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Biomedical subjects

Y Mahler

Publications and source records attributed to Y Mahler.

At least 73 records · Page 4Linked to original sources
PubMedDate not supplied

The effect of heart rate on the termination of electrically induced ventricular fibrillation in the isolated perfused rat heart.

Ventricular fibrillation (VF) which is normally sustained in large animals and humans, is transient in small animals. The purpose of the present study was to evaluate the possible effect of changing cardiac rate on spontaneous ventricular defibrillation. In isolated perfused rat heart, VF was electrically induced during normal spontaneous rhythm of the heart at normal rate and at various ventricular pacing rates. It was found that: 1) Electrically induced VF in isolated perfused, non-ischemic rat heart spontaneously terminated in 88% of the hearts; 2) Ventricular pacing rhythm of spontaneous rate plus 10% caused VF to be sustained in 26% of the hearts (which defibrillated spontaneously during normal rates); 3) Ventricular pacing at 200% of the basic rate led to sustained VF in about half the VF episodes (14 out of 33, p less than 0.005). In the remainder, which defibrillated spontaneously, a sustained VF could be achieved by further increase in ventricular pacing rate; 4) Slow pacing rate, as a result of the surgical production of atrioventricular (A-V) block, enhanced the probability of spontaneous defibrillation (21 of 21 episodes after slow pacing vs 24 of 34 episodes following pacing at previous normal sinus rhythm, p less than 0.05). Selective modulation of conduction velocity, refractory period or both, achieved by changes in ventricular pacing rate was assumed to play an important role in determining whether electrically-induced VF would be transient or sustained.

Animals↗
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The effect of calcium concentration on spontaneous ventricular defibrillation and VF threshold.

In order to investigate the influence of the effective refractory period on spontaneous ventricular defibrillation, isolated rat hearts were perfused with Krebs-Henseleit solution containing 0.5, 2.7 and 5.1 mM calcium. After measuring the fibrillation threshold at spontaneous rate (SR), ventricular fibrillation (VF) was induced during basic ventricular pacing of 110% SR, or the highest rate permitting 1:1 electromechanical coupling. The VF threshold was significantly reduced from 13.6 +/- 3.5 to 7.9 +/- 5.3 and 5.1 +/- 3.4 mA at 0.5, 2.7 and 5.1 mM Ca++ concentrations, respectively. The incidence of spontaneous recovery from VF, induced during basic pacing, was 100%, 83% and 50% at calcium concentrations of 0.5, 2.7 and 5.1 mM, respectively, (p less than 0.01 for the incidences at 0.5 mM versus 5.1 mM Ca++). The incidence of spontaneous defibrillation decreased when the hearts were driven rapidly, with spontaneous recovery rates of 92%, 58% and 0% (p less than 0.0001] for corresponding increases in Ca++ concentration. Induced ventricular fibrillation of fine morphology was frequently observed at 5.1 mM Ca++. It appears that progressive impairment of spontaneous defibrillation is caused by an increase in calcium concentration, this effect being more pronounced at high ventricular rates. Variations in the effective refractory period, caused by alterations in extracellular calcium concentration and differences in intracellular Ca++ accumulation, may account for the above results.

Animals↗
PubMedDate not supplied

Evaluation of time-dependent and quantitative properties of contraction from left ventricular pressure.

Several mechanical indices predicted from a model of cardiac muscle contraction are tested. In the in-vivo canine heart, dp/dtmax and t-dp/dt, the time interval from onset of contraction to dp/dtmax, were measured. The product of these parameters Ap = t-dp/dtmax was calculated. t-dp/dt was shortened when heart rate was elevated and remained constant when ventricular end diastolic volume was changed. Ap increased with augmentation of ventricular end diastolic volume. To achieve constant muscle length when heart rate is changed, analogous tension measurements (assigned as dT/dtmax, t-dT/dt and AT) of prestretched Walton Brodie strain-gauge arch had been taken instead of pressure measurements. In the experiments in which Tmax, maximal isometric tension, was not changed for various heart rates, AT was also unchanged. These results are consistent with the predictions that t-dp/dt and Ap can be used as two independent mechanical indices: 1) t-dp/dt for the evaluation of the "time-dependent properties" of contraction and 2) Ap, for the evaluation of the "quantitative properties" of contraction. The advantages of applying these two mechanical indices for use in the intact ejecting heart, instead of the well-established parameters Vmax and Po are emphasized.

Animals↗
PubMedDate not supplied

Correlation of blood temperature fluctuations with blood pressure waves.

Constant blood temperature in the pulmonary artery is assumed when the thermal dilution method is used for cardiac output determination. In some cases, however, slow temperature fluctuations (2-6 cycles per min.) occur in arterial and venous blood and interfere in the measurement. Those thermal fluctuations were investigated in the pulmonary artery and venae cavae of dogs. The temperature variations were found to be correlated with blood pressure waves: an increase of blood pressure was accompanied by an increase in the blood temperature in the pulmonary artery and a decrease in the blood temperature in the venae cavae. Therefore, measurement of the temperature of the pulmonary artery relative to that of the venae cavae does not rule out those fluctuations, and will not improve the thermal dilution method.

Animals↗
PubMedDate not supplied

Faster procedure for deriving regional blood flow by the noninvasive transient thermal clearance method.

The noninvasive transient clearance method provides absolute quantitative measurement of skin blood flow. It is based on thermally insulating the skin under investigation and measuring the time constant of the resultant exponential skin temperature increase. Conventional assessment of the time constant, by measuring the temperature increase until final equilibrium skin temperature is achieved, has the disadvantage of long time of measurement. A procedure shortening the measurement time is presented. The time constant of the exponential temperature increase is calculated without obtaining the final equilibrium temperature. The method can be used for calculating the time constant of an exponential change of any variable, even though the asymptotic value of that variable is not known.

Evaluation Studies as Topic↗
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Monitoring of erythrocyte aggregate morphology under flow by computerized image analysis.

The morphology of red blood cell (RBC) aggregates was studied by direct visualization of RBC aggregation at different flow conditions in a computerized image analyzer. The aggregate morphology is expressed by an Aggregate Shape Parameter (ASP), defined as the ratio of the aggregate projected area to its square perimeter. Aggregation was induced by either dextran-70 (m.w. 70,000) or dextran-500 (m.w. 500,000), and compared to that in plasma. It was found that the aggregate morphology is a characteristic of the aggregating agent--in dextran-500, the RBC form rouleau aggregates as in plasma, while in dextran-70, they form clusters. In each system, while maintaining the overall typical morphology, the ASP decreases (i.e., the aggregate becomes longer) as the aggregate size is increased. The distribution of the ASP as a function of the aggregate size remains unchanged when the aggregate size is changed by modulation of the dextran concentration or the shear stress. Stretching of a rouleau aggregate by application of shear stress is reflected by a corresponding change in the ASP. It is suggested that the ASP is a characteristic of intercellular interactions. A theoretical model is proposed for evaluation of the deviation of aggregate shape from that of rouleau structure.

Erythrocyte Aggregation↗
PubMedDate not supplied

Computer processing of artifact and arrhythmias in heart rate variability analysis.

Analysis of heart rate variability (HRV) with Holter monitoring is often difficult due to excessive artifacts and arrhythmias. While short sudden surges are treated successfully by most methods, slow heart rate (HR) variations, nocturnal trapezoidally-shaped HR increases and special types of arrhythmias which are similar to normal HRV fluctuations may distort further time domain and spectral analysis. This paper examines the advantages and disadvantages of different methods for preprocessing of HR data. We have developed the following approach to the analysis of HRV. (1) A combination method based on the absolute difference between HR values and both the last normal HR value and an updated mean is used for removal of artifacts and arrhythmias. This method can detect both sudden surges in HR values as well as longer periods of noise combined with slow normal variations. An additional stage of wild point removal is then optionally applied. (2) Certain special problems such as large T-waves, bigeminal rhythm, slow HR variations and nocturnal trapezoidally-shaped HR increases are also identified. Although none of the algorithms can be applied successfully to all cases, the final computer analysis for preprocessing described in the present study has proved to be superior to the simplified methods which are usually used and provides more suitable data for HRV analysis.

Algorithms↗
PubMedDate not supplied

The relationship between systolic blood pressure and microvascular resistance in non-diabetic and diabetic subjects.

Skin blood flow and microvascular resistance were measured in the feet of hypertensive and normotensive subjects by the non-invasive transient thermal clearance method. Skin blood flow decreased and microvascular resistance increased as a function of systolic brachial blood pressure for the non-diabetic subjects who were not treated by vasoactive medication. The relationship between blood flow and systolic blood pressure for diabetic patients and for patients who were treated by enalapril was poor. The average resistance for non-diabetic patients treated by enalapril was lower than that of untreated non-diabetic subjects. For diabetic patients no significant difference in resistance between enalapril treated and untreated patients was found. The results demonstrate that for non-diabetic patients the role of microvascular resistance in hypertension is significant while it is less important in diabetic patients.

Adult↗
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Effect of filter setting on the electromyographic parameters of muscles contracting to fatigue.

The effects of various filter settings on the electrophysiological behavior of the development of muscular fatigue were studied. Eleven healthy volunteers were examined during isometric contraction of biceps brachii and rectus femoris against a constant load until fatigue occurred. The electrical activity was taped and computer processing was carried out at the basic setting of 15-5000 cycles and at low (15-200 Hz) and high (200-5000 Hz) frequence filter. The results support the hypothesis that in the low range of frequencies there is a high density of large slow motor units, while in the high range of frequencies there are numerous small fast motor units.

Action Potentials↗