and low airway resistance
Reduced in pregnancy, obesity, severe obstruction or proximal (of trachea/bronchi obstruction)
|Residual volume/reserve volume||Volume remaining after maximum expiration||1.5L||Cannot be measured by spirometry|
Capacities are composites of 2 or more lung volumes. They are fixed as they do not change with the pattern of breathing
|Vital capacity/forced vital capacity||Volume that can be exhaled after maximum inspiration (to maximum expiration)||Inspiratory reserve volume + tidal volume + expiratory reserve volume||4.5L||
Often changes in disease
Requires adequate compliance, force of muscles and low airway resistance
|Inspiratory capacity||Volume breathed in from quiet expiration to maximum inspiration||Tidal volume + inspiratory reserve volume||3L|
|Functional residual capacity||Volume remaining after quiet expiration||Expiratory reserve volume + residual volume||3L||Many things affect this|
|Total lung capacity||Volume of air in lungs after maximum inspiration||Sum of all volumes||6L||
Restriction < 80%
Hyperinflation > 120%
Measured with helium dilution
Anatomical (serial) dead space is the volume of air that never reaches alveoli and so never participates in respiration, includes volume in upper and lower respiratory tract up to and including the terminal bronchioles
Alveolar (distributive) dead space is the volume of air that reaches alveoli but never participates in respiration (e.g. due to underperfusion from hypoxic vasoconstriction).
Measuring Volumes and Capacities
Spirometry can measure tidal volume, inspiratory reserve volume and expiratory reserve volume (but it cannot measure residual volume)
Measured values are compared to standards for height, age and sex. Height has the greatest influence on capacities
How it’s done: Subject breathes from a closed chamber over water. The chamber is filled with oxygen and as they breathe they remove/add gas to the chamber. A weight above the chamber changes height with ventilation. Its height is recorded with a pen to obtain volume inspired or expired over time (flow).
This can measure total lung capacity but if there is obstruction then the helium may not reach all areas of the lung producing an underestimate (it measures ventilated lung volume)
How it’s done: After quiet expiration the subject breathes in a gas with a known concentration of helium (an inert gas). They hold their breath for 10 seconds so the helium mixes with air in the lungs, becoming diluted. The concentration of helium is measured after expiration. The dilution factor allows the volume of air participating in ventilation to be calculated.
This represents serial/anatomical dead space in the conducting airways up to and including the terminal bronchioles, usually 150mL
How it’s done: Subject takes normal breaths of pure O2. During this time the concentration of nitrogen in the expired air is measured over time. The nitrogen comes from the alveolar air. Initially dead space air is expired which never reached the alveolar air and so is still pure O2. Then a mixture of dead space air and alveolar air is expired so the nitrogen concentration rises. Then purely alveolar air is expired so the nitrogen concentration reflects that of alveolar air. Can determine from a graph of the results the volume of air that contributes to dead space.
NB: There is a variation of this test where nitrogen is measured over only one breath of extended expiration
Visualising lung volumes
A vitalograph (also known as a spirometer) plots volume expired over time from spirometry tests. There is initially a rapid rise which then trails to a plateau. It changes predictably in disease
Important things to note are:
- FVC (forced vital capacity)
- FEV1 (forced expiratory volume in 1 second, the volume expired in the first second)
- Ratio FEV1/FVC
Flow volume loop
This plots flow over volume (shows inspiratory flow and expiratory flow). It is more sensitive than spirometry and can tell you where in the respiratory tract the disease lies
Important things to note are:
- PEFR (peak expiratory flow rate) but this is very insensitive to pathologies of the lower respiratory tract (only affected by changed in the trachea and bronchi of the upper respiratory tract)
- Vital capacity (can be read off x-axis)
- Shape of the curve (eg. spooning in obstructive disease)
Nitrogen washout graph
This plots %N over volume expired
Important things to note:
- Volume at which A1 = A2 (represents dead space volume)
Clinical relevance – Obstructive and Restrictive Deficits
|Process||FEV1/FVC||Key changes||Other volume changes|
|Obstructive||<0.7||FEV1 is low (30-35% expected in severe disease)||Residual volume high|
|Restrictive||>=0.7||FVC is low||
Residual volume low
Inspiratory reserve volume low
In obstructive disease the FEV1 is reduced due to increased resistance (and possibly even complete obstruction of airways) during expiration. Air trapping can also occur where more air is inspired than is expired, which can cause the residual volume to increase. In asthma the obstruction is reversible which can aid in diagnosis. This means that FEV1/FVC will recover on re-test after the application of a bronchodilator such as salbutamol.
Examples of obstructive diseases are asthma, COPD (chronic bronchitis, emphysema), tracheal stenosis and large airway tumors.
In restrictive disease the FVC is reduced due to restricted inflation of the lungs and chest, eg from weak inspiratory muscles or an anatomical deformity. This causes the inspiratory reserve volume to be reduced as the lungs can’t inflate as much during maximum inspiration. Residual volume can also be reduced as expiration is more effective than inspiration
Examples of restrictive diseases are interstitial pulmonary fibrosis, muscle weakness, kyphoscoliosis, obesity, tense ascites.