Interact CardioVasc Thorac Surg 2009;9:635-639. doi:10.1510/icvts.2009.208231
© 2009 European Association of Cardio-Thoracic Surgery
Institutional report - Cardiac general |
Mechanoenergetic function and troponin T release following cardioplegic arrest induced by St Thomas' and histidine-tryptophan-ketoglutarate cardioplegia – an experimental comparative study in pigs ,
Erling Aarsæthera,*,
Thor Allan Stenbergb,
Øyvind Jakobsenb and
Rolf Busunda,b
a Department of Cardiothoracic and Vascular Surgery, University Hospital of North Norway, N-9038 Tromsø, Norway
b Institute of Clinical Medicine, Medical Faculty, University of Tromsø, N-9037 Tromsø, Norway
Received 25 March 2009;
received in revised form 6 July 2009;
accepted 7 July 2009
 The study was supported by a grant from the Northern Norway Regional Health Authority. 
The work was presented in part at the 84th meeting of the Norwegian Surgical Society, Oslo, Norway, October 20–24, 2008.
*Corresponding author. Tel.: +47 77 66 93 56; fax: +47 77 62 82 98.
E-mail address: erling.johan.aarsaether{at}unn.no (E. Aarsæther).
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Abstract
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The study compares the single dose histidine-tryptophan-ketoglutarate (HTK) cardioplegia to the repeatedly delivered St Thomas' Hospital Solution (STHS) with respect to preservation of left ventricular mechanoenergetics and leakage of troponin T in a porcine experimental model. Fourteen pigs were randomized to a single infusion of 30 ml/kg HTK cardioplegia (n=7) or 500 ml STHS (n=7) followed by 200 ml after 20 and 40 min. After 1 h of aortic cross-clamping on cardiopulmonary bypass (CPB), the pigs were weaned and the hearts reperfused for 4 h. Stroke work (SW) was determined by a conductance catheter in the left ventricle. Myocardial oxygen consumption (MvO2) was measured as a function of coronary blood flow and arterial-to-coronary sinus oxygen saturation difference. Troponin T was sampled from the coronary sinus. The slope of the SW-MvO2 relationship increased by 1.09 (±0.53) in the HTK group compared with 0.33 (±0.70) in the STHS group following ischemia and 4 h of reperfusion (P=0.04). Troponin T was significantly higher in the HTK group compared with the STHS group (P=0.04). Repeatedly delivered STHS gives better preservation of postischemic mechanoenergetic function and lower troponin T release compared with single dose HTK cardioplegia, indicating improved cardioprotection with STHS.
Key Words: Cardiac function; Cardioplegia; Ischemia/reperfusion; Myocardial protection
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1. Introduction
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Although off-pump surgery has evolved as an alternative operative technique, cardiac surgery on cardiopulmonary bypass (CPB) under cardioplegic arrest remains the gold standard in open heart surgery. Two of the most commonly employed crystalloid cardioplegic solutions are the St Thomas' Hospital Solution (STHS) and the histidine-tryptophan-ketoglutarate (HTK) cardioplegia, both of which were developed in the 1970s [1]. In spite of a common aim of arresting and protecting the heart during cardioplegic arrest, the mechanism by which they do so differs. STHS utilizes a high potassium concentration to arrest the heart whereas cardiac arrest by HTK cardioplegia is primarily afforded by a low concentration of sodium inhibiting the rapid phase of the action potential, thus enabling it to increase buffer capacity without creating a hyperosmolar solution. The chemical composition of the HTK cardioplegia and the STHS used in the present study are shown in Table 1. While the HTK cardioplegia is administered as a single dose, the STHS is typically repeated every 20 min. This is an evident advantage of the HTK cardioplegia compared with STHS, as repeated delivery of cardioplegia interrupts and prolongs the surgical procedure. Several studies have indicated that the single dose HTK cardioplegia offers superior cardioprotection when compared with the single dose STHS [2–5]. However, a comparative study of the single dose HTK concept to the multi-dose STHS concept has, to our knowledge, not been performed. Cardioplegically induced ischemia by STHS has previously been shown to reduce contractile efficiency in the heart following ischemia and reperfusion in three different studies from our lab when compared to other experimental cardioplegic solutions [6–8]. The aim of the present study was to compare the cardioprotective effect of the single dose HTK cardioplegia concept to the repeatedly delivered STHS as expressed by preservation of contractile efficiency and leakage of troponin T following ischemia and reperfusion in a porcine experimental model.
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2. Material and methods
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2.1. Anesthesia and surgical preparation
The experimental protocol was approved by the local steering committee of the Norwegian Animal Experiments Authority. Animal care was conducted according to Guide for the Care and Use of Laboratory Animals published by the U.S. National Institutes of Health. Twenty-three domestic pigs with mean weight 44 kg (±6.5) were fasted overnight, but with free access to water. The anesthesia protocol and the surgical preparation have been described previously [8]. Briefly, the pigs received premedication with im. ketamine and iv. bolus injections of fentanyl and pentobarbital before they were tracheostomized and ventilated on a respirator with 50% oxygen. The left and right jugular veins were cannulated for measurements of central venous pressure (CVP) and for administration of continuous anesthesia with fentanyl, midazolam and pentobarbital. The heart was exposed through a median sternotomy and the hemiazygous vein was ligated at its entrance into the coronary sinus. A band was placed around the inferior vena cava for preload reductions. Transit time flow probes (Cardio-Med CM 4000, Medi-Stim AS, Norway) were placed snugly around the pulmonary trunk for measurement of cardiac output and around the three coronary arteries for measurement of coronary blood flow. Cardiac function was measured by a conductance catheter (CD Leycom, The Netherlands) in the left ventricle through the right carotid artery. After baseline measurements, the pigs received 380 IU/kg heparin and the axillary artery was cannulated for CPB with a 22 Fr cannula (Aortic Perfusion Cannula, Edwards Lifesciences, USA). For venous drainage, a two stage venous cannula (34x46 Fr Dual Stage Venous Drainage Cannula, Edwards Lifesciences) was inserted through the right atrium. After activated clotting time (ACT) had reached 300 s, normotherm CPB (38 °C) was initiated with a roller pump (Stockert/Shiley Caps Roller Pump, Soma Technology, USA) and a membrane oxygenator (Synthesis, Sorin Biomedica, Italy). Flow was kept between 80 and 110 ml/kg/min. A 9 Fr aortic root cannula (Medtronic, USA) was inserted into the ascending aorta for administration of cardioplegia.
2.2. Protocol
After ACT had reached 480 s, the aorta was clamped and animals were randomized to receive either one single infusion of HTK 30 ml/kg (Custodiol, Normedica, Denmark) or 500 ml STHS followed by 200 ml after 20 and 40 min. Both cardioplegic solutions had a temperature of 6 °C. An insulation pad was placed behind the heart to prevent it from rewarming during ischemia. Infusion time in the HTK group was aimed at 7 min, according to manufacturer's instructions. The cardiac temperature was measured by application of a temperature probe in the apex of the heart following 30 min and 59 min of aortic clamping. The aortic clamp was removed after 60 min. The pigs were weaned from CPB after 20 min of reperfusion. In the case of unsuccessful weaning, the pigs were allowed another 20 min of reperfusion before a second attempt was made. Pigs unable to wean from CPB were excluded and replaced until a sample size of 7 in each group was reached.
2.3. Left ventricular contractility and mechanoenergetic calculations
The first derivative of left ventricular pressure with respect to time (dP/dtmax) and preload recruitable SW were determined by a conductance catheter in the left ventricle as previously described [8]. Left ventricular mechanoenergetics was measured at baseline and 4 h after removal of the aortic clamp. Steady state recordings from five different preload conditions were made by stepwise and transient snaring of the inferior vena cava. Ten to fifteen beats were recorded in each run. Cardiac output and myocardial blood flow were measured during each recording and blood from the coronary sinus was drawn simultaneously for determination of myocardial venous blood oxygen saturation. Stroke work (SW, mmHg·ml) was assessed by the conductance catheter as the area of pressure volume loops and was converted to Joule/beat/100 g by the pig specific constant 1.33 · 10–4 J/mmHg/ml. Left ventricular coronary blood flow (LVCBF) was calculated as left ventricular weight/heart weight multiplied by total coronary blood flow. Left ventricular myocardial oxygen consumption (MvO2) was calculated according to the following formula: MvO2=(LVCBF·avo2·Hb·1.39)/(HR·20.2), where avo2 represents arterial to coronary sinus O2 saturation difference, Hb represents haemoglobin concentration (g/ml), 1.39 the constant oxygen binding capacity for haemoglobin (ml O2/g), HR represents heart rate and 20.2 is a constant in Joule/ml O2 that converts MvO2 to mechanical energy.
2.4. Statistical analysis
Statistical analysis was performed with the SPSS statistical software programme (SPSS 14.0, SPSS Inc, USA). All data are presented as mean±standard deviation (S.D.). Group data comparisons were made with the unpaired t-test and two-way analysis of variance (ANOVA) for repeated measurements and within subjects contrast test. Within groups comparisons between baseline and measurements following ischemia and reperfusion were performed with the one-way ANOVA test. Fisher's exact test was used for categorical data. P<0.05 were considered statistically significant.
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3. Results
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Twenty-three pigs were used, of which 14 completed the protocol successfully. Three pigs were excluded due to technical difficulties during the surgical preparation; one because of major bleeding and two because of ventricular fibrillation during the surgical preparation. Observational data obtained from the experiments are summarized in Table 2. The cardioplegic volume was significantly larger in the HTK group compared to the STHS group (P=0.002). Mean time required to arrest the heart was significantly shorter in the STHS group compared with the HTK group (P=0.01). Cardiac temperature increased after 59 min of aortic clamping in the HTK group compared to the STHS group, in which cardiac temperature was kept constant (P=0.02). Five of 12 pigs in the HTK group displayed signs of inadequate cardiac arrest by fine fibrillation or occasional contractions during ischemia compared to none in the STHS group (P=0.06). Five pigs in the HTK group and one pig in the STHS group failed to wean from CPB (P=0.33). Two pigs in the STHS group needed an extra 20 min to wean from CPB.
3.1. Hemodynamic variables and left ventricular contractility
Hemodynamic variables and left ventricular contractility indices at baseline and after 1 and 4 h of reperfusion are listed in Table 3. CVP was significantly higher in the STHS group compared with the HTK group after 1 h of reperfusion, but the difference was aligned at the end of the experiment. No significant differences were found with respect to left ventricular contractility, as expressed by dP/dtmax, preload recruitable SW or SW between the groups.
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Table 3 Hemodynamic variables and left ventricular contractility at baseline and following ischemia and 1 and 4 h of reperfusion in two groups of pigs receiving HTK cardioplegia (n=7) or STHS (n=7)
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3.2. Left ventricular mechanoenergetics
Data from the SW-MvO2 relationship obtained at baseline and 4 h after aortic unclamping are shown in Table 4. There were no statistically significant differences between the groups with respect to Y-intercept or slope at baseline. The slope of the SW-MvO2 relationship increased significantly in the HTK group (P=0.002) compared to a non-significant increase in the STHS group, indicating a greater loss of contractile efficiency in the HTK group (Fig. 1).
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Table 4 Left ventricular energetics at baseline and 4 h after aortic unclamping in pigs receiving HTK cardioplegia (n=7) or STHS (n=7)
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Fig. 1. Stroke work-MvO2 relationship after 1 h of ischemia and 4 h of reperfusion in porcine hearts. Lines indicate the mean value for the HTK group (solid, n=7) and the STHS group (dashed, n=7). The slope of the stroke work–MvO2 relationship increased more in the HTK group ( ) compared with the STHS group (•) (P=0.04, two way analysis of variance with repeated measures design), indicating a greater loss of contractile efficiency. HKT, histidine-tryptophan-ketoglutarate cardioplegic solution; MvO2, myocardial oxygen consumption; STHS, St Thomas' Hospital Solution.
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3.3. Cardiac troponin T
Results from the analysis of troponin T sampled from the coronary sinus are displayed in Fig. 2. Pigs in the HTK group had significantly higher levels of troponin T in the coronary sinus than pigs in the STHS group (P=0.04).
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Fig. 2. Mean troponin T from the coronary sinus at baseline and after 1 h ischemia and 4 h of reperfusion for porcine hearts protected by single dose HTK cardioplegia (n=7) or repeatedly delivered STHS (n=7). Troponin T in the coronary sinus was significantly higher in the HTK group () than in the STHS group (•) (*P=0.04, two way analysis of variance with repeated measures design). Error bars indicate S.D. HKT, histidine-tryptophan-ketoglutarate cardioplegic solution; STHS, St Thomas' Hospital Solution.
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4. Discussion
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The present study demonstrates better post-ischemic preservation of contractile efficiency, a highly sensitive measure of ischemia reperfusion injury [9], as well as lower leakage of troponin T in the STHS group compared with the HTK group following ischemia and reperfusion. Although five pigs in the HTK group were unable to wean from CPB compared to only one in the STHS group, this difference was not statistically significant. The present study was, however, not designed with a group size intended to demonstrate differences in categorical data, and we speculate that this may be due to a type II statistical error, which could have been avoided by increasing the number of pigs in each group.
Possible explanations for the differences in contractile efficiency and leakage of troponin T between the groups are improved washout of acidic metabolites and more efficient cooling through the repeated delivery of cardioplegia in the STHS group.
The failure of the HTK cardioplegia to induce complete cessation of electric and mechanical activity in the heart, may also contribute to these differences and coincide with clinical experiences from our department. Three of the seven pigs that were successfully weaned from CPB in the HTK group showed recurrent electromechanical activity during ischemia. In contrast, none of the hearts in the STHS group demonstrated signs of inadequate cardiac arrest. The same problem was encountered by Tait et al. who reported resumption of electromechanical activity in four of five dog hearts 5 min after the infusion of HTK cardioplegia [10]. The different ability of these cardioplegic solutions to maintain complete cardiac arrest probably reflects differences in potassium concentration.
Jynge et al. compared one of the precursors to the HTK cardioplegia, i.e. Bretschneider's solution number 3, to an early version of STHS in a rodent model in 1978, and concluded that STHS conferred superior cardioprotection to the Bretschneider solution [11]. Ever since this report, several publications have concluded that the HTK cardioplegia provides better cardioprotection than STHS [2–5]. However, these studies have in common that they have compared a single dose of HTK cardioplegia with a single dose of STHS. Since these cardioplegic solutions are differently administered in clinical practice, i.e. as a single infusion for HTK cardioplegia and as a repeatedly delivered infusion for STHS, we believe that experimental comparisons that also reflect these differences in administration are more clinically relevant.
Our study demonstrates better preservation of postischemic mechanoenergetic function and lower troponin T release following repeatedly delivered STHS compared with single dose HTK cardioplegia. We could not, however, demonstrate any difference in left ventricular function between the groups, which may be due to the relative small sample size and limited observation period following ischemia and reperfusion.
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Acknowledgements
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Custodiol was kindly provided by Normedica. The authors are indebted to the staff at the Surgical Research Laboratory at the University of Tromsø for excellent technical assistance. The Department of Medical Biochemistry, University Hospital of North Norway is acknowledged for analysis of blood samples. We thank Professor Dag Sørlie for advice during preparation of this manuscript.
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Abstract
1. Introduction
