Vasovagal syncope

BACKGROUND

Vasovagal syncope is the most common type of syncope and is one of the most difficult types to manage.

PURPOSE

This article reviews the status of mechanisms, diagnosis, and management of vasovagal syncope.

DATA SOURCES

MEDLINE search for English-language and German-language articles on vasovagal syncope published up to June 1999.

STUDY SELECTION

Case reports and series, clinical trials, research investigations, and review articles from peer-reviewed journals.

DATA EXTRACTION

Findings were summarized and discussed individually. Summaries were made in table format. Statistical analysis of combined data was inappropriate because of differences among studies in patient selection, testing, and follow-up.

DATA SYNTHESIS

The population of patients with vasovagal syncope is highly heterogeneous. Triggers of vasovagal syncope are likely to be protean, and many potential central and peripheral triggers have been identified. The specific mechanisms underlying the interactions among decreased preload, sympathetic and parasympathetic modulation, vasodilation, and cardioinhibition remain unknown. Tilt-table testing is a widely used diagnostic tool. The test results should be interpreted in the context of patients' clinical presentations and with an understanding of the sensitivity and specificity of the test. Assessment of therapeutic outcomes has been difficult, primarily because of patient heterogeneity, the large number of pharmacologic agents available for therapy, and the sporadic nature of the syndrome complex.

CONCLUSIONS

Vasovagal syncope is a common clinical syndrome that has complex and variable mechanisms and is difficult to manage. Advancements are being made in laboratory investigations of its triggering mechanisms. Randomized, controlled trials of pharmacologic and nonpharmacologic interventions are needed. Mechanism-targeted therapeutic trials may improve clinical outcomes.

Global right atrial mapping of human atrial flutter: the presence of posteromedial (sinus venosa region) functional block and double potentials : a study in biplane fluoroscopy and intracardiac echocardiography

BACKGROUND

Previous studies of atrial flutter have found linear block at the crista terminalis; this was thought to predispose the patient to the arrhythmia. More recent observations, however, have demonstrated crista conduction. We sought to characterize the posterior boundary of atrial flutter.

METHODS AND RESULTS

Patients with counterclockwise flutter (n=20), clockwise flutter (n=3), or both (n=5) were studied using two 20-pole catheters. Biplane fluoroscopy determined catheter positions. During counterclockwise flutter, craniocaudal activation occurred along the entire lateral and posterior right atrial walls. Septal activation proceeded caudocranially. In all patients, a line of block was seen in the posteromedial (sinus venosa) right atrium; this was manifested by the presence of double potentials where the upward and downward activations collided. Anatomic location was confirmed by intracardiac echocardiography in 9 patients. In patients with clockwise flutter, the line of block and double potentials were seen in the same location during counterclockwise flutter, but the activation sequence around the line of block was reversed. Pacing near the site of double potentials during sinus rhythm excluded a fixed line of block, and premature atrial complexes demonstrated functional block with manifest double potentials. In 2 patients, posterior ectopy organized to subsequently initiate isthmus-dependent atrial flutter.

CONCLUSIONS

(1) A functional line of block is seen at the posteromedial (sinus venosa region) right atrium during counterclockwise and clockwise atrial flutter. (2) All lateral wall right atrial activation can be uniform during flutter, without linear block or double potentials in the region of the crista terminalis. (3) Activation at the site of posteromedial right atrial functional block can organize to subsequently initiate isthmus-dependent atrial flutter.

Bradykinin modulates arginine vasopressin-induced calcium influx in vascular myocytes

We investigated direct, endothelium-independent effects of bradykinin on arginine vasopressin-induced calcium influx in vascular smooth muscle cells. We studied cultured rat vascular smooth muscle cells by using the whole-cell voltage-clamp and calcium fluorescence imaging methods. Exposing cultured vascular smooth muscle cells (A7r5 cell line) to arginine vasopressin (100 nM) produced a transient increase in [Ca2+]i, followed by a sustained increase in [Ca2+]i. This was readily reversible (n=28). At a holding potential of -40 to -60 mV, arginine vasopressin induced a sustained inward current correlated with a sustained increase in [Ca2+]i. Bradykinin (30 nM to 30 microM) had no effect on arginine vasopressin-induced [Ca2+]i transients. However, during the sustained phase of increased [Ca2+]i, bradykinin reversibly attenuated relative fluorescence and inward current in the presence of arginine vasopressin (n=14). This was concentration dependent and inhibited by [D-Phe7]-bradykinin (30 microM), a kinin receptor antagonist. Also, sustained arginine vasopressin-mediated increases in [Ca2+]i and inward current were attenuated by Ca2+-free or La3+-supplemented perfusate but not by nifedipine (n=5).

CONCLUSIONS

(1) Bradykinin can attenuate arginine vasopressin-induced and sustained Ca2+ influx and sustained inward current through a novel endothelium-independent process. (2) The direct effect of bradykinin on arginine vasopressin-induced increases in [Ca2+]i sustained Ca2+ influx in vascular smooth muscle cells is concentration dependent and kinin-receptor mediated. (3) Arginine vasopressin-induced sustained [Ca2+]i elevation correlates with the activation of a dihydropyridine-insensitive, Ca2+-conducting inward current.

Hemodynamic analysis during isoproterenol-induced vasovagal syncope

An isoproterenol-mediated increase in cardiomotor tone and a decrease in afterload contribute to the induction of vasovagal syncope. Contrary to conventional belief, a significant decrease in preload is not observed immediately before isoproterenol-induced syncope.

Gender differences in response to endothelin-1 in coronary arteries: transcription, receptors and calcium regulation

1. Experiments were designed to determine how sex hormonal status may influence production and response to endothelins. 2. Circulating concentrations of endothelin-1 were not different between sexually mature male and female pigs and did not differ between females of low- and high-oestrogen status. 3. Expression of messenger RNA for preproendothelin was not different in aortic endothelial cells derived from sexually mature male and female pigs. 4. Endothelin-1, endothelin-3 and sarafotoxin S6c caused concentration-dependent contractions of rings of coronary arteries suspended for the measurement of isometric force. Only in response to endothelin-1 were contractions greater in arteries from female than in male pigs. This difference was not related to oestrogen levels in females. 5. The intrinsic force-calcium relationship defined in smooth muscle cells permeabilized with either triton-X or beta-escine was not different in coronary arteries from male and female pigs. The calcium sensitivity was increased to the same extent by endothelin-1 in smooth muscle from male and female pigs. 6. Competitive inhibition of radiolabelled endothelin-1 by unlabelled endothelin-1 was not different in membranes prepared from coronary arteries from male and female pigs. However, competitive inhibition of radiolabelled endothelin-1 by unlabelled endothelin-3 showed two binding sites. The affinity of the high affinity binding site was increased only in females with high oestrogen levels. 7. These results suggest that there are gender differences in contractions of porcine coronary arteries to endothelin-1. These differences may not relate to transcriptional regulation for the production of the peptide in endothelial cells, to the regulation of the number or affinity of endothelin receptors or to intrinsic calcium-sensitivity of the contractile proteins. Rather they are probably due to differences in the regulation of intracellular calcium.

Cyclic ADP-ribose stimulates sarcoplasmic reticulum calcium release in porcine coronary artery smooth muscle

Cyclic ADP-ribose (cADPR) was shown to induce calcium release from the endoplasmic reticulum via ryanodine-sensitive pathways. In smooth muscle, two pathways for calcium release from the sarcoplasmic reticulum (SR) have been previously demonstrated: D-myo-inositol 1, 4, 5-trisphosphate-gated and ryanodine-gated. However, evidence for cADPR as a regulator for SR Ca2+ release in smooth muscle is lacking. We used permeabilized porcine coronary artery smooth muscle cells to directly examine the stimulation of SR Ca2+ release by cADPR. The results provide direct evidence that cADPR stimulates SR Ca2+ release and that this response is not inhibited by heparin, by depletion of the caffeine-sensitive Ca2+ pool, or by blockade or ryanodine receptors. These results indicate a novel mechanism for Ca2+ release from the SR of vascular smooth muscle.

Exploitation of gene(s) involved in 2,4-diacetylphloroglucinol biosynthesis to confer a new biocontrol capability to a Pseudomonas strain

Tn5 mutagenesis and complementation analysis were used to clone a 6-kb genomic fragment required for biosynthesis of 2,4-diacetylphloroglucinol (Phl) from fluorescent Pseudomonas sp. strain F113. A recombinant plasmid, pCU203, containing this region partially complemented a Phl production-negative mutant (F113G22) derived from strain F113. When sugar beet seeds were sown into an unsterilized soil, in which sugar beet was subject to damping-off by Pythium ultimum, the emergence of sugar beet seeds inoculated with strain F113 was significantly greater than that of seeds inoculated with F113G22. Transfer of pCU203 into eight other Pseudomonas strains conferred the ability to synthesize Phl in only one of these strains, Pseudomonas sp. strain M114. Strain M114(pCU203) showed enhanced antagonism towards P. ultimum in vitro and significantly increased the emergence of sugar beet seeds in the same soil compared with emergence induced by the parent strain M114.