Cmr42

All cardiac ¹³C spectra were analyzed using the AMARES algorithm in the jMRUI software package.30 Figure 1D shows example spectra summed over 30 seconds of acquisition in normoxic animals, acutely hypoxic animals and animals housed in hypoxia for 1 and 3 weeks, showing cardiometabolic conversion of the injected hyperpolarized pyruvate into the downstream products, lactate, alanine and bicarbonate. Spectra were DC offset‐corrected based on the last half of acquired points. The peak areas of [1‐¹³C] pyruvate, [1‐¹³C] lactate, [1‐¹³C] alanine and [¹³C] bicarbonate at each timepoint were quantified and used as input data for a kinetic model based on that developed by Zierhut et al31 and Atherton et al.32 PDH flux was quantified as the rate of ¹³C label transfer from pyruvate to bicarbonate. The rate of ¹³C label transfer from pyruvate to lactate and alanine was used as a marker of lactate dehydrogenase activity and alanine aminotransferase activity, respectively. CINE images were analyzed using cmr42 software (Circle Cardiovascular Imaging, Calgary, Canada) by an experienced analyst blinded to experimental group.

After BSPM, MRI markers were applied to replace all torso electrodes. These markers were used to locate the electrode positions on the MRI images, thereby minimizing the systematic error in the inverse procedure. Before entering the MRI room, pacing thresholds, P- and R-wave amplitude, and lead impedance were determined and the pacemaker system was programmed into MRI SureScan® mode [8 ].
In order to control lung volume, breath holding was practiced with the patient prior to the examination. A stack of ECG-triggered, T2-weighted, bright blood images (slice thickness, 6 mm) was acquired to record the anatomy of the thorax and register the position of the RV lead tip and torso electrodes. Subsequently, cardiac function was assessed using steady state free precession (SSFP) short-axis, three- and four-chamber cines (slice thickness, 6 mm, temporal resolution <50 ms). All images were obtained during breath-hold on a Siemens Aera 1.5 Tesla MRI scanner (Siemens Healthcare, Erlangen, Germany). Cardiac function was analysed using CMR42® software (Circle Cardiovascular Imaging, Calgary, Alberta, Canada).
After the examination, pacing thresholds, P- and R-wave amplitude and lead impedance were determined and compared to the initial values. Finally, original programming of the pacemaker was restored.

CMR of the volunteers was performed using either a Magnetom Aera 1.5T (Siemens) or Ingenia 3T (Phillips) scanner under previously described settings [23 (link)]. Parameters such as cardiac volumes and mass were analyzed from imaging data using the CMR42 software (Circle Cardiovascular Imaging) and standardized protocols [23 (link)]. Four cardiac parameters were considered in this study: LVM, LVEDV, RVEDV, and AoF, with the first three being indexed to BSA according to the Dubois formula [47 ]. When performing multiple linear regression, adjustment was made for age, gender, and SBP. In the case of AoF, additional adjustment was made for weight and height. Only cardiac parameters with values within 2 SDs from their mean value were included for analysis. Numbers of data points analyzed for each cardiac parameter are as follows: LVM (n = 202), LVEDV (n = 216), RVEDV (n = 126), and AoF (n = 203). For logistic regression analysis of abnormally high BSA-indexed LVM against volunteer physical activity, only those of Chinese ethnicity were considered (n = 192). Cutoffs for defining abnormal BSA-indexed LVM were obtained from a study of 180 healthy Singaporeans (70 g/m² for males, 50 g/m² for females) [23 (link)].