The McGill Physiology Virtual Lab

Respiration Laboratory

Pneumotachometry> Experiments
  • in the first exercise, air flow and tidal volume signals are recorded during normal respiration and the vital capacity manoeuvre.
  • in the second exercise, the Forced Vital Capacity manoeuvre is recorded. The rate of the expired air flow will depend on the elastic recoil properties of the lung and the resistance to air flow as well as on the voluntary expiratory effort.
  • the third exercise will show the expiratory flow vs. volume changes with increased expiratory resistance.
  • CO2 and O2 concentrations will be recorded during different breathing patterns.
Lung volume and capacities
The following values are measured:
  1. duration of the respiratory cycle
  2. inspiratory time and expiratory time
  3. peak inspiratory and expiratory flows
  4. tidal volume
The following values are calculated:
  1. rate of breathing or frequency
  2. minute ventilation


The red trace on top shows flow on Channel 1.
Where are peak inspiratory and peak expiratory flows on this trace?

The blue trace (Channel 2) shows corresponding volumes with normal respiration followed by maximal inspiration and maximal expiration.

Refer to the Theory section to review concepts.

Forced Vital Capacity manoeuvre

The Forced Vital Capacity (FVC) manoeuvre is based on the Vital Capacity manoeuvre with an added element of speed; an individual exhales with a maximal force and speed. The rate of the expired air flow will depend on the elastic recoil properties of the lung and the resistance to airflow as well as on the voluntary expiratory effort. A healthy individual is able to expel at least 80% of his/her vital capacity during the first second of forced expiration.

The start of the forced expiration is obtained by linear extrapolation of the steepest part of the volume-time diagram.

FVC is the volume change of the lung between a full inspiration to total lung capacity and a maximal expiration to residual volume. The measurement is performed during forceful expiration; the preceding maximal inspiration does not need to be done forcefully . The manoeuvre is performed together with the assessment of the FEV1 and of maximum expiratory flow-volume curves.

Questions to think about:
-How would FEV1 change with airflow limitations: airway obstruction, bronchoconstriction or bronchodilatation?
-What does FEV1 expressed as a percentage of the FVC show?
-How does FEV1as a percentage of VC change with age in the adult?

Flow-volume curve

It is the graph produced by plotting the instantaneous flow of respiratory gas against the simultaneous lung volume. The principal advantage of the flow-volume curve is that it can show whether flows are appropriate for a particular lung volume.

Study the shape of the expiratory flow trace (top part of diagram A above): after a rapid rise, there is a slow decline unlike the round inspiratory flow. Why? As you forcefully expire, the small airways are closing (pressure outside the small airways is greater than inside the small airways), therefore there is a further restriction to air flow (slow decline of last part of expiratory flow).
During inspiration, the airways are patent (unobstructed) because intrathoracic pressure opens the airways.

Can you identify the volume quantities (1, 2 and 3) on the flow-volume curve below constructed while the tidal volume and maximal inspiration/expiration were recorded?

Flow-volume plot changes with different airway conditions

Questions to think about:
How would the Flow-Volume curve change with restrictive lung disease, obstructive lung disease, and fixed major airway obstruction?

Obstructive disease: Although all flow rates are diminished, expiratory prolongation predominates. What are examples of obstructive lung disease?

Restrictive disease: the flow-volume curve is narrowed because of diminished lung volumes, but the shape is generally the same as in normal volume. Flow rates are greater than normal at comparable lung volumes because the elastic recoil of lungs holds the airways open. What are examples of restrictive lung disease?

Fixed obstruction: the flow is equally limited during expiration and inspiration.

Breathing patterns and O2/CO2 analysis

Technical detail of the apparatus used to measure O2/CO2 : it operates with an infra-red transducer to measure CO2 concentration and a visible spectrum transducer to measure O2 concentration. It also consists of a variable pump which draws sample gas through the two transducers. The pump can be set to any flow rate in the range 35 ml/min to 200 ml/min. The analyzer has a response time of about 0.2 s (at ~200 ml/min).

The detail above shows the delay between respiratory phases and the rate of gas sampling: at the end of maximal expiration, a maximal CO2 content would be recorded.

Notice and explain differences in the gas concentrations in relation to the following breathing patterns:

  • during normal breathing

  • during a shallow and rapid respiration

  • following the VC manoeuvre

  • during and following inspiratory apnea