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(Circulation. 1999;99:903-908.)
© 1999 American Heart Association, Inc.
Clinical Investigation and Reports |
From the Division of Cardiology, Department of Medicine, St Luke'sRoosevelt Hospital Center, and Columbia University College of Physicians and Surgeons, New York, NY.
| Abstract |
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Methods and ResultsThis prospective study enrolled
consecutive patients undergoing CABG at a single institution. Patients
were followed up for the development of sustained VT, and a detailed
analysis of clinical, angiographic, and surgical variables
associated with the occurrence of VT was performed. A total of 382
patients participated, and 12 patients (3.1%) experienced
1 episode
of sustained VT 4.1±4.8 days after CABG. In 11 of 12 patients, no
postoperative complication explained the VT; 1 patient had a
perioperative myocardial infarction. The in-hospital
mortality rate was 25%. Patients with VT were more likely to have
prior myocardial infarction (92% versus 50%, P<0.01),
severe congestive heart failure (56% versus 21%,
P<0.01), and ejection fraction <0.40 (70% versus
29%, P<0.01). When all 3 factors were present, the
risk of VT was 30%, a 14-fold increase. Patients with VT had more
noncollateralized totally occluded vessels on angiogram (1.4±0.97
versus 0.54±0.7, P<0.01), a bypass graft across a
noncollateralized occluded vessel (1.50±1.0 versus 0.42±0.62,
P<0.01), and a bypass graft across a noncollateralized
occluded vessel to an infarct zone (1.50±1.0 versus 0.17±0.38,
P<0.01). By multivariate
analysis, the number of bypass grafts across a
noncollateralized occluded vessel to an infarct zone was the only
independent factor predicting VT.
ConclusionsThe first presentation of sustained monomorphic VT in the recovery period after CABG is uncommon, but the incidence is high in specific clinical subsets. Placement of a bypass graft across a noncollateralized total occlusion in a vessel supplying an infarct zone was strongly and independently associated with the development of VT.
Key Words: bypass surgery tachyarrhythmias
| Introduction |
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CABG clearly performs an antiarrhythmic function in selected patients who have experienced previous ventricular tachycardia (VT) and/or ventricular fibrillation (VF). There is, however, a specific subset of patients whose ventricular arrhythmias are aggravated by the surgical procedure. This may take the form of the benign presence of increased ventricular ectopy,4 but more importantly, the de novo occurrence of ventricular tachyarrhythmia as a consequence of bypass tract surgery has been reported.5 6 7 8 9 10 Several case reports and small series5 6 7 8 9 10 have anecdotally described the development of sustained VT in the recovery period after CABG, but the incidence, mortality rate, long-term follow-up, and mechanism underlying arrhythmia occurrence are poorly defined.
The present prospective study was designed to fully characterize the clinical characteristics and course of patients who develop sustained monomorphic VT in the recovery period after CABG. In particular, a detailed analysis was performed in an attempt to better understand the mechanism underlying VT development in this unique group of patients, with a specific emphasis on the relationship of coronary reperfusion and its potential proarrhythmic effect.
| Methods |
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30 seconds unless terminated earlier
because of hemodynamic collapse. There was no upper
rate cutoff for VT rate. VT occurring
24 hours after surgery during
the recovery phase was included. VT/VF on the day of surgery was
excluded because of the difficulty in differentiating causation in the
very early postoperative setting. VT occurring within 30 days of
surgery after patients were discharged from the hospital also met
end-point criteria. Events occurring beyond 30 days were excluded.
Patients who had a previous history of sustained VT or cardiac arrest
before the performance of CABG were excluded from the
study. With the development of VT, standard advanced cardiac life support interventions were made to terminate the arrhythmia and to stabilize the patient. Clinical management of the tachycardia, including diagnostic evaluation, electrophysiology investigation, and long-term treatment, were performed at the discretion of the primary physician and appropriate consultants and according to generally accepted clinical principles. The records were reviewed in detail at the time of each event to assess the presence of any acute reversible or correctable cause of the arrhythmia, including metabolic alterations, myocardial ischemia/infarction, pericarditis, congestive heart failure (CHF), and potentially proarrhythmic medications.
Analysis of Coronary Arteriogram and CABG
Procedure
The results of the preoperative coronary arteriogram
were reviewed and recorded for each patient. The presence of
significant stenoses, defined as
70% stenosis of a
major epicardial vessel, was noted. The number and location of these
lesions were described. In the presence of a total occlusion, the
presence of collateral circulation to the occluded bed was also
recorded. Vessels were defined as a noncollateralized occluded
vessel if there was no visible distal flow from any source, including
collaterals, in a major epicardial vessel distribution.
Surgical variables were routinely collected. The surgical cross-clamp duration and details of bypass graft placement and numbers were noted. Bypass grafts were arterial or venous conduits placed distal to a stenosed segment of a coronary artery or arteries. Placement of a graft into a vessel that subtended an infarct zone was carefully correlated. An infarct zone was defined as the presence of a severely hypokinetic, akinetic, or dyskinetic myocardial segment.
Follow-Up
The nature of medication use during and after hospitalization
was monitored. In-hospital and out-of-hospital mortality and attention
to the cause were tracked. The development of recurrent
ventricular tachyarrhythmias, both in
hospital and after discharge, was followed and investigated.
Statistics
Continuous data are reported as mean±SD. Categorical data were
compared statistically by
2 test. Continuous
data were compared statistically by Student's t test.
Univariate and multivariate associations of
clinical, angiographic, and surgical variables with the defined end
point were tested with a logistical regression analysis.
Statistical significance was present when the probability value
was P<0.05.
| Results |
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Development of VT and Clinical Course
A total of 12 patients (3.1%) developed sustained monomorphic VT
during the in-hospital postoperative period. In all 12 patients,
1
episode of monomorphic VT occurred. In 4 of these 12 patients, there
were additional episodes of polymorphic VT or VF during their
postoperative period. The day of the first episode ranged between day 2
and day 18. None of these patients with VT had experienced VT/VF on
postoperative day 1. VT occurred 4.1±4.8 days after
surgery, and a histogram displaying the day of onset is shown in the
Figure
. No additional events in these 12
patients or new events in the other patients occurred after hospital
discharge for up to 30 days after surgery.
|
Of the 12 patients, the review of the medical records and the events surrounding the initiation of VT revealed no evidence of an electrolyte or metabolic abnormality, myocardial ischemia or infarction, new or worsening CHF, the presence of potentially proarrhythmic medication (including intravenous pressors and inotropes and antiarrhythmic agents), or the presence of any serious postoperative complication in 11 patients. In 1 patient, perioperative MI occurred 2 days after surgery as documented by ECGs and enzymes, and VT had its onset 4 days after MI.
The initial VT event had a rate of 199±28 bpm; the morphology of VT was usually not available. All patients were successfully resuscitated from the initial episode of VT. Three patients (25%) died during their hospital stay after CABG. One patient died on the third postoperative day of recurrent and refractory VT, despite administration of multiple intravenous antiarrhythmic agents, including amiodarone. The 2 remaining patients had 4 to 6 episodes of sustained VT. One of these patients died of cardiogenic shock on the fourth postoperative day, and the other died later during the hospitalization of respiratory failure. Of the 370 patients without VT, the 30-day mortality rate was 3.2%.
Among the 9 survivors, 5 patients underwent electrophysiological study before hospital discharge, and all had induction of sustained monomorphic VT that was similar to the clinical VT. Three of the 9 surviving patients received an implantable cardioverter-defibrillator (ICD) before hospital discharge. The other 6 patients received chronic therapy with antiarrhythmic drugs, generally amiodarone. Four patients were treated with ß-blocker therapy. All 9 patients are alive, with a mean follow-up of 2.5 years. Two patients, 1 with an ICD and the other on amiodarone, have experienced recurrent VT during follow-up.
Comparison of Clinical Characteristics Between Group 1 and Group
2 Patients
Group 1 comprises those patients who have experienced
1 episode
of sustained monomorphic VT in the postoperative period, and group 2
patients are those who have not experienced any episodes of VT.
Clinical characteristics of these groups of patients are compared in
Table 2
. There were no significant
differences in demographics, medical history, preoperative medications,
or preoperative ECG findings between the 2 groups. However, there were
substantial differences in these 2 groups when the presence and
severity of preexisting myocardial disease were analyzed. Prior
MI was present in 92% of group 1 patients (100% if the
perioperative MI is included) versus only 50% of the
group 2 patients (P<0.01). Group 1 was more likely than
group 2 to have recent MI, 17% versus 9%, respectively, but this
difference was not statistically significant. A history of severe CHF,
ie, NYHA functional class III/IV, was present in 58% of group 1
patients and only 21% of group 2 patients (P<0.01). A
reduced LV ejection fraction (EF), <0.40, was present in 67% of
group 1 patients but only 29% of group 2 patients
(P<0.01). In group 2, 2 patients had experienced VF in the
first hours after surgery; none of group 1 had early VT/VF. There was
no relationship of the development of VT with preoperative
antiarrhythmic drugs, postoperative ß-blockers, or ß-blocker
withdrawal.
|
The likelihood of developing an episode of VT when no prior MI was
present was only 1%. In the setting of prior MI, that incidence
increased to 7%. However, in patients whose prior MI was associated
with severe LV dysfunction (EF<0.40) as well as a history of severe
CHF, the VT risk increased to 30%. The clinical values of these
individual clinical risk factors and their combinations are detailed in
Table 3
. Of note, the individual factor
associated with the greatest odds ratio for the development of VT was
prior MI, and the presence of either low EF or a history of CHF gave
the highest odds ratio, a 16-fold increase in risk.
|
Comparison of Angiographic Characteristics Between Group 1 and
Group 2 Patients
The numbers of native vessels with a significant stenosis
were not different between the 2 groups (see Table 4
). The numbers of vessels with a
noncollateralized occlusion were different: in the group 1 patients, a
noncollateralized occlusion was present in 1.40±0.97 vessels,
compared with 0.54±0.70 vessels in the group 2 patients
(P<0.01).
|
Comparison of Surgical and Bypass Graft Characteristics Between
Groups 1 and 2.
The numbers of bypass grafts placed, both arterial and
venous conduits, were similar between groups (see Table 4
).
Aortic valve replacement, mitral valve replacement, and LV
aneurysmectomy did not differ between groups. The amount of
cross-clamp time required for bypass graft placement was also similar
between the 2 groups. There were, however, substantial differences when
the specifics of bypass graft placement across an occluded native
vessel were examined. The placement of
1 graft across an occluded
vessel was significantly more common in the group with VT, who averaged
1.50 grafts, compared with the group without VT, who averaged 0.42
graft (P<0.01). Specifically, a contrast was seen in the
graft placement beyond a totally occluded left anterior descending
coronary artery; 67% of the patients in group 1 had a graft
placed to an occluded LAD, compared with only 11% of group 2
(P<0.01). Similar contrasts were seen for grafts to
occluded right coronary arteries and left circumflex arteries,
although these differences did not reach statistical significance. If
this occluded vessel was also supplying an infarct zone, with a graft
placed across the occlusion, marked differences were observed; patients
in group 1 had 1.50 grafts to an occluded vessel supplying an infarct
zone, whereas patients in group 2 had a 10-fold lower average of 0.17
(P<0.01).
The relative risk of developing
1 episode of postoperative sustained
monomorphic VT based on the presence of the significant
coronary graft characteristics is outlined in Table 5
. Of note, there was a 4-fold increase
in risk of VT if a bypass graft was placed to
1 occluded vessel. This
risk was even higher, 16-fold, if the graft was placed to an occluded
LAD. The risk was greatest, a 22-fold increase, if the graft was placed
to
1 occluded vessel that supplied an infarct zone. In the absence of
a bypass graft to an occluded vessel, only 0.8% of patients developed
VT. When
1 graft was placed to an occluded vessel, that risk
increased to 7% and increased further to 14% when the graft was
placed to an occluded vessel in an infarct zone.
|
Multivariate Analysis
Multivariate logistic regression analysis
of angiographic and CABG variables, alone or in combination, and
significant clinical variables yielded the number of bypass grafts
to occluded vessels perfusing an infarct zone as the only independent
predictor among the 7 significant univariate predictors
(see Tables 2
and 4
) of new-onset sustained VT after
bypass surgery (
2=4.6, P=0.03).
| Discussion |
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Incidence and Hospital Course of New-Onset VT
In this prospective series of consecutively enrolled patients, a
more accurate representation of VT incidence can be determined,
for the first time. Previous series, all retrospective or based on
referral to a consultation service, may be subject to underestimation
and in fact predicted an incidence of 0.7% to 1.4%.7 8
The nature of our study design permitted a more accurate
representation of risk: 3.1% of the CABG patients in the
present series. It is also possible that the true incidence has
increased since the publication of the earlier series, possibly because
of a greater proportion of patients with prior MI undergoing surgery in
the present era.
The VT events in all patients were unstable and required resuscitation. This is not surprising, given the significant LV dysfunction present in all patients and the rapid rate of the tachycardias.11 Although the acute VT event was not fatal in any patient, a substantial mortality rate was nonetheless found; 25% of patients who experienced VT died during the hospitalization of recurrent ventricular tachyarrhythmias or of complications of cardiac arrest. Recurrent VT was not infrequent, observed in 33%, necessitating aggressive efforts at control with antiarrhythmic therapy. Recurrent arrhythmia (which was always multiple) was associated with a high mortality; 3 of 4 patients with these characteristics did not survive to hospital discharge. Previous series have often also commented on the catastrophic outcome when VT interrupts a stable postoperative recovery.7
Identification of High-Risk Patients by Clinical Profile
There were clear-cut distinguishing features between patients who
developed VT and those who did not. The prospective nature of this
study, with enrollment of a consecutive series of patients, made risk
factor analysis possible. In all but 1 patient, MI was
present before surgery, with no evidence of a new ischemic
event in the perioperative period. In addition, most VT
patients had significant LV dysfunction and clinical evidence of severe
CHF. These 3 factors were all present more frequently in the group
with VT than the group without VT. The absence of prior MI was very
useful to exclude risk of VT, because only 0.8% went on to have a
postoperative arrhythmic event. Although statistically significant, the
risk of VT in those with a prior MI was still relatively low,
7%.
However, when all 3 risk factors (prior MI, EF<0.40, and severe CHF)
were present simultaneously, the risk of VT was
ominously high,
30%. Clearly, this preoperative profile, with its
risk of VT, would warrant careful and prolonged observation with
continuous ECG telemetry to avoid an unmonitored serious,
life-threatening ventricular arrhythmia.
Results of Electrophysiological Study and
Long-Term Management
Not all patients were able to undergo
electrophysiological study. Some patients
required aggressive antiarrhythmic therapy to stabilize and were then
treated with empirical amiodarone therapy. However, 5 patients
were studied, and all exhibited provokable monomorphic VT that
resembled the clinical VT, a finding similar to some previous
reports,7 8 although others report a lower frequency of
inducible VT.9 10 Along with the clinical data described
above, the presence of inducible VT supports the premise that a fixed
substrate for VT was present that could represent a
long-term risk; thus, all were treated with an implantable
defibrillator or amiodarone. When inducible VT is absent, some
have suggested that long-term therapy is not required, presumably
indicating that a transient high-risk process had
resolved.9 10 In the patients in our series who received
ICDs, 1 of 3 patients subsequently experienced an appropriate
therapeutic intervention by the ICD. We cannot be certain as to the
long-term need or benefit of either ICDs or amiodarone in these
patients, but the presence of sustained VT, LV dysfunction, and prior
MI prompted these therapeutic decisions.
Potential Role of Chronic Coronary Occlusion and Bypass
Graft Placement
We performed a detailed analysis of factors defined
by the preoperative angiographic study and those defined by the
surgical placement of grafts. We hypothesized that resumption of blood
flow to areas previously poorly perfused could potentially restore
electrophysiological function to cells
embedded within the borders of myocardial scar and create the
possibility of reentrant circuits. In patients who develop VT in the
setting of chronic MI, it has been demonstrated previously that a
source of coronary blood flow to myocardial scar (and
presumed origin of VT) is usually identified.12 When blood
flow is interrupted, VT can be rendered noninducible; conversely, when
blood flow resumes, VT can recur.12 These findings point
to the importance not only of coronary perfusion to maintain
the electrical viability of critical arrhythmogenic tissue but also of
a reversible state that may chronically exist: in essence, a state of
electrical hibernation.
One particular factor was most strongly associated with the development
of VT: placement of a bypass graft across
1 noncollateralized totally
occluded artery that was supplying an infarct zone. This finding
strongly supports the hypothesis stated above. When present, this
factor imparted a 22-fold increase in risk; the very high negative
predictive accuracy (99%) was supportive of the notion that in the
absence of resumption of coronary blood flow to an infarct
zone, VT was very unlikely to develop in the in-hospital recovery
phase. The potential for the critical role played by reperfusion has
been raised in the past7 8 13 ; in addition, a case-control
retrospective study13 demonstrated the association between
revascularization of an area of previous MI and the
development of sustained VT/VF.
Limitations
The data presented in this article can only be applied to
the patients defined by our inclusion and exclusion criteria.
Specifically, our conclusions reflect observations on patients who
experienced VT >24 hours after surgery and within a 30-day window,
with VT having occurred in hospital. Patients with early VT (<24
hours) or VT at >30 days may differ from the cohort analyzed
in this paper. VT was detected predominantly by rhythm strips rather
than full 12-lead ECGs. We cannot be certain that the uniform nature of
VT on the 1 or 2 leads available to us at the time of VT would be
present on a multilead ECG recording. The diagnosis of new
or worsening CHF in the postoperative period was made on clinical
grounds. Hemodynamic data were not routinely
recorded in our database and thus were not included in our
analysis of postoperative risk factors. Finally, a
postoperative coronary angiogram was not performed in the
majority of patients with VT because of the absence of clinical
evidence of acute coronary ischemia that would
reflect graft occlusions or acute coronary occlusion.
Nonetheless, we cannot assume that these ischemic events did
not occur without definitive proof from coronary angiographic
results.
Conclusions
The first presentation of sustained VT in the recovery
period after CABG is relatively uncommon in the general population but
is much higher in specific subsets of patients. High-risk subsets of
patients could be characterized by clinical, angiographic, and surgical
data. In particular, placement of a bypass graft across a
noncollateralized occlusion in a vessel supplying an infarct zone was
strongly and independently associated with development of VT.
Reperfusion via CABG placement may restore electrical function to
previously quiescent myocardium and thus create reentrant
circuits. Because VT risk and fatal outcome are substantial in this
subgroup, this hazard should be considered in planning graft placement
and postoperative care.
| Acknowledgments |
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| Footnotes |
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Received August 7, 1998; revision received October 26, 1998; accepted November 16, 1998.
| References |
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