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PubMed2004

Demarcation of diapause development by cold and its relation to time-interval activation of TIME-ATPase in eggs of the silkworm, Bombyx mori.

We investigated the mode of action of winter cold in the termination of diapause by investigating Time-Interval-Measuring Enzyme (TIME). First, we determined the period of cold required for the completion of diapause development. Synchronously developing egg batches of a pure strain (C108 Bombyx mori silkworm) were used to minimize variations in hatching time. Hatching occurred with only 18 days chilling at 5 degrees C when the incubation at 25 degrees C after the chilling was elongated. The 18-day period was much shorter than we expected; diapause in B. mori is known to terminate completely with about 100 days of chilling. Even in such a short period of chilling, no sporadic hatching occurred. Moreover, we determined that a temperature-insensitive stage, which we called "Neboke", followed the short cold-requiring stage. Thus, the stage of diapause development was demarcated from other stages of diapause. While the length of diapause development was elongated when chilling was delayed after oviposition, the Neboke stage length was invariant. Cold evidently exerts its effect only on diapause development. When TIME was purified from eggs and chilled in test tubes, a transitory burst of its ATPase activity occurred at a time equivalent to shortly before the completion of diapause development; this was an interval-timer activation. The mechanism by which cold activates TIME to measure the time interval may help explain in biochemical terms the insect's adaptation to its seasonal environments.

Acclimatization↗
PubMed1995

The rise times of miniature endplate currents suggest that acetylcholine may be released over a period of time.

Models of miniature endplate currents predict 20-80% rise times of 100 microseconds or less. These predictions are substantially less than most of the rise times recorded in the literature. New measurements were made of rise times at the frog neuromuscular junction using extracellular recording. The mean 20-80% rise time was 250 microseconds. Rise times were variable; at 20 degrees C, 95% of them fell in a range from 140 to 460 microseconds. The most questionable assumption in the models is that the acetylcholine (ACh) is released instantaneously. Modifying the model, so that ACh diffuses from the vesicle through a pore, lengthens the rise time to observed levels. It has been proposed that ACh is released from the vesicle in exchange for Na+. However, the rise times of miniature endplate currents recorded in solutions in which the Na+ is replaced by sucrose are in the normal range. The Q10 for the rise of miniature endplate currents is approximately 2, which is consistent with the models and with temperature effects on pore formation in mast cells.

Acetylcholine↗
PubMed1999

Prospective, randomized, crossover study to evaluate the benefit of human chorionic gonadotropin-timed versus urinary luteinizing hormone-timed intrauterine inseminations in clomiphene citrate-stimulated treatment cycles.

OBJECTIVE: To compare two methods of timing IUI, urinary LH monitoring and transvaginal ultrasonography/ hCG timing of ovulation, in patients receiving clomiphene citrate. DESIGN: Prospective, randomized, crossover study. SETTING: Yale University Reproductive Medicine Center. PATIENT(S): Infertile couples undergoing IUI because of unexplained infertility, anovulation, or male factor infertility. INTERVENTION(S): Patients received clomiphene citrate on days 3-7 of the menstrual cycle and were randomized initially to one of two monitoring protocols. In protocol A, urinary LH monitoring was used to time IUI. Urinary LH levels were determined daily with the use of commercial kits, starting on day 10 of the cycle. When urinary LH was detected, IUIs were performed daily for the next 2 days. In protocol B, ultrasound monitoring of folliculogenesis was performed until a leading follicle of > or = 18 mm was noted, at which time hCG (10,000 IU) was given intramuscularly and IUIs were performed daily for the next 2 days. If no pregnancy occurred, the couple crossed over to the alternate protocol for the next cycle and continued this alternating therapy for a total of four cycles. MAIN OUTCOME MEASURE(S): Pregnancy rate per cycle. RESULT(S): One hundred forty-one cycles were completed. In these cycles, six pregnancies occurred, for an overall pregnancy rate of 4.26% per cycle. The pregnancy rate with LH-timed IUI was 4.29% (3/70) and that with hCG-induced ovulation was 4.23% (3/71); the difference was not statistically significant. CONCLUSION(S): Timing IUI with the use of a relatively expensive and time-consuming method such as ultrasound monitoring of folliculogenesis and hCG induction of ovulation does not appear to produce an increased pregnancy rate over urinary LH monitoring of ovulation.

Adult↗
PubMed1988

On the time-dependent diffusion of macromolecules through transient open junctions and their subendothelial spread. I. Short-time model for cleft exit region.

In this two-part study we shall quantitatively study, using time-dependent models, the hypothesis that transient open junctions associated with widely scattered endothelial cells undergoing mitosis are the structural equivalent for the large pore pathway via which macromolecules the size of albumin or larger cross the vascular endothelium. In an earlier steady-state model [Am. J. Physiol. 248, H945-960 (1985)], the authors demonstrated that such an open-junction pathway could quantitatively account for the regional differences in macromolecular permeability observed in various mammalian arteries in regions of enhanced cell turnover as indicated by 3H-thymidine although these cells were less than 1% of the population and the open junctions occupied less than 10(-5) of the endothelial surface. The time-dependent models described herein have been used to identify a time window and size of probe molecule wherein this hypothesis could be tested experimentally in the larger blood vessels. The first stages of these experiments have now been completed and provide convincing evidence that the junctions of virtually all endothelial cells in the M phase of the cell cycle are leaky to macromolecules (Lin et al., 1988). The statistical frequency of such leakage sites has also been determined. The time-dependent models developed herein contain two important refinements that were not contained in the earlier steady state model. First the finite resistance of the open cleft as a function of molecular size is accounted for by introducing a diffusion coefficient ratio Dj/Dz describing the relative resistance of the open cleft compared to the subendothelial tissue in the direction normal to the endothelial surface. Second the non-isotropy of the vessel wall due to the elastic lamina is considered by introducing a second diffusion coefficient ratio Dx/Dz describing the relative resistance in the lateral as compared to the normal direction. This second ratio can be as large as 100 for the arterial intima, but is of order unity for capillaries. In Part I a short time model is presented to describe the initial labeling of the open cleft and the subendothelial space in the vicinity of the cleft exit following the introduction of a tracer macromolecule. This model is valid for both larger vessels and capillaries since wall thickness and curvature and the interaction between leakage sites does not enter into the model description. In Part II (Wen et al., 1988) a long-time model is developed for larger vessels only which is valid for greater times including steady-state labeling.

Albumins↗
PubMed2000

Time window control: a model for cerebellar function based on synchronization, reverberation, and time slicing.

We present a new hypothesis of cerebellar function that is based on synchronization, delayed reverberation, and time windows for triggering spikes. Our model suggests that granule cells admit mossy fiber activity to the parallel fibers only if the Golgi cells are firing synchronously and if the mossy-fiber spikes arrive within short and well-defined time windows. The concept of time window control organizes neuronal activity in discrete 'time slices' that can be used to discern meaningful information from background noise. In particular, Purkinje cell activity can trigger rebound spikes in deep cerebellar nuclei cells, which project via brain stem nuclei and mossy fibers back to the cerebellar cortex. Using a detailed model of deep cerebellar nuclei cells, we demonstrate that the delayed firing of rebound spikes is a robust mechanism so as to ensure that the reverberated activity re-arrives in the mossy fibers just during the granule-cell time window. Large network simulations reveal that synaptic plasticity (LTD and LTP) at the parallel fiber/Purkinje cell synapses that relies on the timing of the parallel fiber and climbing fiber activities allows the system to learn, store, and recall spatiotemporal patterns of spike activity. Climbing fiber spikes function both as teacher and as synchronization signals. The temporal characteristics of the climbing fiber activity are due to intrinsic oscillatory properties of inferior olivary neurons and to reverberating projections between deep cerebellar nuclei, the mesodiencephalic junction, and the inferior olive. Thus, the reverberating loops of the mossy fiber system and climbing fiber system may interact directly with the time windows provided by the circuitry of the cerebellar cortex so as to generate the appropriate spatio-temporal firing patterns in the deep cerebellar nuclei neurons that control premotor systems. In future studies the model will be extended in that high frequency simple spike activities will be included and that their relevance for motor control will be addressed.

Animals↗
PubMed2003

Motor timing deficits in community and clinical boys with hyperactive behavior: the effect of methylphenidate on motor timing.

In a previous paper we showed that community children with hyperactive behavior were more inconsistent than controls in the temporal organization of their motor output. In this study we investigated: (1) various aspects of motor timing processes in 13 clinically diagnosed boys with attention deficit hyperactivity disorder (ADHD) who were compared to 11 community boys with hyperactive behavior and to a control group and (2) the effect of methylphenidate on the motor timing processes in the clinical group with ADHD in a double blind, cross-over, medication-placebo design, including 4 weeks of medication. The clinical group with ADHD, like the community group with hyperactivity, showed greater variability in sensorimotor synchronization and in sensorimotor anticipation relative to controls. The clinical group was also impaired in time perception, which was spared in the community group with hyperactivity. The persistent, but not the acute dose, of methylphenidate reduced the variability of sensorimotor synchronization and anticipation, but had no effect on time perception. This study shows that motor timing functions are impaired in both clinical and community children with hyperactivity. It is the first study to show the effectiveness of persistent administration of methylphenidate on deficits in motor timing in ADHD children and extends the use of methylphenidate from the domain of attentional and inhibitory functions to the domain of executive motor timing.

Analysis of Variance↗
PubMed2006

Real-time and time-integrated PM2.5 and CO from prescribed burns in chipped and non-chipped plots: firefighter and community exposure and health implications.

In this study, smoke data were collected from two plots located on the Francis Marion National Forest in South Carolina during prescribed burns on 12 February 2003. One of the plots had been subjected to mechanical chipping, the other was not. This study is part of a larger investigation of fire behavior related to mechanical chipping, parts of which are presented elsewhere. The primary objective of the study reported herein was to measure PM(2.5) and CO exposures from prescribed burn smoke from a mechanically chipped vs. non-chipped site. Ground-level time-integrated PM(2.5) samplers (n=9/plot) were placed at a height of 1.5 m around the sampling plots on the downwind side separated by approximately 20 m. Elevated time-integrated PM(2.5) samplers (n=4/plot) were hung atop approximately 30 ft poles at positions within the interior of each of the plots. Real-time PM(2.5) and CO data were collected at downwind locations on the perimeter of each plot. Time-integrated perimeter 12-h PM(2.5) concentrations in the non-chipped plot (AVG 519.9 microg/m(3), SD 238.8 microg/m(3)) were significantly higher (1-tail P-value 0.01) than those at the chipped plot (AVG 198.1 microg/m(3), SD 71.6 microg/m(3)). Similarly, interior time-integrated 8-h PM(2.5) concentrations in the non-chipped plot (AVG 773.4 microg/m(3), SD 321.8 microg/m(3)) were moderately higher (1-tail P-value 0.06) than those at the chipped plot (AVG 460.3 microg/m(3), SD 147.3 microg/m(3)). Real-time PM(2.5) and CO data measured at a position in the chipped plot were uniformly lower than those observed at the same position in the non-chipped plot over the same time period. These results demonstrate that smoke exposures resulting from burned chipped plots are considerably lower than from burned non-chipped plots. These findings have potentially important implications for both firefighters working prescribed burnings at chipped vs. non-chipped sites, as well as nearby communities who may be impacted from smoke traveling downwind from these sights.

Air Pollutants↗
PubMed2000

Operative times, postanesthesia recovery times, and complications during sinonasal surgery using general anesthesia and local anesthesia with sedation.

OBJECTIVE: The goal was to compare complication rates and recovery times in patients undergoing elective septoplasty or endoscopic sinus surgery using local anesthesia with sedation (LAS) versus general anesthesia (GA). METHODS AND PATIENTS: A retrospective chart review of a consecutive sample of 177 patients undergoing elective septoplasty or endoscopic sinus surgery between July 1, 1994, and June 30, 1996, was carried out at our university-based outpatient surgery unit. Outcome measures included total operative time, surgical time, recovery time, and perioperative complications. RESULTS: Total operative and recovery times were shorter in patients undergoing LAS. The frequency of emesis, epistaxis, and nausea were less in the LAS population than in the GA population. Three patients who underwent GA required unplanned admissions. CONCLUSION: This study suggests that in selected patients undergoing sinonasal surgery, LAS may result in shorter total operative times, shorter recovery times, and less frequent nausea, emesis, and epistaxis than GA.

Adolescent↗
PubMed2003

Analysis of global systolic and diastolic left ventricular performance using volume-time curves by real-time three-dimensional echocardiography.

BACKGROUND: Left ventricular (LV) volume-time curves (VTC) have been described to provide quantitative data on the dynamics of global LV performance beyond ejection fraction. However, generation of VTCs by conventional 2-dimensional imaging techniques is inherently limited because of inaccurate geometric volume assumptions. We, therefore, studied whether the new concept of volumetric scanning as realized by real-time 3-dimensional echocardiography (RT-3DE) can be used to provide accurate VTCs. METHODS: In 30 healthy participants, VTCs were generated from 18 to 24 absolute LV volumes per second by transthoracic RT-3DE and compared with magnetic resonance imaging (MRI) used for reference. LVs were traced manually in 9 to 11 parallel, short-axis planes and volumes calculated by disk method. From VTCs, we determined peak ejection rate (PER), peak early filling rate (PFR), time to PER and PFR, and end-diastolic and end-systolic volumes. For initial clinical application, 2 patient groups of coronary (n = 15) and hypertensive heart disease (n = 16) were studied. RESULTS: In healthy participants, VTCs agreed with MRI (mean errors: PER, -39 +/- 67 mL/s; PFR, -18 +/- 84 mL/s; time to PER, 8 +/- 21 milliseconds; time to PFR 4 +/- 18 milliseconds [not significant vs 0]) whereas VTCs in coronary and hypertensive groups revealed significantly impaired diastolic function. Scanning time for VTCs was only 1 to 2 minutes by RT-3DE and 8 +/- 2 minutes by MRI (P <.001) and time for offline analysis was 22 +/- 5 minutes versus 24 +/- 4 minutes by MRI (not significant). CONCLUSIONS: Generation of VTCs by RT-3DE is feasible and shows excellent agreement with MRI used for reference. Thus, VTCs by RT-3DE is a promising new approach providing access to quantitative information on global LV performance such as LV filling rates that is currently unavailable for the cardiologist.

Adult↗
PubMed1993

Four questions of time to contact: a critical examination of research on interceptive timing.

Four questions concerning the use and perception of time to contact, tc, are identified. (i) Is tc information used in the timing of interceptive actions? (ii) If so, what control strategies are used? (iii) What are the perceptual sources of tc information and which of them do people use? (iv) How is the information extracted by the perceptual systems? Research relevant to these questions is reviewed and analysed. In connection with question (i), theoretical work on the special case of catching a moving object is analysed. It is concluded that treatments of catching which involve the use of tc information provide the best account of timing. In connection with question (ii), two types of control strategy suggested in the literature are identified: an intermittent strategy and a continuous strategy. Evidence for a continuous strategy is reconsidered and shown to be at least as well if not better accounted for by an intermittent strategy. Other empirical evidence for intermittent control is also discussed. In connection with question (iii) a simple unifying method is outlined with which all tc information so far presented in the literature can be derived, and examples are given. The viability of various types of information as sources of tc is examined by considering the errors which would result from their use. Finally, in connection with question (iv) the role of 'looming detectors' in the extraction of tc information is considered. These are frequently proposed as mechanisms for extracting the tc information provided by Lee's optic variable, tau. The analysis provided indicates that, despite the existence of a well-known and popular theory, due mainly to Lee, about how interceptive actions are timed, very little is actually known about perceptual timing. It is not yet certain whether tc information is used in interceptive timing tasks, what kinds of control strategies are involved, what sources of information people use to time their actions, or what perceptual processing is involved in the extraction of tc information.

Feedback↗
PubMedDate not supplied

Assessment of physical education time and after-school outdoor time in elementary and middle school students in south Mexico City: the dilemma between physical fitness and the adverse health effects of outdoor pollutant exposure.

Strategies to promote lifelong physical activity among children are needed to stem the adverse health consequences of inactivity. However, the health effects in growing children of long-term exposure to a polluted atmosphere are of deep concern. The atmosphere of south Mexico City (SMC) is characterized by a complex mixture of air pollutants, including ozone, particulate matter, and aldehydes. Radiological evidence suggests that small-airway disease could be present in clinically healthy, tobacco unexposed SMC children. The aim of this study was to assess, by means of a self-reported questionnaire, the physical education class times, daily outdoor after-school exposure time, and tobacco exposure in students attending public elementary and middle schools in SMC. Additionally, the time each student spent viewing television was assessed, and the authors measured each student's weight and height to determine body mass index (BMI, weight in kg divided by height in m2). The survey included 1,159 students in grades 7-9. The authors identified 2 critical periods of outdoor exposure in SMC children that coincided with significant concentrations of both ozone and particulate matter with diameters less than 10 micrometers (PM10): during school time after 11:00 A.M. and in the after-school outdoor activity period, usually extending from 1:00 P.M. to 6:00 P.M. Thirty-two percent of elementary and 61% of middle school students have physical education classes after 11:00 A.M. Students in SMC spend an average of 19.6 hr/wk outdoors in the after-school period, during which time they are engaged in light to moderate physical activities. Half of the students are exposed to tobacco smoke at home, and 7% of middle school students smoke. On the basis of BMI, 60% of students were classified as undernourished, overweight, or obese. No correlations were found between BMI and time spent viewing TV, time outdoors (on weekdays and weekends), or exposure to environmental tobacco smoke. Children and adolescents in SMC are participating in physical activities that enhance multiple components of health-related fitness. However, their activities occur outdoors, where they are exposed to high concentrations of air pollutants throughout the year. The authors believe that SMC children and adolescents must be educated, through both the school and health systems, regarding ways to obtain the necessary exercise while protecting themselves from the high concentrations of pollutants. Individuals should instruct and encourage young people to be involved in lifetime fitness activities and to eat balanced diets, if the goal is to control health-care costs, reduce disease incidence, and improve the overall quality of life of the Mexico City population.

Adolescent↗
PubMed2006

The effect of pedal rate and time of day on the time to exhaustion from high-intensity exercise.

The aim of this study was to examine the supposed influence of pedal rate on the diurnal fluctuation of the time to exhaustion from high-intensity exercise. Eleven male cyclists performed three tests at 06:00 h and three at 18:00 h at a free pedal rate (FPR) and two imposed pedal rates (80% and 120% of the FPR). They performed the tests until exhaustion using a power output corresponding to 95% maximal power (Pmax). Time to exhaustion, rectal temperature, oxygen consumption (.VO2), M. quadriceps, vastus medialis, M. biceps femoris electromyographic Root Mean Square activity rise (RMS slope), and blood lactate concentration were measured. The mean time to exhaustion recorded at 18:00 h (270.6+/-104.8 sec) was greater than at 06:00 h (233.9+/-84.9 sec). The time to exhaustion was significantly greater when the pedal rate was imposed at 80% versus 120% FPR. The blood lactate concentration and absolute core temperature at the point of exhaustion were significantly higher during tests done at 18:00 h. There was no diurnal variation in core temperature increase, .VO2, and RMS slope. The time-of-day effect for every variable did not depend on pedal rate. Diurnal variations in maximal aerobic endurance cannot be explained by a change in aerobic metabolism or in muscular fatigue. The origin of the diurnal variation in the time to exhaustion is likely to lie in greater participation in anaerobic metabolism. Also, the influence of temperature on neuromuscular functioning as an explanation for the diurnal variation in performance cannot be excluded in this study. The hypothesis on the basis of which pedal rate would influence diurnal variations in time to exhaustion in cycling was not validated by this research.

Adult↗
PubMed1992

Simple reaction time to focal transcranial magnetic stimulation. Comparison with reaction time to acoustic, visual and somatosensory stimuli.

We studied the effect of different go-signals on the reaction time in nine normal human subjects trained to respond by rapidly flexing one arm. Reaction times to auditory stimuli were shorter than those to visual or somatosensory stimuli, and were inversely correlated with the stimulus intensity. The reaction time was longest to a transcranial (magnetic or electric) stimulus delivered over the contralateral motor cortex that was sufficiently strong to induce a motor evoked potential in the responding biceps. Conversely, reaction time was shortest to either subthreshold transcranial stimulation over the same scalp position or to transcranial stimulation over the ipsilateral motor cortex regardless of intensity. Suprathreshold transcranial stimulation to the motor cortex seems to transiently inhibit the neurons responsible for initiation of motor programs involving muscles in which motor evoked potentials have been induced, thereby prolonging the reaction time. On the other hand, a subthreshold stimulus either disinhibits or directly activates such neurons leading to a shorter reaction time. Transcallosal connections between the motor cortices may account for the short reaction time to ipsilateral transcranial stimulation.

Acoustic Stimulation↗
PubMed2005

Effects of seed origin and sowing time on timing of height growth cessation of Betula pendula seedlings.

We studied the effects of seed origin and sowing time on height development and timing of height growth cessation of first-year silver birch (Betula pendula Roth) seedlings in a greenhouse experiment. Seeds of seven origins ranging in latitudes from 58 degrees to 67 degrees N were sown at 1-2-week intervals eight times from May 21 to July 30, 2001. The day/night temperature in the greenhouse was set at 20/10 degrees C, but lighting was natural and day length varied accordingly. Seedling height was measured twice a week. The interaction term between seed origin and sowing date was significant, but the pattern of height development and timing of growth cessation depended systematically on latitude of seed origin and sowing date. As seed origin became increasingly northern, growth cessation began earlier and resulted in shorter growth periods. Later sowing dates delayed growth cessation but also shortened the growth period. Final seedling height systematically decreased with increasingly northern origins and with later sowings. Linear regression analysis predicted timing of growth cessation, night length at growth cessation, length of growth period and final seedling height with high precision when the latitude of seed origin and sowing time were predictor variables. The timing of height growth cessation was determined by the seed origin, night length and developmental stage of the seedlings.

Betula↗
PubMed1993

The association between CNS direct care time and total time and very low birth weight infant outcomes.

This study examined the association between the amount of direct care time and total time (direct and administrative) spent by CNSs with families of 39 very low birth weight infants (< or = 1500 grams) and infant outcomes (rehospitalizations and acute care visits). Documentation of the CNSs' time spent with infants and families was recorded during infant hospitalization and for 18 months after infant discharge. The number of infant rehospitalizations and acute care visits was recorded from hospital and physician records. The mean total time the CNS spent per infant for the 18-month period was 27.3 hours. After discharge, there were significant relationships between the amount of CNS direct care time and the number of acute care visits (r = 0.45, p < 0.01) and infant rehospitalizations (r = 0.51, p < 0.01). Although the study findings did not demonstrate improved infant outcomes associated with the amount of time the CNS spent with this high risk population, they document the amount of CNS time spent in intervention and follow-up of these infants. This study provides data for costing home visiting by the CNS in this patient population.

Adolescent↗
PubMed1975

Depression: influence on time estimation and time experience

Time studies are partly concerned with time estimation partly with time experience. In a study of depression both principles were applied. Depression did not influence time estimation, but, on the other hand, the patients did report a slowing down of time (change in time experience). An item analysis of Beck's and Hamilton's rating scales showed that the disturbance in time experience accompanies items referring mainly to mood changes, only to a small extent to somatic symptoms, and not at all to changes in the psychomotor activity.

Adjustment Disorders↗
PubMedDate not supplied

Tumor kinetics and timing of the second dose to improve tumor radiocurability--a prediction of the best timing for fractionation by measuring 125I-iododeoxyuridine uptake in situ.

Experimental radiotherapy using two fractions of X-rays was carried out to study the best time of the second dose to improve the radiocurability of squamous cell carcinoma in WHT/Ht strain mice. Second doses of 27 Gy were given at 0, 6, 12, 24, 60 and 100 h after the first irradiation of 10 Gy. Each time interval relates to a specific change in tumor cell kinetics after irradiation of 10 Gy, that is, completion of PLD and SLD repair, most depression of 125I-IUdR uptake, beginning of increase of clonogenic cells, maximum cell loss rate and bottom regrowth curve, for 6, 12, 24, 60 and 100 h respectively. The local tumor control rates were 100, 78.1 and 96.8% for 0, 6 and 12 h respectively, and then control rates gradually decreased as interval times got longer. A second time window of local control was found 12 h after the first irradiation. This time corresponded to minimal uptake of 125I-IUdR, implying a decrease of S stage cells and depressed DNA synthesis, as suggested from labeling index and grain count studies. Twelve hours may be also the time when cells accumulate at the G2 stage (G2 block) or just after release of the G2 block from the kinetic viewpoint. The 125I-IUdR uptake method may be useful in determining the best time interval for fractionation in situ.

Animals↗
PubMed1989

Time-invariant oxygen cost of mechanical energy in dog left ventricle: consistency and inconsistency of time-varying elastance model with myocardial energetics.

We studied whether the oxygen cost of mechanical energy is time-invariant in the excised, cross-circulated canine heart. The total mechanical energy generated by ventricular contraction can be quantified by the total pressure-volume area (PVA) according to the time-varying elastance model. In this model, mechanical energy generated until a specified time (t) during systole can be quantified by the partial pressure-volume area, PVA(t). PVA(t) was obtained by quickly releasing ventricular volume at a varied time during isovolumic contraction. The quick release aborted further development of mechanical energy. We found that PVA(t) at a constant end-diastolic volume linearly correlated with myocardial oxygen consumption (VO2). This indicates that the oxygen cost of mechanical energy is time-invariant. However, we also found that the slope of the VO2-PVA(t) relation decreased with increasing quick-release speed. This indicates a decrease in VO2 by the quick release despite the same PVA(t). The time-invariant oxygen cost of mechanical energy is consistent with the time-varying elastance model of the ventricle, but the decreased VO2 with increasing quick-release speed despite the same PVA(t) is not.

Animals↗