Respiratory Physiology - Types of Hypoxia Flashcards
Define hypoxia
Insufficient oxygen supply to meet the demands of the tissues for normal cellular function
Can be categorised as:
Hypoxic
Anaemic
Stagnant (or Ischaemic)
Hystotoxic
What is hypoxic hypoxia?
A pathological reduction in the partial pressure of oxygen in the blood (hypoxaemia), associated with reduced saturation of Hb
Causes:
Hypoventilation or reduced FiO2
VQ mismatching
Gas transfer impairment
What is anaemic hypoxia?
A reduction in the blood’s oxygen carrying capacity as a result of reduced functional or total amounts of Hb. The available Hb is fully saturatid, leading to false-normal SpO2
Causes:
Severe chronic anaemia (total Hb reduction)
CO poisoning (functional Hb reduction)
Some sources will put CO poisoning in the histotoxic section. It can probably be argued both ways, but it makes more sense to put it here because the Hb can’t bind to the oxygen, rather than the tissues not being able to absorb it from the blood
What is stagnant (ischaemic) hypoxia?
A failure of perfusion, with inadequate delivery of well-oxygenated blood to the necessary tissues
Causes:
Regional - Limb ischaemia (PVD or compartment syndrome), tourniquet
Systemic - Microcirculatory failure (sepsis), cardiac failure, any organ failure syndrome involving shock
What is histotoxic hypoxia?
A failure of oxygen consumption in the tissues, where they are unable to take up and use the oxygen for oxidative phosphorylation.
Arterial SpO2 is normal, and venous SpO2 is high as there is minimal oxygen extraction.
Causes:
Cyanide toxicity (uncouples oxidative phosphorylation)
Malignant hyperthermia
For bonus marks, technically malignant hyperthermia demonstrates both histotoxic hypoxia as a result of mitochondrial failure, and ischaemic hypoxia due to localised compartment syndrome induced by muscle swelling
How does the body detect hypoxia?
Low PaO2 is detected, primarily via type 1 glomus cells in the carotid body, which activates the carotid sinus branch of the glossopharyngeal nerve. Also via the aortic bodies, which signal via the vagus nerve. Both relay impulses to the nucleus tractus solitarius in the medulla.
These only detect hypoxic hypoxia, increasing discharge rate rapidly below 13kPa, with the highest firing rate around the point at which the oxygen dissociation curve is steepest.
Anaesthetic agents depress this response significantly.
The kidneys can detect chronic hypoxia and chronic anaemia.
How does the body respond to hypoxia?
Divided into multiple systems:
Respiratory - Hyperventilation via increased RR and TV - limited by respiratory alkalosis
Cardiovascular - Sympathetic stimulation causes Increased HR and BP, and peripheral vasoconstriction
Neurovascular - Generealised neuroendocrine stress response (catecholamines and cortisol). Cerebral vasodilation can nearly double blood flow if required
