Heart rate variability (HRV) refers to variations in the time intervals between two consecutive heart beats and serves as a diagnostic and prognostic tool for cardiac as well as non-cardiac diseases, such as heart failure, aging, Parkinson's, diabetes, and sepsis. While HRV is typically investigated by determining time intervals from electrode measurements, scientists at the Medical University of Graz in Austria recently took the unprecedented step of using high-speed video captured with a Mikrotron MotionBlitz® LTR Long Time Recording System to study the temporal variability of beating heart cells.
"In this study, we utilized video recordings to investigate the variability of intervals as well as mechanical contraction strengths and relative contraction strengths with nonlinear analyses," said Helmut Ahammer, the Medical University of Graz professor who headed the study. "Additionally, the video setup allowed us simultaneous electrode registrations of extracellular potentials."
Professor Ahammer's team evaluated beat to beat interval and contraction strength variabilities with the camera. Each image recorded contractions of the tissue as changes in average gray values. Simultaneous measurements of extracellular potentials using a cardiac-near-field electrode validated beat to beat intervals of the video recordings.
For the study, an experimental chamber containing the intact tissue was mounted on the stage of an Olympus upright microscope with tissue superfused with oxygenated standard external solution kept at a constant temperature of 23°C. Recordings were started 20 minutes after the onset of superfusion in order to allow the tissue to establish and maintain a stable beating rhythm. Close to the primary pacemaking site of the tissue, a smaller region of interest (ROI) showing distinct contractions was selected for recording. After recording of the first video, acetylcholine was added to the perfusion solution. After a 5 minute superfusion time the second video was recorded.
Video recordings of beating tissue samples were taken by using the Mikrotron MotionBlitz combined with a Mikrotron EoSens CL camera. This system hardware recording unit avoided erroneous jitter effects of software trigger events. The resolution of the camera was set to 1,280 × 1,024 pixels. An ROI with a pixel size of 160 × 160 was selected to get maximal gray level changes during beating of the tissue.
Professor Ahammer commented: "The Mikrotron high-speed video recordings allowed us to extract beat to beat intervals as well as the contraction strengths and the relative contraction strengths, because the average gray value of an image was directly proportional to the mechanical contraction."
For capturing multiple events and analyze changes over a longer period of time, the long time recorders of the Mikrotron MotionBlitz LTR series allow detailed insights into fast processes – not only for a couple of seconds, but for several hours. Long-term processes can be recorded in their entirety at full resolution. The unique combination of high speed and long time recording provides a tremendous amount of data for further analysis. Recorded images are stored directly on a RAID protected drive array, therefore eliminating any download times.
The size of the ROI was optimized by inspections of the temporal signals gained. Larger ROIs yielded very large video files while smaller ROIs yielded too low signal to noise ratios. Regions with thick tissue layers moving into the ROI turned out to yield the highest signal to noise ratios. With a sample rate of 1,000 fps and a recording duration of 5 min, 300,000 single uncompressed images (160 × 160 resolution) = 25,600 pixels, were taken and stored on the MotionBlitz's hard disk in an avi format. One avi file needed 21 GB of memory. The Mikrotron system saved the individual images in RGB format although the EoSens CL camera was a monochrome model. Monochrome cameras usually give higher signal to noise ratios than color cameras which is important for high-speed acquisitions with very small exposure times. Thus, images were converted in a first step to 8 bit, lowering the memory demand to 7.6 GB per video. The setup was tested against electrical and optical inferences coming from ambient light sources such as the laboratory light or the microscope light source itself. Fourier analyses of videos capturing a static scene revealed no residual frequency components of power supply frequencies or other additional noise components.
The Mikrotron video recording technique is a promising tool to investigate beat to beat behavior regarding absolute values of beating rate and contraction strength, as well as their variabilities in autorhythmic tissue.