Pulmonary function testing

Physiological basis and diagnostic goals

The respiratory system performs many tasks: from participating in heat exchange and water balance to synthesizing biologically active substances and regulating acid-base balance. However, its main function is gas exchange.

For oxygen to reach the tissues and carbon dioxide to leave the body, three sequential stages must function correctly:

1. Ventilation — the mechanical delivery of fresh air through the airways into the alveoli.
2. Diffusion — the direct penetration of gas molecules through the thin alveolar-capillary membrane.
3. Perfusion — adequate blood supply to the lung tissue (influx of venous blood rich in $CO_2$ and outflow of arterial blood saturated with $O_2$).

Clinical assessment of pulmonary function testing (PFT) aims to check each of these stages and identify the severity of potential respiratory failure.

Spirometry: static and dynamic volumes

The primary tool for checking ventilatory capacity is spirometry. The device (spirometer) records changes in lung volumes over time, outputting the result as a graph—the spirogram. Patient data is always compared against predicted (ideal for their sex, age, and height) values.

All measured pulmonary parameters are divided into two large groups:

• Static volumes show lung capacity without accounting for the time factor. These include:
• Tidal volume (TV): the portion of air inhaled during normal quiet breathing (about 400–500 mL, sometimes up to 800 mL). At the same time, only a portion of TV reaches the alveoli and participates in gas exchange. About 30% remains in the airways—this is the so-called "anatomic dead space".
• Vital capacity (VC): the maximum possible amount of air a person can exhale after the deepest possible inhalation. Normal values range between 3200–4800 mL.
• Residual volume (RV): air that always remains in the lungs even after a maximal exhalation (1000–1200 mL).
• Dynamic volumes reflect the speed capabilities of breathing and the patency of the bronchial tree.
• The main indicator here is forced vital capacity (FVC). This is the same maximal exhalation after a full inhalation, but performed with maximal force and speed.

How to differentiate obstruction from restriction

For the differential diagnosis of types of respiratory failure, physicians analyze forced expiration using the Tiffeneau test.

Two parameters are of key importance:

1. FEV₁ (Forced Expiratory Volume in 1 second): shows how much air out of the FVC the patient was able to exhale during the first second. It directly reflects the patency of large bronchi and normally accounts for about 75% of the total FVC.
2. Tiffeneau index: the percentage ratio of FEV₁ to FVC. In a healthy person, this index does not drop below 70%.

By comparing these values, the type of ventilation disorder can be determined:

Pneumotachometry and the flow-volume loop

For a more detailed assessment of airflow velocity, pneumotachometry is used. Velocity depends directly on how clear the bronchi are. During quiet breathing, the flow rate is 300–500 mL/sec, and during a forced maneuver, it shoots up to 5–7 L/sec. A decrease in these parameters is a clear sign of bronchial obstruction.

Modern devices construct a flow-volume loop graph, where the flow rate is plotted on the ordinate axis and the volume of exhaled air is plotted on the abscissa axis.

• In obstruction, the curve shifts to the left, and its shape during expiration becomes flat (concave).
• In restriction, the shape of the curve remains normal, but the graph itself shifts to the right due to an overall reduction in lung volumes.

Diffusion and Perfusion Evaluation

If ventilation is not impaired, the problem may lie at the level of the alveoli or vessels. Special methods exist for this:

• Diffusion capacity testing: The patient inhales a safe mixture containing carbon monoxide (CO). The state of the alveolar-capillary membrane is assessed by the amount of CO in the blood. A decrease in this indicator is characteristic of fibrosis or alveolitis.
• Assessment of perfusion: A test with pure oxygen inhalation is performed. If the partial pressure of oxygen ($p_aO_2$) in arterial blood fails to reach expected high values afterward, it indicates a blood supply issue in the lung tissue.
• Blood gas analysis: The final determination in assessing gas exchange is made by arterial blood gas analysis for $p_aO_2$, partial pressure of carbon dioxide ($p_aCO_2$), and pH (acid-base balance).

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Graphical analysis of the flow-volume loop and instantaneous forced expiratory flows (FEF)
• Detailed method for assessing the diffusing capacity of the lungs using CO
• The role of blood gas partial pressures ($p_aO_2$, $p_aCO_2$) in the diagnosis of respiratory failure
• Non-respiratory functions of the lungs: synthesis of biologically active substances and water balance
• Peak flow meter monitoring as a method for outpatient control of obstructive diseases

Other topics in the "Respiratory System" section

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