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Diagnosing Respiratory Failure
Shortness of breath, fatigue and headaches are key warning signs when assessing the severity of the situation. A comprehensive evaluation is essential to confirm respiratory failure. Diagnosis is based on specialist respiratory investigations that identify the precise cause and allow the necessary interventions to be introduced promptly to support breathing effectively.
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Steps in Diagnosing Respiratory Failure
Diagnosis follows a logical sequence, with each stage building on the previous one. The physician first combines the clinical examination, objective test results and imaging to confirm respiratory failure. The cause is then identified to guide treatment. The four main stages of a comprehensive respiratory assessment are outlined below.
Clinical Assessment and Medical History
Clinical assessment is the essential starting point for diagnosis. The physician first asks detailed questions about your symptoms and looks for characteristic signs: shortness of breath during exertion or at rest, progressive breathing difficulty, persistent cough, wheezing, unusual fatigue or cyanosis. This initial assessment establishes a comprehensive clinical profile.
The physician also reviews your medical history, including previous respiratory or cardiac conditions, tobacco use, occupation, exposure to pollutants such as asbestos, dust or chemicals, living environment and family history. This personalized approach significantly improves diagnostic accuracy.
Detailed Physical Examination
The physician then uses a stethoscope to examine your lungs and heart carefully. This physical examination can detect specific abnormal sounds:
- Crackles : Velcro-like sounds that may suggest pulmonary fibrosis or pulmonary edema
- Wheezing : may suggest airway obstruction
- Abnormal heart sounds : may suggest heart failure or associated right-sided heart failure
Arterial Blood Gas Analysis
Arterial blood gas analysis is a key and highly informative test in the diagnostic assessment of respiratory failure. It is a rapid, specialized blood test performed on an artery, usually at the wrist, using a small arterial puncture. The test measures three fundamental parameters that directly reflect how well your lungs are functioning:
- PaO₂ (arterial oxygen level) : indicates whether your blood is adequately oxygenated and carrying sufficient oxygen to the vital organs
- PaCO₂ (arterial carbon dioxide level) : shows whether your lungs are effectively removing this metabolic waste gas produced by the body
- Blood pH : measures blood acidity and identifies a possible acid–base imbalance caused by ineffective breathing
Interpreting Arterial Blood Gas Results
This test is decisive because it directly measures the effectiveness of pulmonary gas exchange. A low oxygen level (PaO₂ below 60 mmHg) confirms respiratory failure. An elevated carbon dioxide level (PaCO₂ above 45 mmHg) may also indicate respiratory failure. These objective results help the physician determine whether oxygen therapy is required and guide the necessary treatment intensity and oxygen flow rate.
Pulmonary Function Tests (PFTs)
Pulmonary function testing is a simple, non-invasive and painless way to assess how well your lungs work. You breathe into a device called a spirometer, which automatically measures airflow and records the results. The test takes only a few minutes and can be repeated if necessary.
The test measures several important parameters:
- FEV₁ (Forced Expiratory Volume in One Second) : the amount of air forcibly exhaled during the first second of a forced breath
- Vital capacity : the total volume of air you can exhale after taking a deep breath
- Expiratory flow rates : measure how quickly you can breathe out
PFTs help identify the type of respiratory impairment: an obstructive disorder, such as asthma or COPD, restricts airflow through the airways; a restrictive disorder, such as pulmonary fibrosis, reduces the lungs’ capacity to fill with air; and a mixed disorder combines both patterns. The tests also determine disease severity and allow progression to be monitored over time through repeated measurements.
Interpreting Pulmonary Function Test Results
PFT results are compared with predicted values based on your age, sex and height. A reduced FEV₁ (below 80% of the predicted value) may suggest airway obstruction. A reduced vital capacity may suggest a restrictive lung disorder. The FEV₁/vital capacity ratio also helps distinguish obstructive from restrictive patterns.
These results can also be used to classify the severity of respiratory impairment:
- Mild — FEV₁ ≥ 80% predicted
- Moderate — FEV₁ 50–79% predicted
- Severe — FEV₁ 30–49% predicted
- Very severe — FEV₁ < 30% predicted
This classification helps determine treatment intensity and the medical follow-up required.
Chest Imaging: X-Ray and CT Scan
A chest X-ray or CT scan provides direct images of the lungs, chest wall, mediastinum and heart. These imaging investigations complement the previous tests by providing a detailed anatomical view of the respiratory structures.
In practice, these tests help identify the specific cause of respiratory failure:
- COPD (Chronic Obstructive Pulmonary Disease) : characterized by progressive damage to the lung parenchyma
- Pulmonary fibrosis : characterized by thickening and scarring of lung tissue
- Acute or chronic lung infection : pneumonia or tuberculosis
- A cardiac condition affecting breathing : pulmonary edema or heart failure
- Structural abnormalities of the chest wall : congenital abnormalities or deformities
- Lung tumours : cancer or other neoplasms
Each underlying cause requires an appropriate, specific diagnostic and therapeutic approach.
A Comprehensive Diagnosis to Tailor Treatment
Together, these complementary tests allow the physician to establish a complete and accurate diagnosis by integrating your reported symptoms, objective arterial blood gas results, pulmonary function parameters and anatomical chest imaging. On this sound diagnostic basis, the physician selects the treatment best suited to your situation.
If arterial blood gas analysis confirms inadequate oxygenation, oxygen therapy may be offered. It may be temporary, for example during an acute exacerbation, or long term as continuous treatment. For most patients it is provided at home, although hospital treatment may occasionally be required in extremely severe cases. A comprehensive diagnosis and appropriate investigations support personalized, effective and safe management of respiratory failure.
Common Mistakes in Diagnosing Respiratory Failure
Several mistakes can delay diagnosis and the start of treatment:
- Downplaying respiratory symptoms by attributing them to ‘reduced fitness’ or ageing, when they may reflect undiagnosed respiratory failure
- Ignoring a persistent cough or increasing shortness of breath without investigating the medical cause
- Not completing the recommended investigations because you ‘do not feel that unwell’, even though early diagnosis can change the prognosis
- Waiting too long before seeking medical advice while long-term damage gradually accumulates
- Not following HAS recommendations for arterial blood gas analysis and pulmonary function tests
Advice: If you have lung or heart disease, or unexplained fatigue, ask your physician whether arterial blood gas analysis and pulmonary function tests are appropriate. These straightforward investigations provide valuable information. According to HAS guidance, earlier detection supports better long-term control and a more favourable cardiovascular prognosis.
Next Steps
Once your diagnosis has been confirmed and the severity of respiratory failure assessed, the next step is to understand how it can be treated effectively. Learn about the different treatment approaches and oxygen therapy options suited to your clinical situation.
Official and Scientific Resources
- HAS – Severe chronic respiratory failure secondary to COPD
- Ameli – Diagnosing and assessing chronic shortness of breath
This content is provided for information only and is not a substitute for medical advice. If you have symptoms of respiratory failure, consult a general practitioner or respiratory specialist promptly for a professional diagnosis and treatment adapted to your situation.
FAQ
Why are several tests needed to establish the diagnosis?
No single test is sufficient on its own. Symptoms, clinical examination, arterial blood gases, pulmonary function tests (PFTs) and chest imaging provide complementary information. Together, they confirm respiratory failure, assess its severity and help identify the cause.
Is arterial blood sampling for blood gas analysis painful?
It may be slightly more uncomfortable than a standard blood test because the sample is taken from an artery, usually at the wrist, rather than from a vein. The procedure is quick and is performed by trained healthcare professionals.
What is the difference between arterial blood gas analysis and pulmonary function tests?
Arterial blood gas analysis directly measures oxygen and carbon dioxide levels in the blood. Pulmonary function tests assess how air moves through the respiratory system by measuring lung volumes and airflow while you breathe into a spirometer. The tests answer different questions and complement one another.
Where are these tests performed?
Clinical examination and part of the assessment may be carried out by your general practitioner or respiratory specialist. Depending on local arrangements, arterial blood gases, PFTs and certain imaging tests are often performed in a pulmonary function laboratory or hospital.
Do I need to fast before these tests?
This depends on the protocol and any associated investigations. Fasting is not always required for PFTs alone, although you may be asked to avoid certain medications or smoking before the test. Your physician or the testing department will provide precise instructions.
Does a diagnosis of respiratory failure mean that I will definitely need oxygen at home?
Not necessarily. The diagnosis first identifies the cause and severity of the respiratory disorder. Oxygen therapy is one possible option, prescribed by the physician according to the results, the underlying condition and your individual needs.
What happens after the results are available?
Once the results are available, the physician will review them with you, explain the cause of respiratory failure and recommend appropriate management, which may include medication, pulmonary rehabilitation, oxygen therapy and specialist follow-up.
