Cardiac lipid levels show diurnal changes and long‐term variations in healthy human subjects

¹H‐MRS is regularly applied to determine lipid content in ectopic tissue – mostly skeletal muscle and liver – to investigate physiological and/or pathologic conditions, e.g. insulin resistance. Technical developments also allow non‐invasive in vivo assessment of cardiac lipids; however, basic data about methodological reliability (repeatability) and physiological variations are scarce. The aim of the presented work was to determine potential diurnal changes of cardiac lipid stores in humans, and to put the results in relation to methodological repeatability and normal physiological day‐to‐day variations. Optimized cardiac‐ and respiratory‐gated ¹H‐MRS was used for non‐invasive quantification of intracardiomyocellular lipids (ICCL), creatine, trimethyl‐ammonium compounds (TMA), and taurine in nine healthy young men at three time points per day on two days separated by one week. This design allowed determination of (a) diurnal changes, (b) physiological variation over one week and (c) methodological repeatability of the ICCL levels. Comparison of fasted morning to post‐absorptive evening measurements revealed a significant 37 ± 19% decrease of ICCL during the day (p = 0.0001). There was a significant linear correlation between ICCL levels in the morning and their decrease during the day (p = 0.015). Methodological repeatability for the ICCL/creatine ratio was excellent, with a coefficient of variance of ~5%, whereas physiological variation was found to be considerably higher (22%) in spite of a standardized physiological preparation protocol. In contrast, TMA levels remained stable over this time period. The proposed ¹H‐MRS technique provides a robust way to investigate relevant physiological changes in cardiac metabolites, in particular ICCL. The present results suggest that ICCL reveal a diurnal course, with higher levels in the morning as compared to evening. In addition, a considerable long‐term variation of ICCL levels, in both the morning and evening, was documented. Given the high methodological repeatability, these effects should be taken into account in studies investigating the metabolic role of ICCL. Copyright © 2014 John Wiley & Sons, Ltd.     

Estimation of the postmortem interval by means of ¹H MRS of decomposing brain tissue: influence of ambient temperature

Standard methods for the estimation of the postmortem interval (PMI, time since death), based on the cooling of the corpse, are limited to about 48 h after death. As an alternative, noninvasive postmortem observation of alterations of brain metabolites by means of (1)H MRS has been suggested for an estimation of the PMI at room temperature, so far without including the effect of other ambient temperatures. In order to study the temperature effect, localized (1)H MRS was used to follow brain decomposition in a sheep brain model at four different temperatures between 4 and 26°C with repeated measurements up to 2100 h postmortem. The simultaneous determination of 25 different biochemical compounds at each measurement allowed the time courses of concentration changes to be followed. A sudden and almost simultaneous change of the concentrations of seven compounds was observed after a time span that decreased exponentially from 700 h at 4°C to 30 h at 26°C ambient temperature. As this represents, most probably, the onset of highly variable bacterial decomposition, and thus defines the upper limit for a reliable PMI estimation, data were analyzed only up to this start of bacterial decomposition. As 13 compounds showed unequivocal, reproducible concentration changes during this period while eight showed a linear increase with a slope that was unambiguously related to ambient temperature. Therefore, a single analytical function with PMI and temperature as variables can describe the time courses of metabolite concentrations. Using the inverse of this function, metabolite concentrations determined from a single MR spectrum can be used, together with known ambient temperatures, to calculate the PMI of a corpse. It is concluded that the effect of ambient temperature can be reliably included in the PMI determination by (1)H MRS.     

Fuel metabolism during exercise in euglycaemia and hyperglycaemia in patients with type 1 diabetes mellitus—a prospective single-blinded randomised crossover trial

AIMS/HYPOTHESIS: We assessed systemic and local muscle fuel metabolism during aerobic exercise in patients with type 1 diabetes at euglycaemia and hyperglycaemia with identical insulin levels. METHODS: This was a single-blinded randomised crossover study at a university diabetes unit in Switzerland. We studied seven physically active men with type 1 diabetes (mean +/- SEM age 33.5 +/- 2.4 years, diabetes duration 20.1 +/- 3.6 years, HbA1c 6.7 +/- 0.2% and peak oxygen uptake [VO2peak] 50.3 +/- 4.5 ml min(-1) kg(-1)). Men were studied twice while cycling for 120 min at 55 to 60% of VO2peak, with a blood glucose level randomly set either at 5 or 11 mmol/l and identical insulinaemia. The participants were blinded to the glycaemic level; allocation concealment was by opaque, sealed envelopes. Magnetic resonance spectroscopy was used to quantify intramyocellular glycogen and lipids before and after exercise. Indirect calorimetry and measurement of stable isotopes and counter-regulatory hormones complemented the assessment of local and systemic fuel metabolism. RESULTS: The contribution of lipid oxidation to overall energy metabolism was higher in euglycaemia than in hyperglycaemia (49.4 +/- 4.8 vs 30.6 +/- 4.2%; p < 0.05). Carbohydrate oxidation accounted for 48.2 +/- 4.7 and 66.6 +/- 4.2% of total energy expenditure in euglycaemia and hyperglycaemia, respectively (p < 0.05). The level of intramyocellular glycogen before exercise was higher in hyperglycaemia than in euglycaemia (3.4 +/- 0.3 vs 2.7 +/- 0.2 arbitrary units [AU]; p < 0.05). Absolute glycogen consumption tended to be higher in hyperglycaemia than in euglycaemia (1.3 +/- 0.3 vs 0.9 +/- 0.1 AU). Cortisol and growth hormone increased more strongly in euglycaemia than in hyperglycaemia (levels at the end of exercise 634 +/- 52 vs 501 +/- 32 nmol/l and 15.5 +/- 4.5 vs 7.4 +/- 2.0 ng/ml, respectively; p < 0.05). CONCLUSIONS/INTERPRETATION: Substrate oxidation in type 1 diabetic patients performing aerobic exercise in euglycaemia is similar to that in healthy individuals revealing a shift towards lipid oxidation during exercise. In hyperglycaemia fuel metabolism in these patients is dominated by carbohydrate oxidation. Intramyocellular glycogen was not spared in hyperglycaemia.     

Inflammation and atrial remodeling after a mountain marathon

Endurance athletes have an increased risk of atrial fibrillation. We performed a longitudinal study on elite runners of the 2010 Jungfrau Marathon, a Swiss mountain marathon, to determine acute effects of long‐distance running on the atrial myocardium. Ten healthy male athletes were included and examined 9 to 1 week prior to the race, immediately after, and 1, 5, and 8 days after the race. Mean age was 34.9 ± 4.2 years, and maximum oxygen consumption was 66.8 ± 5.8 mL/kg*min. Mean race time was 243.9 ± 17.7 min. Electrocardiographic‐determined signal‐averaged P‐wave duration (SAPWD) increased significantly after the race and returned to baseline levels during follow‐up (128.7 ± 10.9 vs. 137.6 ± 9.8 vs. 131.5 ± 8.6 ms; P < 0.001). Left and right atrial volumes showed no significant differences over time, and there were no correlations of atrial volumes and SAPWD. Prolongation of the SAPWD was accompanied by a transient increase in levels of high‐sensitivity C‐reactive protein, proinflammatory cytokines, total leucocytes, neutrophil granulocytes, pro atrial natriuretic peptide and high‐sensitivity troponin. In conclusion, marathon running was associated with a transient conduction delay in the atria, acute inflammation and increased atrial wall tension. This may reflect exercise‐induced atrial myocardial edema and may contribute to atrial remodeling over time, generating a substrate for atrial arrhythmias.     

Morphological and functional 3-Tesla magnetic resonance imaging of saphenous vein coronary artery bypass grafts

Evaluation of a novel non-invasive tool for postoperative follow-up of patients postelective saphenous vein coronary artery bypass graft (CABG) was performed. Ten patients were included. Their bypass grafts supplied the right coronary artery (7), marginal branches (1), diagonal branches (2), and the circumflex artery (n=1). Each bypass was examined intraoperatively using Doppler flow measurement. Patients were examined with a 3-Tesla magnetic resonance imaging (MRI) scanner (MAGNETOM Verio, Siemens, Erlangen, Germany) within one week postsurgery using MR-angiography with an intravasal contrast agent and velocity encoded phase-contrast flow measurements. Intraoperative Doppler flow measurements revealed regular flow patterns in all vascular territories supplied. The median intraoperative flow rate was 50 ml/min with an inter-quartile range (IQR) of 42-70 ml/min. The clinical postoperative course was uneventful. MRI showed all grafts to be patent. The median postoperative flow rate was 50 ml/min (IQR: 32-65 ml/min). MRI flow rates agreed well with intraoperative Doppler flow measurements (mean difference: -2.8±20.1 ml/min). This initial study demonstrates that 3-Tesla MRI flow measurements correlated well with Doppler thus reconfirming the graft patency postCABG. Further refinement and broader application of this technique may facilitate follow-up postCABG potentially replacing empiric clinical judgment by reliable non-invasive imaging.     

Comparison of arterial and venous coronary artery bypass flow measurements using 3-T magnetic resonance phase contrast imaging

Objective

Comparison of arterial and venous coronary artery bypass flow measurements using 3-T magnetic resonance (MR) phase contrast in correlation with intraoperative Doppler flow measurements.

Methods

Fifty-six coronary bypasses (right coronary artery n = 18, left internal mammary artery to left anterior descending artery n = 16, marginal artery n = 7, circumflex artery n = 7, diagonal artery n = 6, left anterior descending artery n = 1, and right internal mammary artery to right coronary artery n = 1) were studied in 27 asymptomatic patients. In this prospective study, each bypass was studied intra-operatively using Doppler flow measurement. Within one week post surgery, patients were studied using a 3-T MR scanner (Magnetom Verio, Siemens, Erlangen, Germany) using velocity encoded phase-contrast flow measurements.

Results

Intraoperative Doppler flow measurements demonstrated regular flow patterns in all vascular territories supplied. All bypasses were patent on MRI and flow measurement results were as follows: median flow 60 ml/min (interquartile range (IQR): 37.5–78.5 ml/min). For comparison, the corresponding median intraoperative flow was 58 ml/min (IQR: 41–80 ml/min) (p < 0.001; R = 0.44). Linear regression analysis demonstrated a significant correlation for venous bypasses (p = 0.0002; R = 0.48), but not for arterial bypasses (p = 0.09; R = 0.24).

Conclusion

This study demonstrated that MR flow measurements of venous bypass grafts agreed more with Doppler than arterial bypass grafts. However, bypass patency was confirmed for all patients. In the future, this technique may be used for non invasive coronary bypass graft follow-up.     

Standardized protocol for a depletion of intramyocellular lipids (IMCL)

Intramyocellular lipids (IMCL) are flexible fuel stores that are depleted by physical exercise and replenished by fat intake. IMCL or their degradation products are thought to interfere with insulin signaling thereby contributing to insulin resistance. From a practical point of view it is desirable to deplete IMCL prior to replenishing them. So far, it is not clear for how long and at which intensity subjects have to exercise in order to deplete IMCL. We therefore aimed at developing a standardized exercise protocol that is applicable to subjects over a broad range of exercise capacity and insulin sensitivity and allows measuring reliably reduced IMCL levels. Twelve male subjects, including four diabetes type 2 patients, with wide ranges of exercise capacity (VO₂peak per total body weight 27.9–55.8 ml*kg^?1*min^?1), insulin sensitivity (glucose infusion rate per lean body mass 4.7–15.3 mg*min^?1*kg^?1), and BMI (21.7–31.5 kg*m^?2), respectively, were enrolled. Using ¹H magnetic resonance spectroscopy (¹H‐MRS), IMCL was measured in m.tibialis anterior and m.vastus intermedius before and during a depletion protocol of a week, consisting of a moderate additional physical activity (1 h daily at 60% VO₂peak) and modest low‐fat (10–15%) diet. Absolute IMCL‐levels were significantly reduced in both muscles during the first 3 days and stayed constant for the next 3 days of an identical diet/exercise‐scheme. These reduced IMCL levels were independent of insulin sensitivity, yet a tendency to lower depleted IMCL levels has been observed in subjects with higher VO₂peak. The proposed protocol is feasible in subjects with large differences in exercise capacity, insulin sensitivity, and BMI, leading to reduced IMCL levels that neither depend on the exact duration of the depletion protocol nor on insulin sensitivity. This allows for a standardized preparation of IMCL levels either for correlation with other physiological parameters or for replenishment studies. Copyright © 2010 John Wiley & Sons, Ltd.     

Statistical evaluation of time-dependent metabolite concentrations: estimation of post-mortem intervals based on in situ 1H-MRS of the brain

Knowledge of the time interval from death (post-mortem interval, PMI) has an enormous legal, criminological and psychological impact. Aiming to find an objective method for the determination of PMIs in forensic medicine, 1H-MR spectroscopy (1H-MRS) was used in a sheep head model to follow changes in brain metabolite concentrations after death. Following the characterization of newly observed metabolites (Ith et al., Magn. Reson. Med. 2002; 5: 915-920), the full set of acquired spectra was analyzed statistically to provide a quantitative estimation of PMIs with their respective confidence limits. In a first step, analytical mathematical functions are proposed to describe the time courses of 10 metabolites in the decomposing brain up to 3 weeks post-mortem. Subsequently, the inverted functions are used to predict PMIs based on the measured metabolite concentrations. Individual PMIs calculated from five different metabolites are then pooled, being weighted by their inverse variances. The predicted PMIs from all individual examinations in the sheep model are compared with known true times. In addition, four human cases with forensically estimated PMIs are compared with predictions based on single in situ MRS measurements. Interpretation of the individual sheep examinations gave a good correlation up to 250 h post-mortem, demonstrating that the predicted PMIs are consistent with the data used to generate the model. Comparison of the estimated PMIs with the forensically determined PMIs in the four human cases shows an adequate correlation. Current PMI estimations based on forensic methods typically suffer from uncertainties in the order of days to weeks without mathematically defined confidence information. In turn, a single 1H-MRS measurement of brain tissue in situ results in PMIs with defined and favorable confidence intervals in the range of hours, thus offering a quantitative and objective method for the determination of PMIs.