Activation pattern of the avian left ventricle during ventricular pacing.
Objective: This study was planned to investigate ventricular myocardial excitation in birds in which Purkinje fibres penetrate into the ventricular wall and reach the epicardium to advance our knowledge about the evolution of the ventricular activation process in vertebrates.
Methods: A depolarization pattern of the left ventricular free wall in seven open-chest laying hens was mapped by 14 seven-electrode plunge needles under ventricular pacing from different sites.
Results: Duration of activation of the left ventricular free wall is significantly increased during ventricular ectopic excitation as compared with sinus rhythm. Its lowest increase occurs during subendocardial pacing of the middle part of the left ventricle, but its greatest increase is observed during subepicardial pacing of the left ventricular base. Multifocality and mosaicity of depolarization of the left ventricular myocardium are expressed in a considerably less degree during ventricular pacing in comparison with sinus rhythm.
Conclusion: Ectopic excitation of avian heart ventricles occurs mostly due to successive spreading of the activation wave from a pacing site during both ipsi- and contraventricular pacing. During ipsiventricular pacing at least, ectopic excitation of the heart ventricles with the "rich" Purkinje network behaves like one of the mammalian ventricles with the subendocardial Purkinje network. (Anadolu Kardiyol Derg 2007.7 Suppl 1, 95-7)
Keywords: myocardium, heart ventricles, pacing, bird, chicken
In theory, in multifocal ("flash") activation of avian ventricular myocardium (1-3) caused by the specific distribution of Purkinje fibres that penetrate deeply into the ventricular wall and run close to the epicardium (1, 4, 5) there must not appear considerable disturbances in the myocardial activation pattern under local disturbances of conduction. However, in spite of the "rich" network of Purkinje fibres, various disturbances of electrical conduction including ventricular extrasystoles are observed in avian hearts (6-8). Thus, an avian heart is an interesting and unique model to investigate mechanisms of disturbances of electrical conduction within the heart and to advance our knowledge about the evolution of the cardiac electrical function in vertebrates. A study of myocardial excitation features under ventricular premature beats in animals with the multifocal activation pattern of the ventricular myocardium is of significant interest for comparative and comprehensive electrocardiology. The aim of the present study is to investigate a myocardial activation pattern of the chicken left ventricular (LV) free wall during ventricular pacing.
In adult laying hens Gallus gallus domesticus (n=7; age, 9-12 months; body mass, 1.3-1.8 kg) purchased from a poultry farm, general anesthesia was induced with sodium thiopental (120-150 mg/kg i.m.). The chicken was then placed ventral side up, tracheotomized and artificially ventilated. The heart was exposed through a bilateral approach (2). After pericardectomy, fourteen plunge needles (diameter 0.35-0.40 mm), each containing seven electrodes (interelectrode distance 0.65, 0.80 or 1 mm), was inserted into the LV free wall according to one of schemes (Fig. 1A, B). Steel needle electrodes were inserted subcutaneously to obtain standard bipolar limb lead electrocardiograms.
[FIGURE 1 OMITTED]
Limb lead electrocardiograms and intracardiac electrograms were amplified and recorded by means of a custom-designed 128-channel computerized multiplexer recording system with a bandwidth of 0.05-1000 Hz at a sampling rate of 4000 Hz and an accuracy of 16 bits. Ventricular pacing was performed by means of right-angled 3-7 V impulses of 3 ms duration at a frequency of 3 Hz. The base (n=6), middle part (n=6) and apex (n=7) of the left ventricle and the base (n=3) of the right ventricle (Fig. 1C) were paced from different depths (epicardium, mid-myocardium, endocardium). Body temperature was kept in the range of 41-42[degrees]C, the heart was prevented from cooling and drying by means of a warm (41-42[degrees]C) 0.85% NaCl solution. After the experiment, the heart was excised and cut to specify positions of plunge needles.
Local activation times relative to the S peak in the II limb lead electrocardiogram (sinus rhythm) or to the onset of a pacing impulse (ventricular pacing) were determined automatically on the basis of the minimum of the first derivative of the (IRS complex and reviewed and revised if necessary. During construction of isochronal activation maps, all local activation times were referred to the lowest local activation time, which was considered to be the zero value. Duration of activation of the LV free wall was defined as the difference of the greatest and lowest local activation time.
Data are expressed as mean [+ or -] standard deviation. Paired t-test was used for comparisons. P value <0.05 was considered statistically significant.
Depending on the pacing site, duration of activation of the LV free wall was 14.8 [+ or -] 2.8 to 22.4 [+ or -] 3.3 ms being 9.3 [+ or -] 1.0 ms during the sinus rhythm (150 [+ or -] 14 beats per minute) (Table 1). Its lowest (1.6 [+ or -] 0.5 times as much) and greatest (2.4 [+ or -] 0.3 times as much) increases were observed under endocardial pacing of the LV middle part and epicardial pacing of the LV base, respectively. Under sinus rhythm, activation propagated in radial and tangential directions from foci appearing within the LV free wall myocardium during 2-3 ms after the onset of its depolarization (Fig. 2A); a general sequence of activation common to all birds was identified, with interindividual differences in specific details of the patterns. In contrast, multifocality and mosaicity of the activation pattern were expressed in a considerably less degree during ectopic excitation, and the LV free wall was mostly excited due to successive spreading of the depolarization wave from a pacing site (Fig. 2B, 2C). Activation patterns were similar during ventricular pacing from different depths (epicardial, mid-myocardial, endocardial) of each pacing area.
[FIGURE 2 OMITTED]
Within the LV free wall, activation spread from the base to the apex and from the left to the right during pacing from the LV base. The opposite sequence was observed during pacing from the LV apex and the right ventricular base (Fig. 2B, C). Discussion
The non-multifocal activation pattern of the ventricular myocardium during ventricular pacing might signify that Purkinje fibres do not participate in myocardial activation. This supposition is proved by following facts: 1) similar activation patterns of the left ventricle during ventricular pacing from different depths; 2) similar activation patterns of the LV free wall during ipsiventricular (LV apical) pacing and contraventricular (right ventricular basal) pacing; 3) activation within the left ventricle spreads from the apex to the base during apical pacing in both chickens and animals with other ventricular activation patterns (3, 9, 10, 11).
On the other hand, participation of the Purkinje network in ventricular activation of the avian heart during ventricular pacing can not be fully denied for following reasons: 1) the obtained values of duration of activation during ventricular pacing (Table 1) are 2-4 times less than values (50-60 ms) calculated taking into account the conduction velocity in the contractile myocardium of -0.4 m/s and the length of the heart ventricles of 20-25 mm in chickens; 2) in so me cases, foci of activation were located ahead of the main depolarization front and their merging for one millisecond were observed in the ventricular wall during pacing; 3) duration of activation of the LV free wall is minimal during pacing of subendocardial layers of the middle part of the LV free wall that are the most "rich" in Purkinje fibres as compared to the located on a level with them intramural and subepicardial layers (Table 1). A probable cause of insignificant participation of the Purkinje network in myocardial activation during ventricular pacing is that an electrical impulse spreads along Purkinje fibres in the retrograde direction as on a cable without interaction with neighbouring myocardial cells. Probably, during ectopic excitation most of the ventricular myocardium is depolarized by the myogenic way (muscle fibre to muscle fibre) with simultaneous activation of some parts of it through the Purkinje network. Earlier, suggestions about the combined way of myocardial activation during ventricular ectopic excitation were made for ungulates, in particular sheep (9), goats (12), reindeers (Shmakov DN, unpublished), having the "rich" Purkinje network (13-16). Taking into account the similarity of ventricular activation patterns in chickens (2) and wild avian species (1, 3), it can be suggested that activation of the ventricles during ectopic excitation in the latter occurs in the same manner as in chickens.
The data obtained reveals that multifocality and mosaicity of activation of the LV myocardium in chickens are expressed in a considerably less degree during ventricular pacing in comparison with sinus rhythm. Ectopic excitation of the heartventricles in birds occurs mostly due to successive spreading of the activation wave from a pacing site during both ipsi- and contraventricular pacing. The results convince that during ipsiventricular pacing at least, ectopic excitation of the heart ventricles with the "rich" Purkinje network behaves like one of the mammalian ventricles with the subendocardial Purkinje network.
This study was supported by the Russian Foundation for Basic Research (grant N 06-04-48022), the Grant (MK-5347.2006.4) of President of the Russian Federation, and the program of support for basic research performed in the Ural Branch of the Russian Academy of Sciences in association with the Far Eastern Branch of RAS during 2005-2007.
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Sergey Kharin, Nadezhda Antonova, Dmitry Shmakov
Laboratory of Cardiac Physiology, Institute of Physiology of the Komi Science Centre, Ural Branch of the Russian Academy of Sciences, Syktyvkar, Russia
Address for Correspondence: Dr. Sergey N. Kharin, Institute of Physiology of the Russian Academy of Sciences, 50 Pervomayskaya st., Syktyvkar, 167000, Komi Republic, Russia Phone/Fax: +7(8212)240085 E-mail: email@example.com
Table 1. Duration (ms) of ectopic excitation of the left ventricular free wall myocardium in chickens Site of the ectopic beat Epicardium Mid-myocardium The apex of the 19.0 [+ or -] 19.4 [+ or -] 5.1 (a) left ventricle 5.4 (a) The middle part of 19.3 [+ or -] 2.2 19.2 [+ or -] 3.6 (a, b) the left ventricle (a, b, c) The base of the 22.4 [+ or -] 3.3 21.4 [+ or -] 4.7 (a) left ventricle (a, c) The base of the 17.3 [+ or -] 2.2 20.0 [+ or -] 4.0 (a) right ventricle (a, d) Site of the ectopic beat Endocardium The apex of the left ventricle 18.3 [+ or -] 3.8 (a) The middle part of the left ventricle 14.8 [+ or -] 2.8 (a) The base of the left ventricle 20.3 [+ or -] 5.1 (a) The base of the right ventricle 18.4 [+ or -] 1.2 (a, d) Mean [+ or -] standard deviation. (a) p < 0.01 compared to sinus rhythm (9.3 [+ or -] 1.0 ms); (b) p < 0.05 compared to endocardial stimulation; (c) p < 0.05 compared to stimulation of the base of the right ventricle; (d) p < 0.05 compared to midmyocardial stimulation of the middle part of the left ventricle (Reproduced from Comparative Biochemistry and Physiology, 145A (4), Kharin S., Antonova N., Shmakov D., Left ventricular myocardial activation under ventricular paced beats in chickens Gallus gallus domesticus, 540-545, [c] 2006 Elsevier Inc., with permission from Elsevier.)
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|Title Annotation:||Original Investigation|
|Author:||Kharin, Sergey; Antonova, Nadezhda; Shmakov, Dmitry|
|Publication:||The Anatolian Journal of Cardiology (Anadolu Kardiyoloji Dergisi)|
|Article Type:||Clinical report|
|Date:||Jul 1, 2007|
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