In this article we will look at the volumes and capacities within the lungs, how they are measured and how they are affected by pathology.
It is useful to divide the total space within the lungs into volumes and capacities. This allows for an assessment of the mechanical condition of the lungs, its musculature, airway resistance and the effectiveness of gas exchange at the alveolar membrane. These can be determined by simple, cheap and non-invasive tests.
|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 inspired 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 expired 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 composed of 2 or more lung volumes. These 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 (ie. maximum inspiration to maximum expiration)||Inspiratory reserve volume + tidal volume + expiratory reserve volume||4.5L||
Often changes in disease
Requires adequate compliance, muscle strength 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||Affected by height, gender, posture, changes in lung compliance. Height has the greatest influence.|
|Total lung capacity||Volume of air in lungs after maximum inspiration||Sum of all volumes||6L||
Restriction < 80% predicted
Hyperinflation > 120% predicted
Measured with helium dilution
Anatomical (serial) dead space is the volume of air that never reaches alveoli and so never participates in respiration. It 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. This can reflect alveoli that are ventilated but not perfused, for example secondary to a pulmonary embolus.
Measuring Volumes and Capacities
Simple spirometry can measure tidal volume, inspiratory reserve volume and expiratory reserve volume. However, it cannot measure residual volume.
Measured values are standardised for height, age and sex. Of these, height is the factor with the greatest influence upon capacities.
The subject breathes from a closed circuit over water. The chamber is filled with oxygen and as they breathe, gas increased and reduces the volumes within the circuit. A weight above the chamber changes height with each ventilation according to the circuit volume. Its height is recorded with a pen to reflect the volume inspired or expired over time.
Helium dilution is used to measure total lung capacity. However, it is only accurate if the lungs are not obstructed. If there is a point of obstruction, helium may not reach all areas of the lung during a ventilation, producing an underestimate as only ventilated lung volumes are measured.
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, allowing helium to mix with air in the lungs, diluting the concentration of helium. The concentration of helium is then measured after expiration. The volume of air which is ventilated is then calculated according to the degree of dilution of the helium.
A method for calculating serial/anatomical dead space in the conducting airways up to and including the terminal bronchioles (usually 150mL).
The subject takes a breath of pure oxygen and then exhales through a valve which measures nitrogen levels. At first, pure oxygen is exhaled, representing the dead space volume as the air exhaled never reached the alveoli and underwent gaseous exchange.
Then, a mixture of dead space air and alveolar air is expired, meaning the detected concentration of nitrogen increases as nitrogen rich air from the dead space reaches the valve. After a few breaths, the lungs are washed out of pure oxygen, meaning that purely alveolar air is expired, with the nitrogen levels reflecting that of alveolar air. The levels of nitrogen measured over time can be used to calculate the anatomical dead space volume of the lungs.
Visualising lung volumes
A vitalograph creates plots of volume against time, using data collected from spirometry tests.
Two important spirometry volumes that can be measured from a Vitalograph are:
- FVC (forced vital capacity) – the maximal volume of air that a subject can expel in one maximal expiration from a point of maximal inspiration.
- FEV1 (forced expiratory volume in one second) – the maximal volume of air that a subject can expel in one second from a point of maximal inspiration.
The proportion of air that can be exhaled in the first second compared to the total volume of air that can be exhaled is important in assessing for possible airway obstruction. This proportion is known as the FEV1/FVC ratio. This ratio is important in clinically for diagnosis of respiratory conditions.
Flow volume loop
This plots flow over volume (showing expiratory flow and inspiratory flow as positive and negative values respectively).
Important factors to consider when assessing flow-volume curves are as follows:
- Peak Expiratory Flow Rate (PEFR) – the rate of flow.
- Vital capacity – the volume expired, calculated from the X-axis.
- Shape of the curve – ‘spooning’ in obstructive disease, small overall loop in restrictive disease.
Nitrogen washout graph
This plots the percentage concentration of nitrogen in exhaled air (%N) against the total volume of air expired.
The anatomical dead space is determined by the volume of exhaled air at which the volume below the washout curve (A1) is equal to the volume above the washout curve (A2).
Clinical relevance – Obstructive and Restrictive Deficits
|Obstructive||<80% of predicted||Reduced, but not to same degree as FEV1||<0.7|
|Restrictive||<80% of predicted||<80% of predicted||>=0.7|
In obstructive disease, the FEV1 is reduced due to increased resistance during expiration. Air trapping can also occur where more air is inspired than is expired. This 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.
The so-called ‘spooning‘ of a flow-volume curve in obstructive disease arises when the affected small airways begin to collapse.
As air exits the thorax in expiration, the pressure within the small airways reduces and thus the small airways are no longer propped open. This increases resistance to expiration and therefore reduces flow.
Examples of obstructive diseases are asthma, COPD (chronic bronchitis, emphysema), tracheal stenosis and large airway tumours.
In restrictive disease, the FVC is reduced due to poor lung expansion. This can be neurological, due to weak inspiratory muscles or due to 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.