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The question this chapter answers

An ECMO circuit is a suction line joined to a pressure hose, and every breach behaves according to which half it is in. A hole before the pump draws air in; a hole after the pump pushes blood out. Air that enters before the membrane lung is usually caught there; air that gets past it goes to the patient, and on venoarterial support it goes to the arterial tree.

So the bedside question when something goes wrong is not what broke? but which side of the pump is the breach on, which side of the membrane lung is the air on, and does this patient survive the next minute better with the pump running or with it stopped?

This chapter covers air in the circuit, circuit rupture and breach, and the two ways of separating a patient from the circuit in an emergency — emergent and controlled.

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Evidence search date: 24 September 2026. One source is read as complete full text and is the Verified source new to this chapter: an in vitro study of air entry through a central venous catheter and of what measured volumes of air do to a running circuit. The Japanese national accident analysis read in full for Chapter 34 is used again for its air and circuit-damage events. Primary sources are the ELSO Red Book 6th edition (the circuit chapter, the circuit malfunction and crisis management chapter, the cannulation chapter's airless connection techniques, and the registry complication definitions), ECPR and Resuscitative ECMO (initiation and the circuit catastrophes chapter, including its emergent and controlled separation protocols), ECMO: A Practical Guide to Management (the complications and emergencies algorithm), Comprehensive Healthcare Simulation: ECMO Simulation, and the ISCCM Manual of RRT and ECMO in ICU.

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What this chapter covers — and what it does not

35.1 One circuit, two pressure worlds

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The structural claim of this chapter · The pump divides the circuit into a side that sucks and a side that pushes

A centrifugal pump draws blood from the drainage cannula and pushes it through the membrane lung to the return cannula. Upstream of the pump the pressure is below atmospheric; downstream it is above. The Red Book gives the range at the inlet: subatmospheric pressures down to minus 250 to minus 300 mmHg can be generated (Chapter 34 §34.2). The ECPR text states the consequence for air in one line: "due to the negative pressure in the drainage limb, the venous side of the circuit can be a continual source of air entrainment. Typically, this indicates a leak in the system."

So the same defect has opposite presentations depending on where it is.

• A breach on the drainage side draws air in. Nothing leaks out. The first sign may be bubbles, a change in the pump's sound, or a falling flow — not blood.
• A breach on the return side pushes blood out. The ECPR text divides its temporising measures exactly this way: occlusion stops "further air entrainment (pre-pump side) or hemorrhage (post-pump side)."

And the membrane lung divides the positive side a second time. The Red Book's terms: air between the membrane lung and the patient is "arterial air"; air before the membrane lung is "venous air." Venous air may be caught by the membrane lung. Arterial air has nothing between it and the patient.

Three consequences, and the chapter is built on them.

1. Before touching anything, place the problem in one of three zones: before the pump, between pump and membrane lung, or after the membrane lung. The zone decides the danger and the first move.
2. A drainage-side leak is silent and continuous. It keeps drawing air for as long as the pump runs and the hole is open.
3. The Red Book's judgement on the whole subject: this complication "is nearly always avoidable." Its prevention list is three items — limit access points on the negative-pressure side, take care with central lines, and handle stopcocks meticulously.

Certainty: high for the pressure physics and the zone definitions, which the Red Book and the ECPR text state concordantly; the three-zone ordering is this book's framing.

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Evidence · How air got into circuits, when a national system recorded it

From the Japanese national accident analysis read in full for Chapter 34 (Hadano 2023, J Artif Organs, DOI 10.1007/s10047-023-01386-z): air in the circuit was 7 percent (12) of 178 reported ECMO accidents. The reported causes:

Emergency circuit change was needed in half (6 of 12). Separately, 9 of 26 bleeding events were due to circuit damage, including a fissure in the connection between the arterial cannula and the circuit and cracks in a flow sensor.

The reading this chapter takes. Two thirds of the reported air events were a hand on a clamp or a stopcock. That matches the Red Book's prevention list, which names stopcock technique explicitly. The machine was the cause in one sixth.

Certainty: low for proportions — twelve events, no denominator, voluntary reporting. The direction — air is mostly a handling error — is consistent with every primary text in this chapter.

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35.2 Where air comes from

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The sources of air, by the side of the circuit they act on

Notice what most of the list has in common. Most items are an opening to the atmosphere on the negative-pressure side of the circuit, or in the patient's venous system upstream of it. The patient's great veins are, functionally, part of the drainage limb.

Certainty: moderate — each item is named by at least one primary text; no source ranks them by frequency apart from the twelve Japanese events.

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Pitfall · The patient's own veins are on the negative side

The drainage cannula does not only pull from the circuit's tubing; it pulls from the vena cava. Any opening into the central veins near the drainage cannula — a central line hub, an introducer sheath, a pulmonary artery catheter port, a surgical opening in the atrium — is an opening in the drainage limb.

The consequence the sources draw. Yan and colleagues conclude that more attention should be paid to patients with a communication between the right and left heart, "especially in situations when venous accesses' exposure to air could not be avoided"; their first case was suspected of cerebral air embolism through a transseptal right-to-left shunt (the abstract does not state the configuration). ECPR and Resuscitative ECMO makes the same point from the other direction: lowering the head of the bed may keep air that reached the patient out of the cerebral circulation "if on V-A ECMO, or on V-V ECMO via a patent foramen ovale."

So venovenous support is not immune to arterial air. A patent foramen ovale or an atrial septal defect turns venous air into systemic air.

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35.3 How much air, and what it does

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Evidence · Measured volumes of air in a running circuit

Franco D, Krasna N, Novak R, Esposito G, Izzo R, Belohlavek J, Noc M, Goslar T. "Prevention of Air Embolism in Extracorporeal Membrane Oxygenation Systems: An In Vitro Study on Protection of Central Venous Catheter Lumen." Medicina (Kaunas) 2024;60(11):1883. DOI 10.3390/medicina60111883, PMC11596159. Full text read.

The design. A venovenous in vitro model: a Cardiohelp console and HLS set, a 23 Fr 55 cm multistage drainage cannula and a 17 Fr return cannula in latex "vessels", a 40 percent glycerol and water mixture at 36 °C, and a reservoir set to simulate a central venous pressure of 10 cm H₂O. Pump speeds of 1,500, 2,500, 3,500 and 4,500 rpm gave flows of 1.61, 3.15, 4.56 and 5.91 L/min. Every experiment was repeated three times; results are descriptive.

Experiment 1 — a central line beside the drainage cannula. A 7 Fr triple-lumen catheter was placed with its tip 0, 1 or 2 cm from the drainage cannula's distal hole or its proximal side hole, with the hub left open, closed with a three-way stopcock, or fitted with a needle-free valve (clave).

Experiment 2 — known volumes of air injected into the drainage cannula.

• The characteristic sound of air in the pump occurred with as little as 1 mL, and at every larger volume. It was the first warning sign.
• Air passed through the oxygenator and reached the return-side bubble detector with 1 mL at 4,500 rpm, and with 5 mL and 25 mL at 3,500 and 4,500 rpm. At lower speeds, smaller volumes were retained by the membrane lung.
• 25 mL or 50 mL stopped flow completely at every pump speed tested. Smaller volumes reduced flow briefly; the oxygenator then vented slowly and flow recovered.

The authors' reading, which this book adopts. At lower speed the pump is more likely to airlock; at higher speed air is more likely to be driven through the membrane lung to the patient. And once the pump has stopped from air, "increasing the pump speed risks restarting the pump and forcing air through the membrane lung, so the only way to avoid air embolism in the patient is to clamp the circuit."

Certainty: low to moderate. In vitro, venovenous only, one catheter size, one console, a blood substitute, no respiration and a fixed venous pressure; descriptive with three repeats. The authors themselves expect a vigorous inspiration to make entry through the line far more likely than their model shows. What it establishes firmly is direction and order of magnitude: millilitres are audible, tens of millilitres stop the pump, and a closed hub prevents all of it.

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Physiology · Why the membrane lung is a trap, and why the trap is not reliable

The membrane lung is a large chamber in which blood spreads out and slows down, and gas rises (this book's description of the geometry). So venous air that reaches it tends to collect at its top rather than pass through. The Red Book: air within the membrane lung will usually rise to the highest point and may be aspirated through a pigtail or stopcock attached for this purpose. ECPR and Resuscitative ECMO: air bubbles on the drainage side "may not lead to patient harm, as they may be removed by the membrane lung. This should not be relied upon."

Three things defeat the trap.

• Speed. Franco's model shows air passing the membrane at 1 mL at the highest speed tested, and at 5 mL from 3,500 rpm. The faster blood moves through the membrane, the less time a bubble has to rise out of it.
• Volume. Larger volumes overwhelm the membrane's capacity to hold them; in the same model 25 mL reached the return detector at 3,500 rpm and above.
• Microbubbles. The Red Book notes that micro-bubbles from cavitation "may pass the artificial lung, increasing the risk of clotting and gaseous embolism." Born and colleagues (Artif Organs 2020, abstract read), in an in vitro venoarterial circuit, found microbubble generation highly dependent on negative pressure at the inflow line, identified the centrifugal pump as a main source, and showed that the oxygenator's ability to withhold larger microbubbles is limited.

The practical form: venous air is an emergency on a timer. It is less dangerous than arterial air only for as long as it stays in the membrane lung, and pump speed shortens the timer.

Certainty: moderate — mechanism described concordantly by two primary texts and two in vitro studies; no in vivo measurement of how much venous air reaches patients was retrieved.

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