13- Hypoxia Flashcards
What is the difference between hypoxia and hypoaemia
Hypoxia
Describes a specific
environment
Hypoxaemia
Describes the blood environment
What is the o2 cascade
The O2 cascade describes the decreasing oxygen tension from inspired air to respiring cells
What 4 factors determine effectiveness of the o2 cascade
“Alveolar ventilation 2. Ventilation-perfusion matching 3. Diffusion capacity 4. Cardiac output ”
What happens as altitude is increased initially
“As altitude increases, the barometric pressure reduces (the air becomes thinner) and according to Dalton’s law, this means that the content of atmospheric gases is reduced (although the proportions remain unchanged). A reduced PIO2 causes a reduced PaO2. A lower PaO2 will reduce the concentration gradient and slow the rate of O2 diffusion from the alveoli to the capillaries (Fick’s law).
”
How are some highlanders adapted
Barrel chest’ – larger TLC, more alveoli and greater capillarisation
Increased haematocrit – greater oxygen carrying-capacity of the blood
Larger heart to pump through vasoconstricted pulmonary circulation
Increased mitochondrial density – greater oxygen utilisation at cellular level
What drug can be used to treat altitude sickness
Acetazolamide
Carbonic anhydrase inhibitor – accelerates the slow renal compensation to hypoxia-induced hyperventilation
What is acclimation
Acclimation
Like acclimatisation but stimulated by an artificial environment (e.g. hypobaric chamber or breathing hypoxic gas)
Describe chronic mountain sickness
Causes: unknown
Pathophysiology: secondary polycythaemia increases blood viscosity, which sludges through systemic capillary beds impeding O2 delivery (despite more than adequate oxygenation)
Symptoms: cyanosis, fatigue
Consequences: ischaemic tissue damage, heart failure, eventual death
Treatment: no interventional medical treatment – sufferers are exiled to lower altitudes
Describe acute mountain sickness
Causes: maladaptation to the high-altitude environment. Usually associated with recent ascent - onset within 24 hours and can last more than a week
Pathophysiology: probably associated with a mild cerebral oedema
Symptoms: nausea, vomiting, irritability, dizziness, insomnia, fatigue, and dyspnoea – ‘hangover’
Consequences: development into HAPE or HACE
Treatment: monitor symptoms, stop ascent, analgesia, fluids, medication (acetazolamide) or hyperbaric O2 therapy
symptoms tend to subside after 48 hrs of increased renal compensation
Describe high altitude cerebral oedema
Causes: rapid ascent or inability to acclimatise
Pathophysiology: vasodilation of vessels in response to hypoxaemia (to increase blood flow)
more blood going into the capillaries increases fluid leakage
cranium is a ‘sealed box’ – no room to expand so intracranial pressure increases
Symptoms: confusion, ataxia, behavioural change, hallucinations, disorientation
Consequences: irrational behaviour, irreversibal neurological damage, coma, death
Treatment: immediate descent, O2 therapy, hyperbaric O2 therapy, dexamethasone
Describe high altitude pulmonary oedema
Causes: rapid ascent or inability to acclimatise
Pathophysiology: vasoconstriction of pulmonary vessels in response to hypoxia
increased pulmonary pressure, permeability and fluid leakage from capillaries
fluid accumulates once production exceeds the maximum rate of lymph drainage
Symptoms: dyspnoea, dry cough, bloody sputum, crackling chest sounds
Consequences: impaired gas exchange, impaired ventilatory mechanics
Treatment: descent, hyperbaric O2 therapy, nifedipine (CCB), salmeterol, sildenafil
What is type 1 resp failure
TYPE IHypoxic
respiratory failure PaO2 < 8 kPa
PaCO2 = low/normal
Caused by Pulmonary oedema
Pneumonia
Atelectasis
What is type 2 resp failure
TYPE IIHypercapnic respiratory failure PaO2 < 8 kPa
PaCO2 > 6.7 kPa
Caused by Decreased CNS drive Increased work of breathing Pulmonary fibrosis Neuromuscular disease Increased physiological dead space Obesity
What are the o2 pressure in the cascade
Atmosphere (21.3 kPa) -> Upper airways (20.0 kPa) -> Alveolus (13.5 kPa) -> Post-alveolar capillary (13.5 kPa) -> Pulmonary vein (13.3 kPa) -> Systemic artery (13.3 kPa) -> Cells (5.3 kPa)
How does gas transport change during exercise
Exercise increases the oxygen demand; RF increases; TV increases; Q increases; ODC curve shifts right
