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Biomedical subjects

M Akhtar

Publications and source records attributed to M Akhtar.

At least 289 records · Page 16Linked to original sources
PubMed1989

Electrophysiological determinants of antidromic reentry induced during atrial extrastimulation. Insights from a pacing model of Wolff-Parkinson-White syndrome.

The electrophysiology of antidromic reentry, a less common phenomenon than orthodromic reentry, remains a poorly understood aspect of the Wolff-Parkinson-White (WPW) syndrome. We used a pacing model of ventricular preexcitation in patients without WPW, so that electrophysiological events in the normal pathway during atrial extrastimulation (A1-A2 technique) could be precisely delineated without the obscuring effect of an actual accessory pathway. Ventricular preexcitation was simulated by an A1-V1 sequential basic drive with A2-V2 extrastimulation at progressively shorter A1-A2 (equal to V1-V2) coupling intervals. At each coupling interval tested within the zone of atrioventricular (A-V) nodal effective refractory period (since anterograde block of A2 was considered mandatory for manifestation of antidromic reentry), responses were assessed after A2 alone (method I), V2 alone (method II), and A2 plus V2 (method III, the complete preexcitation model). The entire pacing protocol was performed at two A-V intervals, short (50 msec) and long (150-180 msec), thereby simulating different proximities between the A pacing site and "accessory pathway" location. Of 47 consecutive unmedicated patients screened for the study protocol, 38 failed to meet minimal prerequisites for possible initiation of antidromic reentry because of failure in 18 (38% of total) to achieve anterograde A-V nodal block of A2, even though 1:1 ventriculoatrial conduction to cycle lengths less than or equal to 500 msec (less than or equal to 400 msec in 12) was present; and poor or absent ventriculoatrial conduction in the others. The nine remaining candidates underwent the full pacing protocol. Antidromic reentry (retrograde atrial response following V2 in method III) was observed in only two cases (4% of total), and both were associated with retrograde His-Purkinje system delays (documented by method II) occurring in tandem with a long A-V interval, thereby allowing for completion of retrograde A-V nodal recovery after penetration by A2. Indeed, such a prolonged recovery time prevented initiation of antidromic reentry in six of the nine patients (proven by intact ventriculoatrial conduction in method II). Retrograde A-V nodal block of V2, independent of A2, prevented an antidromic echo in one case. Findings in our model help to clarify the various factors, including specific anterograde and retrograde A-V nodal properties; anatomic relation between the accessory and normal pathways; and the retrograde His-Purkinje system delays, that must prevail in a concerted fashion to permit the initiation of antidromic reentry during the A1-A2 technique in patients with the WPW syndrome.

Atrioventricular Node↗
PubMed1989

Electrophysiologic characteristics of sudden QRS axis deviation during orthodromic tachycardia. Role of functional fascicular block in localization of accessory pathway.

We analyzed the effect of functional fascicular block (FFB) on ventriculoatrial conduction time (VACT) during orthodromic tachycardia (OT) in 32 patients with single accessory pathway (AP) of the Kent bundle type. The location of AP was left free wall (LFW-AP) in 21 patients, left posteroseptal in 6, right free wall in 2, and right anteroseptal in 3. FFB either alone or in combination with functional left or right bundle branch block (LBBB or RBBB) occurred predominantly at the onset of OT and was initiated with ventricular extrastimulus technique more often than with atrial extrastimulation. In patients with LFW-AP, isolated functional left anterior fascicular block (LAFB) produced significant prolongation in VACT (15-35 ms). A similar magnitude of VACT increase (20-35 ms) was also observed when LAFB was associated with RBBB. Although 25-45-ms prolongation in VACT occurred with functional LBBB and normal axis, an additional 20-55-ms VACT increase was seen when LAFB accompanied LBBB. Functional LAFB, alone or in combination with bundle branch block, however, did not prolong VACT in patients with other AP locations. Furthermore, left posterior fascicular block did not produce prolongation of VACT in any of the cases. It is concluded that in patients with the Wolff-Parkinson-White syndrome, evaluation of VACT during functional LAFB provides important information regarding AP localization and a clear separation of LFW-AP from all other locations.

Adolescent↗
PubMed1989

Psychological support and psychiatric management of patients with automatic implantable cardioverter defibrillators.

There are well over 6,000 automatic implantable cardioverter defibrillators (AICD) that have been implanted in the United States since 1980. The device clearly reduces arrhythmic mortality in high risk patients. Many AICD patients have other cardiac diseases, most commonly, coronary artery disease with associated prior myocardial infarctions. These patients have special psychosocial stresses that may make them vulnerable to depression and other psychological disturbances. Psychological support together with psychiatric treatment, if needed, should be provided to these patients. Use of psychotropic medications should be closely monitored, especially in this population, as many of these drugs can exacerbate ventricular tachyarrhythmias. Special precautions should also be taken when administering electroconvulsive therapy in these patients.

Adjustment Disorders↗
PubMed1989

Susceptibility of Brucella melitensis to fluoroquinolones.

In vitro activity of four fluoroquinolones and four other antibacterial agents was tested against clinical isolates of Brucella melitensis. Initially all the 146 isolates studied were inhibited by 0.06-0.5 mg/l of ciprofloxacin and fleroxacin and 0.12-0.5 mg/l of pefloxacin or norfloxacin. One of these isolates developed resistance during therapy with ciprofloxacin, with a rise in MIC from 0.06 mg/l to more than 5.0 mg/l. This strain also showed cross-resistance to all other quinolones. All the isolates remained susceptible to tetracycline, gentamicin, rifampicin and trimethoprim-sulfamethoxazole. None of the quinolones showed in vitro synergy with other antibiotics.

4-Quinolones↗
PubMed1989

Use of calcium channel entry blockers in the treatment of cardiac arrhythmias.

In clinical arrhythmias, the main therapeutic role of calcium channel entry blockers is related to their effect on the sinus and atrioventricular (AV) node. Consequently, in cardiac arrhythmias where the AV node is part of the reentry circuit, a beneficial effect of diltiazem and verapamil can be demonstrated. These include AV nodal reentry and orthodromic tachycardia in patients with Wolff-Parkinson-White syndrome. In addition, the ventricular response by the AV node during atrial tachycardias can also be controlled with these agents. A specific type of ventricular tachycardia seen in the absence of structural heart disease has also been reported to respond to intravenous and oral verapamil. Calcium channel blockers have no proven depressant effect on accessory pathway conduction. Similarly, the value of these agents in the treatment of ventricular tachycardia in association with chronic coronary artery disease and idiopathic dilated cardiomyopathy is rather limited. The use of calcium entry blockers in patients with wide QRS tachycardia, therefore, is to be discouraged unless it can be proved that supraventricular tachycardia with aberrant conduction is the underlying basis.

Anti-Arrhythmia Agents↗
PubMed1988

Efficacy and electrophysiologic effects of encainide for atrioventricular nodal reentrant tachycardia.

To prospectively determine the clinical efficacy and electrophysiologic effects of encainide in atrioventricular nodal reentrant tachycardia (AVNRT), 49 patients refractory to 2.7 +/- 1.5 previous antiarrhythmic drug trials underwent electrophysiologic study before and 47 did so after administration of oral encainide (75 to 240 mg/day). Encainide prolonged the minimum atrial pacing cycle length maintaining 1:1 atrioventricular (AV) nodal conduction from 334 +/- 55 to 391 +/- 55 ms (p = 0.0001). Encainide induced ventriculoatrial (VA) block in 12 patients (25%) and slowed the minimum ventricular pacing cycle length maintaining 1:1 VA conduction from 315 +/- 46 to 485 +/- 89 ms (p = 0.0001) in the remaining 35 patients. After encainide, AVNRT was not inducible in 32 of 47 patients (68%) primarily because of the effects on retrograde AV nodal conduction. In the remaining 15 (32%) patients, AVNRT remained inducible; however, the tachycardia cycle length slowed from 397 +/- 86 to 492 +/- 90 ms (p = 0.0001). There was no significant difference in the baseline minimum ventricular pacing cycle length maintaining 1:1 VA conduction in patients whose inducible tachycardia was or was not suppressed. Forty-seven patients were treated for 18.9 +/- 12.9 months (range 1 to 50) with oral encainide. Encainide was completely effective in eliminating recurrences of supraventricular tachycardia in 26 of 47 patients (55%) and partially effective in an additional 42%. Recurrences of arrhythmia occurred in 15 of 32 patients (47%) whose inducible tachycardia was suppressed by encainide and 7 of 15 patients (47%) whose inducible tachycardia was not suppressed by encainide (p = not significant).(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1988

Role of adrenergic stimulation by isoproterenol in reversal of effects of encainide in supraventricular tachycardia.

Reversal of the electrophysiologic effects of oral encainide with isoproterenol was evaluated in 16 patients with atrioventricular (AV) nodal reentry (group A) and in another 16 patients with Wolff-Parkinson-White syndrome (group B). Sustained AV nodal reentry was induced in all group A cases before administration of encainide, in 2 cases after oral encainide, and in 12 patients during infusion of isoproterenol. Among group B cases, 14 of 16 had sustained AV reentry during control, 6 of 16 after receiving encainide, and 8 of 16 with addition of isoproterenol. During a mean follow-up of 19 +/- 10 months in group A and 17 +/- 9 months in group B, clinical tachycardia recurred in 8 patients (4 from each group). These 8 patients were among the 20 patients who demonstrated (1) isoproterenol-induced reversibility of encainide-suppressed tachycardia, or (2) acceleration of tachycardia rate slowed by encainide. No recurrences were seen among any of the 12 cases in which isoproterenol failed to reverse the encainide-induced tachycardia suppression. Patients with clinical recurrences were controlled with a variety of means, which included beta blockers in 3 and nonpharmacologic methods in the remaining 5. In patients with AV junctional tachycardia treated with oral encainide, our findings suggest that lack of tachycardia inducibility with isoproterenol predicts freedom from clinical recurrences. Furthermore, addition of a beta blocker to oral encainide may prevent clinical recurrence in some who demonstrate adrenergic reversal of encainide effect.

Administration, Oral↗
PubMed1988

Wide QRS complex tachycardia. Reappraisal of a common clinical problem.

BACKGROUND AND PURPOSE: Despite available criteria, diagnosis of the mechanisms of wide complex tachycardia is often incorrect. We aimed in this study to identify reasons for misdiagnoses and the value and limitations of clinical and surface electrocardiographic criteria. DATA IDENTIFICATION: The analyzed data of 150 consecutive patients with wide QRS tachycardia from this study and a literature search of key papers in English since 1960 on clinical and surface electrocardiographic criteria form the basis of this report. The final correct diagnosis was made with intracardiac electrograms. DATA EXTRACTION AND ANALYSIS: Among the 150 patients, 122 had ventricular tachycardia, 21 had supraventricular tachycardia with aberrant conduction, and 7 had accessory pathway conduction. Only 39 of 122 patients with ventricular tachycardia were correctly diagnosed initially. In others, the diagnoses were supraventricular tachycardia with aberrant conduction (43 of 122) or simply a wide QRS tachycardia (40 of 122). Misdiagnosis in patients with aberrant or accessory pathway conduction was also common. Standard electrocardiographic criteria for ventricular tachycardia had unacceptable sensitivity, poor specificity, or both. Collectively such criteria allowed a correct diagnosis of ventricular tachycardia in 92% of cases. Diagnosis of ventricular tachycardia was also suggested by its association with structural heart disease. Criteria suggestive of ventricular tachycardia included atrioventricular dissociation, positive QRS concordance, axis less than -90 deg to +/- 180 deg, combination of left bundle branch block and right axis, QRS duration of greater than 140 ms with right bundle branch block and greater than 160 ms with left bundle branch block and, a different QRS during tachycardia compared to baseline preexisting bundle branch block. CONCLUSIONS: Ventricular tachycardia is the commonest underlying mechanism for wide QRS tachycardia. A correct diagnosis can usually be made from clinical and surface electrocardiographic criteria.

Adolescent↗
PubMed1988

Automatic implantable cardioverter defibrillators and survival of patients with left ventricular dysfunction and malignant ventricular arrhythmias.

STUDY OBJECTIVE: To assess survival after insertion of automatic implantable cardioverter defibrillators in high-risk patients who have malignant ventricular arrhythmias and left ventricular dysfunction. DESIGN: Actual survival time compared with arrhythmia-free time in a single group of patients. SETTING: Inpatient services of a tertiary referral center and outpatient follow-up. PATIENTS: Seventy consecutive patients with clinical sustained ventricular tachycardia or fibrillation whose arrhythmia could not be controlled by medication as determined by programmed electrical stimulation, and who had an automatic cardioverter defibrillator implanted. INTERVENTION: All patients received an implantable defibrillator. MEASUREMENTS AND MAIN RESULTS: Two-year survival was 93.4% (95% CI, 87 to 99.8) and projected survival based on recurrence of malignant arrhythmias was 60.3% (CI, 47.3 to 73.3; P less than 0.001). In the 25 patients with left ventricular ejection fraction less than 30%, actual survival was 86.7% (CI, 72.3 to 91.1) and projected survival was 56.9% (CI, 35.9 to 77.9; P = 0.025). Projected survival percentages are similar to survival figures reported in the literature for such high-risk patients. There was only one sudden death; the remaining deaths were not arrhythmic in nature. Of the 65 patients who were alive at the end of follow-up, 13 were in New York Heart Association Class I; 44, Class II; 5, Class III; and 3, Class IV. CONCLUSIONS: The automatic implantable cardioverter defibrillator is probably highly effective in preventing arrhythmic mortality even in high-risk patients. Such treatment does not appear to significantly impair a patient's functional status.

Actuarial Analysis↗
PubMed1988

Substrate specificity of the 3-methylaspartate ammonia-lyase reaction: observation of differential relative reaction rates for substrate-product pairs.

A range of substituted fumaric and aspartic acid substrates for the enzyme 3-methylaspartate ammonia-lyase (EC 4.3.1.2) have been synthesized and used to study the kinetics of the catalyzed reaction in both the forward (deamination) and reverse (conjugative amination) reaction directions. The rates of amination for all of the alpha, beta-unsaturated substrates studied (bearing substituents the size of an ethyl group or smaller) were similar under [s] much greater than KM conditions although KM values for the substrates varied by a factor of 25. The rates of deamination for the corresponding 3-substituted amino acid substrates varied widely with structure under [s] much greater than KM conditions, and thus for substrate-product pairs the ratio for V(forward)/V(reverse) also varied. These differential reaction rates indicate that there is a step in the deamination direction that is especially sensitive to the size of the 3-substituent of the substrate and that a relatively large group (methyl to ethyl in size) is required for binding in order to reduce the activation energy for this step. Given that it is proposed that the enzyme operates via an E1cb-type mechanism where C-N bond cleavage is rate limiting, it is likely that binding of the C-3 substituent of aspartic acid substrates affects the alignment of the nascent carbanion with the C-N bond for elimination.

Ammonia-Lyases↗
PubMed1988

Primary deuterium isotope effects for the 3-methylaspartase-catalyzed deamination of (2S)-aspartic acid, (2S,3S)-3-methylaspartic acid, and (2S,3S)-3-ethylaspartic acid.

3-Methylaspartate ammonia-lyase catalyzes the deamination of (2S)-aspartic acid 137 times more slowly than the deamination of (2S,3S)-3-methylaspartic acid but catalyzes the amination of fumaric acid 1.8 times faster than the amination of mesaconic acid [Botting, N.P., Akhtar, M., Cohen, M. A., & Gani, D. (1988) Biochemistry (preceding paper in this issue)]. In order to understand the mechanistic basis for these observations, the deamination reaction was examined kinetically with (2S)-aspartic acid, (2S,3S)-3-methylaspartic acid, (2S,3S)-3-ethylaspartic acid, and the corresponding C-3-deuteriated isotopomers. Comparison of the double-reciprocal plots of the initial reaction velocities for each of the three pairs of substrates revealed that the magnitude of the primary isotope effect on both Vmax and V/K varied with the substituent at C-3 of the substrate. 3-Methylaspartic acid showed the largest isotope effect (1.7 on Vmax and V/K), 3-ethylaspartic acid showed a smaller isotope effect (1.2 on Vmax and V/K), and aspartic acid showed no primary isotope effect at all. These results, which are inconsistent with earlier reports that there is no primary isotope effect for 3-methylaspartic acid [Bright, H. J. (1964) J. Biol. Chem. 239, 2307], suggest that for both 3-methylaspartic acid and 3-ethylaspartic acid elimination occurs via a predominantly concerted mechanism whereas for aspartic acid an E1cb mechanism prevails.(ABSTRACT TRUNCATED AT 250 WORDS)

Ammonia-Lyases↗
PubMed1988

Mechanisms of clinical tachycardias.

Animal data suggest that cardiac arrhythmias can result from a variety of mechanisms. In clinical settings, arrhythmias that are easily initiated and terminated with programmed electrical stimulation are often designated as reentry tachycardias. However, proof of reentry is contingent upon demonstration of the entire circuit; this relation has been proposed for arrhythmias associated with large circuits, such as those seen in the Wolff-Parkinson-White syndrome. Reentry has also been proposed as the mechanism responsible for a variety of other tachycardias, including bundle branch and atrioventricular nodal reentry tachycardia, permanent junctional reentrant tachycardia, reentry tachycardia associated with nodoventricular Mahaim fibers and inducible atrial and ventricular tachycardia. Documentation of triggered rhythms as the mechanism responsible for clinical arrhythmias has been even more difficult. Examples of arrhythmias resulting from triggered activity may include those associated with digitalis toxicity arising from the atria, the atrioventricular junction or the ventricles. Clinical arrhythmias due to triggered activity in the absence of digitalis have also been described. Cardiac arrhythmias that cannot be induced by electrical stimulation are presumably due to normal or abnormal automaticity. Examples of normal automaticity in the human heart are sinus rhythm and junctional and idioventricular escape rhythms. Tachycardias by abnormal automaticity have seldom been investigated for the purpose of documenting the mechanism and therefore the limited data available make it difficult to draw any final conclusions.

Animals↗
PubMed1988

Aspiration cytology of retinoblastoma: light and electron microscopic correlations.

A series of 16 cases of retinoblastoma diagnosed by fine-needle aspiration biopsy (FNAB) and confirmed by histologic examination is reviewed, and the salient cytomorphologic features are described. Two types of cells were encountered in the aspiration smears; type I cells were undifferentiated while type II cells showed more differentiation and frequently revealed cytoplasmic processes that are probably indicative of early photoreceptor differentiation. Flexner-Wintersteiner rosettes characteristic of retinoblastoma were found in 10 of 16 cases. These findings were further correlated with ultrastructural examination of the tumors in nine cases. It is concluded that the presence of rosettes and type II cells with cytoplasmic processes are the two features that are most helpful in the FNAB diagnosis of retinoblastoma.

Biopsy, Needle↗
PubMed1988

Aspiration cytology of neuroblastoma: light microscopy with transmission and scanning electron microscopic correlations.

Fine-needle aspiration biopsies from three patients with neuroblastoma were studied by light microscopy, and the morphologic findings were correlated with those from transmission and scanning electron microscopy. Light microscopic examination of the aspiration smears from all three cases revealed small and large round cells with variable numbers of intertwining cytoplasmic processes. Transmission electron microscopy confirmed the light microscopic finding of cytoplasmic processes; in addition, it revealed the presence of other diagnostic morphologic features, including neurosecretory granules, microtubules, and synaptic cell junctions. Scanning electron microscopy demonstrated that the tumors were composed of a mixture of undifferentiated round cells and more differentiated cells with long cytoplasmic processes. The morphologic spectrum of these processes and their interrelationships with one another and with other cells could be studied in detail. These findings indicate that scanning electron microscopy may be used effectively in the morphologic evaluation and pathologic diagnosis of neuroblastoma.

Biopsy, Needle↗
PubMed1988

Absence of significant postdefibrillation bradyarrhythmias in patients with automatic implantable defibrillators.

Because postdefibrillation bradycardia or asystole is of clinical concern in patients with implanted automatic cardioverter defibrillators, we analyzed the cardiac rhythm after 157 intraoperative defibrillations in 50 patients undergoing implantation of the device. Factors that could influence the heart rhythm immediately after defibrillation were also analyzed. The postdefibrillation heart rate and the duration of postdefibrillation asystole did not correlate with age, conduction abnormalities, type or duration of arrhythmia before defibrillation, delivered energy, lead system, or left ventricular ejection fraction, but they did correlate with the resting preoperative heart rate and the concomitant performance of coronary artery bypass surgery at the time of defibrillator implantation. Temporary pacing (for prolonged asystole) was required in only two patients, both of whom had also undergone bypass surgery. Fourteen patients being treated with amiodarone had slower resting and postdefibrillation heart rates, but the mean duration of posdefibrillation asystole was similar to that seen in patients not receiving any antiarrhythmic medication. On follow-up, 25 monitored and 178 unmonitored automatic defibrillator discharges occurred in 24 patients, without evidence of symptomatic postdefibrillation bradyarrhythmias. Thus significant postdefibrillation bradyarrhythmias were uncommon, but when such a propensity existed it could not be predicted reliably on clinical grounds alone.

Adult↗
PubMed1988

Useful clinical criteria for the diagnosis of ventricular tachycardia.

Misdiagnosis occurs upon initial presentation to medical attention in a considerable number of patients referred for evaluation of wide QRS tachycardia. In order to improve diagnostic accuracy (ventricular versus supraventricular tachycardia), the answers to two key bedside questions were prospectively evaluated: (1) Had the patient experienced a prior myocardial infarction? (2) Did symptoms of tachyarrhythmia start only after the infarction? A patient presenting with a wide QRS tachycardia was considered to have ventricular tachycardia if he or she answered in the affirmative to both of these questions. Of 31 consecutive patients referred with electrocardiographically documented sustained wide QRS tachycardia that was reproduced in the electrophysiology laboratory, the diagnoses made when the patients first presented to medical attention were ventricular tachycardias in 17 patients and supraventricular tachycardias in 14 patients. Following electrophysiologic evaluation, 29 were diagnosed as having ventricular tachycardia and two as supraventricular tachycardia. If the diagnoses were made solely on the basis of responses to the bedside questions mentioned earlier, 28 of the 29 patients having a final diagnosis of ventricular tachycardia would have been correctly identified. It is concluded that the use of these two questions can be very helpful in improving the clinical diagnosis of ventricular tachycardia.

Adult↗
PubMed1988

Kaposi's sarcoma: the most common tumor after renal transplantation in Saudi Arabia.

Between September 1975 and November 1986, 263 renal transplant recipients at the King Faisal Specialist Hospital and Research Center were followed; 82 procedures were done by the authors using live related donors. Among the 263 patients, 14 cases of Kaposi's sarcoma were identified, an incidence of 5.3 percent compared with an incidence of 0.4 percent in renal transplant recipients from Western countries. In addition, two more patients had other types of tumors. Thus, Kaposi's sarcoma represents 87.5 percent of tumors in the King Faisal Hospital renal transplant population, in contrast to 3.7 percent in the Cincinnati Transplant Tumor Registry. The mean period between transplantation and diagnosis of Kaposi's sarcoma was 12.5 months (range, one to 37 months). Eleven patients were Saudis and three were other Arab nationals. Seven of the 11 Saudi patients were from the southwestern region of the country. Cytomegalovirus titers were not elevated in six of 10 patients. Results of tests for human immunodeficiency virus were negative in seven of eight patients. HLA-A2 antigen frequency was significantly increased in the King Faisal Hospital renal transplant patients with Kaposi's sarcoma as compared with a control population (83.3 percent versus 43.6 percent, p value = 0.006 [P = 0.06 with Bonferroni adjustment]), and increased, though nonsignificantly, compared with the live related kidney transplant recipients without Kaposi's sarcoma (83.3 percent versus 49.4 percent, p value = 0.058 [P = 0.58 with Bonferroni adjustment]), suggesting a genetic predisposition to Kaposi's sarcoma in these patients.

Adult↗