Cardioplegia
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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.