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15 May 2020

ECT4Health - A 5th category of shock: Pneumogenic

Are we missing a vital category of shock?  Forever - we have had shock classified into 4 main categories, but is there a fifth?
Hypovolaemic - that is a reduction in blood volume.
Cardiogenic - that is a reduction in cardiac output.
Obstructive - which is an obstruction to venous return and this preload.
Distributive - which is a venous dilation which causes a maldistribution of blood between arterial and venous capitance.
The point is - they are all shock which can be defined as Global Cellular Hypoxia - lack of oxygen to all cells is the fundamental definition of shock.
So with shock being one thing (global hypoxia), why wouldn’t Respiratory failure contribute to shock?   It should.  I think the clever books have missed a category.  So I’m advocating a 5th category - Pneumogenic shock.
All tissue/organ oxygenation starts with delivery of oxygen via the arterial network.  The right heart receives blood from the tissues, pumps it into the lungs, to be oxygenated, receives it into the left heart, then pumps this oxygen to tissues - thus completing the circuit.
Traditionally in shock we have categorised every step in this circuit, except the glaringly obvious issue to oxygenation of blood inside pulmonary vessels.  The world of respiratory medicine goes to great pains to highlight the importance of understanding the physiology of VQ ratios, PF Ratios and A-a gradients, but stop short when translating this hypoxaemic discourse of respiratory failure to the concept of shock.
It therefore seems rational to add a unique category into the age old shock paradigm.  I propose “pneumogenic shock” to describe an aetiology of profound  global hypoxia born from a respiratory failure, not a cardiovascular failure.
Definition - Pneumogenic Shock, is a syndrome of hypoxaemic hypoxia caused by pathophysiological conditions that prevent adequate oxygenation of blood  (external respiration).  Pneumogenic shock would be classified as pre-pulmonary or extrinsic, and intrapulmonary (intrinsic).
Extrinsic Pneumogenic shock (EPS):  EPS occurs when a person is exposed to a reduction of oxygen in the gas mix being breathed.  EPS could manifest with toxic gas inhalation, or hypoxia environments.  It’s estimated that Fio2 of less than 0.15 (15%) is not sustainable for life at sea level barometric pressure.  Therefore breathing a smoke filled toxic gas environment where partial pressures of oxygen are too low would be considered to constitute EPS.
Prepulmonary Pneumogenic shock would manifest as a factor of airway obstruction, or restriction.  Foreign body airway occlusion (FBAO), mechanical suffocation, strangulation/hanging, and upper airway, larynx, tracheal or bronchial oedema, are all examples.  They may be manifestations of trauma, infection or other triggers of inflammation to the upper airway.  Fundamentally, shock caused by this subcategory are not associated with low Fio2, or alveolar dysfunction.
Intrinsic Pneumogenic Shock:  This Pneumogenic shock is intrapulmonary.  Where gas exchange occurs in alveoli, a mismatch of VQ ratio leads to an A-a gradient that is incompatible with survival.  In instances of severe pulmonary oedema, pulmonary embolism, low airway injury, ARDS or acute lung injury; gas exchange may become so impaired that hypoxaemia is critical.  A measure of PF ratio and V:Q would be assessment data that differentiates Pneumogenic shock from other syndromes.