COPD Explained: Airflow Limitation, Air Trapping and Gas Exchange

COPD makes much more sense when you stop thinking of it as simply “damaged lungs” and follow what happens to a breath.

Normally, air moves into the lungs during inspiration and leaves again during expiration. Elastic lung tissue helps the lungs recoil, small airways remain open long enough for air to escape, and the alveoli provide a large surface for oxygen and carbon dioxide to move between air and blood. In chronic obstructive pulmonary disease (COPD), several parts of that process can change at the same time.

Follow a breath through a COPD lung

Long-term exposure to harmful particles or gases can cause persistent inflammation and structural change within the airways. Smoking is an important cause, but it is not the only possible exposure. Small airway walls can thicken, mucus production can increase and the space available for airflow becomes narrower.

At the same time, emphysema can destroy alveolar walls and reduce the lung's normal elastic recoil. That matters because recoil does more than help push air out: it also helps keep small airways open during expiration.

Now follow the patient's breath out. As they exhale, narrowed small airways already resist airflow. If elastic recoil has also been lost, those airways are less well supported and can narrow or close earlier during expiration. Air therefore leaves the lungs more slowly.

Follow the physiology: small-airway narrowing + loss of elastic recoil → expiratory airflow limitation → incomplete emptying → air trapping → hyperinflation.

This is why a patient with COPD may describe feeling as though they cannot get enough air in, even though part of the problem is that they cannot get the previous breath out efficiently.

During exercise or an acute exacerbation, breathing becomes faster. The next inspiration may begin before the previous breath has fully left the lungs. More air becomes trapped and the lungs operate at a progressively higher volume. This is called dynamic hyperinflation. The diaphragm can become mechanically disadvantaged and the work required for each breath increases, contributing to the exhausting sensation of breathlessness.

Emphysema can also affect gas exchange. Destruction of alveolar walls reduces available surface area and can damage the surrounding capillary network. Alongside uneven airway obstruction, this contributes to ventilation-perfusion mismatch: some lung regions are relatively well ventilated but poorly perfused, while others receive blood flow but inadequate ventilation. Hypoxaemia can result, and carbon dioxide may rise if effective alveolar ventilation becomes insufficient.

Older descriptions sometimes divide people into “pink puffers” and “blue bloaters”. These labels oversimplify COPD. Chronic bronchitis, small-airway disease and emphysema can overlap, and the clinical picture varies considerably between patients.

From stable COPD to an exacerbation

COPD is not diagnosed from symptoms or an X-ray alone. NICE recommends post-bronchodilator spirometry to confirm persistent airflow obstruction, interpreted alongside symptoms, exposure history and the wider clinical picture. A reduced FEV1/FVC ratio supports airflow obstruction, but spirometry quality and alternative diagnoses still matter.

An exacerbation is an acute, sustained worsening from the patient's usual stable state, commonly involving increased breathlessness, cough, sputum production or a change in sputum colour. The important phrase for a student nurse is from their usual state. A respiratory rate, oxygen saturation or degree of breathlessness that looks abnormal on paper may be usual for one patient, while a seemingly modest change may represent significant deterioration in another.

Imagine a patient who normally walks to the bathroom independently and speaks comfortably in full sentences. Today they need to stop halfway, are using accessory muscles and can only answer you in short phrases. That change matters, even before you have looked at a number on the monitor.

Do not automatically label every deterioration as “their COPD”. Infection commonly triggers exacerbations, but pneumonia, pulmonary embolism, pneumothorax, heart failure and other acute problems can mimic or complicate one. Sudden pleuritic chest pain, haemoptysis, unilateral changes in breath sounds, haemodynamic instability or deterioration that seems disproportionate to the usual pattern should make the team consider another or additional cause.

Oxygen is another area where COPD is often taught badly. Oxygen treats hypoxaemia; COPD is not a reason to leave a severely hypoxaemic patient untreated. However, some people with COPD are at risk of hypercapnic respiratory failure, so oxygen needs to be controlled and the response assessed. NICE's quality standard for emergency oxygen during an acute COPD exacerbation uses a target saturation of 88–92%. Individualised prescribed targets and local emergency guidance should still be followed, and arterial blood gases are important in hospital because pulse oximetry tells you oxygen saturation but not the PaCO2 or pH.

If carbon dioxide rises, do not reduce the explanation to the old idea that oxygen has simply “removed the patient's drive to breathe”. Oxygen-induced hypercapnia is more complex and includes worsening ventilation-perfusion matching and the Haldane effect. The practical nursing point is simpler: give oxygen when indicated, titrate it appropriately, assess the patient and escalate deterioration.

What this physiology looks like on placement

Once you understand the mechanics, the nursing assessment becomes easier to connect rather than memorise. Respiratory rate tells you something about demand. Work of breathing shows how hard the patient is working to ventilate. Their ability to speak, mobilise and eat tells you about the functional effect of breathlessness. Oxygen saturation helps assess oxygenation, while a blood gas can show whether carbon dioxide is accumulating and whether respiratory acidosis is developing.

Start with the patient's baseline. Ask about their usual breathlessness and mobility, usual oxygen saturation if known, home oxygen if prescribed, normal cough and sputum, inhalers and recent exacerbations. Then compare that baseline with what is happening now.

Listen to what the patient tells you. “I'm always breathless” does not mean today's breathlessness is normal. Useful questions include whether it is worse than usual, how quickly it changed, whether sputum has increased or changed colour, whether there is chest pain or fever, and whether they can do the things they normally manage.

Observe respiratory rate, work of breathing, oxygen saturation, speech, colour, mental state and fatigue. New confusion or drowsiness is particularly important because deterioration in gas exchange can affect consciousness. Where blood gases are being used, understand the overall pattern and escalate abnormal results within your competence rather than trying to manage respiratory failure independently.

Treatment depends on the individual clinical picture and may include bronchodilators, corticosteroids during appropriate exacerbations, antibiotics when indicated, controlled oxygen, non-invasive ventilation for persistent hypercapnic ventilatory failure despite optimal medical therapy, and treatment of another identified cause. Longer-term management can include support to stop smoking, vaccination, inhaled therapy, pulmonary rehabilitation and carefully assessed long-term oxygen therapy.

For a student nurse, inhaler technique is not a small detail. A prescribed inhaler cannot work properly if the medication is not being delivered effectively. NICE specifically includes assessment of inhaler technique within COPD care, so watching how the patient actually uses their device can uncover an important problem.

Escalate significant deterioration such as increasing oxygen requirement, severe or increasing work of breathing, exhaustion, new confusion or drowsiness, cyanosis, haemodynamic instability, significant chest pain, suspected pneumothorax or pulmonary embolism, or concerning blood-gas changes. You are not expected to diagnose the cause independently. Your role is to recognise that the patient's physiology or function has changed, perform appropriate observations and assessment within your competence, communicate clearly and seek help.

If you remember one chain, remember this: narrowed small airways + reduced elastic recoil → difficulty getting air out → air trapping and hyperinflation → increased work of breathing, with impaired gas exchange in some patients.

References

National Institute for Health and Care Excellence (NICE). Chronic obstructive pulmonary disease in over 16s: diagnosis and management (NG115). Available at: NICE NG115.

National Institute for Health and Care Excellence (NICE). Chronic obstructive pulmonary disease in adults (QS10). Available at: NICE QS10.

British National Formulary. Chronic obstructive pulmonary disease. Available at: BNF treatment summary.

Nursing and Midwifery Council (NMC). The Code: Professional standards of practice and behaviour for nurses, midwives and nursing associates. Available at: NMC Code.