Lung Volumes and Spirometry

Spirometry is a technique to measure the mechanical properties of the respiratory system by measuring a variety of flows and volumes.

Commonly measured parameters include:

  • Lung volumes and capacities
  • The volume of air forcibly exhaled in one second (FEV1)
  • The total volume of air forcibly exhaled (FVC)
  • The peak expiratory flow rate (PEFR)
  • Forced expiratory flow 25-75 (the average flow during the middle 25-75% of FVC)
    • Patients can then be given bronchodilators to assess ‘reversibility’ of airways obstruction when determining whether a patient has asthma or COPD
    • More than 12% increase in FEV1 after bronchodilator therapy is considered reversible

Lung Volumes and Capacities

The different stages of inspiration and expiration can referred to as volumes or capacities, where a capacity is simply the sum of at least two volumes.

Tidal volume (VT)

This is the volume of gas exhaled during a normal breath at rest.

Residual volume (RV)

This is the volume of gas remaining in the thorax after a full forced expiration.

Inspiratory reserve volume (IRV)

This is the maximum amount of gas that can be inhaled on top of a normal tidal breath inhalation. That is, at the end of passive inspiration, the volume that can be inhaled until the lung is maximally inflated.

Functional residual capacity

This is the volume of gas remaining in the lungs at end tidal expiration. It is of particular importance to the anaesthetist, as it forms the reservoir for oxygen during apnoeic intubation. General anaesthesia causes a profound decrease in functional residual capacity compared to the upright position, as the diaphragm is pushed upwards by the abdominal contents, and the respiratory and spinal muscles relax, causing a reduction in the antero-posterior diameter of the thoracic cage.

Functional residual capacity = Residual Volume + ERV

Expiratory reserve volume (ERV)

The expiratory equivalent of IRV, in that it is the maximum volume of gas that can be exhaled after a passive tidal volume exhalation. It is the difference between the functional residual capacity and the residual volume.

Vital capacity (VC)

The volume of gas exhaled from full inspiration to full expiration (or inhaled vice versa)

Vital Capacity = ERV + Tidal Volume + IRV

Total lung capacity (TLC)

This is the volume of gas in the thorax at maximal inspiration.

Total Lung capacity = RV + ERV + VT + IRV

Note the important difference between TLC and VC. The total lung capacity includes the residual volume, as it is the total volume of gas in the thorax at maximal inspiration. The vital capacity meanwhile, is the volume that can be actively exhaled by the patient, and does not include the residual volume.

Closing volume (CV)

The volume of gas in the lungs at which the small airways will begin to close, as the balance between elastic recoil of the lung parenchyma, the chest wall and the resulting intrapleural pressure changes, meaning the pressure outside the small airways exceeds the airway pressure, causing the airway to collapse.

  • Closing volume equals FRC when standing at approximately 65-70 years of age
  • Closing volume equals FRC when lying supine at 40 years of age

Closing capacity is usually smaller than FRC in young healthy people, meaning that the airways don’t start to collapse unless there is forced expiration below FRC. However as patients age, or develop lung disease, CV can exceed FRC, and lead to small airway collapse during normal breathing. The numbers to remember are as follows:

Graph and worked example

  • Remember that a capacity is the sum of two or more volumes
    • Total Lung capacity = volume of gas in lungs at maximal inspiration = 80ml/kg = 6000ml
      • Inspiratory Capacity = 3000ml
        • Inspiratory reserve volume = 2500ml
        • Tidal volume = 500ml
      • Functional Residual Capacity = 3000ml = 30ml/kg
        • Expiratory reserve volume = 1500ml
        • Residual volume = 1500ml
    • Vital Capacity = volume increase from forced expiration to maximal inspiration = 60-70ml/kg
      • Inspiratory reserve volume + Tidal volume + Expiratory reserve volume
        • 2500 + 500 + 1500 = 4500ml

A normal vitalograph spirometry trace for a single exhaled breath

  • Flow is maximal at the start and gradually decreases in a negative hyperbolic pattern
  • Asymptote is at FVC
  • 75% of the forced vital capacity can be expired actively in one second, giving the FEV1 value

What would an obstructive pathology look like?

  • Generally speaking obstructive lung pathology will result in a reduced FVC
  • Given the rate at which gas can be exhaled is reduced, the FEV1/FVC ratio is therefore reduced as well

What would a restrictive pathology look like?

  • Again the patient is likely to have a reduced FVC
  • Expiration not as affected as obstructive pathology, especially early expiration, so FEV1/FVC ratio can be normal or even increased

What volumes cannot be measured using spirometry?

  • The residual volume, and any capacity that includes the residual volume
    • Total lung capacity
    • Functional residual capacity

The residual volume is the volume of gas that remains in the lungs at maximal expiration, and it is approximately 15-20ml/kg.

It would require one of the following three methods to measure it:

Helium dilution

A known quantity and concentration of inert gas is delivered to the patient to breathe while inside the box of known volume. The new concentration measured will then tell you the new volume, and therefore the intrathoracic capacity.

Body plethysmography

The patient sits in a sealed box of known volume, with pressure measurement equipment. Since Boyle’s law states that P1V1 = P2V2, if we know the pressure and volume of the box, and the pressure in the airways, then we can calculate the intrathoracic volume.

Nitrogen washout

Patient breathes 100% oxygen while the concentration of nitrogen they exhale is measured. They keep breathing until all nitrogen has been exhaled. This tells you the total quantity of nitrogen in the lungs. Dividing this by the nitrogen concentration on the first exhalation will give the total volume of the thorax.


What is compliance?

Compliance is the change in lung volume per unit pressure, measured in litres per cmH2O. Lung compliance depends on two factors:

  • The elastance of the lung tissue itself
    • i.e. how much the tension changes as volume increases
  • The surface tension in the alveoli
    • This is greatly reduced by surfactant

Lung compliance can also be dynamic or static. Static compliance is the compliance of the lung when there is no gas flow. It therefore measures alveolar ‘stretch’. Dynamic compliance, meanwhile, is the compliance of the lung during the respiratory cycle. It therefore measures the airway resistance during equilibration of gases


Total thoracic compliance includes lung and chest wall compliance

Chest wall compliance is approximately 150-200 ml per cmH2O

Lung compliance is similar

They are summed as reciprocals where

1/total thoracic compliance = 1/chest wall + 1/lung

= 1/200 + 1/200

= 100 ml per cm H2O

What is hysteresis?

Hysteresis occurs when a value of a parameter differs depending on whether it is increasing or decreasing. The hysteresis curve seen for the pressure volume loop is caused by two factors:

  • Energy required to overcome resistance
    • This is known as frictional loss
  • Energy lost due to stretch and recoil of elastic tissues
    • This is known as viscous loss

Flow-Volume Curves

This is a normal vital capacity breath, and the Y axis is in litres per second. The positive area of the graph is expiration and inspiration is negative, and the graph moves in a clockwise direction. There is a maximum achievable flow rate caused by dynamic airway compression, due to raised intrathoracic pressure, that cannot be exceeded no matter how much effort is exerted by the patient

Obstructive

Obstructive lung disease results in a reduced flow rate due to obstructed airways and reduced elastic recoil of the lung tissue itself. The residual volume may actually be increased due to gas trapping, and so total lung capacity may also increase slightly for the same reason. At all points on the expiratory limb, the flow rate is reduced, while the flows during inspiration are relatively unaffected.

Restrictive

Restrictive lung disease results in a dramatically reduced total lung capacity. Max flow rate is also reduced, but there is a slightly higher flow late in expiration due to increased elastic recoil of lung tissue, resulting in less airway obstruction.

Fixed large airway obstruction

An example of a fixed upper airway obstruction is tracheal stenosis. The residual volume and total lung capacity remain relatively unaffected, however peak flows in both inspiratory and expiratory phases are reduced according to the size of the airway orifice.

Variable obstruction

As previously, residual volume and total lung capacity are relatively unaffected. Flow is easier in inspiration as the negative intrathoracic pressure helps to pull open the small obstructed airways, however during expiration the increased (or less negative) intrathoracic pressure allows the small airways to collapse and obstruct, so flow rate is severely limited during expiration.

Extrathoracic obstruction

An example of this would be a laryngeal mass sat immediately above the vocal cords. Again residual volume and total lung capacity are normal. Flow is easier in expiration as the positive intrathoracic pressure pushes the obstruction out of the way, so it remains relatively unaffected. During inspiration however the obstruction worsens and airway collapse or obstruction means the flow rate is severely reduced.


How can you measure Gas exchange?

  • VQ mismatch
    • Using radioisotope scanning
  • Arterial Blood gas analysis
  • Diffusion capacity
  • Pulse oximetry

What is Transfer factor?

The volume of carbon monoxide transferred across the alveolar membrane into the blood per minute per unit of partial pressure of carbon monoxide (TLCO). Since CO is so rapidly adsorbed by haemoglobin, the rate limiting factor in this process is the membrane diffusion.

Normal value is 17-25 ml/min/mmHg

Any process that thickens the alveolar membrane, reduces the amount of membrane available, or reduces blood flow to the membrane will reduce TLCO, such as:

  • Fibrosis
  • Pneumonectomy
  • Pulmonary embolism

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