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Ann Thorac Surg 1998;65:1231-1234
© 1998 The Society of Thoracic Surgeons
a Cardiovascular Surgery Department, Türkiye Yüksek Ihtisas Hospital, Ankara, Turkey
Accepted for publication November 28, 1997.
Address reprint requests to Dr Vural, N. Tandogan cad. 5/6 Kavaklidere 06540, Ankara, Turkey
e-mail: (kvural{at}tr-net.net.tr)
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Abstract |
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Methods. We studied 10 patients who were in their first 2 years postoperatively. Three values for SD (SD = 0 ms, 45 to 60 ms, and 150 to 160 ms) were echocardiographically evaluated for their influence on both systolic and diastolic left ventricular parameters.
Results. Ejection fractions were 0.27 ± 0.07, 0.28 ± 0.07, and 0.32 ± 0.06; peak aortic velocities were 0.85 ± 0.8, 0.86 ± 0.11, and 0.92 ± 0.8 m/s; and velocity-time integrals were 0.16 ± 0.03, 0.16 ± 0.03, and 0.19 ± 0.03 m for the SD values of 0, 45 to 60 ms, and 150 to 160 ms, respectively. Diastolic parameters were also measured. Isovolumetric diastolic relaxation time was 97.5 ± 49, 97.20 ± 44, and 111.8 ± 49 ms; deceleration time was 83.67 ± 32, 88.48 ± 35, and 92.68 ± 34 ms; and ratio or velocity-time integral of e wave to velocity-time integral of a wave was 3.09 ± 0.98, 2.48 ± 0.69, and 2.38 ± 0.65 for the SD values of 0, 45 to 60 ms, and 150 to 160 ms, respectively. Systolic functions were better when SD was set at 150 to 160 ms, but there was a diastolic compromise. On the other hand, diastolic parameters were more favorable when SD = 0 (ie, cardiomyostimulator triggered without delay) but the systolic assist was suboptimal. Systolic and diastolic parameters seemed relatively well-balanced with the current practice of setting the synchronization delay at 45 to 60 ms.
Conclusions. The most favorable systolic effects were obtained with a prolonged delay of synchronization (150 to 160 ms), at some expense of diastolic functions. On the other hand, with a short or absent delay, diastolic parameters were improved but systolic parameters became suboptimal. Therefore, the current practice of setting the SD between 45 and 60 ms after echocardiographic mitral valve closure is suggested for the optimal timing for cardiomyostimulator stimulation in patients who have undergone latissimus dorsi dynamic cardiomyoplasty. Yet a great deal of individualization is necessary, and fixed preset values cannot definitely be determined because one setting does not fit all patients.
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Introduction |
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TopAbstractIntroductionMaterial and methodsResultsCommentReferences |
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Optimal hemodynamic assistance by the LDM after dynamic cardiomyoplasty depends on the appropriate synchronization of the cardiac and LDM contraction cycles. The optimization of many characteristics of the pulse train (eg, pulse amplitude, pulse width, number of pulses per burst) generated by the cardiomyostimulator has been investigated by a number of studies. However, the optimal delay time for electrical stimulation of the wrapped LDM after mitral valve closure to achieve the best performance has not been clearly defined. Our current practice is to set the synchronization delay (SD) at 45 to 60 ms, so the cardiomyostimulator stimulates the LDM 45 to 60 ms after mitral valve closure on M-mode echocardiography. We studied the optimization of the SD between the mitral valve closure and the electrical stimulus generation by the implanted cardiomyostimulator.
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Material and methods |
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TopAbstractIntroductionMaterial and methodsResultsCommentReferences |
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Echocardiography protocol
Systolic (ejection fraction, peak aortic velocity, velocity-time integral of aortic velocity) and diastolic parameters (e/a wave velocity-time integral ratio, isovolumetric relaxation time, deceleration time) were examined. All examined parameters were measured at assisted beats. Digital stress echocardiography (a Freeland Cineview device; Prism Imaging) was employed for obtaining the left ventricular ejection fraction and the left ventricular end-systolic and end-diastolic volume indices. Eight frames per cardiac cycle, triggered from the R wave at 50-ms intervals, were recorded in a continuous loop format. Apical two- and four-chamber views were acquired at rest. For calculation of diastolic and systolic left ventricular volumes, endocardial borders were traced digitally in diastole and systole. Commercially available software (Cineview Version 5.05; Prism Imaging) was used for the calculations of the left ventricular volumes and thus the ejection fraction, according to the modified Simpson’s rule [7]. All these measurements were repeated for the three values of SD. Obtained data for systolic and diastolic parameters for all three SD values are presented as the means and the standard deviations.
Statistical methods
Statistical analysis was performed by SPSS software (release 6.0; SPSS Inc, Chicago, IL). The Wilcoxon matched-pairs signed-ranks test was used for the analysis of the obtained data. A p value smaller than 0.05 was considered statistically significant.
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Results |
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Peak aortic velocity and velocity-time integral
Peak aortic velocities were 0.85 ± 0.8, 0.86 ± 0.11, and 0.92 ± 0.8 m/s; and velocity-time integrals were 0.16 ± 0.03, 0.16 ± 0.03, and 0.19 ± 0.03 m for the SD values of 0, 45 to 60 ms, and 150 to 160 ms, respectively. Peak aortic velocity was higher with a prolonged SD of 150 to 160 ms when compared with 0 (p = 0.02) and 45 to 60 ms (p = 0.05) values. There was no difference in peak aortic velocity between those obtained with the SD values of 0 and 45 to 60 ms.
Diastolic parameters
Velocity-time integral of e wave/velocity-time integral of a wave
The e/a velocity-time integral ratio was 3.09 ± 0.98, 2.48 ± 0.69, and 2.38 ± 0.65 for the SD values of 0, 45 to 60 ms, and 150 to 160 ms, respectively. This ratio was higher with an SD of 0 ms when compared with 150 to 160 ms (p = 0.008) values. There was no difference in velocity-time integral ratios between those obtained with the SD values of 0 and 45 to 60 ms (p = 0.08).
Isovolumetric diastolic relaxation time
Isovolumetric diastolic relaxation time was 97.5 ± 49, 97.20 ± 44, and 111.8 ± 49 ms for the SD values of 0, 45 to 60 ms, and 150 to 160 ms, respectively. Relaxation time was higher with a prolonged SD of 150 to 160 ms when compared with 0 (p = 0.005) and 45 to 60 ms (p = 0.01) values. There was no difference in isovolumetric diastolic relaxation time between those obtained with the SD values of 0 and 45 to 60 ms. Thus, the best diastolic function was obtained with SD settings of 0 and 45 to 60 ms.
Deceleration time
Deceleration time was 83.67 ± 32, 88.48 ± 35, and 92.68 ± 34 ms for the SD values of 0, 45 to 60 ms, and 150 to 160 ms, respectively. Deceleration time was shorter with an SD of 0 ms when compared with 150 to 160 ms (p = 0.007) of SD, demonstrating better preserved diastolic functions with SD = 0. There was no difference in deceleration time between those obtained with the SD values of 45 to 60 and 150 to 160 ms.
Summary of the hemodynamic effect of changing synchronization delay
Systolic functions were better when SD was set at 150 to 160 ms, but there was a diastolic compromise. On the other hand, diastolic parameters were more favorable when SD = 0 (ie, cardiomyostimulator triggered without delay). Systolic and diastolic parameters seemed relatively well-balanced with the current practice in which synchronization delay was set at 45 to 60 ms.
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Comment |
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Hamilton and associates [12] have studied the effect of cardiomyoplasty on the heart muscle when the skeletal muscle was being stimulated to coincide with alternate natural beats. Left ventricular function was assessed with radionuclide angiography during the beat immediately after skeletal muscle stimulation and during the beat immediately before stimulation. When the supported beat was compared with the unsupported beat, the results demonstrated that cardiomyoplasty improved the global and regional ejection fraction. The systolic peak emptying rate improved, but the diastolic peak filling rate worsened.
In our previous study, the results yielded evidence for an active systolic assistance generated by the synchronously stimulated LDM contractions [1]. Further, electrical stimulation of the LDM resulted in better exercise response when compared with nonstimulated LDM. We concluded that the so-called girdling effect by itself cannot match the requirements of individuals during daily stresses and exertions, and electrical stimulation of LDM is necessary. That brings up another issue that concerns the optimization of the electrical stimulus generated by the cardiomyostimulator. The most appropriate settings for timing, amplitude, and duration of the pulse train should be defined. Even an individualization of these preset values depending on the patient’s requirements may become necessary.
In many previous studies, optimal settings for parameters such as pulse amplitude, pulse width, number of pulses per burst, and synchronization ratio were reported. They are mostly experimental studies, and most of them did not deal with the optimization of SD in clinical settings, which appears as an important determinant for the maximal systolic assistance. Helou and coworkers [13] compared the different modes used to determine the delay period between the R-wave sensing and the onset of burst stimulation during cardiac systole in 3 patients. These modes included the fixed-time mode (in which the timing of the burst stimulus is fixed at 25 ms, corresponding to the usual time interval between the R-wave and the onset of mechanical cardiac systole in a normal conduction system), the valve-synchronized mode (in which two-dimensional echocardiography is used to time the onset of the burst stimulus to occur at mitral valve closure), and the flow-optimized mode (the burst stimulus is programmed to obtain maximal aortic flow velocity as measured by Doppler echocardiography). They stated that, despite its simplicity in programming and the adequacy in many patients, difficulties may arise with the fixed-time mode. In those patients whose mechanical cardiac systole is delayed because of conduction defects such as left bundle-branch blocks, LDM contraction may occur in late diastole before the onset of mechanical cardiac systole, interfering with the diastolic filling. Contraction of the LDM before mitral valve closure and the increased left ventricular pressure while the mitral valve remains open could aggravate the mitral regurgitation that is so common in patients with dilated cardiomyopathy [13]. Helou and coworkers concluded that the valve-synchronized mode ensures that the burst stimulus is delivered at the start of mechanical cardiac systole, and thus prevents interference with diastolic ventricular filling. However, due to additional delays that are not accounted for with the valve-synchronized mode (these include the conduction velocity of the thoracodorsal nerve, the transmission of the impulse across the neuromuscular junction, the contraction velocities of the myocardium and the skeletal muscle wrap, the vector of the force related to the muscle fiber orientation, the fluid dynamics of blood in the ventricle, and so on, all of which exhibit interpatient variability), the delay periods chosen either by the fixed time mode or by the valve-synchronized mode may not induce the heart to deliver maximum output on the assisted beats [13]. They found in 1 patient that the maximum flow velocity in the left ventricular outflow tract was obtained with a 75-ms delay period, which resulted in a flow that was 30% greater than that obtained when the onset of the burst stimulus was synchronized with mitral valve closure (45 ms of delay).
Molteni and colleagues [14] have shown that changes in the delay time between the QRS sensing and single-pulse stimulation in cardiomyoplasty significantly affect the peak flow velocity of blood in the ascending aorta.
We investigated the influence of changing the SD on hemodynamics in 10 cardiomyoplasty patients who recently underwent a dynamic cardiomyoplasty procedure. All the SD values described in this study are the intervals between the echocardiographic mitral valve closure and the first spike of the pulse train burst. We studied the diastolic parameters as well as the systolic parameters. The most favorable systolic effects were obtained with a prolonged SD (150 to 160 ms), at some expense of diastolic functions. This finding supports the conclusion of Geddes and associates [15]. They studied the timing of muscle contraction in a canine cardiomyoplasty model and found the optimal delay period to be an average of 58 ms (range, 40 to 80 ms), which produced maximal augmentation of left ventricular function, including the stroke volume. They theorized that to obtain the maximum precontraction load on the muscle encircling the ventricle, it is desirable to cause it to contract late in the isovolumic period when the ventricle bulges maximally. Similarly, we found that prolonged SD may augment the systolic assist, but a diastolic compromise to some extent should be anticipated. This is most probably due to an interference with the following diastolic filling in the cardiac cycle.
The influence on diastolic parameters is another important subject to consider. Our study revealed that, with a short or absent delay, diastolic parameters may improve but systolic parameters may worsen. This worsening may be related to a premature ending of the preceding diastolic filling causing a submaximal stretching in the muscle fibers. The improvement in diastolic parameters is hard to explain, but may partly be related to the lower degrees of distention due to shortened, incomplete filling of the ventricle. This point needs further evaluation.
In conclusion, systolic and diastolic parameters seemed relatively well-balanced with the current practice of setting the SD at 45 to 60 ms. Therefore, the current practice of setting the SD between 45 and 60 ms after echocardiographic mitral valve closure is suggested for the initial timing for cardiomyostimulator stimulation in most cardiomyoplasty patients. However, should an individualization of these preset values depending on the patient’s requirements become necessary, adjustments may be made, in favor of diastolic or systolic functions, in the light of the present study. A great deal of individualization is necessary, and fixed preset values cannot definitely be determined because one setting does not fit all patients. Further, the delay period may change in the same individual in the months to years after the operation, when the type of fiber conversion, the left ventricular function, and other conditions associated with the initial operation have changed. Certainly programming should be evaluated on at least a 6-month basis to obtain the ideal delay period for stimulating the muscle and synchronizing this with mitral valve closure. We believe that the optimal programming of the electrical stimulus generated by the implanted cardiomyostimulator is an important part of obtaining maximal benefit after dynamic cardiomyoplasty. Thus, further studies focused on all aspects of this subject are necessary.
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  S. Salmons Cardiac assistance from skeletal muscle: a reappraisal Eur. J. Cardiothorac. Surg., February 1, 2009; 35(2): 204 - 213. [Abstract] [Full Text] [PDF]
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