Shortness of Breath: Causes, Tests & When to Worry

Seek Emergency CareSudden severe breathlessness at rest, SpO₂ below 92 %, bluish lips or fingertips (cyanosis), or breathlessness with chest pain: call 999/911 immediately.

Causes by Onset Pattern

Onset	Likely Causes
Sudden (seconds–minutes)	Pulmonary embolism, pneumothorax, anaphylaxis, foreign body
Acute (hours)	Asthma attack, acute heart failure, pneumonia, panic attack
Subacute (days–weeks)	Pleural effusion, worsening COPD, anaemia
Chronic (months–years)	COPD, heart failure, obesity, interstitial lung disease

Key Diagnostic Tests

Pulse oximetry (SpO₂): measures oxygen level in blood
Chest X-ray: detects pneumonia, fluid, pneumothorax
ECG: detects cardiac causes including PE clues
Blood tests: FBC (anaemia), D-dimer (PE), BNP (heart failure)
Spirometry: measures lung function for asthma/COPD
CT pulmonary angiogram: confirms PE

Rule of Thirds

Roughly: ⅓ of chronic breathlessness is cardiac, ⅓ is respiratory, ⅓ is other (anaemia, deconditioning, anxiety). A thorough history usually narrows it quickly.

What SpO₂ level should I worry about?

A reading below 94 % warrants medical attention; below 92 % is an emergency. Normal is 95–100 %.

Can anxiety cause shortness of breath?

Yes. Hyperventilation in panic attacks is real. But cardiac and pulmonary causes must always be ruled out first, especially if the symptom is new.

Does anaemia cause breathlessness?

Yes. Severe anaemia (haemoglobin <8 g/dL) often causes breathlessness on exertion because the blood carries less oxygen.

When is breathlessness an emergency?

Sudden onset at rest, accompanying chest pain, cyanosis, SpO₂ <92 %, or confusion – call emergency services immediately.

Frequently Asked Questions

When is breathlessness a medical emergency?

Call emergency services for sudden severe breathlessness, breathlessness with chest pain, blue lips, confusion, or inability to speak in full sentences. These can indicate a heart attack, pulmonary embolism, or severe asthma attack.

Which tests are used to investigate breathlessness?

Common first-line tests include a full blood count (for anaemia), chest X-ray, ECG, oxygen saturation, and depending on the picture, BNP for heart failure, D-dimer for suspected clot, and spirometry for lung function.

Can anxiety cause genuine breathlessness?

Yes. Anxiety and panic commonly cause real breathlessness with rapid, shallow breathing and tingling. However, this diagnosis should only be made once cardiac and respiratory causes have been considered, especially with a first episode.

References

Sources cited on this page. PubMed links open the original abstract.

Parshall MB, Schwartzstein RM, Adams L, et al; American Thoracic Society Committee on Dyspnea. An official American Thoracic Society statement: update on the mechanisms, assessment, and management of dyspnea. Am J Respir Crit Care Med. 2012;185(4):435–452. PMID 22336677 · doi:10.1164/rccm.201111-2042ST

Acute versus chronic breathlessness – different emergencies

The pace of onset fundamentally changes the differential diagnosis and urgency:

Sudden (seconds to minutes): Pulmonary embolism, anaphylaxis, foreign body aspiration, acute severe asthma, tension pneumothorax. All are emergencies requiring immediate assessment.
Acute (hours to days): Acute heart failure ("flash pulmonary oedema"), pneumonia, COPD exacerbation, pneumothorax, cardiac tamponade, severe anaemia.
Subacute to chronic (weeks to months): Slowly progressive heart failure, COPD or asthma, interstitial lung disease (ILD), pleural effusion, pulmonary hypertension, anaemia, obesity, deconditioning, anxiety.

In emergency assessment, three questions help rapidly prioritise: Is the patient hypoxic (SpO₂ below 94%)? Is there wheeze (bronchospasm – asthma/COPD), stridor (upper airway obstruction – anaphylaxis, croup), or silence (severe obstruction)? Are there signs of shock (altered consciousness, cold peripheries, mottled skin, tachycardia)?

Blood tests in breathlessness investigation

BNP or NT-proBNP: The most diagnostically useful test in undifferentiated breathlessness. BNP below 100 pg/mL (or NT-proBNP below 300 pg/mL) has a high negative predictive value for heart failure – if this low, heart failure is unlikely. NT-proBNP above 900 pg/mL in a breathless patient is highly suggestive of heart failure. The NICE heart failure guidelines recommend NT-proBNP as first-line investigation in suspected heart failure alongside ECG and echocardiography.
D-dimer: In low-to-moderate pre-test probability PE (calculated by Wells PE score), a negative D-dimer (below 500 µg/L FEU) safely excludes pulmonary embolism. In high pre-test probability, skip D-dimer and proceed directly to CT pulmonary angiogram (CTPA).
Troponin: Elevated in myocardial infarction causing acute left heart failure and pulmonary oedema. Troponin is also mildly elevated in acute PE (right heart strain), pulmonary hypertension, and severe sepsis – indicating myocardial stress rather than primary coronary disease in these contexts.
FBC: Anaemia causes or exacerbates breathlessness – even moderate anaemia (Hb below 90 g/L) significantly impairs exercise tolerance and causes breathlessness on exertion.
Arterial blood gas (ABG): Provides definitive information on gas exchange: PaO₂ (oxygen level), PaCO₂ (carbon dioxide – raised in ventilatory failure), pH (acidosis in severe respiratory failure), and bicarbonate (compensatory response). Essential in severe breathlessness, particularly in suspected COPD exacerbation where CO₂ retention guides the decision to use controlled oxygen therapy and non-invasive ventilation.

Shortness of Breath: What Causes It and When Is It Serious?