Prognostic value of coronary risk factors, exercise capacity and single photon emission computed tomography in liver transplantation candidates: A 5-year follow-up study.

BACKGROUND: Although consensus-based guidelines support noninvasive stress testing prior to orthotopic liver transplantation (OLT), the optimal screening strategy for assessment of coronary artery disease in patients with end-stage liver disease (ESLD) is unclear. This study sought to determine the relative predictive value of coronary risk factors, functional capacity, and single photon emission computed tomography (SPECT) on major adverse cardiovascular events and all-cause mortality in liver transplantation candidates. METHODS: Prior to listing for transplantation, 404 consecutive ESLD patients were referred to a University hospital for cardiovascular (CV) risk stratification. All subjects met at least one of the following criteria: inability to perform > 4 METs by history (62%), insulin-treated diabetes mellitus (53%), serum creatinine > 1.72 mg/dL (8%), history of MI, PCI or CABG (5%), stable angina (3%), cerebrovascular disease (1%), peripheral vascular disease (1%). Subjects underwent Technetium-99m SPECT with multislice coronary artery calcium scoring (CACS) using exercise treadmill or standard adenosine stress in those unable to achieve 85% maximal heart rate (Siemens Symbia T16). Abnormal perfusion was defined as a summed stress score (SSS) ≥ 4. RESULTS: Of the 404 patients, 158 (age 59 ± 9 years; male 68%) subsequently underwent transplantation and were included in the primary analysis. Of those, 50 (32%) died after a mean duration follow-up of 5.4 years (maximal 10.9 years). Most deaths (78%) were attributed to noncardiovascular causes (malignancy, sepsis, renal failure). Of the 32 subjects with abnormal perfusion (20%), nine (6%) had a high-risk perfusion abnormality defined as a total perfusion defect size (PDS) ≥ 15% and/or an ischemic PDS ≥ 10%. Kaplan-Meier survival curves demonstrated abnormal perfusion was associated with increased CV mortality (generalized Wilcoxon, P = 0.014) but not all-cause death. Subjects with both abnormal perfusion and an inability to exercise > 4 METs had the lowest survival from all-cause death (P = 0.038). Abnormal perfusion was a strong independent predictor of CV death (adjusted HR 4.2; 95% CI 1.4 to 12.3; P = 0.019) and MACE (adjusted HR 7.7; 95% CI 1.4 to 42.4; P = 0.018) in a multivariate Cox regression model that included age, sex, diabetes, smoking and the ability to exercise > 4 METs. There was no association between CACS and the extent of perfusion abnormality, nor with outcomes. CONCLUSIONS: Most deaths following OLT are noncardiovascular. Nonetheless, abnormal perfusion is prevalent in this high-risk population and a stronger predictor of cardiovascular morbidity and mortality than functional status. A combined assessment of functional status and myocardial perfusion identifies those at highest risk of all-cause death. (Exercise Capacity and Single Photon Emission Computed Tomography in Liver Transplantation Candidates [ExSPECT]; ClinicalTrials.gov Identifier: NCT03864497).

Interdependent feedback regulation of breathing by the carotid bodies and the retrotrapezoid nucleus.

The retrotrapezoid nucleus (RTN) regulates breathing in a CO2 - and state-dependent manner. RTN neurons are glutamatergic and innervate principally the respiratory pattern generator; they regulate multiple aspects of breathing, including active expiration, and maintain breathing automaticity during non-REM sleep. RTN neurons encode arterial PCO2 /pH via cell-autonomous and paracrine mechanisms, and via input from other CO2 -responsive neurons. In short, RTN neurons are a pivotal structure for breathing automaticity and arterial PCO2 homeostasis. The carotid bodies stimulate the respiratory pattern generator directly and indirectly by activating RTN via a neuronal projection originating within the solitary tract nucleus. The indirect pathway operates under normo- or hypercapnic conditions; under respiratory alkalosis (e.g. hypoxia) RTN neurons are silent and the excitatory input from the carotid bodies is suppressed. Also, silencing RTN neurons optogenetically quickly triggers a compensatory increase in carotid body activity. Thus, in conscious mammals, breathing is subject to a dual and interdependent feedback regulation by chemoreceptors. Depending on the circumstance, the activity of the carotid bodies and that of RTN vary in the same or the opposite directions, producing additive or countervailing effects on breathing. These interactions are mediated either via changes in blood gases or by brainstem neuronal connections, but their ultimate effect is invariably to minimize arterial PCO2 fluctuations. We discuss the potential relevance of this dual chemoreceptor feedback to cardiorespiratory abnormalities present in diseases in which the carotid bodies are hyperactive at rest, e.g. essential hypertension, obstructive sleep apnoea and heart failure.

Nalcn Is a "Leak" Sodium Channel That Regulates Excitability of Brainstem Chemosensory Neurons and Breathing.

UNLABELLED: The activity of background potassium and sodium channels determines neuronal excitability, but physiological roles for "leak" Na(+) channels in specific mammalian neurons have not been established. Here, we show that a leak Na(+) channel, Nalcn, is expressed in the CO2/H(+)-sensitive neurons of the mouse retrotrapezoid nucleus (RTN) that regulate breathing. In RTN neurons, Nalcn expression correlated with higher action potential discharge over a more alkalized range of activity; shRNA-mediated depletion of Nalcn hyperpolarized RTN neurons, and reduced leak Na(+) current and firing rate. Nalcn depletion also decreased RTN neuron activation by the neuropeptide, substance P, without affecting pH-sensitive background K(+) currents or activation by a cotransmitter, serotonin. In vivo, RTN-specific knockdown of Nalcn reduced CO2-evoked neuronal activation and breathing; hypoxic hyperventilation was unchanged. Thus, Nalcn regulates RTN neuronal excitability and stimulation by CO2, independent of direct pH sensing, potentially contributing to respiratory effects of Nalcn mutations; transmitter modulation of Nalcn may underlie state-dependent changes in breathing and respiratory chemosensitivity. SIGNIFICANCE STATEMENT: Breathing is an essential, enduring rhythmic motor activity orchestrated by dedicated brainstem circuits that require tonic excitatory drive for their persistent function. A major source of drive is from a group of CO2/H(+)-sensitive neurons in the retrotrapezoid nucleus (RTN), whose ongoing activity is critical for breathing. The ionic mechanisms that support spontaneous activity of RTN neurons are unknown. We show here that Nalcn, a unique channel that generates "leak" sodium currents, regulates excitability and neuromodulation of RTN neurons and CO2-stimulated breathing. Thus, this work defines a specific function for this enigmatic channel in an important physiological context.

Proton detection and breathing regulation by the retrotrapezoid nucleus.

We discuss recent evidence which suggests that the principal central respiratory chemoreceptors are located within the retrotrapezoid nucleus (RTN) and that RTN neurons are directly sensitive to [H(+) ]. RTN neurons are glutamatergic. In vitro, their activation by [H(+) ] requires expression of a proton-activated G protein-coupled receptor (GPR4) and a proton-modulated potassium channel (TASK-2) whose transcripts are undetectable in astrocytes and the rest of the lower brainstem respiratory network. The pH response of RTN neurons is modulated by surrounding astrocytes but genetic deletion of RTN neurons or deletion of both GPR4 and TASK-2 virtually eliminates the central respiratory chemoreflex. Thus, although this reflex is regulated by innumerable brain pathways, it seems to operate predominantly by modulating the discharge rate of RTN neurons, and the activation of RTN neurons by hypercapnia may ultimately derive from their intrinsic pH sensitivity. RTN neurons increase lung ventilation by stimulating multiple aspects of breathing simultaneously. They stimulate breathing about equally during quiet wake and non-rapid eye movement (REM) sleep, and to a lesser degree during REM sleep. The activity of RTN neurons is regulated by inhibitory feedback and by excitatory inputs, notably from the carotid bodies. The latter input operates during normo- or hypercapnia but fails to activate RTN neurons under hypocapnic conditions. RTN inhibition probably limits the degree of hyperventilation produced by hypocapnic hypoxia. RTN neurons are also activated by inputs from serotonergic neurons and hypothalamic neurons. The absence of RTN neurons probably underlies the sleep apnoea and lack of chemoreflex that characterize congenital central hypoventilation syndrome.

The retrotrapezoid nucleus stimulates breathing by releasing glutamate in adult conscious mice.

The retrotrapezoid nucleus (RTN) is a bilateral cluster of neurons located at the ventral surface of the brainstem below the facial nucleus. The RTN is activated by hypercapnia and stabilises arterial Pco2 by adjusting lung ventilation in a feedback manner. RTN neurons contain vesicular glutamate transporter-2 (Vglut2) transcripts (Slc17a6), and their synaptic boutons are Vglut2-immunoreactive. Here, we used optogenetics to test whether the RTN increases ventilation in conscious adult mice by releasing glutamate. Neurons located below the facial motor nucleus were transduced unilaterally to express channelrhodopsin-2 (ChR2)-enhanced yellow fluorescent protein, with lentiviral vectors that employ the Phox2b-activated artificial promoter PRSx8. The targeted population consisted of two types of Phox2b-expressing neuron: non-catecholaminergic neurons (putative RTN chemoreceptors) and catecholaminergic (C1) neurons. Opto-activation of a mix of ChR2-expressing RTN and C1 neurons produced a powerful stimulus frequency-dependent (5-15 Hz) stimulation of breathing in control conscious mice. Respiratory stimulation was comparable in mice in which dopamine-ß-hydroxylase (DßH)-positive neurons no longer expressed Vglut2 (DßH(C) (re/0);;Vglut2(fl/fl)). In a third group of mice, i.e. DßH(+/+);;Vglut2(fl/fl) mice, we injected a mixture of PRSx8-Cre lentiviral vector and Cre-dependent ChR2 adeno-associated virus 2 unilaterally into the RTN; this procedure deleted Vglut2 from ChR2-expressing neurons regardless of whether or not they were catecholaminergic. The ventilatory response elicited by photostimulation of ChR2-positive neurons was almost completely absent in these mice. Resting ventilatory parameters were identical in the three groups of mice, and their brains contained similar numbers of ChR2-positive catecholaminergic and non-catecholaminergic neurons. From these results, we conclude that RTN neurons increase breathing in conscious adult mice by releasing glutamate.

Monosynaptic glutamatergic activation of locus coeruleus and other lower brainstem noradrenergic neurons by the C1 cells in mice.

The C1 neurons, located in the rostral ventrolateral medulla (VLM), are activated by pain, hypotension, hypoglycemia, hypoxia, and infection, as well as by psychological stress. Prior work has highlighted the ability of these neurons to increase sympathetic tone, hence peripheral catecholamine release, probably via their direct excitatory projections to sympathetic preganglionic neurons. In this study, we use channelrhodopsin-2 (ChR2) optogenetics to test whether the C1 cells are also capable of broadly activating the brain's noradrenergic system. We selectively expressed ChR2(H134R) in rostral VLM catecholaminergic neurons by injecting Cre-dependent adeno-associated viral vectors into the brain of adult dopamine-ß-hydroxylase (DßH)(Cre/0) mice. Most ChR2-expressing VLM neurons (75%) were immunoreactive for phenylethanolamine N-methyl transferease, thus were C1 cells, and most of the ChR2-positive axonal varicosities were immunoreactive for vesicular glutamate transporter-2 (78%). We produced light microscopic evidence that the axons of rostral VLM (RVLM) catecholaminergic neurons contact locus coeruleus, A1, and A2 noradrenergic neurons, and ultrastructural evidence that these contacts represent asymmetric synapses. Using optogenetics in tissue slices, we show that RVLM catecholaminergic neurons activate the locus coeruleus as well as A1 and A2 noradrenergic neurons monosynaptically by releasing glutamate. In conclusion, activation of RVLM catecholaminergic neurons, predominantly C1 cells, by somatic or psychological stresses has the potential to increase the firing of both peripheral and central noradrenergic neurons.

Direct excitation of parvalbumin-positive interneurons by M1 muscarinic acetylcholine receptors: roles in cellular excitability, inhibitory transmission and cognition.

Parvalbumin-containing (PV) neurons, a major class of GABAergic interneurons, are essential circuit elements of learning networks. As levels of acetylcholine rise during active learning tasks, PV neurons become increasingly engaged in network dynamics. Conversely, impairment of either cholinergic or PV interneuron function induces learning deficits. Here, we examined PV interneurons in hippocampus (HC) and prefrontal cortex (PFC) and their modulation by muscarinic acetylcholine receptors (mAChRs). HC PV cells, visualized by crossing PV-CRE mice with Rosa26YFP mice, were anatomically identified as basket cells and PV bistratified cells in the stratum pyramidale; in stratum oriens, HC PV cells were electrophysiologically distinct from somatostatin-containing cells. With glutamatergic transmission pharmacologically blocked, mAChR activation enhanced PV cell excitability in both CA1 HC and PFC; however, CA1 HC PV cells exhibited a stronger postsynaptic depolarization than PFC PV cells. To delete M1 mAChRs genetically from PV interneurons, we created PV-M1 knockout mice by crossing PV-CRE and floxed M1 mice. The elimination of M1 mAChRs from PV cells diminished M1 mAChR immunoreactivity and muscarinic excitation of HC PV cells. Selective cholinergic activation of HC PV interneurons using Designer Receptors Exclusively Activated by Designer Drugs technology enhanced the frequency and amplitude of inhibitory synaptic currents in CA1 pyramidal cells. Finally, relative to wild-type controls, PV-M1 knockout mice exhibited impaired novel object recognition and, to a lesser extent, impaired spatial working memory, but reference memory remained intact. Therefore, the direct activation of M1 mAChRs on PV cells contributes to some forms of learning and memory.

Vesicular glutamate transporter 2 is required for the respiratory and parasympathetic activation produced by optogenetic stimulation of catecholaminergic neurons in the rostral ventrolateral medulla of mice in vivo.

Catecholaminergic neurons of the rostral ventrolateral medulla (RVLM-CA neurons; C1 neurons) contribute to the sympathetic, parasympathetic and neuroendocrine responses elicited by physical stressors such as hypotension, hypoxia, hypoglycemia, and infection. Most RVLM-CA neurons express vesicular glutamate transporter (VGLUT)2, and may use glutamate as a ionotropic transmitter, but the importance of this mode of transmission in vivo is uncertain. To address this question, we genetically deleted VGLUT2 from dopamine-ß-hydroxylase-expressing neurons in mice [DßH(Cre/0) ;VGLUT2(flox/flox) mice (cKO mice)]. We compared the in vivo effects of selectively stimulating RVLM-CA neurons in cKO vs. control mice (DßH(Cre/0) ), using channelrhodopsin-2 (ChR2-mCherry) optogenetics. ChR2-mCherry was expressed by similar numbers of rostral ventrolateral medulla (RVLM) neurons in each strain (~400 neurons), with identical selectivity for catecholaminergic neurons (90-99% colocalisation with tyrosine hydroxylase). RVLM-CA neurons had similar morphology and axonal projections in DßH(Cre/0) and cKO mice. Under urethane anesthesia, photostimulation produced a similar pattern of activation of presumptive ChR2-positive RVLM-CA neurons in DßH(Cre/0) and cKO mice. Photostimulation in conscious mice produced frequency-dependent respiratory activation in DßH(Cre/0) mice but no effect in cKO mice. Similarly, photostimulation under urethane anesthesia strongly activated efferent vagal nerve activity in DßH(Cre/0) mice only. Vagal responses were unaffected by α1 -adrenoreceptor blockade. In conclusion, two responses evoked by RVLM-CA neuron stimulation in vivo require the expression of VGLUT2 by these neurons, suggesting that the acute autonomic responses driven by RVLM-CA neurons are mediated by glutamate.

Non-traumatic thoracic aortic emergencies: imaging diagnosis and management.

The major component of non-traumatic thoracic aortic emergencies is the acute aortic syndromes. These include acute aortic dissection, intramural haematoma and penetrating atherosclerotic ulcer, grouped together because they are indistinguishable clinically and highly fatal. All three entities involve disruption to the tunica intima and media and may be complicated by rupture, end-organ ischaemia or aneurysmal transformation. Early diagnosis is vital to allow timely and appropriate management. Paired unenhanced and electrocardiogram-gated computed tomography angiography of the chest, extending more distally if required, is recommended for diagnosis. Specific computed tomography features of all three entities are reviewed, with a focus on morphological features associated with complications. Those with type A pathology are usually managed with open surgery because this has a high risk of complication. Patients with uncomplicated type B pathology are usually managed with best medical therapy whereas those with complicated type B pathology are usually offered either surgery or thoracic endovascular aortic repair. The limited evidence regarding the use of thoracic endovascular aortic repair in patients with subacute uncomplicated type B pathology is briefly discussed.

Results of Serial Myocardial Perfusion Imaging in End-Stage Renal Disease.

For patients awaiting renal transplantation, there is guideline consensus on the need for ischemia testing but no agreement on the frequency of repeat testing. Moreover, there are no data in this population evaluating changes in ischemia assessed with serial myocardial perfusion imaging. Consecutive patients (n = 649) with end-stage renal disease (ESRD) were referred for cardiovascular risk stratification before renal transplantation between 2007 and 2013. Of these, 151 patients (54 ± 9 years) underwent 2 stress-rest technetium-99m single-photon emission computed tomographic (SPECT) studies with CT attenuation correction in accordance with regional guidelines, which recommend repeat imaging in high-risk subjects who have not undergone renal transplantation within 3 years. An abnormal perfusion result was defined as a summed stress score ≥4. The median interval between imaging was 39 months. At baseline, 28% of patients (42/151) had abnormal SPECT perfusion, half with a fixed defect. Nine subjects (6%) underwent revascularization between SPECT studies after the baseline imaging demonstrated an ischemic perfusion defect size affecting ≥10% of the myocardium. On repeat imaging, 60% (25/42) had abnormal perfusion. In the 72% (109 of 151) with normal baseline SPECT perfusion, 19% (21/109) demonstrated new ischemia at follow-up and 3% (3/109) had an ischemic perfusion defect size ≥10%. The development of new-onset ischemia was associated with systolic hypertension (p = 0.015), serum phosphate (p = 0.043), and Agatston score (p = 0.002), but not diabetes (p = 0.12). In conclusion, there is a high frequency of new-onset ischemia in patients with ESRD awaiting renal transplantation. Further study is needed to define the optimal timing for repeat stress testing.

Prognostic Utility of Calcium Scoring as an Adjunct to Stress Myocardial Perfusion Scintigraphy in End-Stage Renal Disease.

Coronary artery calcium score (CACS) is a strong predictor of adverse cardiovascular events in the general population. Recent data confirm the prognostic utility of single-photon emission computed tomographic (SPECT) imaging in end-stage renal disease, but whether performing CACS as part of hybrid imaging improves risk prediction in this population is unclear. Consecutive patients (n = 284) were identified after referral to a university hospital for cardiovascular risk stratification in assessment for renal transplantation. Participants underwent technetium-99m SPECT imaging after exercise or standard adenosine stress in those unable to achieve 85% maximal heart rate; multislice CACS was also performed (Siemens Symbia T16, Siemens, Erlangen, Germany). Subjects with known coronary artery disease (n = 88) and those who underwent early revascularization (n = 2) were excluded. The primary outcome was a composite of death or first myocardial infarction. An abnormal SPECT perfusion result was seen in 22% (43 of 194) of subjects, whereas 45% (87 of 194) had at least moderate CACS (>100 U). The frequency of abnormal perfusion (summed stress score ≥4) increased with increasing CACS severity (p = 0.049). There were a total of 15 events (8 deaths, and 7 myocardial infarctions) after a median duration of 18 months (maximum follow-up 3.4 years). Univariate analysis showed diabetes mellitus (Hazard ratio [HR] 3.30, 95% CI 1.14 to 9.54; p = 0.028), abnormal perfusion on SPECT (HR 5.32, 95% CI 1.84 to 15.35; p = 0.002), and moderate-to-severe CACS (HR 3.55, 95% CI 1.11 to 11.35; p = 0.032) were all associated with the primary outcome. In a multivariate model, abnormal perfusion on SPECT (HR 4.18, 95% CI 1.43 to 12.27; p = 0.009), but not moderate-to-severe CACS (HR 2.50, 95% CI 0.76 to 8.20; p = 0.130), independently predicted all-cause death or myocardial infarction. The prognostic value of CACS was not incremental to clinical and SPECT perfusion data (global chi-square change = 2.52, p = 0.112). In conclusion, a perfusion defect on SPECT is an independent predictor of adverse outcome in potential renal transplant candidates regardless of the CACS. The use of CACS as an adjunct to SPECT perfusion data does not provide incremental prognostic utility for the prediction of mortality and nonfatal myocardial infarction in end-stage renal disease.

Measuring Aggregation of Events about a Mass Using Spatial Point Pattern Methods.

We present a methodology that detects event aggregation about a mass surface using 3-dimensional study regions with a point pattern and a mass present. The Aggregation about a Mass function determines aggregation, randomness, or repulsion of events with respect to the mass surface. Our method closely resembles Ripley's K function but is modified to discern the pattern about the mass surface. We briefly state the definition and derivation of Ripley's K function and explain how the Aggregation about a Mass function is different. We develop the novel function according to the definition: the Aggregation about a Mass function times the intensity is the expected number of events within a distance h of a mass. Special consideration of edge effects is taken in order to make the function invariant to the location of the mass within the study region. Significance of aggregation or repulsion is determined using simulation envelopes. A simulation study is performed to inform researchers how the Aggregation about a Mass function performs under different types of aggregation. Finally, we apply the Aggregation about a Mass function to neuroscience as a novel analysis tool by examining the spatial pattern of neurotransmitter release sites as events about a neuron.

Noninvasive imaging for the diagnosis and prognosis of pulmonary hypertension.

Pulmonary hypertension (PH) is a major cause of morbidity and mortality, but it often presents with nonspecific symptoms, thereby delaying diagnosis and treatment. While invasive cardiac catheterization is essential to confirm the diagnosis, patients with breathlessness are commonly encountered in clinical practice creating a demand for noninvasive screening methods. Preliminary investigations such as the electrocardiogram and chest radiograph lack sensitivity even in advanced cases. Echocardiography is used to screen patients; however, over-reliance on a single estimation of pulmonary artery systolic pressure is unwise, instead multiple parameters should be assessed. Once a diagnosis of PH is made, radionuclide imaging should be performed to exclude chronic thromboembolic disease, and computed tomography is vital for eliminating parenchymal lung disease as a potential etiology. Currently, the primary contribution of cardiac MRI is the accurate assessment of right ventricular size and function. In this respect, cardiac MRI may be supportive during diagnosis of PH, but the main importance of this is in defining prognosis although new outcome variables are anticipated.

Incidental cor triatriatum discovered on multidetector computed tomography.

A 78-year-old woman presented with increasing dyspnea on walking, occasional chest pain, and a single episode of presyncope. An echocardiogram showed a calcified aortic valve with severe aortic stenosis. A computed tomography scan showed a functionally bicuspid aortic valve and moderate calcification of the leaflets. An incidental fine linear membrane was seen traversing the left atrium, dividing the chamber consistent with cor triatriatum. The imaging appearance of cor triatriatum is characteristic and can be easily differentiated from supravalvular mitral ring and intraatrial thrombus. In adult patients, it usually has a large fenestration and is of no significance. In children, the fenestration may be small or absent, and surgical treatment may be required.