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Reduced Periinfarction Mortality as a Result of Long-Term Therapy with Captopril But Not Hydralazine or Propranolol in Spontaneously Hypertensive Rats

Nguyen, Tan*†; El Salibi, Elias*; Rouleau, Jean L.*†

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Journal of Cardiovascular Pharmacology: December 1998 - Volume 32 - Issue 6 - p 884-895
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Abstract

Hypertension and left ventricular hypertrophy (LVH) are known to be important cardiac risk factors (1,2). Hypertension and LVH increase not only the risk of developing cardiac diseases, but also the risk of periinfarction mortality, should a patient sustain an acute myocardial infarction (MI; 3). Studies in the spontaneously hypertensive rat (SHR) model of hypertension and LVH suggested that much of the periinfarction risk associated with hypertension and LVH is the result of an increased risk of malignant ventricular arrhythmias (4). Hypertension and LVH are known to be associated with changes in the myocardium and the interstitial space (5-10), and these changes are in turn known to be associated with an increased risk of developing left ventricular dysfunction and malignant ventricular arrhythmias (1,11,12).

Regression of LVH due to hypertension has been found to occur as the result of treatment with a number of agents and has been advocated as a therapeutic goal in clinical practice (11-14). However, although some clinical evidence exists that regression of LVH has an impact on the prognosis of patients (15), more work must be done before it becomes a major therapeutic goal for many physicians. Nevertheless, there is some experimental evidence that regression of LVH is associated with a decreased susceptibility to ventricular arrhythmias before (16) and during (17) acute myocardial ischemia. Work by Kohya et al. (17) suggested that the major mechanism by which LVH regression is beneficial is by reducing myocardial hypertrophy, thereby preventing the prolongation of action-potential duration and reducing action-potential duration dispersion during acute myocardial ischemia. Work by Pahor et al. (16) and Bélichard et al. (4) suggested that interstitial fibrosis also plays an important role in the genesis of LVH-related arrhythmias; however, they did not test this association in the acute ischemic situation. Thus the relative importance of regression of cardiac fibrosis, as opposed to regression of myocardial hypertrophy, in the acute ischemic situation remains to be determined.

Other equally if not more important issues pertaining to regression of LVH and periinfarction arrhythmias also remain to be addressed. Perhaps the foremost of these is to what extent inducibility of ventricular arrhythmias in a Langendorff-perfused heart studied during 30 min of ischemia predicts periinfarction survival. In vivo hearts are exposed to various neurohumoral stimuli and to loading conditions that can modify susceptibility to ventricular arrhythmias that are not found in vitro. More important, many periinfarction arrhythmias, including ventricular fibrillation (VF), are self-limiting in the rat, do not lead to death, and occur in a majority of rats whether they survive or not (18-20). Also, two distinct postinfarction arrhythmogenic periods exist, one that occurs during the first 30 min and another that occurs between 90 min and 9 h after infarction (18). Such a bimodal arrhythmia time course suggests that two distinct mechanisms exist for the genesis of postinfarction ventricular arrhythmias and cannot be adequately addressed in studies of 30 min of ischemia.

In this study, we evaluated the effects of long-term control of systemic arterial blood pressure, of regression of myocardial hypertrophy, and of regression of cardiac fibrosis on the susceptibility to ventricular arrhythmias and periinfarction mortality in the SHR model of hypertension and LVH. To do this, we compared the effects of 12 weeks of treatment with various clinically used antihypertensive medications on heart rate and blood pressure, on the degree of myocardial hypertrophy and cardiac fibrosis, on inducibility of ventricular arrhythmias by programed electrical stimulation (PES) and on 3-h periinfarction mortality. The medications used were the β-blocker propranolol, which is known to decrease heart rate, to have only mild effects on blood pressure and myocardial hypertrophy in the SHR, and to have poorly described effects on cardiac fibrosis (14,21-24); hydralazine, which increases heart rate, normalizes blood pressure, but has little effect on myocardial hypertrophy or cardiac fibrosis (14,17,22,25,26); and captopril, which has little effect on heart rate, decreases blood pressure, and decreases both myocardial hypertrophy and cardiac fibrosis (27-31). The various effects of these medications on the parameters assessed were then correlated with inducibility of ventricular arrhythmias before ischemia and 3-h postinfarction mortality, a period which includes the two distinct periinfarction arrhythmogenic phases.

METHODS

Preparation of animals

A total of 86 male SHRs (body weight, 250-300 g; 13 weeks old) was obtained from Charles River Breeding Laboratories (Saint-Constant, Quebec, Canada). All SHRs were housed in clear plastic cages, fed normal rat chow (Purina, St. Louis, MO, U.S.A.) and water ad libitum, with a light/dark cycle of 12 h, in accordance with the Société Protectrice Canadienne des Animaux and in-house guidelines. SHRs were allowed to rest 1 week before the start of the experiments.

Drug randomization

After the baseline measurement of systolic blood pressure and heart rate (week 0), the rats were randomly divided into four groups. One group received normal drinking water (control group; n = 25). The second group received captopril (Bristol-Myers-Squibb), 2 g/L (32) in the drinking water (n = 20). The third group received propranolol (Sigma), 750 mg/L (21), in the drinking water (n = 20), and the last group received hydralazine (Sigma), 80 mg/L (33), in the drinking water (n = 21).

Systolic blood pressure and heart rate monitoring

Indirect systolic blood pressure and heart rate were determined by the tail-cuff method (Harvard Apparatus, South Natick, MA, U.S.A.; 34). Vasodilatation was first performed by local warming with a heating pad, and unanesthetized rats were placed in a plastic restraining holder from which the tail protruded. The cuff and transducer were then put around the tail, and the cuff was inflated until the pulse disappeared. When the cuff was slowly deflated, the point of reappearance of the pulse indicated the value of systolic blood pressure (mm Hg). The heart rate also was calculated by counting the pulses recorded over the time when the cuff was completely deflated and converted into beats per minute. The reported values are the mean of at least three recordings taken at the same time of day every 2 weeks during the 12 weeks of treatment.

Programed electrical stimulation

After 12 weeks of antihypertensive therapy, all drugs were then stopped 72 h before the PES to permit adequate washout. The rats were anesthetized with an intramuscular injection of a ketamine HCl (87 mg/kg) and rompun xylazine (13 mg/kg) mixture, intubated, and mechanically ventilated with room air, supplemented with low-flow oxygen with a small-rodent ventilator (Harvard Apparatus) at a rate of 70 cycles per minute and a tidal volume of 8 ml/kg. Body temperature was maintained by lamps and a heating pad. A lead II electrocardiogram (ECG) was obtained with standard limb electrodes and continuously recorded on a Gould recorder. The thorax was then opened by sternotomy, and PES was done through Biomed electrodes (Cooner Wire Company, Chatworth, CA, U.S.A.) sewn on the epicardial surface of the right ventricular outflow tract, and recordings were made at the left ventricular apex. Pacing was performed by means of a Bloom programable stimulator (World Precision Instruments, Sarasota, FL, U.S.A.). The protocol for PES used in this study was similar to that described by Bélichard et al. (32). The effective refractory period was determined by premature stimulation with a single extrastimulus after 20 paced beats at a basic cycle length of 100 ms. Induction of ventricular arrhythmias was then attempted by ventricular stimulation at a basic cycle length of 100 ms (S0) with single (S1), double (S2), and triple (S3) extrastimuli delivered after 20 paced beats (Fig. 1).

FIG. 1
FIG. 1:
Tracings from a control spontaneously hypertensive rat (SHR) with sustained ventricular arrhythmias (>15 beats). A: Three premature stimuli (S1, S2, and S3) after 20 paced stimuli (S0) induced ventricular fibrillation, which gives way to ventricular tachycardia and resolves spontaneously. B: One premature stimulus (S1) after 20 paced stimuli (S0) induced ventricular tachycardia that resolved spontaneously.

The end point of PES was induction of a ventricular tachyarrhythmia consisting of at least six consecutive nondriven ventricular extrastimuli beats. A preparation was considered noninducible when PES produced either no ventricular premature beat or only self-terminating salvos of five or fewer beats. Distinction was not made between ventricular tachycardia (VT) and VF because, in the rat heart, transitions between VT and VF occur spontaneously, and the latter can terminate spontaneously as well (32). A ventricular tachyarrhythmia was considered nonsustained when it lasted ≤15 beats before spontaneous self-termination, and it was considered sustained when it lasted >15 beats before terminating spontaneously or by overdrive pacing. This 15-beat criterion allowed us to differentiate between brief but significant responses and protracted VT, against which overdrive pacing was usually required.

Arrhythmia scoring allowed the use of parametric statistical tests to compare the various treatment groups with respect to their susceptibility to develop PES-induced ventricular tachyarrhythmias: 0, noninducible preparations; 1, nonsustained tachyarrhythmias induced with three extrastimuli; 2, sustained tachyarrhythmias induced with three extrastimuli; 3, nonsustained tachyarrhythmias induced with two extrastimuli; 4, sustained tachyarrhythmias induced with two extrastimuli; 5, nonsustained tachyarrhythmias induced with one extrastimulus; 6, sustained tachyarrhythmias induced with one extrastimulus; and 7, tachyarrhythmias induced during the 20 paced beats at a basic cycle length of 100 ms. This scoring system reflects the fact that specificity for detection of arrhythmogenic substrates associated with spontaneous arrhythmias and sudden death is reduced, but sensitivity is increased as the number of extrastimuli of the PES protocol increases from one to three (35).

Myocardial infarction

At the end of PES, MI was induced in all rats according to methods previously described by Pfeffer et al. (36), with slight modifications. In brief, while the rats were still anesthetized, ventilated, and monitored by ECG, the left anterior descending coronary artery was then carefully ligated at its origin with a 40 silk black braided suture (Ethicon, Somerville, NJ, U.S.A.). The heart was replaced into the chest in its normal position, and the thorax was closed with a Chromic 000 suture (Ethicon). The wound was kept humid with a wet gauze. The time course of postinfarction mortality, if it occurred, was documented for the first 3 h after infarction.

Morphologic studies

Tissue preparation. For rats surviving 3 h after infarction, the heart was stopped in diastole with potassium chloride, removed, and rinsed in saline solution. For rats dying before 3 h, the heart was simply removed and rinsed in saline solution. The left ventricle of the heart was then filled with saline solution to a pressure of 15 mm Hg, sealed, and fixed in formalin for ≥24 h. After dissection of the atria and great vessels, the right ventricle was isolated along its septal insertion, and both right and left ventricular weights were determined. Two cross sections were obtained at 1-mm intervals midway between the base and the apex of the left ventricle.

Morphometric analysis. To determine the degree of hypertrophy, morphometry of the left ventricle was performed on tissues obtained from the two cross sections. These tissues were dehydrated at room temperature through graded ethanol series and embedded in paraffin. Sections 4-μm thick were cut, and a representative section of these from the two cross sections of each left ventricle was stained with hematoxylin and mounted on a slide for projection to a magnification ×10. Sections were traced on a calibrated digitizing tablet and calculated directly by planimetry. The endocardial and epicardial surfaces were numerically summed separately, as were the endocardial and epicardial circumferences.

Collagen quantification by computer-assisted image analysis. This procedure used samples from both cross sections. These were cut into 8-μm thick slices and stained with Sirius red F3BA as a 0.1% solution in saturated aqueous picric acid. The Sirius red staining technique has a good affinity for collagen fibers and gives pictures of high contrast by using polarized light, as described previously by Junqueira et al. (37,38). Each slide was examined under a Carl-Zeiss microscope fitted with cross-polarizing filters at a final magnification of ×200, giving a final resolution of 0.72 μm2 per pixel. The collagen network was quantified by computer-assisted image analysis. The automated system included a digital image-analysis device (Samba 4000; Imaging Products International, Inc., Chantilly, VA, U.S.A.), based on morphologic mathematic principles, connected to an IBM-compatible personal computer running customized algorithms written in C language. Each microscopic field of observation sent to the image analyzer was transmitted by a Sony 3CCD video camera (Montvale, NJ, U.S.A.) and transformed into a binary image. Then a sequence of mathematical and morphologic operations allowed us to identify and quantify the collagen network. Collagen volume density fraction was then determined by measuring the area of stained tissue within a given field and expressing that area as a proportion of the total area under observation. The collagen-rich border zone of vessels was not included in the calculations. Ten fields were analyzed in the subendocardial layer and 10 fields in the subepicardial layer in each left ventricle.

All samples were then sent to an independent investigator who was unaware of the nature of the experimental procedure (Laboratory of Dr. Brazeau, Notre-Dame Hospital, Montreal, Quebec, Canada) for verification of results by using the same image-analysis technique just described but with slightly different equipment. Slides were examined under a Nikon Optiphot microscope (Tokyo, Japan) at a final resolution of 0.66 μm2 per pixel. The automated system included a digital image-analysis device connected to an IBM-compatible computer running software Vision 2.0 (Clemex Technologie Inc., Longueuil, Quebec, Canada). The algorithm was customized to distinguish three different parameters such as the color tint, color saturation, and light intensity. This permitted the separate measurement of the two principal types of collagen (types I and III; 8,39)

The mean value of the two laboratories was used as the final morphologic assessment of percentage collagen by volume density. An approximation of total collagen volume was obtained by multiplying left ventricular weight/body weight ratio by collagen volume density percentage.

Statistics

All values are expressed as mean ± SD. A χ2 test was used to evaluate the effects of different drugs on inducibility of ventricular arrhythmias and final mortality figures. One-way analysis of variance (ANOVA) was used to assess the effects of multiple comparisons, followed by a two-sided Dunnett comparison test. For the Kaplan-Meier survival curves, a log-rank comparison test was used. Significance at either p < 0.1 or p < 0.05 level is indicated.

RESULTS

Heart rate and blood pressure

All four groups of rats started the study with a similar heart rate and degree of systolic blood pressure. In the control SHR group (n = 25), both heart rate and systolic blood pressure increased over the 12-week course of the study (p < 0.05), indicating a progression of the disease process in these rats (Table 1).

TABLE 1
TABLE 1:
Effect of pharmacologic interventions on heart rate and systolic blood pressure in four SHR groups

In the propranolol-treated rats (n = 20), heart rate decreased within the first 2 weeks of therapy and thereafter remained significantly lower than baseline (p < 0.05) and lower than control SHRs (p < 0.05). The effect of propranolol on systolic arterial pressure was not so impressive. Propranolol did not reduce systolic blood pressure but prevented the increase documented in control SHRs. Thus at various time points (e.g., weeks 8 and 10), systolic blood pressure was lower in the propranolol group as compared with controls (p < 0.05).

Treatment with hydralazine (n = 21) caused systolic blood pressure to decrease within the first 2 weeks of therapy and to remain decreased as compared with baseline (p < 0.05) and control SHRs (p < 0.05). This was accompanied by an increase in heart rate, which was even greater than that seen in control SHRs (p < 0.05).

Captopril (n = 20) reduced systolic blood pressure within the first 2 weeks of therapy. This decrease was sustained throughout the course of the study (p < 0.05). This was accompanied by a progressive increase in heart rate similar to that of the control SHRs.

Programed electrical stimulation

PES studies were not performed on two control-, two propranolol-, and one hydralazine-treated rat, all of which died during surgery before PES testing. The remaining 81 rats had PES testing (Table 2, Fig. 2).

TABLE 2
TABLE 2:
Ventricular arrhythmias induced with programmed electrical stimulation
FIG. 2
FIG. 2:
Inducibility of ventricular tachycardia or ventricular fibrillation by programed electrical stimulation (inducibility quotient). Values expressed as mean ± SD. Captopril and possibly propranolol decreased inducibility quotient (rats that died during the surgery before programed electrical stimulation are excluded).

Of the rats that had PES, the majority (65%) of the control SHRs (n = 23) did not have arrhythmias inducible by PES. Nevertheless, eight (35%) of 23 rats had inducible ventricular arrhythmias, of which seven (31%) were sustained. Only one (6%) of the 18 propranolol-treated rats that had PES had inducible ventricular arrhythmias. As compared with controls, this represented a significant reduction (p = 0.02) in inducibility of ventricular arrhythmias. Captopril treatment (n = 20) also significantly decreased arrhythmia inducibility (p = 0.02) as compared with control, with one (5%) of 20 rats having sustained ventricular arrhythmias by PES. Hydralazine (n = 20) had no effect (p = 0.49) on inducibility of ventricular arrhythmias, with five (25%) of 20 rats having inducibility of ventricular arrhythmias, four (20%) of these being sustained.

If we consider rats dying during the surgical preparation for PES as dying of ventricular arrhythmias and attribute to them a score of 8, then 10 (40%) of 25 control SHRs had inducible arrhythmias, three (15%) of 20 propranolol-treated rats had inducible arrhythmias (p = 0.066 vs. control), six (29%) of 21 hydralazine-treated rats had inducible ventricular arrhythmias (p = 0.42 vs. control), and one (5%) of 20 captopril-treated rats had inducible ventricular arrhythmias (p < 0.05 vs. control; p = 0.045 vs. hydralazine; p = 0.29 vs. propranolol).

Arrhythmia score decreased significantly only in rats treated with captopril as compared with control (p = 0.044; Fig. 2). Again, a beneficial trend was noted with propranolol (p = 0.058), although this did not reach statistical significance. Hydralazine had no effect on inducibility score (p = 0.421).

Mortality after infarction

After left anterior descending ligation, SHRs were monitored via ECG, and deaths were recorded. Survival over the first 3 h after infarction was analyzed by standard Kaplan-Meier analysis with the log-rank test and χ2 analysis (Fig. 3).

FIG. 3
FIG. 3:
Kaplan-Meier curves of survival after acute myocardial infarction according to treatment groups. At 1 h, survival appears to be better in captopril- and hydralazine-treated rats. However, by 3 h, only captopril-treated rats had a better survival. The biphasic response in hydralazine-treated rats would suggest two temporally different arrhythmia-related mechanisms for death, the first, affected by hydralazine treatment, and the second, not.

At 1 h after MI, six (24%) of 25 SHRs had died in the control group compared with six (30%) of 20 in the propranolol-treated, two (9.5%) of 21 in the hydralazine-treated, and only one (5%) of 20 in the captopril-treated groups. At this time, there was a trend toward improved survival in the hydralazine-treated (p = 0.20 vs. control and p = 0.10 vs. propranolol) and captopril-treated (p = 0.08 vs. control and p = 0.03 vs. propranolol) groups. At 2 h after MI, 10 (40%) of 25 SHRs had died in the control group compared with eight (40%) of 20 in the propranolol-treated, four (19%) of 21 in the hydralazine-treated, and four (20%) of 20 in the captopril-treated groups. Again a trend toward improved survival was noted with hydralazine versus propranolol (p = 0.12) and versus control (p = 0.10), with captopril versus propranolol (p = 0.15) and versus control (p = 0.1). Finally, at 3 h after MI, a total of 18 (72%) of 25 SHRs had died in the control group, compared with 14 (70%) of 20 in the propranolol-treated, 14 (68%) of 21 in the hydralazine-treated, and eight (40%) of 20 in the captopril-treated groups. Thus only long-term treatment with captopril improved 3-h postinfarction survival in SHRs (p = 0.022 vs. control and p = 0.048 vs. propranolol).

Morphologic changes

As compared with control SHRs, left ventricular weight/body weight ratio decreased from 2.8 ± 0.2 to 2.6 ± 0.2 mg/g (p < 0.05) in the hydralazine-treated and to 2.2 ± 0.2 mg/g (p < 0.05) in the captopril-treated SHRs. Left ventricular weight/body weight ratio did not change in the propranolol-treated SHRs (2.8 ± 0.3 mg/g). Both endocardial and epicardial circumferences of the left ventricle were decreased in hydralazine- and captopril-treated rats (Table 3, Figs. 4 and 5).

TABLE 3
TABLE 3:
Cardiac morphologic characteristics
FIG. 4
FIG. 4:
Midventricular cross sections of hearts from all four groups. A: Saline. B: Propranalol treated. C: Hydralazine treated. D: Captopril treated. Although some degree of regression of hypertrophy occurred in the hydralazine- and propranalol-treated rats, the most marked decrease was in the captopril-treated rats.
FIG. 5
FIG. 5:
Midventricular cross sections of hearts from all four groups. A: Saline. B: Propranolol treated. C: Hydralazine treated. D: Captopril treated. Slight regression of fibrosis occurred with hydralazine and propranolol and marked regression occurred with captopril.

Collagen quantification by computer-assisted image analysis

Cardiac fibrosis decreased most in captopril-treated hearts. Propranolol-treated hearts also had a significant decrease in fibrosis. The decrease with hydralazine was more marginal and reached statistical significance in only one of the two laboratories. This decrease was largely limited to type I collagen, the volume percentage decreasing from 3.0 in controls to 1.64 in the propranolol group, to 1.51 in the captopril group, and to 2.13 in the hydralazine group, all with p < 0.05. An assessment of total cardiac fibrosis was obtained by multiplying collagen volume percentage by heart weight and correcting for body weight. With this method, again captopril decreased fibrosis the most (>50%). Both other interventions also decreased cardiac fibrosis (hydralazine 15% and propranolol 30%; Table 4, Fig. 5).

TABLE 4
TABLE 4:
Cardiac fibrosis by computer-assisted image analysis

DISCUSSION

This study indicates that only the changes caused by 12 weeks of angiotensin-converting enzyme (ACE)-inhibition therapy results in a decrease of both early (<1 h) and later (≤3 h) periinfarction mortality in the SHR. The changes induced by hydralazine tended to reduce early mortality, but this advantage was lost by 3 h. Propranolol had no beneficial effects on either early or later mortality despite decreasing preinfarction inducibility of ventricular arrhythmias. If we consider the hemodynamic and morphologic changes caused by these various interventions, it would appear that regression of myocardial hypertrophy or long-term normalization of arterial pressure or both are the major determinants of early mortality and that longer-term mortality (3 h) may be the result of a more complex interplay of long-term normalization of arterial pressure, of regression of myocardial hypertrophy and cardiac fibrosis, and perhaps of the direct vascular effects of the drugs used.

The hemodynamic and morphologic effects of the various pharmacologic interventions that were performed on these SHRs are compatible with those reported by others (14,17,21-26). Captopril decreased arterial pressure without increasing heart rate, and resulted in marked regression of both LVH and cardiac fibrosis. Propranolol decreased heart rate and decreased cardiac fibrosis but caused only an insignificant decrease in LVH. Hydralazine decreased blood pressure but caused reflex tachycardia. This was accompanied by a reduction of LVH less than that by captopril, and only a marginal decrease in fibrosis. The very high periinfarction mortality rate in the SHRs (72% at 3 h) was consistent with previous reports of a high mortality in hypertension and hypertrophied hearts (40). This study indicates that the control of blood pressure and regression of myocardial hypertrophy and cardiac fibrosis with captopril resulted in a reduction in periinfarction mortality to levels similar to those of normal rats (40% at 3 h; 18).

Thirty-minute survival

Arrhythmias in normal rats with an acute MI occur in two distinct phases (18). The results of this in vivo study suggest that two distinct arrhythmogenic phases also occur in the SHR, although the temporal distinction between the two may not be as clear as in normal rats. The first phase occurs within the first 5 min of the onset of ischemia and persists for ≥30 min. It is characterized by a marked spatial heterogeneity, slowed conduction, and activation delay in the epicardial segments of the underperfused zone (41-43). This study suggests that mortality in the first phase in the SHR appears to be largely dependent on the degree of myocardial hypertrophy, as the two interventions that decreased myocardial hypertrophy the most, captopril and hydralazine, resulted in the greatest decrease in early mortality (<1 h). These findings are consistent with the findings of Kohya et al. (17) that the spontaneous development of ventricular arrhythmias in a Langendorff-perfused heart during an acute MI are closely related to the degree of myocardial hypertrophy. In their study, they demonstrated that myocardial hypertrophy leads to prolongation of action-potential duration, which during ischemia shortens action-potential duration to a greater extent than it does in nonhypertrophied myocardium. This results in greater dispersion of action-potential duration between ischemic and nonischemic hypertrophied myocardium as compared with what happens in nonhypertrophied myocardium, thereby creating a substrate for arrhythmias. ACE inhibition, by decreasing LVH, was shown to reduce action-potential duration and dispersion and to reduce the risk of arrhythmias (17,44). In our study, we did not, however, find a close relation between mortality and the degree of regression of myocardial hypertrophy, as was found by Kohya et al. (17). Regression was much greater in captopril-treated rats as opposed to hydralazine-treated rats, but early mortality tended to be decreased to a similar extent in both groups. As nonlethal arrhythmias are frequent in the early postinfarction period in the rat, our results are not necessarily incompatible with those of Kohya et al. (17) but suggest that in vivo, other factors not present in the in vitro situations may come into play, which may complicate the relation between the incidence of arrhythmias and survival.

Our results also raise the possibility that long-term normalization of arterial blood pressure may contribute to improved survival, perhaps by improving endothelial function. In this study, the two interventions that had the greatest hypotensive effect also appeared to cause the greatest decrease in early postinfarction mortality. Against this possibility is the lack of a simple correlation between normalization of blood pressure and inducibility of ventricular arrhythmias found by Kohya et al. (17). The relation between blood pressure control and survival in our study is complicated by the fact that all medications were stopped 72 h before the PES and the production of the infarction. As we did not measure arterial pressure at the time of the PES and the creation of the MI, some carry-over of the long-term hypotensive effects of these drugs cannot be ruled out. Indeed, long-term treatment of the SHRs with captopril has been shown effective in producing a persistent decrease in blood pressure long after the medication is stopped (45,46), although this effect is much less when treatment is started in older SHRs (47), as was the case in this study (13-week-olds). Any persistent decrease in arterial pressure may be expected to reduce infarction size (40). However, against an important effect of any carry-over hypotensive effect on early mortality, our studies indicated a rapid loss of the hypotensive effects of hydralazine in the SHRs when hydralazine is stopped (45). Finally, it is possible that the normalization of vascular endothelium-dependent vasodilatation as a result of normalization of arterial pressure may have played a contributory role in the hydralazine and captopril groups (48-54).

Regression of cardiac fibrosis did not appear significantly to modify early survival: the decrease in cardiac fibrosis was most consistent in captopril- and propranolol-treated hearts, but mortality was decreased only in captopril-treated hearts. However, consistent with previous studies, susceptibility to complex ventricular arrhythmias induced by PES was significantly correlated to cardiac fibrosis (4,16), susceptibility being decreased in only propranolol- and captopril-treated hearts. Unfortunately, it would appear that susceptibility to ventricular arrhythmias induced by PES was not necessarily correlated with early survival. Taken together, these findings suggest that early postinfarction survival in the SHR is most closely related to regression of myocardial hypertrophy, the extent of regression needed to improve survival not being entirely clear. The possibility that long-term normalization of arterial pressure has a beneficial effect also cannot be ruled out.

Three-hour survival

In the normal Wistar rat, a second much more severe phase of ectopic activity begins 90 min after occlusion (18), by which time the necrotic wavefront of irreversible cell damage has penetrated most of the ischemic area (55). This follows a quiescent period of 60-90 min and lasts up to 9 h after occlusion (18). The origin of these arrhythmias may be the interface between dead and still viable but ischemic myocardium, where depolarized myocytes can develop abnormal automaticity (56). In this study, a clear demarcation between the earlier phase and the later phase was not so obvious as in normal rats, perhaps because in SHRs, greater myocardial damage occurs more quickly, action-potential duration dispersion is marked, and greater fibrosis increases the risk of reentry arrhythmias. Another possibility is that anesthesia influenced the duration or transition or both between phases. Nevertheless, from this study, a second phase also appears to occur in the SHR (4), at least as it relates to the mechanism of arrhythmias. That a second mechanistic phase exists in the SHR also is supported by the loss of the apparent early protective effect of hydralazine between 2 and 3 h after infarction.

What morphologic or electrophysiologic differences between captopril- and hydralazine-treated rats influence the lethality of later arrhythmias is uncertain and cannot be clearly identified in this study. Captopril-treated hearts had significantly less myocardial hypertrophy, which may have reduced action-potential duration dispersion around the necrotic wavefront. Because of hypertension and LVH, the wavefront may be greater in the SHR than in normal rats and may extend over a longer period. The appearance of afterpotentials, which are much more frequent in the untreated SHR (4), may also have been the origin of some of the severe and lethal arrhythmias during this second phase. The other possible mechanism is a greater decrease in cardiac fibrosis in the captopril-treated rats. Hydralazine-treated hearts had much less decrease in fibrosis than did captopril-treated hearts. In the SHR, a good correlation exists between inducibility of ventricular arrhythmias by PES and cardiac fibrosis (4,16) and, although we could find no good correlation between inducibility of ventricular arrhythmias and early-phase mortality, as captopril caused the greatest decrease in fibrosis, this may have played a role in its beneficial effects.

As was the case with the earlier-phase survival, because some carry-over effect on blood pressure likely occurred in the captopril-treated SHRs, an effect of reduced blood pressure on later-phase mortality remains a possibility. Another possibility related to this last one is the chronic coronary vascular changes caused by long-term captopril use in the SHR. Long-term captopril use in the SHR has been shown to reduce vascular hypertrophy and fibrosis as well as cardiac hypertrophy and fibrosis (26,50,52,53,56,57) and to normalize vascular endothelial function (58). Such vascular changes may be expected to reduce infarct size and may have contributed to our findings. Interestingly, hydralazine has not been shown to do this as well as captopril, despite decreasing blood pressure to a similar extent (34,49). Considering the evidence for all these mechanisms, it is likely that the lower mortality in the captopril-treated rats is a result of a combination of all these mechanisms.

Study limitations

The decrease in blood pressure with propranolol was not so great as with the other interventions. This is a well-known finding in the SHR and limits some of the comparisons between treatment groups. However, because propranolol reduced inducibility of arrhythmias by PES without so great a decrease in LVH and blood pressure, we were able to dissociate the effects of modified PES inducibility from those of LVH and blood pressure to a degree not otherwise possible. As already discussed, although all medications were discontinued 72 h before the interventions, some carry-over of the chronic cardiovascular changes caused by these drugs on arterial pressure cannot be ruled out. Also, our rats were anesthetized, which is known to modify the incidence of arrhythmias somewhat and may also have modified mortality (19). Finally, we did not measure infarct size, and an imbalance between groups is possible and may have influenced our results, particularly if it occurred in the captopril group. However, in our hands, moderate and large infarctions in the SHR are produced in >90% of cases (unpublished results), making this possibility quite unlikely.

Potential clinical implications

Results from this study suggest that, as occurs in normal hearts, two distinct phases of arrhythmias occur after infarction in hypertrophied hearts and that, as opposed to normal hearts, the temporal difference in mortality between phases is less clear. Nevertheless, a clear temporal distinction in mechanisms seems to occur. Control of hypertension and regression of LVH with captopril reduces the risk of periinfarction (3-h) mortality in the SHR, suggesting that such an effect could be seen in the hypertensive patient. However, the effects of captopril cannot be extrapolated to those of other antihypertension agents, suggesting that changes specific to ACE inhibitors may be important. Regression of myocardial hypertrophy appears to be the major factor determining overall prognosis, although some contributory role for regression of fibrosis and blood pressure control appears likely. Results from this study thus suggest that much of the excess periinfarction risk associated with LVH can successfully be eliminated by its regression with an ACE inhibitor and supports regression of LVH as being a valid therapeutic goal.

Acknowledgment: This work was supported by the Medical Research Council of Canada.

We are grateful to Dr. Jean-Gilles Latour (Montreal Heart Institute) and to Dr. Paul Brazeau and Ms. Soraya Allas (Notre-Dame Hospital) for collagen quantification. We thank Bristol-Myers Squibb for the generous supply of captopril for this study, and Carolyn Gillis for her secretarial work.

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Keywords:

SHR; Captopril; Ventricular arrhythmias; Mortality periinfarction; Concentric myocardial hypertrophy; Cardiac fibrosis

© 1998 Lippincott Williams & Wilkins, Inc.