CPB Pumps
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Direct answer
Two rollers set 180 degrees apart take turns squeezing a length of tubing against a curved raceway — the roller pump, a positive-displacement device whose flow depends only on rotation speed, delivering the set litres whether the downstream line is open or blocked. A centrifugal pump instead spins cones or an impeller to create a constrained vortex: it is non-occlusive and afterload-dependent, so its flow falls when resistance rises, and it simply stalls rather than bursting a clamped line — but it must be measured, with an electromagnetic or ultrasonic flow probe, because rotation speed alone does not equal output. Every other pump question — occlusion adjustment, spallation, haemolysis, the hand crank — follows from this single contrast.
What you must remember
- Roller pump: flow = revolutions per minute × swept volume, independent of afterload; against a clamped line it generates pressures high enough to rupture tubing — hence the arterial line pressure alarm.
- Occlusion is adjusted and checked each case (falling-column or pressure-decay test): under-occlusion leaks flow backward and under-delivers; over-occlusion haemolyses blood and spalls tubing — sheds microscopic particles into the circuit.
- Centrifugal pump: magnetically coupled, afterload-dependent, non-occlusive — rising systemic resistance silently reduces flow unless RPM is raised; a flow probe (electromagnetic or transit-time ultrasound) is mandatory.
- Air behaviour: a roller pump will propel a large air bolus without complaint; in a centrifugal pump air migrates to the low-pressure centre and is less readily ejected — a relative safety margin, not a guarantee.
- Haemolysis drivers: over-occlusion, excessive RPM, small-gauge tubing, and above all highly negative inlet pressure (venous drainage beyond roughly −80 to −100 mmHg causes microcavitation).
- Dedicated smaller roller heads run cardiotomy suction (regulated to avoid excessive negative pressure), vents and, on many systems, cardioplegia delivery.
- Power failure drill: the hand crank lives with the machine — roughly one crank revolution per second approximates adult full flow on a roller head; one perfusionist cranks while another watches reservoir level and pressures.
- Pump choice in practice: either type for routine adult bypass; centrifugal heads preferred for long support runs (less blood trauma over days) and by many paediatric programmes.
Two pumps, one occlusion event
Imagine the arterial line kinking during preparation for weaning. On the roller pump, the display keeps reading 4.0 L/min because a displacement pump delivers what it is told — while the line pressure rockets toward the alarm; the machine insists nothing has changed until the transducer screams. On the centrifugal pump, the same kink shows immediately as falling measured flow at unchanged RPM — the kinetic pump surrenders to resistance and reports its own defeat. Same event, opposite signatures, and the perfusionist who knows both signatures reads either machine correctly.
The same rehearsed pattern governs the blackout. Mains and battery fail: the roller head stops and the monitors run on battery. The crank comes off its bracket — one perfusionist turns it steadily, about a revolution a second, while the second confirms venous level and arterial pressure aloud. Teams drill this yearly because the machine's obedience, its virtue in normal times, becomes the emergency's engine.
Where the viva goes
"Which is safer against accidental line occlusion?" — the centrifugal, because it stalls rather than over-pressurises. "Why must a centrifugal pump have a flow probe?" — because RPM predicts flow only at fixed afterload; without measurement, rising resistance masquerades as steady perfusion. "When did you last check occlusion?" — the safe answer is every case, since tubing lots differ and the falling-column method takes under a minute. The subtler trap is complacency about air: candidates overstate the centrifugal pump's resistance to embolism — air accumulates centrally until enough collects to break through, so the bubble detector and level sensor remain the actual defences. A final favourite: define spallation in one sentence — particles shed from tubing by repeated roller compression — and note that over-occlusion causes it, which ties the whole topic back to the daily occlusion check.
Frequently asked questions
What is spallation?
Shedding of microscopic tubing particles caused by repeated roller compression, worst with over-occlusion; the particles add to the embolic load.
How is roller pump occlusion set and verified?
By the falling-column (or pressure-decay) test each case — adjusted until the rollers just arrest a falling fluid column without over-compressing the tubing.
Why does centrifugal pump flow fall when arterial pressure rises?
It is a kinetic, afterload-dependent pump: increasing downstream resistance reduces output at constant RPM unless speed is increased.
Why is a flow probe mandatory with a centrifugal pump?
Rotation speed does not equal output when afterload varies; only electromagnetic or ultrasonic flow measurement shows delivered flow.
What is done in a total power failure?
Hand-crank the roller head at roughly one revolution per second while a second perfusionist monitors venous level and pressures.
Which pump is preferred for prolonged circulatory support?
A centrifugal pump — lower blood trauma and haemolysis over long runs, with magnetic coupling and no tubing raceway to wear.