Shock Series 3 of 6 Anaphylaxis

Part 3 of 6 - Anaphylaxis a Distributive Shock 

Anaphylactic Shock part 3 of a 6 part shock series

Recapping: Shock is a syndrome characterised by poor cellular perfusion. It can be preceded by

• blood volume loss (Hypovolaemia),
• obstruction of blood flow into the heart (obstructive),
• failure to pump (cardiogenic), or
• available blood being maldistributed (distributive).

This trauma pearl edition continues to explore distributive shock; specifically anaphylactic shock.
Anaphylactic shock, is the severest manifestation of an allergic reaction. As a shock, it characteristically results in life threatening cellular hypoxia. It is classified as a type of distributive shock because it causes a wide spread vasodilation resulting in severe hypotension.
Similar to sepsis discussed in part 2, anaphylactic shock vasodilates because of immune system activation the difference I'd that in sepsis the endotoxins released from bacteria are the stimulus, but in anaphylactic reactions, it is the activation of immunoglobulin E.
When the body is exposed to an allergen (antigen), the plasma protein Immunoglobulin E, binds to it.
This activates white blood cells in the blood stream (basophils) and in tissue (Mast cells) to degranulate or burst open. These cells are full of Histamine and Heparin. Once released, heparin slows down coagulation (promoting blood glow locally), and histamine is a potent vasodilator. Blood vessels dilate and become engorged, causing plasma water to leak into the interstitial spaces between cells.
Oedema is experienced as hives and angioedema as seen in the image.
In severe swelling, airway obstruction is the rapid killer. A wide spread vasodilation doesn't just result in maldistribution of blood volume, but capillary leakage can result in Hypovolaemia, giving a dual shock syndrome resulting in catastrophic loss of blood pressure.
Anaphylaxis is life threatening and treated using :

• Antihistamine medication counters the vasodilation effect of histamine released by basophils and mast cells.
• IV or IO Fluid resuscitation may be required to restore volume.

Drug of choice (first line)

• Adrenaline IM or SC as a chemical vasoconstrictor, thus restoring vascular tone and BP, and reducing capillary leakage, plasma loss.
• Adrenaline dose: over 13yrs
• 0.5ml of 1:1000 =500mcg

Dose is age dependant and doses recommended by the NH&MRC, and the Australian Resuscitation council, can be found here for children.
The patients vital signs should be closely monitored, and repeat adrenaline doses can be given every 5 minutes.