Effect of Left Ventricular Hypertrophy and Its Regression on Ventricular Electrophysiology and Vulnerability to Inducible Arrhythmia in the Feline Heart
Background Left ventricular hypertrophy (LVH) is associated with an increased risk of death, susceptibility to ventricular arrhythmia, and multiple electrophysiological abnormalities. The purpose of the present study was to determine whether the susceptibility to arrhythmia and electrical abnormalities persists after regression of hypertrophy in an animal model of LVH.
Methods and Results We placed constricting bands on the ascending aorta of cats (n=9) or performed sham operations (n=9). Serial cardiac echocardiography was performed to measure left ventricular wall thickness. After LVH had developed in the banded animals, the constricting bands were removed and serial echocardiograms were used to monitor for regression of hypertrophy. Electrophysiological studies were performed in cats that showed regression of LVH (Regress, n=5), those that showed no change in LV wall thickness (No Regress, n=4), and in the sham-operated animals (Sham). Cats with persistent LVH had a higher incidence of inducible polymorphic ventricular tachycardia (4 of 4) compared with Regress (1 of 5) or Sham (1 of 9) cats (P<.05) and had lower ventricular fibrillation thresholds (9±2 mA) than Regress (17±4 mA) or Sham (16±3 mA) cats (P<.05). Persistent LVH in the No Regress group was associated with prolongation of epicardial monophasic action potential duration (MAPD) in the left but not the right ventricle. Dispersion of refractoriness was greater in the No Regress group (P<.05 versus Regress or Sham). Regress cats were identical to Sham cats in having a low incidence of inducible polymorphic ventricular arrhythmia, high fibrillation threshold, and MAPD measurements (P=NS versus Sham).
Conclusions LVH produces multiple electrophysiological abnormalities and increased vulnerability to inducible polymorphic ventricular arrhythmia in this model of LVH. Cats that show regression of hypertrophy have normal ventricular electrophysiology and have the same low vulnerability to inducible ventricular arrhythmia as Sham animals.
Left ventricular hypertrophy (LVH) is a known risk factor for cardiovascular disease and is associated with an increased risk of sudden death.1 2 3 The increased risk of sudden death is thought to be due to malignant ventricular arrhythmia because of an increased frequency and complexity of ventricular ectopy detected by Holter monitoring in patients with LVH.4 5 Further, electrophysiological study of symptomatic patients whose only structural abnormality is an increased left ventricular mass has shown an increase in inducible arrhythmia compared with similar patients with normal LV mass.6
Animal models of LVH have shown multiple electrical abnormalities that may explain the increased incidence of arrhythmia in patients with LVH. These include increased vulnerability to inducible ventricular arrhythmia,7 8 9 dispersion of monophasic action potential duration and refractoriness,8 9 action potential prolongation,10 11 12 and an increased susceptibility to afterdepolarizations,12 13 triggered activity,14 ischemia,15 16 and reperfusion arrhythmia.17 18 The most frequently observed finding from several animal models of LVH has been action potential prolongation.7 8 9 10 11 12 19 Voltage clamp studies of hypertrophied cells suggest that this may be due to a decrease in outward current via the delayed rectifier channels.18 19
If increased myocardial mass alters ventricular electrophysiology so as to increase the risk of malignant tachyarrhythmia, then interventions that reduce ventricular mass may decrease that risk. Clinical studies have shown that LVH will regress when patients are treated with a variety of antihypertensive agents20 and have shown a decrease in spontaneous ventricular ectopy in patients with hypertension who were treated with agents that normalize LV mass.21 22 However, it is not clear whether the decrease in ectopy is due to regression of hypertrophy or to a direct antiarrhythmic effect of the drugs.
We have observed an increased incidence of inducible arrhythmia in a cat aortic-band model of LVH. To determine whether inducible arrhythmia persists after regression of hypertrophy, we removed the aortic band after LVH had developed. We then performed electrophysiological studies (EPS) in animals that showed regression of hypertrophy, animals that showed persistent hypertrophy, and in sham-operated animals.
This study was carried out in conformance with the guidelines published by the American Heart Association, and the protocol was approved by the Institutional Animal Use Committee. Semiconditioned cats of either sex were sedated with ketamine (10 mg/kg IM) and underwent transthoracic M-mode echocardiography using an imaging system with a 5-MHz phased-array transducer (Sonos 1000, Hewlett Packard) by experienced echocardiographers (F.J.P., S.V.A.) who were blinded to treatment group. Diastolic measurements of the LV posterior wall thickness were made from the M-mode image in the parasternal short-axis view at the level between the mitral valve and papillary muscle by use of the leading-edge method. Adequate windows to allow estimation of LV mass were not obtainable.
After baseline echocardiography had been performed, the cats underwent aortic banding or a sham operation. The cats were anesthetized with ketamine (20 mg/kg IV) and acepromazine (1.1 mg/kg IV), intubated with a cuffed endotracheal tube, and ventilated with room air with a Harvard respirator. Under sterile conditions, a right anterior thoracotomy was performed one interspace superior to the cardiac apical impulse. The pericardium was opened, and the plane between the aorta and pulmonary artery was dissected. In 14 animals, a band consisting of polyethylene tubing with 24-gauge copper wire and 000 polypropylene suture through its lumen was preformed into a circle with an internal diameter of 3.2 to 3.5 mm, then placed around the ascending aorta and sutured into place. In the other 9 animals, banding was not carried out. The chest was closed, and the animals were allowed to recover. Buprenex (0.06 to 0.09 mg SC) was administered as needed for analgesia until the cats had returned to normal activity. Penicillin (300 000 U SC) was given the day before, the day of, and the day after surgery as prophylaxis against wound infection.
Biweekly echocardiographic studies were then performed. When the banded animals had shown at least a 40% increase in posterior LV wall thickness, the cats were anesthetized and ventilated as before, repeat right anterior thoracotomy was performed, the aortic band was dissected free, and the chest was closed. The animals were allowed to recover, and biweekly echocardiographic studies were resumed.
Electrical testing was performed when LV wall thickness either had returned to baseline or had shown no significant change (defined as <20% change from peak LVH value) after three consecutive echocardiograms. Animals were then anesthetized with 70 mg/kg IP of α-chloralose. The femoral artery and vein were cannulated to measure blood pressure and administer drugs. A thoracotomy was performed, and the pericardium was opened. Paired transmural plunge electrodes were positioned on the anterior, posterior, lateral, and apical left ventricle, right ventricular (RV) outflow tract, and RV apex. Endocardial pacing catheters were passed from the atrial appendages to the right and left ventricular apexes and secured with purse-string sutures.
After electrical testing, the animals were killed and the hearts removed. The LV posterior wall thickness was measured and the left ventricle weighed after total heart weight was recorded and both atria and the free wall of the right ventricle were excised.
A Bloom stimulator (Bloom Ltd) was used to deliver constant-current rectangular impulses with pulse widths of 2 ms at twice diastolic threshold. Electrograms were amplified and recorded on a VR 12 Physiologic Recorder (Electronics-for-Medicine). Ventricular effective refractory periods (ERP) were obtained by use of extrastimuli delivered in 10-ms decrements after ventricular drive trains at the longest cycle length that consistently captured the ventricle. The ventricular drive cycle lengths were not significantly different between groups. The ERP was recorded as the longest S1-S2 coupling interval that failed to capture the ventricle. ERP measurements were made with each of the paired transmural plunge electrodes and the RV endocardial electrode. Thus, ERP measurements were made at four LV sites and three RV sites.
Monophasic action potentials were recorded on the epicardial surface adjacent to the site of plunge electrode placement by use of a handheld Franz contact electrode (model 501, EP Technologies Inc) during ventricular pacing at the same cycle length as used to measure ERP. Recordings were obtained adjacent to the epicardial plunge electrodes, yielding 4 LV sites and 2 RV sites. Satisfactory recordings were obtained from every animal. Monophasic action potential durations (MAPD) were measured where the downward deflection of the signal had reached 50% (MAPD50) and 90% (MAPD90) of complete repolarization. Dispersion of ERP or of MAPD was defined as the maximum difference among all sites tested either within the right ventricle (3 sites), within the left ventricle (4 sites), or between the right and left ventricles (7 sites).
Inducibility of ventricular arrhythmia was assessed with the endocardial catheters. With extrastimuli of twice threshold intensity, a 2-ms extrastimulus was delivered after 8 beats of ventricular drive. The method of extrastimulation used was the same as reported previously.6 An initial extrastimulus (S2) was set 250 ms after the pacing artifact of the last drive train beat and delivered decrementally until refractoriness. If a sustained arrhythmia was not provoked, a second (S3) and then a third (S4) extrastimulus were introduced. The end point of testing was the induction of a sustained arrhythmia (lasting >30 seconds or causing hemodynamic instability). If an arrhythmia was induced, the testing was repeated to ensure reproducibility.
The endocardial catheters were also used to measure ventricular fibrillation thresholds (VFT). For this purpose, we used the single-stimulus technique described previously.6 A 5-ms extrastimulus was introduced in mid electrical diastole during ventricular pacing at an initial intensity of 2 mA. The extrastimulus was decremented by 10 ms until refractoriness was reached. The current was increased in 2-mA increments, and electrical diastole was scanned as before. Current intensity was increased by 2 mA, and scanning continued until VF was induced. The arrhythmia was terminated within 15 seconds of its initiation with a 5-J shock applied directly to the heart. Testing was repeated to ensure reproducibility.
Results are expressed in text, tables, and the Figure⇓ as mean±SD. Statistical analysis was performed with the True Epistat 4.0 statistical package. A Student’s t test was used for simple two-way comparisons. Multiple comparisons over time or between groups were first performed with repeated-measures ANOVA or one-way ANOVA, respectively. If ANOVA indicated that differences between all means existed, the Newman-Keuls procedure for multiple comparisons was then performed. Results were considered significant at P<.05.
Echocardiographic data were normalized for each animal’s baseline measurement to adjust for baseline differences in ventricular wall thickness. The data and statistical analysis are those of the normalized values. The number of cats inducible to sustained ventricular arrhythmia was analyzed as a contingency table using the contingency table randomization test, since the small numbers in each group precluded use of a χ2 test.
Fourteen cats underwent aortic banding, and 9 cats underwent sham operation. Thirteen of the 14 banded cats developed LVH after a mean of 93 days (range, 67 to 136 days). One of the 13 died of CHF, and another died suddenly. The remaining 11 cats had the aortic band removed, and 9 survived to completion of the protocol. These 9 animals and the 9 sham-operated animals form the basis of this report.
Five of the banded cats showed complete or partial regression of LVH within 78 days (defined as at least a 20% reduction of wall thickness from peak hypertrophy) after removal of the aortic band (Regress group), and 4 showed no regression of LVH (No Regress group) (Table 1⇓). At peak hypertrophy, wall thickness had increased to 158% of baseline in the Regress group, 163% in the No Regress group, and 117% during the same time interval in the sham-operated (Sham) group (Table 1⇓). The increase in posterior wall thickness was significantly greater in both banded groups (Regress and No Regress) compared with the Sham group (P<.05).
By definition, posterior wall thickness decreased in the Regress group after removal of the aortic band, from 158% of baseline at peak LVH to 121% of baseline at the time of EPS, and was not significantly different from that of Sham animals at that time (Table 1⇑). No Regress cats showed no significant change in posterior wall thickness after removal of the aortic band and had significantly thicker walls than either the Regress animals or the Sham animals at the time of EPS. The No Regress cats had higher ratios of LV wall thickness to body weight than the Regress or Sham animals at the time of EPS (Table 1⇑). Measurement of peak LV systolic pressure and peak femoral artery pressure indicated that a modest gradient persisted in the No Regress group. Sham animals showed a 23% increase in posterior wall thickness over the course of the experimental protocol (P<.05 versus baseline).
Inducibility of Ventricular Arrhythmia and VFT Measurements
At EPS, cats that showed regression of hypertrophy (Regress) had a low incidence of inducible arrhythmia (1 of 5 induced to polymorphic ventricular tachycardia) and had a VFT that was not significantly different from that of Sham animals (Table 2⇓). In contrast, all four cats with persistent LVH (No Regress) were inducible to sustained polymorphic ventricular tachycardia that degenerated into ventricular fibrillation and had significantly lower VFTs than either Sham cats or Regress cats (Table 2⇓).
MAPD and ERP Measurements in Cats With Persistent LVH
Persistent LVH was associated with multiple electrophysiological abnormalities. Cats with persistent LVH showed prolonged MAPD50 and MAPD90 in the hypertrophied LV compared with Sham animals (Figure⇑; Table 3⇓). Maximum dispersion of MAPD within the LV alone was not increased, but dispersion of MAPD between the left and right ventricles was increased in the No Regress group. Mean LV ERP was also increased in the No Regress group compared with Sham, as was dispersion of refractoriness within the LV and between the LV and RV (Table 3⇓). Mean RV ERP and dispersion of RV ERP were not increased.
MAPD and ERP Measurements in Cats With Regression of LVH
Cats in the Regress group did not show the abnormal ventricular electrophysiology seen in No Regress cats (Table 3⇑). In addition to the low incidence of inducible arrhythmia and normal VFT already mentioned, LV MAPD at either 50% or 90% repolarization and LV ERP were not prolonged, dispersion of MAPD within the LV or between the LV and RV was not increased, and dispersion of refractory periods was not increased (P<.05 Regress versus No Regress). No statistical difference was found in these parameters when the Regress group and the Sham group were compared (P=NS).
LVH is associated with multiple electrophysiological abnormalities and an increased risk of sudden death. However, we do not know whether the electrical abnormalities or increased risk of death persists after regression of hypertrophy. This issue is of considerable clinical significance, since some therapies for hypertension, the most common cause of LVH, produce regression of hypertrophy and some do not. If the electrical abnormalities produced by LVH are responsible for the increased risk of sudden death and if these abnormalities resolve with regression of LVH, then therapies that produce regression should reduce the risk of arrhythmic death. If the electrical abnormalities persist despite reduction in LV wall thickness, then patients may remain at risk for sudden death regardless of which therapy is used.
In this animal model, cats that show regression of hypertrophy have the same low vulnerability to ventricular arrhythmia as Sham animals. Cats that do not show regression of hypertrophy remain vulnerable to ventricular arrhythmia despite removal of the stimulus for LVH. These results suggest that the vulnerability to ventricular arrhythmia produced by LVH is reversible. In addition, our data suggest that of the multiple electrical abnormalities associated with LVH, prolongation of MAPD and the subsequent dispersion of ERP are also reversible. Whether the electrical abnormalities produced by LVH in other models (eg, afterdepolarizations) are reversible remains to be determined.
The novel finding that ventricles that show regression of hypertrophy do not have prolongation of MAPD suggests that the mechanism responsible for action potential prolongation is reversible. Voltage-clamp studies suggest that action potential prolongation associated with hypertrophy is primarily due to altered outward potassium current.18 19 Possible mechanisms by which this current could be reversibly altered are numerous. The stimulus to hypertrophy might cause altered expression of genes that encode for potassium channels, leading to a different population of ion channels when the cell is hypertrophied.23 Alternatively, the behavior of existing channels could be modified by reversible fibrosis, neurotransmitter activity, or endocrine factors.
The reduced vulnerability to ventricular arrhythmia in cats that show regression of hypertrophy is consistent with two clinical studies.21 22 Both found a decrease in ventricular ectopy in hypertensive patients with echocardiographic evidence of LVH who were treated with drugs that produced regression of LVH.21 22 Our data suggest that the reduction in ventricular ectopy in the clinical studies is due to normalization of ventricular electrophysiology and not to an antiarrhythmic effect of the drugs used. Indeed, one study21 used a calcium channel blocker, which we have previously shown to have no direct antiarrhythmic effect in this animal model of LVH.9
The vulnerability to ventricular arrhythmia and electrophysiological abnormalities observed in cats with persistent LVH reproduces many of the findings of our previous work with this animal model.8 9 In the present study, mean MAPD was prolonged, but dispersion of MAPD was not. This may be because of a shorter period during which the animals were banded in the present study (mean, 93 days) compared with our earlier studies (Reference 8, mean, 150 days; Reference 9, mean, 140 days). Further, the finding of prolonged MAPD in the present study is consistent with similar observations in other animal models of LVH.7 10 11 12 19 Prolongation of action potential duration may not be uniform at the cellular level,18 and this may explain the dispersion of MAPD and ERP noted in our work and that of others.24 25
Ventricular hypertrophy is associated with nonuniform action potential prolongation, dispersion of refractoriness, transient depolarizations, early afterdepolarizations, triggered activity, and an increased sensitivity to both ischemia and reperfusion. The extent to which each individual electrophysiological abnormality contributes to the increased susceptibility to ventricular arrhythmia in our animal model, in other animal models, or in humans is not clear. Further, LVH is often associated with neuroendocrine, metabolic, and structural abnormalities that also resolve with regression of hypertrophy. Whether these abnormalities contribute to vulnerability to arrhythmia should also be considered.
Dispersion of action potential duration and refractoriness have been reported in clinical24 and in other animal studies of LVH.8 9 25 This dispersion may contribute to vulnerability to arrhythmia by creating pathways for functional reentry. We postulate that dispersion of refractoriness would juxtapose areas of relatively short refractoriness with areas of longer refractoriness and lead to slowing or block of conduction. Since hypertrophy in response to increased afterload is a diffuse process, the number of possible reentrant pathways might be quite large. The availability of multiple functional reentrant pathways would be expected to make the ventricles susceptible to polymorphic arrhythmia, as was observed in this study.
Persistent LVH in our study was associated with a residual pressure gradient between the LV and the femoral artery. Postmortem examination of these animals showed a persistent stenosis at the site at which the aortic band had been placed that was the result of fibrous tissue. The reverse gradient in peak systolic pressure measured between the LV and femoral artery in the Sham and Regress groups is probably due to systolic pressure amplification that is thought to result from reflection of pressure waves.26 27 We also observed that posterior wall thickness increased over the course of the experiment in the Sham group as the animals aged. This paralleled an increase in total body weight.
Our study has several important limitations. We used a constricting band placed on the ascending aorta of adult cats to provoke LVH. The abrupt increase in afterload produces LVH in the clinically relevant ventricle in a manner that is reversible without drugs but produced a greater degree of hypertrophy than is typically seen in clinical situations (58% increase in wall thickness). Results observed in this animal model may not apply to clinical LVH or to other animal models of LVH in which there is a gradual increase in afterload or a less pronounced degree of hypertrophy. For example, although one canine model of LVH has demonstrated action potential prolongation,12 there is not an increased incidence of inducible arrhythmia in that model.28 Whether this discrepancy is due to the method of producing LVH or the degree of LVH or whether it is species related is unclear, but this uncertainty emphasizes the caution with which our results should be interpreted.
The development of LVH and its subsequent regression was assessed in this study by M-mode echocardiographic techniques used by experienced echocardiographers. Limited echocardiographic windows in the cat precluded more sophisticated assessments of LV mass. The finding of an increase in LV wall thickness per kilogram body weight and electrophysiological abnormalities in animals identified by M-mode echocardiography to have LVH reproduces our previous studies and supports the validity of our experimental approach. Finally, our study involved relatively small numbers of animals in each group. The dramatic differences between cats with and cats without regression allowed us to attain statistical significance despite this limitation.
The authors wish to thank Rose Marie Wells for her expert help in the preparation of the manuscript and the staff of the Animal Care Facility at the Lankenau Medical Research Center for their help in the care of our experimental animals.
Presented in part at the 13th Annual Scientific Sessions of the North American Society of Pacing and Electrophysiology, May 1992, Chicago, Ill.
- Received May 3, 1994.
- Accepted August 8, 1994.
- Copyright © 1995 by American Heart Association
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