How do you record inspiratory capacity on a spirometer?

Recording Inspiratory Capacity on a Spirometer

To record inspiratory capacity (IC) on a spirometer, have the patient breathe normally at rest, then from the end of a normal passive exhalation (functional residual capacity), instruct them to take a slow, full inspiration without hesitation to total lung capacity. 1

Definition and Clinical Significance

• Inspiratory capacity is the volume change recorded at the mouth when taking a slow full inspiration with no hesitation, from a position of passive end-tidal expiration (FRC) to maximum inspiration, expressed in liters at BTPS. 1
• IC serves as an indirect estimate of lung hyperinflation at rest and is particularly useful for assessing changes in FRC with pharmacological interventions and physical exercise. 1

Equipment Requirements

• The spirometer or flow meter must comply with FVC requirements and be capable of accumulating volume for 30 seconds. 1
• Volume accuracy must be within ±3.5% of the reading or ±65 mL, whichever is greater. 1, 2
• The display should show both inspiratory and expiratory maneuvers ideally, though at minimum the entire recorded maneuver must be displayed. 1

Step-by-Step Procedure

Patient Preparation:

• Position the patient seated upright in a chair with armrests (without wheels for safety). 2
• Apply a nose clip or manually occlude the nares. 2
• Have the patient place the mouthpiece in their mouth with lips sealed around it. 1

Maneuver Execution:

• Instruct the patient to breathe normally (tidal breathing) for several breaths. 1
• At the end of a normal passive exhalation (at FRC), immediately instruct the patient to inhale slowly and completely to total lung capacity without any hesitation. 1
• The inspiration should be slow and complete, taking approximately 5-6 seconds in healthy subjects. 1
• Ensure no air is exhaled while performing the inspiratory maneuver. 1

Quality Control and Common Pitfalls

Critical Technical Points:

• The maneuver must start from passive end-tidal expiration (FRC), not from residual volume. This distinguishes IC from inspiratory vital capacity (IVC). 1
• There must be no hesitation during the inspiratory maneuver, as this can underestimate IC. 1
• Ensure there are no leaks at the mouth and no obstruction of the mouthpiece. 1

Avoiding Underestimation:

• IC may be underestimated if the inspiratory maneuver is too slow due to poor effort or hesitation. 1
• Premature closure of the glottis can also lead to underestimation. 1
• The technician must observe the patient's inhalation to ensure it is complete and that air is not exhaled while the mouthpiece is being inserted. 1

Graphical Recording

• Figure 12 from the European Respiratory Society guidelines illustrates the proper tracing: tidal breathing followed by an inspiratory maneuver to TLC to record IC, which can then be followed by a full expiration to RV if measuring expiratory reserve volume. 1
• The maneuver is not forced; it should be performed in a relaxed manner except at the point of reaching maximum inspiration where extra effort is required. 1

Safety Considerations

• Monitor patients closely for dizziness, especially older subjects, as prolonged interruption of venous return can cause syncope. 1, 2
• If dizziness occurs, stop the maneuver immediately. 2