SEPT 10 Built from the handwritten pages you uploaded on 10 September 2026.

Hypoxaemia & the A–a gradient

The five mechanisms, and how the A–a gradient tells you which one you are looking at.

The gradient

A–a gradient = P_AO₂ − PaO₂ — alveolar minus arterial.
Alveolar gas equation: P_AO₂ = FiO₂ × (P_atm − P_H₂O) − PaCO₂ / R
On air at sea level: P_AO₂ ≈ 0.21 × (101 − 6.3) − PaCO₂ / 0.8 ≈ 20 − PaCO₂/0.8 kPa
Normal: about 1–2 kPa (8–12 mmHg) in a young adult, rising with age — roughly (age/4 + 4) mmHg.
It is the single most useful step in working out why a patient is hypoxaemic.

Hypoxaemia with a NORMAL A–a gradient → the lung is working; the problem is the gas going in. Either low inspired oxygen (altitude) or hypoventilation (opioids, residual block, neuromuscular disease, obesity). Look at the PaCO₂ — it will be high in hypoventilation and low or normal at altitude.
Hypoxaemia with a RAISED A–a gradient → the problem is within the lung. Then give 100% oxygen: if PaO₂ rises it was V/Q mismatch or diffusion; if it barely moves it is shunt.

Mechanism	Typical causes	Perioperative example
Hypoventilation	Opioids, residual neuromuscular block, high neuraxial block, obesity, neuromuscular disease	The slow, snoring patient in recovery
V/Q mismatch	COPD, asthma, pulmonary embolism, atelectasis (partial)	The commonest cause under anaesthesia
Shunt	Pneumonia, ARDS, complete atelectasis or collapse, pulmonary oedema, right-to-left cardiac shunt	Endobronchial intubation, lobar collapse
Diffusion	Fibrosis, emphysema, ILD	Rarely the sole cause at rest
Low FiO₂	Altitude, hypoxic gas mixture	Equipment failure, crossed pipeline

A low mixed venous oxygen saturation amplifies any existing shunt — because the shunted blood is more desaturated to begin with. Causes: low cardiac output, sepsis, anaemia, raised oxygen consumption (shivering, pyrexia, thyrotoxicosis).
Conversely a high SvO₂ (sepsis, cyanide poisoning, arteriovenous shunting) means tissue is not extracting oxygen — the arterial figure can look reassuring while the tissues are hypoxic.
Raised PvO₂ with a normal A–a gradient in a shocked or poisoned patient should prompt thought about cyanide and about impaired cellular utilisation.

A–a P_AO₂ − PaO₂

Normal 1–2 kPa, rises with age

Normal gradient altitude or hypoventilation

Shunt won't correct with O₂

COPD V/Q — responds

ARDS shunt — doesn't

Built from your A–a gradient page. See also Pulmonary function tests.