Comparison of the effects of right atrial, right ventricular apex and atrioventricular sequential pacing on myocardial oxygen consumption and cardiac efficiency: a laboratory investigation

Relation between regional electrical activation time and subepicardial fiber strain in the canine left ventricle

To determine the relation between regional electrical activation time and fiber strain, epicardial electrical activation and deformation were measured in six open-chest dogs at the left ventricular anterior free wall after 15 min of right atrial, left ventricular free wall, left ventricular apex, or right ventricular outflow tract pacing, when end-diastolic pressure was normal or elevated (volume-loading). Regional electrical activation was measured using a 192-electrode brush. Regional subepicardial fiber strain (ef) was measured simultaneously in 16 regions, using optical markers which were attached to the epicardial surface and recorded on video. When relating regional ef during the ejection phase to regional activation time, the best correlation was found when a hemodynamic time reference rather than an electrophysiological one is used. Using the moment of the maximum rate of change of left ventricular pressure as the time reference for electrical activation, regional electrical activation time (t(ea)) and the degree of ef during the ejection phase could be fitted by a linear regression equation ef = a t(ea) + b, in which a = -3.46 +/- 0.73 s-1 an b = -0.28 +/- 0.05. For electrical activation times ranging from -40 to -80 ms, fiber strain was estimated with an accuracy of +/- 0.026 (+/- SE) with this relation. During right atrial pacing, t(ea) and ef were on the average -48 ms and -0.10 respectively. On further investigation, the relation between ef and t(ea) appeared to be influenced by end-diastolic pressure. For normal (1.1 kPa) and elevated end-diastolic pressure (1.8 kPa), the slope of the linear regression line was -3.96 and -2.86 s-1, respectively. Three conclusions may be drawn. Firstly, the time interval between the moment of regional electrical activation and the moment of the maximum rate of change of left ventricular pressure is an index of regional fiber strain. Secondly, it can be concluded from the above equations that electrical asynchrony of more than 30 ms causes non-uniformities in the degree of ef of the order of mean ef during pacing from the right atrium. Finally, differences in fiber strain during asynchronous electrical activation are less pronounced at larger filling pressures.

Evaluation by cardiopulmonary exercise test of DDDR versus DDD pacing

In eight patients (age 62 +/- 6 years) a DDDR pacemaker was implanted for sick sinus syndrome (three cases) or second- and third-degree AV block (five cases). In five subjects chronotropic incompetence (maximal heart rate on effort < 110 beats/min) was present before implantation. One month after implantation the patients were randomized to DDDR or DDD pacing for 3 weeks each, with subsequent crossover, and at the end of each period a symptom limited cardiopulmonary exercise test (25 watts/2 min) was performed and the patients were requested to fill a symptoms questionnaire.

RESULTS

DDDR pacing, compared to DDD, was associated with higher maximal heart rates (127 +/- 20 vs 110 +/- 27 beats/min, P < 0.02), higher [VO2 max (25.4 +/- 6.1 vs 21.5 +/- 7.8 mL/kg/per min, P < 0.03) and higher VO2 at the anaerobic threshold (20.3 +/- 5.0 vs 15.8 +/- 4.9 mL/kg per min, P < 0.03), without significant differences in mean exercise time (526 +/- 193 vs 472 +/- 216 sec, NS). The increase in VO2 max obtained in DDDR versus DDD was significantly related to the increase in maximal heart rate (r = 0.72, P < 0.05) and the increase in VO2 at the anaerobic threshold obtained in DDDR versus DDD was related to the increase in heart rate at the anaerobic threshold (r = 0.81, P < 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)

Single-lead VDD pacing: excellence or expedience?

The interest in VDD pacemakers has been renewed by the introduction of single-pass leads, and continuing modifications and technical developments promise significant improvement. However, convincing long-term data confirming the reliability and the use of newer leads are not yet available. Compared with VVIR pacemakers VDD pacing has theoretical advantages over the VVIR mode, offering better hemodynamic and endocrine responses. However, its range of indications is narrow and rate adaptation is essential when sinus node chronotropic inadequacy is present or likely to occur. In addition, on certain occasions a VDD pacemaker may not maintain persistent atrial-synchronous pacing, thus necessitating the use of a DDD unit.

VVIR or DDD(R): does it matter?

A continuing emphasis on cost effectiveness in health care may require that we use more expensive pacing systems only in situations where there is clear medical and scientific evidence of increased efficacy. Although dual-chamber and/or sensor-based, rate-modulating pacing systems are electronically no less reliable, they are part of a more complex pacing system. The requirement for two leads, one of which must maintain both pacing and sensing in the atrium, will inevitably impact the cost and reliability of such systems compared with a single-chamber ventricular system. Yet, there is clear evidence that AV synchrony is important at rest, particularly in patients susceptible to pacemaker syndrome, and there is mounting evidence that AV synchrony during exercise is beneficial independent of rate response. Finally, and perhaps most important, there is the suggestion that patient longevity may be extended by using pacing systems that preserve AV synchrony and/or minimize ventricular pacing.

Improved dual chamber pacing mode in paroxysmal atrioventricular conduction disorders

Dual chamber pacing may sometimes be directly indicated for carotid sinus hypersensitivity, vasovagal syndrome, and certain cases of sinoatrial block and intermittent atrioventricular (AV) block, although AV conduction is dominantly normal. At times of normal AV conduction, competition between ventricular pacing and spontaneous ventricular depolarization may occur, with its adverse hemodynamic effects on ventricular function and unnecessary drainage of pacemaker battery energy. A new mode of stimulation is described, called automatic DDD mode, which functions in 'pseudo-AAI' mode during normal AV conduction and reverts to classical DDD function during episodes of AV blocks. Furthermore, during pseudo-AAI function, the pacemaker measures certain physiological parameters that serve to automatically program certain parameters used in DDD mode. Preliminary clinical evaluation has shown that this new mode functions satisfactorily.

Effects of increasing heart rate induced by efferent sympathetic neuronal stimulation, isoproterenol or cardiac pacing on myocardial function and oxygen utilization

The effects of increasing heart rate by six different methods on cardiac function were investigated in 17 open-chest anesthetized dogs. Heart rate was increased approximately 30% by (1) right interganglionic nerve stimulation, (2) atrial pacing, (3) ventricular pacing, (4) atrioventricular sequential pacing, (5) right stellate ganglion stimulation, and (6) isoproterenol administration. During heart rate increases induced by atrial pacing left ventricular intramyocardial pressure, coronary blood flow, oxygen delivery per unit of myocardial oxygen consumption, and myocardial efficiency were unchanged. Ventricular pacing reduced left ventricular cavity and septal intramyocardial pressure, while circumflex coronary flow increased, resulting in reduced oxygen delivery relative to myocardial oxygen consumption. Similarly, atrioventricular sequential pacing increased circumflex coronary artery flow and myocardial oxygen consumption, and decreased septal intramyocardial pressure and oxygen delivery per unit of myocardial oxygen consumption. Right stellate ganglion stimulation and isoproterenol increased left anterior descending and circumflex coronary artery blood flow, intramyocardial pressure, and myocardial oxygen consumption. Estimated myocardial efficiency (left ventricle) was decreased by ventricular pacing and isoproterenol, and was unchanged by atrial pacing and right interganglionic nerve stimulation. Increases in heart rate induced by right interganglionic nerve stimulation did not alter myocardial oxygen consumption, or the index of cardiac efficiency. It is concluded that augmentation of heart rate by either ventricular or atrioventricular pacing impairs myocardial function so that there is a decrease of left ventricular efficiency and isoproterenol augments chronotropism and myocardial force relative to cardiac external work so there is a reduction in cardiac efficiency. In contrast, atrial pacing or right interganglionic nerve stimulation augments chronotropism such that myocardial oxygen consumption and efficiency are unchanged.

Current status of sensor-modulated rate-adaptive cardiac pacing

In one third of patients requiring permanent cardiac pacing, sinoatrial response to exercise is inadequate. This has led to the development of a number of nonatrial sensors capable of increasing stimulation rate in response to metabolic demand. Initial sensor-modulated pacemakers were designed for use in the ventricle. This review was undertaken to compare ventricular rate-adaptive pacing with dual chamber and nonrate-adaptive atrial modes. Rate-adaptive ventricular pacing is primarily indicated in patients with complete heart block and nonfunctional atria in whom increased rates are not associated with the development of myocardial ischemia or retrograde conduction. A responsive atrium remains the best measure of metabolic demand, and devices capable of maintaining atrioventricular (AV) sequencing provide superior hemodynamic performance at usual activity levels. In addition, preserved AV relations reduce the incidence of new atrial fibrillation and stroke. Newer devices combining the advantages of atrial and nonatrial rate modulation with the preservation of AV synchrony will best utilize the developing sensor technologies.