Risk factors, biomarkers, and mechanisms for persistent inflammation, immunosuppression, and catabolism syndrome (PICS): a systematic review and meta-analysis

INTRODUCTION

Persistent inflammation, immunosuppression, and catabolism syndrome (PICS) has been proposed as an endotype of chronic critical illness (CCI). The aim of this systematic review is to synthesise the available evidence of risk factors, biomarkers, and biological mechanisms underlying PICS.

METHODS

MEDLINE, CENTRAL, and EMBASE were searched on June 2, 2023. Our population of interest was adult intensive care unit survivors. The exposure group was patients with PICS and the comparator group was patients with no PICS, CCI, or rapid recovery. Mean differences were pooled for each biomarker using a random effects DerSimonian-Laird method. Risk of bias assessment was done using the Newcastle-Ottawa Scale.

RESULTS

Six papers were included. Five were single-centre retrospective cohort studies, and one was a prospective cohort study, with sample sizes ranging from 22 to 391 patients. Two studies showed an increased incidence of PICS with age, and two studies showed an association between PICS and Charlson Comorbidity Index scores. PICS was associated with requiring mechanical ventilation in four studies. Meta-analysis showed a 34.4 mg L-1 higher C-reactive protein (95% confidence interval [CI] 12.7-56.2 mg L-1; P<0.01), a 4.4 g L-1 lower albumin (95% CI 0.5-8.3 g L-1; P<0.01), and a 0.36×109 L-1 lower lymphocyte count (95% CI 0.25-0.47×109 L-1; P=0.01) in the PICS compared with the non-PICS group. There are a large variety of other potential biomarkers but limited validation studies. The overall quality of evidence is limited, and these results should be interpreted accordingly.

CONCLUSIONS

While older patients and those with co-morbidities could be at greater risk for PICS, acquired risk factors, such as injury severity, are potentially more predictive of PICS than intrinsic patient characteristics. There are many potential biomarkers for PICS, but limited validation studies have been conducted. Persistent myeloid-derived suppressor cell expansion, the continual release of danger-associated molecular patterns and pathogen-associated molecular patterns propagating inflammation, and bioenergetic failure are all mechanisms underlying PICS that could offer potential for novel biomarkers and therapeutic interventions.

CLINICAL TRIAL REGISTRATION

International Prospective Register of Systematic Reviews (PROSPERO; CRD42023427749).

Persistent inflammation, immunosuppression, and catabolism syndrome (PICS): a review of definitions, potential therapies, and research priorities

Persistent Inflammation, Immunosuppression, and Catabolism Syndrome (PICS) is a clinical endotype of chronic critical illness. PICS consists of a self-perpetuating cycle of ongoing organ dysfunction, inflammation, and catabolism resulting in sarcopenia, immunosuppression leading to recurrent infections, metabolic derangements, and changes in bone marrow function. There is heterogeneity regarding the definition of PICS. Currently, there are no licensed treatments specifically for PICS. However, findings can be extrapolated from studies in other conditions with similar features to repurpose drugs, and in animal models. Drugs that can restore immune homeostasis by stimulating lymphocyte production could have potential efficacy. Another treatment could be modifying myeloid-derived suppressor cell (MDSC) activation after day 14 when they are immunosuppressive. Drugs such as interleukin (IL)-1 and IL-6 receptor antagonists might reduce persistent inflammation, although they need to be given at specific time points to avoid adverse effects. Antioxidants could treat the oxidative stress caused by mitochondrial dysfunction in PICS. Possible anti-catabolic agents include testosterone, oxandrolone, IGF-1 (insulin-like growth factor-1), bortezomib, and MURF1 (muscle RING-finger protein-1) inhibitors. Nutritional support strategies that could slow PICS progression include ketogenic feeding and probiotics. The field would benefit from a consensus definition of PICS using biologically based cut-off values. Future research should focus on expanding knowledge on underlying pathophysiological mechanisms of PICS to identify and validate other potential endotypes of chronic critical illness and subsequent treatable traits. There is unlikely to be a universal treatment for PICS, and a multimodal, timely, and personalised therapeutic strategy will be needed to improve outcomes for this growing cohort of patients.

Childhood outcomes after low-grade intraventricular haemorrhage: A systematic review and meta-analysis

AIM

To undertake a systematic review and meta-analysis exploring school-age neurodevelopmental outcomes of children after low-grade intraventricular haemorrhage (IVH).

METHOD

The published and grey literature was extensively searched to identify observational comparative studies exploring neurodevelopmental outcomes after IVH grades 1 and 2. Our primary outcome was neurodevelopmental impairment after 5 years of age, which included cognitive, motor, speech and language, behavioural, hearing, or visual impairments.

RESULTS

This review included 12 studies and over 2036 infants born preterm with low grade IVH. Studies used 30 different neurodevelopmental tools to determine outcomes. There was conflicting evidence of the composite risk of neurodevelopmental impairment after low-grade IVH. There was evidence of an association between low-grade IVH and lower IQ at school age (-4.23, 95% confidence interval [CI] -7.53, -0.92, I2  = 0%) but impact on school performance was unclear. Studies reported an increased crude risk of cerebral palsy after low-grade IVH (odds ratio [OR] 2.92, 95% CI 1.95, 4.37, I2  = 41%). No increased risk of speech and language impairment or behavioural impairment was found. Few studies addressed hearing and visual impairment.

INTERPRETATION

This systematic review presents evidence that low-grade IVH is associated with specific neurodevelopmental impairments at school age, lending support to the theory that low-grade IVH is not a benign condition.

WHAT THIS PAPER ADDS

The functional impact of low-grade intraventricular haemorrhage (IVH) at school age is unknown. Low-grade IVH is associated with a lower IQ at school age. The risk of cerebral palsy is increased after low-grade IVH. Low-grade IVH is not associated with speech and language impairment.

Preterm Brain Injury and Neurodevelopmental Outcomes: A Meta-analysis

CONTEXT

Preterm brain injuries are common; neurodevelopmental outcomes following contemporary neonatal care are continually evolving.

OBJECTIVE

To systematically review and meta-analyze neurodevelopmental outcomes among preterm infants after intraventricular hemorrhage (IVH) and white matter injury (WMI).

DATA SOURCES

Published and grey literature were searched across 10 databases between 2000 and 2021.

STUDY SELECTION

Observational studies reporting 3-year neurodevelopmental outcomes for preterm infants with IVH or WMI compared with preterm infants without injury.

DATA EXTRACTION

Study characteristics, population characteristics, and outcome data were extracted.

RESULTS

Thirty eight studies were included. There was an increased adjusted risk of moderate-severe neurodevelopmental impairment after IVH grade 1 to 2 (adjusted odds ratio 1.35 [95% confidence interval 1.05-1.75]) and IVH grade 3 to 4 (adjusted odds ratio 4.26 [3.25-5.59]). Children with IVH grade 1 to 2 had higher risks of cerebral palsy (odds ratio [OR] 1.76 [1.39-2.24]), cognitive (OR 1.79 [1.09-2.95]), hearing (OR 1.83 [1.03-3.24]), and visual impairment (OR 1.77 [1.08-2.9]). Children with IVH grade 3 to 4 had markedly higher risks of cerebral palsy (OR 4.98 [4.13-6.00]), motor (OR 2.7 [1.52-4.8]), cognitive (OR 2.3 [1.67-3.15]), hearing (OR 2.44 [1.42-4.2]), and visual impairment (OR 5.42 [2.77-10.58]). Children with WMI had much higher risks of cerebral palsy (OR 14.91 [7.3-30.46]), motor (OR 5.3 [3-9.36]), and cognitive impairment (OR 3.48 [2.18-5.53]).

LIMITATIONS

Heterogeneity of outcome data.

CONCLUSIONS

Mild IVH, severe IVH, and WMI are associated with adverse neurodevelopmental outcomes. Utilization of core outcome sets and availability of open-access study data would improve our understanding of the nuances of these outcomes.

The clinical utility of dysregulated microRNA expression in paediatric solid tumours

MicroRNAs (miRNAs) are short, non-protein-coding genes that regulate the expression of numerous protein-coding genes. Their expression is dysregulated in cancer, where they may function as oncogenes or tumour suppressor genes. As miRNAs are highly resistant to degradation, they are ideal biomarker candidates to improve the diagnosis and clinical management of cancer, including prognostication. Furthermore, miRNAs dysregulated in malignancy represent potential therapeutic targets. The use of miRNAs for these purposes is a particularly attractive option to explore for paediatric malignancies, where the mutational burden is typically low, in contrast to cancers affecting adult patients. As childhood cancers are rare, it has taken time to accumulate the necessary body of evidence showing the potential for miRNAs to improve clinical management across this group of tumours. Here, we review the current literature regarding the potential clinical utility of miRNAs in paediatric solid tumours, which is now both timely and justified. Exploring such avenues is warranted to improve the management and outcomes of children affected by cancer.

Evaluation of prognostic risk models for postoperative pulmonary complications in adult patients undergoing major abdominal surgery: a systematic review and international external validation cohort study

BACKGROUND

Stratifying risk of postoperative pulmonary complications after major abdominal surgery allows clinicians to modify risk through targeted interventions and enhanced monitoring. In this study, we aimed to identify and validate prognostic models against a new consensus definition of postoperative pulmonary complications.

METHODS

We did a systematic review and international external validation cohort study. The systematic review was done in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. We searched MEDLINE and Embase on March 1, 2020, for articles published in English that reported on risk prediction models for postoperative pulmonary complications following abdominal surgery. External validation of existing models was done within a prospective international cohort study of adult patients (≥18 years) undergoing major abdominal surgery. Data were collected between Jan 1, 2019, and April 30, 2019, in the UK, Ireland, and Australia. Discriminative ability and prognostic accuracy summary statistics were compared between models for the 30-day postoperative pulmonary complication rate as defined by the Standardised Endpoints in Perioperative Medicine Core Outcome Measures in Perioperative and Anaesthetic Care (StEP-COMPAC). Model performance was compared using the area under the receiver operating characteristic curve (AUROCC).

FINDINGS

In total, we identified 2903 records from our literature search; of which, 2514 (86·6%) unique records were screened, 121 (4·8%) of 2514 full texts were assessed for eligibility, and 29 unique prognostic models were identified. Nine (31·0%) of 29 models had score development reported only, 19 (65·5%) had undergone internal validation, and only four (13·8%) had been externally validated. Data to validate six eligible models were collected in the international external validation cohort study. Data from 11 591 patients were available, with an overall postoperative pulmonary complication rate of 7·8% (n=903). None of the six models showed good discrimination (defined as AUROCC ≥0·70) for identifying postoperative pulmonary complications, with the Assess Respiratory Risk in Surgical Patients in Catalonia score showing the best discrimination (AUROCC 0·700 [95% CI 0·683-0·717]).

INTERPRETATION

In the pre-COVID-19 pandemic data, variability in the risk of pulmonary complications (StEP-COMPAC definition) following major abdominal surgery was poorly described by existing prognostication tools. To improve surgical safety during the COVID-19 pandemic recovery and beyond, novel risk stratification tools are required.

FUNDING

British Journal of Surgery Society.

Long COVID-19 and Postural Orthostatic Tachycardia Syndrome- Is Dysautonomia to Be Blamed?

While the increased arrhythmic tendency during acute COVID-19 infection is recognised, the long-term cardiac electrophysiological complications are less well known. There are a high number of patients reporting ongoing symptoms post-infection, termed long COVID. A recent hypothesis is that long COVID symptoms could be attributed to dysautonomia, defined as malfunction of the autonomic nervous system (ANS). The most prevalent cardiovascular dysautonomia amongst young people is postural orthostatic tachycardia syndrome (POTS). Numerous reports have described the development of POTS as part of long COVID. Possible underlying mechanisms, although not mutually exclusive or exhaustive, include hypovolaemia, neurotropism, inflammation and autoimmunity. Treatment options for POTS and other long COVID symptoms are currently limited. Future research studies should aim to elucidate the underlying mechanisms of dysautonomia to enable the development of targeted therapies. Furthermore, it is important to educate healthcare professionals to recognise complications and conditions arising from COVID-19, such as POTS, to allow prompt diagnosis and access to early treatment.

Gene and protein expression profiles of selected molecular targets mediating electrophysiological function in pgc-1-alpha deficient murine ventricles

Background

The risk of cardiac arrhythmias increases significantly in patients with metabolic disorders such as obesity and diabetes mellitus. The mechanisms linking metabolic conditions and electrophysiological changes underlying cardiac arrhythmias remain poorly understood. Central to the energetic abnormalities characterising these conditions is mitochondrial dysfunction. Peroxisome proliferator activated receptor-γ (PPARγ) coactivator-1 (Pgc-1) regulate mitochondrial biogenesis and function. Their expression is impaired in metabolic disorders. Murine Pgc-1α−/− hearts replicate disrupted mitochondrial function and model the associated pro-arrhythmic electrophysiological abnormalities.

Purpose

To explore the molecular mechanisms underlying the pro-arrhythmic electrophysiological changes in the Pgc-1α−/− murine model of mitochondrial dysfunction.

Methods

Ventricular tissue samples were obtained from aged (&gt;12 months) wild-type (WT) and homozygous Pgc-1α−/− mice. Quantitative PCR was used to examine transcription of 60 genes underlying cardiac tissue excitability, western blotting was used to examine expression of proteins relating to cardiac conduction velocity, and histological analysis was used to examine cardiac tissue fibrotic change.

Results

qPCR analysis implicated downregulation of genes related to Na+-K+ ATPase activity (Atp1b1), surface Ca2+ entry (Cacna1c), action potential repolarisation (Kcnn1), autonomic function (Adra1d, Adcy4, Pde4d, Prkar2a), and morphological properties (Myh6, Tbx3) in murine Pgc-1α−/− ventricles. Western blotting revealed reduced NaV1.5 but normal Cx43 expression. Histological analysis revealed increased tissue fibrosis in the Pgc-1α−/− ventricles.

Conclusions

These results identified molecular mechanisms underlying previously reported electrophysiological abnormalities such as impaired ventricular activation and reduced conduction velocity in arrhythmic substrate associated with Pgc-1 deficiencies. Results also correlated with earlier findings including functional effects of elevated Ca2+ concentrations. The present findings clarify possible mechanisms by which mitochondrial dysfunction affects electrophysiological function and identify potential pharmacological targets for anti-arrhythmic therapy.

Funding Acknowledgement

Type of funding sources: Public Institution(s). Main funding source(s): Medical Research Council; the Wellcome Trust; British Heart Foundation

Sequential Phase II clinical trials evaluating CRLX101 as monotherapy and in combination with bevacizumab in recurrent ovarian cancer

BACKGROUND

Topoisomerase-1 inhibitors are an important class of cytotoxics associated with toxicity that limits their use. CRLX101 is a novel cyclodextrin-containing polymer conjugate of camptothecin (CPT) that self-assembles into nanoparticles to deliver sustained levels of active CPT into cancer cells while substantially reducing systemic exposure.

METHODS

We conducted sequential phase II, open label, single arm clinical trials to evaluate CRLX101 as a single agent (n = 29) and with bevacizumab (Bev) (n = 34). Patients (pts) had measurable recurrent epithelial ovarian, tubal or primary peritoneal cancer, that could be platinum refractory, resistant or sensitive. Cohort A (Single agent CRLX101) allowed up to 3 prior chemotherapy regimens, but no prior topo-1 inhibitors. Pts received CRLX101 15 mg/m² IV every 14 days Q28 with response evaluation every 2 cycles. Cohort B also received Bev 10 mg/kg D1,15 Q28, and included only platinum resistant disease with up to 2 prior lines, and more rigorous eligibility criteria.

RESULTS

CRLX101 was well tolerated other than nausea, fatigue and anemia. 29 pts. received a median of 3 (1-16) cycles with a clinical benefit rate (CBR) of 68% and overall response rate (ORR) of 11%. With the addition of Bev in Cohort B (n = 34), the CBR was increased to 95% and the ORR to 18%. PFS was 4.5 months (0.9 to 15.9 months) in Cohort A and 6.5 months (2.8 to 14.4 months) in Cohort B. Bev increased the incidence of hypertension and qualitatively increased bladder toxicities, but without SAEs.

CONCLUSIONS

CRLX101 meets the clinical need for an effective and tolerable topoisomerase I inhibitor and can be safely combined with bevacizumab.

Molecular basis of ventricular arrhythmogenicity in a Pgc-1α deficient murine model

Mitochondrial dysfunction underlying metabolic disorders such as obesity and diabetes mellitus is strongly associated with cardiac arrhythmias. Murine Pgc-1α-/- hearts replicate disrupted mitochondrial function and model the associated pro-arrhythmic electrophysiological abnormalities. Quantitative PCR, western blotting and histological analysis were used to investigate the molecular basis of the electrophysiological changes associated with mitochondrial dysfunction. qPCR analysis implicated downregulation of genes related to Na+-K+ ATPase activity (Atp1b1), surface Ca2+ entry (Cacna1c), action potential repolarisation (Kcnn1), autonomic function (Adra1d, Adcy4, Pde4d, Prkar2a), and morphological properties (Myh6, Tbx3) in murine Pgc-1α-/- ventricles. Western blotting revealed reduced NaV1.5 but normal Cx43 expression. Histological analysis revealed increased tissue fibrosis in the Pgc-1α-/- ventricles. These present findings identify altered transcription amongst a strategically selected set of genes established as encoding proteins involved in cardiac electrophysiological activation and therefore potentially involved in alterations in ventricular activation and Ca2+ homeostasis in arrhythmic substrate associated with Pgc-1α deficiency. They complement and complete previous studies examining such expression characteristics in the atria and ventricles of Pgc-1 deficient murine hearts.

GLP-1 agonists for obesity and type 2 diabetes in children: Systematic review and meta-analysis

Pharmacological options for management of obesity and type 2 diabetes mellitus (T2DM) in children are limited. We aimed to synthesize published randomized controlled trial (RCT) evidence on the efficacy of glucagon-like peptide-1 (GLP-1) agonists in T2DM, pre-diabetes, and obesity in children aged <18 years. Inclusion criteria were RCTs of any GLP-1 agonist, solely or in conjunction with other drugs, for the treatment of obesity, pre-diabetes, and/or T2DM in children aged <18 years old. Nine studies met the inclusion criteria (two for T2DM, one for pre-diabetes, and six for obesity without diabetes). In total, 286 children were allocated to GLP-1 agonist therapy. Compared with controls, GLP-1 agonist therapy reduced HbA1c by -0.30% (95% confidence interval [CI] -0.57, -0.04) with a larger effect in children with (pre-)diabetes (-0.72%; 95% CI -1.17, -0.28; three studies) than in children with obesity (-0.08%; 95% CI -0.13, -0.02; four studies). Conversely, GLP-1 agonist therapy reduced body weight more in children with obesity (-2.74 kg; 95% CI -3.77, -1.70; six studies) than in children with T2DM (-0.97 kg; 95% CI -2.01, 0.08; two studies). Adverse effects included gastrointestinal symptoms and minor hypoglycemic episodes, but not severe hypoglycemia. GLP-1 agonists are efficacious in treating children with obesity and/or T2DM. Effect sizes are comparable with those reported in adults.

A rare case of paediatric astroblastoma with concomitant MN1-GTSE1 and EWSR1-PATZ1 gene fusions altering management

In a case of astroblastoma, methylation analysis was uninformative, with no clustering with known CNS-HGNET-MN1 cases. Whole genome sequencing however identified a novel MN1-GTSE1 gene fusion (image), confirming the diagnosis of astroblastoma, as well as an EWSR1-PATZ1 gene fusion. Whole genome sequencing, alongside methylation profiling and conventional neuropathology, will continue to lead to improved diagnostics and prognostication for children with brain tumours.

Arrhythmogenic mechanisms of obstructive sleep apnea in heart failure patients

Heart failure (HF) affects 23 million people worldwide and results in 300000 annual deaths. It is associated with many comorbidities, such as obstructive sleep apnea (OSA), and risk factors for both conditions overlap. Eleven percent of HF patients have OSA and 7.7% of OSA patients have left ventricular ejection fraction <50% with arrhythmias being a significant comorbidity in HF and OSA patients. Forty percent of HF patients develop atrial fibrillation (AF) and 30%-50% of deaths from cardiac causes in HF patients are from sudden cardiac death. OSA is prevalent in 32%-49% of patients with AF and there is a dose-dependent relationship between OSA severity and resistance to anti-arrhythmic therapies. HF and OSA lead to various downstream arrhythmogenic mechanisms, including metabolic derangement, remodeling, inflammation, and autonomic imbalance. (1) Metabolic derangement and production of reactive oxidative species increase late Na+ currents, decrease outward K+ currents and downregulate connexin-43 and cell-cell coupling. (2) remodeling also features downregulated K+ currents in addition to decreased Na+/K+ ATPase currents, altered Ca2+ homeostasis, and increased density of If current. (3) Chronic inflammation leads to downregulation of both Nav1.5 channels and K+ channels, altered Ca2+ homeostasis and reduced cellular coupling from alterations of connexin expression. (4) Autonomic imbalance causes arrhythmias by evoking triggered activity through increased Ca2+ transients and reduction of excitation wavefront wavelength. Thus, consideration of these multiple pathophysiological pathways (1-4) will enable the development of novel therapeutic strategies that can be targeted against arrhythmias in the context of complex disease, such as the comorbidities of HF and OSA.

Rare case of paraneoplastic cerebellar degeneration secondary to high-grade serous carcinoma of tubo-ovarian origin

This case describes a 69-year-old woman, who presented with rapidly progressive cerebellar symptoms and unintentional weight loss. Full neurological assessment excluded space-occupying lesions, vascular accidents and infection. Surprisingly, a chest, abdomen and pelvis CT showed a left hemipelvis mass, which was subsequently biopsied. A high-grade serous carcinoma of tubo-ovarian origin was found, diagnosing paraneoplastic cerebellar degeneration (PCD) secondary to this. The exact mechanism is not known, but is thought to be immune-mediated. In cases of PCD, after cancer treatment, the neurological disability stabilises to a severe level and will unfortunately be lifelong. Our patient continues to make great progress with intensive rehabilitation for her ongoing balance issues. Early recognition of PCD can lead to a prompt diagnosis of the underlying malignancy and hence subsequent management. This can at least limit the extent of the neurological disability of the disease and increase the survival rate from cancer.

Atrial Transcriptional Profiles of Molecular Targets Mediating Electrophysiological Function in Aging and Pgc-1β Deficient Murine Hearts

BACKGROUND

Deficiencies in the transcriptional co-activator, peroxisome proliferative activated receptor, gamma, coactivator-1β are implicated in deficient mitochondrial function. The latter accompanies clinical conditions including aging, physical inactivity, obesity, and diabetes. Recent electrophysiological studies reported that Pgc-1β^-/- mice recapitulate clinical age-dependent atrial pro-arrhythmic phenotypes. They implicated impaired chronotropic responses to adrenergic challenge, compromised action potential (AP) generation and conduction despite normal AP recovery timecourses and background resting potentials, altered intracellular Ca²⁺ homeostasis, and fibrotic change in the observed arrhythmogenicity.

OBJECTIVE

We explored the extent to which these age-dependent physiological changes correlated with alterations in gene transcription in murine Pgc-1β^-/- atria.

METHODS AND RESULTS

RNA isolated from murine atrial tissue samples from young (12-16 weeks) and aged (>52 weeks of age), wild type (WT) and Pgc-1β^-/- mice were studied by pre-probed quantitative PCR array cards. We examined genes encoding sixty ion channels and other strategic atrial electrophysiological proteins. Pgc-1β^-/- genotype independently reduced gene transcription underlying Na⁺-K⁺-ATPase, sarcoplasmic reticular Ca²⁺-ATPase, background K⁺ channel and cholinergic receptor function. Age independently decreased Na⁺-K⁺-ATPase and fibrotic markers. Both factors interacted to alter Hcn4 channel activity underlying atrial automaticity. However, neither factor, whether independently or interactively, affected transcription of cardiac Na⁺, voltage-dependent K⁺ channels, surface or intracellular Ca²⁺ channels. Nor were gap junction channels, β-adrenergic receptors or transforming growth factor-β affected.

CONCLUSION

These findings limit the possible roles of gene transcriptional changes in previously reported age-dependent pro-arrhythmic electrophysiologial changes observed in Pgc-1β^-/- atria to an altered Ca²⁺-ATPase (Atp2a2) expression. This directly parallels previously reported arrhythmic mechanism associated with p21-activated kinase type 1 deficiency. This could add to contributions from the direct physiological outcomes of mitochondrial dysfunction, whether through reactive oxygen species (ROS) production or altered Ca²⁺ homeostasis.

Ageing, the autonomic nervous system and arrhythmia: From brain to heart

An ageing myocardium possesses significant electrophysiological alterations that predisposes the elderly patient to arrhythmic risk. Whilst these alterations are intrinsic to the cardiac myocytes, they are modulated by the cardiac autonomic nervous system (ANS) and consequently, ageing of the cardiac ANS is fundamental to the development of arrhythmias. A systems-based approach that incorporates the influence of the cardiac ANS could lead to better mechanistic understanding of how arrhythmogenic triggers and substrates interact spatially and temporally to produce sustained arrhythmia and why its incidence increases with age. Despite the existence of physiological oscillations of ANS activity on the heart, pathological oscillations can lead to defective activation and recovery properties of the myocardium. Such changes can be attributable to the decrease in functionality and structural alterations to ANS specific receptors in the myocardium with age. These altered ANS adaptive responses can occur either as a normal ageing process or accelerated in the presence of specific cardiac pathologies, such as genetic mutations or neurodegenerative conditions. Targeted intervention that seek to manipulate the ageing ANS influence on the myocardium may prove to be an efficacious approach for the management of arrhythmia in the ageing population.

Gene and Protein Expression Profile of Selected Molecular Targets Mediating Electrophysiological Function in Pgc-1α Deficient Murine Atria

Increases in the prevalence of obesity, insulin resistance, and metabolic syndrome has led to the increase of atrial fibrillation (AF) cases in the developed world. These AF risk factors are associated with mitochondrial dysfunction, previously modelled using peroxisome proliferator activated receptor-γ (PPARγ) coactivator-1 (Pgc-1)-deficient murine cardiac models. We explored gene and protein expression profiles of selected molecular targets related to electrophysiological function in murine Pgc-1α^−/− atria. qPCR analysis surveyed genes related to Na⁺-K⁺-ATPase, K⁺ conductance, hyperpolarisation-activated cyclic nucleotide-gated (Hcn), Na⁺ channels, Ca²⁺ channels, and indicators for adrenergic and cholinergic receptor modulation. Western blot analysis for molecular targets specific to conduction velocity (Na_v1.5 channel and gap junctions) was performed. Transcription profiles revealed downregulation of molecules related to Na⁺-K⁺-ATPase transport, Hcn-dependent pacemaker function, Na⁺ channel-dependent action potential activation and propagation, Ca²⁺ current generation, calsequestrin-2 dependent Ca²⁺ homeostasis, and adrenergic α_1D dependent protection from hypertrophic change. Na_v1.5 channel protein expression but not gap junction expression was reduced in Pgc-1α^−/− atria compared to WT. Na_v1.5 reduction reflects corresponding reduction in its gene expression profile. These changes, as well as the underlying Pgc-1α^−/− alteration, suggest potential pharmacological targets directed towards either upstream PGC-1 signalling mechanisms or downstream ion channel changes.

Ventricular pro-arrhythmic phenotype, arrhythmic substrate, ageing and mitochondrial dysfunction in peroxisome proliferator activated receptor-γ coactivator-1β deficient (Pgc-1β-/-) murine hearts

INTRODUCTION

Ageing and age-related bioenergetic conditions including obesity, diabetes mellitus and heart failure constitute clinical ventricular arrhythmic risk factors.

MATERIALS AND METHODS

Pro-arrhythmic properties in electrocardiographic and intracellular recordings were compared in young and aged, peroxisome proliferator-activated receptor-γ coactivator-1β knockout (Pgc-1β-/-) and wild type (WT), Langendorff-perfused murine hearts, during regular and programmed stimulation (PES), comparing results by two-way ANOVA.

RESULTS AND DISCUSSION

Young and aged Pgc-1β-/- showed higher frequencies and durations of arrhythmic episodes through wider PES coupling-interval ranges than WT. Both young and old, regularly-paced, Pgc-1β-/- hearts showed slowed maximum action potential (AP) upstrokes, (dV/dt)max (∼157 vs. 120-130 V s-1), prolonged AP latencies (by ∼20%) and shortened refractory periods (∼58 vs. 51 ms) but similar AP durations (∼50 ms at 90% recovery) compared to WT. However, Pgc-1β-/- genotype and age each influenced extrasystolic AP latencies during PES. Young and aged WT ventricles displayed distinct, but Pgc-1β-/- ventricles displayed similar dependences of AP latency upon (dV/dt)max resembling aged WT. They also independently increased myocardial fibrosis. AP wavelengths combining activation and recovery terms paralleled contrasting arrhythmic incidences in Pgc-1β-/- and WT hearts. Mitochondrial dysfunction thus causes pro-arrhythmic Pgc-1β-/- phenotypes by altering AP conduction through reducing (dV/dt)max and causing age-dependent fibrotic change.

Cardiac electrophysiological adaptations in the equine athlete-Restitution analysis of electrocardiographic features

Exercising horses uniquely accommodate 7–8-fold increases in heart rate (HR). The present experiments for the first time analysed the related adaptations in action potential (AP) restitution properties recorded by in vivo telemetric electrocardiography from Thoroughbred horses. The horses were subjected to a period of acceleration from walk to canter. The QRS durations, and QT and TQ intervals yielded AP conduction velocities, AP durations (APDs) and diastolic intervals respectively. From these, indices of active, λ = QT/(QRS duration), and resting, λ₀ = TQ/(QRS duration), AP wavelengths were calculated. Critical values of QT and TQ intervals, and of λ and λ₀ at which plots of these respective pairs of functions showed unity slope, were obtained. These were reduced by 38.9±2.7% and 86.2±1.8%, and 34.1±3.3% and 85.9±1.2%, relative to their resting values respectively. The changes in λ were attributable to falls in QT interval rather than QRS duration. These findings both suggested large differences between the corresponding critical (129.1±10.8 or 117.4±5.6 bpm respectively) and baseline HRs (32.9±2.1 (n = 7) bpm). These restitution analyses thus separately identified concordant parameters whose adaptations ensure the wide range of HRs over which electrophysiological activation takes place in an absence of heart block or arrhythmias in equine hearts. Since the horse is amenable to this in vivo electrophysiological analysis and displays a unique wide range of heart rates, it could be a novel cardiac electrophysiology animal model for the study of sudden cardiac death in human athletes.

Age-dependent atrial arrhythmic phenotype secondary to mitochondrial dysfunction in Pgc-1β deficient murine hearts

INTRODUCTION

Ageing and several age-related chronic conditions including obesity, insulin resistance and hypertension are associated with mitochondrial dysfunction and represent independent risk factors for atrial fibrillation (AF).

MATERIALS AND METHODS

Atrial arrhythmogenesis was investigated in Langendorff-perfused young (3-4 month) and aged (>12 month), wild type (WT) and peroxisome proliferator activated receptor-γ coactivator-1β deficient (Pgc-1β-/-) murine hearts modeling age-dependent chronic mitochondrial dysfunction during regular pacing and programmed electrical stimulation (PES).

RESULTS AND DISCUSSION

The Pgc-1β-/- genotype was associated with a pro-arrhythmic phenotype progressing with age. Young and aged Pgc-1β-/- hearts showed compromised maximum action potential (AP) depolarization rates, (dV/dt)max, prolonged AP latencies reflecting slowed action potential (AP) conduction, similar effective refractory periods and baseline action potential durations (APD90) but shortened APD90 in APs in response to extrasystolic stimuli at short stimulation intervals. Electrical properties of APs triggering arrhythmia were similar in WT and Pgc-1β-/- hearts. Pgc-1β-/- hearts showed accelerated age-dependent fibrotic change relative to WT, with young Pgc-1β-/- hearts displaying similar fibrotic change as aged WT, and aged Pgc-1β-/- hearts the greatest fibrotic change. Mitochondrial deficits thus result in an arrhythmic substrate, through slowed AP conduction and altered repolarisation characteristics, arising from alterations in electrophysiological properties and accelerated structural change.

Cardiac Potassium Channels: Physiological Insights for Targeted Therapy

The development of novel drugs specifically directed at the ion channels underlying particular features of cardiac action potential (AP) initiation, recovery, and refractoriness would contribute to an optimized approach to antiarrhythmic therapy that minimizes potential cardiac and extracardiac toxicity. Of these, K⁺ channels contribute numerous and diverse currents with specific actions on different phases in the time course of AP repolarization. These features and their site-specific distribution make particular K⁺ channel types attractive therapeutic targets for the development of pharmacological agents attempting antiarrhythmic therapy in conditions such as atrial fibrillation. However, progress in the development of such temporally and spatially selective antiarrhythmic drugs against particular ion channels has been relatively limited, particularly in view of our incomplete understanding of the complex physiological roles and interactions of the various ionic currents. This review summarizes the physiological properties of the main cardiac potassium channels and the way in which they modulate cardiac electrical activity and then critiques a number of available potential antiarrhythmic drugs directed at them.

Ion channels, long QT syndrome and arrhythmogenesis in ageing

Ageing is associated with increased prevalences of both atrial and ventricular arrhythmias, reflecting disruption of the normal sequence of ion channel activation and inactivation generating the propagated cardiac action potential. Experimental models with specific ion channel genetic modifications have helped clarify the interacting functional roles of ion channels and how their dysregulation contributes to arrhythmogenic processes at the cellular and systems level. They have also investigated interactions between these ion channel abnormalities and age-related processes in producing arrhythmic tendency. Previous reviews have explored the relationships between age and loss-of-function Na_v 1.5 mutations in producing arrhythmogenicity. The present review now explores complementary relationships arising from gain-of-function Na_v 1.5 mutations associated with long QT3 (LQTS3). LQTS3 patients show increased risks of life-threatening ventricular arrhythmias, particularly after 40 years of age, consistent with such interactions between the ion channel abnormailities and ageing. In turn clinical evidence suggests that ageing is accompanied by structural, particularly fibrotic, as well as electrophysiological change. These abnormalities may result from biochemical changes producing low-grade inflammation resulting from increased production of reactive oxygen species and superoxide. Experimental studies offer further insights into the underlying mechanisms underlying these phenotypes. Thus, studies in genetically modified murine models for LQTS implicated action potential recovery processes in arrhythmogenesis resulting from functional ion channel abnormalities. In addition, ageing wild type (WT) murine models demonstrated both ion channel alterations and fibrotic changes with ageing. Murine models then suggested evidence for interactions between ageing and ion channel mutations and provided insights into potential arrhythmic mechanisms inviting future exploration.

Sodium channel biophysics, late sodium current and genetic arrhythmic syndromes

Arrhythmias arise from breakdown of orderly action potential (AP) activation, propagation and recovery driven by interactive opening and closing of successive voltage-gated ion channels, in which one or more Na⁺ current components play critical parts. Early peak, Na⁺ currents (I_Na) reflecting channel activation drive the AP upstroke central to cellular activation and its propagation. Sustained late Na⁺ currents (I_Na-L) include contributions from a component with a delayed inactivation timecourse influencing AP duration (APD) and refractoriness, potentially causing pro-arrhythmic phenotypes. The magnitude of I_Na-L can be analysed through overlaps or otherwise in the overall voltage dependences of the steady-state properties and kinetics of activation and inactivation of the Na⁺ conductance. This was useful in analysing repetitive firing associated with paramyotonia congenita in skeletal muscle. Similarly, genetic cardiac Na⁺ channel abnormalities increasing I_Na-L are implicated in triggering phenomena of automaticity, early and delayed afterdepolarisations and arrhythmic substrate. This review illustrates a wide range of situations that may accentuate I_Na-L. These include (1) overlaps between steady-state activation and inactivation increasing window current, (2) kinetic deficiencies in Na⁺ channel inactivation leading to bursting phenomena associated with repetitive channel openings and (3) non-equilibrium gating processes causing channel re-opening due to more rapid recoveries from inactivation. All these biophysical possibilities were identified in a selection of abnormal human SCN5A genotypes. The latter presented as a broad range of clinical arrhythmic phenotypes, for which effective therapeutic intervention would require specific identification and targeting of the diverse electrophysiological abnormalities underlying their increased I_Na-L.