Spirometry Lab

What does spirometry measure?

airflow and corresponding changes in lung volume

Inspiratory Reserve volume (IRV)

The maximum volume above the tidal volume that we can inspire into our lungs (~3L)

Tidal Volume (V_T)

• Volume we inspire and expire during restful breathing.

• Normally the rate of breathing is 10-12 respiratory cycles per minute and V_t is ~0.5L

Expiratory reserve volume (ERV)

Maximum volume below the tidal volume that we can expire from our lungs (~1.5L)

Residual Volume (RV)

Volume of air remaining in the lungs after full expiration (~1.2L) as we can never empty the lungs completely

Inspiratory capacity (IC)

• the total amount of air that can be inspired after a tidal expiration:

• = TV + IRV

Expiratory Capacity (EC)

• All the air exhaled in a maximal expiration after a normal inspiration.

• = V_T + ERV

Functional Residual Capacity (FRC)

• the amount of air in the lungs after a tidal expiration:

• = ERV + RV

Vital Capacity

• All the air that can be expired from a maximal inspiration

• = IRV + V_T + ERV

Total lung capacity

• All the air that it is possible for the lungs to contain

• = IRV + V_T + ERV + RV

Directly measurable lung volumes

• VC

• IC

• IRV

• V_T

• ERV

Indirectly measurable lung volumes

• RV

• TLC

• FRC

FEV₁

Forced Expiratory Volume in One Second:

• The maximal volume of gas, which can be expired from the lungs in the first second of a forced expiration from full inspiration

• Uses maximal expiratory effort

• Represents more than 80% of exhaled volume in healthy young adult

FVC

Forced Vital Capacity:

The maximal volume of gas, which can be expired from the lungs during a forced expiration from full inspiration

FEV₁/FVC %

• The proportion of the FVC, which can be expelled during the first second of expiration – expressed as a percentage

• can indicate if someone has obstructive lung disease

PEF

Peak Expiratory Flow

• Maximum expiratory flow that can be sustained for at least 10msecs (L/min)

Flow volume loops

The y-axis indicates flow rate (L/s). By convention, expiratory flow is positive (above zero) while inspiratory flow is negative (below zero). The x-axis indicates the volume in liters (L).

The loop "begins" when you start to exhale from total lung capacity (TLC). This is where both flow and volume are zero.

Flow volume loop expiration

Flow reaches a peak value, called the peak expiratory flow rate (PEFR). Following this, flow rate decreases linearly as more air is expired (the curve descends in a relatively straight line). 

The flow during the middle half of forced expiration is referred to as forced expiratory flow (FEF). It is common to mark FEF at defined percentages of the forced vital capacity (75%, 50%, and 25%). 

Flow volume loop Inspiration

When the flow reaches zero, you have expired as much air as possible. The only air remaining in the lungs is the RV. The difference in volume between the beginning and end of a maximal expiration is the VC.

If you now make a maximal inspiration you will at some point reach a maximal inspiratory flow rate (PIFR). Flow eventually returns to zero at TLC, effectively closing the loop. But closure of the loop is not necessary, as it is the expiratory information that is diagnostically significant.

Note that time is not shown in flow-volume loops so it is not possible to determine FEV₁. This is determined from the original recordings from which the data for the flow-volume loops is derived.

Criteria for acceptable spirograms

1. Free from artefacts e.g.cough, glottis closure, early termination or cut-off, effort not maximal throughout, leak, obstructed mouthpiece

2. Good starts; low extrapolated volume

3. Satisfactory exhalation; duration >6s or a plateau in the volume-time curve

Use of peak flow meters

• Reflects mainly the calibre of the bronchi / larger bronchioles

• Mainly used in the management of patients with variable airflow limitation (asthma)

• Very effort dependent

• Records first few milliseconds

• Record the best of 3 attempts

Common errors of using peak flow meters

• Coughing

• Spitting

• Poor seal

• Insufficient inspiration

• Slow expiration

Obstructive lung disease

• e.g. COPD, Asthma

• Narrowing of airways

Restrictive lung diseases

• Pulmonary fibrosis: reduction of lung volume or elasticity

• DMD: Weakened respiratory muscles

• Kyphoscoliosis: Thorax deformity

Obstructive flow volume loop

Emphysema flow volume loop

Restrictive flow volume loop

During an asthma attack, it is more difficult to expire than to inspire. Why might explain this phenomenon?

As bronchi are narrowed, they cannot widen as much with the expansion of the lungs leading to longer expiration and increase in resistance of airflow.

COPD flow volume loop features

• scooped out appearance of expiratory curve

• increased FRC

• increased RV

Lower airway obstruction diseases

• Asthma

• Chronic bronchitis

• Emphysema

• Cystic fibrosis

Upper airway obstruction disorders

• epiglottitis

• upper airway tumour

• foreign body obstruction