How to differentiate heart n lung causes of breathlessness

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I now have excellent content from Fishman's and Murray & Nadel's. Let me compile the full, detailed answer.

Differentiating Cardiac vs. Pulmonary Causes of Breathlessness (Dyspnoea)

Breathlessness (dyspnoea) is one of the most common presenting symptoms in medicine and can arise from disorders of the heart, lungs, blood, metabolism, or neuromuscular system. The key challenge is distinguishing a cardiac cause from a pulmonary cause. Here is a systematic approach:

1. History and Character of the Breathlessness

Feature	Cardiac	Pulmonary
Onset	Often sudden (flash pulmonary oedema) or subacute	Variable; often chronic and progressive (COPD, ILD)
Exertional dyspnoea	Present in both; cardiac patients often slow down gradually without noticing	Present in both; may be accompanied by wheeze, cough
Orthopnoea	Classic feature - worsening lying flat, relieved by 2-3 pillows	Can occur in severe COPD/asthma (difficulty using chest bellows in recumbent position), but less common
Paroxysmal Nocturnal Dyspnoea (PND)	Hallmark of left ventricular failure - patient wakes gasping, must sit up or stand; may open windows	Can occur in lung disease (pooling of secretions, nocturnal aspiration, sleep-induced airway resistance increase) but less typical
Platypnea	Not typical	Dyspnoea worse on sitting up, better lying down - suggests arteriovenous malformations at lung bases or right-to-left shunting
Trepopnea	Not typical	Dyspnoea worse when the diseased lung is in the dependent position
Wheezing	Can occur ("cardiac asthma") in pulmonary oedema	Common in asthma, COPD
Productive cough	Pink frothy sputum in pulmonary oedema	Purulent in infection; chronic in COPD
Haemoptysis	Mitral stenosis, pulmonary oedema	Malignancy, TB, bronchiectasis
Chest pain	Ischaemic / pleuritic in cardiac disease	Pleurisy, pneumothorax
Ankle swelling	Right heart failure	Cor pulmonale in chronic lung disease

Fishman's Pulmonary Diseases and Disorders, 2-Volume Set, block5.md

Murray & Nadel's Textbook of Respiratory Medicine, block4.md

2. Key Symptom Patterns

Orthopnoea

In cardiac disease: caused by pulmonary congestion (increased venous return in recumbency increases pulmonary capillary pressures, stiffening lungs and increasing work of breathing).
In lung disease: caused by mechanical difficulty using the chest bellows flat, or gravity-induced reduction in lung volumes.

Paroxysmal Nocturnal Dyspnoea (PND)

Cardiac (classic): patient wakes after 1-2 hours of sleep, gasping, drenched in sweat, must sit or stand. May hear bilateral wheeze. Often associated with profuse sweating. Represents acute left ventricular decompensation from surge in venous return (mobilisation of peripheral oedema on lying).
Pulmonary: can mimic PND but typically due to pooling of airway secretions, nocturnal bronchoconstriction (nocturnal asthma), or aspiration.

3. Associated Symptoms

Symptom	Points to Cardiac	Points to Pulmonary
Bilateral ankle oedema	Right heart failure	Cor pulmonale
Palpitations / syncope	Arrhythmia, HF	Less typical
Fatigue (low cardiac output)	Common	Less prominent unless severe
Chronic productive cough	Less typical	COPD, bronchiectasis
Wheeze	Cardiac asthma	Asthma, COPD
Clubbing	Cyanotic heart disease	ILD, bronchiectasis, malignancy
Cyanosis	Central (cardiac)	Central (respiratory)

4. Physical Examination

Sign	Cardiac	Pulmonary
JVP elevation	Present (RHF)	Cor pulmonale
S3 gallop	Left ventricular failure	Absent
Displaced apex beat	Cardiomegaly	Absent
Basal crackles	Pulmonary oedema	ILD, infection
Wheeze	Cardiac asthma	Asthma/COPD
Prolonged expiration	Absent	COPD
Chest hyperinflation	Absent	Emphysema
Pleural effusion	Bilateral (cardiac)	Unilateral (infection, malignancy)

5. Investigations

Investigation	Cardiac finding	Pulmonary finding
ECG	LVH, ischaemia, arrhythmia, AF	P-pulmonale, right axis deviation (cor pulmonale)
CXR	Cardiomegaly, upper lobe diversion, Kerley B lines, bilateral perihilar oedema	Hyperinflation, infiltrates, effusions, consolidation
Echocardiogram	Reduced EF, diastolic dysfunction, valve disease	Pulmonary hypertension, right heart dilatation
Spirometry / PFTs	Normal or mild restriction	Obstruction (FEV1/FVC <0.7) or restriction
BNP / NT-proBNP	Markedly elevated in heart failure	Normal or mildly elevated in severe lung disease
ABG	Hypoxia ± hypocapnia; in severe HF, hypercapnia	Hypoxia, hypercapnia (type 2 RF in COPD) or hypocapnia (type 1)
D-dimer / CTPA	Normal	Elevated in PE
6-minute walk / CPET	Low peak VO2, VE/VCO2 elevated	O2 desaturation, low FEV1

BNP is a key discriminator: A BNP <100 pg/mL makes heart failure unlikely; >500 pg/mL strongly suggests it. This is referenced in Fishman's Pulmonary Diseases (the BREATHING NOT PROPERLY study is cited therein for BNP in emergency breathlessness assessment).

6. Quick Clinical Mnemonic

For cardiac breathlessness, think "SOAP":

Swelling (ankle oedema)
Orthopnoea / PND
Arrhythmia / gallop S3
Pink frothy sputum

For pulmonary breathlessness, think "CHAP":

Chronic productive cough
Hyperinflation / wheeze
Abnormal PFTs
Purulent sputum

7. The Difficult Overlap

Both conditions can coexist. Cor pulmonale (right heart failure from chronic lung disease) and patients with COPD who also develop ischaemic heart disease create diagnostic challenges. In these cases:

BNP/NT-proBNP and echocardiogram are the most useful discriminators.
Cardiopulmonary exercise testing (CPET) can definitively identify the limiting system (cardiac vs. ventilatory).
High-resolution CT (HRCT) and spirometry complete the pulmonary evaluation.

Sources: Fishman's Pulmonary Diseases and Disorders, 2-Volume Set | Murray & Nadel's Textbook of Respiratory Medicine

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