Absolute risk reduction in total mortality with implantable cardioverter defibrillators: analysis of primary and secondary prevention trial data to aid risk/benefit analysis.

AIMS: Absolute risk reduction (ARR) and number needed to treat (NNT) are considered by many to be the most appropriate figures to use for the informed consent process, yet the results of published implantable cardioverter defibrillators (ICD) trials are frequently presented as relative risk reduction or odds ratio. The period over which risk reduction is calculated also varies between trials, making comparison difficult. METHODS AND RESULTS: Published ICD trials used to formulate national and international guidelines were examined for 1, 2, and 3 year total mortality in ICD and medically treated patients. The number of patients enrolled and at risk at these time points were also sought. Where the raw data were not included in the original text, estimates were taken from published Kaplan-Meier graphs. Eight primary prevention (PP) trials, three secondary prevention (SP) trials, and one SP meta-analyses were included. For PP, ARR at 3-year follow-up ranged from 0 (no benefit) to 24.6% (NNT = 4). For SP, ARR at 3-year follow up ranged from 3.7% (NNT = 27) to 11.3% (NNT = 9). Absolute risk reduction increased with follow-up in PP trials, whereas there was considerable variation in SP trials. Overall, very few trial patients received 3-year follow-up, giving wide confidence intervals (CIs). CONCLUSION: Absolute risk reduction from ICD trials varies significantly depending upon trial entry criteria, subgroup characteristics, and duration of follow-up. The relatively small number of patients followed for 2 or more years leads to wide CIs. Despite these limitations, the standardized ARR and NNT data presented may give a more individualized estimate of risk/benefit that could potentially aid an informed consent process.

Living without creatine: unchanged exercise capacity and response to chronic myocardial infarction in creatine-deficient mice.

RATIONALE: Creatine is thought to be involved in the spatial and temporal buffering of ATP in energetic organs such as heart and skeletal muscle. Creatine depletion affects force generation during maximal stimulation, while reduced levels of myocardial creatine are a hallmark of the failing heart, leading to the widely held view that creatine is important at high workloads and under conditions of pathological stress. OBJECTIVE: We therefore hypothesised that the consequences of creatine-deficiency in mice would be impaired running capacity, and exacerbation of heart failure following myocardial infarction. METHODS AND RESULTS: Surprisingly, mice with whole-body creatine deficiency due to knockout of the biosynthetic enzyme (guanidinoacetate N-methyltransferase [GAMT]) voluntarily ran just as fast and as far as controls (>10 km/night) and performed the same level of work when tested to exhaustion on a treadmill. Furthermore, survival following myocardial infarction was not altered, nor was subsequent left ventricular (LV) remodelling and development of chronic heart failure exacerbated, as measured by 3D-echocardiography and invasive hemodynamics. These findings could not be accounted for by compensatory adaptations, with no differences detected between WT and GAMT(-/-) proteomes. Alternative phosphotransfer mechanisms were explored; adenylate kinase activity was unaltered, and although GAMT(-/-) hearts accumulated the creatine precursor guanidinoacetate, this had negligible energy-transfer activity, while mitochondria retained near normal function. CONCLUSIONS: Creatine-deficient mice show unaltered maximal exercise capacity and response to chronic myocardial infarction, and no obvious metabolic adaptations. Our results question the paradigm that creatine is essential for high workload and chronic stress responses in heart and skeletal muscle.

Regulation of ß-adrenergic control of heart rate by GTP-cyclohydrolase 1 (GCH1) and tetrahydrobiopterin.

AIMS: Clinical markers of cardiac autonomic function, such as heart rate and response to exercise, are important predictors of cardiovascular risk. Tetrahydrobiopterin (BH4) is a required cofactor for enzymes with roles in cardiac autonomic function, including tyrosine hydroxylase and nitric oxide synthase. Synthesis of BH4 is regulated by GTP cyclohydrolase I (GTPCH), encoded by GCH1. Recent clinical studies report associations between GCH1 variants and increased heart rate, but the mechanistic importance of GCH1 and BH4 in autonomic function remains unclear. We investigate the effect of BH4 deficiency on the autonomic regulation of heart rate in the hph-1 mouse model of BH4 deficiency. METHODS AND RESULTS: In the hph-1 mouse, reduced cardiac GCH1 expression, GTPCH enzymatic activity, and BH4 were associated with increased resting heart rate; blood pressure was not different. Exercise training decreased resting heart rate, but hph-1 mice retained a relative tachycardia. Vagal nerve stimulation in vitro induced bradycardia equally in hph-1 and wild-type mice both before and after exercise training. Direct atrial responses to carbamylcholine were equal. In contrast, propranolol treatment normalized the resting tachycardia in vivo. Stellate ganglion stimulation and isoproterenol but not forskolin application in vitro induced a greater tachycardic response in hph-1 mice. ß1-adrenoceptor protein was increased as was the cAMP response to isoproterenol stimulation. CONCLUSION: Reduced GCH1 expression and BH4 deficiency cause tachycardia through enhanced ß-adrenergic sensitivity, with no effect on vagal function. GCH1 expression and BH4 are novel determinants of cardiac autonomic regulation that may have important roles in cardiovascular pathophysiology.

Cavotricuspid isthmus dependent flutter is associated with an increased incidence of occult coronary artery disease.

AIMS: Atrial flutter (AFl) and atrial fibrillation (AFib) share many clinical risk factors and potential mechanisms with atherosclerosis. Despite this, an association between stable coronary artery disease (CAD) and atrial arrhythmias has not previously been documented. To investigate this hypothesis we measured the incidence of occult coronary atheroma on coronary angiography inpatients undergoing radiofrequency ablation procedures. METHODS AND RESULTS: Consecutive coronary angiograms performed on patients with no history or symptoms of CAD undergoing elective ablation of arrhythmias were analysed. Patients were divided into three groups according to their arrhythmia: Typical right AFl, AFib, and a matched control group undergoing ablation for either atrioventricular node-dependent supraventricular tachycardia (SVT) or idiopathic right ventricular outflow tract tachycardia (RVOT). Atherosclerosis on angiography was graded according to the most severe stenosis. A total of 138 patients were included. Groups were evenly matched for age (P = 0.4), risk factors for coronary disease including hypertension (P = 0.38) and diabetes (P = 0.2). The incidence of asymptomatic, occult coronary atheroma was significantly greater in patients with AFl (AFl 54%, AFib 26%, SVT/RVOT 21%, P = 0.005). In contrast there was no higher incidence of occult atheroma in patients with AFib than those with SVT/RVOT (P = 0.68). The majority of atherosclerosis observed was mild, non-obstructive plaque disease (AFl 75%, AFib 44%, SVT/RVOT 67%). CONCLUSION: There was a significantly greater incidence of occult coronary atheroma in asymptomatic patients undergoing ablation for AFl, suggesting that the mechanism underlying the development of atherosclerosis may also be important in creating the substrate that allows typical right AFl to develop.

Transcriptional response of the neuromuscular system to exercise training and potential implications for ALS.

The transcriptional adaptive response of motoneurons and muscles to voluntary exercise has been investigated by using laser capture microdissection and microarray analysis. Our results show that motoneurons respond to physical activity by activating a complex transcriptional plan, with changes involved in neurotrophic factor signalling, electrophysiological changes and synaptic reorganization. Gastrocnemius muscle shows increases in transcripts responsible for neovascularization and new myogenesis. Both tissues show transcriptional changes involved in the growth and reinforcement of the neuromuscular junction. This study indicates that the neuromuscular system undergoes significant structural and functional alterations, aiming to optimize the transmission of both chemical and electrical stimuli, thus prompting axonal outgrowth and mechanisms similar to long-term potentiation in hippocampal neurons. Understanding the response of these cells during exercise has potentially important implications for human neuromuscular disease, including amyotrophic lateral sclerosis, by highlighting candidate genes pivotal for the balance between the physiology and the pathology of the neuromuscular system in terms of the stress response to physical exercise.

Presence Of Left Atrial Appendage Thrombus In Patients Presenting For Left Atrial Ablation Of Atrial Fibrillation Despite Pre-Operative Anticoagulation.

Background: One of the recognised complications of left atrial ablation for atrial fibrillation (AF) is stroke. Left atrial (LA) thrombus, which may be dislodged by catheter manipulation, is an absolute contraindication to ablation. It is unclear whether imaging of the left atrial appendage (LAA) by transesophageal echo (TEE) is mandatory to exclude LA clot prior to ablation, particularly in "low-risk" patients with paroxysmal AF and normal left ventricular (LV) function. Methods and Results: We carried out a retrospective analysis of pre-ablation TEE in patients presenting for ablation of AF. Images from 244 ablation procedures carried out in 148 patients were examined, including 106 patients with paroxysmal AF and normal LV function. Despite at least 4 weeks of pre-operative therapeutic anticoagulation with Warfarin (INR>2.0), LAA thrombus was identified in 4 patients (2.7% (0.1-5.3%)). These included 2 patients with paroxysmal AF and normal LV function, although both had a high arrhythmia burden. The thrombi regressed with intensification of anticoagulation. Conclusions: Pre-operative imaging of the LAA remains advisable to exclude thrombus prior to ablation for AF even in patients with paroxysmal AF and normal LV function, especially if there is a high AF burden.

Impaired synaptic plasticity and motor learning in mice with a point mutation implicated in human speech deficits.

The most well-described example of an inherited speech and language disorder is that observed in the multigenerational KE family, caused by a heterozygous missense mutation in the FOXP2 gene. Affected individuals are characterized by deficits in the learning and production of complex orofacial motor sequences underlying fluent speech and display impaired linguistic processing for both spoken and written language. The FOXP2 transcription factor is highly similar in many vertebrate species, with conserved expression in neural circuits related to sensorimotor integration and motor learning. In this study, we generated mice carrying an identical point mutation to that of the KE family, yielding the equivalent arginine-to-histidine substitution in the Foxp2 DNA-binding domain. Homozygous R552H mice show severe reductions in cerebellar growth and postnatal weight gain but are able to produce complex innate ultrasonic vocalizations. Heterozygous R552H mice are overtly normal in brain structure and development. Crucially, although their baseline motor abilities appear to be identical to wild-type littermates, R552H heterozygotes display significant deficits in species-typical motor-skill learning, accompanied by abnormal synaptic plasticity in striatal and cerebellar neural circuits.

Abnormal sympathoadrenal development and systemic hypotension in PHD3-/- mice.

Cell culture studies have implicated the oxygen-sensitive hypoxia-inducible factor (HIF) prolyl hydroxylase PHD3 in the regulation of neuronal apoptosis. To better understand this function in vivo, we have created PHD3(-/-) mice and analyzed the neuronal phenotype. Reduced apoptosis in superior cervical ganglion (SCG) neurons cultured from PHD3(-/-) mice is associated with an increase in the number of cells in the SCG, as well as in the adrenal medulla and carotid body. Genetic analysis by intercrossing PHD3(-/-) mice with HIF-1a(+/-) and HIF-2a(+/-) mice demonstrated an interaction with HIF-2alpha but not HIF-1alpha, supporting the nonredundant involvement of a PHD3-HIF-2alpha pathway in the regulation of sympathoadrenal development. Despite the increased number of cells, the sympathoadrenal system appeared hypofunctional in PHD3(-/-) mice, with reduced target tissue innervation, adrenal medullary secretory capacity, sympathoadrenal responses, and systemic blood pressure. These observations suggest that the role of PHD3 in sympathoadrenal development extends beyond simple control of cell survival and organ mass, with functional PHD3 being required for proper anatomical and physiological integrity of the system. Perturbation of this interface between developmental and adaptive signaling by hypoxic, metabolic, or other stresses could have important effects on key sympathoadrenal functions, such as blood pressure regulation.

Radiochemical HPLC detection of arginine metabolism: measurement of nitric oxide synthesis and arginase activity in vascular tissue.

Nitric oxide (NO) plays a key role in vascular homeostasis. Accurate measurement of NO production by endothelial nitric oxide synthase (eNOS) is critical for the investigation of vascular disease mechanisms using genetically modified animal models. Previous assays of NO production measuring the conversion of arginine to citrulline have required homogenisation of tissue and reconstitution with cofactors including NADPH and tetrahydrobiopterin. However, the activity and regulation of NOS in vivo is critically dependant on tissue levels of these cofactors. Therefore, understanding eNOS regulation requires assays of NO production in intact vascular tissue that do not depend on the addition of exogenous cofactors and have sufficient sensitivity and specificity. We describe a novel technique, using radiochemical detection of arginine to citrulline conversion, to measure NO production within intact mouse aortas, without exogenous cofactors. We demonstrate the presence of arginase activity in mouse aortas which has the potential to confound this assay. Furthermore, we describe the use of N-hydroxy-nor-L-arginine (nor-NOHA) to inhibit arginase and permit specific detection of NO production in intact mouse tissue. Using this technique we demonstrate a 2.4-fold increase in NO production in aortas of transgenic mice overexpressing eNOS in the endothelium, and show that this technique has high specificity and high sensitivity for detection of in situ NO synthesis by eNOS in mouse vascular tissue. These results have important implications for the investigation of NOS regulation in cells and tissues.