Q.30 Inspiratory Capacity (IC) can be calculated as
- Tidal Volume (TV) + Inspiratory Reserve Volume (IRV)
- Tidal Volume (TV) + Expiratory Reserve Volume (ERV)
- Tidal Volume (TV) − Inspiratory Reserve Volume (IRV)
- Expiratory Reserve Volume (ERV) − Inspiratory Reserve Volume (IRV)
Correct Answer: Tidal Volume (TV) + Inspiratory Reserve Volume (IRV)
Inspiratory Capacity (IC) measures maximum air inhalation from resting expiratory level, calculated as TV + IRV, typically ~3500 mL in adults.
Option Analysis
Option 1: Correct
IC equals tidal volume (normal breath, ~500 mL) plus inspiratory reserve volume (extra forceful inhalation after normal breath, ~3000 mL), representing total inspiratory capacity from functional residual capacity.
Option 2: Incorrect
TV + ERV calculates Expiratory Capacity (EC), the maximum exhalable volume from normal inhalation end-point (~1500 mL), not inspiratory measure.
Option 3: Incorrect
Subtracting IRV from TV yields no physiological lung volume; it would produce a negative or meaningless value unrelated to any capacity.
Option 4: Incorrect
ERV minus IRV lacks respiratory significance; ERV (~1000 mL) is post-normal exhalation extra volume, while IRV is inhalation reserve—subtraction irrelevant.
Introduction to Inspiratory Capacity Calculation
Inspiratory capacity calculation defines IC as tidal volume (TV) plus inspiratory reserve volume (IRV), quantifying maximum inhalation from normal exhalation. Typical adult IC is 3000-3500 mL, key in spirometry for assessing restrictive lung diseases.
Lung Volume Definitions
TV (~500 mL) is normal breath air; IRV (~2500-3000 mL) adds forceful inspiration beyond TV. IC = TV + IRV measures from end-normal expiration to maximum inspiration.
Clinical Measurement
Spirometry directly measures TV and IRV via forced maneuvers; IC assesses inspiratory muscle strength and lung expansion.
Lung Parameter Formula Typical Volume (mL) Tidal Volume (TV) Measured 500 Inspiratory Reserve (IRV) Measured 3000 Inspiratory Capacity (IC) TV + IRV 3500 Expiratory Reserve (ERV) Measured 1000 Vital Capacity (VC) TV + IRV + ERV 4500
