Lung volumes and spirometry

The core idea

The lung never empties completely. Each breath moves air in and out on top of a volume that always stays behind. Learn the four basic volumes; every capacity is a sum of two or more of them, and a question that describes a manoeuvre ("breathed out normally, then in maximally") is asking you to name the matching capacity.

Volumes and capacities (typical adult male)

• Rearranging: IRV = IC − TV, or IRV = VC − (TV + ERV). ERV = FRC − RV.
• Volumes are about 20–25% lower in women.
• FRC is the resting point of the respiratory system: the inward elastic recoil of the lungs exactly balances the outward recoil of the chest wall, with no muscle acting. It is the oxygen store between breaths, which is why preoxygenation fills the FRC with oxygen before intubation.
• Residual volume keeps alveoli open after maximal expiration and prevents complete lung collapse; it exists because small airways close (and the chest wall cannot be compressed further) before the lung empties.

Muscles that move these volumes

• Quiet inspiration is active: the diaphragm produces about two-thirds to three-quarters of tidal volume, helped by the external intercostals.
• Quiet expiration is passive elastic recoil.
• Forced expiration (FVC, peak flow, coughing) uses the abdominal wall muscles (rectus abdominis, external and internal obliques, transversus abdominis) and the internal intercostals.
• Forced inspiration adds sternocleidomastoid, scalenes and other accessory muscles.

What spirometry can and cannot measure

A spirometer records only air that moves, so it measures TV, IRV, ERV, IC, VC, FEV₁, FVC and peak flow. It cannot measure residual volume, and therefore cannot measure FRC or TLC. These need:

• Helium dilution: measures gas in airways that communicate with the mouth; it underestimates volume in severe COPD with trapped air.
• Body plethysmography: measures all gas in the thorax, including trapped gas (based on Boyle's law).
• Nitrogen washout.

Dead space

• Anatomical dead space: the conducting airways from nose to terminal bronchioles, about 150 mL (roughly 2 mL/kg). Measured by Fowler's method (single-breath nitrogen washout).
• Alveolar dead space: alveoli that are ventilated but not perfused. Negligible in health.
• Physiological (total) dead space = anatomical + alveolar. In health the two are nearly equal (about 30% of tidal volume). It rises in pulmonary embolism, emphysema, low cardiac output and positive-pressure ventilation. It is measured with the Bohr equation: VD/VT = (PaCO₂ − PECO₂)/PaCO₂.

• Alveolar ventilation = (tidal volume − dead space) × respiratory rate. At the same minute volume, rapid shallow breathing wastes a larger fraction on dead space and ventilates alveoli less than slow deep breathing. Alveolar ventilation, not minute ventilation, sets PaCO₂.

Spirometry patterns

• In obstruction, airways close early in expiration and trap air, so RV, FRC and TLC increase; vital capacity falls because RV takes up more of TLC.
• In restriction, all volumes fall together; FEV₁ and FVC fall in proportion, so the ratio is preserved or high.

Reversibility and gas transfer

• Reversibility: a rise in FEV₁ of at least 12% and 200 mL after a bronchodilator shows reversible airflow obstruction and supports asthma. An FEV₁ rising from 2.0 to 2.5 L (25%, 500 mL) is clearly reversible. A large response with a still-low ratio (for example FEV₁/FVC 40% improving by 18%) is reversible obstruction. A negative test does not exclude asthma, especially when the patient is well or already treated.
• COPD is largely irreversible. An infective exacerbation is treated with bronchodilators, antibiotics if sputum is purulent, and oral prednisolone 30 mg daily for 5 days (check glucose in diabetics).
• Transfer factor (TLCO, DLCO) measures carbon monoxide uptake across the alveolar membrane. It falls when the membrane thickens (pulmonary fibrosis), when surface area is lost (emphysema), when the pulmonary capillary bed is lost (pulmonary embolism, pulmonary hypertension) and in anaemia (correct for haemoglobin). It rises with polycythaemia, alveolar haemorrhage, and sometimes asthma. KCO is transfer factor corrected for alveolar volume.

Flow–volume loops

• Obstruction: a scooped-out (concave) expiratory limb.
• Restriction: a small, tall, narrow loop (small volumes, flows preserved for volume).
• Fixed upper airway obstruction (tracheal stenosis, goitre): both limbs flattened into a box.
• Variable extrathoracic obstruction (vocal cord palsy): flattened inspiratory limb. Variable intrathoracic obstruction: flattened expiratory limb.

Changes worth knowing

• Posture: FRC falls on lying down because abdominal contents push the diaphragm up. It falls further under anaesthesia, in obesity and in late pregnancy, so these patients desaturate quickly.
• Age: loss of elastic recoil raises RV and FRC and lowers VC; TLC stays roughly constant.
• Pregnancy: FRC, ERV and RV fall; tidal volume and minute ventilation rise (progesterone); VC is unchanged.

Remember Capacities are sums of volumes; anything that contains RV (FRC, TLC) cannot be measured by a spirometer.

Exam trap In restrictive disease the FEV₁/FVC ratio is normal or raised, not reduced; the defining finding is a low TLC.

Exam trap Extending the neck increases anatomical dead space; a tracheostomy reduces it.

How FCPS asks about this

• Volume left after a normal expiration: FRC (ERV + RV); after maximal expiration: residual volume.
• Normal expiration then maximal inspiration: inspiratory capacity (TV + IRV); IRV is IC minus TV.
• Cannot be measured by spirometry: RV, FRC, TLC; use helium dilution or body plethysmography.
• Capacity that increases in obstructive disease: TLC, FRC and RV rise from air trapping.
• Restrictive disease: low TLC, low FVC, normal or high FEV₁/FVC; fibrosis also lowers TLCO.
• Physiological dead space: anatomical plus alveolar; measured by the Bohr equation; rises in PE and emphysema.
• Muscles of forced expiration: abdominal wall muscles and internal intercostals.