In this phase the cardiac output falls due to myocardial depression and decreased intravascular volume. Sympathetic responses produce marked vasoconstriction, narrowing of the pulse pressure, and marked sinus tachcardia. Significant hypotension occurs due to the increased afterload and decreased output. Skin is pale and cool and the respirations are rapid and shallow. 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.