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Biomedical subjects

M Akhtar

Publications and source records attributed to M Akhtar.

At least 361 records · Page 20Linked to original sources
PubMed1984

Functional characteristics of retrograde conduction in a pacing model of "endless loop tachycardia".

A pacing model was designed that stimulated "endless loop tachycardia," a complication found in the new generation of DDD (atrioventricular [AV] universal) pacemakers. The functional characteristics of the train of ventricular impulses simulating endless loop tachycardia were studied during both AV sequential pacing and basic ventricular drive. AV sequential pacing, by causing a decrease in ventriculoatrial (VA) conduction time of the first beat of the endless loop tachycardia, was associated with a decrease in the cycle length at which VA block occurred in 9 of 12 patients. The site of block was the His-Purkinje system in 4 of these 12 patients and the AV node in the remaining 8. At a cycle length with 1:1 VA conduction, a steady state VA conduction time was achieved in 2 to 4 beats (VA conduction time accommodation). The pattern of such accommodation depended on the site (His-Purkinje system versus AV node) of the maximal conduction delay. The steady state VA conduction time itself was altered with AV sequential pacing in patients showing His-Purkinje system delay, but not in patients with AV nodal delay. The results suggest that in most patients, the cycle length of VA block and the longest steady state VA conduction time will depend on the retrograde conduction time of the first beat of the tachycardia. In addition, pharmacologic measures to prevent or terminate endless loop tachycardia will have to take into account the fact that both the His-Purkinje system and the AV node can be the site of initial block.

Atrioventricular Node↗
PubMed1984

Effect of amiodarone on conduction and refractoriness of the His-Purkinje system in the human heart.

Although the antiarrhythmic aspect of amiodarone has been extensively studied, its effects on His-Purkinje system conduction and refractoriness have not been systematically investigated in human beings. In 24 patients, anterograde His-Purkinje system conduction (HV intervals) and variables of His-Purkinje system refractoriness using the ventricular extrastimulus (V2) technique were analyzed before and after long-term therapy with amiodarone. The mean duration of amiodarone therapy at the time of repeat study was 16.2 +/- 7.7 weeks (range 11 to 42). The anterograde His-Purkinje system conduction time (HV interval) measured 49.6 +/- 9.5 ms (range 40 to 80) before and 60.6 +/- 10.7 ms (range 45 to 90) after amiodarone (p less than 0.005). During retrograde refractory period studies, the longest V1V2 interval at which a retrograde His bundle potential (H2) emerged from the V2 electrogram (relative refractory period of the His-Purkinje system) was consistently longer after amiodarone as compared with the control period (376.4 +/- 46.6 versus 318.8 +/- 33.1 ms, p less than 0.005). Similarly, the shortest and longest His-Purkinje system conduction times ( V2H2 interval) at comparable V1V2 intervals were uniformly and significantly prolonged after administration of the drug. Amiodarone also abolished macroreentry in the His-Purkinje system in six of the nine patients who showed such reentry during the control period. The effective refractory period of the ventricular myocardium was also increased from a mean of 227.1 +/- 13.9 to 259.2 +/- 20.2 ms (p less than 0.005) in this series of patients.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1984

A variant of Bartter's syndrome. Bartter's syndrome associated with hydramnios, prematurity, hypercalciuria and nephrocalcinosis.

A case of early onset Bartter's syndrome associated with hydramnios, prematurity, hypercalciuria and nephrocalcinosis is reported. A literature review of Bartter's syndrome supports the hypothesis that the findings in this infant constitute a specific variant of Bartter's syndrome inherited in an autosomal recessive mode. Fetal polyuria in Bartter's syndrome leads to hydramnios, and the excess fluid causes premature birth. This variant of Bartter's syndrome should be included in the differential diagnosis of hydramnios, especially if the woman has had previous hydramnios resulting in a perinatal death. The disorder responds to treatment with indomethacin.

Bartter Syndrome↗
PubMed1984

Facilitation of macroreentry within the His-Purkinje system with abrupt changes in cycle length.

We have recently described the ability of abrupt short-to-long changes in atrial cycle length (CL) to prolong refractoriness of the His-Purkinje system (HPS) and increase the likelihood of aberrant ventricular conduction. We have also shown similar functional behavior in retrograde refractoriness of the HPS during changes in ventricular CL. To further assess these characteristics we evaluated the effect of abrupt short-to-long change in ventricular CL on the phenomenon of macroreentry within the HPS (Re-HPS) in 20 patients in whom Re-HPS occurred during application of a ventricular extrastimulus (V2) at a constant ventricular CL (method I) and/or with abrupt short-to-long change in CL (method II). For both methods V2 was coupled to a CL of identical duration, designated the reference CL (CLR). In method I the CLs preceding (CLP) the CLR equaled CLR, whereas in method II CLP was less than CLR. The results showed a dramatic increase in occurrence of Re-HPS with abrupt short-to-long change in CL with Re-HPS occurring in 19 patients with this method compared with in 11 patients during constant CL. In 10 patients manifesting Re-HPS with both methods the associated retrograde conduction (V2H2) delays were equal or less during abrupt short-to-long changes in CL and, remarkably, there were concomitantly shorter antegrade conduction (H2V3) delays compared with at a constant CL. Moreover, despite the resulting shorter V2V3, additional Re-HPS beats were also more likely to occur with abrupt short-to-long change in CL compared with at a constant CL.(ABSTRACT TRUNCATED AT 250 WORDS)

Bundle of His↗
PubMed1984

Postextrasystolic alterations in refractoriness of the His-Purkinje system and ventricular myocardium in man.

The changes in refractoriness of the His-Purkinje system (HPS) and ventricular myocardium (VM) that are associated with the occurrence of postextrasystolic beats in man are unknown. Accordingly, using a pacing model of the cycle length changes created by a ventricular extrasystole-postextrasystole sequence, we measured retrograde HPS and VM relative and effective refractory periods (RRP and ERP) in 15 patients with the use of ventricular test extrastimuli during preextrasystolic basic control drive (method I) and after programmed extrasystolic (method II) and postextrasystolic (method III) beats. The basic cycle length (same for all three methods) ranged from 500 to 700 msec (mean 613 +/- 74 msec) and the extrasystolic coupling interval (identical for methods II and III) comprised 68 +/- 4% of the basic cycle length. In method III the postextrasystolic pause was programmed to equal the basic cycle length (i.e., noncompensatory) so that method I could serve as control for method III. RRP-HPS decreased from 331 +/- 37 msec in method I to 245 +/- 37 msec or less during the extrasystolic beat (p less than .001). A less dramatic corresponding shortening of ERP-VM and RRP-VM was observed, i.e., from 245 +/- 21 and 264 +/- 23 msec (method I) to 233 +/- 23 and 251 +/- 22 msec (method II), respectively (p less than .001). With the postextrasystolic beat, however, RRP-HPS increased to exceed the control value of method I by 23 +/- 11 msec (p less than .001). This greater-than-expected RP prolongation was also associated with significantly increased retrograde HPS conduction times (in method III vs method I) at both long and short test stimulus coupling intervals.(ABSTRACT TRUNCATED AT 250 WORDS)

Bundle of His↗
PubMed1984

Retrograde concealed conduction in the atrioventricular node: differential manifestations related to level of intranodal penetration.

Although concealed conduction in the atrioventricular node (AVN) has been the focus of numerous experimental and clinical studies, little is known about the differential effects on AVN functional behavior of prior concealed retrograde impulse penetration alone vs prior anterograde or dual AVN activation. This study was undertaken specifically to investigate this aspect of human AVN physiology in patients without intact ventriculoatrial conduction to provide a model with which to analyze concealed retrograde AVN conduction. In 13 such patients AVN behavior during a subsequent anterograde input was characterized for each of three different manners of prior AVN activation: anterograde alone (method I), simultaneous anterograde and retrograde excitation (method II), or equivalently timed retrograde concealed AVN penetration alone (method III). In all patients evidence for retrograde AVN concealment was documented by longer anterograde conduction in the AVN during method III than that observed when the retrograde impulse was omitted (method IV). Furthermore, in these patients lacking intact ventriculoatrial conduction, anterograde conduction in the AVN was facilitated and refractoriness was decreased during dual excitation vs anterograde activation alone (method II vs method I). However, when each of methods I and II were compared with concealed retrograde AVN penetration alone (method III), two patterns of AVN responses were observed: In 11 of 13 patients (group A) conduction in the AVN was "worse" and refractoriness was greater both in methods I and II compared with method III; in the remaining two patients (group B) the opposite relationships were observed. The results suggest that among patients with no ventriculoatrial AVN conduction there exist at least two subpopulations whose AVN functional characteristics differ markedly when the effects of anterograde and dual excitation are compared with equivalently timed concealed retrograde activation alone. These findings may be related to differences in level of retrograde AVN concealment in groups A and B, which can be unmasked by the outlined pacing techniques.

Adult↗
PubMed1984

Effects of alternating cycle lengths on refractoriness of the His-Purkinje system.

The effects of alternating cycle lengths (bigeminal rhythm) on His-Purkinje system refractoriness were studied in 14 patients using His bundle and right bundle recordings. Programmed atrial stimulation at constant cycle length (method I) was scanned using the atrial extrastimulus technique (A2) and compared with an atrial cycle length of identical duration coupled to A2 on alternate beats (method II). The results showed that (a) despite shorter cycle length of the His-Purkinje system with method II due to effect of A2 on atrioventricular nodal conduction (699 +/- 90 vs. 743 +/- 87 ms, P less than 0.001), the relative refractory period of the His-Purkinje system was always longer with method II (463 +/- 52 vs. 440 +/- 43 ms, P less than 0.001). A similar increase also occurred in effective refractory period of the His-Purkinje system; (b) while functional right bundle branch block occurred in eight patients and functional left bundle branch block in two patients with method I, functional right bundle branch block occurred in all 14 patients and left bundle branch block in seven patients with method II; (c) in two patients where both functional right and left bundle branch block occurred with method I this never was manifest at identical degree of abbreviation of His-Purkinje system cycle length but was manifest at identical abbreviation in each of seven patients with method II; (d) site of conduction delay and/or block during functional right bundle branch block was always proximal, i.e., between the His bundle and right bundle recordings with both methods. During method II this resulted in shortening of the subsequent right bundle cycle length relative to the subsequent His bundle (and of necessity left bundle) cycle length. The finding of increased His-Purkinje system refractoriness despite shorter preceding cycle length of the His-Purkinje system during atrial bigeminy has never been previously described and suggests that classical concepts of His-Purkinje system behavior may require revision in this setting. Secondly, during atrial bigeminy the occurrence of alternating functional bundle branch block cannot be accounted for solely by the degree of abbreviation of His-Purkinje system cycle length, but may be explained by a relative shortening of the next ipsilateral bundle branch cycle length in the bundle manifesting block.

Atrial Function↗
PubMed1984

Effect of digoxin on serum and urinary cation changes on acute induction to high altitude.

Forty subjects each were rapidly inducted by road to altitudes of 3200 and 3771 meters (m). Half of subjects at each altitude were maintained on digoxin and the other half received placebos. The digoxin group showed higher serum potassium at both altitudes. Serum sodium remained unaltered. When compared to control groups, urinary volume changes were less marked in the digoxin group. Similarly, the alterations in calcium and magnesium in serum and urine at 3771 m were less pronounced. However, none of these changes in the digoxin groups were statistically significant when compared with control groups.

Adult↗
PubMed1983

Secretory carcinoma of the breast in adults. Light and electron microscopic study of three cases with review of the literature.

Three adult cases of secretory carcinoma of the breast were studied by both light and electron microscopy. Histologically the tumors revealed three patterns, namely, solid, microcystic and ductal. Ultrastructurally the tumor cells contained large numbers of membrane-bound intracytoplasmic secretory vacuoles. In addition, numerous intracytoplasmic as well as intercellular lumina containing abundant secretory material were noted. Secretory carcinoma in adults is rare with only twenty cases reported previously in the literature. It is known to have a better prognosis than the usual ductal carcinoma. The most appropriate treatment appears to be an extended simple mastectomy.

Adult↗
PubMed1983

Structural studies on membrane-bound bovine rhodopsin.

Our previous work has shown that the treatment of bovine rhodopsin with the proteolytic enzyme papain gives rise to a cleaved, but fully functional, complex consisting of three fragments, H, M and L (heavy, medium and light), held together by strong non-covalent forces. By using some of the chemical and physical differences between the three fragments, a protocol for the preparative isolation of each fragment was devised. Purified M-fragment, which had been radiochemically labelled at the retinal-binding site was treated with CNBr and the mixture subjected to a multi-step separation to furnish a retinyl peptide. The sequence analysis of the latter showed that the retinal-binding lysine residue was located at position 296 from the N-terminal of rhodopsin (or residue 53 from the C-terminal). In order to ascertain the position of the cytoplasmic loop which exists between the M- and L-fragments, radiochemically labelled L-fragment was isolated from the cleaved complex. The purified L-fragment was shown to consist of two populations of peptides which were produced by the action of papain on the bonds between Lys-311 and Gln-312 and between Gln-312 and Phe-313.

Amino Acid Sequence↗
PubMed1983

Anaerobic and aerobic coproporphyrinogen III oxidases of Rhodopseudomonas spheroides. Mechanism and stereochemistry of vinyl group formation.

An improved method for the preparation of various species of porphobilinogen stereospecifically labelled with 3H in the side chains (at C-6, C-7 and C-8) is described. These labelled samples were used to study the mechanism and stereochemistry of anaerobic as well as aerobic coproporphyrinogen III oxidase of light-grown Rhodopseudomonas spheroides. It was shown that both the oxidases catalyse the conversion of the propionate side chains of coproporphyrinogen III into the vinyl groups of protoporphyrinogen IX, (formula; see text) with the labilization of the pro-S-hydrogen atom at the beta-position. These results are similar to those previously recorded for such conversions in animal and plant systems. In the light of the cumulative information available to date, mechanisms for the conversion, (formula; see text) are discussed and doubt is cast on the participation of hydroxylated intermediates in the process.

Aerobiosis↗
PubMed1983

Report on evaluation of hydron film as burn wound dressing.

Hydron Burn Dressing, an amalgam of Polyethylene Glycol-400, and Polyhydroxyethyl methacrylate formed directly on the burn wound has been used in 50 cases with comparable control areas on the same patients which were managed by Polybactrin spray and exposure. The 4-layer film was found to be easily applicable in indoor patients and was pain-free during application and reduced pain after application in superficial burns. It was found to be translucent, pliable, less liable to crack and did not peel off once dried. The rate of infection was found to be 12.5 per cent more than the controls but the healing time was shorter by 2.8 days. Patient's comfort and compliance was better than the scab of the control areas.

Adolescent↗
PubMed1983

An electrocardiographic--electrophysiologic correlation of aberrant ventricular conduction in man.

Surface electrocardiographic (ECG) features of aberrant ventricular conduction (VAb) were correlated with His bundle electrograms (HBE) in 16 patients to assess the usefulness and limitations of ECG in the diagnosis of VAb. VAb was studied in these patients using the technique of atrial premature stimulation (A2) during paced atrial cycle lengths (A1). VAb in the form of right bundle branch block (RBBB) was noted in 12/16 patients, left (L) BBB in 7/16, whereas 3/16 showed R as well as LBBB following A2. The longest atrial coupling (A1A2) intervals resulting in VAb approximated 50% of the basic cycle length (BCL) and VAb was frequently abolished at closer A1A2 intervals due to excessive AV nodal delays. While the PR interval preceding an aberrant QRS complex always exceeded the PR of the prior CL, the two major components of the PR [i.e. AV node and His Purkinje system (HPS)] showed different and unpredictable degrees of delays. In comparison to the beats of BCL, during VAb the AV nodal delays amounted to a maximum of a twofold increase, whereas a greater than fourfold increase was noted in the HPS in some instances. The constant finding of PR prolongation and frequent conduction delay in the HPS (HV prolongation) associated with VAb suggested that the appearance of an unexpectedly shortened PR interval, though not usually considered in the differentiation of VAb from premature ventricular beats, may prove extremely useful; yet the often utilized R-R interval relationship poorly reflects the H-H interval relationship (which determines VAb) and therefore may be frequently unreliable.

Adult↗
PubMed1983

Role of cardiac electrophysiologic studies in patients with unexplained recurrent syncope.

Cardiac electrophysiologic studies (EPS) with programmed electrical stimulation (PES) were performed in 30 patients with recurrent syncope to uncover possible arrhythmic etiology. All patients had undergone thorough medical and neurologic evaluation prior to EPS without finding a definitive cause for syncope. In the majority of patients an arrhythmic etiology for syncope was suspected but could not be documented utilizing the 12-lead surface ECG, extended in-hospital and/or ambulatory monitoring (for greater than or equal to 48 hours) and exercise testing prior to the EPS. The studies provided a clue to the possible underlying rhythm disturbance which could have caused syncope in 16/30 patients. Sustained or nonsustained ventricular tachycardia and/or ventricular fibrillation was induced in 11/30, sinus node dysfunction in 4/30 and intra-His block in the remaining one. Fourteen of the 16 have remained free of symptoms following therapy based on results of EPS during a follow-up period ranging from 6-30 months (mean 16.5 +/- 7.8). In 2/16 syncope recurred (one arrhythmic and one non-arrhythmic) despite pacemaker therapy for sinus node dysfunction detected during EPS. In the remaining 14/30 patients, EPS and PES did not induce arrhythmia which could account for patient symptomatology and therefore no specific therapy could be recommended. Eleven of these 14 patients experienced a recurrence of symptoms within a 6-25 month period (mean 16.2 +/- 6.8). Of the 16 patients with inducible arrhythmias considered clinically significant, 15 had associated structural heart disease. On the other hand, of the 14 patients without clinically significant arrhythmias, structural heart disease could be detected in only three. It is concluded that cardiac arrhythmias constitute a common cause of unexplained syncope, particularly in patients with structural heart disease, and that EPS with PES can uncover the type of arrhythmic disturbance in a significant number of cases.

Adult↗