Physiology, lung function & ABGs flashcards
19 high-yield MRCP Part 1 cards from the Intellect Medicos Respiratory Medicine course. Try to answer each question before you open it — then study them properly with spaced repetition, which brings each card back just before you would forget it.
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Which mnemonic lists the causes of a RIGHT shift of the oxygen dissociation curve, and what does it stand for?
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CADET: CO2 raised, Acid (low pH), 2,3-DPG raised, Exercise, Temperature raised.
Why? A right shift lowers haemoglobin affinity for oxygen, so oxygen is released more easily to metabolically active tissues.
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Name three causes of a LEFT shift of the oxygen dissociation curve.
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Any three of: carbon monoxide poisoning, methaemoglobinaemia, fetal haemoglobin (HbF), alkalosis, low CO2, hypothermia, low 2,3-DPG.
Why? A left shift means haemoglobin holds on to oxygen more tightly, so less oxygen is released to the tissues.
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Raised CO2 and H+ lowering haemoglobin's affinity for oxygen in the tissues is the ______ effect; oxygenated haemoglobin carrying less CO2 in the lungs is the Haldane effect.
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Bohr
Why? Bohr helps oxygen unloading in tissues (right shift). Haldane helps CO2 unloading in the lungs because oxygenated Hb binds less CO2 and H+.
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Raised CO2 and H+ lowering haemoglobin's affinity for oxygen in the tissues is the Bohr effect; oxygenated haemoglobin carrying less CO2 in the lungs is the ______ effect.
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Haldane
Why? Bohr helps oxygen unloading in tissues (right shift). Haldane helps CO2 unloading in the lungs because oxygenated Hb binds less CO2 and H+.
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Where are the central chemoreceptors, and what stimulus do they mainly respond to?
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Ventral surface of the medulla; they respond to a rise in H+ (fall in pH) in CSF caused by raised PaCO2.
Why? CO2 crosses the blood–brain barrier and forms H+, so central chemoreceptors are the main minute-to-minute controllers of ventilation via CO2.
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What ABG values define type 2 respiratory failure?
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PaO2 < 8 kPa with PaCO2 > 6.0 kPa (45 mmHg); some sources, including the course notes, use > 6.5 kPa.
Why? Type 2 failure is hypercapnic: alveolar hypoventilation (COPD, OHS, CNS depression, neuromuscular disease) fails to clear CO2.
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What is the main mechanism of type 1 respiratory failure?
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Ventilation/perfusion (V/Q) mismatch.
Why? Pneumonia, pulmonary oedema, ARDS, PE and ILD impair oxygen transfer; CO2 is still cleared, so PaCO2 is normal or low.
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What spirometry ratio confirms an obstructive defect?
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FEV1/FVC < 0.7.
Why? In obstruction FEV1 falls much more than FVC, so the ratio drops.
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Which lung-volume measurement is the gold standard for confirming a restrictive defect?
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A reduced total lung capacity (TLC).
Why? A low FVC on spirometry suggests restriction, but only a low TLC on lung-volume testing confirms it.
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Name two causes of a RAISED transfer factor (TLCO).
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Any two of: pulmonary haemorrhage, polycythaemia, left-to-right cardiac shunt, asthma, hyperkinetic states.
Why? More blood (or haemoglobin) in the alveoli or pulmonary capillaries takes up more carbon monoxide during the test.
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How does TLCO help to distinguish asthma from emphysema?
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TLCO is normal or raised in asthma but reduced in emphysema.
Why? Emphysema destroys alveolar walls and the capillary bed, so gas transfer falls. Asthma affects airways, not the gas-exchange surface.
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In extrapulmonary restriction (e.g. obesity, scoliosis, neuromuscular weakness), what is the typical gas-transfer pattern?
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Raised KCO with a normal or reduced TLCO.
Why? The alveoli and capillaries are healthy but lung volume (VA) is small. As TLCO = KCO × VA, KCO rises while TLCO is normal or low.
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What does the flow–volume loop show in a VARIABLE EXTRAthoracic upper airway obstruction (e.g. vocal cord paralysis)?
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A flattened (truncated) inspiratory limb with a normal expiratory limb.
Why? During inspiration, negative airway pressure narrows the extrathoracic airway further; during expiration positive pressure holds it open.
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What does the flow–volume loop show in a FIXED upper airway obstruction (e.g. tracheal stenosis)?
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Flattening of BOTH the inspiratory and expiratory limbs (a "box-shaped" loop).
Why? A rigid narrowing limits flow equally in both directions, regardless of intrathoracic pressure.
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Name three causes of bilateral hilar lymphadenopathy.
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Any three of: sarcoidosis, tuberculosis, lymphoma, histoplasmosis, berylliosis.
Why? These cause granulomatous or malignant hilar node enlargement visible on chest X-ray.
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Winter's formula: in metabolic acidosis the expected PaCO2 (mmHg) = ______.
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1.5 × HCO3− + 8 (± 2)
Why? It tells you whether respiratory compensation is appropriate. Divide mmHg by 7.5 to convert to kPa.
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In a metabolic acidosis, the measured PaCO2 is HIGHER than predicted by Winter's formula. What does this mean?
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There is an additional (co-existing) respiratory acidosis.
Why? The lungs are not blowing off as much CO2 as expected, so a second, respiratory process is present (a mixed disorder).
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How much does bicarbonate rise per 1.3 kPa (10 mmHg) rise in PaCO2 in CHRONIC respiratory acidosis?
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About 3.5–4 mmol/L (versus about 1 mmol/L in acute respiratory acidosis).
Why? Renal compensation (retaining bicarbonate) takes 3–5 days, so a big bicarbonate rise implies a chronic process such as COPD.
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How is the anion gap calculated?
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Na+ − (Cl− + HCO3−); potassium is usually omitted.
Why? A raised anion gap metabolic acidosis points to unmeasured acids such as lactate, ketones, toxins (methanol, ethylene glycol, salicylates) or renal failure.