# Cardioplegia > Cardioplegia for Perfusion Technology: del Nido and St Thomas solutions, antegrade and retrograde delivery, potassium arrest, doses, pressures and redosing. - Canonical URL: https://prepelephant.com/topics/allied/perfusion-technology/cardioplegia-pt - Exam / course: Allied Health · Subject: Perfusion Technology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Cardioplegia", PrepElephant, https://prepelephant.com/topics/allied/perfusion-technology/cardioplegia-pt ## Direct answer Hyperkalaemia is deliberate here: cardioplegia floods the myocardium with a cold, potassium-rich solution to arrest the heart in diastole, collapsing its oxygen demand to a fraction of working levels for as long as the cross-clamp stays on. Modern practice divides between del Nido solution — a single-dose strategy, about 1,000 mL in adults (or roughly 20 mL/kg in children), protecting for 90-120 minutes — and St Thomas-type multidose strategies, re-dosed every 20-30 minutes, usually as 4:1 blood cardioplegia. Delivery is antegrade into the aortic root or coronary ostia, retrograde through the coronary sinus, or a combination, each with its own pressure rules and failure modes. The whole subject is a small set of numbers — potassium, interval, pressure, temperature — that examiners probe in exactly that order. ## What you must remember - Extracellular-type solutions carry potassium roughly 15-30 mmol/L — enough to depolarise the myocyte and hold diastolic arrest; del Nido contains about 26 mEq/L, St Thomas-type solutions around 16-20 mmol/L. - del Nido: Plasma-Lyte base with mannitol, magnesium and lignocaine; single dose of about 1,000 mL in adults (roughly 20 mL/kg paediatric) protects for 90-120 minutes without re-dosing in routine cases. - St Thomas-type multidose: induction 500-1,000 mL, then about 250 mL every 20-30 minutes; commonly delivered as 4:1 blood-to-crystalloid — blood carries oxygen, buffers better and causes less oedema. - Temperature: cold 4-10 degrees C remains standard; a terminal warm blood dose ("hot shot") just before clamp release is widely used to support reperfusion. - Antegrade root delivery: root pressure monitored, commonly kept around 60-90 mmHg and not pushed beyond about 100 — higher pressures mean oedema and, in diseased aortas, embolic risk. - Retrograde coronary sinus delivery: pressure 30-40 mmHg (a soft rule with a hard ceiling near 50 to avoid sinus rupture); it perfuses the LV well but the right ventricle poorly, so it pairs with antegrade doses when RV protection matters. - Arrest should confirm within 30-60 seconds on the ECG; a beating heart after delivery is a delivery problem until proven otherwise. - Volumes are recorded separately from the pump volume balance; repeated doses load potassium — expect and treat hyperkalaemia at weaning (calcium chloride, insulin-dextrose, ultrafiltration). ## One CABG, two strategies A three-vessel CABG with an anticipated clamp time of 75 minutes can run either script. Del Nido plan: after cross-clamp, 1 L into the root over two to three minutes; asystole within 45 seconds; no further dose for the case; systemic temperature allowed to drift to 32-34 degrees C. St Thomas plan: induction with about 700 mL of 4:1 blood cardioplegia, a 250 mL re-dose after each distal anastomosis (roughly every 25-30 minutes), and a terminal warm blood dose before release. Both are defensible; the choice follows surgeon preference, case length and teaching tradition — beyond about 90-120 minutes, the single-dose strategy needs a re-dose or a switch. Now the failure branch: the heart keeps fibrillating after the first dose. Work the delivery, not the bottle. Is the line flowing (air lock, kink, pump)? Is the root pressure rising as it should — if not, aortic insufficiency is venting cardioplegia into a visibly distending left ventricle, and the answers are ostial cannulae or retrograde delivery with LV venting. Only then does the solution itself come under suspicion. That branch — line, root, solution — is what the examiner is listening for. ## Where students slip The interval reversal is the classic written-paper error: del Nido is single-dose at 90-120 minutes, St Thomas is multidose at 20-30 minutes — reversing them is the classic lost mark. Second, retrograde cardioplegia gets oversold: the right ventricle and part of the interventricular septum drain through Thebesian veins directly into the chambers, so retrograde flow protects them poorly; the complete answer adds antegrade graft doses. Third, the pressure question: the delivery line pressure and the root pressure are different numbers — line pressure includes tubing resistance, and the number that matters is the one transduced at the root or sinus. Finally, the "hot shot": the safe formulation is a terminal warm blood dose supporting metabolic recovery before reperfusion, used variably by programme. ## Frequently asked questions ### Why does potassium arrest the heart in diastole? High extracellular potassium depolarises the resting membrane potential, inactivating fast sodium channels and silencing systole — flaccid diastolic arrest that collapses oxygen demand. ### What is the redosing interval for del Nido solution? Single dose of about 1,000 mL in adults (20 mL/kg in children) protecting 90-120 minutes; longer clamp times require re-dosing or a strategy change. ### At what pressure is retrograde cardioplegia delivered? Coronary sinus pressure of 30-40 mmHg, not exceeding about 50 — higher risks sinus injury and oedema. ### What is a "hot shot"? A terminal dose of warm blood cardioplegia given just before cross-clamp release to support metabolic recovery during early reperfusion. ### Why is blood cardioplegia often preferred? Blood carries oxygen to the arrested myocardium, buffers well, limits oedema, and reduces the volume of crystalloid added to the pump. ### Why does antegrade root cardioplegia fail in aortic regurgitation? The regurgitant jet diverts solution into the left ventricle instead of the coronaries, distending the ventricle — the fixes are ostial cannulae or retrograde delivery with LV venting.