Circadian Variations of Heart Rate and STIMT Interval: Adaptation for Nighttime Pacing

In order to determine the optimal pacing rate for pacemaker patients at night, 150 normal subjects with regular sinus rhythm and free of manifest heart disease, were studied using 24-hour Holter monitoring. Minimum and average heart rates were analyzed on an hourly basis. The study group was divided into six age groups, 25 subjects each, ranging from 20-29 years to 60-69 years. The minimum heart rate during the night was found to be lower than 65 ppm for all groups. The youngest subjects showed the largest variation in the minimum heart rate. The results suggest that an automatic lowering of the pacing rate during the night would allow for longer periods of sinus rhythm, thereby improving hemodynamic performance and reducing pacemaker power consumption. Suitable sensors for automatic lowering of the pacing rate include inbuilt 24-hour clock systems and the QT interval that lengthens during sleep.     

Physiological Relationship Between AV Interval and Heart Rate in Healthy Subjects: Applications to Dual Chamber Pacing

The purpose of this paper is to specify the mathematical relationship between spontaneous AV interval (AVI) and heart rate (HR), the amplitude and rate of variation of AVI, and the physiological factors likely to affect these characteristics. Ten patients with healthy hearts were studied. Two catheter electrodes were positioned in the right atrium and at the tip of the right ventricle respectively, allowing the detection of endocardial signals. The AV and AA intervals for each heart cycle were digitized to on accuracy of ± 1 msec. Measurements were made at rest, then during a stress test on an exercise bicycle, and finally during the recovery phase. The results show that adaptation is very precise and takes place instantly. Any variation in heart rate causes an immediate, inversely proportional variation in AVI. Adaptation follows a linear pattern, generally with relatively low amplitude and an average AVI reduction of 27.5 ±11.2 msec for an average HR increase of 78.7 ± 22.5 bpm, i.e., a decrease of 4 ± 2.1 msec for an HR variation 0f 10 bpm. The amplitude and variation rate of AVI seem to be independent 0f the age and base value of the PR interval. These observations may be useful for designing new VDD or DDD pacemakers that automatically adapt the AV interval to the instantaneous heart rate. The hemodynamic benefits 0f this adaptation were also demonstrated.     

The A-R Interval as Exercise Indicator: A New Option for Rate Adaptation in Single and Dual Chamber Pacing

RUITER, J.H., ET AL.: The A-R Interval as Exercise Indicator: A New Option for Rate Adaptation in Single and Dual Chamber Pacing. We investigated the possibility to use the interval from an atrial stimulus to the Ventricular R wave [A-R interval) as an indicator of physical stress, in 16 patients with pacemakers implanted for severe atrial bradycardia but with intact AV conduction. The A-R interval was studied during incremental atrial pacing at rest and during exercise with a constant workload. In addition, the atrial pacing rate was kept constant just above spontaneous sinus rate and the dynamics of the A-R interval were studied during exercise with a low constant workload and during a maximal exercise test with increasing workload. Incremental atrial pacing prolonged the A-R interval and this response was blunted during exercise [p < 0.003). Atrial pacing at a constant rate and during a constant workload resulted in an almost direct shortening of the A-R interval. When the workload was increased but the atrial rate kept constant, a pronounced shortening of the A-R interval was noted [p < 0.0001). It is concluded that changes of the A-R interval during different kinds of exercise were prompt and predictable in patients with sinus node dysfunction but intact AV conduction. In these patients the shortening of the A-R interval during exercise may be a suitable indicator for rate adaptive atrial pacing.     

Atrioventricular Interval Optimization and Exercise Tolerance

Modern pacemakers offer many programming options regarding the AV interval including the ability to vary AV intervals depending on whether atrial activity is paced or spontaneous and to shorten AV intervals with increasing rates. To determine if optimization of these features improves exercise tolerance, 14 patients with intact sinus node function and AV block treated with dual chamber pacemakers were enrolled in a randomized double-blind crossover trial. Doppler echocardiographic measurements of cardiac index and mitral flow were assessed over a range of programmable AV intervals at rest to determine each patient's optimal AV interval. Eleven patients completed serial graded exercise tests with spiroergometry after randomly programming the AV interval three ways in a crossover manner: fixed AV interval = 150 ms without rate adaptation (150/Fixed), fixed AVinterval = 150 ms with rate adaptation (150/R), or optimized AV interval with rate adaptive AV interval shortening (optimized/R). Exercise capacity was determined by maximum oxygen uptake. Ten men and four women, age 64 +/- 8 years, were enrolled. At rest, optimization of the AVintervalimproved the cardiac index by 21% (P < 0.001) and mitral flow by 13.4% (P < 0.001) when compared to least-favorable AV intervals. During exercise, no differences in maximum heart rates were noted. Maximum oxygen uptake was increased in both groups with rate adaptive AVinterval shortening when compared tofixed AVinterval without rate adaptation: 13.9% (adjusted P < 0.04) and 14.6% (adjusted P < 0.02) in optimized/R and 150/R, respectively. No differences were noted between optimized/R and 150/R. In conclusion, rate adaptive AV interval shortening improved exercise tolerance independent of changes in heart rate. However, optimization of the AV interval with Doppler echocardiography at rest did not further improve exercise capacity.     

An Unusual PR Interval Alternans During 2:1 Atrio-Ventricular Block

In a case of a 2:1 second degree A-V block during sinus rhythm, PR intervals of two different durations occur, either separately or alternating beat-by-beat. The longer intervals are not caused by concealed conduction in the A-V junction of the preceding blocked impulses and the shorter ones are not due to supernormal conduction induced by the blocked impulses. This primary PR interval alternans is alternating from a pothophysiological point of view; it only concerns every other impulse alternately traveling along the faster and slower A-V functional pathway.     

Lack of Physiological Adaptation of the Atrioventricular Interval to Heart Rate in Patients Chronically Paced in the AAIR Mode

Seventeen consecutive patients, aged 56 +/- 12, were chronically paced in the AAIR mode for a symptomatic sinus node disease with atrial chronotropic incompetence defined by a peak exercise heart rate (HR) less than 75% of the maximal predicted heart rate (MPHR) mean = 65 +/- 10%). Sensors used were activity sensing (n = 7), minute ventilation (n = 6), or respiratory rate (n = 4). Basic pacing rate was programmed at 71 +/- 5 beats/min and the maximal sensor rate at approximately 85% MPHR (143 +/- 10); other sensor parameters were programmed individually. Six months after implant, two standardized and symptom limited exercise tests were performed in random order, AAI and AAIR modes, respectively. AAIR pacing significantly improved peak exercise HR (139 +/- 14 vs 112 +/- 30 beats/min; P less than 0.01), maximal sustained workload (132 +/- 42 vs 110 +/- 38 watts; P less than 0.02), and total exercise duration (724 +/- 299 vs 594 +/- 245 sec; p less than 0.02) compared to the AAI mode. In all 17 patients, HR was continuously sensor driven in the AAIR mode, making it possible to precisely study the adaptation of the stimulus-R interval and of the stimulus-R:RR ratio during exercise. Six patients normally adapted with a progressive shortening. Six others did not adapt at all without any variation of interval. Five patients paradoxically increased their stimulus-R interval (286 +/- 10 msec at peak E vs 220 +/- 19 msec at rest) and their stimulus-R:RR ratio (67 +/- 20% vs 29 +/- 4%), producing P waves occurring immediately after, or even within the R wave of the preceding cycle; two patients complained of severe exercise related symptoms corresponding to the so-called "AAIR pacemaker syndrome." The principal factors involved in the nonadaptation of AV interval to HR were related to the patient (organic heart disease, with the particular problem of the denervated heart; the bradytachy syndrome; and the use of drugs, especially beta blockers and Class I antiarrhythmic drugs) or to the pacemaker ("overstimulation" phenomenon). These observations constitute an additional argument for wider indications of implanting DDDR units in these patients.     

Comparison of Time-Domain Short-Term Heart Interval Variability Analysis Using a Wrist-Worn Heart Rate Monitor and the Conventional Electrocardiogram

Background: Wrist-worn heart rate monitors have not been extensively validated for heart rate variability analysis. The purpose of this study was to compare time-domain variability of heart interval series (R-Ri) recorded by the Polar S810 monitor (Polar Electro Oy, Kempele, Finland) and the conventional electrocardiogram (ECG).
Methods: Agreement was verified between variability indices of 5-minute R-Ri simultaneously recorded by both devices and processed by unique software, from 33 subjects aged 18 to 42 years, normal or with different clinical conditions, in rest supine and active standing. ECG minus Polar differences were quantified by the Bland-Altman analysis, and tested by the one-sample t-test or Wilcoxon test.
Results: In the supine position, the Polar overestimates (P < 0.0001) the absolute and percentage mean or median of the number (−2.00; −0.49%) and mean of R-Ri (–1.85 ms; –0.20%) and pNN50 (−2.20%; −8.68%), and underestimates the standard deviation (SDNN) (0.32 ms; 0.59%) (P = 0.08; P = 0.02) and root mean square successive difference (RMSSD) (0.90 ms; 1.56%) (P = 0.0008; P < 0.0001). The coefficient of variation (CV) showed null difference. On standing, differences were overestimated for the number (−2.61 intervals; −0.64%) and mean of R-Ri (−0.70 ms; −0.09%), and underestimated for rMSSD (1.70 ms; 10.84%) (P < 0.0001 to < 0.02). The SDNN, CV, and pNN50 indices did not show differences (P = 0.12 to 0.73).
Conclusions: The Polar S810 monitor was feasible and reliable for recording short-term R-R interval series, showing excellent agreement with the ECG in providing the time-domain indexes of heart interval variability with differences functionally not relevant. The CV showed the higher agreement in both postures, and the SDNN and pNN50 in the standing posture.     

Optimum AV Interval in Dual Chamber Pacemakers

Approximately 50-70% of permanent implanted pacemakers are dual chamber pacemakers. However, little is known concerning adjustment of the atrioventricular (AV) interval to maximize cardiac output. Ten consecutive patients with complete heart block and dual chamber pacemakers were paced at heart rates of 80, 100, and 118 beats/minute and at five AV intervals with simultaneous measurements of cardiac output using pulsed Doppler. Maximum cardiac output occurred at AV intervals of 150 and 200 ms at rates of 80 and 100 beats/minute, and at 150 ms at a rate of 118 beats/minute (p less than .05). An increase in the AV interval to 250 ms resulted in a decrease in cardiac output at all heart rates (p less than .01). We recommend the noninvasive measurement of cardiac output, if available, for determining the optimum AV interval in an individual patient; otherwise, an AV interval of 150 or 200 ms will provide the highest cardiac output in most patients.     

Influence of Sympathetic Heart Rate Modulation on RT Interval Rate Adaptation in Conscious Dogs

PLADYS, P., et al. : Influence of Sympathetic Heart Rate Modulation on R Interval Rate Adaptation In Conscious Dogs . The objective was to test if changes in autonomic tone still influenced the RT-RR relationship when full RT adaptation is completed, when heart rate is controlled, and when beat-to-beat variability is abolished by atrial pacing. Eight dogs (8–11 kg) were chronically instrumented with atrial pacing electrodes. Digital ECG (1,000 Hz, 12 bits) were recorded from healthy conscious dogs during spontaneous sinus rhythm and during atrial pacing. The protocol was repeated before and after atenolol (2 mg/kg), prazosin (0.5 mg/kg), or atenolol + prazosin. A vocal incitation was used as sympathetic stimulation. Beat-to-beat quantitative analysis of the RT interval (from QRS apex to end of T wave) was correlated with the preceding RR by linear regression. In spontaneous rhythm, atenolol increased RR (P < 0.001), RT (P < 0.001), and short-term heart rate variability (P < 0.01) and decreased RT-RR slopes (P < 0.001). Prazosin did not significantly modify any parameter. Sympathetic stimulation decreased RR (P < 0.001), RT (P < 0.05), and short-term heart rate variability (P < 0.01) and increased RT-RR slopes (P < 0.001). In atrial pacing, the RTRR slopes were steeper during pacing than during spontaneous rhythm but were not modified by pharmacological manipulation of the autonomic nervous system. During sinus rhythm the RT-RR relationship is increased by sympathetic stimulation and decreased by β–blockade. When heart rate modulation and the effects of the time delay in RT rate adaptation are abolished by atrial pacing, the influence of autonomic tone on RT rate adaptation disappears.     

PR Interval Behavior During Exercise: Implications for Physiological Pacemakers

LUCERI, R., ET AL.: PR Interval Behavior During Exercise: Implications for Physiological Pacemakers. The relationship between heart rate response and the dynamic changes in the PR interval was assessed in 631 patients undergoing routine cardiac exercise tests for a variety of clinical indications. Patients were stratified into four subsets: nonmedicated normals [n = 437), patients on beta-antagonist agents (n = 118), those on antiarrhythmic agents alone (n = 62) and those with a clinical diagnosis of advanced (New York Heart Association [NYHA] Class 111 or IV) congestive heart failure. All patients were in stable sinus rhythm throughout the test. PR intervals were measured at rest, at mid-exercise and at peak exercise. Mean PR intervals shortened to a statistically significant degree in most subgroups. This effect was predominantly observed in the earlier stages of exercise. In patients with advanced heart failure, there was no statistically significant shortening of exercise PR intervals later in exercise, demonstrating a parallel with their relatively blunted heart rate response. These changes in exercise PR intervals suggest that implanted pacemaker algorithms may be constructed to maximize hemodynamic benefit in patients requiring physiological pacemakers.     

The Relationship Between Heart Rate and QT Interval During Atrial Stimulation

The relationship between heart rate and QT interval was investigated during atrial stimulation (intrinsic effect of heart rate) in ten healthy male volunteers prior to and after administration of sotaloI. The QT interval in the ECG (paper speed 200 mm/s) was determined at rates of 70, 85, 100, 115, 130, 145, and 160 beats/min and at pacing periods of 180 s each at 30, 60, 120, and 180 s. After a 15-minute period, 2.0 mg sotalol/kg body weight were administered iv and the stimulation protocol was repeated. The analysis of QT interval behavior reveals contradictions to the mathematical implications of Bazett's equation     , so that the relationship between heart rate and QT interval is not adequately described under the given conditions. After examination of approaches reported in the literature and our own approaches, the expression QT = a e^−b(HR-60) is used as a possibility differentially to describe the data by nonlinear regression. The parameters a and b may be interpreted as QT reference value and shortening parameter. The QT reference value a, a parameter in reference to heart rate of 60 beats/min, has a comparable significance to the expression QT, in the Bazett equation. A reduction in the shortening parameter b indicates whether substances influencing the QT interval additionally produce overproportional shortening of the QT interval with increasing heart rate. After administration of sotalol, an increase can be observed in both the QT reference value and also in the shortening parameter. The suggested approach is an attempt to provide a more precise assessment of the QT interval under different conditions.     

Safety of Slow Pathway Ablation in Patients with Long PR Interval: Further Evidence of Fast and Slow Pathway Interaction

Whether the presence of abnormal PR before selective slow pathway ablation for AV node reentrant tachycardia increased the risk of complete heart block remains controversial. We report our experience in seven patients with prolonged PR intervals undergoing catheter ablation for AV reentry tachycardia. Their mean age was 66 ± 12 years; four patients were female and three were male. RF ablation was performed using an anatomically guided stepwise approach. In six patients, common type AV node reentry was induced and uncommon type was observed in the remaining patient. In all seven patients, successful selective slow pathway ablation was associated with no occurrence of complete heart block and was followed by shortening of the AH interval in five patients. In all seven patients, successful ablation was achieved at anterior sites (M₁ in two patients and M₂ in five patients). Despite AH shortening after ablation, the 1:1 AV conduction was prolonged after elimination of the slow pathway, excluding either sympathetic tone activation or parasympathetic denervation. In conclusion, selective slow pathway ablation can be performed safely in the majority of patients with prolonged PR interval before the procedure. Because successful ablation is achieved at anterior sites in most patients, careful selection and monitoring of catheter position is required.     

Unusual Mechanism of PR Interval Variation and Nonreentrant Supraventricular Tachycardia as Manifestation of Simultaneous Anterograde Fast and Slow Conduction Through Dual Atrioventricular Nodal Pathways

Noninvasive recordings in a 69-year-old woman showed two distinct PR intervals of about 0.21 and 0.58 s, suggestive of dual AV nodal conduction. Various unusual mechanisms of transition from short to long and from long to short conduction intervals and phenomena of concealed conduction were due to the presence of two functionally separated intranodal pathways. Refractoriness of the slow pathway was associated with bradycardia. Episodes of tachycardia exhibited a one-to-two relationship between P-waves and ventricular activations as a consequence of simultaneous anterograde fast and slow conduction leading to double ventricular responses to single P-waves.     

Long-Term Pacing in Heart Transplant Recipients is Usually Unnecessary

The indications for and timing of permanent pacing were reviewed in all 17 of 154 adult heart transplant recipients at this center who have had permanent pacemakers implanted. Resting 12-lead ECGs recorded during routine follow-up were examined. A prospective study of pacing requirement was then undertaken. Holter monitoring was performed before and after reprogramming the pacemakers to VVI mode at 50 beats/min. Exercise responses in various pacing modes were then assessed in seven patients with rate responsive pacemakers using a standard Bruce protocol treadmill test. The indication for pacing was sinus node dysfunction in 59% (10/17) and atrioventricular (AV) block in 41% (7/17). The majority of pacemakers were implanted between seven and 21 days after transplantation. There was a progressive reduction in the frequency of pacing on 12-Jead ECGs with time after transplantation. Eight of 14 patients with empirically selected programming paced during Holter monitoring. After reprogramming to 50 beats/ min VVI mode only three of 14 patients, all with sinus node dysfunction, paced. Rate responsive pacing made no difference to exercise time. The requirement for long-term pacing in cardiac transplant recipients is small (3/154) and is limited lo patients with sinus node dysfunction. Rate responsive pacing did not increase exercise tolerance.     

Ventilation and Heart Rate Response During Exercise in Normals: Relevance for Rate Variable Pacing

The observation of a close relationship of heart rate to oxygen uptake (HH-VO₂) and heart rate to minute ventilation (HR-VE)has been shown to be of particular value in rate variable pacing. However, the impact of anaerobic threshold (AT)for the HH-VO₂ and HH-VE slope has been studied Jess. Twenty-three male and 16 female subjects, mean age 52 ± 7 years, were selected in whom complete heart catherization and extensive noninvasive sludies excluded major cardiopulmonary disease. Semisupine bicycle exercise testing with analysis of respiratory gas exchange was performed using a ramping work rate protocol with work increments of 20 watts/min. At the respiratory AT, determined by the V slope method, oxygen uptake (VO₂-AT)was 15.2 ± 3.0 mL/kg in males versus 13.8 ± 2.3 mL/kg in females and heart rate (HR-AT)was 109 ± 18 beats/min versus 119 ± 20 beats/min, respectively. Heart rate was highly correlated (r ±0.9)to VO₂ and minute ventilation (VE). A linear regression for HR-VO₂, however, was found only in 16/39 and for HR-VE in 11/39 subjects. Assuming the AT as the breakpoint of two linear curves, it could be demonstrated that compared to low exercise HR appeared to increase at maximal exercise more in relation to VO₂ but less in relation to VE; in men the individual slopes for HR-VO₂ were 2.6 ± 0.7 below but 3.2 ± 1.0 above AT (P < 0.05) and the slopes for HH-VE were 1.6 ± 0.5 below but 1.0 ± 0,4 above AT (P < 0.05). Similarly, in women the individual slopes for HR-VO₂ were 3.7 ± 1.4 below but 4.3 ± 1.4 above AT (P < 0.05)and the slopes for HR-VE were 2.1 ± 0.9 below but 1.3 ± 0.4 above AT (P < 0.05). The differences between male and female subjects were significant. The nonlinear behavior of the HB-VO₂ and HR-VE relation from rest to maximal exercise should have a particular impact in respiratory controlied pacing systems.     

冠心病患者运动试验早期心率快速增加意义探讨

【目的】探讨平板运动试验早期心率快速增加与冠心病（coronary heart disease,CHD）的关系。【方法】选择122例进行了平板运动试验与冠状动脉造影的胸痛患者,根据冠脉造影结果,分为冠心病组和非冠心病组,冠心病组中又进一步分为单支病变、双支病变及三支病变组,对比分析各组平板运动试验开始后第1分钟心率的增加量（ΔHR1minute）。【结果】冠心病组ΔHR1minute明显高于非冠心病组（P〈0.01）,随着冠脉病变支数的增加,ΔHR1minute逐渐增大,组间差异均有显著性（P〈0.05）。【结论】冠心病患者运动试验早期心率快速增加,且ΔHR1minute随着冠脉病变支数的增加而增大,提示ΔHR1minute能反映心肌缺血及冠脉病变程度。     

Changes in QT Interval during Exercise Testing in Patients with VVI Pacemakers

. The changes in QT intervals were studied in nine patients with normal sinus node function who had VVI pacemakers. Though PP intervals uniformly shortened during exercise, the change in QaT* intervals during exercise was variable. The correlation between PP and QaT intervals varied from case to case. A good correlation was found in only two cases (r =+0.816 or +0.897); a fair correlation was found in four cases (r =+0.672, +0.615, +0.615, or −0.669) and in the remaining three, the correlation was poor (r =+0.494, +0.467 or−0.424). In patients who are candidates for VTI pacemaker implantation, changes in QaT intervals should be assessed during exercise stress testing to determine if the intervals shorten during exercise or not.
(QaT*: Interval from the pacing spike to the apex of T-wave.)     

Doppler Echocardiographic Assessment of the Hemodynamic Benefits of Rate Adaptive AV Delay During Exercise in Paced Patients with Complete Heart Block

To determine if rate adaptation of the atrioventricular (AV) delay (i.e., linearly decreasing the AV interval for increasing sinus rate) improves exercise left ventricular systolic hemodynamics, we performed paired maximal semi-upright bicycle exercise tests (EXTs) on 14 chronotropically competent patients with dual chamber pacemakers. Nine patients with complete AV block (CAVB) and total ventricular pacing dependence during exercise comprised the experimental group. Pacemakers in these patients were programmed randomly to rate adaptive AV delay (AVDR) for one EXT and fixed AV delay (AVDF) for the other EXT. AVDF was 156 msec; AVDR decreased linearly from 156–63 msec from rates of 78–142 beats/min. The other five patients had intact AV conduction and comprised the control group who were exercised in identical fashion while their pacemakers were inhibited throughout exercise io assure reproducibility of hemodynamic measurements between EXTs. Cardiac hemodynamics were calculated using measured Doppler echocardiographic systolic aortic valve flows recorded suprasternally with an independent 2-MHz Doppler transducer during a graded ramp exercise protocol. For analysis, exercise was divided into four phases to compare Doppler measurements at submaximal and maximal levels of exercise, rest, early exercise (1st stage), late exercise (stage preceding peak), and peak. Patients achieved statistically similar heart rates between EXTs at each phase of exercise. Although at lower levels of exercise cardiac hemodynamics did not differ, experimental patients (with CAVB) showed a statistically significant benefit to cardiac output at peak exercise with heart rates of 129 ± 13 beats/min (AVDR: 9.4 ± 2.8 L/min; AVDE: 8.2 ± 2.6 L/min, P = 0.002), stroke volume (AVDR: 74.1 ± 25.6 mL; AVDF: 64.3 ± 24.4 mL, P = 0.0003), and aortic ejection time (AVDR: 253.3 ± 35.7 msec; AVDF: 226.7 ± 35.0 msec, P = 0.002). Duration of exercise, peak rate pressure product, peak aortic flow velocities, and acceleration times did not differ. In contrast, control group patients (intact AV conduction throughout exercise) showed no statistical differences between any hemodynamic parameters measured at any phase of exercise from the first to second exercise test. These data demonstrate that systolic cardiac hemodynamics measured echocardia-graphically at the high heart rates achieved with peak exercise are improved with AVDR compared to AVDF in chronofropically competent patients with complete AV block. This is due primarily to improved stroke volume and a longer systolic ejection time with AV delay rate adaptation.     

Prognostic Value of Heart Rate Variability in Patients Awaiting Cardiac Transplantation

Time and frequency domain parameters of heart rate variability (HRV) were determined in patients with severe end stage heart failure awaiting cardiac transplantation (HTX). These parameters were then correlated with mortality to investigate the performance of HRV in discriminating between groups with high and low risk of death. The standard deviation of five consecutive RR intervals (SDANN) was found to be the parameter with the greatest sensitivity (90%) and specificity (91%). Patients with SDANN values of < 55 msec had a twenty-fold increased risk of death (90% confidence limits: 4–118, P < 0.001). The results furthermore suggest that measurements of HRV are superior to other prognostic markers such as left ventricular ejection fraction, pulmonary artery wedge pressure, cardiac index, and serum sodium levels. We conclude that HRV is a powerful, noninvasive tool to assess the risk of death in candidates for HTX. HRV measurements can therefore be used as a supplement to other markers of risk to determine the optimal therapeutic strategy in patients with severe congestive heart failure.     

Normal values for QT intervals in ECG during ramp exercise on bicycle

Summary. The relation between QT interval and heart rate during ramp exercise tests on a bicycle was investigated in 37 healthy individuals (21 women) without regular medication and with a normal thallium-201 exercise scintigram (mean age 52–9 ± 8–3, range 38–68). The test started at 20 W and the load increased by 10 W min^-1. A 12-lead ECG was recorded twice every min and mean complexes (during a 15 s period) were calculated by computer. At rest the QT interval (in s) corrected for heart rate (QT_C) for women and men was 0–408 ± 0–004 and 0–399 ± 0–005, respectively, P > 0–05). During exercise there was no difference in QT interval between women and men or between younger (<50 years) and older (> 50 years) individuals. A straight line was used to describe the relation between QT interval and heart rate (beats min^-1; QT = 0–459–12–3xlO^-4*HR). A 95% prediction interval around the regression line was determined using a non-parametric statistical method. When QT_C was calculated using Bazett's formula with a cut-off value of QT_c= 0–46, 19 individuals (11 women) had a prolonged QT interval during exercise. It is concluded that the relation between QT interval and heart rate can during exercise be described by a straight line for normal individuals. It is not valid to use Bazett's formula for correction of QT intervals during ramp exercise tests.