Warm (32 degrees celcius) polarised arrest: towards a new cardioprotective strategy using normokalaemic adenosine-lignocaine cardioplegia

Background and Aims: Cardiac surgeons and anaesthetists are facing unprecedented challenges from increasing numbers of higher-risk adult and paediatric patients. Currently, cardiac surgeons rely on depolarising potassium cardioplegia and hypothermia to protect the heart during surgical cardiac arrest. A range of adverse outcomes may result from prolonged hyperkalaemia and cold temperatures, however no alternative to high potassium cardioplegia has successfully translated into clinical use. There is a compelling need for new evidence-based myocardial protection strategies. In 2004 our laboratory developed a novel polarising cardioplegia using adenosine and lignocaine (AL) as the arresting agents in a normokalaemic solution. The main aim of this thesis was to develop normokalaemic AL cardioplegia for arrest at warm (32oC) or cold (8-12oC) temperatures using intermittent delivery over two hours or ‘one-shot’ delivery to extend the single-dose arrest interval to 50 minutes in the isolated working rat heart. The efficacy of this AL cardioplegia protocol was then compared with del Nido hyperkalaemic cardioplegia solution at warm and cold temperatures. Methods: Experiments were conducted on male adult Sprague Dawley rats using isolated heart perfused models (Langendorff model and working heart model modified by Neely). Arrest was induced and maintained with intermittent (20 minute intervals) or continuous cardioplegia delivery for up to two hours at 32°C, or one-shot delivery for 40 and 50 minutes at 32oC and 8-12oC. Functional recovery was assessed during reanimation in Langendorff mode and 60 minutes of working mode reperfusion with primary endpoints of heart rate, aortic flow, coronary flow, cardiac output, systolic pressure and diastolic pressure. The alpha level of significance for all experiments was set at p<0.05. Experimental design: The thesis was divided into five study arms: Study 1: The first study compared recovery of function following 40 or 60 minutes of warm arrest with intermittent or continuous delivery of AL cardioplegia. Study 2: The second study evaluated the effect of varying the potassium concentration in AL or potassium alone cardioplegia solution on arresting membrane potential and functional recovery following two hours of warm intermittent arrest. Study 3: In the third study a warm AL 40-minute one-shot arrest protocol was developed, and the AL cardioplegia modified with an antioxidant adjunct to improve recovery of post-arrest function. Study 4: In the fourth study the AL solution and arrest protocols were modified to optimise 50-minute warm and cold one-shot arrest. Study 5: The fifth study compared recovery of function following warm and cold oneshot arrest with AL cardioplegia or hyperkalaemic del Nido solution. Results: The first major result from this thesis was that AL cardioplegia delivered continuously or intermittently at 32°C resulted in no significant difference in coronary vascular resistance during cardioplegia delivery, or recovery of heart rate, aortic flow, coronary flow or rate pressure product following arrest. The second study showed that increasing or decreasing the potassium concentration in AL cardioplegia above or below normokalaemia (5.9mM K⁺) at 32°C led to depolarised and hyperpolarised states respectively, which increased coronary vascular resistance and decreased functional recovery. High potassium alone solutions (16 and 25mM K⁺) also resulted in reduced recovery of function. After 1- or 2-hour arrest, optimal coronary vascular resistance and functional recovery (cardiac output and stroke volume) occurred when the heart was arrested at polarised membrane potentials. Notably, AL with low potassium (AL 3mM K⁺) produced significantly worse outcomes than all other groups after 2-hour arrest. In the third study, increasing the cardioplegia delivery interval from 20 to 40 minutes with warm one-shot AL arrest significantly decreased function (increased the time to reanimation at reperfusion and decreased functional recovery early in the reperfusion period) compared with continuous or intermittent AL delivery. AL one-shot hearts recovered 37 ± 13% of pre-arrest cardiac output at 10 minutes reperfusion and 76 ± 2% of cardiac output at 60 minutes reperfusion, compared with 89 ± 4% (p<0.05) and 90 ± 2% (p<0.05) recovery of cardiac output respectively for hearts arrested with intermittent AL cardioplegia. The addition of N-(2-mercaptopropionyl)-glycine (MPG), a hydroxyl radical scavenger, to the terminal flush and reperfusion solutions of the 40- minute one-shot protocol significantly improved recovery of cardiac output to 83 ± 3% (p<0.05) at 10 minutes and 87 ± 3% (p<0.05) at 60 minutes reperfusion, which was not significantly different from recovery following intermittent warm AL arrest. Increasing the one-shot arrest interval to 50 minutes also decreased recovery which was improved by doubling the cardioplegia adenosine and lignocaine concentrations to AL(400:1000). Functional recovery was not further improved with addition of MPG during warm arrest with higher AL concentrations. However, at cold arresting temperatures increased magnesium (2.5mM) was required in the higher AL concentrations (ALM) for comparable recovery of function to warm AL (400:1000) arrest. Study 5 compared AL cardioplegia with hyperkalaemic del Nido solution (24mM K+) for 50-minute one-shot arrest at warm (32oC) and cold (8-12oC) temperatures. Warm arrest with del Nido solution significantly reduced functional recovery with 27 ± 2% return of aortic flow and 68 ± 3% coronary flow at 60 minutes reperfusion compared with recovery of 69 ± 2% aortic flow (p<0.05) and 105 ± 4% coronary flow (p<0.05) following del Nido arrest at cold temperatures. In contrast, hearts arrested with AL cardioplegia at warm or cold temperatures recovered 76 ± 2% (p<0.05) and 73 ± 2% (p<0.05) of pre-arrest aortic flow respectively, and 92 ± 5% (p<0.05) and 121 ± 5% (p