Pacing Conditions Research Articles

Computing and visualizing electric potentials and current pathways in the thorax

The long-term goal of electrocardiography is to relate electric potentials on the body surface with activities in the heart. Many previously reported studies have focused on direct links between heart and body surface potentials. The goals of this study were first to validate computational methods of determining volume potentials and currents with high-resolution experimental measurements and then to use interactive visualization of thoracic currents to understand features of the electrocardiographic fields from measured cardiac sources. We developed both simulation and experimental studies based on a realistic shaped torso phantom containing an isolated, perfused dog heart. Interventions included atrial pacing, single pacing and simultaneously pacing at multiple locations on the ventricles. Simulated torso volume potentials closely matched measured potentials in the torso-tank preparation (mean correlation coefficients of 0.95). Simulation further provided a means of estimating the current field in the torso from the computed torso volume potentials and the local geometric and conductive properties of the medium. Applying these techniques to the torso electric fields under a variety of pacing conditions, we have further demonstrated that thoracic current can provide many insights into the relationship between heart surface potential and body surface potentials. Specifically, we have shown that geometric factors including cardiac source configuration and location play an important role in determining to what extent electric activity in the heart is directly visible on the body surface electrocardiogram. The computation and visualization toolkit we developed in this study to explore current fields associated with cardiac events may provide new insights into electrocardiology.

Temporal aspects of female musk shrew (Suncus murinus) sexual behavior.

The effects of testing condition on sexual behavior were examined in female musk shrews (Suncus murinus). Females were tested in 2 conditions, a pair test and a paced test. The pair test was similar to traditional sex test conditions in which the female and male are placed into the same chamber together until mating occurs. The paced condition allowed the female to leave the male's chamber and revisit him at will, thus "pacing" the interaction. Females displayed receptivity continuously for 14 days in both conditions. In the paced condition, females were less likely to become receptive within 30 min and mate to ejaculation. However, few additional differences were found between test conditions. Because this is the 1st experiment to use a pacing test paradigm in a species with induced ovulation, the authors speculate that the absence of pacing behavior during mating may be shown by other species that have induced ovulation.

Shortening and [Ca2+] dynamics of left ventricular myocytes isolated from exercise-trained rats.

The effects of run endurance training and fura 2 loading on the contractile function and Ca2+ regulation of rat left ventricular myocytes were examined. In myocytes not loaded with fura 2, the maximal extent of myocyte shortening was reduced with training under our pacing conditions [0.5 Hz at 2.0 and 0.75 mM external Ca2+ concentration ([Ca2+]o)], although training had no effect on the temporal characteristics. The "light" loading of myocytes with fura 2 markedly suppressed (approximately 50%) maximal shortening in the sedentary and trained groups, although the temporal characteristics of myocyte shortening were significantly prolonged in the trained group. No discernible differences in the dynamic characteristics of the intracellular Ca2+ concentration ([Ca2+]) transient were detected at 2.0 mM [Ca2+]o, although peak [Ca2+] and rate of [Ca2+] rise during caffeine contracture were greater in the trained state at 0.75 mM [Ca2+]o. We conclude that training induced a diminished myocyte contractile function under the conditions studied here and a more effective coupling of inward Ca2+ current to sarcoplasmic reticulum Ca2+ release at low [Ca2+]o, and that fura 2 and its loading vehicle DMSO significantly alter the intrinsic characteristics of myocyte contractile function and Ca2+ regulation.

The Effect of Pacing and Explicit Costs on Human Decision Making Performance in Inspection

Decision making is a critical component of visual inspection and a primary determinant of inspection quality. Previous studies in inspection have shown that pacing and costs have an effect on decision performance. The objective of this study was to evaluate human decision making performance under different conditions of pacing and cost tradeoffs. The study utilized twelve subjects who performed a decision-making (line judgment) task under paced and unpaced conditions and under various reward and penalty structures associated with various decision making outcomes (hits, correct accepts, false alarm, misses). The results were analyzed using Signal Detection Theory (SDT) measures of sensitivity and response criterion. Explicit costs had a significant effect on the response criterion, whereas pacing had a significant effect on the sensitivity.

Interactions: timing and force control of finger-tapping sequences.

This study examined effects of combinations of intertap interval and muscle force on interactions between two factors in sequences of equally paced finger taps. 12 male college students tapped a force plate connected to strain gauges. Subjects firstly tapped the plate at a preferred pace and force for 12 sec. Next, subjects tapped the plate by half or double the preferred pace. A series of finger-tapping tasks the consisted of nine combinations of pace and force. Analysis showed that, although variations in intertap interval were considerably accurately controlled across conditions, those in peak forces were not. Movement timing of tapping sequences hence appeared to be independent of force control. For six of 12 subjects, on the other hand, positive correlations between spontaneous variations in intertap interval and in forces were noted. Then, although motor timing was independent of force control in controls of low pace and weak forces, there were strong interactions between the two factors under high pace conditions.

BLUNTED NATRIURETIC RESPONSE TO ENDOGENOUS ATRIAL NATRIURETIC PEPTIDE DURING RAPID CARDIAC PACING IN ANAESTHETIZED DOGS

1. We investigated whether diuresis and natriuresis induced by endogenous atrial natriuretic peptide (ANP) were blunted during rapid cardiac pacing. 2. Changes in plasma ANP, renal function and haemodynamics during rapid cardiac pacing were studied in anaesthetized closed-chest dogs. Dogs were paced via the right ventricle at a rate of 200 b.p.m. (moderate pacing) or 250 b.p.m. (severe pacing) for 180 min. 3. The maximal increases in plasma ANP and urinary excretion of cGMP during severe pacing were four- and three-fold higher, respectively, than those during moderate pacing. Despite the higher concentration of plasma ANP, the maximal increases in urine volume, urinary excretion of sodium and fractional excretion of sodium during severe pacing were similar to those during moderate pacing. Mean arterial pressure and renal vascular resistance were decreased only by severe pacing. The increase in total peripheral resistance during severe pacing was significantly smaller than that during moderate pacing. However, the glomerular filtration rate was kept at basal levels by both moderate and severe pacing. 4. These results suggest that there are certain mechanisms that counteract renal tubular sodium reabsorption induced by endogenous ANP under conditions of severe pacing. The suppression occurs at tubular sites but at glomerular sites. One of the possibilities for the suppression is the decrease in renal perfusion pressure accompanied by decreases in peritubular capillary hydrostatic pressure.

Auditory vs visual speech timing cues as external rate control to enhance verbal intelligibility in mixed spastic ataxic dysarthric speakers: a pilot study

Metronome, singing, and board pacing were used as external rate control techniques for the purpose of comparing the effectiveness of auditory and visual speech timing cues for reducing speech rate and increasing intelligibility in three traumatically brain injured mixed spastic ataxic dysarthric speakers. A single system design with baseline reversal (ABACAD) was used in this preliminary investigation. Results demonstrated statistically significant (p < 0.05) changes in increased speech intelligibility during all three pacing conditions for the two more involved subjects. Differences between treatment conditions were not statistically significant. However, auditory metronome cuing showed the best results for the two subjects who benefited from rate control. Lower baseline intelligibility was strongly correlated with higher benefit from rate control. Furthermore, the two auditory rhythmic pacing conditions exhibited a close synthronization effect between the frequency rate of the cue and speech rate. Significant correlation coefficients between decreased speech rate and increased intelligibility were only found for the two more involved subjects. These findings suggested a differential benefit of slowing speech rate to improve intelligibility contingent upon severity of speech deficits.

Effects of quinidine on repolarization in canine epicardium, midmyocardium, and endocardium: II. In vivo study.

In the companion article, we report a significant difference in quinidine effects on the action potential duration between surface (epicardial and endocardial) cells and midmyocardial cells (M cells) of canine left ventricle in vitro. This article considers two questions raised by the previous study: (1) Are the complex quinidine effects in vitro reflected in its actions on the heart in situ? (2) What are the cellular determinants of quinidine effects on QT interval in ECG? We used plunge and surface electrodes to measure activation-recovery intervals (ARIs) of bipolar electrograms obtained from epicardium, endocardium, and midmyocardium (3, 5, and 9 mm from epicardium) of canine left ventricle in conditions of AV block and right ventricular pacing. Quinidine was infused continuously; its plasma level increased from 1.6+/-0.1 microg/mL at 30 minutes to 7.6+/-0.7 microg/mL at 180 minutes. At cycle lengths (CLs) from 300 to 1500 ms, there was no ARI gradient across the ventricular wall before and during quinidine infusion. At a CL of 300 ms, therapeutic concentrations of quinidine prolonged ARIs and QT intervals. At a CL of 1500 ms, ARIs were significantly prolonged at low quinidine concentrations. With an increase of quinidine concentration, this effect subsided and disappeared. In situ, quinidine-induced prolongation of repolarization is uniform in all myocardial layers and follows the pattern observed in M cells in vitro. The ability of quinidine in therapeutic concentrations to prolong repolarization at rapid heart rates can contribute to its antiarrhythmic efficacy.

Hemodynamic Effects of Chronic Prenatal Ventricular Pacing for the Treatment of Complete Atrioventricular Block

Increasing the heart rate of the fetus with cardiac failure caused by complete AV block (CAVB) may allow delivery of a full-term, stable neonate with preserved ventricular function. Direct fetal pacing may be a feasible method to achieve this, but the effect of pacing on the structure and function of the rapidly developing fetal heart is unknown. CAVB was created in fetal lambs at 80% gestation by cryoablating the AV node. Epicardial ventricular pacing at 130 bpm was achieved by use of a pacemaker placed under the pectoral muscles. The fetus was returned to the uterus and allowed to continue to term. Ventricular function was assessed 1 week after birth in 7 lambs with CAVB and 10 control lambs. By use of the conductance catheter technique, the end-systolic pressure-volume relationship was determined at different heart rates, pacing conditions, and inotropic states. The contractility was not different between the two groups at their baseline heart rates and rhythms or when they were paced synchronously compared with asynchronously. Also, both groups responded significantly and similarly to inotropic manipulation, indicating preserved contractile reserve. Finally, in both groups, increased heart rates were associated with increased contractility, indicating an intact force-frequency relationship. We conclude that chronic epicardial ventricular pacing is well tolerated by the fetus, can be successfully applied as a treatment for CAVB, and does not adversely affect myocardial function in the rapidly developing, immature heart.

CHRONOTROPIC RESPONSE TO EXERCISE USING THREE PACING MODES VERSUS A PREDICTIVE HEART RATE 954

This study compared the chronotropic response of pacemaker dependent patients in three different sensor modes to expected heart rates calculated from an equation designed to predict appropriate rate response. Ten patients age 74 ± 2 yrs, implanted with a dual sensor rate adaptive pacemaker(Vitatron Topaz™ model 515) underwent three modified Chronotropic Assessment Exercise Protocol tests with sensor programming randomly assigned. For a given treadmill test the pacemaker was programmed into activity sensing(ACT), QT length sensing (QT) or, dual sensing (Dual). HR response was measured continuously and averaged over the last 15 seconds of exercise during two 4 minute submaximal workloads (workload 1 = 1.3 mph & 0.5% grade; workload 2 = 3.0 mph & 1.5% grade). The HR responses were then compared to predicted HR's computed using the equation of Wilkoff et al's 1989(Journal of Electrophysiology. 3,176-180). ACT pacing yielded HRs higher than predicted in the initial workload while the QT and Dual pacing conditions yielded HRs lower than expected for both workloads (p<0.05). Results indicate that, compared to the predicted HRs. ACT sensor driven pacing, overpaced during very light exercise while QT and Dual sensor driven pacing appeared to underpace for a given submaximal metabolic demand.

Regional differences in electrophysiological properties of epicardium, midmyocardium, and endocardium. In vitro and in vivo correlations.

Microelectrode studies have described a population of cells within the midmyocardium (M cells) displaying a steep rate dependence of action potential duration (APD) and high Vmax compared with endocardial (Endo) and epicardial (Epi) cells. We studied repolarization in different myocardial layers in vitro and in situ. In addition to confirming the results of earlier studies, we found that after abrupt lengthening of the cycle length (CL), APDs in M cells reached a new steady state faster than in Epi or Endo cells: the time to achieve 90% of the difference in APD (t90) was 13.3 +/- 0.7 minutes in Endo cells, 12.8 +/- 1.1 minutes in Epi cells, and 2.6 +/- 0.4 minutes in M cells (P < .05 compared with Epi or Endo) when CL changed from 400 to 1000 ms. In situ, we registered activation-recovery intervals (ARIs) in bipolar electrograms obtained from different myocardial layers in conditions of AV block and His-bundle pacing. At all CLs from 300 to 2000 ms, ARIs were equal in all myocardial layers from Epi to Endo cells. Steady-state ARIs coincided with APD of M cells registered in vitro in the physiological range of CL from 300 to 700 ms. When CL was changed from 300 to 1000 ms, the ARI followed the rapid time course typical of M cells (t90 = 2.6 +/- 0.5, 2.2 +/- 0.4, 2.5 +/- 0.4, 2.6 +/- 0.5, and 2.3 +/- 0.4 minutes for Epi; 3-, 5-, and 7-mm sub-Epi; and Endo cells, respectively). In contrast to in vitro results, there is no significant difference in repolarization among myocardial layers in the intact normal canine heart.

Excitability and Repolarization in an Ionic Model of the Cardiac Cell Membrane

The main objective of this study was to investigate the possibility of expressing the activation and repolarization processes of a realistic ionic model of the myocyte membrane in terms of simplified dynamic equivalents. The modified Beeler-Reuter model (MBR) of the ventricular membrane was selected for this purpose because its action potential upstroke, plateau and repolarization phase occur along sufficiently well separated timescales. The information on the MBR model dynamics was obtained by premature stimulation at various coupling intervals, under stable conditions of regular pacing at different cycle lengths. A general method was developed to study the threshold behavior of the system. As a first step, a pair of complementary threshold criteria was defined in terms of peak ionic current and time to repolarization in order to reliably distinguish between classes of sub-threshold and supra-threshold responses. One of the main conclusions is that the activation of the MBR model by short-duration stimuli (< 5 msec) can be accurately represented by a one-variable or a two-variable dynamic equivalent. In addition, because of the large surge of Na + current at threshold, the recovery of excitability is essentially independent of the conditioning action potential waveform (no threshold-memory effect). Another major result pertains to the higher complexity of the repolarization process, stressing the critical role played by the activation dynamics of the secondary inward current. There is a substantial dependence of the action potential duration (APD) on the conditioning action potential waveform (APD-memory effect), and at least a three-variable model is necessary for a reasonable approximation.

Functional magnetic resonance imaging of complex human movements.

Functional magnetic resonance imaging (FMRI) is a new, noninvasive imaging tool thought to measure changes related to regional cerebral blood flow (rCBF). Previous FMRI studies have demonstrated functional changes within the primary cerebral cortex in response to simple activation tasks, but it is unknown whether FMRI can also detect changes within the nonprimary cortex in response to complex mental activities. We therefore scanned six right-handed healthy subjects while they performed self-paced simple and complex finger movements with the right and left hands. Some subjects also performed the tasks at a fixed rate (2 Hz) or imagined performing the complex task. Functional changes occurred (1) in the contralateral primary motor cortex during simple, self-paced movements; (2) in the contralateral (and occasionally ipsilateral) primary motor cortex, the supplementary motor area (SMA), the premotor cortex of both hemispheres, and the contralateral somatosensory cortex during complex, self-paced movements; (3) with less intensity during paced movements, presumably due to the slower movement rates associated with the paced (relative to self-paced) condition; and (4) in the SMA and, to a lesser degree, the premotor cortex during imagined complex movements. These preliminary results are consistent with hierarchical models of voluntary motor control.

Automatic Reduction of Stimulus Polarization Artifact for Accurate Evaluation of Ventricular Evoked Responses

The ventricular evoked response, the cardiac depolarization generated in response to a pacing stimulus, is potentially useful as a sensor for rate responsive pacing and automatic threshold tracking. It is necessary to minimize the polarization artifact that results from pacing in order to sense cardiac depolarizations from the same electrodes that pace the heart. To accomplish this, a triphasic stimulus waveform consisting of precharge, stimulus, and postcharge was used. An algorithm was developed that introduced pacing stimuli during the refractory period of sensed beats, when cardiac depolarization could not occur by definition and polarization artifact could be evaluated. Precharge duration was varied until the amplitude of the polarization artifact was small compared to the evoked response. In 18 patients with temporary electrode catheters, polarization artifact was reduced from 6.8 +/- 3.4 mV to 1.9 +/- 1.1 mV after balancing (P less than 0.005). Initial precharge duration was 3200 mu sec and the mean final precharge duration was 3551 +/- 516 mu sec. In 14 patients with permanent bipolar pacing leads, polarization artifact was reduced from 3.2 +/- 3.5 mV to 0.7 +/- 0.6 mV (P less than 0.025). Final precharge duration averaged 3440 +/- 310 mu sec. Under a wide variety of pacing conditions, this algorithm simply and quickly reduces polarization artifact to a minimum to allow accurate analysis of evoked responses.

The Effect of Rate Control on the Intelligibility and Naturalness of Dysarthric Speech

Speaking rates of individuals with severe ataxic dysarthria (n = 4) and severe hypokinetic dysarthria (n = 4) were reduced to 60% and 80% of habitual rates using four different pacing strategies (Additive Metered, Additive Rhythmic, Cued Metered, and Cued Rhythmic). Effects of rate control on sentence and phoneme intelligibility and speech naturalness were examined. Sentence intelligibility improved for both groups, with metered pacing conditions associated with the largest improvement in scores. Similar improvements as speaking rates were reduced were not seen for the phoneme intelligibility task; however, one must recognize that sentence and phoneme intelligibility tasks are different. Slowing the rate of dysarthric speakers did not have as marked an impact on speech naturalness as it did for normal speakers whose naturalness decreased at slowed rates. Metered rate control strategies were associated with the lowest ratings of naturalness for all subject groups. A potential explanation for the discrepancies between the findings for sentence and phoneme intelligibility is offered.

Hypertension-induced cardiac hypertrophy. Oxygen supply and consumption with pacing.

The purpose of this study was to determine if hypertrophied myocardium was associated with diminished cardiac function, restricted oxygen supply, or oxygen consumption during tachycardia. Myocardial oxygen supply and oxygen consumption were determined during baseline and atrial pacing conditions 30 days after New Zealand White rabbits were prepared as one-kidney, one clip Goldblatt hypertensive or uninephrectomized sham control rabbits. Coronary blood flow and cardiac output, using radioactive microspheres, and small vessel oxygen saturations, using microspectrophotometry, were measured in hypertrophied and nonhypertrophied hearts. After 30 days, baseline blood pressure was significantly higher in the Goldblatt rabbits compared with sham controls, and hypertension was maintained during pacing. The myocardium was hypertrophied in the Goldblatt hypertensive rabbits compared with sham controls. Baseline heart rates were not different between animal groups (242 +/- 32 and 244 +/- 24 beats/min, respectively). Both groups were paced 35% above baseline heart rates; during pacing, cardiac output was similar to baseline values in the sham controls (304 +/- 99 versus 321 +/- 116 ml/min, respectively) but reduced in the hypertensive rabbits (248 +/- 43 versus 325 +/- 62 ml/min). Myocardial oxygen consumption increased twofold in both nonhypertrophied and hypertrophied ventricles during tachycardia. Oxygen extraction was significantly elevated, but coronary blood flow was not altered during pacing in either animal group. Therefore, at the pacing level chosen the diminished function in cardiac hypertrophy was not associated with reduced oxygen consumption. Conversely, reduced efficiency during pacing in the hypertrophied myocardium was suggested.

Classification of Multidimensional Data under Time Constraints: Evaluating Digital and Configural Display Representations

The purpose of this research was to evaluate differences in processing ability between integral and separable representations of process control data under conditions of time constraints. Subjects were trained as “operators” to identify system states and subsequently participated in time constrained (100%, 50% and 25% paced) sessions. Findings revealed that operators in both display conditions were able to respond significantly faster (while maintaining fairly high degrees of accuracy) under 100% and 50% time constrained conditions than in self-paced conditions. Classification accuracy suffered more for the separable than the integral display during the 25% paced condition.

Personal control over work pace--circulatory, neuroendocrine and subjective responses in borderline hypertension.

Cardiovascular and neuroendocrine reactivity during arithmetic task performance were studied in 24 patients with borderline hypertension and normotensive controls. Personal control (self-paced versus externally paced performance) resulted in an attenuated blood pressure (BP) response to task performance in normotensives but not in borderline hypertensives. In response to self-paced work, systolic blood blood pressure (SBP) reactivity was significantly greater in borderline hypertensives, but the heart rate (HR) reactivity was similar in borderline hypertensives and normotensives under the different pacing conditions. Task difficulty (pace variation during external pacing) did not differentially affect reactivity in borderline hypertensives and normotensives. Venous plasma noradrenaline and adrenaline were unaffected by task performance. At rest after the task performance plasma noradrenaline was elevated in both normotensives and borderline hypertensives. The latter group also showed a persistent elevation of diastolic blood pressure after task performance, suggesting a prolonged vasoconstrictor response. The present findings indicate that, in terms of circulatory responses, borderline hypertensives may profit less than normotensives from personal control over environmental demands.

The effects of pacing on complex decision-making inspection performance

The effects of time constraints in a complex decision-making inspection task were assessed utilizing a number of performance, physiological, and subjective measures. Twelve subjects were trained to classify a simulated product into one of four quality control categories, then tested on six pacing conditions varying in pacing speed and pacing rigidity. Subjects were able to maintain classification accuracy despite considerable variations in the time constraints of the task. The index of mental effort—heart rate variability—was the only measure which was found to be sensitive to the differences in pacing conditions. The subjective measures were not affected by pacing conditions but exhibited trends toward increasing postural discomfort, fatigue and boredom from the beginning to the end of each condition. A model of the inspector is tested which is based upon resource-limited models of human information processing.

External and internal attentional environments. II. Reconsideration of the relationship between sinus arrhythmia and information load.

As part of an investigation aimed at assessing differences in mental workload between machine-paced (MP) and self-paced (SP) work, this study focused on the relative usefulness of sinus arrhythmia (SA) and mean heart rate (MHR) as measures of informational load. Two tasks having contrasting attentional demands were performed both MP and SP by 32 subjects. Physiologically speaking, these tasks were characterized as having either cardiac deceleratory or acceleratory response patterns associated with them. An analysis of non-random trends, motivated by the presence of these response patterns, revealed the standard measure of variance, s2, to be less appropriate than the mean-square successive difference (MSSD) for scoring SA. Utilizing the MSSD, results indicated that MH R was far more sensitive than SA to the differences in informational load resulting from the intrinsic processing demands of these two tasks. SA, on the other hand, was more sensitive to the two pacing conditions. These latter findings, howe...