Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: UMLS:C0038454 (stroke)
 147,016 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Exercise (rate) responsive pacemakers benefit patients by providing increased cardiac output when needed and permitting lower rate during rest. This paper briefly reviews trends in reported studies on rate responsive pacemakers. For patients with reliable atrial rhythms, atrial-triggered pacemakers (DDD) provide physiological ventricular rates unless complications arise. At low rates, A-V synchrony benefits patients with refractory cardiac decompensation; however, in patients with healthy myocardiums, achieving higher pacing rates is more significant than maintaining synchrony. If atrial rhythms are unreliable, an alternative sensor for determining pacing rate is indicated. Pacemakers that respond to changes in right ventricular blood temperature, respiratory rate, QT interval, body vibration (motion), and pH have been implanted in humans. Clinical evaluations have shown that increased pacing rate leads to increased exercise tolerance and cardiac output when needed, independent of the sensor type (DDD, QT, respiratory rate, etc.). The effectiveness of any sensor type to increase pacing rate appropriately requires reliable sensors that respond specifically to the need for increased pacing rate. Sensors for stroke volume, venous oxygen saturation, right atrial or ventricular pressure and catecholamines are also under preclinical investigation. The availability of a reliable, long-term sensor is a key limitation to several techniques including pH, stroke volume, oxygen saturation, pressure, and catecholamines. Sensor technology and clinical effectiveness are the keys to rate responsive pacing.
 ...
PMID:Trends in pacemakers which physiologically increase rate: DDD and rate responsive. 243 72

Hemodynamics, myocardial oxygen consumption, and lactate concentration were determined by cardiac catheterization at rest and during exercise in eight patients treated with AV universal pacemakers (DDD) for high degree AV block. The pulse generator was alternately programmed in ventricular inhibited (VVI) or atrial synchronous (VAT) mode. During VVI pacing, the cardiac output rose between rest and exercise (4.3-7.6 L/min) due to increased stroke volume. VAT pacing gave significantly greater increase (4.5-8.8 L/min) which, as the stroke volume was unchanged, resulted from accelerated heart rate. The myocardial oxygen consumption and the coronary blood flow did not differ between VVI and VAT mode at rest or during exercise, nor did the modes make a difference in arterial systolic and pulmonary wedge pressures. These observations suggested that VAT pacing offers higher cardiac output than VVI pacing, but with similar demands on myocardial oxygen consumption.
 ...
PMID:Myocardial demands of atrial-triggered versus fixed-rate ventricular pacing in patients with complete heart block. 244 40

The beneficial haemodynamic effects of sequential atrioventricular (AV) pacing have been clearly established and are dependent on the AV delay and pacing rate. However, the optimal AV delay is difficult to determine in each particular patient. We used a modified impedance plethysmographic method to assess variations in stroke volume for different AV delay and pacing rate settings. Impedance measurements showed a good correlation with CO2 rebreathing stroke volume measurements in VVI patients. Impedance variations were then used to set the optimal AV delay at different pacing rates in DDD patients. The inverse relationship between the optimal AV delay and the pacing rate has been accurately identified in most of the patients but is not predictable. In all cases, the cardiac output was higher in DDD mode at the optimal AV delay than in VVI mode. In some patients with a damaged myocardium, the stroke volume appeared to be highly sensitive to multiple AV delay settings. Impedance plethysmography can permit such repetitive non-invasive quick measurements, increasing the accuracy of optimal AV delay determination and is well suited for routine examination of patients with cardiac dual chamber pacemakers.
 ...
PMID:Assessment of the optimal atrio-ventricular delay in DDD paced patients by impedance plethysmography. 270 72

The increase in stroke volume with DDD compared with VVI pacing was measured at rest using pulsed Doppler echocardiography in 23 patients at a pacing rate of 70 beats min-1. Stroke volume was assessed by measuring the velocity integral of the flow at the aortic annulus using the apical five-chamber window. Pulsed Doppler echocardiography allowed determination of the least and most favourable AV delay haemodynamically. TVI was also measured at each nominal value of AV delay. The percentage increase in stroke volume was determined in every patient changing from VVI to optimum DDD pacing and was used as a measurement of the 'sensitivity' to optimum DDD pacing; the mean increase was 27 +/- 19%. The increase in stroke volume accompanying the change from DDD pacing with the least favourable to the optimum AV delay was also measured, and used as a measurement of 'sensitivity' to changes in AV delay; the mean increase was 23.7 +/- 16.3%. Clinical and standard echocardiographic parameters were studied in order to determine which variable might best identify the patients more likely to benefit from DDD pacing, and to identify those more sensitive to the AV delay setting. With respect to sensitivity to DDD pacing, three echocardiographic variables were selected by linear discriminant analysis from 11 clinical and echocardiographic variables. These were, in order of importance, left ventricular systolic diameter (LVSD), left ventricular wall thickness (LVWT) and left atrial size (LAS) which allowed the prediction of a good or a bad response to optimal DDD pacing with an accuracy of 91.3%.(ABSTRACT TRUNCATED AT 250 WORDS)
 ...
PMID:Non-invasive study of dual chamber pacing by pulsed Doppler. Prediction of the haemodynamic response by echocardiographic measurements. 275 13

In dual chamber pacing, an improvement of exercise capacity is expected when the atrial refractory period is shortened, because the 2/1 point is increased. This objective can be achieved by greatly reducing atrioventricular delay (AVD) on exercise. Are such variations (up to 100-120 ms) detrimental from a haemodynamic standpoint? This study was performed to analyse this particular aspect of DDD pacing. Three DDD pacing modes, differing by their AVDs (fixed 200 ms AVD, fixed 150 ms AVD, and rate-adapted AVD) were tested in random order, with a haemodynamic protocol including ten patients with chronic atrio-ventricular (A-V) block. For the rate-adapted AVD pacing mode, AVD was reduced by 20 ms every 10 beats min-1 increment (from 220 ms at 90 beats min-1 to 100 ms at 150 beats min-1). Pacing rate was increased from 90 to 150 beats min-1 by increments of 10 beats min-1 every 5 min. Cardiac performance was significantly improved with the rate-adapted AVD above the two fixed AVDs, despite a large AVD variation. When AVD was rate adapted, cardiac index, stroke volume index and left ventricular systolic work index were generally higher and pulmonary capillary wedge pressure, pulmonary arterial pressure and systemic vascular resistances were generally lower, especially at 120, 130 and 140 beats min-1. Comparing the two fixed AVDs, 200 AVD improved cardiac function more at lower heart rates, whereas 150 AVD improved cardiac function more at higher heart rates. Despite its limitations, this study demonstrates that the potential benefits of reducing AVD with increasing heart rates should be twofold in dual chamber pacing: (a) haemodynamic, optimizing cardiac performance on exercise for all heart rates, especially in cases of organic heart disease; (b) electrophysiologic, permitting a sufficiently rapid maximal tracking rate in cases with long post-ventricular atrial refractory periods, allowing a satisfactory level of exercise.
 ...
PMID:Haemodynamic benefit of a rate-adapted A-V delay in dual chamber pacing. 276 75

To determine the importance of different atrioventricular intervals during exercise in patients with dual chamber pacemakers, seven patients with complete heart block and sinus rhythm were exercised in different pacing modes and atrioventricular intervals: (a) ventricular inhibited (VVI) pacing with no synchronous atrial augmentation or rate responsiveness; (b) atrial synchronous ventricular or DDD pacing with a short mean (SD) atrioventricular interval of 66 (4) ms; and (c) DDD pacing with a long atrioventricular interval of 168 (12) ms. Pacing with a short or long atrioventricular interval gave similar maximum heart rates, oxygen uptake at the anaerobic threshold, end tidal pressure of carbon dioxide or oxygen pulse (a measure of stroke volume). Pacing with either a short or long atrioventricular interval produced a significantly higher oxygen consumption and anaerobic threshold and less lactate production than VVI pacing. During exercise a short atrioventricular interval does not provide a better cardiopulmonary performance than a long atrioventricular interval.
 ...
PMID:Physiological importance of different atrioventricular intervals to improved exercise performance in patients with dual chamber pacemakers. 291 98

The optimal atrioventricular delay at rest and during exercise was investigated in nine patients with heart block and implanted dual chamber pacemakers. All patients studied had normal left ventricular function and a normal sinus node rate response to exercise. Cardiac output was measured by continuous wave Doppler and was calculated as the product of stroke distance measured by Doppler at the left ventricular outflow, aortic root area measured by M mode echocardiography, and heart rate. Pacemakers were programmed in the DDD mode. Cardiac output was measured with the patient at rest while supine and while erect and at the peak of submaximal exercise (the end of stage 1 of the Bruce protocol) with the pacemakers programmed to the following atrioventricular intervals: 75-80 ms, 100-110 ms, 140-150 ms, and 200 ms. During exercise the basic pacing rate was programmed to 70 beats/min. Cardiac output at rest while supine and erect was greatest with an atrioventricular delay of 140-150 ms and it was significantly higher than that with an atrioventricular delay of 75-80 ms. On average there was a 31% decrease in cardiac output when patients stood up. During treadmill exercise, however, cardiac output was greatest when the atrioventricular delay was 75-80 ms, and this was significantly higher than the cardiac output with atrioventricular delays of 150 and 200 ms. During exercise 1:1 atrioventricular relations were maintained in patients at all atrioventricular intervals. In patients with atrioventricular sequential pacemakers cardiac output at rest is greatest with an atrioventricular delay of 140-150 ms but during exercise the optimal atrioventricular delay is shorter. Rate modulation of the atrioventricular interval may improve the haemodynamic response and possibly exercise tolerance in patients with dual chamber pacemakers.
 ...
PMID:Optimal atrioventricular delay at rest and during exercise in patients with dual chamber pacemakers: a non-invasive assessment by continuous wave Doppler. 292 53

Cardiac sympathetic function was assessed by measuring the coronary sinus overflow of noradrenaline and dopamine at rest and during supine exercise in eight patients with high degree atrioventricular block treated with dual chamber pacemakers (DDD). Patients exercised (30-60 W) during both ventricular inhibited (VVI) and atrial synchronous (VAT) pacing. During exercise cardiac output increased less markedly in the VVI mode than in the VAT mode. The cardiac output response was entirely stroke volume dependent in the VVI mode and mainly heart rate dependent in the VAT mode. Coronary sinus noradrenaline concentrations were higher in the VVI mode at rest and during exercise. Noradrenaline overflow from the heart was enhanced during VVI pacing and increased from about 100 pmol/min (17 ng/min) at rest to 1087 pmol/min during exercise (60 W) in the VVI mode and 545 pmol/min in the VAT mode. Dopamine overflow from the heart was less than 5 pmol/at rest but increased 2-5 fold during exercise. Also arterial concentrations of catecholamine increased more during exercise in the VVI mode, but the differences between pacing modes were less pronounced. Circulating adrenaline seems to be of little importance for cardiac function under these conditions; in healthy individuals the arterial concentrations of adrenaline attained in this study have small effects. Cardiac noradrenaline overflow correlated with pulmonary capillary venous pressures and atrial rates in both pacing modes, indicating a relation between cardiac sympathetic activity and cardiac function. Enhanced cardiac release of noradrenaline may increase cardiac contractility and thereby partially compensate for the lack of heart rate responsiveness to exercise during VVI pacing.
 ...
PMID:A comparison of sympathoadrenal activity and cardiac performance at rest and during exercise in patients with ventricular demand or atrial synchronous pacing. 317 37

To determine whether survival following permanent ventricular demand pacing differs from survival following permanent dual-chamber pacing in patients with symptomatic sinus node dysfunction (unexplained sinus bradycardia, subsidiary rhythms, sinus arrest, sinoatrial block, or the bradycardia/tachycardia syndrome), we followed 79 patients who received a VVI pacemaker (group 1) and 49 patients who received a DVI or DDD pacemaker (group 2) for 1 to 5 years. There was no significant difference in sex distribution, mean age, or the incidence of coronary heart disease, hypertension, valvular heart disease, diabetes mellitus, stroke, or renal failure between groups 1 and 2. Overall, the predicted cumulative survival rates at 1, 3, and 5 years were 89%, 82%, and 74%, respectively, for group 1 and 94%, 86%, and 78%, respectively, for group 2. In patients with preexistent congestive heart failure (CHF), predicted cumulative survival rates at 1, 3, and 5 years were 78%, 69%, and 57%, respectively, for group 1 (n = 23) and 90%, 83%, and 75%, respectively, for group 2 (n = 16). Five-year predicted cumulative survival was significantly lower in group 1 patients with CHF than in group 2 patients with CHF (p less than 0.03). There was no significant difference in 5-year cumulative survival rates between groups 1 and 2 in patients without CHF. The results suggest that permanent dual-chamber pacing enhances survival to a greater extent than permanent ventricular demand pacing in patients with chronic symptomatic sinus node dysfunction and CHF.
 ...
PMID:Comparative survival following permanent ventricular and dual-chamber pacing for patients with chronic symptomatic sinus node dysfunction with and without congestive heart failure. 356 45

To determine whether survival after permanent ventricular demand (VVI) pacing differs from survival after permanent dual chamber (DVI or DDD) pacing in patients with chronic high degree atrioventricular (AV) block (Mobitz type II or trifascicular block), 132 patients who received a VVI pacemaker (Group 1) and 48 patients who received a DVI or DDD pacemaker (Group 2) were followed up for 1 to 5 years. There was no significant difference in sex distribution, mean age or incidence of coronary heart disease, hypertension, valvular heart disease, diabetes mellitus, stroke or renal failure between Groups 1 and 2. Overall, the predicted cumulative survival rate at 1, 3 and 5 years was 89, 76 and 73%, respectively, for Group 1 and 95, 82 and 70%, respectively, for Group 2. In patients with preexistent congestive heart failure, the predicted cumulative survival rate at 1, 3 and 5 years was 85, 66 and 47%, respectively, for Group 1 (n = 53) and 94, 81 and 69%, respectively, for Group 2 (n = 20). The 5 year predicted cumulative survival rate was significantly lower in Group 1 patients with preexistent congestive heart failure than in Group 2 patients with the same condition (p less than 0.02). There was no significant difference in 5 year cumulative survival rate between Groups 1 and 2 for patients without preexistent congestive heart failure. The results suggest that permanent dual chamber pacing enhances survival to a greater extent than does permanent ventricular demand pacing in patients with high degree AV block and preexistent congestive heart failure.
 ...
PMID:Comparative survival after permanent ventricular and dual chamber pacing for patients with chronic high degree atrioventricular block with and without preexistent congestive heart failure. 395 51


<< Previous 1 2 3 4 5 6 7 8 Next >>