Four volumes combining into four capacities
Mnemonic
“A capacity is two or more volumes”, and the rule that matters is which ones contain the residual volume:
- Vital capacity = tidal + inspiratory reserve + expiratory reserve. Measurable by spirometry
- Functional residual capacity = expiratory reserve + residual
- Total lung capacity = vital capacity + residual
“Spirometry cannot measure residual volume, so it cannot measure FRC or TLC either.” Those require helium dilution, nitrogen washout or body plethysmography.
Plethysmography measures all intrathoracic gas; dilution measures only communicating gas, so a large difference between the two is itself a measure of trapped gas in bullae or severe airflow obstruction.
FRC is the equilibrium point where inward lung recoil balances outward chest wall recoil. It falls with obesity, pregnancy, lying supine, fibrosis and anaesthesia, and rises in emphysema and acute asthma.
Expansion
The four volumes
- Tidal volume: normal breath, about 500 ml
- Inspiratory reserve volume: further air that can be inspired, about 3000 ml
- Expiratory reserve volume: further air that can be expired, about 1100 ml
- Residual volume: air remaining after maximal expiration, about 1200 ml
The four capacities, each the sum of two or more volumes
- Inspiratory capacity = tidal + inspiratory reserve
- Functional residual capacity = expiratory reserve + residual
- Vital capacity = tidal + inspiratory reserve + expiratory reserve
- Total lung capacity = vital capacity + residual
Any capacity containing the residual volume cannot be measured by spirometry, so functional residual capacity and total lung capacity require helium dilution, nitrogen washout or body plethysmography.
Plethysmography versus dilution: dilution techniques measure only communicating gas, so they underestimate volumes when there are bullae or severe air trapping. Plethysmography measures all intrathoracic gas, so a large difference between the two is itself a measure of trapped gas.
Functional residual capacity is the equilibrium point where the inward elastic recoil of the lung balances the outward recoil of the chest wall. It is reduced by obesity, pregnancy, supine posture, fibrosis and general anaesthesia, and raised in emphysema and acute asthma through hyperinflation.
Raised residual volume and total lung capacity with a low FEV1/FVC ratio confirms hyperinflation and air trapping in obstructive disease; a reduced total lung capacity confirms a genuine restrictive defect.