2017A Question 11
Draw an expiratory flow-volume curve obtained from a maximal expiratory effort after a vital capacity breath, for a person with:
a) normal lungs
b) restrictive lung disease
c) obstructive lung disease (10 marks)
Explain how and why these curves (and the derived parameters) are different for each disease state (15 marks)
Examiner Report
39.9% of candidates achieved a pass in this question.
Good answers included a representative diagram and demonstrated understanding of how the disease states alter respiratory mechanics and why this leads to the differences seen between the three curves. Almost all candidates that failed drew a diagram with significant errors and their subsequent explanations were insufficient to make up for this.
Common problems:
- Incorrect units, units without values or units with grossly incorrect values. A peak expiratory flow of 6 - 10L/min was frequently quoted despite normal minute ventilation at rest being in this range
- Drawing three separate graphs without making it clear how they differ from each other
- Drawing flow-time or volume-time curves
- Drawing peak flows that where the same or higher in the disease states compared to the normal lung curve
- Incorrect changes to residual volume and total lung capacity
- Confusing vital capacity and total lung capacity
- Incorrect or vague explanations as to why the curves differ
No marks were awarded for drawing inspiratory curves or discussing work of breathing, tidal volumes or functional residual capacity. Discussion of time related values such as FEV1 and FEV1/FVC also attracted no marks as they cannot be readily obtained from these curves.
Model Answer
Structure:
- Diagram
- Relevant physiology
- Normal flow-volume loop
- Obstructive pathophysiology
- Restrictive pathophysiology
Diagram
Relevant Physiology
| Factor | Mechanism |
|---|---|
| Flow Determinants | - Ohm’s law: - Normal values - PAlv: 38cmH2O - Patm: 0cmH2O by definition - AWR: 2cmH2O.L-1.s-1 - Factors ↑ Q: ↑ PAlv, ↓ Patm, ↓ AWR |
| Resistance | - Laminar flow (small airways): - Turbulent flow (large airways): - Factors ↓ AWR - ↑ Radius (most important since raised to power 4 or 5) - ↓ Viscosity (e.g. Temp) - ↓ Density (e.g. Heliox vs. air) - ↓ Length (not under control) |
| Dynamic Airways Compression (DAC) | - Pressure drop occurs between alveolus and mouth due to airway resistance - Where airway pressure = intrapleural pressure, unsupported airways collapse - i.e. Equal pressure point (EPP) - Starling resistor mechanism; P2 = PIP not Patm - Cartilage absent after generation 11 |
Normal Flow-Volume Loop
| Variable | Mechanism |
|---|---|
| TLC | - Lung volume after maximal inspiratory effort - Note unable to measure with flow-volume loop |
| Upward slope | - Effort-dependent - Radial traction distends airways |
| Max PEFR | - Occurs at beginning of forced expiration. - Highest lung elastic recoil - Highest airway radius - ?High expiratory muscle mechanical advantage - Reflects larger airway function - Mainly effort-dependent - Limited by onset of DAC |
| Linear decline | - Mainly effort-independent, especially FEF25-75% - Decline due to increasing DAC as lung volume and airway radii fall - Mostly effort independent - Reflects smaller airway function |
| RV | - Maximal expiration - Limited by small airway closure - Note unable to measure with flow-volume loop |
| FVC |
Obstructive Pathophysiology
| Factor | Detail |
|---|---|
| Example | - Asthma, COPD |
| ↑/↓ TLC | - COPD with emphysema: ↓ Elastic recoil - Asthma: Gas trapping, elastic recoil may be maintained |
| ↓ Upward slope | - Cause: ↓ Airway radius |
| ↓ PEFR | - ↓ Airway radius - Earlier and exaggerated DAC, distal migration of EPP |
| Linear decline | - Scooped out appearance, ↓ FEF25-75% - Earlier and exaggerated DAC, distal migration of EPP - (If emphysema: ↓ Radial traction, ↓ lung recoil) - Changes ∝ severity |
| ↑ RV | - ↓ Airway radius → Earlier airway closure - (If emphysema: ↓ lung recoil, earlier airway closure) |
| ↓ FVC | - |
Restrictive Pathophysiology
| Factor | Detail |
|---|---|
| Example | - Pulmonary fibrosis |
| ↓ TLC | - ↓ Lung compliance |
| ↔↑ Upward slope | - ↑ Lung recoil |
| ↔↓ Max PEFR | - (affected in severe disease) - ↓ Lung compliance |
| Linear decline | - ↔↑ Slope, especially at end-expiration - ↑ Lung recoil → ↑ Transpulmonary pressure - Stiff, less collapsible tissue |
| ↓ RV | - ↑ Lung recoil → Airway closure at lower lung volume |
| ↓ FVC | - ∆TLC > ∆RV |