Postural tachycardia syndrome: Diagnostic sensitivity altered by diurnal variability

S146 of the uncorrected QT interval with IVA is associated with reduction in heart rate. Objective: to investigate whether changes in the QT interval observed with IVA are entirely due to bradycardia or also to the direct effects of IVA on ventricular repolarisation. Methods: Because the non-linear relationship between the QT interval and the heart rate may often result in under- or overestimation of the corrected QTc interval by the conventional Bazett (QTcB) and Fridericia (QTcF) formulae, respectively, a population correction formula (QTcP) was used. The QTcP formula was developed and validated in 23,997 QT/RR paired values from the ECGs in 1216 patients with coronary artery disease randomised to the placebo arm of several IVA efficacy studies. The QTcP formula was then used for rate correction of the QT interval in 995 patients randomised to treatment with IVA 10 mg bid. Eighty-three patients treated with atenolol 100 mg od served as active controls. Results: Based on the analysis of the QT/RR values in the placebo group, the QTcP formula resulted in a correction exponent of 0.384. Prolongation of the uncorrected QT interval was observed in patients treated with IVA or atenolol consistent with a reduction in heart rate (Figure). There were no changes in the corrected QTc interval throughout the follow-up period. Conclusion: The effects of IVA and atenolol on the QT interval is solely related to bradycardia with no evidence of any direct effect on repolarisation. The QTcP correction formula allows accurate QTc interval assessment.

P2-37 OBESITY CONTRIBUTES TO RISK OF SCD IN THE GENERAL POPULATION AND PROLONGATION OF VENTRICULAR REPOLARIZATION IS A POTENTIAL MECHANISM Kyndaron Reinier, Eric C. Stecker, Kalpana Narapasetty, Catherine Vickers, Justin Waltz, Carmen Miu, Benjamin T. John, Ron Mariani, Karen Gunson, Jonathan Jui, John H. McAnulty and Sumeet S. Chugh. Oregon Health and Science University, Portland, OR. Background: The Paris Prospective cohort study reported an association between increased body mass index (BMI) and sudden cardiac death (SCD). We sought to evaluate the potential contribution of obesity (BMI ⱖ 30) to the burden of SCD in the general population. Methods: In the ongoing Oregon Sudden Unexplained Death Study (OreSUDS) adult residents of Multnomah County, OR (population 660,486) who suffered SCD during 2002 and 2003 were identified prospectively using emergency medical services, medical examiner and 16 area hospitals. All available medical records and autopsy data were reviewed and SCD cases identified by in-house adjudication based on the WHO definition. BMI (kg/m2) was calculated for all SCD cases and compared with BMI of randomly selected residents of the same community. The latter data were obtained from the ongoing Behavioral Risk Factor Surveillance System (BRFSS) project from the US Centers for Disease Control. The corrected QT interval (QTc, Bazett’s method) was determined for all SCD cases that underwent ECGs. Results: There were a total of 720 cases of SCD over two years. Of the 701 cases among adults, 326 (47%) had BMI data available. Obesity prevalence was available for 2503 BRFSS respondents from the same community. Obesity was significantly more prevalent in SCD vs. the overall community (37% vs. 19%; p ⬍0.001). Among SCD cases, QTc was available in 77 with BMI ⬍ 30 and 43 with obesity. In non-obese patients (BMI⬍30) mean QTc was 452 ms vs. 473 ms in obese subjects (p⫽0.06). When stratified by gender, QTc was longer in obese men compared to non-obese men (482 vs. 443, p⫽0.002), but in women, QTc was unaffected by obesity (456 vs. 466, p⫽0.57). Conclusion: The occurrence of SCD in the general population was strongly associated with obesity. Overall, SCD cases had significantly prolonged

Heart Rhythm, Vol 2, No 5, May Supplement 2005 mean QTc, but obesity was associated with a further prolongation of QTcan effect that was restricted to obese males. P2-38 APPLICATION OF THE AMERICAN COLLEGE OF EMERGENCY PHYSICIANS (ACEP) RECOMMENDATIONS AND A RISK STRATIFICATION SCORE (OESIL) FOR PATIENTS WITH SYNCOPE ADMITTED IN INTERNAL MEDICINE SERVICE Adrian Baranchuk, MD, Rajesh Hiralal, MD, William Harper, MD, Jeff Healey, MD, Stuart J. Connolly, MD and Carlos A. Morillo, MD. McMaster University, Hamilton, Ontario, Canada. Background: Syncope represents 1-6% of total admissions to the hospital. Identification of high-risk patients (pts) at the ER is essential for avoiding unnecessary admissions. Aims: To retrospectively evaluate the ACEP recommendations for syncope admissions and the OESIL risk score for syncope stratification in pts with syncope admitted to the Internal Medicine Service. Methods: Two blinded investigators review all the charts and retrospectively applied the guidelines. ACEP recommendations were divided into level B (high sensitivity and specificity to detect cardiac syncope) and C (high sensitivity, low specificity). OESIL risk score was divided into 0-1 points (less than 1% of mortality risk) and 2-4 (more than 20% of mortality risk). We assumed that pts with an OESIL ⬎ 2 should be admitted due to high risk of cardiac mortality. Results: Between June 2003-July 2004, 75 pts were admitted. Mean age was 68⫾14 years, 41% were female. Structural heart disease was present in 60% and ECG was abnormal in 25%. A diagnosis was achieved in 40 pts (54%), vasovagal syncope 22 (55%), cardiac 6 (15%), orthostatic hypotension 7 (18%), drug-induced 2 (5%) and neurologic 2 (5%). The average length of stay was 4.2⫾3.7 days.

ACEP level B Sensitivity 100%, Specificity 81%, ACEP level C; Sensitivity 100%, Specificity 26%. Conclusions: The majority of syncope admissions to an Internal Medicine Service were low risk. ACEP level B recommendations had a good sensitivity identifying cardiac causes of syncope. However, ACEP guidelines overestimate cardiac causes leading to unnecessary admissions. Level C recommendations have poor specificity (26%) leading to unnecessary admissions. OESIL score identified 30% of pts with very low mortality that may have been unnecessarily admitted. P2-39 POSTURAL TACHYCARDIA SYNDROME: DIAGNOSTIC SENSITIVITY ALTERED BY DIURNAL VARIABILITY Satish R. Raj, MD, John F. Ling, III, Bonnie K. Black, RN, Italo Biaggioni, MD and David Robertson, MD. Vanderbilt University, Nashville, TN. Background: Patients with the postural tachycardia syndrome (POTS) experience considerable disability. The syndrome is defined by an excessive tachycardia on standing (ⱖ30 bpm), without significant orthostatic hypotension, and with hyperadrenergic symptoms on standing. We hypothesized that the magnitude of orthostatic tachycardia in these patients would vary at different times of day, and that standardizing the time of assessment might be important in the diagnosis of POTS. Methods: Eleven patients with symptoms of POTS were admitted to the Vanderbilt University General Clinical Research Center for evaluation. In the initial 24 hours, non-invasive supine and standing (for at least 5 minutes) heart rate (HR) and blood pressure were measured on the afternoon (A1) and evening (E1) of admission, and on the following early morning (M2). Data are presented as mean⫾SD. P⬍0.05 using a repeated measured ANOVA model was considered significant.

Poster 2 Results: The standing HR was greater in the morning than in the afternoon and evening (A1:100⫾14, E1:99⫾13, M2:117⫾19 bpm; ANOVA p⫽0.007, A1vs. M2 p⫽0.019, E1 vs. M2 p⫽0.003). In contrast, the supine HR did not vary between the 3 time points (A1:77⫾9, E1:77⫾9, M2: 81⫾21 bpm; P⫽0.636). As a result, the orthostatic increase in HR varied with time of day (A1:23⫾12, E1:22⫾10, M2:37⫾17 bpm; ANOVA p⫽0.034), with the early morning increase in HR greater than in the evening (M2 vs. E1 p⫽0.020). There were no changes in systolic or diastolic blood pressure between these time points. While only 2 patients (18%) met the POTS HR criterion (ⱖ30 bpm) in the evening, 7 of the same patients (64%) met the criterion in the morning. Conclusion: Orthostatic tachycardia in suspected POTS subjects is 50% greater in the morning. Using a 30 bpm orthostatic tachycardia as a criterion for POTS diagnosis, more than twice as many subjects will test positive in the morning as compared to later in the day. Drug trials conducted in the morning with continued HR monitoring over succeeding hours will likely yield false positive evidence of benefit, unless controlled by placebo. This diurnal pattern may underlie controversies about therapeutic benefit in POTS. P2-40

S147 to small changes in left ventricular activation sequence, even in the setting of bundle branch block. We hypothesized that this vector loop is useful for assessing the response to CRT. Thirteen patients (age 69 years ⫹/- 11, LVEF 20 % ⫹/- 8) undergoing biventricular ICD implant were studied. Each patient underwent baseline and post-CRT scalar and vector ECG’s and TDI. The maximum peak systolic velocity difference between 4 basal and mid LV segments (anterior, inferior, septal, lateral) were measured. Changes in the horizontal plane QRS vector loop were quantified by measuring its axis and the surface area within the loop before and after CRT. The QRS vector loop was directed posteriorly at baseline and rotated counterclockwise with CRT (-79 degrees ⫹/- 62 vs. -144 degrees ⫹/- 45, p⫽0.01). The area within the loop decreased markedly with CRT (243 mm2 ⫹/- 142 vs 162 mm2 ⫹/-154, p⫽0.028) (see figure). The change in the area of the horizontal plane QRS vector loop was correlated with maximum peak systolic velocity difference on TDI after CRT (r⫽0.5, p⫽0.006). There was no relationship between scalar QRS duration and TDI. CRT causes the horizontal plane vector QRS loop to decrease in area and shift counterclockwise. The change in vector loop area correlates with resynchronization as quantified by TDI. Vectorcardiography may prove to be useful for screening patients for CRT and for assessing its effect.

MODIFICATIONS OF CARDIAC CONTRACTILITY ASSESSED BY TISSUE-DOPPLER IN TILT-INDUCED SYNCOPE Antonio F. Folino, MD, PhD, Giulia Russo, MD, Gianfranco Buja, MD and Sabino Iliceto, MD. University of Padua, Padova, Italy. Different mechanisms are implicated in the genesis of neurally-mediated syncope. The precise origin of the afferent traffic is controversial, and simultaneous involvement of various peripheral receptors is likely to be present. However, it is generally accepted that an essential role is played by ventricular mechanoreceptors. We evaluated the modifications of myocardial performance during upright tilt test (UTT), by tissue Doppler echocardiography (TDI), in 47 patients (mean age 43.2⫾19) with unexplained syncope. Three echocardiographic recordings were collected: at baseline, during the initial 5 minutes of test, and after 15 minutes of tilt. The parameters considered were: left ventricle end diastolic volume (LVEDV) and ejection fraction (LVEF), left atrial area (LAA) and volume (LAV). Samples for TDI were collected at anterior and inferior walls. UTT was positive in 28 patients (59.6%). At baseline patients with positive or negative test had comparable echocardiographic parameters. During test, both groups had a similar, significant, progressive reduction of LVEDV, LAA, LAV, whereas LVEF showed an early, more pronounced decrease in positive. Systolic waves at all sites remained almost unchanged, while early filling waves decreased similarly, in either positive and negative patients. On the contrary, the atrial filling waves showed a significant decrease only in positive patients, on inferior (baseline 8.61⫾2 vs 15’ 7.08⫾2, p⬍0.005) and anterior (baseline 6.52⫾2 vs 15’ 4.94⫾2, p⬍0.002) walls, whereas negative subjects had minimal changes (inferior wall, baseline 8.43⫾3 vs 15’ 8.13⫾3, p⫽ NS; anterior wall, baseline 6.31⫾2 vs 15’ 6.96⫾3, p⫽ NS).In conclusion, our study showed that both patients with positive and negative UTT had a similar decrease of left atrial and ventricular dimensions, and a moderate increment of systolic waves. Nevertheless, patients with tilt-induced syncope were characterized by a significant reduction of atrial waves, reflecting a decreased atrial contribute to ventricular filling, that may be a contributory factor to tilt-induced syncope. P2-41 VECTORCARDIOGRAPHY PREDICTS RESPONSE TO CARDIAC RESYNCHRONIZATION THERAPY Joon Ahn, MD, Safwat Gassis, MD, Maria A. Pernetz and Jonathan J. Langberg, MD. Emory University, Atlanta, GA. Tissue Doppler imaging (TDI) is useful to quantify left ventricular dyssynchrony and the effects of cardiac resynchronization therapy (CRT). The utility of TDI as a screening tool is limited by its expense, operator dependence, and limited availability. Vectorcardiography provides a three dimensional representation of the temporal-spatial activation of the heart. Unlike the scalar ECG, the horizontal plane QRS vector loop is sensitive

P2-42 QRS DURATION, QRS COMPLEXITY AND REPOLARIZATION HETEROGENEITY IN BIVENTRICULAR PACING IN CHRONIC HEART FAILURE Bart Hooft van Huysduynen, MD, Cees A. Swenne, PhD, Jeroen J. Bax, MD, Sander G. Molhoek, MD, Gabe B. Bleeker, Arie C. Maan, PhD, Hedde Van de Vooren, BSc, Lieselot Van Erven, MD, Ernst E. Van der Wall, MD and Martin J. Schalij, MD, PhD. Leiden University Medical Center, Leiden, Netherlands. Background: By overruling delayed ventricular depolarization, biventricular (BiV) pacing aims to improve mechanical synchronization in chronic heart failure. Often, but not always, BiV pacing shortens QRS duration. Inevitably, BiV pacing influences ventricular repolarization as well, and changes in repolarization heterogeneity, an arrhythmogenic factor, may be expected. Our study sought to answer the question how BiV pacing induces changes in ventricular repolarization heterogeneity. Methods: We analyzed 12-lead ECG recordings of 28 patients (22 male/6 female, age 69⫾9 year), 2 days after BiV pacemaker implantation. Patients were in NYHA class 3.0⫾0.5 with an ejection fraction of 21⫾7 %. The ECGs, with and without BiV pacing, were recorded in random order. Analyses were done on a PC, using Matlab programs. Heterogeneity of the repolarization was assessed by computing the square root of the summed second to eighth singular values divided by the first singular value of the T wave, which is mathematically related to repolarization heterogeneity. Similarly, QRS complexity was calculated. Results: QRS duration was smaller during BiV pacing than during sinus rhythm (SR) (141⫾20 vs.172⫾31 ms, P⬍0.001). QRS complexity was larger during BiV pacing than during SR (0.52⫾0.19 vs. 0.36⫾0.14, P⬍0.001). Repolarization heterogeneity was larger during BiV pacing than during SR (0.34⫾0.20 vs. 0.24⫾0.11, P⬍0.001). BiV pacing induced changes in QRS complexity correlated significantly with changes in repolarization heterogeneity (r⫽0.55, P⬍0.01). Conclusions: BiV pacing induced amelioration of ventricular depolarization (measured as QRS duration decrease). However, BiV pacing tends to entail an increased repolarization heterogeneity (measured as the ratio of