Continuous Resetting Research Articles

Entrainment of ventricular tachycardia with V-shaped diastolic activation pattern: Is the pacing site in or out?

The use and interpretation of entrainment mapping, or continuous resetting, of a reentrant ventricular tachycardia, has been regarded as the gold standard for delineation of the components of a reentrant circuit. However, there are both inherent limitations and interpretative pitfalls to entrainment mapping. Beyond the far-field (FF) signals, the multiple near-field (NF) components may also be recorded at a given site and pacing typically interacts and captures only one component of the local activation. In the evaluation of the entrainment response, it is crucial to distinguish the NF electrogram captured from FF or other NF components to accurately measure the post-pacing interval, particularly in the presence of multiple NF signals This article is protected by copyright. All rights reserved.

Cooperative Branch Migration: A Mechanism for Flexible Control of DNA Strand Displacement.

DNA strand displacement plays an essential role in the field of dynamic DNA nanotechnology. However, flexible regulation of strand displacement remains a significant challenge. Most previous regulatory tools focused on controllable activation of toehold and thus limited the design flexibility. Here, we introduce a regulatory tool termed cooperative branch migration (CBM), through which DNA strand displacement can be controlled by regulating the complementarity of branch migration domains. CBM shows perfect compatibility with the majority of existing regulatory tools, and when combined with forked toehold, it permits continuous fine-tuning of the strand displacement rate spanning 5 orders of magnitude. CBM manifests multifunctional regulation ability, including rate fine-tuning, continuous dynamic regulation, reaction resetting, and selective activation. To exemplify the powerful function, we also constructed a nested if-function signal processing system on the basis of cascading CBM reactions. We believe that the proposed regulatory strategy would effectively enrich the DNA strand displacement toolbox and ultimately promote the construction of DNA machines of higher complexity in nucleic acid research and biomedical applications.

Self-Centered Function of Adaptive Immunity in Regulation of Immune Responses and in Tolerance.

The search for common mechanisms underlying the pathogenesis of chronic inflammatory conditions has crystalized the concept of continuous dual resetting of the immune repertoire (CDR) as a basic principle of the immune system function. Consequently, outlined was the first dynamic comprehensive picture of the immune system function. The goal of this study is to elaborate on regulation of immune responses and mechanisms of tolerance, particularly focusing on adaptive immunity. It is well established that the T/B cell repertoire is selected and maintained based on interactions with self. However, their activation also requires interaction with a self-specific major histocompatibility complex (MHC) “code,” i.e., the context of MHC molecules. Therefore, not only repertoire selection and maintenance but also the T/B cell activation and function are self-centered. Thus, adaptive effectors may be primarily focused on the state of self and maintenance of integrity of the self, and only to a certain degree on elimination of the foreign. As examples of such function are used immunologically poorly understood MHC-disparate settings typical for transplantation and pregnancy. Transplantation represents an extreme setting of strong systemic compartment-level adaptive/MHC-restricted immune responses. Described are clinically identified conditions for operational tolerance of MHC-disparate tissues/living systems in allotransplantation, which are in line with the CDR-proposed self-centered regulatory role of T/B cells. In contrast, normal pregnancy is coexistence of semiallogeneic or entirely allogeneic mother and fetus, but without alloreactivity akin to transplantation settings. Presented data support the notion that maintenance of pregnancy is a process that relies predominantly on innate/MHC-independent immune mechanisms. By the inception of hemotrophic stage of pregnancy (second and third trimester), both mother and child are individual living systems, with established adaptive immune repertoires. Although mother-fetus interactions at that point become indirect systemic compartment-level communications, their interactions throughout gestation remain within the innate realm of molecular-level adaptations.

A Comparative Study of Adaptive Interlimb Coordination Mechanisms for Self-Organized Robot Locomotion.

Walking animals demonstrate impressive self-organized locomotion and adaptation to body property changes by skillfully manipulating their complicated and redundant musculoskeletal systems. Adaptive interlimb coordination plays a crucial role in this achievement. It has been identified that interlimb coordination is generated through dynamical interactions between the neural system, musculoskeletal system, and environment. Based on this principle, two classical interlimb coordination mechanisms (continuous phase modulation and phase resetting) have been proposed independently. These mechanisms use decoupled central pattern generators (CPGs) with sensory feedback, such as ground reaction forces (GRFs), to generate robot locomotion autonomously without predefining it (i.e., self-organized locomotion). A comparative study was conducted on the two mechanisms under decoupled CPG-based control implemented on a quadruped robot in simulation. Their characteristics were compared by observing their CPG phase convergence processes at different control parameter values. Additionally, the mechanisms were investigated when the robot faced various unexpected situations, such as noisy feedback, leg motor damage, and carrying a load. The comparative study reveals that the phase modulation and resetting mechanisms demonstrate satisfactory performance when they are subjected to symmetric and asymmetric GRF distributions, respectively. This work also suggests a strategy for the appropriate selection of adaptive interlimb coordination mechanisms under different conditions and for the optimal setting of their control parameter values to enhance their control performance.

Challenges and Pitfalls of Entrainment Mapping of Ventricular Tachycardia: Ten Illustrative Concepts.

The use and interpretation of entrainment mapping, or continuous resetting, of a reentrant tachycardia has been regarded as the gold standard for delineation of the components of a reentrant circuit.1,2 The response during and after overdrive pacing, whereby 2 wavefronts enter the circuit antidromically (with fusion) and orthodromically, is used to confirm reentry as the arrhythmia mechanism and determine the relationship of the pacing site to the circuit. The fulfillment of classical criteria outlined by Waldo and colleagues3,4 were synthesized into an anatomic concept for scar-related ventricular tachycardia (VT) by Stevenson et al5 to portray the structural and architectural basis of circuit conduction meandering between regions of fibrosis. In this construct, a central corridor, or protected isthmus, is bordered between 2 regions of dense scar with a single entrance that is distinct from a single exit, which yields the QRS morphology.6 This reentrant model has been central to our current mechanistic understanding of scar-mediated VT and is critically important for differentiating critical sites from bystander sites and regions that are unlikely to interrupt or eliminate reentry.7 However, the nature of reentrant VT in man is more complex than our idealized working construct for many reasons. In clinical practice, the majority of VT is hemodynamically unstable, which precludes the ability to perform entrainment mapping and activation mapping of the entire circuit.8 Differences in the circuit between patients with untolerated and tolerated VT are not well understood. VT circuits are 3 dimensionally complex with transmural conduction and circuit conduction is unlikely to be planar, as depicted by electroanatomic mapping of the myocardial surface. Exits may be multiple9,10 and patterns other than loop reentry around scar are likely. Channels of preserved myocardium are frequently not “normal” in voltage (>1.5 mV) …

Continuous Dual Resetting of the Immune Repertoire as a Basic Principle of the Immune System Function.

Idiopathic chronic inflammatory conditions (ICIC) such as allergy, asthma, chronic obstructive pulmonary disease, and various autoimmune conditions are a worldwide health problem. Understanding the pathogenesis of ICIC is essential for their successful therapy and prevention. However, efforts are hindered by the lack of comprehensive understanding of the human immune system function. In line with those efforts, described here is a concept of stochastic continuous dual resetting (CDR) of the immune repertoire as a basic principle that governs the function of immunity. The CDR functions as a consequence of system's thermodynamically determined intrinsic tendency to acquire new states of inner equilibrium and equilibrium against the environment. Consequently, immune repertoire undergoes continuous dual (two-way) resetting: against the physiologic continuous changes of self and against the continuously changing environment. The CDR-based dynamic concept of immunity describes mechanisms of self-regulation, tolerance, and immunosenescence, and emphasizes the significance of immune system's compartmentalization in the pathogenesis of ICIC. The CDR concept's relative simplicity and concomitantly documented congruency with empirical, clinical, and experimental data suggest it may represent a plausible theoretical framework to better understand the human immune system function.

Unusual response to entrainment of ventricular tachycardia: In or out?

FEATURED ARRHYTHMIA Unusual response to entrainment of ventricular tachycardia: In or out? Roderick Tung, MD, FHRS, Kalyanam Shivkumar, MD, PhD, FHRS From the UCLA Cardiac Arrhythmia Center, David Geffen School of Medicine at UCLA, Los Angeles, California. Case presentation A 70-year-old man with history of inferior myocardial infarction with an ejection fraction of 25% was urgently transferred for ablation of incessant slow ventricular tachycardia (VT). Electroanatomic mapping revealed extensive inferolateral scar extending to the inferior septum. Entrainment mapping demonstrated an entrance in periannular inferolateral scar with an exit anterior to the aortomitral continuity. Entrain- ment mapping was performed in the scar at a site in between these 2 regions. Is this response in or out of the circuit? (Figure 1) Commentary Entrainment mapping, or continuous resetting of VT, is the gold standard technique for characterizing the components within a reentrant circuit. Demonstration of an isthmus requires the presence of (1) concealed fusion, (2) postpacing interval within 30 ms of the tachycardia cycle length, and (3) stimulus-QRS (S-QRS) interval equal to electrogram-QRS (EGM-QRS) interval. 1 The presence of these responses is highly predictive of termination with ablation. 2,3 In this case, overdrive pacing of the slow VT (680 ms) is performed at 620 ms (10 mA @ 2 ms). Two distinct diastolic potentials are seen during VT, and it is important to distinguish the near-field EGM captured from far-field components in order to accurately measure the postpacing interval. 4 Disappearance of a local EGM during pacing represents either direct capture or saturation of the channel from pacing. Two features confirm that the early diastolic EGM is not captured during entrainment: (1) the S-QRS interval would be longer as the EGM-QRS occurs in early diastole and (2) analysis of the onset of pacing reveals KEYWORDS Ventricular tachycardia; Entrainment ABBREVIATIONS EGM-QRS ¼ electrogram-QRS; MES ¼ multiple exit site; S-QRS ¼ stimulus-QRS; VT ¼ ventricular tachycardia (Heart Rhythm Address reprint requests and correspondence: Dr Roderick Tung, UCLA Cardiac Arrhythmia Center, David Geffen School of Medicine at UCLA, 100 Medical Plaza, Suite 660, Los Angeles, CA 90095. E-mail address: rtung@mednet.ucla.edu. 1547-5271/$-see front matter B 2014 Heart Rhythm Society. All rights reserved. saturation (Figure 2, dashed circle) from the pacing current in the absence of local and QRS capture. The last entrained beat demonstrates overt or manifest fusion. The postpacing interval measured to the peak component of the return sensed EGM is within 30 ms of the tachycardia cycle length (680 ms). The S-QRS interval appears to match the EGM-QRS interval when measuring the peak deflection of the late diastolic EGM. These responses suggest entrainment from an outer loop site on the periphery of the reentrant circuit with antidromic fusion outside the circuit. However, careful analysis of the first 2 paced beats on the tracing demonstrates a QRS morphology with an exact 12-lead match for the VT. The possibility of transient loss of capture during these 2 complexes is ruled out by acceleration of the VT to the pacing cycle length (620 ms). These 2 beats represent concealed fusion with a perfect match for the VT, and the S-QRS interval (145 ms) matches the EGM-QRS interval when the EGM is timed to the onset of the return diastolic potential rather than the peak. Analysis of the diastolic EGM before pacing is useful because the EGM is not yet saturated by the pacing channel, which allows more detailed analysis of multicomponent local EGMs (Figure 2). Closer analysis of this EGM confirms a fractionated compo- nent of the late diastolic potential 35 ms before the peak deflection (EGM-QRS interval 145 ms). Note that the first entrained beat after the onset of pacing has the same morphology as the last entrained beat. Interpretation of this entrainment response using the first 2 complexes in Figure 1 suggests a critical isthmus site. Ablation at this site resulted in the abrupt termination of VT within 4 seconds (Figure 3). We have previously reported the observation of multiple exit site (MES) morphologies when pace mapping within scar during sinus rhythm. 5 These sites are likely specific for reentrant isthmuses and importantly can be identified as surrogates for critical sites in sinus rhythm. However, pacing during sinus rhythm from a point-like source is likely to have a radial spread of activation in contrast to a broad reentrant wave front with a preferential exit during VT. Therefore, activation through MESs out of scar is more likely to be seen during pace mapping. The entrainment response in this case likely represents an alternate exit site from a common http://dx.doi.org/10.1016/j.hrthm.2013.09.006

The geological significance of 40Ar/39Ar and Rb–Sr white mica ages from Syros and Sifnos, Greece: a record of continuous (re)crystallization during exhumation?

AbstractThe Attic‐Cycladic crystalline belt in the central Aegean region records a complex structural and metamorphic evolution that documents Cenozoic subduction zone processes and exhumation. A prerequisite to develop an improved tectono‐metamorphic understanding of this area is dating of distinct P–T–D stages. To evaluate the geological significance of phengite ages of variably overprinted rocks, 40Ar/39Ar and Rb–Sr analyses were undertaken on transitional blueschist–greenschist and greenschist facies samples from the islands of Syros and Sifnos. White mica geochronology indicates a large age variability (40Ar/39Ar: 41–27 Ma; Rb–Sr: 34–20 Ma). Petrologically similar samples have either experienced greenschist facies overprinting at different times or variations in ages record variable degrees of greenschist facies retrogression and incomplete resetting of isotopic systematics. The 40Ar/39Ar and Rb–Sr data for metamorphic rocks from both islands record only minor, localized evidence for Miocene ages (c. 21 Ma) that are well documented elsewhere in the Cyclades and interpreted to result from retrogression of high‐pressure mineral assemblages during lower pressure metamorphism. Field and textural evidence suggests that heterogeneous overprinting may be due to a lack of permeability and/or limited availability of fluids in some bulk compositions and that retrogression was more or less parallel to lithological layering and/or foliation as a result of, possibly deformation‐enhanced, channelized fluid ingress. Published and new 40Ar/39Ar and Rb–Sr data for both islands indicate apparent age variations that can be broadly linked to mineral assemblages documenting transitional blueschist‐to‐greenschist‐ and/or greenschist facies metamorphism. The data do not record the timing of peak HP metamorphism, but may accurately record continuous (partial) resetting of isotopic systematics and/or (re)crystallization of white mica during exhumation and greenschist facies retrogression. The form of 40Ar/39Ar phengite age spectra are complex with the lowest temperature steps yielding Middle to Late Miocene ages. The youngest Rb–Sr ages suggest maximum ages of 20.6 ± 0.8 Ma (Syros) and 22.5 ± 0.6 Ma (Sifnos) for the timing of greenschist facies overprinting. The results of this study further accentuate the challenges of interpreting isotopic data for white mica from polymetamorphic terranes, particularly when mixing of populations and/or incomplete resetting of isotopic systematics occurs during exhumation. These data capture the full range of isotopic age variations in retrogressed HP rocks documented in previous isotopic studies, and can be interpreted in terms of the geodynamic evolution of the Aegean.

Self-arrangement of cellular circadian rhythms through phase-resetting in plant roots

We discovered a striped pattern of gene expression with circadian rhythms in growing plant roots using bioluminescent imaging of gene expression. Our experimental analysis revealed that the stripe wave in the bioluminescent image originated at the root tip and was caused by a continuous phase resetting of circadian oscillations. Some complex stripe waves containing arrhythmic regions were also observed. We succeeded in describing the formation mechanisms of these patterns using a growing phase oscillator network with a phase-resetting boundary condition.

Exercise‐induced neuronal plasticity in central autonomic networks: role in cardiovascular control

It is now well established that brain plasticity is an inherent property not only of the developing but also of the adult brain. Numerous beneficial effects of exercise, including improved memory, cognitive function and neuroprotection, have been shown to involve an important neuroplastic component. However, whether major adaptive cardiovascular adjustments during exercise, needed to ensure proper blood perfusion of peripheral tissues, also require brain neuroplasticity, is presently unknown. This review will critically evaluate current knowledge on proposed mechanisms that are likely to underlie the continuous resetting of baroreflex control of heart rate during/after exercise and following exercise training. Accumulating evidence indicates that not only somatosensory afferents (conveyed by skeletal muscle receptors, baroreceptors and/or cardiopulmonary receptors) but also projections arising from central command neurons (in particular, peptidergic hypothalamic pre-autonomic neurons) converge into the nucleus tractus solitarii (NTS) in the dorsal brainstem, to co-ordinate complex cardiovascular adaptations during dynamic exercise. This review focuses in particular on a reciprocally interconnected network between the NTS and the hypothalamic paraventricular nucleus (PVN), which is proposed to act as a pivotal anatomical and functional substrate underlying integrative feedforward and feedback cardiovascular adjustments during exercise. Recent findings supporting neuroplastic adaptive changes within the NTS-PVN reciprocal network (e.g. remodelling of afferent inputs, structural and functional neuronal plasticity and changes in neurotransmitter content) will be discussed within the context of their role as important underlying cellular mechanisms supporting the tonic activation and improved efficacy of these central pathways in response to circulatory demand at rest and during exercise, both in sedentary and in trained individuals. We hope this review will stimulate more comprehensive studies aimed at understanding cellular and molecular mechanisms within CNS neuronal networks that contribute to exercise-induced neuroplasticity and cardiovascular adjustments.

Dynamic arterial baroreflex function during high intensity exercise in humans: insights into sympathetic control

Dynamic arterial baroreflex function during high intensity exercise in humans: insights into sympathetic control

Arterial baroreflex control of muscle sympathetic nerve activity in the transition from rest to steady-state dynamic exercise in humans

We sought to investigate arterial baroreflex (ABR) control of muscle sympathetic nerve activity (MSNA) in the transition from rest to steady-state dynamic exercise. This was accomplished by assessing the relationship between spontaneous variations in diastolic blood pressure (DBP) and MSNA at rest and during the time course of reaching steady-state arm cycling at 50% peak oxygen uptake (VO(2peak)). Specifically, DBP-MSNA relations were examined in eight subjects (25 +/- 1 yr) at the start of unloaded arm cycling and then during the initial and a later period of arm cycling once the 50% VO(2peak) work rate was achieved. Heart rate and arterial blood pressure were progressively increased throughout exercise. Although resting MSNA [16 +/- 2 burst/min; 181 +/- 36 arbitrary units (au) total activity] was unchanged during unloaded cycling, MSNA burst frequency and total activity were significantly elevated during the initial (27 +/- 4 burst/min; 367 +/- 76 au; P < 0.05) and later (36 +/- 7 burst/min; 444 +/- 91 au; P < 0.05) periods of exercise. The relationships between DBP and burst incidence, burst strength, and total MSNA were progressively shifted rightward from unloaded to the initial to the later period of 50% VO(2peak) arm cycling without any changes in the slopes of the linear regressions (i.e., ABR sensitivity). Thus a continuous and dynamic resetting of the ABR control of MSNA occurred during the transition from rest to steady-state dynamic exercise. These findings indicate that the ABR control of MSNA was well maintained throughout dynamic exercise in humans, progressively being reset to operate around the exercise-induced elevations in blood pressure and MSNA without any changes in reflex sensitivity.

The NTS and integration of cardiovascular control during exercise in normotensive and hypertensive individuals

Due to upward resetting of baroreceptors, tachycardia coexists with increased pressure during dynamic exercise. This review critically evaluates current knowledge of proposed mechanisms to explain the continuous resetting of baroreflex control of heart rate and sympathetic nerve activity during and after exercise and exercise training. Of interest is the exercise-induced upward resetting that occurs in hypertensive and normotensive individuals. Accumulated evidence indicates that not only somatosensory afferents, but also inputs from central command projecting to the nucleus tractus solitarius (NTS) in the dorsal brainstem may mediate inhibition of excitatory neurotransmission on barosensitive neurons. Specific coordinated activation of vasopressinergic and oxytocinergic projections to the NTS is essential to tonically maintain baroreflex sensitivity and to adjust heart rate and cardiac output to circulatory demand at rest and during exercise in both sedentary and trained individuals. These findings reinforce the paramount importance of the NTS in integration of cardiovascular control during exercise.

From bedside to bench: entrainment and other stories

The concepts of transient entrainment of reentrant rhythms started with studies of overdrive pacing of atrial flutter (AFL) in patients in the immediate period after open heart surgery. Initial studies demonstrated the need to achieve a critical pacing rate and a critical duration of pacing at the critical pacing rate to interrupt AFL. Further pacing studies of AFL, ventricular tachycardia, atrioventricular (AV) reentrant tachycardia, AV nodal reentrant tachycardia, and atrial tachycardia refined the understanding of what occurs during overdrive pacing of reentrant tachycardias, and permitted a mechanistic understanding of transient entrainment as continuous resetting of a reentrant tachycardia to a pacing rate that is faster than the rate of the tachycardia, but which fails to interrupt it. The demonstration of transient entrainment of a tachycardia provides a reliable clinical tool to establish the presence of a reentrant rhythm. Moreover, the principles of entrainment have also been applied clinically to assist in effective application of antitachycardia pacing and catheter ablation techniques.

Bipolar feedback transistor integrated on detector with JFET for continuous reset

A monolithic system with a silicon detector, front-end electronics and a BJT reset device to be operated in a charge amplifier and in continuous resetting mode is presented.

Resetting of the carotid arterial baroreflex during dynamic exercise in humans.

Recent investigations have demonstrated that at the onset of low-to-moderate-intensity leg cycling exercise (L) the carotid baroreflex (CBR) was classically reset in direct relation to the intensity of exercise. On the basis of these data, we proposed that the CBR would also be classically reset at the onset of moderate- to maximal-intensity L exercise. Therefore, CBR stimulus-response relationships were compared in seven male volunteers by using the neck pressure-neck suction technique during dynamic exercise that ranged in intensity from 50 to 100% of maximal oxygen uptake (VO(2 max)). L exercise alone was performed at 50 and 75% VO(2 max), and L exercise combined with arm (A) exercise (L + A) was performed at 75 and 100% VO(2 max). O(2) consumption and heart rate (HR) increased in direct relation with the increases in exercise intensity. The threshold and saturation pressures of the carotid-cardiac reflex at 100% VO(2 max) were >75% VO(2 max), which were in turn >50% VO(2 max) (P < 0.05), without a change in the maximal reflex gain (G(max)). In addition, the HR response value at threshold and saturation at 75% VO(2 max) was >50% VO(2 max) (P < 0.05) and 100% VO(2 max) was >75% VO(2 max) (P < 0.07). Similar changes were observed for the carotid-vasomotor reflex. In addition, as exercise intensity increased, the operating point (the prestimulus blood pressure) of the CBR was significantly relocated further from the centering point (G(max)) of the stimulus-response curve and was at threshold during 100% VO(2 max). These findings identify the continuous classic rightward and upward resetting of the CBR, without a change in G(max), during increases in dynamic exercise intensity to maximal effort.

ELECTROPHYSIOLOGIC EVALUATION OF SUPRAVENTRICULAR TACHYCARDIA

The evolution of intracardiac recording and programmed stimulation has allowed a better definition of the mechanisms and types of a variety of supraventricular tachyarrhythmias . This article describes the evaluation of supraventricular tachycardias resulting from atrioventricular (AV) nodal reentry, circus movement tachycardia using a concealed accessory pathway , and atrial tachycardia. The appropriate interpretation of intracardiac recordings and programmed stimulation to assess the mechanism of arrhythmia is based on understanding of the characteristic responses of known mechanisms of arrhythmias to programmed stimulation as well as response to drugs. 8 , 19 The mechanisms of all paroxysmal tachycardias are due to either abnormalities of impulse formation or abnormalities of conduction leading to reentry. Enhanced impulse formation may be due to abnormal automaticity or triggered activity as a result of either delayed afterdepolarizations or early afterdepolarizations. In the atrium and in particular for supraventricular arrhythmias , early afterdepolarizations probably do not play a role. The characteristics of automatic rhythms are that they can neither be initiated nor terminated in a reproducible way by programmed stimulation. Although annihilation of an automatic focus may be possible in experimental models, it is not predictable and has not yet been described in humans. The most common response of automatic rhythms to overdrive pacing is overdrive suppression or, in mildly depolarized tissue, no effect or, rarely, acceleration. Triggered rhythms caused by delayed afterdepolarizations are characterized by their ability to be initiated by programmed stimulation; however, such tachycardias are more readily induced by overdrive pacing rather than timed extrastimuli and do not require conduction delay or block for initiation. A direct relationship between the coupling interval or paced cycle length initiating rhythm and the interval to the onset of the rhythm and early cycle lengths of the rhythm as well as overdrive acceleration in response to rapid pacing are typical of arrhythmias related to delayed afterdepolarizations. Reentrant arrhythmias are most well understood and constitute the vast majority of clinical arrhythmias. The requirements for reentry include the presence of at least functional or anatomically distinct potential pathways that are linked proximally and distally to form a closed circuit of conduction, unidirectional block in one of these pathways, and slow conduction over the unblocked pathway allowing the previous blocked pathway time to recover. Reentrant arrhythmias can be reproducibly initiated and terminated by timed extrastimuli (usually in association with conduction delay or block) or rapid pacing. Frequently the conduction delay can be seen as an inverse relationship between the coupling interval of the initiating extrastimulus and the interval to the onset of the first complex of the tachycardia. In addition, during reentrant arrhythmias, continuous resetting of the tachycardia with fusion (i.e., entrainment) is diagnostic of reentry because fusion represents the presence of wavefronts, orthodromic and antidromic , occurring at the same time. This cannot happen in a focal tachycardia. Thus, the electrophysiologic evaluation of supraventricular tachycardia should include analysis of (1) mode of initiation of tachycardia, (2) atrial activation sequence during the tachycardia, (3) effect of atrial or ventricular stimulation during the tachycardia, (4) requirement of the atrium or ventricles to initiate and sustain the tachycardia, (5) influence of bundle-branch block on conduction and cycle length of the tachycardia, and (6) effect of drugs or physiologic perturbation of the autonomic nervous system on the tachycardia. 8 , 19

Continuous resetting of the human carotid baroreceptor-cardiac reflex.

Although human baroreflex responses have been studied during night as well as day, there has been no attempt to distinguish circadian changes of baroreflex function from those related to sleep. We measured carotid baroreceptor-cardiac reflex responses serially during a 24-h period in 11 normotensive volunteers who were awake and cooperative during testing. We applied sequences of ramped R-wave-triggered neck chamber pressure changes from +40 to -65 mmHg, during held expiration, at 3-h intervals. Subjects maintained their usual sleep-wake cycles but were awakened for three 30-min periods for night testing. There was no systematic change of baroreflex slope during the 24-h period. There were, however, parallel shifts of the entire sigmoid baroreceptor-cardiac reflex response relation along its R-R interval and arterial pressure axes associated with small, but significant, circadian changes of baseline R-R intervals and arterial pressures. Thus, although our data do not point toward major circadian variability of baro-reflex responsiveness, they provide evidence for an ongoing process of human baroreflex resetting.

THE LOWEST TEMPERATURE IN ANTARCTICA

The IGY period 1957-68 wa,s the first time in history that continuous meteorologicaLl observations were made in the interior of Antarctica,. The U.S. I G Y AmundsenScott Station (90 s., elev. 2,800 ni.) and the Soviet, l G P stations, Vostok (78'27' S., 106'52' E., elev. 3,420 m.) and Sovetskaya (78'24' S., 87'35' E., elev. 3,570 m.) are located on a high plateau of the Antarct,ic Continent and the minimum temperatures observed there have set new world records. I n fact, the Antarctic winter season of 1958 saw a continuous breaking and resetting of the world minimum temperature record. a s it now stands, the 87.4 C. ( 125.3' F.) recorded at Vostok on August 25 is the world record. Now that this eventful winter is over, the still fragmentary information from Sntarctica can be analyzed and summarized. However, a reservation must be made as to the reliability of sources of this information. We are compelled to resort to a practice usually avoided in 'scientific analysis; that is, to a utilization of newspaper and radio reports. At this early date these are our only sources for some of the data which may be long in reaclling publication in technical journals. In a previous article [l] on the lowest temperatures reported on earth, we published the then latest record low air temperature for the world. It was -74.5' C. (-102.1' F.) observed at the U.S. Antarctic station at, the South Pole (elev. 2,800 m.) on September 17, 1957. This minimum broke the previous rec.ord of -67.7' C. (-89.9' F.) observed at Oimekon in eastern Siberia on February 6, 1933. Carrying on from where we left off in the previous article, we show in tab'le 1 a chronological list of the later world-record-breaking minima observed in Antarctica during the, winter of 1958. The new record of -187.4~ C. has, exceeded a theoretical estimate of the lowest possible Antarctic temperature made by Sllliakhov [ 2 ] . His estimate was based on the assumption that radiation balance becomes zero at the theoretical minimum temperat.ure ; i.e., the back radiation from the atmosphere equals the upcoming radiation from the surfa,ce. Using actual radiation measurements from Vostok 1 and theoretical values derived from soundings macle a t Vostok 1 in May 1957 and at the South Pole in ,July 1957, and using the Shekhter radiation nomogram [ 3 ] , Shliakhov computed the minimum temperature to vary from -78'+-2 C. to -80'+.1' C. He then performed another calculation based on greater station elevation (5 km. m.s.1.) and using a mean lapse rate of 5 C. per 1,000 m. above the inversion layer , a s determined from radiosoundings made at the interior stations. He assumed the temperature of the snow surface to be -75O C., the strength of the inversion -30' C., the depth of the inversion layer 1 km., and also assumed that