scholarly journals A random cycle length approach for assessment of myocardial contraction in isolated rabbit myocardium

2009 ◽  
Vol 297 (5) ◽  
pp. H1940-H1948 ◽  
Author(s):  
Kenneth D. Varian ◽  
Ying Xu ◽  
Carlos A. A. Torres ◽  
Michelle M. Monasky ◽  
Paul M. L. Janssen
Keyword(s):  
Steady State ◽  
Cycle Length ◽  
Relaxation Times ◽  
Myocardial Contraction ◽  
Cardiac Contraction ◽  
Force Frequency ◽  
Cycle Lengths ◽  
Length Changes ◽  
Primary Cycle ◽  
The Impact

It is well known that the strength of cardiac contraction is dependent on the cycle length, evidenced by the force-frequency relationship (FFR) and the existence of postrest potentiation (PRP). Because the contractile strength of the steady-state FFR and force-interval relationship involve instant intrinsic responses to cycle length as well as slower acting components such as posttranslational modification-based mechanisms, it remains unclear how cycle length intrinsically affects cardiac contraction and relaxation. To dissect the impact of cycle length changes from slower acting signaling components associated with persisting changes in cycle length, we developed a novel technique/protocol to study cycle length-dependent effects on cardiac function; twitch contractions of right ventricular rabbit trabeculae at different cycle lengths were randomized around a steady-state frequency. Patterns of cycle lengths that resulted in changes in force and/or relaxation times can now be identified and analyzed. Using this novel protocol, taking under 10 min to complete, we found that the duration of the cycle length before a twitch contraction (“primary” cycle length) positively correlated with force. In sharp contrast, the cycle length one (“secondary”) or two (“tertiary”) beats before the analyzed twitch correlated negatively with force. Using this protocol, we can quantify the intrinsic effect of cycle length on contractile strength while avoiding rundown and lengthiness that are often complications of FFR and PRP assessments. The data show that the history of up to three cycle lengths before a contraction influences myocardial contractility and that primary cycle length affects cardiac twitch dynamics in the opposite direction from secondary/tertiary cycle lengths.

scholarly journals Effect of twitch interval duration on the contractile function of subsequent twitches in isolated rat, rabbit, and dog myocardium under physiological conditions

2011 ◽  
Vol 111 (4) ◽  
pp. 1159-1167 ◽  
Author(s):  
Ying Xu ◽  
Michelle M. Monasky ◽  
Nitisha Hiranandani ◽  
Kaylan M. Haizlip ◽  
George E. Billman ◽  
...  
Keyword(s):  
Cycle Length ◽  
Contractile Function ◽  
Stimulation Frequency ◽  
Mediated Effects ◽  
Relative Contribution ◽  
Intracellular Ca ◽  
State Frequency ◽  
Cycle Lengths ◽  
Primary Cycle ◽  
Contractility Of The Myocardium

Many studies have shown that a change in stimulation frequency leads to altered contractility of the myocardium. However, it remains unclear what changes occur directly after a change in frequency and which ones are a result of the slow processes that lead to the altered homeostasis, which develops after a change in stimulation frequency. To distinguish the immediate from the slow responses, we assessed contractile function in two species that have distinctively different calcium (Ca2+)-handling properties using a recently developed, randomized pacing protocol. In isolated dog and rat right ventricular trabeculae, twitch contractions at five different cycle lengths within the physiologic range of each species were randomized around a steady-state frequency. We found, in both species, that the duration of the cycle length just prior to the analyzed twitch (primary) positively correlated with the increased force of the analyzed twitch. In sharp contrast, the cycle lengths, one and two more removed from the analyzed twitch (“secondary” and “tertiary”), displayed a negative correlation with force of the analyzed twitch. In additional experiments, assessment of intracellular Ca2+ transients in rabbit trabeculae revealed that diastolic Ca2+ levels were closely correlated to contractile function outcome. The relative contribution of the primary cycle length was different between dog (51%) and rat (71%), whereas in neither species was a significant effect on relaxation time observed. With the use of randomized cycle lengths, we have distinguished the intrinsic response from the signaling-mediated effects of frequency-dependent activation on myofilament properties and Ca2+ handling.

Impact of Phase Sequence on Cycle Length Resonance

2019 ◽  
Vol 2673 (11) ◽  
pp. 398-408
Author(s):  
Christopher M. Day ◽  
A. M. Tahsin Emtenan
Keyword(s):  
Cycle Length ◽  
Strong Impact ◽  
Optimization Strategy ◽  
Sequence Optimization ◽  
Substantial Impact ◽  
Phase Sequence ◽  
Cycle Lengths ◽  
Potential Strategy ◽  
The Impact ◽  
Selection Of

The concept of resonant cycle length, that there are certain cycle lengths that may provide excellent progression owing to corridor geometry and other factors, has some currency as a potential strategy for cycle length selection. Past studies have identified resonant cycles under certain conditions and demonstrated benefits from use of the strategy as a means of selecting cycle length. The present study revisited the concept in application to flow-based models of traffic signal performance, highlighting the impact of phase sequence optimization. The phenomenon of cycle length resonance was explored for corridors with equal and randomly generated spacing between intersections, and finally for a field-calibrated corridor. Under each scenario, the performance of different cycle lengths was explored under two optimization strategies: optimization of only offsets, and optimization of both offsets and phase sequence. It was found that phase sequence has a substantial impact on the performance of coordination. Optimized phase sequences were found to yield 8% to 14% improvement in performance compared with use of the default sequence. For corridors where a resonant cycle length was evident, when phase sequences could also be adjusted, the poorer performance of non-resonant cycle lengths could be mitigated by optimizing phase sequence. Although use of a resonant cycle length is likely to yield good performance for some corridors under appropriate conditions, the use of a phase sequence optimization strategy is likely to have a strong impact on most corridors, and could be more impactful than selection of a resonant cycle length.

Regional adaptations of rabbit diaphragm muscle fibers to unilateral denervation

1995 ◽  
Vol 79 (3) ◽  
pp. 941-950 ◽  
Author(s):  
W. Z. Zhan ◽  
G. A. Farkas ◽  
M. A. Schroeder ◽  
L. E. Gosselin ◽  
G. C. Sieck
Keyword(s):  
Relaxation Times ◽  
Diaphragm Muscle ◽  
Type I ◽  
Force Frequency ◽  
Tetanic Force ◽  
Before And After ◽  
Length Changes ◽  
Main Determinants ◽  
Type I Fibers ◽  
Minimal Stress

We hypothesized that adaptations of the rabbit diaphragm (Dia) after unilateral denervation (DNV) result from removal of a neural influence rather than from passive stress. Length changes of midcostal and sternal Dia regions were measured before and after DNV by using sonomicrometry. Midcostal fibers passively lengthened after DNV, whereas sternal fibers shortened. In both regions, these length changes were associated with minimal stress, as estimated from passive force-length relationships. Morphological and contractile adaptions of midcostal and sternal Dia regions were examined after 1 and 4 wk of DNV. In both Dia regions, type I fibers progressively hypertrophied, whereas type IIb fibers atrophied. After DNV, changes in isometric contraction were similar in both Dia regions. Twitch contraction and half-relaxation times increased, force-frequency relationships shifted leftward, and maximum tetanic force decreased. We conclude that passive length changes and mechanical stress are not the main determinants of the morphological and contractile adaptations of the Dia after unilateral DNV but that these adaptations result from DNV itself.

Development of Pedestrian Recall Versus Actuation Guidelines for Pedestrian Crossings at Signalized Intersections

2021 ◽  
pp. 036119812110028
Author(s):  
Burak Cesme ◽  
Peter G. Furth ◽  
Ryan Casburn ◽  
Kevin Lee
Keyword(s):  
Cycle Length ◽  
Operational Efficiency ◽  
Signalized Intersections ◽  
Fairfax County ◽  
Time Duration ◽  
Real Network ◽  
Moderate Reduction ◽  
Cycle Lengths ◽  
Pedestrian Crossing ◽  
The Impact

At signalized intersections, pedestrian phases can be configured as recall or pushbutton actuated. While pedestrian recall results in a moderate reduction in pedestrian delay because, with recall, a pedestrian arriving during the time nominally reserved for the Walk interval will be served immediately rather than waiting to be served in the next cycle, it can also lead to longer cycle lengths, increasing delay for all users, including pedestrians. This research explores the impact of pedestrian recall along a coordinated-actuated arterial for pedestrians crossing the mainline (i.e., crossing the coordinated phase) to provide pedestrian recall versus actuation guidelines for agencies. The guidance was developed with the aim of balancing pedestrian delay with operational efficiency for vehicles. Two criteria were considered while developing the guidance: (1) pedestrian demand; and (2) vehicular green time duration for the concurrent vehicle phase that is parallel to the pedestrian crossing. VISSIM microsimulation software was used on a real network in Fairfax County, Virginia to model the effects of pedestrian recall and actuation. Results showed that pedestrian recall should be considered when pedestrian demand is large enough that there is a pedestrian call in most cycles (pedestrian probability in a given cycle is greater than 0.6 or pedestrian volume per cycle is greater than 0.9). The guidance also suggests setting pedestrian phases on recall when the length of the vehicular green for the concurrent phase is long enough in most cycles that a pedestrian phase would fit without constraining the signal cycle length.

Slow rate during AF improves ventricular performance by reducing sensitivity to cycle length irregularity

2002 ◽  
Vol 283 (6) ◽  
pp. H2706-H2713 ◽  
Author(s):  
Zoran B. Popović ◽  
Kent A. Mowrey ◽  
Youhua Zhang ◽  
Shaowei Zhuang ◽  
Tomotsugu Tabata ◽  
...  
Keyword(s):  
Cycle Length ◽  
Left Ventricular ◽  
Ventricular Performance ◽  
Nonlinear Fitting ◽  
Beneficial Effects ◽  
Sinus Cycle Length ◽  
Cycle Lengths ◽  
Anesthetized Dogs ◽  
Heart Rate Slowing ◽  
The Impact

Atrial fibrillation (AF) is characterized by short and irregular ventricular cycle lengths (VCL). While the beneficial effects of heart rate slowing (i.e., the prolongation of VCL) in AF are well recognized, little is known about the impact of irregularity. In 10 anesthetized dogs, R-R intervals, left ventricular (LV) pressure, and aortic flow were collected for >500 beats during fast AF and when the average VCL was prolonged to 75%, 100%, and 125% of the intrinsic sinus cycle length by selective atrioventricular (AV) nodal vagal stimulation. We used the ratio of the preceding and prepreceding R-R intervals (RRp/RRpp) as an index of cycle length irregularity and assessed its effects on the maximum LV power, the minimum of the first derivative of LV pressure, and the time constant of relaxation by using nonlinear fitting with monoexponential functions. During prolongation of VCL, there was a pronounced decrease in curvature with the formation of a plateau, indicating a lesser dependence on RRp/RRpp. We conclude that prolongation of the VCL during AF reduces the sensitivity of the LV performance parameters to irregularity.

Isoproterenol antagonizes drug-induced prolongation of action potential duration in humans

1993 ◽  
Vol 71 (10-11) ◽  
pp. 755-760 ◽  
Author(s):  
David Newman ◽  
Paul Dorian ◽  
Randi Feder-Elituv
Keyword(s):  
Steady State ◽  
Action Potential ◽  
Action Potential Duration ◽  
Refractory Period ◽  
Cycle Length ◽  
Monophasic Action Potential ◽  
Class Iii ◽  
State Action ◽  
Cycle Lengths ◽  
Prolongation Of Action Potential

The effects of an isoproterenol infusion on the duration of the human right ventricular endocardial monophasic action potential at 90% repolarization were recorded in the absence and in the presence of an antiarrhythmic drag regimen containing class III effects in two similar groups of patients. The drugs used were amiodarone (N = 3, 300 ± 50 mg), sotalol plus quinidine (N = 11, 156 ± 13 mg sotalol, 1688 ± 594 mg quinidine), and sotalol alone (N = 3, 300 ± 20 mg). All patients had underlying coronary disease but no evidence of inducible ischemia. In the absence of antiarrhythmic drug, isoproterenol did not significantly change the relationship of action potential duration at 90% repolarization to cycle length; there was a linear decrease in action potential duration by 19.8% between a paced cycle length of 600 and 300 ms. Isoproterenol did not significantly shorten the action potential duration at any cycle length. However, isoproterenol decreased the ventricular effective refractory period at 400 ms drive from 240 ± 5.0 to 225 ± 6.0 ms (p < 0.05) accompanied by no change in the ratio of refractory period to steady-state action potential duration. In the presence of class III drug effects, the action potential duration was increased by an average of 9.2% at all paced cycle lengths longer than 300 ms (p < 0.05). The ventricular refractory period was increased from 240 ± 5 to 269 ± 9.0 ms (p < 0.05 compared with baseline) with a concomitant increase in the ratio of refractory period to action potential duration from 96 ± 2 to 103 ± 2% (p < 0.05 compared with baseline). With infusion of isoproterenol in the presence of a class III containing regimen, the drug-prolonged action potential duration was shortened (p < 0.05) by an average of 8.1% at all cycle lengths longer than 300 ms. These results suggest that isoproterenol simulation of enhanced sympathetic tone can antagonize drug prolongation of action potential duration and that in the absence of drugs, the effects of isoproterenol on the steady-state action potential duration are modest. The clinical utility of class III agents may be augmented by the addition of concomitant β-blockade.Key words: action potential duration, antiarrhythmic drugs, isoproterenol.

scholarly journals Advanced Constitutive Modeling of the Thixotropic Elasto-Visco-Plastic Behavior of Blood: Steady-State Blood Flow in Microtubes

Materials ◽  
2021 ◽  
Vol 14 (2) ◽  
pp. 367
Author(s):  
Konstantinos Giannokostas ◽  
Yannis Dimakopoulos ◽  
Andreas Anayiotos ◽  
John Tsamopoulos
Keyword(s):  
Blood Flow ◽  
Steady State ◽  
Constitutive Model ◽  
Blood Plasma ◽  
Constitutive Modeling ◽  
Flow Direction ◽  
Momentum Balance ◽  
Plastic Behavior ◽  
Flow Investigation ◽  
The Impact

The present work focuses on the in-silico investigation of the steady-state blood flow in straight microtubes, incorporating advanced constitutive modeling for human blood and blood plasma. The blood constitutive model accounts for the interplay between thixotropy and elasto-visco-plasticity via a scalar variable that describes the level of the local blood structure at any instance. The constitutive model is enhanced by the non-Newtonian modeling of the plasma phase, which features bulk viscoelasticity. Incorporating microcirculation phenomena such as the cell-free layer (CFL) formation or the Fåhraeus and the Fåhraeus-Lindqvist effects is an indispensable part of the blood flow investigation. The coupling between them and the momentum balance is achieved through correlations based on experimental observations. Notably, we propose a new simplified form for the dependence of the apparent viscosity on the hematocrit that predicts the CFL thickness correctly. Our investigation focuses on the impact of the microtube diameter and the pressure-gradient on velocity profiles, normal and shear viscoelastic stresses, and thixotropic properties. We demonstrate the microstructural configuration of blood in steady-state conditions, revealing that blood is highly aggregated in narrow tubes, promoting a flat velocity profile. Additionally, the proper accounting of the CFL thickness shows that for narrow microtubes, the reduction of discharged hematocrit is significant, which in some cases is up to 70%. At high pressure-gradients, the plasmatic proteins in both regions are extended in the flow direction, developing large axial normal stresses, which are more significant in the core region. We also provide normal stress predictions at both the blood/plasma interface (INS) and the tube wall (WNS), which are difficult to measure experimentally. Both decrease with the tube radius; however, they exhibit significant differences in magnitude and type of variation. INS varies linearly from 4.5 to 2 Pa, while WNS exhibits an exponential decrease taking values from 50 mPa to zero.

Impact of Turning Lane Storage Length and Turning Proportions on Throughput at Oversaturated Signalized Intersections

2021 ◽  
pp. 036119812110171
Author(s):  
A. M. Tahsin Emtenan ◽  
Christopher M. Day
Keyword(s):  
Flow Rate ◽  
Cycle Length ◽  
Signal Timing ◽  
Real World Data ◽  
Left Turn ◽  
Common Response ◽  
Cycle Lengths ◽  
Saturation Flow Rate ◽  
Saturation Flow ◽  
Timing Strategies

During oversaturated conditions, common objectives of signal timing are to maximize vehicle throughput and manage queues. A common response to increases in vehicle volumes is to increase the cycle length. Because the clearance intervals are displayed less frequently with longer cycle lengths and fewer cycles, more of the total time is used for green indications, which implies that the signal timing is more efficient. However, previous studies have shown that throughput reaches a peak at a moderate cycle length and extending the cycle length beyond this actually decreases the total throughput. Part of the reason for this is that spillback caused by the turning traffic may cause starvation of the through lanes resulting in a reduction of the saturation flow rate within each lane. Gaps created by the turning traffic after a lane change may also reduce the saturation flow rate. There is a relationship between the proportions of turning traffic, the storage length of turning lanes, and the total throughput that can be achieved on an approach for a given cycle length and green time. This study seeks to explore this relationship to yield better signal timing strategies for oversaturated operations. A microsimulation model of an oversaturated left-turn movement with varying storage lengths and turning proportions is used to determine these relationships and establish a mathematical model of throughput as a function of the duration of green, storage length, and turning proportion. The model outcomes are compared against real-world data.

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