Dr Henry Jefferson, Consultant Anaesthetist
#QuestionFeedback 2. Shunt & Oxygen At what shunt fraction does hypoxaemia become refractory to 100% oxygen? A figure of approximately 30% is sometimes quoted, but this is a teaching approximation rather than a strict physiological threshold. As shunt fraction increases, the effectiveness of increasing FiO₂ progressively decreases. Even with substantial shunting, increasing FiO₂ may still produce some improvement in arterial oxygenation. The key principle is that shunted blood bypasses ventilated alveoli, limiting the benefit of increasing inspired oxygen.