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Experimental cardiac tachyarrhythmias in guinea pigs
Journal of Electrocardiology Vol. 32 Supplement 1999
Experimental Cardiac Tachyarrhythmias in Guinea Pigs
Robert
A. Malkin,
PhD
Abstract: Despite years of intense research into the mechanisms of defibril-
lation, there remain many unanswered questions. In many fields, hypotheses are first tested in rodent models before confirming the results in larger animals. This work suggests the guinea pig as a rodent model for defibrillation. Twenty-eight guinea pigs were studied, all male retired breeders weighing over 900 g. T-wave stimuli (upper limit of vulnerability [ULV] ) were given after 15 rapid pacing beats, since the rapid pacing has been suggested to extend the tachyarrhythmia. Defibrillation (DF) was attempted after 5 seconds. The correlation between the ULV50 and DFS0 in guinea pigs (0.82, n = 8) is very close to that seen in dogs (0.85). Also, the sensitivity of the DFS0 to waveform is similar (476 -+ 176 for monophasic vs 364 -+ 94 V for biphasic P < 0.005, n = 10). The dose-response curve widths (2.3 _+ 1.7 for ULV vs 1.9 _+ 1.8 for defibrillation, n = 10) show the same trend of increasing curve widths for ULV, and similar magnitude to dogs (mean 1.8). We rarely (<1.5%) observed spontaneous conversion in less than 10 seconds. The guinea pig can be used as a model for defibrillation as it shows many of the same characteristics as dogs. K e y w o r d s : ventricular fibrillation, defibrillation, rodents.
G u i n e a pigs are f r e q u e n t l y u s e d as m o d e l s for v e n t r i c u l a r t a c h y a r r h y t h m i a s (1) s u c h as p o l y m o r phic v e n t r i c u l a r t a c h y c a r d i a (VT) a n d v e n t r i c u l a r fibrillation (VF). H o w e v e r , t h e r e h a v e b e e n reports t h a t in intact rodents, VF can be difficult to initiate a n d typically s p o n t a n e o u s l y reverts to n o r m a l sinus r h y t h m (2). S p o n t a n e o u s r e v e r s i o n c o u l d limit the application of the m o d e l to t h e d e v e l o p m e n t of
t e c h n o l o g i e s for a c u t e t h e r a p y b e c a u s e s p o n t a n e ous r e v e r s i o n c o u l d be c o n f u s e d w i t h a successful treatment. H o w e v e r , based o n w o r k first d o n e in dogs to p r o m o t e initiation of VF (3), w e s h o w t h a t s p o n t a n e o u s r e v e r s i o n c a n be e l i m i n a t e d or a v o i d e d in g u i n e a pigs. Thus, t h e g u i n e a pig m o d e l p r e s e n t e d h e r e can be used to test h y p o t h e s e s or d e v e l o p t e c h n o l o g i c a l a d v a n c e s in defibrillation or cardioversion.
From the Joint Department of Biomedical Engineering, The University of Tennessee-Memphis and The University of Memphis, Memphis, Tennessee. Supported in part by NIH research grant HL56308, a faculty research grant from the University of Memphis, and a grant-inaid from the American Heart Association. Reprint requests: Robert A. Malkin, The University of Memphis, Department of Biomedical Engineering, ET330, Memphis, TN 38152.
Materials and Methods W e h a v e successfully used this m o d e l in t h a n 100 g u i n e a pigs to date. The w o r k h e r e marizes t h e results of 28 of these animals. retired b r e e d e r g u i n e a pigs ( 8 0 0 - 1 , 2 0 0 g)
Copyright © 1999 by Churchill Livingstone ® 0022-0736199/320S-0016510.OO/O
84
more sumMale, were
Tachyarrhythmias in Guinea Pigs
weighed and then anesthetized with intraperitoneal sodium pentobarbital (32 mg/kg initially, i0 rag! kg/h thereafter as indicated). Stainless steel needle electrodes were inserted into the paws for the recording of leads I, II, and III body surface electrograms. Rectal temperature was continuously monitored. Body temperature was maintained using a hot water blanket or a hot water bottle. A heparinized, 22G catheter was introduced into the carotid artery for the continuous monitoring of systemic blood pressure. A tracheotomy or direct laryngeal intubation was performed and an endotracheal tube inserted to facilitate continuous positive pressure ventilation (Small Animal Ventilator, Harvard, Edenbridge, Kent). A pacing electrode was advanced in the esophagus until approximately adjacent to the ventricles. The esophageal pacing electrode was constructed of a coated stainless steel wire (180-/~m diameter, World Precision Instruments, Sarasota, FL) inside a silicone tube (0.02-in diameter, Baxter, Deerfield, IL). The cathode for pacing was advanced through the diaphragm, near the xiphoid, toward the ventricular apex through a 20G catheter. The apical pacing electrode was made of the same coated stainless steel wire inserted into the 20G catheter. The coating was removed from the distal 5 m m and 1 mm of the wire for the esophageal and apical pacing electrodes, respectively. The esophageal/apical pacing configuration was found to offer low pacing thresholds and minimal contamination of the blood pressure recording by skeletal muscle contractions. During pacing threshold testing, the position of the esophageal electrode was adjusted to further minimize skeletal muscle contractions. The intrinsic R-R interval was measured at the beginning of the study. The pacing current threshold for a 10-ms, monophasic stimulus was measured next. In some animals, in order to control the T-wave timing, the guinea pig was continuously paced at 80% of the measured R-R interval for the remainder of the study using a pacing strength of twice the diastolic pacing current threshold. In the remaining animals, continuous pacing was not used. Ten minutes after beginning pacing, the time to the peak of the T wave was measured on the body surface electrogram which showed the cleanest peak. Typically, lead I or III was used. Four measurements of the T wave were taken and averaged. The time to the peak of the T wave was also measured after the rapid pacing sequence. The rapid pacing sequence consisted of a total of 25 beats, similar to that described by Malkin et al. (3). The pacing interval for the last 15 beats was selected
•
Robert A. Malkin
85
to be the shortest interval which did not lead to a loss of electrical capture for any of the 15 beats. The first 10 beats were used to progressively, and uniformly, shorten the pacing interval. Two 12-mm-diameter, sintered silver/silver chloride electrodes (In Vivo Metric, Healdsburg, CA) or stainless steel electrodes of the same size, were placed on the skin at opposing aspects of the thorax for T-wave stimulation and electrical countershock. The electrodes were placed as cranially as possible. A highly conductive gel (SignaGel, Parker, Orange, New Jersey [or similar]) was used between the electrodes and the shaved thorax. The DF50 (the strength that defibrillates 50% of the time) and the ULVS0 (the strength that induces fibrillation 50% of the time) were measured using the Bayesian 10-step approach (4).
Results A tachyarrhythmia was defined as any rapid surface electrocardiographic (ECG) waveform lasting at least 2 sec after the termination of the inducting stimulus. Ventricular fibrillation (VF) was defined as a polymorphic tachyarrhythmia accompanied by complete hemodynamic collapse. Polymorphism was defined as a tachyarrhythmia of irregular ECG morphology on all observed leads. Complete hemodynamic collapse was defined as an arterial pressure below 15 mmHg. All induced tachyarrhythmias seen in all animals (about 600 total) were VF in this study. In some animals, no attempt was made to defibrillate for 30 sec. When a VF spontaneously terminated in these animals, the time of termination was noted. When a rapid pacing sequence was used to initiate the VF, nearly all episodes were sustained for at least 30 sec (79%). In 1 subset of 207 inductions, only 3 episodes spontaneously terminated before 10 sec. In all of the approximately 600 observed VF episodes in these guinea pigs, independent of how the VF was initiated, none spontaneously terminated within 5 sec. Ten seconds is sufficiently long to allow 5 sec for detection of VF, the administration of an electrical countershock, and 5 sec for noting the shock's effect. Since only 3 of 207 episodes spontaneously terminated before 10 seconds, the risk of mistaking spontaneous defibrillation for successful electrical countershock is less than 1.5 %. The correlation between the ULV50 and DF50 in guinea pigs (0.82) is very close to that seen in dogs (0.85). Also, the sensitivity of the DF50 to wave-
86
Journal of Electrocardiology Vol. 32 Supplement 1999
form is similar (476 _+ 176 for m o n o p h a s i c vs 364 + 94 V for biphasic P < 0.005, n = 10). The doseresponse curve widths (2.3 _+ 1.7 for ULV vs 1.9 + 1.8 for defibrillation, n = 10) show the same trend of increasing curve widths for ULV, and similar magnitude to clogs (mean, 1.8).
Discussion It is reported (1,2) that small animals c a n n o t sustain ventricular t a c h y a r r h y t h m i a s in vivo. We show that large, intact guinea pigs can consistently sustain t a c h y a r r h y t h m i a s for 30 sec or more. Ours is the first w o r k to test large, intact guinea pigs as a model for defibrillation. As previously stated, all t a c h y a r r h y t h m i a s were polymorphic VT with complete h e m o d y n a m i c collapse, often lasting 30 sec or more. No pulsatile activity of more t h a n 15 m m H g was observed. The precipitous fall in pressure, the lack of pulsatility, and the p o l y m o r p h i c ECG are consistent with ventricular fibrillation. However, classifying our observations as VF w o u l d contradict a widely supported assumption that small animals, such as rats, mice, and guinea pigs, cannot consistently sustain VF (2). It is believed that there is a m i n i m u m (or critical) mass needed to sustain VF experimentally estimated to be 6 to 25 g. If the arrhythmias induced here are VF, then our data suggest that the critical
mass m a y be m u c h smaller t h a n previously t h o u g h t (ie, less t h a n 2.5 g).
Conclusions In m a n y laboratories, the canine, swine, or, to a limited extent, rabbit models are used to develop n e w technologies for treating VF. While large animal models will not be replaced, this study suggests that the guinea pig m a y be a viable alternative model for some developmental studies because of its low cost, ease of handling, and ready availability.
References 1. Lu HR, Remeysen P, Clerck FD: Antiarrhythmic effects of nebivolol in experimental models in vivo. J Cardiovasc Pharmacol 24:986, 1994 2. Ferris LP, King BG, Spence PW, Williams HB: Effect of electric shock on the heart. Electrical Eng 1936, p. 498 3. Malkin RA, Idriss SF, Walker RG, Ideker RE: The effect of rapid pacing and T-wave scanning on the relationship between the defibrillation and upper limit of vulnerability dose-response curves. Circulation 92: 1291, 1995 4. Compos AT, Malkin RA, Ideker RE: A Bayesian updown defibrillation efficacy estimator. Pacing Clin Electrophysiol 20(5): 1292, 1997
Experimental Cardiac Tachyarrhythmias in Guinea Pigs
Robert
A. Malkin,
PhD
Abstract: Despite years of intense research into the mechanisms of defibril-
lation, there remain many unanswered questions. In many fields, hypotheses are first tested in rodent models before confirming the results in larger animals. This work suggests the guinea pig as a rodent model for defibrillation. Twenty-eight guinea pigs were studied, all male retired breeders weighing over 900 g. T-wave stimuli (upper limit of vulnerability [ULV] ) were given after 15 rapid pacing beats, since the rapid pacing has been suggested to extend the tachyarrhythmia. Defibrillation (DF) was attempted after 5 seconds. The correlation between the ULV50 and DFS0 in guinea pigs (0.82, n = 8) is very close to that seen in dogs (0.85). Also, the sensitivity of the DFS0 to waveform is similar (476 -+ 176 for monophasic vs 364 -+ 94 V for biphasic P < 0.005, n = 10). The dose-response curve widths (2.3 _+ 1.7 for ULV vs 1.9 _+ 1.8 for defibrillation, n = 10) show the same trend of increasing curve widths for ULV, and similar magnitude to dogs (mean 1.8). We rarely (<1.5%) observed spontaneous conversion in less than 10 seconds. The guinea pig can be used as a model for defibrillation as it shows many of the same characteristics as dogs. K e y w o r d s : ventricular fibrillation, defibrillation, rodents.
G u i n e a pigs are f r e q u e n t l y u s e d as m o d e l s for v e n t r i c u l a r t a c h y a r r h y t h m i a s (1) s u c h as p o l y m o r phic v e n t r i c u l a r t a c h y c a r d i a (VT) a n d v e n t r i c u l a r fibrillation (VF). H o w e v e r , t h e r e h a v e b e e n reports t h a t in intact rodents, VF can be difficult to initiate a n d typically s p o n t a n e o u s l y reverts to n o r m a l sinus r h y t h m (2). S p o n t a n e o u s r e v e r s i o n c o u l d limit the application of the m o d e l to t h e d e v e l o p m e n t of
t e c h n o l o g i e s for a c u t e t h e r a p y b e c a u s e s p o n t a n e ous r e v e r s i o n c o u l d be c o n f u s e d w i t h a successful treatment. H o w e v e r , based o n w o r k first d o n e in dogs to p r o m o t e initiation of VF (3), w e s h o w t h a t s p o n t a n e o u s r e v e r s i o n c a n be e l i m i n a t e d or a v o i d e d in g u i n e a pigs. Thus, t h e g u i n e a pig m o d e l p r e s e n t e d h e r e can be used to test h y p o t h e s e s or d e v e l o p t e c h n o l o g i c a l a d v a n c e s in defibrillation or cardioversion.
From the Joint Department of Biomedical Engineering, The University of Tennessee-Memphis and The University of Memphis, Memphis, Tennessee. Supported in part by NIH research grant HL56308, a faculty research grant from the University of Memphis, and a grant-inaid from the American Heart Association. Reprint requests: Robert A. Malkin, The University of Memphis, Department of Biomedical Engineering, ET330, Memphis, TN 38152.
Materials and Methods W e h a v e successfully used this m o d e l in t h a n 100 g u i n e a pigs to date. The w o r k h e r e marizes t h e results of 28 of these animals. retired b r e e d e r g u i n e a pigs ( 8 0 0 - 1 , 2 0 0 g)
Copyright © 1999 by Churchill Livingstone ® 0022-0736199/320S-0016510.OO/O
84
more sumMale, were
Tachyarrhythmias in Guinea Pigs
weighed and then anesthetized with intraperitoneal sodium pentobarbital (32 mg/kg initially, i0 rag! kg/h thereafter as indicated). Stainless steel needle electrodes were inserted into the paws for the recording of leads I, II, and III body surface electrograms. Rectal temperature was continuously monitored. Body temperature was maintained using a hot water blanket or a hot water bottle. A heparinized, 22G catheter was introduced into the carotid artery for the continuous monitoring of systemic blood pressure. A tracheotomy or direct laryngeal intubation was performed and an endotracheal tube inserted to facilitate continuous positive pressure ventilation (Small Animal Ventilator, Harvard, Edenbridge, Kent). A pacing electrode was advanced in the esophagus until approximately adjacent to the ventricles. The esophageal pacing electrode was constructed of a coated stainless steel wire (180-/~m diameter, World Precision Instruments, Sarasota, FL) inside a silicone tube (0.02-in diameter, Baxter, Deerfield, IL). The cathode for pacing was advanced through the diaphragm, near the xiphoid, toward the ventricular apex through a 20G catheter. The apical pacing electrode was made of the same coated stainless steel wire inserted into the 20G catheter. The coating was removed from the distal 5 m m and 1 mm of the wire for the esophageal and apical pacing electrodes, respectively. The esophageal/apical pacing configuration was found to offer low pacing thresholds and minimal contamination of the blood pressure recording by skeletal muscle contractions. During pacing threshold testing, the position of the esophageal electrode was adjusted to further minimize skeletal muscle contractions. The intrinsic R-R interval was measured at the beginning of the study. The pacing current threshold for a 10-ms, monophasic stimulus was measured next. In some animals, in order to control the T-wave timing, the guinea pig was continuously paced at 80% of the measured R-R interval for the remainder of the study using a pacing strength of twice the diastolic pacing current threshold. In the remaining animals, continuous pacing was not used. Ten minutes after beginning pacing, the time to the peak of the T wave was measured on the body surface electrogram which showed the cleanest peak. Typically, lead I or III was used. Four measurements of the T wave were taken and averaged. The time to the peak of the T wave was also measured after the rapid pacing sequence. The rapid pacing sequence consisted of a total of 25 beats, similar to that described by Malkin et al. (3). The pacing interval for the last 15 beats was selected
•
Robert A. Malkin
85
to be the shortest interval which did not lead to a loss of electrical capture for any of the 15 beats. The first 10 beats were used to progressively, and uniformly, shorten the pacing interval. Two 12-mm-diameter, sintered silver/silver chloride electrodes (In Vivo Metric, Healdsburg, CA) or stainless steel electrodes of the same size, were placed on the skin at opposing aspects of the thorax for T-wave stimulation and electrical countershock. The electrodes were placed as cranially as possible. A highly conductive gel (SignaGel, Parker, Orange, New Jersey [or similar]) was used between the electrodes and the shaved thorax. The DF50 (the strength that defibrillates 50% of the time) and the ULVS0 (the strength that induces fibrillation 50% of the time) were measured using the Bayesian 10-step approach (4).
Results A tachyarrhythmia was defined as any rapid surface electrocardiographic (ECG) waveform lasting at least 2 sec after the termination of the inducting stimulus. Ventricular fibrillation (VF) was defined as a polymorphic tachyarrhythmia accompanied by complete hemodynamic collapse. Polymorphism was defined as a tachyarrhythmia of irregular ECG morphology on all observed leads. Complete hemodynamic collapse was defined as an arterial pressure below 15 mmHg. All induced tachyarrhythmias seen in all animals (about 600 total) were VF in this study. In some animals, no attempt was made to defibrillate for 30 sec. When a VF spontaneously terminated in these animals, the time of termination was noted. When a rapid pacing sequence was used to initiate the VF, nearly all episodes were sustained for at least 30 sec (79%). In 1 subset of 207 inductions, only 3 episodes spontaneously terminated before 10 sec. In all of the approximately 600 observed VF episodes in these guinea pigs, independent of how the VF was initiated, none spontaneously terminated within 5 sec. Ten seconds is sufficiently long to allow 5 sec for detection of VF, the administration of an electrical countershock, and 5 sec for noting the shock's effect. Since only 3 of 207 episodes spontaneously terminated before 10 seconds, the risk of mistaking spontaneous defibrillation for successful electrical countershock is less than 1.5 %. The correlation between the ULV50 and DF50 in guinea pigs (0.82) is very close to that seen in dogs (0.85). Also, the sensitivity of the DF50 to wave-
86
Journal of Electrocardiology Vol. 32 Supplement 1999
form is similar (476 _+ 176 for m o n o p h a s i c vs 364 + 94 V for biphasic P < 0.005, n = 10). The doseresponse curve widths (2.3 _+ 1.7 for ULV vs 1.9 + 1.8 for defibrillation, n = 10) show the same trend of increasing curve widths for ULV, and similar magnitude to clogs (mean, 1.8).
Discussion It is reported (1,2) that small animals c a n n o t sustain ventricular t a c h y a r r h y t h m i a s in vivo. We show that large, intact guinea pigs can consistently sustain t a c h y a r r h y t h m i a s for 30 sec or more. Ours is the first w o r k to test large, intact guinea pigs as a model for defibrillation. As previously stated, all t a c h y a r r h y t h m i a s were polymorphic VT with complete h e m o d y n a m i c collapse, often lasting 30 sec or more. No pulsatile activity of more t h a n 15 m m H g was observed. The precipitous fall in pressure, the lack of pulsatility, and the p o l y m o r p h i c ECG are consistent with ventricular fibrillation. However, classifying our observations as VF w o u l d contradict a widely supported assumption that small animals, such as rats, mice, and guinea pigs, cannot consistently sustain VF (2). It is believed that there is a m i n i m u m (or critical) mass needed to sustain VF experimentally estimated to be 6 to 25 g. If the arrhythmias induced here are VF, then our data suggest that the critical
mass m a y be m u c h smaller t h a n previously t h o u g h t (ie, less t h a n 2.5 g).
Conclusions In m a n y laboratories, the canine, swine, or, to a limited extent, rabbit models are used to develop n e w technologies for treating VF. While large animal models will not be replaced, this study suggests that the guinea pig m a y be a viable alternative model for some developmental studies because of its low cost, ease of handling, and ready availability.
References 1. Lu HR, Remeysen P, Clerck FD: Antiarrhythmic effects of nebivolol in experimental models in vivo. J Cardiovasc Pharmacol 24:986, 1994 2. Ferris LP, King BG, Spence PW, Williams HB: Effect of electric shock on the heart. Electrical Eng 1936, p. 498 3. Malkin RA, Idriss SF, Walker RG, Ideker RE: The effect of rapid pacing and T-wave scanning on the relationship between the defibrillation and upper limit of vulnerability dose-response curves. Circulation 92: 1291, 1995 4. Compos AT, Malkin RA, Ideker RE: A Bayesian updown defibrillation efficacy estimator. Pacing Clin Electrophysiol 20(5): 1292, 1997