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J Thorac Cardiovasc Surg 2005;130:615-617
© 2005 The American Association for Thoracic Surgery
Letters to the Editor |
Ospedale San Raffaele, Milan, Italy
We thank Dr Balacumaraswami and colleagues for their interest in our study on the acute effects of left ventricular (LV) restoration on cardiac performance. We prefer to answer in reverse order.
The reason for not giving any information on mitral regurgitation (MR) was the absence of significant perioperative MR in our patient group, all of whom underwent routine echocardiographic examinations. Concerning MR, McCarthy
1
referred to Di Donato and colleagues,
2
revealing that 10% of their patients had preoperative MR and 38% of their patients had had MR develop by 1 year after LV restoration, whereas immediately after surgery the patients were free from MR. When present, MR may offload the left ventricle; however, offloading will not change contractile state. Recently we
3
demonstrated that acute decrease in LV afterload by intra-aortic balloon pumping resulted in instantaneous increases in stroke volume but not in an increase of contractile state.
The second comment suggested that a significant improvement of LV systolic function and ejection fraction may have been attributable to recovery of hibernating myocardium in patients with additional coronary artery bypass grafting. Bax and colleagues
4
demonstrated in patients with ischemic cardiomyopathy undergoing CABG a reduction in wall motion abnormalities, based on hibernation, in 30% of the segments 3 months after bypass grafting. This suggests that in our acute LV restoration study, recovery from hibernation might have occurred; however, its contribution to an increase in contractile state could be doubted. Moreover, LV contractile state is commonly impaired or unchanged immediately after cardiopulmonary bypass.
5,6
In Table 3 of the LV restoration manuscript, we showed that LV mechanical dyssynchrony decreased significantly in the apical and midventricular segments, the areas of the LV restoration, whereas the dyssynchrony of the basal segments was unchanged.
7
The major finding of our study, the marked relationship between LV mechanical dyssynchrony and contractile state of the heart, however, is not affected by a possible decrease in mechanical dyssynchrony due to recovery from hibernation.
The first suggestion by Dr Balacumaraswami and colleagues concerned the significant increase in heart rate, present immediately after bypass, which might have increased contractile state on basis of the force-frequency relationship. Feldman and associates
8
showed that a heart rate increase of 60 beats/min increased +dP/dtmax by 30% in healthy man, whereas an increase of 30 beats/min did not increase +dP/dtmax. In patients with dilated cardiomyopathy, however, a mean increase in heart rate of 55 beats/min from a baseline value of 82 beats/min did not result in any significant change in +dP/dtmax.
In our study, mean heart rate changed from 76 to 99 beats/min after left ventricular restoration in these patients with dilated hearts, whereas mean +dP/dtmax increased by 41%.
7
Figure 1, A, shows the nonsignificant correlated force-frequency relationship from our study (r = 0.138, n = 9, P = .723). Figure 1, B, shows the relationship between change in heart rate and change in end-systolic elastance (Ees) due to LV restoration (r = –0.55, n = 9, P = .12). This suggests that heart rate merely increased as a compensatory mechanism in patients, characterized by a small increase in contractile state (Ees) and small decrease in systolic dyssynchrony, to maintain cardiac index. We
9
previously indicated that stroke volume could only be maintained by decreases in LV mechanical dyssynchrony in patients undergoing partial left ventriculectomy. This reasoning is confirmed by the significant correlation between heart rate change and dyssynchrony change (Figure 1, C, r = 0.715, P = .03) in the patients undergoing the LV restoration. In case mechanical resynchronization was suboptimal and end-diastolic volume decrease was significant, a heart rate increase may have compensated for decreased stroke volume (cardiac index). In the LV restoration manuscript, we indicated that heart rate increase correlated significantly with end-diastolic volume decrease (r = –0.679, P = .044) and that contractile state (Ees) was markedly inversely related (P <.00001) to LV mechanical dyssynchrony.
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References
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%HR); in B, change in end-systolic elastance (