Spirometry values are always interpreted against the predicted value for sex, age, height and ethnicity — not against absolute figures. Since 2022 the z-score with a lower limit of normal at z = −1.64 (the fifth percentile) has been recommended in preference to fixed percentage thresholds.
Bedside parameters
Spirometry
| Parameter | Meaning |
|---|---|
| FEV₁ | Forced expiratory volume in the first second |
| FVC | Forced vital capacity |
| FEV₁/FVC | The ratio that defines obstruction |
| PEF | Peak expiratory flow — variability rather than the absolute value |
| TLC | Total lung capacity, which requires body plethysmography |
| DLCO | Diffusing capacity for carbon monoxide |
An obstructive pattern: FEV₁/FVC below the lower limit of normal (traditionally < 0.70). A restrictive pattern: a reduced TLC — this cannot be established with spirometry alone, but is suspected when the FVC is low with a normal or high ratio.
Reversibility: an increase in FEV₁ or FVC of > 10 % of the predicted value after a bronchodilator suggests asthma.
Severity in COPD
Arterial blood gas
Type 1 respiratory failure: PaO₂ < 8 kPa with a normal or low PaCO₂ — a problem of oxygenation. Type 2: PaO₂ < 8 kPa with a PaCO₂ > 6.5 kPa — a problem of ventilation.
Derived measures
| Measure | Formula | Normal |
|---|---|---|
| P/F ratio | PaO₂ (kPa) / FiO₂ | > 53 kPa (> 400 mmHg) |
| Alveolar gas equation | PAO₂ = FiO₂ × (Patm − PH₂O) − PaCO₂/R | R ≈ 0.8 |
| A–a gradient | PAO₂ − PaO₂ | < 2 kPa in the young; it increases with age |
| Anion gap | Na − (Cl + HCO₃) | 8–16 mmol/L |
ARDS according to the Berlin definition
| Grade | P/F ratio with a PEEP ≥ 5 cmH₂O |
|---|---|
| Mild | 26.6–40 kPa (200–300 mmHg) |
| Moderate | 13.3–26.6 kPa (100–200 mmHg) |
| Severe | ≤ 13.3 kPa (≤ 100 mmHg) |
See the intensive care parameters for ventilator settings.
Oxygen targets
Aim for a saturation of 92–96 % in most patients, and 88–92 % where there is a risk of carbon dioxide retention (COPD, obesity hypoventilation, neuromuscular disease). Both hypoxia and liberal oxygen administration are associated with worse outcomes.
