F4i / 17B18: Spirometry

17B18: Exam Report

Draw and numerically label, on a spirogram, the lung volumes and capacities of a 30 kg child.

87% of candidates passed this question.

This core respiratory physiology topic was well answered by most candidates. Candidates generally were able to draw a spirogram. A common omission was inspiratory capacity.

F4i / 17B18: Draw and numerically label, on a spirogram, the lung volumes and capacities of a 30kg child.

Spirogram

Volume

Definition

mL/kg

VT

Volume of air inspired & expired with a normal breath

7mL/kg

IRV

Volume of air which can be inspired over & above VT when the subject inspires at full force

45mL/kg

ERV

Maximum volume of air which can be expired with forceful expiration at the end of VT

15mL/kg

RV

Volume remaining in lungs after forceful expiration

15mL/kg

VC

IRV + VT + ERV

The maximum amount of air which can be expired after forced inspiration

67mL/kg

IC

IRV + VT

Amount of air that can be breathed in at full inspiration

52mL/kg

FRC

ERV + RV

The amount of air left in the lungs at the end of normal VT

30mL/kg

TLC

Volume of gas in lungs at end of maximal forced inspiration

82mL/kg

  • Pulmonary gas transport depends on the filling capacity of the lungs
  • Markers of lung filling are volumes
  • >2 volumes are capacities
  • Measured non-invasively by spirometry
  • Flow rates & volumes are measured by connecting a spirometry sensor through a mouthpiece
  • Subject takes a normal tidal volume & then maximum inspiration, followed by maximum expiration, then returns to tidal ventilation
  • Recording of the test is taken as a:
    • Forced spirogram (volume vs time)
    • Flow volume loop (flow vs volume)
  • FVC and FEV1 are the most commonly derived, but you can derive dozens of parameters
  • Spirometry can only measure expired gas & after full expiration RV remains in the lungs
  • ∴RV & any capacity containing it cannot be measured → requires gas dilution methods or body plethysmography