Cardiovascular Research I 992;26:487-495
487
Antiarrhythmic effects of preconditioning in anaesthetised dogs and rats Agnes Vegh, Sadayoshi Komori, Laszlo Szekeres, and James R Parratt Objective: The aim was to determine the relationship of the duration of short coronary artery occlusions and of the reperfusion period to the extent of the antiarrhythmic effect of preconditioning. Methods: A prolonged occlusion of a coronary artery in 102 anaesthetised rats and 55 anaesthetised dogs was preceded by a variable number of preconditioning coronary artery occlusions, of varying duration and with variable reperfusion periods between them and the prolonged occlusion. Results: Preconditioning in both species reduced the severity of ischaemia induced arrhythmias, epicardial ST segment changes, and alterations in the degree of inhomogeneity of conduction during a subsequent prolonged coronary artery occlusion, provided that the reperfusion time was less than 30 min (in rats) and 1 h (in dogs). This antiarrhythmic effect of preconditioning was marked; eg, in dogs following two preconditioning occlusions survival from a combined ischaemia-reperfusion insult was 40% (cf, 0% in the controls). Conclusions: Short preconditioning periods of myocardial ischaemia protect the myocardium against the arrhythmogenic effects of a more prolonged occlusion. The optimum time for this preconditioning occlusion in rats is 3 min and protection is still apparent 30 min later. In dogs, the protective effect is especially clear with two short (5 min) coronary artery occlusions. The protection in this species lasts for less than 1 h. the prolonged occlusions. These studies have been undertaken in two species: mongrel dogs, which have a variable coronary collateral circulation; and rats, which have hardly any significant collateral circulation. We have examined the effects of preconditioning both on ischaemia and reperfusion induced arrhythmias in these two species.
Methods Anuesthetised ruts Male Sprague-Dawley rats (body weight 190-300 g) were anaesthetised with sodium pentobarbitone (60 mg.kg-'), given intraperitoneally. The trachea was cannulated for artificial respiration and systemic arterial blood pressure was recorded from a catheter inserted into a carotid artery. Blood pressure, together with a standard limb lead 1 electrocardiogram, was continuously monitored over the experimental period on a Grass model 7D recorder (Grass Instrument Co, USA). The chest was opened using a left thoracotomy at a point approximately 2 mm to the left of the sternum and this was followed by sectioning of ribs 4 and 5. Artificial respiration was immediately started using room air (volume, 1.5 ml 10O.g-l; rate, 54 strokes.min-I). This is sufficient to maintain Pco?, Po?, and pH within normal limits.' After incising the pericardium, the heart was exteriorised using gentle pressure on the rib cage, and a 610 braided silk suture (attached to a 10 mm micropoint reverse cutting needle) was placed around the left coronary artery, as previously described.' ' The heart was replaced in the chest and the animal allowed to stabilise for 15 min. Any animal that had arrhythmias during this recovery period and/or had a mean arterial blood pressure of less than 70 mm Hg was discarded.
Department of Pharmacology, Albert Szent-Gyorgyi Medical University, Dom ter 12, Szeged, Hungary: A Vegh, L Szekeres; Department of Physiology and Pharmacology, Royal College, University of Strathclyde, Glasgow G 1 1XW, United Kingdom: J R Parratt, S Komori. Correspondence to Professor Parratt.
Downloaded from by guest on June 9, 2016
w
e have shown recently' ' that short (5 min) periods of coronary occlusion in anaesthetised dogs reduce the severity of ventricular arrhythmias that occur during a subsequent longer period of ischaemia. This phenomenon is known as preconditioning, a term first used to describe the reduction in ischaemic ultrastructural injury (infarct size) that occurs when a prolonged coronary occlusion is preceded by several brief episodes of ischaemia and reperfusion." Among the several unanswered questions relating to the antiarrhythmic effects of preconditioning are those concerned with the number and duration of the preconditioning occlusions required for optimum protection, and with the duration and mechanisms of this protection. One fundamental question relating to the antiarrhythmic effects of short preconditioning occlusions is concerned with whether this is genuine myocardial protection (ie, a real reduction in the severity and incidence of ventricular arrhythmias) or whether, since the onset of ischaemia may be delayed by the preconditioning occlusions, the time course of ventricular arrhythmias is modified in such a way that they appear much later during the prolonged ischaemic period. Hearse and his colleagues' have raised a similar question regarding drug induced reductions in the severity of reperfusion induced arrhythmias. Is it real protection (reduced severity, improved survival) or simply delay, the arrhythmias being spread over a longer time scale? Other important questions relate to the duration of the protective effect of preconditioning occlusions and whether this might be increased by pharmacological or other interventions. We have examined these questions by ( I ) varying the number and duration of the preconditioning occlusions, (2) extending the prolonged ischaemic period. and ( 3 ) varying the time interval between the preconditioning occlusions and
488
Vegh, Komori, Szekeres, Purrutt
'
Studies in unuesthetised dogs Adult mongrel dogs of either sex, weighing between 16 and 25 kg, were anaesthetised with a mixture of chloralose and urethane (60 and 200 mgkg-' respectively, intravenously) and ventilated with room air using a Harvard ventilator at a rate of 10 to 15 strokes.min-I; the stroke volume was adjusted to maintain pH and blood gases within normal limits: pH 7.40(SEM 0.02); Pa02 85( 1.2) mm Hg [ 11.3(0.16) kPa], Pacoz 30(1.0) mm Hg [4.0(0.13) kPa]. Body temperature was monitored from the oesophagus and maintained at 37(0.4)"C by means of a heating pad. Arterial blood pressure was recorded from the right femoral artery using a Statham P23Db pressure transducer and recorded on a Medicor R-81 recorder. A thoracotomy was performed at the fifth intercostal space and the heart suspended in a pericardial cradle. The left anterior descending coronary artery was dissected free about 2 cm from its origin, just above the first marginal branch, and a silk thread loosely placed around it. To evaluate the severity of myocardial ischaemia following coronary occlusion, a small rubber pad containing four unipolar electrodes was
sutured to the myocardium supplied by the left anterior descending artery; changes in ST segment elevation (mV) were continuously recorded from the same two selected epicardial sites and these were meaned for each dog. The degree of inhomogeneity of conduction within the ischaemic zone was evaluated by means of a composite electrode" I"; this composite electrode was built into the same pad as that containing the unipolar electrodes used for monitoring local ST segment changes. The experiments were carried out in a total of 55 dogs divided into five groups. Fifteen animals (group I), distributed throughout the experimental period, served as controls and, following a 30-40 min stability period, were subjected to a 25 rnin coronary artery occlusion followed by reperfusion. Ten dogs were "preconditioned" by subjecting them to two 5 rnin coronary artery occlusions (with a 20 min reperfusion interval between) followed, 20 min later, by a more prolonged (25 min) occlusion (group 11). These animals were then reperfused. A further group of 10 dogs was also subjected to preconditioning but the prolonged occlusion was maintained for 60 rather than 25 min (group 111). This group was designed to explore the possibility that ventricular ectopic activity was delayed by preconditioning and thus spread over a longer time course. A further group of 10 dogs (group IV) was subjected to only a single preconditioning occlusion; 20 min later the coronary artery was reoccluded for a 25 rnin period. These animals were then reperfused. In order to examine the duration of protection afforded by preconditioning occlusions, a further 10 dogs (group V ) were subjected to two 5 rnin preconditioning occlusions and then, 1 h later, the coronary artery was reoccluded for a 25 rnin period. These animals were also reperfused. Ventricular arrhythmias during ischaemia and reperfusion were analysed according to the Lambeth Convention,' except that (1) couplets and salvos were not analysed separately but were included in the analysis as single ventricular premature (ectopic) beats, and ( 2 ) ventricular tachycardia was defined as a run of four or more premature beats from the same focus at a rate higher than the basal sinus rate. At the end of the experiments the "at risk" area was assessed by the controlled infusion of methylene blue dye into the occluded artery at a pressure similar to that in the vessel before occlusion. The dyed area was cut out and weighed, and the risk area calculated as a percentage of the left ventricular free wall. This was between 34 and 42%. Although the anaesthetised dog experiments were conducted in Szeged, the protocol complies with UK Home Office requirements (Project Licence No 60/00307). Statistics Data are expressed as means(SEM) and differences between means were compared by Student's t test (haemodynamic data) or by the Mann-Whitney U test (for arrhythmias). To compare the between group incidences of ventricular tachycardia and ventricular fibrillation and differences in survival, the Fisher Irwin test (with Yate's correction factor) was used. Differences between groups were considered significant at a level of pe0.05.
Results STUDIES IN ANAESTHETISED RATS
Arrhythmias during the preconditioning occlusions and during the repegusion period As described by Kane et al,' there were only occasional ventricular premature beats during occlusion periods of 1, 3.
Downloaded from by guest on June 9, 2016
Transient regional myocardial ischaemia was induced by passing the threads through a small plastic tube and then pulling the suture while pressing the tube against the surface of the myocardium for periods of 1-5 rnin ("preconditioning"). Reperfusion was initiated by releasing the ligature and removing the plastic tube; it was maintained for periods of between 10 and 30 min, after which the artery was again occluded, this time for a period of 30 min. Full details of this method have been described before, including evidence that reperfusion does indeed take place in this experimental model following periods of occlusion of up to s min.' In the first protocol the duration of the preconditioning coronary occlusion was varied. We used three different periods, 1, 3, and 5 min. In this initial study, the reperfusion period was maintained at 10 rnin before reoccluding the artery for 30 min. The severity of arrhythmias during this 30 min reocclusion period was assessed and compared with control rats in which the coronary artery was also occluded for 30 rnin but without preconditioning. In the second protocol the period of the preconditioning coronary artery occlusion was maintained at 3 min, and the reperfusion period was varied (10, 20, or 30 min) before the artery was reoccluded for a 30 rnin period. The severity of the arrhythmias during the second (prolonged) occlusion period was compared with that of arrhythmias occurring in a group of control rats in which the coronary artery was occluded for the same time period (30 min) but without any preconditioning occlusions. The severity of arrhythmias was assessed as described previously,s except that we did not separate single beats from salvos. Ventricular tachycardia was defined as four or more consecutive ventricular ectopic beats at a rate greater than the preocclusion heart rate. Although, in this species, ventricular fibrillation is not necessarily a terminal event,s we chose not to distinguish between a ventricular fibrillation which was self terminating and one that resulting in death. Both are included as total ventricular fibrillation. In the tables and figures the number of ventricular premature beats during ischaemia is derived only from those animals that survived the occlusion period. The experiments were conducted under a protocol covered by UK Home Office requirements (project licence No 60/00307).
489
Preconditioning of the ischaemic myocardium
Table II The number of ventricular premature beats (VPB), the incidence and duration of ventricular tachycardia (VT), and the incidence qf Ventricularjibrillation (VF) in rats subjected to a 30 rnin occlusion of the Iefi coronrity artery. In the precrinditiotied rats this occIuJion was preceded I0 min earlier by a preconditioning occlusion of I , 3, or 5 tnin. Preconditioning periods o j 3 or 5 min signiJicantly reduced the arrhythmias occurring during a longer occlusion
Table I Reperfusion arrhythmias following I , 3, or 5 min periods of coronar?, artery occlusion in anaesthetised rats. There were on1.y occasional ventricular premature beats ( VPBs) during reperfusion following occlusion periods of I and 3 min but arrhythtnicrs were more severe (high incidences of ventricular tachycardia (VT) and jibrillation (VF)) during reperfusion Jidlowing a 5 min occlusion period Occlusion
tinie (tnin)
1 3
n
8 17 24
5
Incidence oj: VT% VMI
No of' VPBs (SEMJ
0 0 83
IOU) 306(145)
Durution of' n preconditioning occlusion (min)
0 0 71
I 3
S
f
55.1(20.3)
(% incidencrJ
42 50
10
30
Mod$ication of the protective effect o j a 3 min Preconditioning occlusion by varying the duration of the reperjusion period We chose a single 3 rnin occlusion period to examine the duration of the protection afforded by preconditioning. This was done by prolonging the reperfusion period from 10 rnin to 20 min and then to 30 min. The results are summarised in table 111 and the effect on the distribution of ventricular premature beats over the 30 rnin second occlusion period is shown in fig 2. Incidentally, the reliability of this model for evaluating the effect of preconditioning is illustrated by the similarities in arrhythmia severity between the control group used in this series of experiments (table 111) and that used in the experiments summarised in table I1 and also between the two groups in which the artery was occluded 10 rnin after D
308 1 min occl 10 min rep (n = 8)
Control (n = 10)
200
5 min occl 10 min rep (n = 101
3 niin occl 10 min rep (n = 101
I
c
E"
!! n
-m L
.-c0
3
: 100 >
.b
u-
0 L
al
n
E
1 10
20
30
20
30
10
20
30
10
20
30
Time (min)
Figure 1 Ventricular premature bears induced by lefi corona? artery occlusion in anaesthetised male rats under control conditions ( A ) and under condiiions when the prolonged, 30 min, occlusion is preceded by a short preconditioning occlusion of I min (6).3 min (C), or 5 min (D). The reperfusion time in each case was I0 min. A preconditioning occlusion of 3 or 5 tnin markedly reduced the number of premature beats that occurred during a subsequent prolonged occlusion. The optimum preconditioning occlusion time appears to be 3 min in this species. VPC=ventricular premature complexes; VT=ventricular tachycardia.
Downloaded from by guest on June 9, 2016
a P) n
95.6(28.7) 109.8(30.9) 12.6(3.8)1
VF
J ~
Very few animals survived a 5 rnin preconditioning occlusion period because of the high incidence of ventricular fibrillation on reperfusion after this occlusion time.' However, in those animals that did survive reperfusion, this period of preconditioning also reduced the severity of arrhythmias that occurred during the second, prolonged occlusion (table 11; fig 1).
C
3041~rEo 11
v)
VT
Values are rneans(SEM) *p
487
Antiarrhythmic effects of preconditioning in anaesthetised dogs and rats Agnes Vegh, Sadayoshi Komori, Laszlo Szekeres, and James R Parratt Objective: The aim was to determine the relationship of the duration of short coronary artery occlusions and of the reperfusion period to the extent of the antiarrhythmic effect of preconditioning. Methods: A prolonged occlusion of a coronary artery in 102 anaesthetised rats and 55 anaesthetised dogs was preceded by a variable number of preconditioning coronary artery occlusions, of varying duration and with variable reperfusion periods between them and the prolonged occlusion. Results: Preconditioning in both species reduced the severity of ischaemia induced arrhythmias, epicardial ST segment changes, and alterations in the degree of inhomogeneity of conduction during a subsequent prolonged coronary artery occlusion, provided that the reperfusion time was less than 30 min (in rats) and 1 h (in dogs). This antiarrhythmic effect of preconditioning was marked; eg, in dogs following two preconditioning occlusions survival from a combined ischaemia-reperfusion insult was 40% (cf, 0% in the controls). Conclusions: Short preconditioning periods of myocardial ischaemia protect the myocardium against the arrhythmogenic effects of a more prolonged occlusion. The optimum time for this preconditioning occlusion in rats is 3 min and protection is still apparent 30 min later. In dogs, the protective effect is especially clear with two short (5 min) coronary artery occlusions. The protection in this species lasts for less than 1 h. the prolonged occlusions. These studies have been undertaken in two species: mongrel dogs, which have a variable coronary collateral circulation; and rats, which have hardly any significant collateral circulation. We have examined the effects of preconditioning both on ischaemia and reperfusion induced arrhythmias in these two species.
Methods Anuesthetised ruts Male Sprague-Dawley rats (body weight 190-300 g) were anaesthetised with sodium pentobarbitone (60 mg.kg-'), given intraperitoneally. The trachea was cannulated for artificial respiration and systemic arterial blood pressure was recorded from a catheter inserted into a carotid artery. Blood pressure, together with a standard limb lead 1 electrocardiogram, was continuously monitored over the experimental period on a Grass model 7D recorder (Grass Instrument Co, USA). The chest was opened using a left thoracotomy at a point approximately 2 mm to the left of the sternum and this was followed by sectioning of ribs 4 and 5. Artificial respiration was immediately started using room air (volume, 1.5 ml 10O.g-l; rate, 54 strokes.min-I). This is sufficient to maintain Pco?, Po?, and pH within normal limits.' After incising the pericardium, the heart was exteriorised using gentle pressure on the rib cage, and a 610 braided silk suture (attached to a 10 mm micropoint reverse cutting needle) was placed around the left coronary artery, as previously described.' ' The heart was replaced in the chest and the animal allowed to stabilise for 15 min. Any animal that had arrhythmias during this recovery period and/or had a mean arterial blood pressure of less than 70 mm Hg was discarded.
Department of Pharmacology, Albert Szent-Gyorgyi Medical University, Dom ter 12, Szeged, Hungary: A Vegh, L Szekeres; Department of Physiology and Pharmacology, Royal College, University of Strathclyde, Glasgow G 1 1XW, United Kingdom: J R Parratt, S Komori. Correspondence to Professor Parratt.
Downloaded from by guest on June 9, 2016
w
e have shown recently' ' that short (5 min) periods of coronary occlusion in anaesthetised dogs reduce the severity of ventricular arrhythmias that occur during a subsequent longer period of ischaemia. This phenomenon is known as preconditioning, a term first used to describe the reduction in ischaemic ultrastructural injury (infarct size) that occurs when a prolonged coronary occlusion is preceded by several brief episodes of ischaemia and reperfusion." Among the several unanswered questions relating to the antiarrhythmic effects of preconditioning are those concerned with the number and duration of the preconditioning occlusions required for optimum protection, and with the duration and mechanisms of this protection. One fundamental question relating to the antiarrhythmic effects of short preconditioning occlusions is concerned with whether this is genuine myocardial protection (ie, a real reduction in the severity and incidence of ventricular arrhythmias) or whether, since the onset of ischaemia may be delayed by the preconditioning occlusions, the time course of ventricular arrhythmias is modified in such a way that they appear much later during the prolonged ischaemic period. Hearse and his colleagues' have raised a similar question regarding drug induced reductions in the severity of reperfusion induced arrhythmias. Is it real protection (reduced severity, improved survival) or simply delay, the arrhythmias being spread over a longer time scale? Other important questions relate to the duration of the protective effect of preconditioning occlusions and whether this might be increased by pharmacological or other interventions. We have examined these questions by ( I ) varying the number and duration of the preconditioning occlusions, (2) extending the prolonged ischaemic period. and ( 3 ) varying the time interval between the preconditioning occlusions and
488
Vegh, Komori, Szekeres, Purrutt
'
Studies in unuesthetised dogs Adult mongrel dogs of either sex, weighing between 16 and 25 kg, were anaesthetised with a mixture of chloralose and urethane (60 and 200 mgkg-' respectively, intravenously) and ventilated with room air using a Harvard ventilator at a rate of 10 to 15 strokes.min-I; the stroke volume was adjusted to maintain pH and blood gases within normal limits: pH 7.40(SEM 0.02); Pa02 85( 1.2) mm Hg [ 11.3(0.16) kPa], Pacoz 30(1.0) mm Hg [4.0(0.13) kPa]. Body temperature was monitored from the oesophagus and maintained at 37(0.4)"C by means of a heating pad. Arterial blood pressure was recorded from the right femoral artery using a Statham P23Db pressure transducer and recorded on a Medicor R-81 recorder. A thoracotomy was performed at the fifth intercostal space and the heart suspended in a pericardial cradle. The left anterior descending coronary artery was dissected free about 2 cm from its origin, just above the first marginal branch, and a silk thread loosely placed around it. To evaluate the severity of myocardial ischaemia following coronary occlusion, a small rubber pad containing four unipolar electrodes was
sutured to the myocardium supplied by the left anterior descending artery; changes in ST segment elevation (mV) were continuously recorded from the same two selected epicardial sites and these were meaned for each dog. The degree of inhomogeneity of conduction within the ischaemic zone was evaluated by means of a composite electrode" I"; this composite electrode was built into the same pad as that containing the unipolar electrodes used for monitoring local ST segment changes. The experiments were carried out in a total of 55 dogs divided into five groups. Fifteen animals (group I), distributed throughout the experimental period, served as controls and, following a 30-40 min stability period, were subjected to a 25 rnin coronary artery occlusion followed by reperfusion. Ten dogs were "preconditioned" by subjecting them to two 5 rnin coronary artery occlusions (with a 20 min reperfusion interval between) followed, 20 min later, by a more prolonged (25 min) occlusion (group 11). These animals were then reperfused. A further group of 10 dogs was also subjected to preconditioning but the prolonged occlusion was maintained for 60 rather than 25 min (group 111). This group was designed to explore the possibility that ventricular ectopic activity was delayed by preconditioning and thus spread over a longer time course. A further group of 10 dogs (group IV) was subjected to only a single preconditioning occlusion; 20 min later the coronary artery was reoccluded for a 25 rnin period. These animals were then reperfused. In order to examine the duration of protection afforded by preconditioning occlusions, a further 10 dogs (group V ) were subjected to two 5 rnin preconditioning occlusions and then, 1 h later, the coronary artery was reoccluded for a 25 rnin period. These animals were also reperfused. Ventricular arrhythmias during ischaemia and reperfusion were analysed according to the Lambeth Convention,' except that (1) couplets and salvos were not analysed separately but were included in the analysis as single ventricular premature (ectopic) beats, and ( 2 ) ventricular tachycardia was defined as a run of four or more premature beats from the same focus at a rate higher than the basal sinus rate. At the end of the experiments the "at risk" area was assessed by the controlled infusion of methylene blue dye into the occluded artery at a pressure similar to that in the vessel before occlusion. The dyed area was cut out and weighed, and the risk area calculated as a percentage of the left ventricular free wall. This was between 34 and 42%. Although the anaesthetised dog experiments were conducted in Szeged, the protocol complies with UK Home Office requirements (Project Licence No 60/00307). Statistics Data are expressed as means(SEM) and differences between means were compared by Student's t test (haemodynamic data) or by the Mann-Whitney U test (for arrhythmias). To compare the between group incidences of ventricular tachycardia and ventricular fibrillation and differences in survival, the Fisher Irwin test (with Yate's correction factor) was used. Differences between groups were considered significant at a level of pe0.05.
Results STUDIES IN ANAESTHETISED RATS
Arrhythmias during the preconditioning occlusions and during the repegusion period As described by Kane et al,' there were only occasional ventricular premature beats during occlusion periods of 1, 3.
Downloaded from by guest on June 9, 2016
Transient regional myocardial ischaemia was induced by passing the threads through a small plastic tube and then pulling the suture while pressing the tube against the surface of the myocardium for periods of 1-5 rnin ("preconditioning"). Reperfusion was initiated by releasing the ligature and removing the plastic tube; it was maintained for periods of between 10 and 30 min, after which the artery was again occluded, this time for a period of 30 min. Full details of this method have been described before, including evidence that reperfusion does indeed take place in this experimental model following periods of occlusion of up to s min.' In the first protocol the duration of the preconditioning coronary occlusion was varied. We used three different periods, 1, 3, and 5 min. In this initial study, the reperfusion period was maintained at 10 rnin before reoccluding the artery for 30 min. The severity of arrhythmias during this 30 min reocclusion period was assessed and compared with control rats in which the coronary artery was also occluded for 30 rnin but without preconditioning. In the second protocol the period of the preconditioning coronary artery occlusion was maintained at 3 min, and the reperfusion period was varied (10, 20, or 30 min) before the artery was reoccluded for a 30 rnin period. The severity of the arrhythmias during the second (prolonged) occlusion period was compared with that of arrhythmias occurring in a group of control rats in which the coronary artery was occluded for the same time period (30 min) but without any preconditioning occlusions. The severity of arrhythmias was assessed as described previously,s except that we did not separate single beats from salvos. Ventricular tachycardia was defined as four or more consecutive ventricular ectopic beats at a rate greater than the preocclusion heart rate. Although, in this species, ventricular fibrillation is not necessarily a terminal event,s we chose not to distinguish between a ventricular fibrillation which was self terminating and one that resulting in death. Both are included as total ventricular fibrillation. In the tables and figures the number of ventricular premature beats during ischaemia is derived only from those animals that survived the occlusion period. The experiments were conducted under a protocol covered by UK Home Office requirements (project licence No 60/00307).
489
Preconditioning of the ischaemic myocardium
Table II The number of ventricular premature beats (VPB), the incidence and duration of ventricular tachycardia (VT), and the incidence qf Ventricularjibrillation (VF) in rats subjected to a 30 rnin occlusion of the Iefi coronrity artery. In the precrinditiotied rats this occIuJion was preceded I0 min earlier by a preconditioning occlusion of I , 3, or 5 tnin. Preconditioning periods o j 3 or 5 min signiJicantly reduced the arrhythmias occurring during a longer occlusion
Table I Reperfusion arrhythmias following I , 3, or 5 min periods of coronar?, artery occlusion in anaesthetised rats. There were on1.y occasional ventricular premature beats ( VPBs) during reperfusion following occlusion periods of I and 3 min but arrhythtnicrs were more severe (high incidences of ventricular tachycardia (VT) and jibrillation (VF)) during reperfusion Jidlowing a 5 min occlusion period Occlusion
tinie (tnin)
1 3
n
8 17 24
5
Incidence oj: VT% VMI
No of' VPBs (SEMJ
0 0 83
IOU) 306(145)
Durution of' n preconditioning occlusion (min)
0 0 71
I 3
S
f
55.1(20.3)
(% incidencrJ
42 50
10
30
Mod$ication of the protective effect o j a 3 min Preconditioning occlusion by varying the duration of the reperjusion period We chose a single 3 rnin occlusion period to examine the duration of the protection afforded by preconditioning. This was done by prolonging the reperfusion period from 10 rnin to 20 min and then to 30 min. The results are summarised in table 111 and the effect on the distribution of ventricular premature beats over the 30 rnin second occlusion period is shown in fig 2. Incidentally, the reliability of this model for evaluating the effect of preconditioning is illustrated by the similarities in arrhythmia severity between the control group used in this series of experiments (table 111) and that used in the experiments summarised in table I1 and also between the two groups in which the artery was occluded 10 rnin after D
308 1 min occl 10 min rep (n = 8)
Control (n = 10)
200
5 min occl 10 min rep (n = 101
3 niin occl 10 min rep (n = 101
I
c
E"
!! n
-m L
.-c0
3
: 100 >
.b
u-
0 L
al
n
E
1 10
20
30
20
30
10
20
30
10
20
30
Time (min)
Figure 1 Ventricular premature bears induced by lefi corona? artery occlusion in anaesthetised male rats under control conditions ( A ) and under condiiions when the prolonged, 30 min, occlusion is preceded by a short preconditioning occlusion of I min (6).3 min (C), or 5 min (D). The reperfusion time in each case was I0 min. A preconditioning occlusion of 3 or 5 tnin markedly reduced the number of premature beats that occurred during a subsequent prolonged occlusion. The optimum preconditioning occlusion time appears to be 3 min in this species. VPC=ventricular premature complexes; VT=ventricular tachycardia.
Downloaded from by guest on June 9, 2016
a P) n
95.6(28.7) 109.8(30.9) 12.6(3.8)1
VF
J ~
Very few animals survived a 5 rnin preconditioning occlusion period because of the high incidence of ventricular fibrillation on reperfusion after this occlusion time.' However, in those animals that did survive reperfusion, this period of preconditioning also reduced the severity of arrhythmias that occurred during the second, prolonged occlusion (table 11; fig 1).
C
3041~rEo 11
v)
VT
Values are rneans(SEM) *p
