Oxygen delivery is diminished and arterial blood gases reveal combined metabolic and respiratory acidosis with hypoxaemia. Urinary output decreases to the point of oliguria. This phase has a poor prognosis. Anaphylactic Shock which can be classified in the distributive group is characterised by acute changes in vascular permeability and bronchial hyperactivity. These changes occur in response to both antigenic and non-antigenic stimuli.
Chemical mediators cause marked vasodilation and increased capillary permeability.
Bronchial oedema and smooth muscle contraction cause bronchospasm which leads to alveolar hypoventilation and acute respiratory distress. The patient usually detects the onset of symptoms within 20 minutes of exposure to the reactant. Initially the the body feels warm and this is followed by pruritis, urticaria and angioedema. The patient often feels a sense of 'impending doom'. Laryngeal oedema causes hoarseness and inspiratory stridor, whilst bronchoconstriction leads to wheezing, hypoxia and cyanosis.