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Digital Analysis of High Frame Rate 2D Echocardiography Image Loops Provide New Insights into Strain Twist Interaction and Heart Function Muhammad Ashraf, MD; Li Xiong, MD; Zhiwen Zhou, MD; Keith DesRochers; David J Sahn, MD, FAHA Oregon Health & Science University, Portland, OR
Abstract
Background
Methods
Cardiac motion is a dynamic interaction of linear and rotational deformation. Methods; We studied Seven adult pigs that underwent midline sternotomy to allow acquisition of very high frame rate images directly from the heart surface. Left ventricular (LV) pressure volume curves were obtained by Millar conductance catheters and apical short axis views acquired with a 10S probe on a Vivid 7 Dimension ultrasound system at >120 fps. After baseline image acquisition, pigs were subjected to escalating doses of dobutamine and then esmolol. Images for each state were obtained and exported to EchoPac PC for offline analysis using 2D speckle tracking. Results; At baseline, there was a consistent delay of 10-12% in R-R' interval time between peak contraction of the septum and the lateral wall (P<0.05), that was decreased with dobutamine (4-6%) and increased with esmolol (18-22%). Peak rotation coincided with peak strain in lateral wall. At lower doses of dobutamine, the sequential pattern of contraction was preserved with 15-25% increase in strain and 10-15% increase in peak rotation that resulted in 30-40% increase in dP/dt. With higher doses of dobutamine (>50mcg/kg) and esmolol, the sequential pattern of contraction was lost, with 20-25% reduction in rotation and 35-40% reduction in peak dP/dt as compared to baseline. Conclusions; We concluded that coordinated sequential contractile action, with segments moving one after the other like a cascade, is responsible for cardiac rotation which seems to be directly related to systolic function or peak dP/dt.
The dynamic function of the heart is a complex interaction of shortening, thickening and rotational motion. Although there is continuing debate about macroscopic myocardial architecture and its impact on dynamic function of heart, LV twisting motion is believed to be the consequence of oblique myocardial fiber orientation that induces rotation around the long axis during contraction. Since the magnitude of LV twist is determined by contractile force, it is suggested that measurement of LV twist could be implemented as a clinical index of contractility and may also serve as a potential marker of myocardial dysfunction in the diseased heart. This concept is partially supported by earlier experimental work, but none of the methods used in experimental work were either non-invasive , to allow this evaluation without disturbing structural integrity of myocardium, or had high enough temporal resolution to accurately evaluate myocardial motion. With the advent of digital tracking of acoustic tags in scan line echocardiographic image data, computation of rotation and strain (contraction) can be done simultaneously in short axis planes noninvasively using high resolution dynamic images and has been validated against more invasive and high resolution methods. We studied this strain rotation interaction in open chest instrumented pigs subjected to different pharmacological interventions.
We studied seven adult pigs of either sex weighing between 20-25kg. All pigs were intubated, anesthetized and operated for midline sternotomy to expose the heart. Vital signs and oxygen saturation was monitored continuously throughout the procedure while keeping pigs pain free and warm. Millar conductance catheters were introduced from right carotid artery into left ventricular (LV) to obtain pressure-volume curves. Short axis views were acquired with 10 MHz pediatric sector probe just above and below the papillary muscle level on Vivid 7 Dimensions Ultrasound system. Frequency and frame rate was optimized before acquisition to ensure best possible spatiotemporal resolution of 2D images. After acquisition of baseline data, pigs were subjected to pharmacological interventions using escalating doses of dobutamine and then doses of esmolol . Apical and basal short axis 2D views along with pressure-volume data was acquired at each state. Images were exported to EchoPac PC for an offline analysis of strain and rotation dynamics using speckle tracking based method. Hemodynamic data was analyzed to obtain stroke volume and cardiac output at each state.
Results
At baseline, there was a constant delay of 10-12% of R-R' interval time between peak contraction of the septum and the lateral wall (P<0.05), that was decreased with dobutamine (4-6%) and increased with esmolol (18-22%).
Peak apical rotation coincided with peak circumferential strain in lateral wall .
Dobutamine increased the heart rate and shortened the time to peak contraction [circumferential strain] from 31.35% ± 2.65% to 18.90% ± 3.35% of the RR' interval and increased the period of sustained contraction at peak from 16.45% ± 1.60% to 38.90% ± 3.5%. At lower doses of dobutamine, the sequential pattern of contraction was preserved with 15-25% increase in peak strain and 10-15% increase in peak rotation that resulted in 30-40% increase in dP/dt. With higher doses of dobutamine (>50mcg/kg), the sequential pattern of contraction was lost, with 20-25% reduction in rotation and 35-40% reduction in peak systolic dP/dt .
Esmolol decreased heart rate, increased the time to peak strain to 52.90% ± 3.5% and decreased the time of sustained contraction to 8.90% ± 4.65%. The sequential pattern of contraction was also lost, with 20-25% reduction in rotation and 35-40% reduction in peak dP/dt at moderate dose. Further increase in dose decreased heart rate only.
Conclusions
The coordinated sequential contractile action, with segments moving one after the other like a cascade, is responsible for cardiac rotation which seems to be lost with esmolol and higher doses of dobutamine.
Changes in apical rotation seems to be directly related to systolic function or peak dP/dt.
Disclosures
No relationships to disclose:
Muhammad Ashraf Li Xiong
Zhiwen Zhou Keith DesRochers
David J. Sahn