Ventilation — From Lung Function to Dyspnoea

Table of Contents

1. Why Breathing Is the Diagnostic Hinge
2. Tasks of Pulmonary Respiration
3. The Range of Ventilation
4. The Clinical Vocabulary
5. Lung Function Parameters
6. Dead Space, Hypo- and Hyperventilation
7. Resistance and Regulation

1 Why Breathing Is the Diagnostic Hinge

When people are “out of breath” they are, as a rule, either excited, physically exerted, or ill. Against the background of acute respiratory infections — influenza, COVID-19, common colds, other upper and lower airway infections — breathing has acquired a particular diagnostic weight in sports medicine. Breathlessness in healthy people from psychological tension or physical load has to be assessed differently from breathlessness caused by disease of the airways, of the cardiovascular system, or of oxygen transport in the blood, as in anaemia. That discrimination is the diagnostic task, and it returns in Lecture 12 when a post-infectious case is discussed.

2 Tasks of Pulmonary Respiration

Pulmonary respiration loads the blood with O₂ and regulates acid–base balance via blood CO₂ concentration. Breathing mechanics serve the ventilation of the alveoli, through whose walls O₂ diffuses into and CO₂ out of the blood. Transport of the respiratory gases in blood occurs largely in bound form; the transported amount depends on concentrations in blood and alveoli and on pulmonary perfusion. The task of respiratory regulation is to match ventilation to the current requirement.

3 The Range of Ventilation

At rest, ventilation is below 10 l·min⁻¹. Maximal volumes — the breathing limit — are reached only briefly and can exceed 300 l·min⁻¹ in large, trained individuals. Under maximal exercise, as in an exercise ECG or a performance test, very well trained people can exceed 200 l·min⁻¹; people in their third decade with average training status reach more than 100 l·min⁻¹. With ageing, lack of training or disease, maximal ventilation falls below 100 l·min⁻¹, and in very low capacity only slightly above resting ventilation.

A consequence with direct relevance to exercise immunology: during prolonged endurance work in enclosed rooms a considerable volume of room air is “breathed through”, and the room is loaded with exhaled droplets and their nuclei. This connects ventilatory physiology to the infection-transmission context of Lecture 11 and Lecture 12.

4 The Clinical Vocabulary

Clinically the symptoms are ordered along a severity and time axis: exertional dyspnoea, resting dyspnoea and orthopnoea, apnoea; and acute versus chronic dyspnoea. Where a complaint sits on both axes determines which rung of the stepwise model of Lecture 1, §3, it enters at.

5 Lung Function Parameters

• Tidal volume (V_T) — volume inspired and expired per breath
• Vital capacity (VC) — maximal tidal volume
• Breathing limit (V̇_max) — maximal possible ventilatory flow
• Compliance (C) — distensibility of the lung
• Forced expiratory volume in 1 s (FEV₁) — maximal volume expired in one second
• Functional residual capacity (FRC) — end-expiratory lung volume

6 Dead Space, Hypo- and Hyperventilation

Inspired air must pass airways in which no gas exchange occurs — mouth, pharynx, trachea, bronchi, bronchioles — and is warmed, humidified and cleaned on the way. Tidal volume contains both the air reaching the alveoli (V_A) and the air remaining in dead space (V_D, normally ~150 ml). If tidal volume is smaller than dead space the alveoli are not ventilated at all; above it, the alveolar share rises with breathing depth. Even in hyperpnoea alveolar ventilation can be reduced if breathing is shallow and primarily ventilates dead space — a pattern worth remembering when a patient reports rapid breathing but shows unchanged gas exchange.

Changes in alveolar ventilation do not affect O₂ uptake and CO₂ output equally. Because the O₂ binding curve is sigmoid, mild hypoventilation lowers alveolar and arterial PO₂ while haemoglobin saturation — sitting on the flat part of the curve — barely changes; the simultaneous rise in PCO₂, however, noticeably impedes CO₂ output. Massive hypoventilation drives PO₂ into the steep part of the curve and then impairs O₂ uptake far more than CO₂ output. Hyperventilation raises alveolar and arterial PO₂ but cannot meaningfully increase O₂ uptake into already saturated haemoglobin; it does promote CO₂ output. This asymmetry is the physiological reason why capnometric and oximetric findings must be read together rather than separately.

7 Resistance and Regulation

On the way to the alveoli, air passes airways that oppose flow with a resistance dictated by airway lumen, especially below 2 mm, which can be narrowed by mucus and by bronchial muscle contraction. Increased ventilation may arise from physiologically raised demand (work), pathophysiologically raised demand (metabolic acidosis), or inadequate overactivity of the respiration-regulating cells. Reduced ventilation may arise from lowered demand, from damage to respiratory neurons or neuromuscular transmission, from disease of the respiratory muscles, from restricted thoracic mobility (deformity, joint inflammation, obesity), from enlargement of the pleural space by effusion or pneumothorax, and from restrictive and obstructive lung disease. In obesity, respiratory drive falls during sleep and sleep apnoea may result.

One-Minute-Paper Topics

• A tidal volume of 120 ml at a frequency of 30 min⁻¹ gives 3.6 l·min⁻¹. Why is this ventilation ineffective?
• Explain, using the shape of the O₂ binding curve, why mild hypoventilation affects CO₂ output more than O₂ uptake.
• Name three non-pulmonary causes of reduced ventilation and assign each to a rung of the stepwise diagnostic model.
• Distinguish exertional dyspnoea in a healthy runner from exertional dyspnoea in anaemia — which findings separate them?

References

• Puta C, Gabriel HHW. Sports Medical Diagnostics — Basics. Lecture script, Version 1.2. Jena: Friedrich-Schiller-Universität Jena, 2024.
• Silbernagl S, Lang F. Taschenatlas Physiologie. 9th ed. Stuttgart: Thieme, 2019.