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Eur J Cardiothorac Surg 2002;21:975-980
© 2002 Elsevier Science NL
a The Department of Cardiothoracic Surgery, Tel Aviv Sourasky Medical Center, 6 Weizman Street, Tel-Aviv 64239, Israel
b The Department of Cardiothoracic Surgery, Maastricht University, Maastricht, The Netherlands
Received 10 September 2001; received in revised form 14 January 2002; accepted 16 January 2002.
* Corresponding author. Tel.: +972-3-697-3322; fax: +972-3-697-4439
e-mail: bolotin{at}netvision.net.il
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Abstract |
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 Top Abstract 1. Introduction2. Methods3. Results4. DiscussionAppendix A. Conference...References |
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Key Words: Heart failure • Skeletal muscle • Aortomyoplasty • Intra-aortic-balloon-pump
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1. Introduction |
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TopAbstract1. Introduction2. Methods3. Results4. DiscussionAppendix A. Conference...References |
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2. Methods |
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TopAbstract1. Introduction2. Methods3. Results4. DiscussionAppendix A. Conference...References |
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All animals underwent the same general anesthesia induced by i.v. sodium thiopental (Penthotal) 15 mg/kg, and maintained following endotrachial intubation with O2:NO (1:2) and 1.5% Fluothane. Throughout the experiments, lung ventilation was achieved using a positive pressure respirator (Harvard). Body temperature was kept constant using a heating mattress. Prior to surgery, a single dose of 5000 U i.v. heparin was administered.
2.1. Ascending aortomyoplasty: surgical technique
A right-sided midaxillary incision was performed above the right LD muscle and all collateral blood vessels to the distal part of the muscle were ligated or coagulated. All attachments of the muscle were disconnected, except for the axillary pedicle, in order to keep the thoracodorsal artery, vein, and nerve intact. Two intramuscular electrodes (Medtronic SP 5590 stimulation leads) were implanted in the upper part of the LD muscle flap, perpendicular to the main branches of the thoracodorsal nerve, as described previously by Chachques and coworkers [8]. To ensure proper positioning of the electrodes, satisfactory threshold (0.3–0.6 V) and total recruitment (1.0–2.5 V) values were obtained following connection of the electrodes to the stimulator system (Medtronic Cardio-Myo Stimulator SP 3076). A 5 cm segment of the anterior portion of the right second rib, including the periosteum, was then resected to allow transposition of the LD muscle flap into the thorax. The muscle was inserted into the chest cavity, its tendon cut and sutured to the periosteum of the third rib, prior to closing the thoracic window.
The skin incision was then sutured and the animal's position was changed to allow a median sternotomy. After the median sternotomy, the pericardium was opened and a sensing electrode (Medtronic 6500 sensing lead) was implanted in the right ventricular wall (adjacent to the septum) and the sensing threshold (4.5–16.4 mV) was measured. To enlarge the portion of the ascending aorta to be wrapped, a dissection was performed between the aorta and the pulmonary artery. Another portion of the ascending aorta was exposed by dissecting toward the aortic annulus. The right latissimus dorsi muscle was then wrapped around the ascending aorta in a single layer (clockwise) and was fixed using prolene 2/0 interrupted sutures.
2.2. Descending aortomyoplasty: surgical technique
The left latissimus dorsi was dissected and introduced into the left chest using the same technique described above for the right latissimus dorsi during ascending aortomyoplasty. Through the same skin incision the thorax was then opened at the fourth left intercostal space and the pericardium cut open. A sensing electrode (Medtronic 6500 sensing lead) was implanted in the right ventricular wall (adjacent to the septum) and the sensing threshold (4.5–16.4 mV) was measured. A segment of approximately 8–10 cm of the descending aorta (1.5–2.5 cm in diameter), distal to the left subclavian artery, was then mobilized. All intercostal arteries arising from the aorta in this particular portion were ligated and divided. The left LD muscle flap was subsequently wrapped around the exposed descending aorta (single layer) using Prolene 2/0 sutures. The configuration required tight wrapping of the latissimus dorsi around the aorta in a counter-clockwise orientation (best configuration from previous study [9]).
2.3. LD muscle stimulation and IABP setting
Stimulation commenced immediately following the application of each surgical configuration. The first part of this protocol comprised a six-pulse burst (5 V, 165 µs pulse width, 200 ms burst duration) using different delays (150, 200, 250, 300, 350, 400 or 450 ms) after the QRS complex, in order to cover the entire diastolic range. The stimulation burst was applied every third or fourth spontaneous heartbeat.
The IABP was inserted through the femoral artery and its exact position was determined using fluoroscopic guidance. The IABP was activated in a 1:3 ratio, to allow comparison between the assisted beats and the previous, unassisted beats. The balloon was inflated at different delays after the QRS complex (from 200 to 400 ms) and with varying inflation duration (from 150 to 250 ms). The best delay and duration were explored by increments of 50 ms and the decision was made according to the plotted proximal blood pressure curve.
2.4. Instrumentation and data collection
A Millar (Millar Instruments, TX) solid state pressure catheter was inserted via the left carotid artery and advanced into the left ventricle (LV). Two additional solid state pressure catheters were inserted via the right and left femoral arteries and advanced into the aorta, proximally and distally to the wrapping site, respectively.
A Doppler flow probe (Transonic Systems, NY) was placed around the left anterior descending (LAD) coronary artery, proximally to the first diagonal, for measurement of coronary blood flow. Surface ECG was monitored continuously throughout the experiments.
2.5. Data and statistical analysis
Hemodynamic data were gathered on an IBM personal computer using CODAS (DATAQ, Akron, OH) and HDAS (Maastricht, The Netherlands) software. Additional software was designed to detect peak diastolic pressure and coronary blood flow and to calculate diastolic pressure and coronary blood flow curve integral during the diastolic phase (area under the curve). Assisted beats were compared to prior, unassisted beats using Student's paired t-test. Results are expressed as mean±SD. Differences were considered significant at a P value <0.05.
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3. Results |
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TopAbstract1. Introduction2. Methods3. Results4. DiscussionAppendix A. Conference...References |
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Descending aortomyoplasty was found to be a simpler operation compared to ascending aortomyoplasty, performed through one skin incision without the need to re-position the animal on the operating table or to open the pericardium. All descending aortomyoplasty animals survived the entire experimental protocol. Ascending aortomyoplasty was found to be a more demanding operation, done through a right thoracic skin incision followed by mid sternotomy, which necessitates changing the animal's position in the middle of the operation. The main technical problem in the ascending aortomyoplasty operation was the short segment of the ascending aorta before the first branch, which required a wide dissection around the ascending aorta toward the heart, as well as the dissection of parts of the latissimus dorsi muscle in three of the eight experiments, in order to facilitate the wrapping of the muscle around the ascending aorta. This dissection caused both bleeding from the adjacent tissue as well as supra ventricular arrhythmias. Two animals died during the wrapping stage (and were excluded from the study results) due to major bleeding combined with arrhythmias.
3.2. Timing of the counterpulsation
Both the contraction of the LD muscle around the descending or ascending aorta and the IABP counterpulsation induced diastolic pressure augmentation in the proximal aorta. However, exact timing was crucial for achieving the desired hemodynamic benefits. Shorter delays (150 ms following the QRS complex) induced aortic regurgitation before aortic valve closure, identified by the pressure sensor in the LV. Moreover, the shorter delays induced higher proximal aortic pressure during the last part of the systole, resembling aortic narrowing, an effect contrary to the desired afterload reduction. Counterpulsation during the latter part of the ventricular diastole (450 ms following the QRS complex) resulted in a less drastic diastolic pressure augmentation. As found with the shorts delays, this long delay caused partial obstruction of the aorta during the next systole.
3.3. Hemodynamic parameters
Comparison of the diastolic augmentation achieved by the surgical configuration reveals an advantage to the descending technique in the coronary blood flow during the assisted beats as compared to the unassisted beats (Fig. 1)
. While ascending aortomyoplasty generated coronary blood flow augmentation similar to that achieved by the IABP (19 and 18%), descending aortomyoplasty induced a 27% increase in the coronary flow during the assisted beats.
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Proximal aortic end-diastolic pressure reduction resembling afterload reduction was consistently achieved in the assisted beats during the ascending aortomyoplasty operations, comparable to the reduction found in the IABP group (Fig. 2) . In contrast, descending aortomyoplasty failed to induce receptive proximal aortic end-diastolic pressure reduction.
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4. Discussion |
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TopAbstract1. Introduction2. Methods3. Results4. DiscussionAppendix A. Conference...References |
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4.1. Surgical technical considerations
As reported by Mesana and coworkers, in the current study we found the descending aortomyoplasty to be a faster and easier procedure due to the single skin incision, the single position of the surgical candidate throughout the entire operation, and the fact that the pericardium could remain unopened [6]. The avoidance of pericardial opening may be even more important when performing the procedure following a previous cardiac operation.
Another difference which exists between the two techniques is the size and the structure of the descending and ascending aortas. As in humans, the ascending aortic diameter was found to be larger than the descending aortic diameter (the hemodynamic implications of which will be discussed later); however, it was the short length of the ascending aorta before the first branch to be wrapped that caused surgical difficulties. In three of the experiments, parts of the latissimus dorsi muscle had to be removed to facilitate the wrapping, a procedure that may cause long-term damage to the muscle. However, in the last six ascending aortomyoplasty experiments, a wide dissection around the aorta toward the aortic root enabled the wrapping to be completed without cutting of the skeletal muscle, but with more bleeding and supra-ventricular arrhythmias. The death of two animals during this stage was probably related to the learning curve needed in this part of the operation. Cabrera and coworkers described ascending aortomyoplasty with the left latissimus dorsi without any technical problems during the wrapping [10]. This is most probably due to the long segment of the ascending aorta before the first branch in sheep as compared to the animal model used in the current study. Moreover, looking at the different anatomy of the ascending aorta in humans, it is safe to assume that both the diameter and the length of the aorta to be wrapped will be larger in clinical situations.
Descending aortomyoplasty usually requires the ligation of one or more intercostal arteries, with the subsequent potential complication of postoperative neurological disorders (Adamkiewitz artery). Cernaianu reported a modification of the descending aortomyoplasty operation in an attempt to preserve the intercostal arteries [11]. Indeed, the use of several muscle splittings at the level of the intercostal artery and of pericardial ‘bridges" to posteriorly connect the LDM to the wrapped aortic portion has been shown previously. Another clinical issue that should be taken into consideration when comparing the two procedures is the relatively higher prevalence of atherosclerosis in the descending aorta as compared to the ascending aorta [12].
In general, we found the descending aortomyoplasty to be a more simple procedure. These differences may become less important once more experience with the two techniques has been gathered; however, since patients who are candidates for aortomyoplasty are most often at high surgical risk, the implementation of a less invasive procedure may be important [2].
4.2. Timing of the counterpulsation
The timing of skeletal muscle contraction around the descending aorta is as crucial as the timing of the IABP counterpulsation [13,14]. Zelano and coworkers have demonstrated that increased afterload was due to an inappropriate setting of the IABP, a finding that our previous work replicated with regard to descending aortomyoplasty [9,15]. An increased afterload in hemodynamically unstable patients may have detrimental effects. To prevent a possible increase in afterload in certain clinical situations in patients after aortomyoplasty, two issues should be addressed: (1) the chronic decrease in muscle power and speed of contraction; and (2) changes in the patient's heart rate and general hemodynamic situation. Both issues will probably demand frequent clinical evaluation of the patient and modification of stimulation settings.
4.3. Hemodynamic parameters
Descending aortomyoplasty was found to be superior in coronary blood flow augmentation. This may be due to the higher aortic volume to be wrapped in this procedure (26±2 ml as compared to 19±2 ml in the ascending aortomyoplasty). However, since the augmentation was found to be higher than in the IABP control group (using a 30 ml pediatric balloon) another explanation should be proposed. A possible explanation is the orientation of the skeletal muscle fibers. In the case of descending aortomyoplasty, the contraction upward induces squeezing of the aorta proximally, and thus may increase the diastolic augmentation. In ascending aortomyoplasty, the orientation causes squeezing of the aorta distally (although to a lesser extent), which may reduce diastolic augmentation. Cabrera and coworkers report a hemodynamic advantage of ascending aortomyoplasty over abdominal descending aortomyoplasty [10]. However, the use of the abdominal thoracic aorta may result in less prominent augmentation compared to that achieved by wrapping the thoracic descending aorta, as in the current study. The proximal aortic end-diastolic pressure reduction induced by ascending aortomyoplasty was equal to that achieved with the IABP, constituting an important advantage over descending aortomyoplasty. The reasons for that effect may be the proximity of the ascending aorta to the coronary ostea, its larger diameter, which results in higher volume changes due to diameter changes and perhaps differences in aortic wall elasticity. These results suggest the superiority of ascending aortomyoplasty for end-stage heart failure patients, whereas descending aortomyoplasty may be more suitable for end-stage ischemic patients. Cabrera and coworkers did report important hemodynamic effects of ascending versus descending aortomyoplasty; however, their hemodynamic data did not include coronary flow and afterload reduction [10]. Other studies showed important hemodynamic benefits of ascending or descending aortomyoplasty, but since all studies were done using only one of the techniques, no objective comparison between the two is possible [16,17].
4.4. Study limitations
The difference in the starting conditions in the hemodynamic parameters represents the animals’ general hemodynamic status at the time of the measurements. The IABP group was in the best hemodynamic condition while the ascending aortomyoplasty group was found to be in the worst. These differences represent the complexity of the procedure and were discussed in the ‘Surgical technical considerations’ chapter earlier. These changes were checked and were not found to influence the statistical significance of the results. The muscle in our study was untrained. As mentioned earlier, reduction in muscle speed and power should be expected after the conditioning period. Though there are data suggesting the efficacy of the procedure in conditioned muscle [1,17], the difference between ascending and descending aortomyoplasty should be tested both in chronic animal experiments and in future clinical cases.
4.5. Theoretical clinical advantages of aortomyoplasty
The idea of using skeletal muscle as aortic external counter-pulsation was suggested already in 1959 by Kantrowitz and McKinnon, who demonstrated diastolic augmentation while stimulating a wrapped diaphragm muscle around the descending aorta [18]. The chronic hemodynamic effects of aortomyoplasty present several possible advantages for ischemic patients. It is possible to increase the efficacy of aortomyoplasty according to the clinical demands of the patient by changing the amplitude and the rate (telemetric setting changes) of the latissimus dorsi muscle contraction. Lowering the stimulation amplitude and, more importantly, reducing the stimulation/spontaneous beat ratio can protect the muscle from chronic damage while still maintaining some of its protective effects. Once unstable angina or even acute myocardial infarction does occur, increasing aortomyoplasty efficacy may attenuate the angina and reduce infarct size, as has previously been demonstrated with the IABP [19]. Utilization of the IABP is beneficial in acute situations, such as cardiogenic shock, cardio-pulmonary resuscitation, and as an adjuvant treatment to thrombolytic therapy [20,21]. Similarly, exacerbation in a patient's clinical status, even years after aortomyoplasty, may be treated by increasing both the latissimus dorsi muscle power and the ratio of assisted contractions to spontaneous beats.
In conclusion, both surgical techniques were found to generate hemodynamic effects comparable to those of the IABP. However, descending aortomyoplasty was found to be a more simple operation, capable of inducing higher coronary flow augmentation; thus, it may be particularly helpful for treating end-stage ischemic patients. Ascending aortomyoplasty was found to produce hemodynamic effects similar to those of IABP, including after load reduction, and may be more suitable for treating end-stage heart failure patients.
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Acknowledgments |
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Footnotes |
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Appendix A. Conference discussion |
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TopAbstract1. Introduction2. Methods3. Results4. DiscussionAppendix A. Conference...References |
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Dr Bolotin: No, we didn't. We did conversion 1:3, 1:4 in order to divide all the hemodynamic effects on the beat without influence between the next beat or the previous beat, and we compared just the assisted beat to unassisted beat. Of course it is untrained muscle, so if we do it 1:1 for a long time probably the muscle will reduce in power.
Mr Tang: And the second question I was going to ask you is, are you planning to repeat this using trained muscle in a failed circulation?
Dr Bolotin: I think the next step would be to see that the results of descending aortomyoplasty will sustain in chronic animals using trained muscle, however, there are some results from the literature that you can assure the beats will be like that.
Dr B. Walpoth (Bern, Switzerland): I would like you to give us an idea or a statement why there is this difference. Is it a purely anatomical reason that you find more after load reduction if you do the ascending and that you might find more flow or diastolic augmentation if you do the descending?
Dr Bolotin: Yes. Of course, these are the results. We think that for augmentation was higher in the descending aortomyoplasty group for two reasons. First of all, the wrapped section of the descending aorta was much longer, so you shift a higher quantity of blood as compared to the ascending aortomyoplasty, but this is not the only explanation, because according to our calculations, in descending aortomyoplasty you shift an amount of blood equivalent to the intra-aortic balloon pump and go actually higher than the intra-aortic balloon pump. So the second explanation is probably the direction of the muscle fibers and the squeezing effect. Like in humans, in the animal the squeezing effect was in the upward direction in the descending aorta, and I believe this caused higher diastolic augmentation.
Regarding afterload reduction, I think it is the proximity to the aortic valve and perhaps different elasticity of the aorta, ascending versus descending.
Dr L. Lapanashvili (Winterthur, Switzerland): What kind of stimulation do you use?
Dr Bolotin: We used the conventional clinical stimulation as previously reported by Chachques and Carpentier, usually a burst of six pulse that generates for most of the diastole, and the results were after we optimized the timing and after the closing of the aortic valve and before the next diastole. So it is actually stimulation like in the clinical report of cardiomyopathy.
Dr Lapanashvili: So you mean that you used a Medtronic stimulator?
Dr Bolotin: Yes, this was Medtronic's experimental stimulator.
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References |
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TopAbstract1. Introduction2. Methods3. Results4. DiscussionAppendix A. Conference...References |
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