Determinants of DLCO (haemoglobin, altitude, haemorrhage, shunt)
Core concept
- CO is used because it is diffusion-limited - avid Hb binding keeps capillary PCO near zero, so uptake reflects the barrier, not blood flow
- 1/DLCO = 1/Dm + 1/(theta x Vc)
- Dm = membrane conductance -> surface area and thickness
- Vc = pulmonary capillary blood volume, and the haemoglobin inside it
- -> only three ways to lower DLCO: less surface area, a thicker membrane, or less capillary blood/Hb
- VA = alveolar volume accessible to the test gas (helium/methane dilution)
- KCO = DLCO / VA = transfer per unit of accessible lung
- KCO separates "less lung" from "worse lung" - the single most useful derived number
3 more sections, plus exam facts
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