Respiratory - Spirometry

Question 1

Classify the different types of spirometry

Answer 1

Volume-sensing
- E.g. Vitalograph

Flow-sensing
- E.g. Pneumotachograph (more portable)

Question 2

Classify the different variables measured by spirometry

Answer 2

STATIC LUNG VOLUMES
Measured: Vt, IRV, ERV, VC, TLC, IC
Calculated: RV, FRC, TLC

DYNAMIC SPIROMETRY (depend on rate of flow)

1. FEV1
2. FVC
3. PEFR
4. Expiratory flow-volume curve
5. Flow volume loops

SPECIAL TESTS

1. DLCO
2. Gas dilution, N2 washout used to calculate RV/FRC/TLC

Question 3

How can PEFR be calculated from the expiratory volume - time curve?

Answer 3

By calculating the steepest gradient of the curve delta V / delta time.

This is more commonly measured by a separate device: Peak Expiratory Flowmeter

Question 4

Why is the FEV1/FVC ratio used instead of just comparing them separately to the demographic matched predicted values

Answer 4

The ratio identifies the relative difference between FVC and FEV1. E.g. A patient with reduced FVC will have reduced FEV1 as there is overall less volume to expire.

Question 5

What are the forced expiratory flow volume diagnostic criteria for COPD / ASTHMA / RESTRICTIVE LUNG DISEASE

Answer 5

Obstructive

FEV1 /FVC < 0.7
FEV1 < 80% Predicted

FEV1 59 - 70% –> Mild
FEV1 30 - 49 –> Mod
FEV1 < 30% –> Severe

Restrictive (Fibrosis / Kyphoscoliosis / Resp muscle weakness)

FEV 1 / FVC >0.7 (normal or high)
FEV1 < 80% predicted
FVC < 80% predicted

Question 6

How is PEFR used clinically

Answer 6

PEFR used bedside to compare to patient’s PEFR baseline during a COPD exacerbation

PEFR diurnal variation of > 20% is suggestive of asthma

Question 7

How is forced expiratory volume - time graphs and paramaeters used in the diagnosis of asthma

Answer 7

An improvement of FEV1 of 400 mL 15 minutes after the administration of a bronchodilator is said to correspond with significant airway reversibility, suggesting asthma.

Question 8

What are the key components on the flow-volume curve diagnostic of restrictive vs obstructive lung disease

Answer 8

Obstructive

• Air trapping –> increased RV
• Concave effort independent portion of the curve
• Reduced PEFR

Restrictive
- Reduced TLC
- Reduced VC
- Straight effort independent portion of curve (non-concave)
- Normal RV
-

Question 9

Explain the shape of the forced expiratory flow - volume curve and dynamic airway compression

Answer 9

Start: high lung volume –> small airways large radius –> low resistance (HP)

During forced expiration –> high intrapleural pressure generated by resp. muscles. which increase Pressure in alveolus causing rapid ejection of air and the initial steep rise in flow seen on the flow-volume curve.

As expiration continues and the intrapleural pressure remains high with reducing lung volumes, there is dynamic airway compression which increases the resistance to eair flow converting this flow into an EFFORT INDEPENDENT phenomena and a linear decline in flow until residual volume is reached.

Question 10

What is the difference between a flow volume curve and a flow volume loop

Answer 10

Flow volume loop has both an inspiratory flow volume curve superimposed onto an expiratory flow volume curve.