MOLLI and AIR T1 mapping pulse sequences yield different myocardial T1 and ECV measurements

Both post‐contrast myocardial T₁ and extracellular volume (ECV) have been reported to be associated with diffuse interstitial fibrosis. Recently, the cardiovascular magnetic resonance (CMR) field is recognizing that post‐contrast myocardial T₁ is sensitive to several confounders and migrating towards ECV as a measure of collagen volume fraction. Several recent studies using widely available Modified Look‐Locker Inversion‐recovery (MOLLI) have reported ECV cutoff values to distinguish between normal and diseased myocardium. It is unclear if these cutoff values are translatable to different T₁ mapping pulse sequences such as arrhythmia‐insensitive‐rapid (AIR) cardiac T₁ mapping, which was recently developed to rapidly image patients with cardiac rhythm disorders. We sought to evaluate, in well‐controlled canine and pig experiments, the relative accuracy and precision, as well as intra‐ and inter‐observer variability in data analysis, of ECV measured with AIR as compared with MOLLI. In 16 dogs, as expected, the mean T₁ was significantly different (p < 0.001) between MOLLI (891 ± 373 ms) and AIR (1071 ± 503 ms), but, surprisingly, the mean ECV between MOLLI (21.8 ± 2.1%) and AIR (19.6 ± 2.4%) was also significantly different (p < 0.001). Both intra‐ and inter‐observer agreements in T₁ calculations were higher for MOLLI than AIR, but intra‐ and inter‐observer agreements in ECV calculations were similar between MOLLI and AIR. In six pigs, the coefficient of repeatability (CR), as defined by the Bland–Altman analysis, in T₁ calculation was considerably lower for MOLLI (32.5 ms) than AIR (82.3 ms), and the CR in ECV calculation was also lower for MOLLI (1.8%) than AIR (4.5%). In conclusion, this study shows that MOLLI and AIR yield significantly different T₁ and ECV values in large animals and that MOLLI yields higher precision than AIR. Findings from this study suggest that CMR researchers must consider the specific pulse sequence when translating published ECV cutoff values into their own studies. Copyright © 2014 John Wiley & Sons, Ltd.     

Post‐contrast myocardial T1 and ECV disagree in a longitudinal canine study

Both post‐contrast myocardial T₁ and extracellular volume (ECV) measurements have been associated with diffuse interstitial fibrosis. The cardiovascular magnetic resonance (CMR) field is migrating towards ECV, because it is largely insensitive to confounders that affect post‐contrast myocardial T₁. Despite the theoretical advantages of myocardial ECV over post‐contrast myocardial T₁, systematic experimental studies comparing the two measurements are largely lacking. We sought to measure the temporal changes in post‐contrast myocardial T₁ and ECV in an established canine model with chronic atrial fibrillation. Seventeen mongrel dogs, implanted with a pacemaker to induce chronic atrial fibrillation via rapid atrial pacing, were scanned multiple times for a total of 46 CMR scans at 3T. These dogs with different disease durations (0–22 months) were part of a separate longitudinal study aimed at studying the relationship between AF and pathophysiology. In each animal, we measured native and post‐contrast T₁ values and hematocrit. Temporal changes in post‐contrast myocardial T₁ and ECV, as well as other CMR parameters, were modeled with linear mixed effect models to account for repeated measurements over disease duration. In 17 animals, post‐contrast myocardial T₁ decreased significantly from 872 to 698 ms (p < 0.001), which corresponds to a 24.9% relative reduction. In contrast, ECV increased from 21.0 to 22.0% (p = 0.38), which corresponds to only a 4.5% relative increase. To partially investigate this discrepancy, we quantified collagen volume fraction (CVF) in post‐mortem heart tissues of six canines sacrificed at different disease durations (0–22 months). CVF quantified by histology increased from 0.9 to 1.9% (p = 0.56), which agrees better with ECV than with post‐contrast myocardial T₁. This study shows that post‐contrast myocardial T₁ and ECV may disagree in a longitudinal canine study. A more comprehensive study, including histologic, cardiac, and renal functional analyses, is warranted to test rigorously which CMR parameter (ECV or post‐contrast myocardial T₁) agrees better with CVF. Copyright © 2014 John Wiley & Sons, Ltd.     

Dynamic alteration in myocardium creatine during acute infarction using MR CEST imaging

Creatine (Cr) is an essential metabolite in the creatine kinase reaction, which plays a critical role in maintaining normal cardiac function. Chemical exchange saturation transfer (CEST) MRI offers a novel way to map myocardium Cr. This study aims to investigate the dynamic alteration in myocardium Cr during acute infarction using CEST MRI, which may facilitate understanding of the heart remodeling mechanism at the molecular level. Seven adult Bama pigs underwent cardiac cine, Cr CEST, and late gadolinium-enhanced (LGE) T₁-weighted (T₁w) imaging three and 14 days after myocardial infarction induction on a 3 T scanner. Cardiac structural and functional indices, including myocardium mass (MM), end-diastolic volume (EDV), end-systolic volume (ESV), stroke volume (SV), and ejection fraction (EF), were measured from cines. Infarct angle was determined from LGE T₁w images, based on which myocardium was classified into infarct, adjacent, and remote regions. Cr-weighted CEST signal was quantified from a three-pool Lorentzian fitting model and measured within each region and the entire myocardium. Student's t-test was conducted to evaluate any significant differences in measurements between the two time points. Correlation was assessed with Pearson correlation. P values less than 0.05 were considered statistically significant. Over the studied period, MM, EDV, and ESV did not alter significantly (P > 0.05), whereas significant increases of SV and EF and decrease of infarct angle were observed (P < 0.05). Meanwhile, the Cr-weighted CEST signal elevated significantly on Day 14 compared with Day 3 in the infarct (10.00 ± 1.28% versus 6.91 ± 1.54%, P < 0.01), adjacent (11.17 ± 2.00% versus 8.01 ± 1.58%, P = 0.01), and entire myocardium (11.03 ± 1.36% versus 8.19 ± 1.28%, P < 0.01). Moderate negative correlations were shown between the infarct angle and Cr-weighted CEST signals in the infarct (r = −0.80, P < 0.001), adjacent (r = −0.58, P = 0.03), and entire myocardium (r = −0.76, P < 0.01). In conclusion, the dynamic increase of myocardium Cr during acute infarction may interact with cardiac structural and functional recovery. The study provides supplementary insights into the heart remodeling process from the metabolic viewpoint.     

1H NMR and hyperpolarized 13C NMR assays of pyruvate–lactate: a comparative study

Pyruvate–lactate exchange is mediated by the enzyme lactate dehydrogenase (LDH) and is central to the altered energy metabolism in cancer cells. The measurement of exchange kinetics using hyperpolarized ¹³C NMR has provided a biomarker of response to novel therapeutics. However, the observable signal is restricted to the exchanging hyperpolarized ¹³C pools and the endogenous pools of ¹²C‐labelled metabolites are invisible in these measurements. In this study, we investigated an alternative in vitro ¹H NMR assay, using [3‐¹³C]pyruvate, and compared the measured kinetics with a hyperpolarized ¹³C NMR assay, using [1‐¹³C]pyruvate, under the same conditions in human colorectal carcinoma SW1222 cells. The apparent forward reaction rate constants (k_PL) derived from the two assays showed no significant difference, and both assays had similar reproducibility (k_PL = 0.506 ± 0.054 and k_PL = 0.441 ± 0.090 nmol/s/10⁶ cells; mean ± standard deviation; n = 3); ¹H, ¹³C assays, respectively). The apparent backward reaction rate constant (k_LP) could only be measured with good reproducibility using the ¹H NMR assay (k_LP = 0.376 ± 0.091 nmol/s/10⁶ cells; mean ± standard deviation; n = 3). The ¹H NMR assay has adequate sensitivity to measure real‐time pyruvate–lactate exchange kinetics in vitro, offering a complementary and accessible assay of apparent LDH activity. Copyright © 2013 John Wiley & Sons, Ltd.     

Evaluation of bound and pore water in cortical bone using ultrashort‐TE MRI

Bone water exists in different states with the majority bound to the organic matrix and to mineral, and a smaller fraction in ‘free’ form in the pores of cortical bone. In this study, we aimed to develop and evaluate ultrashort‐TE (UTE) MRI techniques for the assessment of T₂*, T₁ and concentration of collagen‐bound and pore water in cortical bone using a 3‐T clinical whole‐body scanner. UTE MRI, together with an isotope study using tritiated and distilled water (THO–H₂O) exchange, as well as gravimetric analysis, were performed on ten sectioned bovine bone samples. In addition, 32 human cortical bone samples were prepared for comparison between the pore water concentration measured with UTE MRI and the cortical porosity derived from micro‐computed tomography (μCT). A short T₂* of 0.27 ± 0.03 ms and T₁ of 116 ± 6 ms were observed for collagen‐bound water in bovine bone. A longer T₂* of 1.84 ± 0.52 ms and T₁ of 527 ± 28 ms were observed for pore water in bovine bone. UTE MRI measurements showed a pore water concentration of 4.7–5.3% by volume and collagen‐bound water concentration of 15.7–17.9% in bovine bone. THO–H₂O exchange studies showed a pore water concentration of 5.9 ± 0.6% and collagen‐bound water concentration of 18.1 ± 2.1% in bovine bone. Gravimetric analysis showed a pore water concentration of 6.3 ± 0.8% and collagen‐bound water concentration of 19.2 ± 3.6% in bovine bone. A mineral water concentration of 9.5 ± 0.6% was derived in bovine bone with the THO–H₂O exchange study. UTE‐measured pore water concentration is highly correlated (R² = 0.72, p < 0.0001) with μCT porosity in the human cortical bone study. Both bovine and human bone studies suggest that UTE sequences could reliably measure collagen‐bound and pore water concentration in cortical bone using a clinical scanner. Copyright © 2015 John Wiley & Sons, Ltd.     

Relationship between blood and myocardium manganese levels during manganese‐enhanced MRI (MEMRI) with T1 mapping in rats

Manganese ions (Mn²⁺) enter viable myocardial cells via voltage‐gated calcium channels. Because of its shortening of T₁ and its relatively long half‐life in cells, Mn²⁺ can serve as an intracellular molecular contrast agent to study indirect calcium influx into the myocardium. One major concern in using Mn²⁺ is its sensitivity over a limited range of concentrations employing T₁‐weighted images for visualization, which limits its potential in quantitative techniques. Therefore, this study assessed the implementation of a T₁ mapping method for cardiac manganese‐enhanced MRI to enable a quantitative estimate of the influx of Mn²⁺ over a wide range of concentrations in male Sprague‐Dawley rats. This MRI method was used to compare the relationship between T₁ changes in the heart as a function of myocardium and blood Mn²⁺ levels. Results showed a biphasic relationship between ΔR₁ and the total Mn²⁺ infusion dose. Nonlinear relationships were observed between the total Mn²⁺ infusion dose versus blood levels and left ventricular free wall ΔR₁. At low blood levels of Mn²⁺, there was proportionally less cardiac enhancement seen than at higher levels of blood Mn²⁺. We hypothesize that Mn²⁺ blood levels increase as a result of rate‐limiting excretion by the liver and kidneys at these higher Mn²⁺ doses. Copyright © 2010 John Wiley & Sons, Ltd.     

Localized rest and stress human cardiac creatine kinase reaction kinetics at 3 T

Changes in the kinetics of the creatine kinase (CK) shuttle are sensitive markers of cardiac energetics but are typically measured at rest and in the prone position. This study aims to measure CK kinetics during pharmacological stress at 3 T, with measurement in the supine position. A shorter “stressed saturation transfer” (StreST) extension to the triple repetition time saturation transfer (TRiST) method is proposed. We assess scanning in a supine position and validate the MR measurement against biopsy assay of CK activity. We report normal ranges of stress CK forward rate (k_f^CK) for healthy volunteers and obese patients. TRiST measures k_f^CK in 40 min at 3 T. StreST extends the previously developed TRiST to also make a further k_f^CK measurement during <20 min of dobutamine stress. We test our TRiST implementation in skeletal muscle and myocardium in both prone and supine positions. We evaluate StreST in the myocardium of six healthy volunteers and 34 obese subjects. We validated MR‐measured k_f^CK against biopsy assays of CK activity. TRiST k_f^CK values matched literature values in skeletal muscle (k_f^CK = 0.25 ± 0.03 s^?1 vs 0.27 ± 0.03 s^?1) and myocardium when measured in the prone position (0.32 ± 0.15 s^?1), but a significant difference was found for TRiST k_f^CK measured supine (0.24 ± 0.12 s^?1). This difference was because of different respiratory‐ and cardiac‐motion‐induced B₀ changes in the two positions. Using supine TRiST, cardiac k_f^CK values for normal‐weight subjects were 0.15 ± 0.09 s^?1 at rest and 0.17 ± 0.15 s^?1 during stress. For obese subjects, k_f^CK was 0.16 ± 0.07 s^?1 at rest and 0.17 ± 0.10 s^?1 during stress. Rest myocardial k_f^CK and CK activity from LV biopsies of the same subjects correlated (R = 0.43, p = 0.03). We present an independent implementation of TRiST on the Siemens platform using a commercially available coil. Our extended StreST protocol enables cardiac k_f^CK to be measured during dobutamine‐induced stress in the supine position.     

In vivo mouse myocardial 31P MRS using three‐dimensional image‐selected in vivo spectroscopy (3D ISIS): technical considerations and biochemical validations

³¹P MRS provides a unique non‐invasive window into myocardial energy homeostasis. Mouse models of cardiac disease are widely used in preclinical studies, but the application of ³¹P MRS in the in vivo mouse heart has been limited. The small‐sized, fast‐beating mouse heart imposes challenges regarding localized signal acquisition devoid of contamination with signal originating from surrounding tissues. Here, we report the implementation and validation of three‐dimensional image‐selected in vivo spectroscopy (3D ISIS) for localized ³¹P MRS of the in vivo mouse heart at 9.4 T. Cardiac ³¹P MR spectra were acquired in vivo in healthy mice (n = 9) and in transverse aortic constricted (TAC) mice (n = 8) using respiratory‐gated, cardiac‐triggered 3D ISIS. Localization and potential signal contamination were assessed with ³¹P MRS experiments in the anterior myocardial wall, liver, skeletal muscle and blood. For healthy hearts, results were validated against ex vivo biochemical assays. Effects of isoflurane anesthesia were assessed by measuring in vivo hemodynamics and blood gases. The myocardial energy status, assessed via the phosphocreatine (PCr) to adenosine 5′‐triphosphate (ATP) ratio, was approximately 25% lower in TAC mice compared with controls (0.76 ± 0.13 versus 1.00 ± 0.15; P < 0.01). Localization with one‐dimensional (1D) ISIS resulted in two‐fold higher PCr/ATP ratios than measured with 3D ISIS, because of the high PCr levels of chest skeletal muscle that contaminate the 1D ISIS measurements. Ex vivo determinations of the myocardial PCr/ATP ratio (0.94 ± 0.24; n = 8) confirmed the in vivo observations in control mice. Heart rate (497 ± 76 beats/min), mean arterial pressure (90 ± 3.3 mmHg) and blood oxygen saturation (96.2 ± 0.6%) during the experimental conditions of in vivo ³¹P MRS were within the normal physiological range. Our results show that respiratory‐gated, cardiac‐triggered 3D ISIS allows for non‐invasive assessments of in vivo mouse myocardial energy homeostasis with ³¹P MRS under physiological conditions. Copyright © 2015 John Wiley & Sons, Ltd.     

Fast volumetric imaging of bound and pore water in cortical bone using three‐dimensional ultrashort‐TE (UTE) and inversion recovery UTE sequences

We report the three‐dimensional ultrashort‐TE (3D UTE) and adiabatic inversion recovery UTE (IR‐UTE) sequences employing a radial trajectory with conical view ordering for bi‐component T₂* analysis of bound water (T₂*^BW) and pore water (T₂*^PW) in cortical bone. An interleaved dual‐echo 3D UTE acquisition scheme was developed for fast bi‐component analysis of bound and pore water in cortical bone. A 3D IR‐UTE acquisition scheme employing multiple spokes per IR was developed for bound water imaging. Two‐dimensional UTE (2D UTE) and IR‐UTE sequences were employed for comparison. The sequences were applied to bovine bone samples (n = 6) and volunteers (n = 6) using a 3‐T scanner. Bi‐component fitting of 3D UTE images of bovine samples showed a mean T₂*^BW of 0.26 ± 0.04 ms and T₂*^PW of 4.16 ± 0.35 ms, with fractions of 21.5 ± 3.6% and 78.5 ± 3.6%, respectively. The 3D IR‐UTE signal showed a single‐component decay with a mean T₂*^BW of 0.29 ± 0.05 ms, suggesting selective imaging of bound water. Similar results were achieved with the 2D UTE and IR‐UTE sequences. Bi‐component fitting of 3D UTE images of the tibial midshafts of healthy volunteers showed a mean T₂*^BW of 0.32 ± 0.08 ms and T₂*^PW of 5.78 ± 1.24 ms, with fractions of 34.2 ± 7.4% and 65.8 ± 7.4%, respectively. Single‐component fitting of 3D IR‐UTE images showed a mean T₂*^BW of 0.35 ± 0.09 ms. The 3D UTE and 3D IR‐UTE techniques allow fast volumetric mapping of bound and pore water in cortical bone. Copyright © 2016 John Wiley & Sons, Ltd.     

Noninvasive mapping of endothelial dysfunction in myocardial ischemia by magnetic resonance imaging using an albumin‐based contrast agent

Noninvasive preclinical methods for the characterization of myocardial vascular function are crucial to an understanding of the dynamics of ischemic cardiac disease. Ischemic heart disease is associated with myocardial endothelial dysfunction, resulting in leakage of plasma albumin into the extravascular space. These features can be harnessed in a novel noninvasive three‐dimensional magnetic resonance imaging method to measure fractional blood volume (fBV) and vascular permeability (permeability–surface area product, PS) using labeled albumin as a blood pool contrast agent. C57BL/6 mice were imaged before and 3 days after myocardial infarction (MI). Following the quantification of endogenous myocardial R₁, the dynamics of intravenously injected albumin‐based contrast agent, extravasating from permeable myocardial blood vessels, were tracked on short‐axis magnetic resonance images of the entire heart. This study successfully discriminated between infarcted and remote regions at 3 days post‐infarct, based on a reduced fBV and increased PS in the infarcted region. These findings were confirmed using ex vivo fluorescence imaging and histology. We have demonstrated a novel method to quantify blood volume and permeability in the infarcted myocardium, providing an imaging biomarker for the assessment of endothelial dysfunction. This method has the potential to three‐dimensionally visualize subtle changes in myocardial permeability and to track endothelial function for longitudinal cardiac studies determining pathophysiological processes during infarct healing.     

Modulation of vascular contractile responses to α1-and α2-adrenergic and neuropeptide Y receptor stimulation in rats with ischaemic heart failure

In order to evaluate adaptational changes in vascular function in congestive heart failure (CHF), we studied the contractile responses of isolated arterial and venous blood vessels from rats suffering from CHF induced by coronary artery ligature, resulting in a myocardial infarction. The contractile responses of the basilar, femoral and renal arteries and of the iliac vein were examined in relation to adrenergic and neuropeptide Y (NPY) receptor function by the action of the α₁ agonist phenylephrine, the α₂ agonist clonidine and NPY. The contractile force was measured (in mN) and in% of K⁺-induced contraction as well as pD₂ to each agonist. When stimulated by a 60 mM K⁺-buffer solution, the femoral and renal arteries from CHF rats responded with a stronger contraction (E_max; 9.4 ± 0.6 and 9.8 ± 0.6mN) than the corresponding Sham vessels (E_max; 6.2 ± 0.7 and 5.6 ± 0.4 mN respectively, P < 0.001). On the contrary, the iliac vein of CHF responded less to K⁺ than the Sham iliac vein (E_mas 2.5 ± 0.2 and 3.7 ± 0.5 mN, P < 0.01). The CHF iliac vein responded with a weaker contraction when stimulated with phenylephrine (E_max 1.9 ± 0.4 mN) and showed a lower sensitivity (pD₂ 5.6 ± 0.1) than the corresponding sham vessel (E_max 5.7 ± 2.3mN and pD₂ 6.3 ± 0.5, P < 0.05). The CHF renal artery was less sensitive to clonidine (pD₂ 6.4 ± 0.6) than the Sham renal artery (pD₂ 7.2 ± 0.1, P < 0.05). The results indicate differences between CHF and Sham vessel segments according to both contractile capacity induced by K⁺-depolarization and to agonist induced contractile capacity and sensitivity. The differences are not of general nature but vary according to the vascular bed examined.     

Multiparametric CMR imaging of infarct remodeling in a percutaneous reperfused Yucatan mini‐pig model

To further understanding of the temporal evolution and pathophysiology of adverse ventricular remodeling over the first 60 days following a myocardial infarction (MI) in both the infarcted and remote myocardium, we performed multi‐parametric cardiac magnetic resonance (CMR) imaging in a closed‐chest chronic Yucatan mini‐pig model of reperfused MI. Ten animals underwent 90 min left anterior descending artery occlusion and reperfusion. Three animals served as controls. Multiparametric CMR (1.5T) was performed at baseline, Day 2, Day 30 and in four animals on Day 60 after MI. Left ventricular (LV) volumes and infarct size were measured. T₁ and T₂ mapping sequences were performed to measure values in the infarct and remote regions. Remote region collagen fractions were compared between infarcted animals and controls. Procedure success was 80%. The model created large infarcts (28 ± 5% of LV mass on Day 2), which led to significant adverse myocardial remodeling that stabilized beyond 30 days. Native T₁ values did not reliably differentiate remote and infarct regions acutely. There was no evidence of remote fibrosis as indicated by partition coefficient and collagen fraction analyses. The infarct T₂ values remained elevated up to 60 days after MI. Multiparametric CMR in this model showed significant adverse ventricular remodeling 30 days after MI similar to that seen in humans. In addition, this study demonstrated that remote fibrosis is absent and that infarct T₂ signal remains chronically elevated in this model. These findings need to be considered when designing preclinical trials using CMR endpoints.     

Initial investigation of glucose metabolism in mouse brain using enriched 17O‐glucose and dynamic 17O‐MRS

In this initial work, the in vivo degradation of ¹⁷O‐labeled glucose was studied during cellular glycolysis. To monitor cellular glucose metabolism, direct ¹⁷O‐magnetic resonance spectroscopy (MRS) was used in the mouse brain at 9.4 T. Non‐localized spectra were acquired with a custom‐built transmit/receive (Tx/Rx) two‐turn surface coil and a free induction decay (FID) sequence with a short TR of 5.4 ms. The dynamics of labeled oxygen in the anomeric 1‐OH and 6‐CH₂OH groups was detected using a Hankel–Lanczos singular value decomposition (HLSVD) algorithm for water suppression. Time‐resolved ¹⁷O‐MRS (temporal resolution, 42/10.5 s) was performed in 10 anesthetized (1.25% isoflurane) mice after injection of a 2.2 M solution containing 2.5 mg/g body weight of differently labeled ¹⁷O‐glucose dissolved in 0.9% physiological saline. From a pharmacokinetic model fit of the H₂¹⁷O concentration–time course, a mean apparent cerebral metabolic rate of ¹⁷O‐labeled glucose in mouse brain of CMR_Glc = 0.07 ± 0.02 μmol/g/min was extracted, which is of the same order of magnitude as a literature value of 0.26 ± 0.06 μmol/g/min reported by ¹⁸F‐fluorodeoxyglucose (¹⁸F‐FDG) positron emission tomography (PET). In addition, we studied the chemical exchange kinetics of aqueous solutions of ¹⁷O‐labeled glucose at the C1 and C6 positions with dynamic ¹⁷O‐MRS. In conclusion, the results of the exchange and in vivo experiments demonstrate that the C6‐¹⁷OH label in the 6‐CH₂OH group is transformed only glycolytically by the enzyme enolase into the metabolic end‐product H₂¹⁷O, whereas C1‐¹⁷OH ends up in water via direct hydrolysis as well as glycolysis. Therefore, dynamic ¹⁷O‐MRS of highly labeled ¹⁷O‐glucose could provide a valuable non‐radioactive alternative to FDG PET in order to investigate glucose metabolism.     

Ambulatory detection of sleep apnea using a non‐contact biomotion sensor

The high prevalence of obstructive sleep apnea has led to increasing interest in ambulatory diagnosis. The SleepMinder? (SM) is a novel non‐contact device that employs radiofrequency wave technology to assess the breathing pattern, and thereby estimate obstructive sleep apnea severity. We assessed the performance of SleepMinder? in the home diagnosis of obstructive sleep apnea. One‐hundred and twenty‐two subjects were prospectively recruited in two protocols, one from an unselected sleep clinic cohort (n = 67, mean age 51 years) and a second from a hypertension clinic cohort (n = 55, mean age 58 years). All underwent 7 consecutive nights of home monitoring (SM_HOME) with the SleepMinder? as well as inpatient‐attended polysomnography in the sleep clinic cohort or cardiorespiratory polygraphy in the hypertension clinic cohort with simultaneous SleepMinder? recordings (SM_LAB). In the sleep clinic cohort, median SM_HOME apnea–hypopnea index correlated significantly with polysomnography apnea–hypopnea index (r = .68; p < .001), and in the hypertension clinic cohort with polygraphy apnea–hypopnea index (r = .7; p < .001). The median SM_HOME performance against polysomnography in the sleep clinic cohort showed a sensitivity and specificity of 72% and 94% for apnea–hypopnea index ≥ 15. Device performance was inferior in females. In the hypertension clinic cohort, SM_HOME showed a 50% sensitivity and 72% specificity for apnea–hypopnea index ≥ 15. SleepMinder? classified 92% of cases correctly or within one severity class of the polygraphy classification. Night‐to‐night variability in home testing was relatively high, especially at lower apnea–hypopnea index levels. We conclude that the SleepMinder? device provides a useful ambulatory screening tool, especially in a population suspected of obstructive sleep apnea, and is most accurate in moderate–severe obstructive sleep apnea.     

Arrhythmogenic mechanisms in the isolated perfused hypokalaemic murine heart

Aim: Hypokalaemia is associated with a lethal form of ventricular tachycardia (VT), torsade de pointes, through pathophysiological mechanisms requiring clarification. Methods: Left ventricular endocardial and epicardial monophasic action potentials were compared in isolated mouse hearts paced from the right ventricular epicardium perfused with hypokalaemic (3 and 4 mm [K⁺]_o) solutions. Corresponding K⁺ currents were compared in whole‐cell patch‐clamped epicardial and endocardial myocytes. Results: Hypokalaemia prolonged epicardial action potential durations (APD) from mean APD₉₀s of 37.2 ± 1.7 ms (n = 7) to 58.4 ± 4.1 ms (n =7) and 66.7 ± 2.1 ms (n = 11) at 5.2, 4 and 3 mm [K⁺]_o respectively. Endocardial APD₉₀s correspondingly increased from 51.6 ± 1.9 ms (n = 7) to 62.8 ± 2.8 ms (n = 7) and 62.9 ± 5.9 ms (n = 11) giving reductions in endocardial–epicardial differences, ΔAPD₉₀, from 14.4 ± 2.6 to 4.4 ± 5.0 and ?3.4 ± 6.0 ms respectively. Early afterdepolarizations (EADs) occurred in epicardia in three of seven spontaneously beating hearts at 4 mm [K⁺]_o with triggered beats followed by episodes of non‐sustained VT in nine of 11 preparations at 3 mm . Programmed electrical stimulation never induced arrhythmic events in preparations perfused with normokalemic solutions yet induced VT in two of seven and nine of 11 preparations at 4 and 3 mm [K⁺]_o respectively. Early outward K⁺ current correspondingly fell from 73.46 ± 8.45 to 61.16±6.14 pA/pF in isolated epicardial but not endocardial myocytes (n = 9) (3 mm [K⁺]_o). Conclusions: Hypokalaemic mouse hearts recapitulate the clinical arrhythmogenic phenotype, demonstrating EADs and triggered beats that might initiate VT on the one hand and reduced transmural dispersion of repolarization reflected in ΔAPD₉₀ suggesting arrhythmogenic substrate on the other.     

Characterization of creatine guanidinium proton exchange by water‐exchange (WEX) spectroscopy for absolute‐pH CEST imaging in vitro

Chemical exchange saturation transfer (CEST) enables indirect detection of small metabolites in tissue by MR imaging. To optimize and interpret creatine‐CEST imaging we characterized the dependence of the exchange‐rate constant k_sw of creatine guanidinium protons in aqueous creatine solutions as a function of pH and temperature T in vitro. Model solutions in the low pH range (pH = 5–6.4) were measured by means of water‐exchange (WEX)‐filtered ¹H NMR spectroscopy on a 3 T whole‐body MR tomograph. An extension of the Arrhenius equation with effective base‐catalyzed Arrhenius parameters yielded a general expression for k_sw(pH, T). The defining parameters were identified as the effective base‐catalyzed rate constant k_b,eff(298.15 K) = (3.009 ± 0.16) × 10⁹ Hz l/mol and the effective activation energy E_A,b,eff = (32.27 ± 7.43) kJ/mol at a buffer concentration of c_buffer = (1/15) M. As expected, a strong dependence of k_sw on temperature was observed. The extrapolation of the exchange‐rate constant to in vivo conditions (pH = 7.1, T = 37 °C) led to the value of the exchange‐rate constant k_sw = 1499 Hz. With the explicit function k_sw(pH, T) available, absolute‐pH CEST imaging could be realized and experimentally verified in vitro. By means of our calibration method it is possible to adjust the guanidinium proton exchange‐rate constant k_sw to any desired value by preparing creatine model solutions with a specific pH and temperature. Copyright © 2014 John Wiley & Sons, Ltd.     

Minimal effects of nitric oxide on spatial blood flow heterogeneity of the dog heart

 Eleven Beagle dogs were studied to elucidate the possible role of L-arginine-derived nitric oxide on local blood flow distribution in left and right ventricular myocardium. Local blood flow was determined in 256 samples from the left and 64 samples from the right ventricle per heart using the tracer microsphere technique (mean sample mass 319 ± 131 mg). Nitric oxide production was effectively inhibited by intravenous infusion of 20 mg/kg nitro-L-arginine methylester (L-NAME) as evidenced by a shift of the dose/response curve for the effect of intracoronary administration of bradykinin (0.004–4.0 nmol/min) on coronary blood flow. L-NAME enhanced left and right ventricular systolic pressures from 132 ± 18 to 155 ± 15 mm Hg and from 26 ± 3 to 29 ± 3 mm Hg respectively (both P = 0.043). Mean left ventricular blood flow was 1.14 ± 0.38 before and 0.99 ± 0.28 ml min^–1 g^–1 after L-NAME (P = 0.068), while right ventricular blood flow fell from 0.72 ± 0.28 to 0.53 ± 0.20 ml min^–1 g^–1 (P = 0.043). Coronary conductance of left and right ventricular myocardium fell by 31 and 43% respectively (both P = 0.043). The coefficient of variation of left ventricular blood flow was 0.26 ± 0.07 before and 0.29 ± 0.07 after L-NAME (P = 0.068), that of right ventricular blood flow was 0.27 before and after L-NAME. Skewness (0.51) and kurtosis (4.23) of left ventricular blood flow distribution were unchanged after L-NAME, while in the right ventricle skewness decreased from 0.54 to 0.09 (P = 0.043) and kurtosis (3.68) tended to decrease after L-NAME (P = 0.080). The fractal dimension (D = 1.20–1.27) and the corresponding nearest-neighbor correlation coefficient (r _n = 0.37–0.53) of left and right ventricular myocardium remained unchanged after infusion of L-NAME. From these results it is concluded that firstly, local nitric oxide release does not explain the higher perfusion of physiological high flow samples and secondly, that spatial myocardial blood flow coordination is not dependent on nitric oxide. Received: 11 July 1996 / Received after revision: 29 October 1996 / Accepted: 17 December 1996     

Non‐inferiority of microencapsulated mesenchymal stem cells to free cells in cardiac repair after myocardial infarction: A rationale for using paracrine factor(s) instead of cells

A major translational barrier to the use of stem cell (SC)‐based therapy in patients with myocardial infarction (MI) is the lack of a clear understanding of the mechanism(s) underlying the cardioprotective effect of SCs. Numerous paracrine factors from SCs may account for reduction in infarct size, but myocardial salvage associated with transdifferentiation of SCs into vascular cells as well as cardiomyocyte‐like cells may be involved too. In this study, bone marrow‐derived rat mesenchymal SC (MSCs) were microencapsulated in alginate preventing viable cell release while supporting their secretory phenotype. The hypothesis on the key role of paracrine factors from MSCs in their cardioprotective activity was tested by comparison of the effect of encapsulated vs free MSCs in the rat model of MI. Intramyocardial administration of both free and encapsulated MSCs after MI caused reduction in scar size (12.1 ± 6.83 and 14.7 ± 4.26%, respectively, vs 21.7 ± 6.88% in controls, P = 0.015 and P = 0.03 respectively). Scar size was not different in animals treated with free and encapsulated MSC (P = 0.637). These data provide evidence that MSC‐derived growth factors and cytokines are crucial for cardioprotection elicited by MSC. Administration of either free or encapsulated MSCs was not arrhythmogenic in non‐infarcted rats. The consistency of our data with the results of other studies on the major role of MSC secretome components in cardiac protection further support the theory that the use of live, though encapsulated, cells for MI therapy may be replaced with heart‐targeted‐sustained delivery of growth factors/cytokines.     

Caffeine stimulates the reverse mode of Na+/Ca2+ exchanger in ferret ventricular muscle

This study investigated the effect of caffeine on the sarcolemmal mechanisms involved in intracellular calcium control. Ferret cardiac preparations were treated with ryanodine and thapsigargin in order to eliminate the sarcoplasmic reticulum (SR) function. This treatment abolished caffeine contracture irreversibly in normal solution. The perfusion with K‐free medium that blocked the Na⁺–K⁺ pump resulted in a recovery of slow relaxing caffeine contractures similar to Na‐free contractures. The amplitude of caffeine contractures was dependent on the bathing [caffeine]o and [Ca²⁺]_o. Divalent cations Ni²⁺ and Cd²⁺, which have an inhibitory effect on the Na⁺/Ca²⁺ exchanger, produced dose‐dependent inhibition of caffeine responses with apparent Ki of 780 ± 19 and 132 ± 5 μM , respectively. Caffeine also caused dose‐dependent inhibition of Na‐free contractures (Ki=4.62 ± 1.5 mM ), and the reduction or removal of [Na⁺]_o exerted an inhibitory effect on caffeine contractures (Ki=73.5 ± 17.12 mM ). These experiments indicate that the increase in resting tension following exposure to caffeine was mediated by Na⁺/Ca²⁺ exchanger, which represents an additional element of complexity in caffeine action on cardiac muscle.     

Stunning and myocardial contractile autoregulation studied on the isolated isovolumic blood‐perfused dog heart

Aim: To study, for the first time, the effects of stunning on homeometric and heterometric autoregulation. Methods and results: Ischaemia (15 min)/reperfusion (30 min) was induced in the isovolumic blood‐perfused dog heart preparation. Heart rate elevations (n = 9) from 60 to 200 beats min^?1, in steps of 20 beats min^?1, promoted the same inotropic stimulation in control (C) and stunning (S), indicating that ischaemia/reperfusion does not affect the changes in calcium kinetics elicited by the Bowditch effect. Sudden ventricular dilation (VD) (n = 10) evoked an instantaneous increase in developed pressure (Δ₁DP) followed by a continuous slow performance increase (Δ₂DP) in C and S. Δ₁DP (C: 35 ± 2.2 mmHg; S: 27 ± 2.1 mmHg; P = 0.002) and Δ₂DP (C: 20 ± 1.6 mmHg; S: 14 ± 1.3 mmHg; P = 0.002) decreased proportionally, while Δ₂/Δ₁DP (C: 0.57 ± 0.13; S: 0.58 ± 0.14) and slow response time course (T/2) were unchanged (C: 55 ± 6.6 s; S: 57 ± 7.7 s) after ischaemia/reperfusion. The reduction of Δ₁DP can be understood as a decline of the myofilaments calcium responsiveness, the main pathophysiological effect of stunning. The reason for the weakening of Δ₂DP, due to intracellular calcium gain, was not determined but it was supposed that its complete manifestation could be restricted by cyclic adenosine monophosphate (cAMP) myocardial content reduction. As reported by others, Δ₂DP depends on myocardial cAMP, and it has been shown that myocardial cAMP is decreased after ischaemia/reperfusion. Conclusions: Contractile depression due to stunning has no effect on the inotropic stimulation generated by the Bowditch phenomenon. Immediate and time‐dependent enhancements of contraction evoked by sudden VD are proportionally reduced and the slow response time course is unaffected in the stunned myocardium.