Search PubMed⌕ Search

Biomedical subjects

L Glass

Publications and source records attributed to L Glass.

At least 73 records · Page 4Linked to original sources
PubMed1989

Complex oscillations in a human motor system.

Experiments were performed to investigate the oscillations arising in a human motor system with delayed visual feedback. Eight subjects were instructed to maintain a constant finger position relative to a stationary baseline. The finger displacement was measured using a microdisplacement transducer connected to the index finger, and was displayed on an oscilloscope. Time delays between 40 and 1,500 ms were inserted in the visual feedback loop for 100 s. Results show that, as the time delays increase, irregular rhythms appear with short intermittent periods of regular oscillations. These regular low-frequency oscillations have an amplitude that increases with the time delays and a period that is consistently about 2 to 4 times the time delay. Fast Fourier Transforms (FFT) show a peak between 8 and 12 Hz, corresponding to physiological tremor in half the subjects. No systematic variations in the FFT for the 2 to 15 Hz range were observed as time delay increased. In the 0 to 2 Hz range, the FFT show a consistent increase in power with the time delay. These results indicate that, under the conditions of this experiment, tremor is not affected by time delays in the visuomotor system, and time delays in the visuomotor feedback loop give rise to complex oscillatory behaviors.

Journal Article↗
PubMed1989

A clinical study of the dynamics of parasystole.

A mathematical model for pure parasystole and modulated parasystole leads to a number of quantitative predictions. The predictive power of the model is examined by confronting it with data obtained from a 16-year-old symptomatic male born with a ventricular septal defect that was surgically closed at 5 years of age. A diagnosis of ventricular parasystole and inducible ventricular tachycardia was made following a syncopal episode. The physiological variables required by the model to make specific predictions are the sinus and ectopic cycle lengths and the ventricular refractory period. From these three variables, a two-dimensional parameter space is constructed consisting of the ratio of the refractory period to the sinus cycle length and the ratio of the ectopic to sinus cycle length. For any set of parameters, predictions are made concerning the number of sinus beats between ectopic beats. The different behaviors exhibited in the electrocardiographic (ECG) data agree with theoretical predictions.

Adolescent↗
PubMed1989

Beyond pure parasystole: promises and problems in modeling complex arrhythmias.

The dynamics of pure parasystole, a cardiac arrhythmia in which two competing pacemakers fire independently, have recently been fully characterized. This model is now extended in an attempt to account for the more complex dynamics occurring with modulated parasystole, in which there exists nonlinear interaction between the sinus node and the ectopic ventricular focus. Theoretical analysis of modulated parasystole reveals three types of dynamics: entrainment, quasiperiodicity, and chaos. Rhythms associated with quasiperiodicity obey a set of rules derived from pure parasystole. This model is applied to the interpretation of continuous electrocardiographic data sets from three patients with complicated patterns of ventricular ectopic activity. We describe several new statistical properties of these records, related to the number of intervening sinus beats between ectopic events, that are essential in characterizing the dynamics and testing mathematical models. Detailed comparison between data and theory in these cases show substantial areas of agreement as well as potentially important discrepancies. These findings have implications for understanding the dynamics of the heartbeat in normal and pathological conditions.

Animals↗
PubMed1989

Ionic basis and analytical solution of the wenckebach phenomenon in guinea pig ventricular myocytes.

The ionic mechanisms of slow recovery of cardiac excitability and rate-dependent activation failure were studied in single, enzymatically dissociated guinea pig ventricular myocytes and in computer simulations using a modified version of the Beeler and Reuter model for the ventricular cell. On the basis of our results, we developed a simplified analytical model for recovery of cell excitability during diastole. This model was based on the equations for current distribution in a resistive-capacitive circuit. A critical assumption in the model is that, in the voltage domain of the subthreshold responses, the sodium and calcium inward currents do not play a significant role, and only the two potassium outward currents, the delayed rectifier (IK) and the inward rectifier, are operative. The appropriate parameters needed to numerically solve the analytical model were measured in the guinea pig ventricular myocyte, as well as in the Beeler and Reuter cell. The curves of recovery of excitability and the rate-dependent activation patterns generated by numerical iteration of the analytical model equations closely reproduced the experimental results. Our analysis demonstrates that slow deactivation of the delayed rectifier current determines the observed variations in excitability during diastole, whereas the inward rectifier current determines the amplitude and shape of the subthreshold response. Both currents combined are responsible for the development of Wenckebach periodicities in the ventricular cell. The overall study provides new insight into the ionic mechanisms of rate-dependent conduction block processes and may have important clinical implications as well.

Action Potentials↗
PubMed1988

Phase-locked rhythms in periodically stimulated heart cell aggregates.

We have studied the effect of injecting a periodic train of current pulses into spontaneously beating aggregates of embryonic chick ventricular heart cells. Over a range of stimulation frequencies around the intrinsic frequency of an aggregate we find one action potential for each stimulus with a fixed latency from each stimulus to the subsequent action potential. For a stimulation frequency higher (lower) than the intrinsic frequency, this corresponds to overdrive (underdrive). At high frequencies of stimulation dropped beats occur leading to complex rhythms analogous to various Wenckebach rhythms observed clinically. At higher stimulation frequencies one can obtain a complete suppression of action potential generation. At low frequencies of stimulation, there are rhythms containing escape beats. Almost every rhythm seen bears a striking resemblance to some cardiac arrhythmia. We present a simple classification scheme that predicts the order of appearance of all the classes of rhythms experimentally observed as one changes the stimulation frequency. We propose that this scheme can be used generally to describe the behavior of other biological oscillators.

Action Potentials↗
PubMed1988

A cohort study of alkaloidal cocaine ("crack") in pregnancy.

The recent dramatic increase in the use of alkaloidal cocaine ("crack") has led to concern about possible deleterious fetal effects associated with its use during pregnancy. Crack, which is not destroyed by heating, can be smoked, and delivers a large quantity of cocaine to the vascular bed of the lung, producing an effect similar to that from intravenous injection. To describe the association of crack use with pregnancy outcome, we conducted a retrospective matched cohort study of 55 women who admitted to the use of crack during pregnancy and 55 non-drug-using women who delivered during the same period. The groups were matched for age, parity, socioeconomic status, alcohol use, and presence or absence of prenatal care. A significantly larger number of women using crack delivered at 37 weeks or earlier (50.9 versus 16.4%; P = .001). Crack-exposed infants were 3.6 times more likely to have intrauterine growth retardation (P less than .006) and 2.8 times more likely to have a head circumference less than the tenth percentile for gestational age (P less than .007). Premature rupture of the membranes was 1.8 times more common in the crack group (P less than .03). Sixty percent of crack-using mothers received no prenatal care. Abnormal neurobehavioral symptoms were present in a minority of infants and were usually mild.

Adult↗
PubMed1987

Prediction of complex atrioventricular conduction rhythms in humans with use of the atrioventricular nodal recovery curve.

Theoretical considerations indicate that complex patterns of atrioventricular conduction produced by rapid atrial stimulation can be predicted from changes in atrioventricular conduction produced by premature stimulation of the atrium. The purpose of this study was to evaluate the validity of this theoretical approach in seven patients undergoing electrophysiologic investigation. The atrioventricular nodal recovery curve was determined at two different basic cycle lengths. Subsequently, periodic atrial stimulation was delivered for 30 sec periods over a range of frequencies giving 11, Wenckebach, reverse Wenckebach, and 21 rhythms. The recovery curve data was then used to compute the response to periodic stimulation by an iterative technique. The conduction patterns actually seen during periodic atrial stimulation showed close agreement with the computed patterns. This work thus provides a unified explanation for the appearance of Wenckebach, reverse Wenckebach, alternating Wenckebach, and high grade block rhythms.

Atrioventricular Node↗
PubMed1986

Death-scene investigation in sudden infant death.

We conducted death-scene investigations in 26 consecutive cases in which a presumptive diagnosis of sudden infant death syndrome (SIDS) was made and the infants were brought to the emergency room of the Kings County Hospital Center between October 1983 and January 1985. In six cases, we observed strong circumstantial evidence of accidental death. In 18 other cases, we discovered various possible causes of death other than SIDS, including accidental asphyxiation by an object in the crib or bassinet, smothering by overlying while sharing a bed, hyperthermia, and shaken baby syndrome. This study suggests that many sudden deaths of infants have a definable cause that can be revealed by careful investigation of the death scene and that the extremely high rate of SIDS (4.2 per 1000 live births) reported in the population of low socioeconomic status served by Kings County Hospital Center should be questioned.

Accidents, Home↗
PubMed1986

Dynamics of pure parasystole.

A mathematical model is proposed for ventricular parasystole. In this model, there are two separate rhythms, a sinus rhythm and a ventricular ectopic rhythm. An ectopic beat will occur if the ectopic depolarization falls during a time interval when the ventricles are not refractory. Following an ectopic beat there is a compensatory pause. Analysis of this model, utilizing numerical simulation and techniques in number theory, demonstrates several new rules for parasystole. Specifically, for any set of fixed values for the sinus and ectopic frequencies and the ventricular refractory time, there are at most three different values for the number of sinus beats between ectopic beats. One and only one of these values is odd, and the sum of the two smaller values is one less than the larger value. The variation in the allowed values of the number of sinus beats between ectopic beats, as a function of the parameters of the model, is classified. Clinical cases found in the literature display certain aspects of the predictions of the theoretical model. Theoretical analysis of this kind provides new approaches to assessing the mechanism of complex ventricular arrhythmias.

Arrhythmias, Cardiac↗
PubMed1986

Phase resetting of spontaneously beating embryonic ventricular heart cell aggregates.

The influence of isolated 20-ms duration current pulses on the spontaneous rhythm of embryonic chick ventricular heart cell aggregates was studied. A pulse could either delay or advance the time of occurrence of the next action potential, depending on whether it fell early or late in the cycle. As the stimulus amplitude was increased, the transition from delay to advance occurred over a narrower range of coupling intervals. At low-stimulus amplitudes the transition from delay to advance occurred in a smooth continuous fashion; at medium-stimulus amplitudes the transition was discontinuous; at high-stimulus amplitudes graded action potentials were seen. It was impossible to annihilate spontaneous activity in aggregates with a single stimulus. The phase-resetting response to hyperpolarizing pulses was qualitatively the reverse of that produced by depolarizing pulses. A very high-amplitude depolarizing or hyperpolarizing pulse could produce rapid repetitive activity. Theoretical aspects of these phenomena are discussed.

Animals↗
PubMed1986

Respiratory phase locking during mechanical ventilation in anesthetized human subjects.

The coupling patterns between the rhythm of a mechanical ventilator and the rhythm of spontaneous breathing were studied in enflurane-anesthetized adult human subjects. The spontaneous breathing pattern was altered in response to different frequencies and amplitudes of forced lung inflations. A 1:1 phase locking (the frequency of the mechanical ventilator is matched by the frequency of spontaneous breathing with a fixed phase between the 2 rhythms) was observed in a range of up to +/- 40% of some of the subject's spontaneous breathing frequencies. During 1:1 phase locking, there were marked changes in the expiratory duration as measured from the electromyogram of the diaphragm. The phase relationship between onset of inflation and onset of inspiration depended on the frequency and amplitude of mechanical inflation. At ventilator settings that did not give 1:1 phase locking, other simple phase-locked patterns, such as 1:2 and 2:1, or irregular non-phase-locked patterns were observed. Reflexes arising from lung inflation, which may underlie the entrainment, are discussed in the context of these results.

Adolescent↗
PubMed1985

Apgar scores and cerebrospinal fluid beta-endorphinlike immunoreactivity during the first day of life. Preliminary observations.

beta-Endorphinlike Immunoreactivity (BLI) was measured in sterile, bloodless samples of cerebrospinal fluid (CSF) during the first 24 hours of life in order to assess the relationship between perinatal asphyxia and endogenous opioid activity within the central nervous system. The median CSF BLI in infants with one-minute Apgar scores of 1 to 4 was 148 pg/mL (range, 96 to 171 pg/mL) and that of infants with Apgar scores of 5 to 9 was 78 pg/mL (range, 25 to 162 pg/mL). The linear regression equation correlating CSF BLI with one-minute Apgar score was y = -10.7x + 169.1. Our findings of a highly significant inverse correlation between one-minute Apgar scores and CSF BLI support the hypothesis that perinatal asphyxia is associated with increased activity of opioid systems in the central nervous system.

Apgar Score↗
PubMed1984

Global bifurcations of a periodically forced nonlinear oscillator.

The effects of periodic pulsatile stimulation on a simple mathematical model of biological oscillations, called the radial isochron clock (RIC), are investigated as a function of stimulus frequency and amplitude. This system can be reduced to a two parameter, one-dimensional circle map. Numerical and topological methods are used to give a very detailed picture of the observed bifurcations over the complete range of parameters. The bifurcations are generic for a class of models which generalize the RIC.

Biological Clocks↗
PubMed1984

Discontinuities in phase-resetting experiments.

The effects of perturbing an on-going biological oscillation with a single brief stimulus are considered. If the time from some observable event before the stimulus to the next event after the stimulus is plotted as a function of the phase of the stimulus, then there may be discontinuities in this plot. The discontinuities reflect the size of the stimulus and the topological properties of the biological oscillation. The implications for experiments are discussed.

Animals↗
PubMed1984

A theory for phase locking of respiration in cats to a mechanical ventilator.

A mathematical model describing the Hering-Breuer reflexes in mechanically ventilated cats is developed. There is good agreement between the properties of the model and experimental studies performed over a wide range of frequencies and volumes of the mechanical ventilator. There is a correspondence between the model and a periodically forced nonlinear oscillator, similar to the van der Pol equation. Brain stem mechanisms underlying the entrainment are discussed.

Animals↗
PubMed1983

Neonatal methadone withdrawal. Effect of two treatment regimens.

Thirty-one infants exposed to methadone in utero who required pharmacologic treatment for withdrawal symptoms were randomly assigned to a paregoric or phenobarbital treatment group. Seven infants had symptoms too mild to require treatment. Respiratory rate, blood pH, PCO2, systolic BP, and serum thyroxine concentrations were measured on the 4th, 7th, and 14th days of life. Platelet counts were performed on the seventh and 14th days of life, and urinary catecholamine excretion was measured on the sixth day of life. Rates of weight gain were recorded during the second and third weeks of life. With the exception of a slightly higher blood PCO2 level in the phenobarbital-treated infants on day 7, no significant intergroup differences were observed in the treated infants. Paregoric-treated infants required a significantly longer period of treatment than phenobarbital-treated infants (22 v 17 days). This may have been due, at least in part, to the prolonged half-life and relatively high blood levels of phenobarbital present after cessation of therapy.

Blood Pressure↗