| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Ann Thorac Surg 1999;68:460-468
© 1999 The Society of Thoracic Surgeons
a Departments of Department of Surgery, Faculty of Medicine, Dalhousie University, Halifax, Nova Scotia, Canada
b Department of Physiology and Biophysics, Faculty of Medicine, Dalhousie University, Halifax, Nova Scotia, Canada
c Department of Anesthesiology, Faculty of Medicine, Dalhousie University, Halifax, Nova Scotia, Canada
Address reprint requests to Dr Hirsch, Division of Cardiovascular Surgery, Department of Surgery, Queen Elizabeth II Health Sciences Center, 1796 Summer St, Room 2271, Halifax, NS B3H 3A7, Canada
 |
Abstract |
|---|
 Top Abstract IntroductionMaterial and methodsResultsCommentReferences |
|---|
Methods. Studies were performed in 9 dogs that were artificially ventilated and underwent thoracotomy. Changes in ventricular dynamics and intrinsic cardiac neuronal activity were monitored before and after creating 20 transmural channels in the left ventricular ventral free wall with a holmium:yttrium-aluminum-garnet laser in response to three stimuli: application of veratridine or bradykinin to the epicardial sensory neurites of intrinsic cardiac afferent neurons; sympathetic or parasympathetic efferent neuronal activation either electrically (4 V, 10 Hz, 5 ms) or chemically (nicotine, 5 µg/kg intravenously), and direct cardiomyocyte ß-adrenergic receptor stimulation (isoproterenol hydrochloride, 5 µg intravenously).
Results. Sensory neurites of right atrial afferent neurons in the studied epicardial region responded similarly to chemical stimulation before and after transmyocardial laser revascularization. Transmyocardial laser treatment did not reduce local ventricular contractile responses to direct activation of sympathetic or parasympathetic efferent neurons by electrical or chemical means, nor did it affect cardiomyocyte augmentor responses elicited by exogenous ß-adrenergic receptor challenge.
Conclusions. As transmyocardial laser revascularization does not affect afferent or efferent axonal function in the affected ventricle, the efficacy of this form of therapy cannot be ascribed to local denervation.
|
Introduction |
|---|
|
TopAbstractIntroductionMaterial and methodsResultsCommentReferences |
|---|
Three mechanisms have been proposed to account for the therapeutic benefits provided by TMLR to patients with myocardial ischemia. First, multiple channels produced in the wall of the ischemic ventricle by TMLR permit blood from the chamber to reach underperfused cardiomyocytes by forming direct communications between the chamber and the ventricular blood vessels [1, 3–7]. Although some investigators [5] have reported that transmyocardial channels remain patent for months, others [8–13] have challenged this by demonstrating only transient patency.
Second, angiogenesis after TMLR has been suggested to account for the beneficial effects that such therapy can impart [10, 11]. Although it has been reported that angiogenesis does not occur after TMLR in animals [9, 12], accumulating evidence does indicate this may be an important mechanism for the therapeutic benefits observed clinically. Thus, convincing evidence of improved myocardial perfusion after TMLR by either direct channel formation or induction of angiogenesis is pending.
Third, it has recently been proposed that laser-induced myocardial injury is nonspecific and that the damage created by the laser involves local neuronal tissue. Using indirect measures of neuronal innervation in the canine model, Kwong and associates [14] found that ventricular nerves were rendered nonfunctional after such therapy. If this is true, alterations in the function of cardiac afferent and efferent neurons would ensue and affect not only symptomatology (afferent neuronal function) but efferent neuronal control of the heart.
To determine whether TMLR influences the functional innervation of the ventricles, the effects of TMLR on the following were studied: the function of local sensory axons associated with intrinsic cardiac afferent neurons using direct neuronal recordings; the capacity of sympathetic or parasympathetic efferent neurons to modulate regional ventricular dynamics with direct electrical or chemical stimulation; and the capacity of laser-treated ventricular muscle to respond to an exogenously applied ß-adrenergic receptor agonist. In this manner, we assessed the effects that TMLR exerts on local ventricular cardiomyocyte function as well as the function of local afferent and efferent nerves involved in the regulation of such cardiomyocytes.
|
Material and methods |
|---|
|
TopAbstractIntroductionMaterial and methodsResultsCommentReferences |
|---|
Experimental model
Nine mongrel dogs of either sex and weighing 19 to 25 kg were anesthetized with the barbiturate sodium thiopental (intravenous bolus of 20 mg/kg), and the trachea was intubated. Positive pressure ventilation with an inspired oxygen fraction of 0.95 and an inspired carbon dioxide fraction of 0.05 was maintained with a Bird Mark 7A ventilator (Bird Corporation, Palm Springs, CA). During the operative phase, anesthesia was maintained with sodium thiopental (10 to 15 mg/kg intravenously every 5 minutes); during the experimental phase, it was changed to 
-chloralose (intravenous bolus of 75 mg/kg) with repeat treatment (12.5 mg/kg intravenously) as required. During the experimental phases, -chloralose was used as opposed to barbiturates because it has fewer neuronal-depressing effects. Depth of anesthesia was assessed by applying noxious stimuli to a paw. Heart rate was monitored with a lead II electrocardiogram. Body temperature was maintained at 37° to 38°C with a circulating hotwater pad.
Direct electrical stimulation of the sympathetic and parasympathetic efferent cardiac nerves is the most effective way to assess efferent innervation of the mammalian heart. Thus, a midline incision was made in the neck to facilitate exposure of the right and left cervical vagosympathetic trunks. These nerves were severed so that the distal end of each could be stimulated electrically later in the experiments without producing afferent axonal-induced reflexes. A bilateral thoracotomy was made in the fifth intercostal space to expose the heart and stellate ganglia. Each stellate ganglion was identified and decentralized. Bipolar electrodes were placed around the ganglia so that they could be stimulated later in the experiments.
The ventral pericardium was incised and retracted laterally to expose the heart and the ventral right atrial ganglionated plexus [15]. Neurons in this ganglionated plexus are representative of those found in the various intrinsic cardiac ganglionated plexuses [15–17]. Left atrial pressure was measured using a PE-50 catheter inserted directly into the left atrial chamber through its appendage. Left ventricular pressure was measured using a 6F Cordis pigtail catheter (Miami, FL) inserted through the femoral artery cannula. Systemic arterial pressure was measured using a 7F Cordis catheter placed in the descending aorta through the other femoral artery. All catheters were attached to Bentley Trantec model 800 transducers (Irvine, CA).
A miniature solid-state pressure transducer measuring 5 x 1.2 mm (model P19; Konigsberg Instruments, Pasadena, CA) was inserted into the midwall region of the right ventricular conus, and two such transducers were inserted into the midwall regions of the left ventricle to record regional intramyocardial pressures. These sensing devices were used because intraventricular systolic pressure represents a less sensitive index for detecting ventricular force changes induced by efferent autonomic neurons [18]. As efferent sympathetic nerve stimulation exerts minimal effects on ventricular segment lengths [19], these indices were not measured. One miniature pressure transducer was placed in the region of the left ventricular ventral wall in which 20 transmural channels were to be created by means of the laser. The second intramyocardial pressure sensor was placed in a region of the left ventricular lateral wall adjacent to that region in which the first 20 laser channels were to be created. All indices, including the lead II electrocardiogram, were recorded on an MT 9500 eight-channel rectilinear recorder (Astro-Med, West Warwick, RI) and stored on VHS tape using a videocassette recorder for later analysis.
Echocardiographic assessment of left ventricular wall motion was performed in four animals using an Ultramark 8 Ultrasound Systems echocardiographic apparatus (model UM-8-OPO1; Advanced Technology Laboratory, Bellevue, WA). The cross-sectional midpapillary view of the left ventricle was obtained before and after TMLR using an Access C ultrasound probe (Advanced Technology Lab, Bothell, WA) placed directly on the heart over the free wall of the right ventricle.
Neuronal recording
The recording of right atrial neuronal activity in situ is an established method to directly assess how intrinsic cardiac neurons respond to various chemical and physical stimuli applied to the heart [16, 17]. Activity generated by ventral right atrial neurons was directly recorded by means of a model PK/12 FHC tungsten microelectrode (Brunswick, ME) with a diameter of 250 µm and an exposed tip of 1 µm (impedance of 9 to 12 M
at 1,000 Hz), as has been described elsewhere [17]. To minimize epicardial motion during each cardiac beat, a circular ring of heavy-gauge wire was gently placed around the epicardial fat of the ventral surface of the right atrium. The tungsten microelectrode, mounted on a micromanipulator, was used to explore the fat at varying depths ranging from the surface of the fat to regions adjacent to the cardiac musculature. Proximity to cardiac musculature was indicated by increases in the amplitude of the electrocardiographic artifact. The reference electrode was attached to the adjacent pericardium and a ground electrode, to subcutaneous fat. Signals generated by intrinsic cardiac neurons were differentially amplified by a Princeton Applied Research model 113 amplifier (Princeton, NJ) with band-pass filters set at 300 Hz to 10 kHz and an amplification range of 100 to 500x. The output of this device, further amplified (50 to 200x) and filtered (bandwidth of 100 Hz to 2 kHz) by means of an optically isolated amplifier (Applied Microelectronics Institute, Halifax, NS, Canada), was led to a Nicolet model 207 oscilloscope (Madison, WI) and to a Grass AM8 audio monitor (Quincy, MA).
Loci in epicardial fat were identified from which action potentials with signal-to-noise ratios greater than 3:1 were recorded and analyzed, individual units being identified by the amplitude and configuration of their action potentials. Because these techniques and criteria were used, the microelectrode does not record action potentials generated by axons of passage; rather it records action potentials generated by somata, dendrites, or both [17]. Periodic motion at the recording site occurred because of cardiac and respiratory dynamics, thereby inducing minor fluctuations in the amplitude of individual action potentials generated by a given unit over time. Fluctuations in the amplitude of action potentials were found to vary by less than 10 µV over several minutes, as action potentials retain the same configurations over time. Thus, action potentials recorded in a given locus with the same configuration and amplitude (±10 µV) were considered to be generated by a single unit.
Interventions
Acutely decentralized right and left stellate ganglia were stimulated individually (10 Hz, 5 ms, 4 V) for 20 seconds. Thereafter, the distal end of the sectioned right or left cervical vagus was stimulated individually for 10 seconds (20 Hz, 5 ms, 4 V). The bipolar electrodes (electrode tips 5 mm apart) were connected to a Grass SD-9 square-wave stimulator, the output of which was monitored on a Telequipment D-54 oscilloscope (Beaverton, OR). At least 5 minutes was allowed to elapse between stimulations of neuronal structures for cardiovascular indices to return to baseline values.
Veratridine (7.5 µmol/L) and bradykinin (100 µmol/L) were then applied individually with 2 x 2-cm pledgets to the affected left ventricular epicardium (1 to 2 minutes) before and after the initial 20 laser-generated channels were produced. The Na+-channel modifier veratridine and the peptide bradykinin are known to activate ventricular sensory neurons when applied to their ventricular sensory neurites in the same concentration [20]. Sensory fields were washed for 30 seconds with normal saline solution (~2 mL/s) after each chemical was removed, and at least 10 minutes was allowed to elapse before the next intervention. Chemicals were reapplied to the same epicardial locus at least twice before and after TMLR to verify reproducibility of induced responses. Reapplication prevents spurious results caused by the tachyphylaxis produced by such agents. Pledgets soaked with room-temperature normal saline solution were also applied to identified epicardial sensory fields to determine whether neuronal responses elicited were due to vehicle effects or the mechanical effects elicited by gauze squares. Thereafter, nicotine (chemical stimulation of autonomic efferent neurons) (5 µg/kg intravenously) and isoproterenol hydrochloride (chemical stimulation of ventricular cardiomyocytes) (5 µg intravenously) were administered individually into the superior vena cava, enough time being allowed between administrations for the preparation to return to baseline. The order of the application of these interventions was randomized among animals.
Laser revascularization
After completion of all of the interventions described, 20 individual channels penetrating through a 4 x 4-cm area of the ventral wall of the left ventricle were produced, as performed clinically. These were created by means of a model TMR 2000 holmium:yttrium-aluminum-garnet laser (
, 2.1 µm; pulse length, 250 µs) (Eclipse Surgical Technologies, Sunnyvale, CA). As the cavity blood readily absorbs excess energy, channels were not made in the opposite wall of the left ventricle (confirmed at postmortem examination). Each epicardial channel was approximately 1 mm in diameter. A few of the transmural channels bled immediately after being created. In such instances, digital pressure was applied to the epicardial aperture until bleeding ceased. After 30 to 45 minutes, each of the interventions already described was repeated. The data from each animal served as its own control. In 7 of the 9 dogs, an additional 40 transmural channels were produced in both ventricles (total, 60 channels). Then, each intervention described was repeated. Postmortem examination was performed at the end of each experiment to confirm the nature of the transmural channels produced.
Data analysis
Heart rate, left atrial systolic pressure, left ventricular systolic pressure, and right and left intramyocardial systolic pressures were measured for 30 seconds before and after each intervention. The frequency of activity generated by identified intrinsic cardiac neurons was analyzed for 30-second periods before and during each intervention. Cardiac, vascular, and neuronal data obtained before and after each intervention are presented as the mean ± the standard error of the mean. One-way analysis of variance and Student’s paired t test were used for statistical analysis. Contingency tables with corrections for continuity were constructed employing analysis of variance so that responses elicited before TMLR could be compared with those elicited after TMLR. A p value of less than 0.05 or less than 0.01 was used for these determinations.
|
Results |
|---|
|
TopAbstractIntroductionMaterial and methodsResultsCommentReferences |
|---|
|
|
|
Nicotine is a cholinergic nicotinic receptor agonist activating both sympathetic and parasympathetic efferent postganglionic neurons that innervate the heart. The nicotine-induced bradycardia, augmentation of ventricular chamber pressure, and enhancement of intrinsic cardiac neuronal activity were similar before and after TMLR (see Tables 1, 2). Treated or nontreated regions of the left ventricle responded similarly too.
Isoproterenol is a ß-adrenergic receptor agonist that enhances ventricular cardiomyocyte contractility directly. Thus, this agent represents a good method to test the effects of TMLR on cardiomyocyte function. Isoproterenol induced similar enhancement of intramyocardial systolic pressures in both regions of the left ventricle as well as other cardiac indices and intrinsic cardiac neuronal activity before and after TMLR (see Tables 1, 2).
The Na+-channel modifier veratridine or the peptide bradykinin, when applied individually to the treated area of the left ventricular epicardium, activated identified right atrial neurons in each animal studied. Reapplication of these chemicals to the affected epicardial field after TMLR activated right atrial neurons to similar degrees as before treatment (Fig 2; see Table 2).
|
|
Comment |
|---|
|
TopAbstractIntroductionMaterial and methodsResultsCommentReferences |
|---|
Data derived from the present experiments using direct neuronal recordings indicate that the capacity of ventricular sensory neurites associated with intrinsic cardiac afferent neurons to sense alterations in their chemical milieu is not affected by TMLR. In situ neuronal recording represents a direct measure of neuronal function. As such, it is more reliable index of afferent innervation than indirect measures such as chemical-induced cardiovascular reflexes used by Kwong and colleagues [14].
We used the peptide bradykinin and the Na+-channel modifier veratridine, as they are known to activate ventricular sensory neurites associated with intrinsic cardiac afferent neurons when administered in doses that do not enter the systemic circulation in sufficient quantities to affect distant tissues [20]. The enhancement of intrinsic cardiac neuronal activity that occurred when these two chemicals were applied individually to TMLR–treated regions of the left ventricle were similar before and after TMLR (Fig 3). These data indicate that TMLR does not alter the chemical response characteristics of cardiac sensory axons or, for that matter, local neurites associated with intrinsic cardiac afferent neurons.
|
Direct electrical stimulation of sympathetic and parasympathetic efferent cardiac neurons is the most effective way to assess the functional innervation of the heart [16, 17, 20]. Although indirect measures of such innervation (ie, tyrosine hydroxylase immunoreactivity, positron emission tomography) have been used in the past [14], these methods cannot directly assess the functional sympathetic innervation of the heart. The capacity of cardiac sympathetic efferent postganglionic neurons to affect contractility in a ventricular region treated with TMLR was unchanged by that procedure, whether the neurons were excited electrically or chemically (see Tables 1, 2; Figs 1, 2). Electrical stimulation of parasympathetic efferent neurons suppressed intramyocardial systolic pressures in treated and nontreated regions of the left ventricular free wall (see Table 2) after, but not before (see Table 1), TMLR. It is presumed this occurred because TMLR affected the capacity of the left ventricular wall to generate pressure in such a manner that the suppressive effects of parasympathetic efferent neurons were made more evident.
Nicotine activates parasympathetic and sympathetic efferent postganglionic neurons directly [22]. Thus, nicotine induced bradycardia (parasympathetic effects on atrial tissues) and ventricular augmentation (sympathetic effects on ventricular tissue) concomitantly, as both populations of autonomic efferent neurons were affected (see Table 1). In accord with data obtained when parasympathetic or sympathetic efferent neurons were activated individually by electrical current, similar results were elicited by nicotine before and after TMLR (see Table 2) in all cardiac indices, including intramyocardial pressures in the treated zone of the left ventricle. Thus, TMLR does not affect the capacity of sympathetic or parasympathetic efferent neurons to influence local ventricular cardiomyocyte contractility, whether such neurons are stimulated electrically or chemically. Even when many laser channels (n = 60) were created through the walls of both ventricles, the capacity of autonomic efferent neurons to modify cardiodynamics remained unimpaired. Thus, the creation of many ventricular channels does not affect the capacity of autonomic efferent neurons to affect ventricular tissues substantially.
Transmyocardial laser revascularization induced a slight suppression in local intramyocardial systolic pressure in nonstimulated states (78 ± 8 to 68 ± 10 mm Hg; p = not significant). Despite that, the enhancement of contractility in the left ventricular region induced by the exogenous ß-adrenergic receptor agonist isoproterenol was similar before (76%) and after (67%) TMLR (see Tables 1, 2). Thus, the creation of laser-induced channels does not affect the capacity of ventricular cardiomyocytes to respond to ß-adrenergic stimulation. Likewise, TMLR did not reduce the capacity of ventricular myocytes in the nontreated region to respond to isoproterenol (+66% before TMLR, +82% after TMLR).
Data derived from studying the function of cardiac afferent and efferent neurons directly demonstrate that the production of multiple transmural channels by a laser does not alter the function of afferent or efferent nerve terminals within the affected ventricular tissue. Although our data reported here do not address the issue of long-term effects that TMLR might exert on ventricular sensory or motor nerves, it seems very unlikely that such revascularization would result in long-term local nerve injury in the absence of direct neuronal effects. It is equally unlikely that TMLR would induce long-term reduction in the capacity of ventricular sensory or motor nerve terminals to sprout and thus reinnervate local sites of injury over time. Given that, any beneficial effects that transmyocardial laser treatment may afford the ischemic ventricle cannot be ascribed to local ventricular denervation.
|
Acknowledgments |
|---|
|
References |
|---|
|
TopAbstractIntroductionMaterial and methodsResultsCommentReferences |
|---|
weeks. Ann Thorac Surg 1996;61:1532-1535.
Related Article
Ann. Thorac. Surg. 1999 68: 468-469.
This article has been cited by other articles:
|
|
 |
|
  K. A. Horvath and Y. Zhou Transmyocardial Laser Revascularization and Extravascular Angiogenetic Techniques to Increase Myocardial Blood Flow Card. Surg. Adult, January 1, 2008; 3(2008): 733 - 752. [Full Text]
|
|
|
|
 |
|
 M. B. Leon, R. Kornowski, W. E. Downey, G. Weisz, D. S. Baim, R. O. Bonow, R. C. Hendel, D. J. Cohen, E. Gervino, R. Laham, et al. A Blinded, Randomized, Placebo-Controlled Trial of Percutaneous Laser Myocardial Revascularization to Improve Angina Symptoms in Patients With Severe Coronary Disease J. Am. Coll. Cardiol., November 15, 2005; 46(10): 1812 - 1819. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 T. Asai, S. Yamamoto, K. Ishino, T. Kohmoto, M. Kuriyama, G. Kato, Y. Oshima, N. Yamamoto, K. Notohara, S. Okada, et al. Time-Dependent Regional Myocardial Denervation as a Nonspecific Response to Transmyocardial Laser Revascularization Ann. Thorac. Surg., October 1, 2005; 80(4): 1362 - 1369. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. Galinanes, M. Loubani, P. R. Sensky, A. Hassouna, G. R. Cherryman, J. N. Leverment, and N. J. Samani Efficacy of transmyocardial laser revascularization and thoracic sympathectomy for the treatment of refractory angina Ann. Thorac. Surg., July 1, 2004; 78(1): 122 - 128. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
C. R. Bridges, K. A. Horvath, W. C. Nugent, D. M. Shahian, C. K. Haan, R. J. Shemin, K. B. Allen, and F. H. Edwards The Society of Thoracic Surgeons practice guideline series: transmyocardial laser revascularization Ann. Thorac. Surg., April 1, 2004; 77(4): 1494 - 1502. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 J. F. Beek, J. A. P. van der Sloot, M. Huikeshoven, H. J. Verberne, B. L. F. van Eck-Smit, J. van der Meulen, J. G. P. Tijssen, M. J. C. van Gemert, and R. Tukkie Cardiac denervation after clinical transmyocardial laser revascularization: Short-term and long-term iodine 123-labeled meta-iodobenzylguanide scintigraphic evidence J. Thorac. Cardiovasc. Surg., February 1, 2004; 127(2): 517 - 524. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. Saririan and M. J. Eisenberg Myocardial laser revascularization for the treatment of end-stage coronary artery disease J. Am. Coll. Cardiol., January 15, 2003; 41(2): 173 - 183. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. Ruel, R. A. Kelly, and F. W. Sellke Therapeutic Angiogenesis, Transmyocardial Laser Revascularization, and Cell Therapy Card. Surg. Adult, January 1, 2003; 2(2003): 715 - 750. [Full Text]
|
|
|
|
|
|
M. Huikeshoven, J. F. Beek, J. A.P. van der Sloot, R. Tukkie, J. van der Meulen, and M. J.C. van Gemert 35 years of experimental research in transmyocardial revascularization: what have we learned? Ann. Thorac. Surg., September 1, 2002; 74(3): 956 - 970. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
A. J. Minisi, O. Topaz, M. S. Quinn, and L. B. Mohanty Cardiac nociceptive reflexes after transmyocardial laser revascularization: Implications for the neural hypothesis of angina relief J. Thorac. Cardiovasc. Surg., October 1, 2001; 122(4): 712 - 719. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 R. C. Arora, G. M. Hirsch, K. Hirsch, and J. A. Armour Transmyocardial Laser Revascularization Remodels the Intrinsic Cardiac Nervous System in a Chronic Setting Circulation, September 18, 2001; 104 (2009): I-115 - I-120. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 B. B. Y. Chiang, A. M. Roberts, A. M. Kashem, W. P. Santamore, S. Chien, L. Gray Jr, and R. Dowling Chemoreflexes: An Experimental Study Arch Surg, May 1, 2000; 135(5): 577 - 581. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| ANN THORAC SURG | ASIAN CARDIOVASC THORAC ANN | EUR J CARDIOTHORAC SURG |
| J THORAC CARDIOVASC SURG | ICVTS | ALL CTSNet JOURNALS |
