Pathophysiology of ischemic reperfusion injuryMechanisms and alternative methods of achieving cardiac arrest
Section snippets
Hyperkalemia
The most commonly used method for inducing rapid diastolic arrest during cardiac surgery is moderate elevation of the extracellular potassium (K+) concentration (usually within the range of 15 to 40 mmol/L). As the extracellular K+ concentration increases, the resting membrane potential (Em) becomes progressively more depolarized [1] and, at each K+ concentration, a new resting Em is established. As the resting Em depolarizes to approximately −65 mV (at K+ concentrations around 10 mmol/L) the
Polarized arrest
An alternative to inducing arrest by depolarization (with elevated K+ concentrations) is to maintain polarization of the Em, close to the resting Em. Polarized arrest should have a number of advantages; ionic movement (particularly Na+ and Ca2+ ions) should be reduced, because the threshold potential for activation of the ion channels will not be reached and window currents will not be activated. This reduction in ionic imbalance should, in turn, reduce myocardial energy utilization for ion
Hypocalcemia
The absence of extracellular Ca2+ induces cardiac arrest in diastole by inhibiting excitation-contraction coupling [40]. This characteristic was used in early cardioplegic solutions (predominantly from Germany), although accompanying low extracellular Na+ also attenuated the Na+-channel current, thereby maintaining Em close to the resting Em. However, the absence of Ca2+ increased the risk of inducing a “calcium paradox” [41], although traces of contaminant Ca2+, hypothermia and low Na+, or
Comment
The induction of rapid depolarized arrest of the heart in diastole by moderately elevated concentrations of K+ is by far the most widely used technique (possibly because it is the simplest to apply and to remove) for cardiac arrest during cardiac surgery; however, it cannot be used haphazardly, has a number of disadvantages, and is not necessarily the best and most optimally protective. The concept of maintaining arrest by induction of polarization (or hyperpolarization) compared with
Acknowledgements
Some of the studies conducted in the author’s laboratory were funded by the British Heart Foundation.
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