The Invisible Threat: Why Microbubbles During Bypass Should Keep Us Honest

If you've spent any time running a heart-lung machine, you'll know the feeling. That moment where you catch a flash of air in the venous line, or the cardiotomy suction starts gulping, and your hand instinctively reaches for the clamp. It's visceral. It's trained into us.

But here's the thing, the air we can see isn't really the problem.

It's the stuff we can't see that should worry us most.

What are microbubbles, and why do they matter?

Microbubbles are tiny gaseous emboli, we're talking less than a millimetre in diameter, that are generated throughout the bypass circuit during cardiac surgery. They form in the oxygenator, at connection points, during drug injection, through cardiotomy suction, and anywhere turbulence meets air-blood interface.

They're invisible to the naked eye, and most of the time, they're invisible to the perfusionist too.

The clinical concern is well established. Gaseous microemboli that reach the arterial side of the circuit can travel to the cerebral vasculature, where they lodge in capillaries, trigger localised ischaemia, activate inflammatory pathways, and contribute to postoperative neurological dysfunction. The literature links microbubble load during CPB to problems with attention, concentration, and memory in the days and weeks following surgery, what's broadly termed "post-perfusion syndrome" or, less charitably, "pump head."

Bubbles larger than 20–40 micrometres are considered potentially hazardous. Arterial line filters will catch most things above 40 micrometres, but anything smaller can slip through and those smaller bubbles can coalesce downstream into something bigger.

Where do they come from?

The research points to a handful of key drivers:

Cardiotomy suction is the biggest offender. Every time the surgeon's suction picks up blood from the operative field, it's also picking up air. That blood gets returned to the venous reservoir carrying a significant microbubble load. The more aggressive the suction flow, the more bubbles you generate, it's essentially that simple.

Venous reservoir level matters more than many of us appreciate. Studies using GAMPT bubble counters have demonstrated a near-linear relationship between dropping reservoir levels and rising microbubble counts. Let your level get low and you're asking for trouble.

Blood temperature and viscosity play a role as well. Cooler blood holds more dissolved gas. Rewarm too quickly and that gas comes out of solution, right into the arterial line.

Perfusion flow rate rounds out the picture. Higher flows through the circuit mean more turbulence and more opportunity for bubble formation.

So what can we do about it?

Honestly? Quite a lot — but it starts with accepting that this is a real and measurable problem, not just an academic curiosity.

First, manage your reservoir level. Keep it at a safe working level and resist the temptation to run it low. It's one of the simplest interventions with a real impact on bubble transmission.

Second, be deliberate with cardiotomy suction. Communicate with your surgeons about suction technique. A bit of coordination here can meaningfully reduce the microbubble load returning to your reservoir.

Third, warm gradually during rewarming. Rapid temperature swings encourage outgassing. Patience pays off.

Fourth, and this is the one closest to my heart, measure what you're actually delivering to the patient. You can't manage what you can't see. Ultrasonic bubble detection gives you real-time visibility of the gaseous microemboli passing through your arterial line, turning an invisible problem into something you can monitor, respond to, and ultimately reduce.

At ABM, we've seen firsthand how continuous bubble monitoring changes perfusionist behaviour at the pump. When you can see the bubble count climbing in real time, you adjust. You manage your reservoir. You talk to the surgeon about suction. You slow down the rewarm. It creates a feedback loop that drives better practice — not through theory, but through visibility.

The bottom line

We spend significant resources on arterial filters, oxygenator design, and circuit optimisation — and rightly so. But if we're not actually monitoring the microbubble load reaching our patients, we're flying partially blind.

The evidence is clear: microbubbles during cardiopulmonary bypass contribute to adverse neurological outcomes. The good news is that the tools and techniques to detect and reduce them are available right now. It's not about perfection — it's about vigilance, measurement, and a willingness to keep improving how we protect our patients on the pump.

Because the air you can't see? That's the air that matters most.