27, 3
Review of extracorporeal carbon dioxide removal technology and indications. Used in Chapter 27 for the physiological separation of the two gases and for the arithmetic behind it.
- Oxygen is carried bound to haemoglobin and mixed venous blood is already 65–70% saturated, which caps how much oxygen can be added per litre — this is why blood oxygenation depends on blood flow (typically 4–7 L/min).
- Carbon dioxide exchange depends on gas flow (sweep). With sweep gas containing little or no carbon dioxide, the gradient across the membrane can greatly exceed the alveolar–capillary gradient of the native lung.
- 1 L of blood carries roughly 500 mL of carbon dioxide — about double the whole-body production of 200–250 mL/min. In a perfectly efficient system a blood flow of 0.5 L/min would remove all of it; in practice carbon dioxide removal devices clear up to about 25% of production.
- Removal follows biphasic kinetics: an initial rapid fall in arterial carbon dioxide tension from the dissolved fraction, then a slower phase as carbon dioxide is liberated from bicarbonate.
Why it matters to this book. It is the clearest published statement of the gap that Chapter 27's organising insight rests on: the blood flow required for full carbon dioxide clearance is an order of magnitude below that required for full oxygenation. That gap is both the rationale for carbon dioxide removal as a therapy and the trap — a patient with perfect gases may be receiving almost no oxygenation support.
NOTE: no chapter in this book's 92-chapter map currently owns extracorporeal carbon dioxide removal as a therapy — established by tracker query during Chapter 27. This row is filed against Chapter 27 and Chapter 3 pending that decision.
Read as indexed full-text passages. DOI not independently resolved.