The study investigated the differences in five different formulae for heart rate correction of the QT interval in serial electrocardiograms recorded in healthy subjects subjected to graded exercise. Twenty-one healthy subjects (aged 37+/-10 years, 15 male) were subjected to graded physical exercise on a braked bicycle ergometer until the heart rate reached 120 beats/min. Digital electrocardiograms (ECG) were recorded on baseline and every 30 seconds during the exercise. In each ECG, heart rate and QT interval were measured automatically (QT Guard package, Marquette Medical Systems, Milwaukee, WI, USA). Bazett, Fridericia, Hodges, Framingham, and nomogram formulae were used to obtain QTc interval values for each ECG. For each formula, the slope of the regression line between RR and QTc values was obtained in each subject. The mean values of the slopes were tested by a one-sample t-test and the comparison of the baseline and peak exercise QTc values was performed using paired t-test. Bazett, Hodges, and nomogram formulae led to significant prolongation of QTc intervals with exercise, while the Framingham formula led to significant shortening of QTc intervals with exercise. The differences obtained with the Fridericia formula were not statistically significant. The study shows that the practical meaning of QT, interval measurements depends on the correction formula used. In studies investigating repolarization changes (e.g., due to a new drug), the use of an ad-hoc selected heart rate correction formula is highly inappropriate because it may bias the results in either direction.
Objective: To prospectively determine whether ventricular rate and regularity are significant determinants of the velocity and magnitude of left atrial appendage (LAA) flow. Design and patients: 12 patients with atrial fibrillation (AF), high degree atrioventricular block, and indwelling permanent pacemakers were studied. Setting: Cardiology department of a tertiary referral centre. Interventions: Pacing was triggered by an external programmable transcutaneous device. Patients were paced at 60, 120, and 150 beats/min in both regular and irregular rhythm. LAA flow velocity and magnitude were assessed with transoesophageal Doppler echocardiography. Main outcome measures: Peak and mean LAA inflow and outflow velocity, and time-velocity interval (TVI) of LAA flow. Results: Increasing ventricular rate was associated with significantly lower peak inflow (p , 0.01), peak outflow (p , 0.05), mean inflow (p , 0.01), and mean outflow (p , 0.05) velocities and with a lower TVI of LAA filling and emptying velocities (p , 0.01). This effect was noted at rates of 60 beats/min compared with both 120 and 150 beats/min. At a pacing rate of 120 beats/min there was a significantly higher total TVI when pacing at a regular than at an irregular rhythm (40.16 (14.6) cm v 30.74 (10.9) cm, p , 0.05). Conclusions: In this study, LAA filling velocities in patients in AF were significantly influenced by paced ventricular rate and to a much lesser extent ventricular rhythm. These results suggest that rapid ventricular rates may predispose to stasis in the LAA in AF.A trial fibrillation (AF) is associated with an increased risk of systemic embolism and stroke, even in the absence of valvar heart disease.1 The majority of these events result from thrombus formation within the left atrium, in particular the left atrial appendage (LAA). LAA flow has been shown to be related to left ventricular (LV) filling and loading, 5 which in turn are related to ventricular rate and regularity of rhythm.6 7 Furthermore, the LAA lies within the confines of the pericardium close to the LV free wall. The purpose of this study was to prospectively determine, in a controlled manner, whether ventricular rate and regularity influence the velocity and magnitude of LAA flow. PATIENTS AND METHODS PatientsThe study group comprised 12 patients aged 43-74 years with high degree atrioventricular (AV) block who were in AF. Five patients had undergone AV nodal ablation, most recently 16 months before the study. All patients had an indwelling permanent pacemaker with the facility to be programmed to triggered (VVT) mode. Full ethical approval was obtained for the study and all patients gave informed written consent. Potential study participants were identified by reviewing records from our pacemaker follow up clinic and records of AV nodal ablation procedures. Patients with permanent pacemakers implanted for high degree AV block who had a history of AF were identified. Patients were excluded if they had significant mitral valve disease; if they were not able to safely tole...
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