A 26-year-old man is brought to the Emergency Department after a diving incident. He is alert but weak in all 4 limbs. His observations are: blood pressure 78/42 mmHg, heart rate 44/min, oxygen saturations 98% on high-flow oxygen, and warm peripheries. After an initial fluid bolus, his blood pressure remains low. During log-roll and suction, his heart rate falls further. Which physiological process best explains this haemodynamic pattern?
