Gabriello Marchetti, Graziana Labanti, Ennio Talamonti, Giancarlo Carini, Stefano Urbinati, Giuseppe Pinelli.
Unita Operativa di Cardiologia, Ospedale Bellaria, Bologna, Italy
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Both a synchronous contraction between the
atria and the ventricles as well as normal ventricular activation sequence converges
and contributes to the normal heart beat. Artificial pacemakers provide a wide
variability of altered contraction pattern. Clinical studies have focused on hemodynamic
importance of normal ventricular activation sequence for example in VVI mode, for the
deterioration in left ventricular function compared to AAI, caused by the asynchronous
pattern of ventricular activation1,2.
Dual chamber pacemakers are increasingly implanted to achieve optimal
hemodynamics by atrio-ventricular synchrony.
Nishimura et al3 demonstrated that dual chamber pacing may improve acute
hemodynamic variables in selected patients with dilated cardiomyopathy, mainly
by optimization of the timing of mechanical atrial and ventricular synchrony
reestablishing the optimal diastolic filling period and abolishing mitral regurgitation.
In patients with wide surface QRS Auricchio et al for the Guidant Congestive Heart
Failure Research Group4 have found the major benefit from atrial synchronous
ventricular pacing, where maximum left ventricle pressure derivative dP/dt and
aortic pulse pressure changed immediately at pacing onset, increasing at a patient
specific optimal A-V delay in patients with wide surface QRS (180 sec) and decreasing
at short A-V delays in patients with narrower QRS (128 ms). This close relationship
between abnormal contraction patterns and electrical activation disturbances, (as a
wide QRS complex) may explain why a sufficient wide QRS and left bundle branch
block predict short-term pacing benefit.
Abnormalities of left ventricular diastolic function may precede systolic dysfunction and
have been demonstrated with pulsed echo-Doppler echocardiography5. The Doppler
pattern of diastolic mitral inflow directly reflects left ventricular filling and it is
influenced by factors such as age, heart rate, loading conditions6. When impaired
relaxation is present the early diastolic pressure gradient between the left atrium and
left ventricle is small, resulting in decreased rate of early filling velocity with a
prolonged deceleration time as well as a greater residual atrial volume. If it is obtained
an increase in early diastolic filling may be obtained a significant reduction in the
isovolumic relaxation time, that is the most energy requiring phase of
excitation-contraction coupling and it is influenced by simpathetic tone. An increase in
myocardial distensibility can contribute to an increase in cardiac output and oxygen
supply to skeletal muscles8. This mechanism may play a role in explaining the
changes in left ventricle diastolic filling in patients with abnormal relaxation4-7. A significant
Doppler predictor of outcome is the early to atrial filling velocity (E/A ratio). In paced
patients mean age is often >60 years and a diastolic dysfunction is a common finding
so that is important to optimize the A-V delay to maximize cardiac output at rest and
during exercise by an automatic decrease in A-V delay during higher rates3,9.
The use of cardiopulmonary exercise testing is useful in the assessment of ambulatory
patients11-14.
Peak VO2 is an objective measure of functional capacity and an independent prognostic
index, but may be influenced by non-cardiovascular factors such as patient’s motivation
and skeletal muscle characteristics8-10.
Previous studies demonstrated that oxygen kinetics is delayed in patients with
congestive heart failure. Oxygen kinetic at exercise onset with a low work is more
dependent on changes of pump function than heart rate, while peak oxygen
consumption is more heart rate dependent. So chronotropic incompetence influences
peak oxygen consumption while oxygen kinetics is unaffected. Higher the work load,
more heart rate dependent oxygen kinetics become.
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