L05 Respiratory Physiology 2 Lung function tests

Overview

This lecture covers how ventilation is assessed clinically: the lung volumes and capacities that make up total lung capacity, how Spirometry measures them, how the volumes it cannot measure directly (residual volume and anything built from it) are obtained instead, how forced spirometry values are interpreted against predicted normals, Peak expiratory flow rate monitoring, and how the resulting patterns distinguish obstructive from restrictive lung disease.

The oxygen cascade and assessing ventilation

• $P{O}_2$ falls in stages along the O2 transport path: Atmosphere -> Air -> Alveolar gas ($PA{O}_2$) -> Capillary -> Arterial ($Pa{O}_2$) -> Tissues -> Mitochondria. The alveolar gas and arterial points are the two key measurement compartments on this cascade.
• Formulas (assuming barometric pressure $P_B$): $PI{O}_2=FI{O}_2\times (P_B-47)$ and $P{O}_2=FI{O}_2\times P_B$.
• Typical values at $P_B=760$ mmHg: inspired $P{O}_2=159$ mmHg, $PI{O}_2=149$ mmHg, $PA{O}_2=100$ mmHg, $Pa{O}_2=95$ mmHg, $Pv{O}_2=40$ mmHg, $PAC{O}_2=40$ mmHg, $PaC{O}_2=40$ mmHg, $PvC{O}_2=46$ mmHg.
• Ventilation is the process governing the atmosphere-to-alveolar step of the cascade.
• Respiration can be considered as five steps; ventilation is the first: moving air in and out of the lungs to supply O2 to and remove CO2 from the alveoli, a process of bulk flow.
• Ventilation is assessed by: blood gases ($Pa{O}_2$ for hypoxia, $PaC{O}_2$ for hypercapnia - $PC{O}_2$ is influenced only by ventilation), lung volumes/flows (spirometry, peak flow rates), and exhaled nitric oxide.

Spirometry: what it measures and how it is done

• Spirometry measures how much air moves and how fast. It is common, simple, mechanical or digital, and can be used to test response to therapy, but it is only part of the clinical picture alongside history, examination, chest X-ray and blood gases.
• Forced expiratory vital capacity manoeuvre: the patient inspires maximally to total lung capacity, then exhales into the spirometer as forcefully, rapidly and completely as possible.
• Quality control: results are technician- and subject-dependent. An acceptable effort has a sharp peak, a gradual return to zero flow, and lasts at least 4 seconds. Three acceptable attempts within 5% of each other are required.

Lung volumes and capacities

Four volumes divide the air in the lungs (typical values):

• Tidal volume (TV), ~500 mL: air moved in and out during normal quiet ventilation.
• Inspiratory reserve volume (IRV), ~3.0-3.3 L: extra volume that can be inspired if the external intercostal muscles also contract.
• Expiratory reserve volume (ERV), ~1.0-1.2 L: extra volume that can be expelled if the internal intercostal and abdominal muscles contract for maximal active expiration.
• Residual volume (RV), ~1.2 L: volume still in the lungs even after maximal expiration.

Capacities combine two or more volumes:

• Vital capacity (VC) = TV + IRV + ERV, ~4.5-5.0 L: maximal breath in to maximal breath out.
• Inspiratory capacity (IC) = TV + IRV, ~3.5-3.8 L.
• Functional residual capacity (FRC) = ERV + RV, ~2.2-2.4 L: the volume in the lungs at the end of a tidal expiration; this is the “equilibrium point” of the respiratory system, so work has to be done to move volume away from it.
• Total lung capacity (TLC) = TV + IRV + ERV + RV = VC + RV, ~5.7-6.2 L.

On a spirogram trace, TV, IRV, ERV, VC and FVC all appear as excursions from the resting baseline. FRC, RV and TLC cannot be read directly off the trace, since the trace has no reference point below the level reached after maximal expiration (see below).

Worked examples

• TLC = 9 L, RV = 1 L. Since TLC = VC + RV, VC = 9 - 1 = 8 L.
• TLC = 8 L, TV = 0.5 L, IRV = 1.5 L. IC = TV + IRV = 2.0 L, and since TLC = IC + FRC, FRC = 8 - 2.0 = 6 L.

Measuring residual volume

• RV cannot be measured directly by spirometry, so any capacity built from RV (FRC, TLC) cannot be read directly off a spirometer trace either.
• Helium dilution method: the subject breathes on a spirometer to which a known amount of helium has been added. Helium concentration is measured at TLC (= VC + RV). The volume the helium has distributed into equals spirometer volume + VC + RV; since the spirometer volume and VC are known/measured, RV is found by difference.
• Body plethysmography is also named as a method for measuring RV. [slide does not elaborate]

Forced spirometry: FVC, FEV1 and flow

• Forced (rapid, maximal) measurements give information about flow, not just volume.
• Forced vital capacity (FVC): the vital capacity measured during a forced expiratory manoeuvre.
• Forced expiratory volume in one second (FEV1): the volume exhaled in the first second of the forced manoeuvre. It is reduced by diseases causing resistance to airflow (airway obstruction) or by small lungs (e.g. scarred or fibrotic lungs).
• FEF25-75%: the flow over the middle (25-75%) portion of the forced expiratory volume-time curve, read from its steepest-slope segment.
• FEV1/FVC ratio: a value below 0.70 defines obstructive dysfunction.
• Normal values: FVC $\ge 80\%$ of predicted; FEV1 $\ge 80\%$ of predicted; FEV1/FVC ratio $\ge 0.70$.

Predicted normal values

• Predicted values are calculated from height, age and sex.
• A nomogram has parallel scales for height, a combined FEV1/FVC%-and-age scale, FEV1 and FVC. A straight edge connecting a person’s age and height reads off their predicted FEV1 and FVC.
• A measured (“actual”) value is expressed as a percentage of the predicted value, because raw absolute values are only meaningful relative to what is expected for that person.
• Worked examples: a 21-year-old, 6’6” male has a predicted FVC of 6.9 L, so a measured FVC of 2.1 L is only 30% predicted (severely abnormal). An 80-year-old, 4’10” female has a predicted FVC of 1.9 L, so the same measured value of 2.1 L is 111% predicted (essentially normal).

Peak expiratory flow rate (PEFR)

• Can be obtained during spirometry, or measured with portable devices at home or in the workplace.
• Measures a rate (e.g. 500 L/min), not a volume.
• The absolute value is not that useful on its own, since the “normal” range is wide (though it should still be “appropriate” for the person); normal values are estimated from a nomogram.
• Change from a person’s own baseline PEFR is more clinically useful than the absolute value: a decrease suggests decreased flow, e.g. worsening asthma, and serial measurements can be used to guide or assess therapy.

Indications for pulmonary function testing

1. Objective assessment of pulmonary symptoms.
2. Categorisation of the type and severity of physiologic abnormalities.
3. Documentation of disease progression.
4. Documentation of the patient’s response to therapy.
5. Preoperative assessment.
6. Screening for subclinical disease.

Obstructive vs restrictive lung disease

• Restrictive pattern: reduced lung volume / stiff lungs, from lung compliance-related disease (pulmonary fibrosis, oedema), chest-wall compliance disease (kyphoscoliosis), or pleural and respiratory muscle disease.
• Obstructive pattern: increased resistance to airflow, from chronic obstructive lung disease (chronic bronchitis, emphysema) or asthma.
• On a % lung volume trace, obstructive disease shifts the whole trace upward (hyperinflated, oscillating in a narrow band near TLC, reflecting air trapping), while restrictive disease compresses the trace into a smaller envelope in a lower-mid range.
• On a forced expiration volume-time curve, normal rises fastest and plateaus highest/earliest; restrictive disease plateaus lowest and earliest (reaches a small volume quickly); obstructive disease rises more slowly and plateaus at a lower, later volume than normal (takes longer to reach a reduced, but not as small, volume).
• Restrictive lung disease: FVC decreased; FEV1 often decreased proportionately to FVC; FEV1/FVC normal or increased; lung volume measurements (RV, FRC, TLC) may be needed to confirm.
• Obstructive lung disease: FEV1/FVC < 0.70 defines it; FEV1 usually decreased; FVC may also be decreased if expiration is incomplete due to air trapping; obstruction and restriction can coexist in the same patient.

Common patterns of abnormal PFTs:

Measurement	Restrictive	Obstructive - mild	Obstructive - severe
FVC	↓↓	Normal	↓
FEV1	↓↓	↓	↓↓
FEV1/FVC%	Normal/↑	↓	↓↓
FEF25-75%	↓↓	↓	↓↓
FRC	↓	Normal	↑
RV	↓	Normal	↑
TLC	↓	Normal	↑

Self-test

1. Describe the steps of the oxygen cascade from atmosphere to mitochondria, and state which two points on it are the key measurement compartments.
2. Explain how ventilation is assessed clinically, including which blood gas value is influenced only by ventilation.
3. Define tidal volume, inspiratory reserve volume, expiratory reserve volume and residual volume, with typical values for each.
4. Distinguish vital capacity, inspiratory capacity and functional residual capacity in terms of how each is built from the four lung volumes.
5. Why is functional residual capacity described as the “equilibrium point” of the respiratory system?
6. A patient has a total lung capacity of 9 L and a residual volume of 1 L. Calculate their vital capacity.
7. A patient has a total lung capacity of 8 L, a tidal volume of 0.5 L and an inspiratory reserve volume of 1.5 L. Calculate their functional residual capacity.
8. Which lung volumes and capacities cannot be measured directly by spirometry, and why not?
9. Describe how the helium dilution method determines residual volume.
10. Distinguish forced vital capacity, FEV1 and FEF25-75% in terms of what each measures.
11. What value of the FEV1/FVC ratio defines obstructive dysfunction, and what are the normal thresholds for FVC, FEV1 and the FEV1/FVC ratio?
12. Why must a measured FVC value be interpreted relative to a predicted value rather than in absolute terms? Give an example from the lecture.
13. Distinguish peak expiratory flow rate from FVC and FEV1 in what it measures, and explain why a change from a patient’s own baseline is more clinically useful than the absolute value.
14. List the criteria a spirometry attempt must meet to count as an acceptable effort.
15. List the indications for pulmonary function testing.
16. Distinguish restrictive from obstructive lung disease in terms of the underlying problem, and give two example conditions of each.
17. Predict the pattern of FVC, FEV1, FEV1/FVC%, FRC, RV and TLC you would expect in severe obstructive disease compared with restrictive disease.

Answers

Reveal answers

1. $P{O}_2$ falls through: Atmosphere, Air, Alveolar gas ($PA{O}_2$), Capillary, Arterial ($Pa{O}_2$), Tissues, Mitochondria. The alveolar gas ($PA{O}_2$) and arterial ($Pa{O}_2$) points are the two key measurement compartments.
2. Ventilation is assessed via blood gases ($Pa{O}_2$ for hypoxia, $PaC{O}_2$ for hypercapnia, since $PC{O}_2$ is influenced only by ventilation), lung volumes/flows (spirometry, peak flow rates), and exhaled nitric oxide.
3. Tidal volume (~500 mL): air moved in/out during normal quiet breathing. Inspiratory reserve volume (~3.0-3.3 L): extra volume inspired with contraction of the external intercostals. Expiratory reserve volume (~1.0-1.2 L): extra volume expelled with active contraction of the internal intercostals and abdominal muscles. Residual volume (~1.2 L): volume remaining after maximal expiration.
4. Vital capacity = TV + IRV + ERV (maximal breath in to maximal breath out). Inspiratory capacity = TV + IRV. Functional residual capacity = ERV + RV (volume remaining at the end of a tidal expiration).
5. Because it is the volume the respiratory system rests at after a normal tidal expiration; work must be done to move the system’s volume away from this point in either direction.
6. Vital capacity = TLC - RV = 9 - 1 = 8 L.
7. Inspiratory capacity = TV + IRV = 0.5 + 1.5 = 2.0 L; since TLC = IC + FRC, FRC = 8 - 2.0 = 6 L.
8. FRC and TLC, because both include residual volume, which cannot be measured directly by spirometry (a spirometer has no reference point below the level reached after maximal expiration).
9. A known amount of helium is added to the spirometer circuit and the subject breathes on it; helium concentration is measured at TLC (= VC + RV); the volume the helium has distributed into equals spirometer volume + VC + RV, and since the spirometer volume and VC are known, RV is obtained by difference.
10. FVC is the vital capacity measured during a forced expiratory manoeuvre. FEV1 is the volume exhaled in the first second of that manoeuvre, reduced by airway obstruction or by small (e.g. fibrotic) lungs. FEF25-75% is the flow over the middle 25-75% portion of the forced volume-time curve, taken from its steepest slope.
11. FEV1/FVC < 0.70 defines obstructive dysfunction. Normal thresholds are FVC ≥ 80% predicted, FEV1 ≥ 80% predicted, and FEV1/FVC ratio ≥ 0.70.
12. Because the same absolute volume can be normal for one person and severely abnormal for another depending on their height, age and sex; e.g. a measured FVC of 2.1 L is only 30% predicted (abnormal) for a 21-year-old 6’6” male, but 111% predicted (normal) for an 80-year-old 4’10” female.
13. PEFR measures a flow rate (e.g. L/min), whereas FVC and FEV1 measure volumes. Its absolute value is not very informative because the normal range is wide; a fall from the patient’s own baseline is more useful because it signals decreased flow, e.g. worsening asthma, and can guide or assess therapy.
14. A sharp peak, a gradual return to zero flow, a duration of at least 4 seconds, and 3 acceptable attempts within 5% of each other.
15. Objective assessment of pulmonary symptoms; categorisation of the type and severity of physiologic abnormalities; documentation of disease progression; documentation of response to therapy; preoperative assessment; screening for subclinical disease.
16. Restrictive disease is a problem of reduced lung volume / stiff lungs (e.g. pulmonary fibrosis, kyphoscoliosis). Obstructive disease is a problem of increased resistance to airflow (e.g. chronic bronchitis/emphysema, asthma).
17. Severe obstructive disease: FVC ↓, FEV1 ↓↓, FEV1/FVC% ↓↓, FRC ↑, RV ↑, TLC ↑. Restrictive disease: FVC ↓↓, FEV1 ↓↓, FEV1/FVC% normal or ↑, FRC ↓, RV ↓, TLC ↓.