Cardiopulmonary Bypass: What It Does to the Patient

"Draw me a cardiopulmonary bypass circuit." Venous cannula, reservoir, pump, oxygenator, back to the aorta, cross-clamp, cardioplegia, vents, suckers. Good — that's a pass. The mark comes when you can say what each component does to the patient rather than what it does to the blood, and that is the difference between a trainee who has memorised a diagram and a senior who understands a machine. This is cardiopulmonary bypass top to bottom, pitched at FRCA level and a little beyond, and built around the questions that have actually appeared in past papers. Almost everything else we discuss on a cardiac unit assumes this episode. Please note: the thresholds, ACT practice and doses are Wythenshawe-specific local practice. Take the principles, and check your own guidelines. We start with the circuit, answering the examiner's real question component by component. The reservoir is a volume buffer — somewhere to hold blood so flow is maintained when the surgeon lifts the heart and venous return suddenly falls — and it is where air gets trapped rather than delivered. The heat exchanger is the patient's entire thermoregulation for the duration, not an accessory. The arterial filter is about the brain, because embolic load is one of the two mechanisms behind neurocognitive injury after bypass, and hypoperfusion is the other. Then cannulation, including the one absolute rule in the whole chapter: never site a right radial arterial line if the axillary artery is going to be cannulated, because clamping it takes out your trace for a prolonged period and you will be flying blind at the worst possible moment. Plus the circuit variants worth knowing — hybrid circuits switchable between full bypass and VA-ECMO for lung transplantation, haemoadsorption for the septic patient, and ultrafiltration, which must be handed over explicitly to intensive care rather than buried in the chart. And a question most people are never taught properly: how is the patient anaesthetised on bypass? An isoflurane vaporiser on the circuit, capnography switched to the oxygenator — and depth monitoring throughout, because the transitions on and off pump are exactly where awareness happens. Haemodilution gets a proper answer rather than a hand-wave. The haematocrit falls the moment bypass starts because you have connected the patient to a crystalloid prime, and it is tolerated because haematocrit is two competing things at once: oxygen-carrying capacity, which you want high, and viscosity, which you want low — especially when cold. Hence a target somewhere around 21–24%, and retrograde autologous priming to reduce the crystalloid load in selected patients. Anticoagulation is given both ways. Heparin 300–400 units/kg and an ACT over 480 seconds is the version the exam wants; locally the threshold is over 400, because our machines double-count. Then the scenario: the ACT is 210 after a full dose. Don't give more heparin. Confirm it reached the circulation — go back to the lumen you used and aspirate it, because a surprising amount of this is a delivery problem rather than a pharmacology one. Then think heparin resistance, which almost always means acquired antithrombin deficiency, and give antithrombin III 500 units. Cardioplegia covers the mechanism precisely — a high extracellular potassium depolarises the membrane, inactivating the fast inward sodium channels so there is no action potential upstroke — the five things cardioplegia actually achieves, the six advantages of blood over crystalloid (the exam wants three), warm versus cold, and the three indications for retrograde delivery with the 40 mmHg coronary sinus limit. Then the material that takes you past Fellowship: del Nido, and why Custodiol arrests the heart by hyperpolarisation rather than depolarisation. Opposite direction, same destination. Hypothermia is delivered in the shape the question asks for — four advantages and three disadvantages — including the paradox worth saying out loud, that you are cooling to reduce oxygen demand while simultaneously making oxygen harder to offload. Plus flow and pressure targets, and alpha-stat versus pH-stat. We finish with an eight-step checklist for separation from bypass, the vasoplegia that commonly follows, and protamine: the electrostatic reversal, why you must account for the heparin in the prime as well as the dose you gave, the four groups of adverse reaction with the Horrow type III as the one to fear, and why too much protamine is itself an anticoagulant. Chapters (00:00) Cold open — "draw me a bypass circuit" (01:00) What each component does to the patient (03:10) Cannulation, and the right radial rule (04:20) Circuit variants: hybrid ECMO, haemoadsorption, ultrafiltration (05:40) How is the patient anaesthetised on bypass? (06:50) Priming, and why the haematocrit falls (08:20) Heparin, the ACT, and what the ACT is actually for (09:50) The ACT is 210 — what do you do, in order? (11:20) Cardioplegia: how potassium arrests the heart (12:30) Blood versus crystalloid, warm versus cold (13:20) Antegrade and retrograde delivery (14:10) del Nido, Custodiol and hyperpolarising arrest (15:20) Laplace, and why we vent (15:50) Hypothermia: four advantages, three disadvantages (17:00) Flow, pressure, alpha-stat and pH-stat (17:40) Coming off bypass — the eight-step checklist (19:10) Vasoplegia (19:40) Protamine: pharmacology, reactions and technique (20:40) What arrives on the unit afterwards (21:40) Wrap-up Key takeaways Name the parts, then say what each does to the patient: the reservoir is a volume buffer and an air trap, the heat exchanger is the patient's thermoregulation, the arterial filter protects the brain Neurocognitive injury after bypass comes from embolic load and hypoperfusion Never site a right radial arterial line if the axillary artery is to be cannulated Hybrid circuits can switch between full bypass and VA-ECMO — used for lung transplantation, because it avoids full-dose anticoagulation and lets the heart keep ejecting If ultrafiltration has been used, hand that over explicitly — over-filtration makes postoperative fluid and electrolyte management difficult Monitor depth of anaesthesia throughout: going on and coming off pump disrupts both volatile and intravenous delivery, and that is where awareness happens The haematocrit falls because of the crystalloid prime, and it is tolerated because haematocrit is oxygen carriage and viscosity at the same time — target around 21–24% Heparin 300–400 units/kg with an ACT over 480 seconds for the exam; over 400 locally because the machines double-count The ACT's genuinely useful role is confirming it is safe to turn the suckers on and go onto bypass If the ACT won't rise, don't give more heparin — confirm it reached the patient, then give antithrombin III 500 units for acquired antithrombin deficiency Potassium cardioplegia depolarises the myocyte membrane and inactivates the fast inward sodium channels, abolishing the action potential upstroke Blood cardioplegia adds oxygen carriage, hydrogen ion buffering, free-radical scavenging, improved microvascular flow, reduced myocardial oedema and nutrient delivery Retrograde delivery is needed for significant aortic regurgitation, root surgery and severely diseased coronaries — keep coronary sinus pressure below 40 mmHg Custodiol arrests the heart by hyperpolarisation through sodium depletion, the opposite mechanism to potassium depolarisation Laplace's law is the basis of every unloading strategy: venting, the IABP, and LV decompression on VA-ECMO Hypothermia reduces cerebral and myocardial oxygen consumption but shifts the oxyhaemoglobin curve left, so offloading is impaired at the same time as demand falls Separation from bypass in eight steps: warm, rhythm and AV synchrony, electrolytes and acid–base, lungs, de-air under TOE, rate to suit the lesion, titrate support, separate incrementally Protamine 300 mg reverses roughly 30,000 units of heparin — and you must account for the heparin in the pump prime too The Horrow type III reaction is pulmonary hypertension with right heart strain, and excess protamine is itself an anticoagulant References / further reading Charlesworth M. CTCCU Handbook, 2nd edition, sections 2b and 2d Wahba A et al. EACTS/EACTAIC/EBCP Guidelines on cardiopulmonary bypass in adult cardiac surgery. Eur J Cardiothorac Surg Murphy GS, Hessel EA, Groom RC. Optimal perfusion during cardiopulmonary bypass: an evidence-based approach. Anesth Analg 2009 Ltaief Z et al. Pathophysiology and clinical implications of vasoplegic syndrome after cardiopulmonary bypass. J Clin Med 2022 Finley A, Greenberg C. Heparin sensitivity and resistance: management during cardiopulmonary bypass. Anesth Analg 2013 Boer C et al. 2017 EACTS/EACTA Guidelines on patient blood management for adult cardiac surgery. J Cardiothorac Vasc Anesth 2018 Follow the podcast Bluesky: @cardiacoutput.bsky.social X: @CardiacOutputMC If you've found this useful, a follow or a share genuinely helps other trainees find it — and do get in touch if there's a topic you'd like covered. This podcast is for medical education for healthcare professionals. It is not clinical advice. All thresholds, doses and practice described reflect local Wythenshawe practice at the time of recording — always follow your own centre's guidelines and current local policy.