Causes of Breathlessness
Breathlessness arises from two primary physiological mechanisms: increased respiratory drive (from stimulation of pulmonary receptors or chemoreceptors) and impaired ventilatory mechanics (from airflow obstruction, muscle weakness, or decreased chest wall compliance), with most cardiopulmonary diseases involving both mechanisms simultaneously. 1
Primary Pathophysiological Categories
Increased Respiratory Drive
This mechanism involves increased afferent input to respiratory centers and includes:
Stimulation of Pulmonary Receptors:
- Interstitial lung disease 1
- Pleural effusion causing compressive atelectasis 1
- Pulmonary vascular disease including thromboembolism and idiopathic pulmonary hypertension 1
- Congestive heart failure (both systolic and diastolic dysfunction) 1
Chemoreceptor Stimulation:
- Acute hypoxemia, hypercapnia, and/or acidemia from impaired gas exchange in asthma, pulmonary embolism, pneumonia, or heart failure 1
- Environmental hypoxia at altitude or in contained spaces 1
- Metabolic acidosis from renal disease (renal failure, renal tubular acidosis) 1
- Decreased oxygen carrying capacity from anemia 1
- Decreased cardiac output 1
- Pregnancy 1
Impaired Ventilatory Mechanics
This mechanism involves reduced afferent feedback for a given efferent output:
Airflow Obstruction:
- Asthma 1
- COPD (the most common cause, particularly in smokers over 40 years with gradual onset over many years) 1
- Laryngospasm 1
- Foreign body aspiration 1
- Bronchitis 1
Muscle Weakness:
- Myasthenia gravis 1
- Guillain-Barré syndrome 1
- Spinal cord injury 1
- Myopathy 1
- Post-poliomyelitis syndrome 1
Decreased Chest Wall Compliance:
Behavioral and Psychological Factors
Clinical Context and Disease Prevalence
Approximately 85% of chronic breathlessness cases are attributable to congestive heart failure, myocardial ischemia, or COPD, with more than 30% of cases being multifactorial 1. Among respiratory causes specifically, respiratory diseases account for 40-57% of breathlessness in the general population, with asthma being most common (approximately 25%) 2.
Key Diagnostic Considerations
In COPD patients specifically, breathlessness develops gradually over many years and typically presents after age 40 in long-term smokers with moderate to severe airflow limitation 1. The breathlessness eventually limits daily activities and should be assessed using questions related to everyday activities 1.
Critical pitfall: Physical examination has poor sensitivity for detecting or excluding moderately severe COPD, and the degree of airways obstruction cannot be predicted from symptoms or signs alone 1. Wheezing during tidal breathing and prolonged forced expiratory time (>5 seconds) are useful indicators of airflow limitation, but their absence does not exclude COPD 1.
Quality of Breathlessness as Diagnostic Clue
The specific descriptors patients use can provide insight into underlying mechanisms 1:
- "Chest tightness" is relatively specific for bronchoconstriction 1
- "Air hunger" and "inability to get a deep breath" suggest dynamic hyperinflation or restrictive mechanics (heart failure, pulmonary fibrosis) 1
- "Effort, suffocation, and rapid breathing" characterize CO2-induced panic attacks but are nonspecific 1
Initial Assessment Priorities
Oxygen saturation should be checked by pulse oximetry in all breathless patients as the "fifth vital sign," supplemented by blood gases when necessary 1. The medical history may point to specific diagnoses such as pneumonia, pulmonary embolism, or exacerbations of chronic conditions like COPD, asthma, or heart failure 1.
For patients with risk factors for hypercapnic respiratory failure (severe or moderate COPD, especially with previous respiratory failure or on long-term oxygen, severe chest wall/spinal disease, neuromuscular disease, severe obesity, cystic fibrosis, or bronchiectasis), target oxygen saturation should be 88-92% rather than 94-98% 1.