• Cross clamping of aorta leads to ventricular fibrillation followed by asystole when cardiac supplies of ATP are depleted: ischaemic arrest

• Objective is to stop the heart as quickly as possible to enjoy the benefits of a quiet, bloodless operative field while minimising ischaemic injury to the heart

• Cold chemical cardioplegia provides better protection than cold ischaemic arrest

• Hypothermia potentiates the protective effects of chemical cardioplegia

• Ideal cardioplegic solution should eliminate external cardiac work by inducing diastolic arrest, minimize myocardial oxygen requirements and cause no myocardial damage itself

Ions & Buffers

• Americans favour a solution that approximates extracellular fluid ionic concentration [ie use potassium] while Europeans favour an intracellular composition [reduce ionic gradients across cell membrane]; comparable results are seen however

• The solution should be slightly hypertonic (315-350 mOsm/l) to minimize myocardial oedema

• Cardioplegia solutions are buffered so as to maintain an alkaline pH; as at 37°C the neutral pH of water is 6.8, at 0°C it rises to 7.5, therefore require to maintain an alkaline solution if neutrality is maintained [ie a pH of 6.8 while hypothermic is acidic which may cause ischaemic myocardial damage]

Composition of Cardioplegia Solutions

KCl

• Cardioplegic agent most common used in USA

• Usually 10 - 30 mEq/l

• High concentrations (30 mEq/l) necessary with blood cardioplegia

Mannitol

• Contributes to a hypertonic cardioplegia

• Also has free oxygen radical scavenging benefits

Mg

• Potentially can induce cardiac arrest by itself

• Stabilises cell membranes

• Blocks phosphorylase action of myosin thereby preserving high energy substrates

• Slow calcium channel blocker

Procaine

• Is by itself a cardioplegic agent; blocks permeability of cell membrane to sodium during repolarisation

• Slow calcium channel blocker

• Also stabilises cell membrane

• Prevents vasoconstriction due to the particle contents of the IV solution thereby improving distribution of cardioplegia in coronary artery disease; coronary vasodilator

Calcium

• Essential to myocardial contractility and to maintain normal membrane functioning

• But implicated in reperfusion injury [“calcium paradox”]

• Therefore most institution include a small amount of calcium in crystalloid cardioplegia (1mM/L)

• Blood cardioplegia does not need any additional calcium

St Thomas’ solution

• Aim is to close to extracellular fluid with added components

Bretsschneider solution

• Similar to intracellular fluid

• Cardiac arrest achieved by absence of calcium, low sodium concentration, addition of procaine, mannitol (to increase osmolality)

Blood Cardioplegia

• Associated with Buckberg

• Consists of oxygenated blood which is cooled & diluted with cardioplegia to produce a solution which is alkaline, raised potassium concentration to induce arrest and a reduced calcium level and haematocrit

• Benefit of oxygenating potential

• Benefit of buffering potential of plasma

• Benefit of being able to alter temperature

• Most effective at 20°C, if too cold have increased blood viscosity and reduced oxygen dissociation from Hb

• Clinically shows little/no benefit over crystalloid cardioplegia

• More complex & expensive than crystalloid

• May show benefits for long ischaemic periods and in poor ejection fraction patients