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Biomedical subjects

D Mehta

Publications and source records attributed to D Mehta.

At least 109 records · Page 6Linked to original sources
PubMed1990

Acute electrophysiologic effects of an HT2-serotonin antagonist, ketanserin, in humans.

The acute electrophysiologic effects of an intravenous bolus of ketanserin, a 5HT2 serotonin blocker, were studied in ten patients (four females, six males) during invasive electrophysiology. Following baseline electrophysiologic measurements during sinus rhythm and fixed-rate atrial pacing at 600 ms, a bolus of 0.2 mg/kg ketanserin was given over a 3-minute period. After 30 minutes all measurements were repeated. Systemic blood pressure was measured at regular intervals throughout. During sinus rhythm, there was no significant change in the basic cycle length or in the PA, AH, HV, QRS, QT, and QTc intervals. During atrial pacing there was a nonsignificant increase in the QT interval, from 342 +/- 13 ms to 366 +/- 16 ms, and a significant increase in the QTc interval, from 422 +/- 27 ms to 449 +/- 29 ms (p less than 0.05). There was no reduction in blood pressure. Thus ketanserin produced a significant prolongation of the QTc interval, in the absence of hypokalemia, in humans.

Adult↗
PubMed1990

Prospective evaluation of a protocol for induction of sustained ventricular tachycardia in patients referred to a tertiary centre.

All eight stages of a stimulation protocol that used one then two extrastimuli from the right ventricular apex in sinus rhythm and three ventricular drive rates (100, 120, and 140 beats/min) were performed in 24 patients with recurrent spontaneous sustained ventricular tachycardia despite drug treatment. Twenty two of the patients had sustained a previous myocardial infarct and 18 were on long term treatment with amiodarone. Sustained (greater than 30 s) ventricular tachycardia was induced in all patients. Two extrastimuli were significantly more likely to induce sustained ventricular tachycardia than one extrastimulus, both overall and individually for the three ventricular drive rates. A ventricular drive rate of 140 beats/min was significantly more likely to induce ventricular tachycardia than ventricular drive rates of 100 and 120 beats/min which were significantly more effective than sinus rhythm. A ventricular drive rate of 140 beats/min with one or two extrastimuli induced ventricular tachycardia in 23/24 (95%) of the patients in this study. The full eight stage protocol was progressive separately for both extrastimuli and ventricular drive rate but the last two stages (ventricular drive rate of 140 beats/min with one or two extrastimuli) were as effective as the entire protocol in inducing ventricular tachycardia.

Aged↗
PubMed1990

Increased leukocyte phospholipase A2 activity and plasma lysophosphatidylcholine levels in asthma and rhinitis and their relationship to airway sensitivity to histamine.

This study was carried out to determine leukocyte phospholipase A2 (PLA2) activity and plasma lysophosphatidylcholine (LPC) levels in normal subjects and in patients with asthma and rhinitis and to examine their relationship to airway sensitivity to histamine. Leukocyte PLA2 activity and plasma LPC levels were highly correlated (rs = 0.90), and were found significantly raised in both the disease groups, more so in the asthmatics. Both PLA2 activity and LPC levels showed overall negative correlations with the log dose of histamine producing a 35% fall in specific airway conductance (PD35 histamine) when the patients and the normal subjects were examined together (rs = -0.77 and rs = -0.83, respectively). The patients with leukocyte PLA2 activity more than 1.8 U (nmol 14C-AA released/mg protein/10 min at 30 degrees C) or plasma LPC levels more than 8.7% of total phospholipids (mostly asthmatics) showed high airway sensitivity to histamine (PD35 histamine less than or equal to 1 mg/ml). On the other hand, the PD35 histamine values of patients with leukocyte PLA2 activity equal to or less than 1.96 U or plasma LPC levels equal to or less than 8.7% overlapped with those of normal subjects in the range of 1.2 to 10 mg/ml. Lowering of plasma LPC levels, which probably reflect tracheobronchial LPC content as shown in guinea pigs, seems to be an essential step in the return of airway reactivity toward normal.

Adult↗
PubMed1989

Echocardiographic and histologic evaluation of the right ventricle in ventricular tachycardias of left bundle branch block morphology without overt cardiac abnormality.

The right ventricle was investigated by multiple biopsies and detailed echocardiographic evaluation, including measurement of cavity dimensions at the level of the inflow, body and outflow tract, in 27 patients with right ventricular tachycardia who had no clinical evidence of an underlying morphologic abnormality. Nine (33%) patients had abnormal biopsy results, with a quantifiable increase in interstitial fibrosis. Abnormal echocardiograms, defined as an increase in greater than or equal to 2 dimensions of the right ventricular cavity or wall motion abnormalities or both, were seen in 9 patients. There was a strong association between abnormal myocardial histologies and abnormal right ventricular echocardiograms (p less than 0.001). An abnormal echocardiogram was 94% specific and 80% sensitive for an abnormal biopsy. The findings of echocardiography and biopsy were correlated with the electrocardiographic features of the tachycardia. Evidence of right ventricular disease was seen in all 6 patients with superior frontal plane axis of clinical tachycardia as compared with 4 of 21 with inferior axis (p less than 0.001). Thus, 2-dimensional echocardiography is a sensitive means of diagnosing right ventricular disease in patients with nonischemic tachycardias of left bundle branch block morphology. A superior frontal plane axis of ventricular tachycardia in this group strongly suggests right ventricular disease, whereas an inferior frontal plane axis is frequently not associated with any morphologic or histologic abnormality of the right ventricle.

Adult↗
PubMed1989

Signal-averaged electrocardiography and the significance of late potentials in patients with "idiopathic" ventricular tachycardia: a review.

High-frequency, low-amplitude electrical activity during and in continuation with the ventricular electrogram in sinus rhythm is now accepted as an indirect marker for reentrant pathways in ventricular muscle. Because the amplitude of these signals is low, much effort has recently been directed toward finding methods of recording these noninvasively. This essentially involves averaging the electrocardiogram, amplifying it, and filtering out the low frequency and nonrepetitive signals. This review discusses the methods presently available for signal-averaged electrocardiography and their background. The clinical application of signal-averaged electrocardiograms in patients with or suspected to have ventricular tachycardia is summarized. Data regarding the incidence of abnormal signal-averaged electrocardiography in patients with "idiopathic" ventricular tachycardia compared with normal subjects and patients with ventricular tachycardia related to ischemic heart disease are presented.

Action Potentials↗
PubMed1989

Megaprostheses in the treatment of primary malignant and metastatic tumors in the hip region.

Twenty patients with malignant bone tumors of the hip region were treated surgically by resection and reconstruction with an endoprosthesis. Histologic types included five primary bone tumors and 15 metastatic lesions. At review four primary bone tumor patients are still alive without evidence of disease. The length of the observation period varied from 26 to 104 months. Eleven patients with metastatic bone disease died. The average postsurgical survival time was 23 months. All patients were able to walk with or without a cane. Failure of an endoprosthesis occurred in one case. According to the Enneking Evaluation System 11 patients had a good and 9 a fair result.

Adolescent↗
PubMed1989

Correlation artifacts in speed of sound estimation in scattering media.

A recently described method for speed of sound estimation in tissues in pulse-echo mode involves reception of echoes generated by an ultrasonic pulse by means of a linearly tracking transducer. When the peaks of echo amplitudes are used as markers of arrival time, stairstep-like artifacts appear in the echo arrival time vs. transducer position plots. We postulate that these artifacts are a consequence of the speckle phenomenon commonly encountered in ultrasonic imaging. To test this hypothesis, we report computer simulations and water tank experiments which demonstrate similarities between the behavior of the stairsteps and the properties of ultrasonic speckle. Additionally, equations describing the precision of the speed of sound estimation in terms of the second order statistical properties of the stairstep artifact are derived.

Computer Simulation↗
PubMed1989

Correction of refraction and other angle errors in beam tracking speed of sound estimations using multiple tracking transducers.

The beam tracking approach to the estimation of the speed of sound has shown potential for making unbiased estimates in tissues. The speed of sound in a medium can be found from the arrival times of echoes as a function of the position of a tracking transducer. There is a problem in this approach if the angle between the direction of tracked beam and the direction of tracking translation is not zero due to refraction or other effects. An angle error as small as 1 degree would result in an error that is too large for diagnostic applications. A modified technique using three or more tracking transducers is described. This yields a corrected speed of sound estimate, and calculates the angle error. A simulation program has shown that this modified technique could indeed correct for the angle errors.

Computer Simulation↗
PubMed1989

Significance of signal-averaged electrocardiography in relation to endomyocardial biopsy and ventricular stimulation studies in patients with ventricular tachycardia without clinically apparent heart disease.

Signal-averaged electrocardiography (ECG) was performed in 38 patients (mean age 38 years, range 15 to 70) with ventricular tachycardia who had no clinical evidence of structural heart disease. Spontaneous ventricular tachycardia was nonsustained in 23 patients and sustained in 15. None of the patients had symptoms of heart failure or ischemic heart disease, and at cardiac catheterization none had significant coronary artery disease or left ventricular wall motion abnormalities. In addition, all patients underwent left and right ventricular endomyocardial biopsy and ventricular stimulation studies. Signal-averaged ECG was performed and late QRS potentials were defined with use of Simson's method. Late QRS potentials were detected in a minority (18%) of patients including 2 of 23 with nonsustained and 5 of 15 with sustained (p = NS) ventricular tachycardia. Fifteen patients (40%) had abnormal endomyocardial biopsy results and these findings were more common in patients with sustained than in those with nonsustained ventricular tachycardia (9 of 15 versus 6 of 23, p less than 0.05). Late potentials were associated with abnormal endomyocardial biopsy findings (6 of 15 versus 1 of 23, p less than 0.01). An increase in fibrous tissue was the most frequent histopathologic abnormality; this increase was quantified by morphometric methods and compared with biopsy findings in normal control subjects. In the control group the proportion of collagen in relation to myocytes was less than 10%. All patients with both late potentials and abnormal biopsy findings had a greater than 15% ratio of collagen to myocytes in at least one specimen and the biopsies revealed marked interstitial fibrosis.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1989

Respiratory responses to sustained isometric muscle contractions in man: the effect of muscle mass.

1. Respiratory responses to sustained isometric contractions of a small mass of muscle (the finger flexors) during handgripping, and of a larger mass of muscle (the quadriceps) during extension of the leg at the knee, have been studied in man. 2. For both masses of muscle the increases of ventilation and of oxygen consumption were greater for contractions at 40% maximum voluntary contraction (MVC) than for contractions at 20% MVC. 3. The increase of ventilation was not related to the mass of muscle involved. 4. At 20% MVC oxygen consumption during contraction of the quadriceps was greater than that during handgripping. At 40% MVC the oxygen consumptions were similar. The oxygen debts following both handgrip and knee extensor contractions at 20% MVC were negligible. Following 40% MVC contractions of the quadriceps a significant oxygen debt was recorded but no debt was apparent following 40% MVC contractions of the finger flexors. 5. The increases of ventilation during isometric exercise were generally inappropriately high for the increases of gas exchange. This led to reductions of the end-tidal carbon dioxide pressure (PET,CO2), especially towards the end of exercise. 6. Following 40% MVC handgripping hyperventilation continued despite the reduced alveolar PCO2. By contrast, following 40% MVC knee extension PET,CO2 transiently rose above the resting level, but did not stimulate ventilation. 7. It appears that following fatiguing isometric contractions hyperventilation continues and appears to be independent of alveolar PCO2. It is suggested that stimuli which increase ventilation during exercise may continue to act during the early phase of recovery.

Adult↗
PubMed1989

Optimal atrioventricular delay at rest and during exercise in patients with dual chamber pacemakers: a non-invasive assessment by continuous wave Doppler.

The optimal atrioventricular delay at rest and during exercise was investigated in nine patients with heart block and implanted dual chamber pacemakers. All patients studied had normal left ventricular function and a normal sinus node rate response to exercise. Cardiac output was measured by continuous wave Doppler and was calculated as the product of stroke distance measured by Doppler at the left ventricular outflow, aortic root area measured by M mode echocardiography, and heart rate. Pacemakers were programmed in the DDD mode. Cardiac output was measured with the patient at rest while supine and while erect and at the peak of submaximal exercise (the end of stage 1 of the Bruce protocol) with the pacemakers programmed to the following atrioventricular intervals: 75-80 ms, 100-110 ms, 140-150 ms, and 200 ms. During exercise the basic pacing rate was programmed to 70 beats/min. Cardiac output at rest while supine and erect was greatest with an atrioventricular delay of 140-150 ms and it was significantly higher than that with an atrioventricular delay of 75-80 ms. On average there was a 31% decrease in cardiac output when patients stood up. During treadmill exercise, however, cardiac output was greatest when the atrioventricular delay was 75-80 ms, and this was significantly higher than the cardiac output with atrioventricular delays of 150 and 200 ms. During exercise 1:1 atrioventricular relations were maintained in patients at all atrioventricular intervals. In patients with atrioventricular sequential pacemakers cardiac output at rest is greatest with an atrioventricular delay of 140-150 ms but during exercise the optimal atrioventricular delay is shorter. Rate modulation of the atrioventricular interval may improve the haemodynamic response and possibly exercise tolerance in patients with dual chamber pacemakers.

Adult↗
PubMed1988

Relative efficacy of various physical manoeuvres in the termination of junctional tachycardia.

The ability of four vagotonic physical manoeuvres to terminate paroxysmal junctional tachycardias was tested in 35 patients with inducible and sustained arrhythmia. Each manoeuvre was used up to three times in an attempt to terminate an induced tachycardia and was judged to be effective if it terminated two out of the three induced episodes. The Valsalva manoeuvre in the supine position was effective in 19 (54%), right carotid sinus massage in 6 (17%), left carotid sinus massage in 2 (5%), and the diving reflex in 6 (17%) cases. 4 of the 6 patients who responded to right carotid sinus massage and all patients who responded to the diving reflex also responded to the Valsalva manoeuvre. The Valsalva manoeuvre while standing was effective in 9 (20%) patients only. Patients in whom the manoeuvres terminated the tachycardias were significantly younger than those who did not respond (median age: 30 vs 45 years, p less than 0.01). Physical manoeuvres were much more successful in terminating atrioventricular re-entry tachycardias (19/24) than atrioventricular nodal re-entry tachycardias (3/11, p less than 0.01). Efficacy of the manoeuvres was related to their bradycardic effect in sinus rhythm.

Adolescent↗
PubMed1988

Clinical electrophysiologic effects of flecainide acetate.

Flecainide acetate depresses the rate of depolarization of action potential (Vmax), the so-called "membrane stabilizing action." In the intact heart it has a unique profile of substantial effect on conduction with modest effect on refractoriness. After intravenous administration, clinical electrophysiologic studies show that conduction through atrial myocardium, atrioventricular (AV) node, His-Purkinje system, and ventricular myocardium is depressed, the most prominent effect being on the His-Purkinje system. Refractorines of the normal atrial and AV nodal myocardium is not prolonged while that of the ventricular muscle is slightly increased. Atrial fibrillation (60% to 70%), atrial tachycardia (90% to 100%), and nodal and AV tachycardia (80% to 90%) are generally terminated, while flutter is usually slowed, but in a small proportion of patients (10% to 20%) might be terminated by the intravenous use of flecainide acetate. This drug has also been shown to be effective in terminating stable ventricular tachycardia (70%). However, it appears to be slightly less effective in suppressing inducibility of ventricular arrhythmias. Administered orally, flecainide is very effective in decreasing ventricular ectopic activity (80% to 95%) and nonsustained ventricular tachycardia. Thus, flecainide has a wide range of antiarrhythmic properties, making it a useful agent in the management of a variety of supraventricular and ventricular arrhythmias. In a small proportion of patients, however, its use can lead to apparent arrhythmogenic effects, the most dangerous being exacerbation of ventricular tachycardia.

Animals↗
PubMed1988

Limitations of rate response of an activity-sensing rate-responsive pacemaker to different forms of activity.

The responses of an activity-sensing rate-responsive system (Activitrax) to various forms of physiological activity were assessed in 15 individuals who had this pacemaker. Nine were patients with complete heart block and atrial arrhythmias; their mean age was 60 years (range, 41-85 years). Six were age-matched healthy volunteers who were exercised with an external Activitrax system attached firmly to the chest wall. The pacemaker was programmed to achieve a pacing rate of about 100 bpm at the end of the first stage of the Bruce protocol (pacemaker settings: rate = 70-150 bpm; threshold = low to medium; response = 6-9). In the activity-sensing ventricular pacing mode, all patients achieved a significant increase in treadmill time compared to constant-rate ventricular pacing (mean +/- SD, 8.0 +/- 3.3 vs 5.4 +/- 2.3 minutes; p less than 0.01), with a mean maximum pacing rate of 123 +/- 18 bpm. Jogging in place produced a prompt increase in pacing rate, with the maximum achieved at the end of the exercise. However, physiological activities such as hand-grip, the Valsalva maneuver and standing resulted in only minimal rate response. Pacing rate after ascending 4 flights of stairs was the same as that achieved after descending the same stairs (100 +/- 8 vs 105 +/- 4 bpm; p = 0.1). All 15 subjects were exercised from resting heart rate for 3 minutes on a treadmill at 1.2 mph and 2.5 mph with four gradients at each speed. Although the pacing rate increased with a faster treadmill speed (p less than 0.005), it did not respond appropriately to a change in gradient compared to the sinus rate. We conclude that although activity-sensing rate-responsive pacing gives a prompt increase in pacing rate and improves maximum exercise tolerance, further refinement is necessary because: (1) physiological activities not associated with significant movement are not detected by this pacing system; (2) detection of vibrations as an indicator of activities does not correlate well with the level of exertion.

Adult↗
PubMed1988

Comparative evaluation of chronotropic responses of QT sensing and activity sensing rate responsive pacemakers.

The rate responses of activity sensing (ATS) and QT sensing (QTS) rate responsive pacemakers to different forms and durations of exercises were compared. Nine patients with ATS and five with QTS were studied. All had complete heart block and atrial arrhythmias. At the onset, the pacemakers were programmed to achieve a pacing rate of 100-110 bpm by the end of stage 1 of the Bruce protocol, and to a pacing rate range of 70-150 bpm. With progressive exercise, using a treadmill (Bruce protocol), the maximum pacing rates in the two groups were not significantly different (mean +/- SD: 123 +/- 18 vs 129 +/- 23 bpm, ATS vs QTS). The time taken to return to the baseline pacing rate during recovery was significantly longer with QTS (178 +/- 70 vs 264 +/- 68 s, p less than 0.05). Brief exercise tests on a treadmill were performed for 3 min each with different combinations of treadmill speeds (1.2 and 2.5 mph) and gradients (0, 5, 10 and 15%). In both groups of patients, faster walking speed was associated with a faster pacing rate at each gradient. However, with increasing gradients, at each speed, there was a rise in the maximum pacing rate only in patients with QTS. During brief exercise tests, the maximum rate was achieved by the end of exercise in patients with ATS, but was delayed by 33 +/- 20 s after exercise in patients with QTS.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1988

Elimination of diffraction error in acoustic attenuation estimation via axial beam translation.

Optimized wideband attenuation estimations were performed on a tissue mimicking phantom in a water tank with and without axial beam translation (ABT), and the results were compared to those from standard substitution measurements. A -17 percent discrepancy between the results of the substitution method and the optimized estimation without ABT was noted in the far field. This discrepancy was eliminated when ABT was utilized.

Models, Structural↗