Normothermic blood polarizing versus depolarizing cardioplegia in a porcine model of cardiopulmonary bypass.

OBJECTIVES: We have previously demonstrated beneficial cardiac protection with hypothermic polarizing cardioplegia compared to a hyperkalemic depolarizing cardioplegia. In this study, a porcine model of cardiopulmonary bypass was used to compare the protective effects of normothermic blood-based polarizing and depolarizing cardioplegia during cardiac arrest. METHODS: Thirteen pigs were randomized to receive either normothermic polarizing (n = 8) or depolarizing (n = 5) blood-based cardioplegia. After initiation of cardiopulmonary bypass, normothermic arrest (34°C, 60 min) was followed by 60 min of on-pump and 90 min of off-pump reperfusion. Primary outcome was myocardial injury measured as arterial myocardial creatine kinase concentration. Secondary outcome was haemodynamic function and the energy state of the hearts. RESULTS: During reperfusion, release of myocardial creatine kinase was comparable between groups (P = 0.36). In addition, most haemodynamic parameters showed comparable results between groups, but stroke volume (P = 0.03) was significantly lower in the polarizing group. Adenosine triphosphate levels were significantly (18.41 ± 3.86 vs 22.97 ± 2.73 nmol/mg; P = 0.03) lower in polarizing hearts, and the requirement for noradrenaline administration (P = 0.002) and temporary pacing (6 vs 0; P = 0.02) during reperfusion were significantly higher in polarizing hearts. CONCLUSIONS: Under normothermic conditions, polarizing blood cardioplegia was associated with similar myocardial injury to depolarizing blood cardioplegia. Reduced haemodynamic and metabolic outcome and a higher need for temporary pacing with polarized arrest may be associated with the blood-based dilution of this solution.

St Thomas' Hospital polarizing blood cardioplegia improves hemodynamic recovery in a porcine model of cardiopulmonary bypass.

OBJECTIVE: Cardiac surgery demands highly effective cardioprotective regimens. We previously demonstrated improved cardioprotection with "polarized" compared with "depolarized" arrest. This study uses a clinically relevant porcine model of cardiopulmonary bypass to compare the efficacy of blood-based St Thomas' Hospital polarizing cardioplegia (STH-Pol-B) with blood-based St Thomas' Hospital hyperkalemic cardioplegia (STH2-B). METHODS: Pigs were monitored and subjected to normothermic cardiopulmonary bypass, cardiac arrest via antegrade cold (4°C) blood cardioplegia (STH2-B, control group: n = 6 or STH-Pol-B, study group: n = 7), and global ischemia (60 minutes) followed by on-pump reperfusion (60 minutes) and subsequent off-pump reperfusion (90 minutes). At termination, tissue samples were taken for analysis of high-energy phosphates, ultrastructure, and microRNAs. The primary endpoint of this study was creatine kinase-muscle/brain release during reperfusion. RESULTS: Creatine kinase-muscle/brain was comparable in both groups. After pigs were weaned from cardiopulmonary bypass, hemodynamic parameters such as mean arterial pressure (P = .007), left ventricular systolic pressure (P < .001), external heart work (P = .012), stroke volume (P = .015), as well as dp/dtmax (P = .027), were improved with polarizing cardioplegia. Wedge pressure was significantly lower in the study group (P < .01). Energy charge was comparable between groups. MicroRNA-708-5p was significantly lower (P = .019) and microRNA-122 expression significantly (P = .046) greater in STH-Pol-B hearts. CONCLUSIONS: Polarized cardiac arrest offers similar myocardial protection and enhances functional recovery in a porcine model of cardiopulmonary bypass. Differential expression of microRNAs may indicate possible new ischemia-reperfusion markers. These results confirm the noninferiority and potential of polarized versus depolarized arrest.

A comparison of haemostatic biomarkers during low-risk patients undergoing cardiopulmonary bypass using either conventional centrifugal cell salvage or the HemoSep device.

BACKGROUND: Cardiac surgery using cardiopulmonary bypass (CPB) is associated with a coagulopathy due to haemodilution, thrombocytopenia and platelet dysfunction and the activation of coagulation and fibrinolysis, despite the use of large doses of unfractionated heparin. Conventional red cell salvage may exacerbate post-operative bleeding as plasma containing haemostatic factors is discarded. We hypothesized that a novel cell salvage device (HemoSep) may attenuate haemostatic changes associated with red cell salvage. We studied haemostatic markers following autologous transfusion from conventional cell salvage or the HemoSep device. METHODS: This randomised, controlled trial compared haemostatic markers in patients undergoing coronary artery bypass grafting or aortic valve replacement who received autologous blood returned from cell salvage (control) or HemoSep (study). Blood samples were taken pre-operatively, end of CPB, post-transfusion of salvaged blood and 3 hours post-operatively and analysed for full blood count (FBC), prothrombin time (PT), activated partial thromboplastin time (aPTT), fibrinogen, D-dimer and endogenous thrombin potential (ETP). RESULTS: Fifty-four patients were recruited (n=28 control, n=26 study). Processed blood volume for transfusion was significantly (p<0.001) higher in the HemoSep group. In the HemoSep group, the PT was shorter (18.7±0.3 vs 19.9±0.3 sec; p<0.05) post-operatively and the aPTT was longer (48.6±3.8 vs 37.3±1.0 sec; p<0.01) following autologous transfusion. In the control group, D-dimer and ETP levels were higher (1903±424 vs.1088±151; p<0.05 and 739±46 vs. 394±60; p<0.001, respectively) following autologous transfusion. CONCLUSIONS: Although centrifuged cell salvage is known to adequately haemoconcentrate and remove unwanted substrates and bacteriological contamination, the process can exacerbate coagulopathy. The HemoSep device demonstrated some increase in haemostatic markers when used in low-risk cardiac surgery patients.

Left ventricular dysfunction after two hours of polarizing or depolarizing cardioplegic arrest in a porcine model.

INTRODUCTION: This experimental study compares myocardial function after prolonged arrest by St. Thomas' Hospital polarizing cardioplegic solution (esmolol, adenosine, Mg2+) with depolarizing (hyperkalaemic) St. Thomas' Hospital No 2, both administered as cold oxygenated blood cardioplegia. METHODS: Twenty anaesthetized pigs on tepid (34°C) cardiopulmonary bypass (CPB) were randomised to cardioplegic arrest for 120 min with antegrade, repeated, cold, oxygenated, polarizing (STH-POL) or depolarizing (STH-2) blood cardioplegia every 20 min. Cardiac function was evaluated at Baseline and 60, 150 and 240 min after weaning from CPB, using a pressure-conductance catheter and epicardial echocardiography. Regional tissue blood flow, cleaved caspase-3 activity and levels of malondialdehyde were evaluated in myocardial tissue samples. RESULTS: Preload recruitable stroke work (PRSW) was increased after polarizing compared to depolarizing cardioplegia 150 min after declamping (73.0±3.2 vs. 64.3±2.4 mmHg, p=0.047). Myocardial tissue blood flow rate was high in both groups compared to the Baseline levels and decreased significantly in the STH-POL group only, from 60 min to 150 min after declamping (p<0.005). Blood flow was significantly reduced in the STH-POL compared to the STH-2 group 240 min after declamping (p<0.05). Left ventricular mechanical efficiency, the ratio between total pressure-volume area and blood flow rate, gradually decreased after STH-2 cardioplegia and was significantly reduced compared to STH-POL cardioplegia after 150 and 240 min (p<0.05 for both). CONCLUSION: Myocardial protection for two hours of polarizing cardioplegic arrest with STH-POL in oxygenated blood is non-inferior compared to STH-2 blood cardioplegia. STH-POL cardioplegia alleviates the mismatch between myocardial function and perfusion after weaning from CPB.

Statistical primer: performing repeated-measures analysis.

Longitudinal data arise when repeated measurements are taken on the same individuals over time. Inference about between-group differences of within-subject change is usually of interest. This statistical primer for cardiothoracic and vascular surgeons aims to provide a short and practical introduction of biostatistical methods on how to analyse repeated-measures data. Several methodological approaches for analysing repeated measures will be introduced, ranging from simple approaches to advanced regression modelling. Design considerations of studies involving repeated measures are discussed, and the methods are illustrated with a data set measuring coronary sinus potassium in dogs after occlusion. Cardiothoracic and vascular surgeons should be aware of the myriad approaches available to them for analysing repeated-measures data, including the relative merits and disadvantages of each. It is important to present effective graphical displays of the data and to avoid arbitrary cross-sectional statistical comparisons.

Efficacy of esmolol cardioplegia during hypothermic ischaemia.

OBJECTIVES: Cardioplegic arrest using a polarizing solution has been shown to have beneficial advantages for cardioprotection compared with depolarizing (potassium-based) arrest; most studies, however, have looked at normothermic ischaemia with short infusion intervals (every 10-15 min). This study examines the protective efficacy of an esmolol-based cardioplegia during hypothermic arrest, together with a prolonged infusion interval (30 min) for increased clinical feasibility. METHODS: Isolated Langendorff-perfused hearts were subjected to arrest with St Thomas' Hospital cardioplegia (STH2), or esmolol cardioplegia (single- or multidose infusion at 32°C) for 60-min, 90-min or 120-min global ischaemia at 32°C, and recovery of function (left ventricular developed pressure) measured. A further study examined the protective efficacy of multidose esmolol cardioplegia compared with hypothermia alone at temperatures of 20°C, 28°C and 32°C compared with 37°C for 120 min of ischaemia. RESULTS: Esmolol cardioplegic arrest with multidose infusion at 32°C significantly improved recovery of function (left ventricular developed pressure) compared with 32°C STH2, at each ischaemic duration (88 ± 3 vs 66 ± 3% at 60 min, 82 ± 3 vs 51 ± 3% at 90 min and 73 ± 6 vs 49 ± 4% at 120 min; P < 0.05). At various hypothermic temperatures, esmolol cardioplegia significantly improved protection compared with hypothermia alone (88 ± 4%, 88 ± 3% and 72 ± 3% vs 60 ± 3%, 30 ± 2% and 15 ± 1% at 20°C, 28°C and 32°C, respectively; P < 0.05); however, at 37°C, there was no difference in protection. Contracture during ischaemia mirrored the effects of left ventricular developed pressure recovery. CONCLUSIONS: Esmolol cardioplegia (a polarizing solution), used as a multidose infusion during hypothermia, significantly improved cardioprotection compared with the depolarizing STH2. An increased infusion interval of 30 min indicates improved clinical feasibility.

Myocardial energy metabolism and ultrastructure with polarizing and depolarizing cardioplegia in a porcine model.

OBJECTIVES: This study investigated whether the novel St. Thomas' Hospital polarizing cardioplegic solution (STH-POL) with esmolol/adenosine/magnesium offers improved myocardial protection by reducing demands for high-energy phosphates during cardiac arrest compared to the depolarizing St. Thomas' Hospital cardioplegic solution No 2 (STH-2). METHODS: Twenty anaesthetised pigs on tepid cardiopulmonary bypass were randomized to cardiac arrest for 60 min with antegrade freshly mixed, repeated, cold, oxygenated STH-POL or STH-2 blood cardioplegia every 20 min. Haemodynamic variables were continuously recorded. Left ventricular biopsies, snap-frozen in liquid nitrogen or fixed in glutaraldehyde, were obtained at Baseline, 58 min after cross-clamp and 20 and 180 min after weaning from bypass. Adenine nucleotides were evaluated by high-performance liquid chromatography, myocardial ultrastructure with morphometry. RESULTS: With STH-POL myocardial creatine phosphate was increased compared to STH-2 at 58 min of cross-clamp [59.9 ± 6.4 (SEM) vs 44.5 ± 7.4 nmol/mg protein; P < 0.025], and at 20 min after reperfusion (61.0 ± 6.7 vs 49.0 ± 5.5 nmol/mg protein; P < 0.05), ATP levels were increased at 20 min of reperfusion with STH-POL (35.4 ± 1.1 vs 32.4 ± 1.2 nmol/mg protein; P < 0.05). Mitochondrial surface-to-volume ratio was decreased with polarizing compared to depolarizing cardioplegia 20 min after reperfusion (6.74 ± 0.14 vs 7.46 ± 0.13 µm 2 /µm 3 ; P = 0.047). None of these differences were present at 180 min of reperfusion. From 150 min of reperfusion and onwards, cardiac index was increased with STH-POL; 4.8 ± 0.2 compared to 4.0 ± 0.2 l/min/m 2 ( P = 0.011) for STH-2 at 180 min. CONCLUSIONS: Polarizing STH-POL cardioplegia improved energy status compared to standard STH-2 depolarizing blood cardioplegia during cardioplegic arrest and early after reperfusion.

HEMO2life as a protective additive to Celsior solution for static storage of donor hearts prior to transplantation.

BACKGROUND: Prior to heart transplantation, static storage of donor hearts is currently limited to 4-5 h, despite profound hypothermia (4-8 °C). Because heart transplantation is an emergency procedure, improved protection to extend safe storage duration would be advantageous. We investigated whether the naturally respiratory pigment HEMO2life®, which is effective at hypothermia for the passive release of oxygen via oxygen gradient, could improve long-term preservation. METHODS: Isolated Langendorff-perfused rat hearts (n = 12/group) were equilibrated (20 min) and function (left ventricular developed pressure: LVDP) measured by intraventricular balloon before arrest with cold (7.5 °C) Celsior® solution, either alone (control) or with the addition of HEMO2life® (Hemarina SA, Morlaix, France) at 1 g/L. Cold storage lasted 8 h prior to reperfusion (60 min) and recovery (as % of pre-ischemic function) was assessed. Hearts (minced and homogenized) were also assessed by TTC staining as a measure of viability and two hearts from each group were sliced and assessed by TTC staining for infarct size. Values are expressed as mean ± standard error of mean and analyzed by Student's t-test. RESULTS: Hearts recovered rapidly in both groups to a plateau by 20 min of reperfusion; control and HEMO2life® final recovery (60 min) was 45 ± 2% and 57 ± 1% (P < 0.05) respectively. Left ventricular end-diastolic pressure recovered to a similar extent in both groups (between 31 to 35 mmHg), as did heart rate (final recovery between 84 to 89% pre-ischemic value); however, coronary flow was significantly (P < 0.05) higher in HEMO2life® group (7.5 ± 0.7 ml/min) compared to control (5.4 ± 0.4 ml/min). Viability and infarct size measurements were similar between groups. CONCLUSION: The addition of the natural oxygen releasing pigment HEMO2life® to Celsior® preservation solution significantly improved post-ischemic recovery of heart function. This additive may have major therapeutic potential for clinical heart transplantation.

Myocardial function after polarizing versus depolarizing cardiac arrest with blood cardioplegia in a porcine model of cardiopulmonary bypass.

OBJECTIVES: Potassium-based depolarizing St Thomas' Hospital cardioplegic solution No 2 administered as intermittent, oxygenated blood is considered as a gold standard for myocardial protection during cardiac surgery. However, the alternative concept of polarizing arrest may have beneficial protective effects. We hypothesize that polarized arrest with esmolol/adenosine/magnesium (St Thomas' Hospital Polarizing cardioplegic solution) in cold, intermittent oxygenated blood offers comparable myocardial protection in a clinically relevant animal model. METHODS: Twenty anaesthetized young pigs, 42 ± 2 (standard deviation) kg on standardized tepid cardiopulmonary bypass (CPB) were randomized (10 per group) to depolarizing or polarizing cardiac arrest for 60 min with cardioplegia administered in the aortic root every 20 min as freshly mixed cold, intermittent, oxygenated blood. Global and local baseline and postoperative cardiac function 60, 120 and 180 min after myocardial reperfusion was evaluated with pressure-conductance catheter and strain by Tissue Doppler Imaging. Regional tissue blood flow, cleaved caspase-3 activity, GRK2 phosphorylation and mitochondrial function and ultrastructure were evaluated in myocardial tissue samples. RESULTS: Left ventricular function and general haemodynamics did not differ between groups before CPB. Cardiac asystole was obtained and maintained during aortic cross-clamping. Compared with baseline, heart rate was increased and left ventricular end-systolic and end-diastolic pressures decreased in both groups after weaning. Cardiac index, systolic pressure and radial peak systolic strain did not differ between groups. Contractility, evaluated as dP/dtmax, gradually increased from 120 to 180 min after declamping in animals with polarizing cardioplegia and was significantly higher, 1871 ± 160 (standard error) mmHg/s, compared with standard potassium-based cardioplegic arrest, 1351 ± 70 mmHg/s, after 180 min of reperfusion (P = 0.008). Radial peak ejection strain rate increased and the load-independent variable preload recruitable stroke work was increased with polarizing cardioplegia after 180 min, 64 ± 3 vs 54 ± 2 mmHg (P = 0.018), indicating better preserved left ventricular contractility with polarizing cardioplegia. Phosphorylation of GRK2 in myocardial tissue did not differ between groups. Fractional cytoplasmic volume in myocytes was reduced in hearts arrested with polarizing cardioplegia, indicating reduction of cytoplasmic oedema. CONCLUSIONS: Polarizing oxygenated blood cardioplegia with esmolol/adenosine/magnesium offers comparable myocardial protection and improves contractility compared with the standard potassium-based depolarizing blood cardioplegia.

Lung injury after simulated cardiopulmonary bypass in an isolated perfused rat lung preparation: Role of mitogen-activated protein kinase/Akt signaling and the effects of theophylline.

OBJECTIVES: Lung deflation and inflation during cardiac surgery with cardiopulmonary bypass contributes to pulmonary dysfunction postoperatively. Theophylline treatment for lung diseases has traditionally been thought to act by phosphodiesterase inhibition; however, increasing evidence has suggested other plausible mechanisms. We investigated the effects of deflation and reinflation on signaling pathways (p38-mitogen-activated protein kinase [MAPK], extracellular signal-regulated kinase 1 and 2 [ERK1/2], and Akt) and whether theophylline influences the deflation-induced lung injury and associated signaling. METHODS: Isolated rat lungs were perfused (15 mL/min) with deoxygenated rat blood in bicarbonate buffer and ventilated. After 20 minutes' equilibration, the lungs were deflated (60 minutes, aerobic perfusion 1.5 mL/min), followed by reinflation (60 minutes, anaerobic reperfusion 15 mL/min). Compliance, vascular resistance, and kinase phosphorylation were assessed during deflation and reinflation. The effects of SB203580 (50 µM), a p38-MAPK inhibitor, and theophylline (0.083 mM [therapeutic] or 3 mM [supratherapeutic]) on physiology and signaling were studied. RESULTS: Deflation reduced compliance by 44% compared with continuously ventilated lungs. p38-MAPK and Akt phosphorylation increased (three to fivefold) during deflation and reinflation, and ERK1/2 phosphorylation increased (approximately twofold) during reinflation. SB203580 had no effect on lung physiology or ERK1/2 and Akt activation. Both theophylline doses increased cyclic adenosine monophosphate, but only 3 mM theophylline improved compliance. p38-MAPK phosphorylation was not affected by theophylline; 0.083 mM theophylline inhibited reinflation-induced ERK1/2 phosphorylation (72%±3%); and 3 mM theophylline inhibited Akt phosphorylation during deflation (75%±5%) and reinflation (87%±4%). CONCLUSIONS: Lung deflation and reinflation stimulates differential p38-MAPK, ERK1/2, and Akt activation, suggesting a role in lung injury during cardiopulmonary bypass. However, p38-MAPK was not involved in the compromised compliance. A supratherapeutic theophylline dose protected lungs against deflation-induced injury and was associated with inhibition of phosphoinositide 3-kinase/Akt rather than phosphodiesterase.

Future perspective of cardioplegic protection in cardiac surgery.

"Depolarized arrest", induced by hyperkalemic (moderately increased extracellular potassium) cardioplegia is the gold standard to achieve elective temporary cardiac arrest in cardiac surgery. Hyperkalemic cardioplegic solutions provide good myocardial protection, which is relatively safe and easily and rapidly reversible. However, this technique has detrimental effects associated with ionic imbalance involving sodium and calcium overload of the cardiac cell induced by depolarization of the cell membrane. Hence, the development of an improved cardioplegic solution that enhances myocardial protection would be expected as an alternative to hyperkalemic cardioplegia. In this review, we assess the potential disadvantages of "depolarized arrest" and the suitability and clinical potential of "non-depolarized arrest". "Magnesium cardioplegia" and "esmolol cardioplegia" has been shown to exert superior protection with comparable safety profiles to that of hyperkalemic cardioplegia. These alternative techniques require further examination and investigation to challenge the traditional view that hyperkalemic arrest is best. Endogenous cardioprotective strategies, termed "ischemic preconditioning" and "ischemic postconditioning", may have a role in cardiac surgery to provide additional protection. The elective nature of cardiac surgery, with the known onset of ischemia and reperfusion, lends it to the potential of these strategies. However, the benefit of preconditioning and postconditioning during cardiac surgery is controversial, particularly in the context of cardioplegia. The clinical application of these strategies is unlikely to become routine during cardiac surgery because of the necessity for repeated aortic crossclamping with consequent potential for embolic events, but offers considerable potential especially if "pharmacological" preconditioning and postconditioning could be established.

Cardioprotection with esmolol cardioplegia: efficacy as a blood-based solution.

OBJECTIVES: Current cardiac surgery patients are older, sicker, with more diffuse disease and hence a reduced tolerance to ischaemia-reperfusion injury. We previously demonstrated that esmolol, an ultra-short-acting ß-blocker, can be used as an arresting agent at high (millimolar) concentrations, and that a crystalloid-based esmolol cardioplegia afforded cardioprotection at least equivalent to hyperkalaemic (St Thomas' Hospital) cardioplegia. Esmolol is rapidly metabolized by blood esterases, so it was important to determine the feasibility of its use in blood-based solutions. This study compared the efficacy of blood-based esmolol cardioplegia with hyperkalaemic cardioplegia in a novel blood-perfused rat heart preparation. METHODS: Isolated rat hearts were Langendorff blood-perfused with a rat blood/buffer perfusate mixture (flow rate, 3.0 ml/min) and pre-ischaemic baseline function (left ventricular developed pressure) assessed. All values are expressed as mean ± SEM. Three studies were conducted: (i) the efficacy of blood-based vs crystalloid-based esmolol or hyperkalaemic cardioplegia (40 min ischaemia) was evaluated (five groups; six hearts/group); (ii) the effect of the mode of cardioplegia delivery (constant flow/pressure), esmolol concentration and extended delivery interval (45 min ischaemia) was evaluated (four groups; six hearts/group); (iii) the efficacy of blood-based esmolol compared with hyperkalaemic cardioplegia over extended (60 min) ischaemia duration was evaluated (two groups; six hearts/group). Hearts were reperfused (60 min) and recovery (percent of pre-ischaemic baseline function) measured at the end of reperfusion. RESULTS: Hearts subjected to blood-based esmolol or hyperkalaemia cardioplegia recovered to 78 ± 4% and 68 ± 6%, whereas crystalloid-based esmolol or hyperkalaemic cardioplegia recovered to 84 ± 1% and 77 ± 2%, respectively [all P < 0.05 vs control (2 ± 2%)]; there were no differences between cardioplegia groups. When infusion duration was extended, a lower (2 mmol/l) esmolol concentration improved recovery compared with the higher (3 mmol/l) concentration (66 ± 4% vs 29 ± 12%, P < 0.05). Extending the ischaemic duration demonstrated enhanced efficacy for blood-based esmolol cardioplegia (70 ± 4%; P < 0.05) compared with hyperkalaemic cardioplegia (47 ± 10%). CONCLUSIONS: Blood-based esmolol cardioplegia improved cardioprotective efficacy compared with hyperkalaemic cardioplegia; the metabolic effects of blood esterase did not appear to influence this efficacy. An esmolol-based cardioplegic solution may be a beneficial alternative to hyperkalaemic solutions.

Ischaemic postconditioning: does cardioplegia influence protection?

OBJECTIVES: Ischaemic postconditioning attenuates reperfusion injury and may be a useful adjunct to cardiac surgery. We examined the efficacy of postconditioning following ischaemic protection with cardioplegic arrest and the importance of index ischaemia and cardioplegia formulation. METHODS: Isolated Langendorff-perfused rat hearts were subjected to varying durations (30, 45 or 60 min) of global (37°C) ischaemia and then reperfused; functions such as left ventricular developed pressure, left ventricular end-diastolic pressure and heart rate and coronary flow were assessed throughout reperfusion, and infarct size measured after 120 min of reperfusion. Two studies were conducted. Study 1 compared recovery of function and infarct size after varying durations (30, 45 or 60 min) of ischaemia alone and ischaemia preceded by protection with St Thomas' Hospital cardioplegic solution, with or without a postconditioning protocol (six cycles of 10-s reperfusion and 10-s ischaemia). Study 2 investigated the importance of magnesium in cardioplegia [low magnesium (1.2 mmol/l) or zero magnesium (0 mmol/l)] on postconditioning protection after varying durations of ischaemia (30, 45 or 60 min). RESULTS: Postconditioning enhanced protection after 30-min ischaemia alone [left ventricular developed pressure at 60-min reperfusion recovered to 34 ± 1% after ischaemia alone and 45 ± 2% after postconditioning (P < 0.05); infarct size was 21 ± 4 and 11 ± 1%, respectively (P < 0.05)], but there were no differences after 45 or 60 min of ischaemia. Increasing ischaemic durations reduced recovery, and cardioplegia was protective during shorter ischaemia but protection diminished as ischaemic duration increased. Postconditioning was ineffective after cardioplegia protection at all ischaemic durations. However, postconditioning after cardioplegia containing zero magnesium was efficacious for 60 min of ischaemia [left ventricular developed pressure at 60-min reperfusion was 41 ± 2% (P < 0.05) vs. 31 ± 3%; infarct size was 30 ± 7 vs. 56 ± 6%, respectively (P < 0.05)], but was ineffective for shorter periods of ischaemia. CONCLUSIONS: We conclude that postconditioning does not enhance the protective effect of St Thomas' Hospital cardioplegia (under these strict experimental conditions); however, the efficacy of postconditioning correlates with the magnesium concentration of the cardioplegic solution, which may imply involvement of magnesium on mitochondria during ischaemia. There is a limited window of postconditioning protection dependent on the duration of the index ischaemia.