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Chapter question: A patient on ECMO can have four different blood gases drawn within the same minute, from four different places, all of them correct and all of them saying different things. Which sample answers which question — and what has changed about the physiology of oxygen and carbon dioxide once a machine, rather than a lung, is doing the exchange?
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Evidence search date: 20 September 2026.
Primary sources: ELSO Red Book 6th edition — Chapter 25 (venovenous management: sweep gas weaning, the sweep-gas oxygen fraction argument, extracorporeal blood flow ranges), Chapter 28 (adult cardiac management: the gas-exchange and oxygen-delivery troubleshooting table), Chapter 45 (the apnoea test on ECMO, owned by Chapter 24 and cited only). ISCCM Manual of RRT and ECMO in ICU — Chapter 26 (extracorporeal physiology, rated flow, the inlet-outlet content difference) and Chapter 33 (metabolic monitoring). ECMO: A Practical Guide to Management (Taha, Caridi-Scheible, Leiendecker) — Chapter 5, the gas-transfer figure, the rated-flow table and the worked recirculation table; Chapter 9, the monitoring-modality table and the pulse-oximetry error. ECMO in the Adult Patient (Core Critical Care, 2017) — circuit blood gas sampling technique and the transmembrane-pressure pairing.
External evidence: a physiological review of extracorporeal gas exchange by the field's founding investigator; a monitoring review of the adult venoarterial patient; an international standardisation document for in vitro evaluation of extracorporeal devices; an experimental study of carbon dioxide removal in 12 pigs; a review of extracorporeal carbon dioxide removal; a pharmacotherapy review of ventilatory strategy on extracorporeal support; a 33-patient study of carboxyhaemoglobin as a marker of haemolysis on ECMO; a registry analysis of pulse-oximeter accuracy by patient race in an ECMO population, read second-hand; and two general reviews of blood gas and oximetry measurement.
Every external reference was read as an indexed full-text passage or, where stated, second-hand through a journal-watch summary. None was retrieved as a complete article. Digital object identifiers are reproduced from the search record and have not been independently resolved. Every database row seeded from this chapter is marked Verified = No.
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Chapter 25 established that every haemodynamic instrument on a circuit was calibrated in a circulation that no longer exists. Chapter 26 established that every echocardiographic number is a measurement at a stated flow. This chapter completes the argument for the two gases, and the conclusion is sharper than for either of the others: on ECMO a blood gas result is not interpretable at all unless you know where the sample came from.
| This chapter owns | Owned elsewhere — cite, do not re-argue |
|---|---|
| Where blood gases are drawn from on a circuit, and what each site measures | The physiology of differential oxygenation and the mixing point — Chapter 13, Chapter 16 |
| Gas transfer across the membrane lung: rated flow, the oxygen and carbon dioxide levers and their independence | The circuit and oxygenator as hardware, and device selection — Chapter 3; oxygen delivery in the intact circulation — Chapter 2; effective flow and the mixing equation on venovenous support — Chapter 6 |
| Oxygenation and carbon dioxide targets, and the disagreement between this book's own primary sources | Managing persistent hypoxaemia on venovenous support — Chapter 9; differential hypoxaemia — Chapter 16 |
| Recirculation as a measured oxygen-transfer loss, and the saturation rule that detects it | Recirculation physiology and effective flow — Chapter 6; its place in the daily round — Chapter 8; its calculation — Appendix 5; the cannula geometry that causes it — Chapter 26, Chapter 34 |
| The pre-membrane and post-membrane pair as physiological information | The same pair as an oxygenator failure diagnosis, with transmembrane pressure — Chapter 32 |
| Pulse oximetry and co-oximetry on a circuit, including occult hypoxaemia and dyshaemoglobins | Haemolysis as a complication, its causes and its management — Chapter 37 |
| Haemoglobin as an oxygen-delivery lever, and the argument about transfusing to raise it | The transfusion strategy itself, thresholds and products — Chapter 46 |
| The rate at which carbon dioxide should be corrected | The neurological consequences of rapid correction after cardiac arrest — Chapter 22; cerebral monitoring — Chapter 28 |
| Assembling a blood gas round, and the errors of interpretation | Ventilator settings on ECMO — Chapter 53; lactate and the laboratory panel — Chapter 29; oxygen content and delivery calculators — Appendix 2, Appendix 3 |
The boundary with Chapter 32 is the one to hold. A post-membrane gas that has deteriorated is a machine diagnosis and belongs to Chapter 32. This chapter owns the same sample read as physiology: what the membrane is delivering to this patient, now. The two readings use the same number and answer different questions — which is this Part's recurring theme.
This is the organising idea, and like Chapter 25's it is mechanical rather than clinical.
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Physiology · One organ becomes two independent knobs
In a native lung, oxygenation and carbon dioxide clearance are performed by the same structure under the same settings. They are not independent: recruit the lung and both improve; lose it and both deteriorate. Clinicians reason about "gas exchange" as one thing because, in a patient, it mostly is one thing.
A membrane lung separates them completely, onto two knobs that do not interact.
The clinical consequences follow immediately and are the whole chapter in miniature.
Source for the physiology and the 500 mL/min figure: Bartlett RH. Comprehensive Physiology 2020;10(3):879–891. DOI 10.1002/j.2040-4603.2020.tb00130.x.
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| Variable | Affects oxygenation | Affects carbon dioxide elimination |
|---|---|---|
| Diffusion gradient | Yes | Yes |
| Membrane surface area | Yes | Yes |
| Oxygen fraction of the sweep gas | Yes in principle — and see §27.3, where a working membrane saturates the outlet blood at a sweep oxygen fraction of 0.21 | No |
| Extracorporeal blood flow | Yes — the dominant lever | No, except at the low end of the blood flow range, where it becomes a major determinant |
| Sweep gas flow rate | No | Yes — the dominant lever |
| Haemoglobin concentration | Yes — the second lever, and the one most often forgotten (§27.4) | Indirectly, through carbon dioxide carriage |
| Inlet (pre-membrane) saturation | Yes, inversely — a higher inlet saturation means less oxygen can be added (§27.5) | Through the inlet carbon dioxide tension, which sets the gradient |
Table adapted from the gas-exchange table in Chung M, Shiloh AL, Carlese A, Butt WW, Spapen H. The Scientific World Journal 2014. DOI 10.1155/2014/393258, with the haemoglobin and inlet-saturation rows added from Bartlett 2020 and from this book's primary textbooks.
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Clinical pearl · The sweep gas is the on-off switch; the blood flow is the volume control
ELSO Red Book Chapter 25 states it in one line, about weaning: "Irrespective of the extracorporeal blood flow, once the sweep gas flow ceases, ECMO support effectively ceases."
This is worth holding onto because it inverts the intuition. Most clinicians watch the blood flow number on the console as the measure of "how much ECMO" a patient is receiving. A circuit running at 5 L/min with the sweep gas off is providing no gas exchange at all — it is an expensive, thrombogenic bypass loop. Conversely, a circuit at 2 L/min with an open sweep is still clearing carbon dioxide efficiently.
When someone asks how much support a patient is on, the answer requires two numbers, and the sweep is the one more often omitted from the handover.
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Pitfall · Turning up the blender for hypoxaemia
The commonest reflex error in this domain, and it is a category error rather than a dosing error. On a functioning membrane lung the outlet blood is already fully saturated (§27.3). Raising the sweep gas oxygen fraction from 0.6 to 1.0 in a hypoxaemic patient on venovenous support adds essentially nothing, because there is no unsaturated haemoglobin left in the outlet blood to fill.
The hypoxaemia is coming from somewhere else — insufficient blood flow relative to cardiac output, recirculation, anaemia, a failing membrane, or native shunt (Chapter 9 owns the differential). Turning the blender up wastes the minutes in which the real cause could have been found.
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A conventional ICU patient has one arterial sampling site and the word "the blood gas" is unambiguous. On ECMO there are at least four, and they are not interchangeable.
| Sample | What it physically is | The question it answers | What it is NOT |
|---|---|---|---|
| Pre-membrane (pre-oxygenator) port | Blood drained from the patient, after the pump, before the gas exchanger | On venoarterial support: the true mixed venous sample. The ISCCM Manual states it directly — venous saturation on venoarterial ECMO is a true reflection of venous oxygen content and can be taken from a central line or pre-membrane gases. This is the oxygen-delivery-versus-consumption measurement (Chapter 25) | On venovenous support it is NOT a mixed venous sample, because it contains recirculated, already-oxygenated blood. It is a mixture whose composition is the recirculation diagnostic (§27.5) |
| Post-membrane (post-oxygenator) port | Blood leaving the gas exchanger, about to re-enter the patient | What the machine is delivering. Should be fully saturated with a high partial pressure of oxygen. Paired with the pre-membrane sample it gives the oxygen and carbon dioxide the device actually transferred | Not the patient's arterial blood. On venovenous support it never is; on venoarterial support it is what reaches the lower body but not necessarily the brain |
| Right radial or right brachial arterial line | Blood supplied to the innominate, right carotid and right subclavian territory | On peripheral venoarterial support: what the brain and the coronary arteries are receiving. Both the ISCCM Manual and the practical-guide textbook state that the arterial line should be on the right arm and that gases should be taken from that side | Not representative of the lower body on venoarterial support, and its divergence from a femoral sample is the definition of the problem Chapter 16 owns |
| Left arm, femoral or other distal arterial site | Blood in the territory supplied predominantly by the circuit on femoral venoarterial support | What the machine's territory is receiving, and by comparison with the right radial sample, where the mixing point is sitting | Not the cerebral supply. A reassuring femoral gas on peripheral venoarterial ECMO is compatible with a hypoxaemic brain |
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Danger · The sampling site must be written on the result, every time
This chapter's version of the rule Chapters 25 and 26 established for circuit flow. "PaO₂ 58" is not a result on ECMO. It is a result on a patient. On a circuit it becomes a result only when accompanied by: the site, the extracorporeal blood flow, the sweep gas flow, the sweep gas oxygen fraction, and the ventilator settings at the time.
The specific harm is not theoretical. A post-membrane sample charted as an arterial gas reads as spectacular oxygenation in a profoundly hypoxaemic patient. A femoral arterial sample charted as "arterial" on peripheral venoarterial support conceals differential hypoxaemia — the exact scenario Chapter 16 exists for, and the one in which the brain is injured while the chart looks normal.
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Clinical pearl · Draw circuit gases with the sweep gas at 100 percent oxygen
A practical convention from ECMO in the Adult Patient, already adopted as part of the daily circuit round in Chapter 8: to ensure reliable and comparable results, blood gases obtained from the circuit are measured with the sweep gas set at 100 percent oxygen. The reason is that a post-membrane partial pressure of oxygen means nothing as a device measurement unless the inspired fraction on the gas side is known and constant — exactly as an arterial partial pressure of oxygen means nothing without the inspired fraction on a ventilator.
The same source adds the practical warning that sampling is done carefully to avoid excessive loss of blood, as the pressure at the sampling point can be very high. Post-pump ports are under positive pressure; an unguarded stopcock sprays.
Set the sweep oxygen fraction to 1.0, wait for equilibration, draw the pair, then return it. And record that you did, because the next person comparing to your value needs to know the conditions were the same.
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