Mayo Clin Proc, May 2002, Vol 77 Infusion QT Stress Testing in LQTS 413 Original rticle -Induced QT Interval Prolongation: Gene-Specific Paradoxical Response in Congenital Long QT Syndrome MICHEL J. CKERMN, MD, PHD; NNT KHOSITSETH, MD; DVID J. TESTER, S; JOSEPH. HEJLIK, RN; WIN-KUNG SHEN, MD; ND CO-URN J. PORTER, MD Objective: To determine the effect of epinephrine on the QT interval in patients with genotyped long QT syndrome (LQTS). Patients and Methods: etween May 1999 and pril 2001, 37 patients (24 females) with genotyped LQTS (19 LQT1, 15 LQT2, 3 LQT3, mean age, 27 years; range, 10-53 years) from 21 different kindreds and 27 (16 females) controls (mean age, 31 years; range, 13-45 years) were studied at baseline and during gradually increasing doses of intravenous epinephrine infusion (0.05, 0.1, 0.2, and 0.3 µg kg 1 min 1 ). The 12-lead electrocardiogram was monitored continuously, and heart rate, QT, and corrected QT interval (QTc) were measured during each study stage. Results: There was no significant difference in resting heart rate or chronotropic response to epinephrine between LQTS patients and controls. The mean ± SD baseline QTc was greater in LQTS patients (500±68 ms) than in controls (436±19 ms, P<.001). However, 9 (47%) of 19 KVLQT1-genotyped LQT1 patients had a nondiagnostic resting QTc (<460 milliseconds), whereas 11 (41%) of 27 controls had a resting QTc higher than 440 milliseconds. During epinephrine infusion, every LQT1 patient manifested prolongation of the QT interval (paradoxical response), whereas healthy controls and patients with ei- ther LQT2 or LQT3 tended to have shortened QT intervals (P<.001). The maximum mean ± SD change in QT ( QT [epinephrine QT minus baseline QT]) was 5±47 ms (controls), +94±31 ms (LQT1), and 87±67 ms (LQT2 and LQT3 patients). Of 27 controls, 6 had lengthening of their QT intervals ( QT >30 milliseconds) during high-dose epinephrine. Low-dose epinephrine (0.05 µg kg 1 min 1 ) completely discriminated LQT1 patients ( QT, +82±34 ms) from controls ( QT, 7±13 ms; P<.001). triggered nonsustained ventricular tachycardia occurred in 2 patients with LQTS and in 1 control. Conclusions: -induced prolongation of the QT interval appears pathognomonic for LQT1. Lowdose epinephrine infusion distinguishes controls from patients with concealed LQT1 manifesting an equivocal QTc at rest. Thus, epinephrine provocation may help unmask some patients with concealed LQTS and strategically direct molecular genetic testing. Mayo Clin Proc. 2002;77:413-421 ECG = electrocardiogram; LQTS = long QT syndrome; NSVT = nonsustained ventricular tachycardia; PVCs = premature ventricular contractions; QTc = corrected QT interval; TW = T-wave alternans The congenital long QT syndrome (LQTS) comprises the first genetically defined type of arrhythmia to be understood at the molecular level as a primary cardiac channelopathy. 1-3 To date, 6 LQTS loci; 5 LQTS genes KVLQT1 (KCNQ1 [LQT1]), HERG (KCNH2 [LQT2]), SCN5 (LQT3), KCNE1 (mink [LQT5]), and KCNE2 From the Division of Cardiovascular Diseases and Internal Medicine (M.J.., D.J.T., J..H., W.-K.S.), Division of Pediatric Cardiology (M.J..,.K., C.J.P.), and Department of Molecular Pharmacology and Experimental Therapeutics (M.J..), Mayo Clinic, Rochester, Minn. Dr ckerman was supported by a Doris Duke Clinical Scientist Development ward from the Doris Duke Charitable Foundation and a Mayo Foundation clinical research award. Presented in part at the Society for Pediatric Research annual meeting, oston, Mass, May 12-16, 2000, and at the North merican Society of Pacing and Electrophysiology annual meeting, oston, Mass, May 2-5, 2001. ddress reprint requests and correspondence to Michael J. ckerman, MD, PhD, Division of Pediatric Cardiology, Mayo Clinic, 200 First St SW, Rochester, MN 55905 (e-mail: ackerman.michael @mayo.edu). (MiRP1 [LQT6]) 4-8 ; and more than 200 LQTS-causing mutations 9 have been identified and compiled in an LQTS database (www.ssi.dk/en/forskning/lqtsdb/lqtsdb.htm). The LQTS subtype 1 (LQT1) is secondary to defects in the cardiac potassium channel encoded by KVLQT1, LQTS subtype 2 (LQT2) is secondary to defects in the cardiac potassium channel gene encoded by HERG, and LQTS subtype 3 (LQT3) is secondary to defects in the cardiac sodium channel gene encoded by SCN5. Unlike the F508del- CFTR mutation in cystic fibrosis that accounts for approximately 70% of cystic fibrosis, there are no hot spots in the LQTS genes. 2 For editorial comment, see page 405. Clinically, LQTS affects approximately 1 in 5000 persons and can cause syncope, seizures, and sudden death. Such events occur when the cardiac rhythm degenerates into the hallmark tachyarrhythmia of LQTS termed torsades de pointes. Certain triggers such as intense adrenergic or auditory stimulation appear particularly arrhyth- Mayo Clin Proc. 2002;77:413-421 413 2002 Mayo Foundation for Medical Education and Research
414 Infusion QT Stress Testing in LQTS Mayo Clin Proc, May 2002, Vol 77 Table 1. Demographics for Controls and Patients With Long QT Syndrome* Control LQT1 LQT2/3 Sex (F/M) 16/11 13/6 11/7 ge (y) 31.3±11.4 26.7±10.4 27.5±12.1 aseline QT interval (ms) 418±28 449±28 535±66 aseline corrected QT interval (QTc, ms) 436±19 463±29 539±74 aseline heart rate (beats/min) 66±10 65±9 61±8 Peak heart rate with epinephrine (beats/min) 90±16 84±15 90±14 Change in heart rate (beats/min) 24±12 19±15 29±11 *ll values are ± SD unless stated otherwise. Patients with LQT2 and LQT3 were combined for analysis. mogenic in LQTS. 10,11 One specific activity, swimming, has been shown to trigger symptoms in nearly 15% of children and young adults with LQTS and is virtually always due to mutations in KVLQT1 (LQT1). 11,12 Natural history studies suggest that 40% of patients remain asymptomatic, 50% have at least 1 cardiac event, and 5% to 10% may present with aborted cardiac arrest as a sentinel event. Thus, an accurate diagnosis of LQTS is of paramount importance. Pronounced prolongation of the corrected QT interval (QTc) (>460 milliseconds) on a resting 12-lead electrocardiogram (ECG) remains the cornerstone of the diagnosis of congenital LQTS. 3 However, even when such prolongation exists, computer diagnostic interpretation of the ECG may be erroneous. 13 Moreover, at least 25% of individuals with LQTS may manifest an ECG with an equivocal or borderline QTc (concealed LQTS). 14,15 New diagnostic modalities are needed for accurate identification of individuals who may harbor this potentially lethal arrhythmogenic substrate. Since many of the observed cardiac events are precipitated by sympathetic activation, exercise stress testing has been suggested to enhance the diagnostic accuracy in LQTS. 10,16,17 Swan et al 18 performed exercise tests in 45 genotyped LQT1 patients, 22 LQT2 patients, and 33 healthy genotype-negative relatives and showed that LQT1 patients had a diminished chronotropic response to exercise and an exaggerated prolongation of the QT interval during recovery from exercise. esides exercise testing, pharmacological provocation tests with use of catecholamines have been conducted in attempts to accurately identify the patient with LQTS. 19,20 Recently, we 21 described a family with concealed LQTS (normal screening ECGs) after an epinephrine provocation elicited significant prolongation of the QT interval during infusion of 0.1 µg kg 1 min 1. In contrast, a study involving 9 healthy volunteers showed that similar infusion rates of epinephrine slightly decreased the QT interval. 22 We speculated that epinephrine-mediated QT prolongation (a paradoxical response) might be an LQT1-specific response to epinephrine. 21 In fact, this abnormal epinephrine study finding fostered a successful molecular autopsy that confirmed a KVLQT1 mutation (LQT1) in the deceased person despite the sentinel event occurring during sleep. We tested our hypothesis by performing epinephrine infusion QT stress tests in 37 genotyped LQTS patients, including 19 with KVLQT1-based LQT1, and 27 age- and sex-matched healthy controls. PTIENTS ND METHODS Study Population etween May 1999 and pril 2001, 64 persons, including 19 (13 female) LQT1 patients (age, 26.7±10.4 years, range, 13-46 years), 18 (11 female) LQT2/3 patients (15 LQT2 and 3 LQT3; age, 27.5±12.1 years, range, 10-53 years), and 27 age- and sex-matched healthy controls (16 females; age, 31.3±11.4 years, range, 13-45 years), underwent an epinephrine infusion QT stress test. Patients with LQTS were genotyped after written informed consent, and the epinephrine studies were conducted as part of their clinical evaluation. Controls underwent epinephrine provocation approved by the Mayo Foundation Institutional Review oard after written informed consent. Controls either (1) had no personal history of syncope, family history of sudden cardiac death, or ECG evidence of diagnostic QT interval prolongation on their baseline 12-lead ECG (n=23) or (2) were genotype-negative LQTS family members (n=4). Of 19 patients with LQT1, 11 were genotyped successfully before the epinephrine study, whereas the specific KVLQT1 mutation was established after the epinephrine study in the remaining 8 LQT1 patients. Similarly, 10 of the 18 LQT2/3 patients were already genotyped as 9 HERG mutations and 1 distinct SCN5 mutation. Patient demographics are shown in Table 1, and the spectrum of mutations represented in this study is depicted in Figure 1. Only 2 LQTS patients (LQT2) were taking β-blockers at the time of their epinephrine QT stress test. The remaining patients had not yet begun β-blocker therapy because the epinephrine challenge was part of the initial evaluation or the patient was noncompliant, had chosen implantable cardioverter-defibrillator device monotherapy, or had testing conducted after monitored β-blocker washout.
Mayo Clin Proc, May 2002, Vol 77 Infusion QT Stress Testing in LQTS 415 Figure 1. Spectrum of channel mutations in the cohort of patients with long QT syndrome (LQTS). Schematic of a prolonged cardiac action potential duration from a ventricular myocyte is shown with the linear topologies of the cardiac channels responsible for LQT1 (KVLQT1), LQT2 (HERG), and LQT3 (SCN5). There were 19 LQT1 patients studied who had 11 distinct KVLQT1 mutations, 15 LQT2 patients who had 9 distinct HERG mutations, and 3 LQT3 patients who had the same missense mutation. Circles denote the missense mutations, and squares indicate insertion/deletion mutations. The lightning bolt in KVLQT1 represents a splicing mutation. Not shown in KVLQT1 is the splicing mutation denoted SP/639+5 g>a. Infusion QT Stress Test These studies were performed in the clinical electrophysiology laboratory with a physician and nurse present. ll LQTS patients and controls were connected to a bedside cardioverter-defibrillator device at the start of study and were monitored by continuous 12-lead ECG and continuous blood pressure monitoring. fter baseline evaluation while supine, an infusion of epinephrine was initiated at 0.05 µg kg 1 min 1, and repolarization measurements (QT, R-R, and QTc) were determined after 10 minutes of low-dose epinephrine. The QTc was calculated by the azett formula: the absolute QT interval divided by the square root of the preceding R-R interval. Next, the epinephrine infusion was increased sequentially to 0.1, 0.2, and 0.3 µg kg 1 min 1 with measurements made after 5 minutes at each subsequent dose and, finally, at 10 minutes into the recovery period. The total duration of epinephrine infusion was 25 minutes, and the overall study was completed in 45 minutes. The interval measurements were obtained from lead II, typically by using digital calipers and a recorded tracing speed of 50 mm/s. t least 4 beats were averaged for each study stage. The change in QT ( QT) and the change in QTc ( QTc) were calculated by the difference between the maximal or minimal QT and QTc during epinephrine infusion compared to baseline. Criteria to discontinue epinephrine infusion included systolic blood pressure 200 mm Hg or higher, nonsustained ventricular tachycardia (NSVT) or polymorphic VT, increasing premature ventricular contractions (PVCs) (>10 PVCs/min), T-wave alternans (TW), or patients intolerance due to headache and/or nausea. Statistical nalysis ll continuous variables were reported as mean ± SD. One-way analysis of variance followed by Tukey-Kramer honestly significant difference test was used to compare
416 Infusion QT Stress Testing in LQTS Mayo Clin Proc, May 2002, Vol 77 Figure 2. Genotype and baseline corrected QT interval (QTc) in patients with long QT syndrome (LQTS). Mean values, standard deviation bar, and individual data points for the resting baseline QTc are indicated for controls, LQT1 patients, and LQT2/3 patients. Patients with LQTS had a greater QTc than controls (P<.001), and patients with LQT2/3 had a longer QTc than those with LQT1 (P<.002). measurements among the 3 groups before and after epinephrine infusion and to compare each parameter among LQT1 and LQT2/3 patients and controls. LQT2 (n=15) and LQT3 (n=3) patients were combined for analysis in order to compare LQTS patients with a defective KVLQT1-mediated cardiac potassium current (I Ks [LQT1]) with those having an intact I Ks (LQT2/3). paired Student t test was used to compare parameters between the 2 groups. P<.05 was considered statistically significant. RESULTS s expected, baseline QT intervals and calculated QTc were significantly longer in LQTS patients compared with controls (P<.001): QT, 490±66 ms (LQTS) vs 418±28 ms (controls) and QTc, 500±68 ms (LQTS) vs 436±19 ms (controls). In addition, baseline QT intervals and QTc in LQT2/3 patients were significantly longer than in LQT1 patients (P<.002): QT, 535±66 ms (LQT2/3) vs 449±28 ms (LQT1) and QTc, 539±74 ms (LQT2/3) vs 463±29 ms (LQT1). Interestingly, nearly half of the LQT1 patients (9/19) had a nondiagnostic resting QTc (<460 milliseconds), whereas 11 (41%) of 27 healthy controls had borderline prolongation (QTc >440 milliseconds) (Figure 2). No significant difference was noted in baseline heart rate between LQT1 (65±9 beats/min), LQT2/3 (61±8 beats/min), and controls (66±10 beats/min). The chronotropic response to epinephrine was similar between LQTS patients and controls, with the heart rate increasing by 19±15 beats/min in LQT1, 29±11 beats/min in LQT2/3, and 24±12 beats/min in controls (P=.06). In comparing only LQT1 and LQT2/3 patients, heart rate increased more during epinephrine infusion in LQT2/3 than in LQT1 patients (P=.02). Effect of on QTc lthough epinephrine tended to increase QTc in all 3 study groups (Figure 3, ), the epinephrine-mediated increase in QTc was significantly greater in LQT1 patients than in LQT2/3 patients and controls ( QTc [ QTc = epinephrine QTc minus baseline QTc], 143±52 ms [LQT1], 37±60 ms [LQT2/3], 65±46 ms [controls], Figure 3, ; P<.001). However, there was substantial overlap among the 3 groups such that the effect of epinephrine on Figure 3. Effect of epinephrine (Epi) on corrected QT interval (QTc) in patients with long QT syndrome (LQTS) and controls., The baseline QTc and the maximal QTc response regardless of Epi dose is displayed for each LQTS patient and control., The mean change in QTc ( QTc), SD, and individual QTc is displayed for the 3 study groups. The mean QTc was significantly greater in patients with LQT1 (P<.001), but overlap is considerable.
Mayo Clin Proc, May 2002, Vol 77 Infusion QT Stress Testing in LQTS 417 Figure 4. Effect of epinephrine on absolute QT interval in patients with long QT syndrome (LQTS)., Schematic of an epinephrine study in a patient with LQT1 showing the baseline and epinephrine traces from lead II. Note on the expanded time scale, an epinephrine QRST complex is superimposed on a baseline complex showing the paradoxical response of epinephrine-induced QT interval prolongation (arrow)., similar depiction is shown from a patient with a truncated C-terminal HERG mutation showing shortening of the QT interval with epinephrine. QTc failed to distinguish between the LQTS genotypes or even LQTS patients from controls. Effect of on the bsolute QT Interval The characteristic paradoxical response to epinephrine (ie, prolongation of the absolute QT interval) seen in every LQT1 patient regardless of their underlying KVLQT1 mutation is depicted in Figure 4,, and a typical epinephrinemediated QT shortening seen in LQT2/3 patients and also most controls is shown in Figure 4,. During epinephrine infusion, the absolute QT interval in LQT1 patients increased uniformly. In every LQT2/3 patient, the QT interval decreased uniformly. In controls, the QT interval response to epinephrine was not uniform (Figure 5, ). There was a marked difference between the mean change in QT ( QT [epinephrine QT minus baseline QT]) among the 3 study groups: +94±31 ms (LQT1), 87±67 ms (LQT2/3), and 5±47 ms (controls); P<.001 (Figure 5, ). Here, the change in QT was derived as the maximum difference obtained during the epinephrine infusion without respect to the dose at which the maximum response occurred. Thus, some patients manifested the greatest response during low-dose infusion (0.05 µg kg 1 min 1 ), whereas the greatest effect was observed in others during high-dose epinephrine infusion (0.3 µg kg 1 min 1 ). Patients with LQT1 and LQT2/3 were distinguished completely by the epinephrine-qt response (Figures 4 and 5). However, nearly half of the LQT1 patients were not discriminated from the controls with use of peak QT effect independent of epinephrine dose (Figure 5, ). Subset analysis of the QT response to only low-dose epinephrine (0.05 µg kg 1 min 1 ) showed a significantly greater response in LQT1 patients than in controls (+82±34 vs 7±13, Figure 6, ; P<.001). More importantly, there was no overlap between LQT1 patients and controls in the effect of low-dose epinephrine on the QT interval (Figure 6, ). lthough the subset analysis of the QT response to low-dose epinephrine demonstrated a significantly greater degree of QT interval shortening in LQT2/3 patients compared with controls, overlaps persisted between these 2 groups (data not shown). In contrast to this pathognomonic gene-specific paradoxical response to low-dose epinephrine, we could show no additive diagnostic utility during infusion of higher doses of epinephrine (0.2 and 0.3 µg kg 1 min 1 ). In fact, as shown in Figure 5,, high-dose epinephrine yielded a false-positive paradoxical response ( QT >30 milliseconds) in 6 (22%) of 27 controls. rrhythmogenicity and Safety of Challenge No LQTS patient or control required defibrillation. The study was discontinued before the entire epinephrine infusion protocol was completed in 11 LQTS patients (30%) because preset end point criteria were reached: 1 (LQT1) due to a 15-beat run of NSVT, 1 (LQT3) due to a 4-beat run of NSVT, 1 (LQT1) due to TW, 1 due to systemic hypertension, 1 due to an increasing number of PVCs, and 6 due to headache and nausea at the highest level of epinephrine infusion (0.3 µg kg 1 min 1 ). The epinephrine study of a 17-year-old female adolescent subsequently found to have a novel I235N-KVLQT1 missense mutation is depicted in Figure 7,. With low-dose epinephrine, the absolute QT interval lengthened from 450 milliseconds at baseline to 609 milliseconds. During infusion of 0.2 µg kg 1 min 1, an asymptomatic 15-beat run of VT occurred. Sinus
418 Infusion QT Stress Testing in LQTS Mayo Clin Proc, May 2002, Vol 77 Figure 5. Summary of the QT response to epinephrine (Epi) in patients with long QT syndrome (LQTS) and controls., The absolute QT interval is displayed on the y-axis for controls, LQT1 patients, and LQT2/3 patients at baseline and after Epi infusion. Note that the maximal effect (either lengthening or shortening) on the QT interval is shown regardless of the dose of Epi at which it occurred., The mean change in QT ( QT), SD, and individual QT is displayed for the 3 study groups. The mean QT was significantly greater in LQT1 patients (P<.001). The maximal QT response to Epi completely distinguishes LQT1 from LQT2/3 patients, but there is considerable overlap with healthy controls. rhythm recurred spontaneously, and the infusion was terminated. Striking macrovoltage TW that occurred during infusion of 0.2 µg kg 1 min 1 of epinephrine in a 37-yearold woman harboring a previously known splice-site mutation in KVLQT1 is shown in Figure 7,. mong the controls, 9 (33%) did not complete the entire epinephrine protocol because of bigeminy (2), an increasing number of PVCs (4), and headache/nausea (3). Of importance, there appeared to be no risk-stratifying value with the observation of epinephrine-induced ventricular ectopy in terms of increased frequency of single PVCs or bigeminy. One control (40-year-old woman) had a 3-beat run of NSVT during the highest dose of epinephrine (0.3 µg kg 1 min 1 ). Of 64 study participants, 61 (95%) tolerated the epinephrine infusion through 0.2 µg kg 1 min 1 without invoking any of the end point criteria. DISCUSSION Provocation and the I Ks Pathway: Molecular and Cellular Considerations KVLQT1 (chromosome 11p15.5) encodes the 676- amino acid α-subunit underlying one of the principal potas- Figure 6. QT prolongation with low-dose epinephrine (Epi) is pathognomonic for long QT syndrome subtype 1 (LQT1)., The QT response during low-dose Epi (0.05 µg kg 1 min 1 ) only is plotted on the y-axis for controls and LQT1 patients (KVLQT1). Change in QT interval ( QT) is displayed on the y-axis for controls and LQT1 patients., paradoxical response to low-dose Epi ( QT >30 milliseconds) now completely distinguishes LQT1 patients from controls (P<.001).
Mayo Clin Proc, May 2002, Vol 77 Infusion QT Stress Testing in LQTS 419 aseline (0.05 µg kg 1 min 1 ) (0.1 µg kg 1 min 1 ) (0.2 VT) aseline (0.05 µg kg 1 min 1 ) (0.1 µg kg 1 min 1 ) (0.2 VT) (0.2 VT) Recovery (5 min) Recovery (5 min) 200 ms 200 ms Figure 7. rrhythmogenicity during epinephrine provocation., -induced nonsustained ventricular tachycardia (VT). n epinephrine QT stress test from a 17-year-old female adolescent with a novel KVLQT1 missense mutation denoted I235N showing a paradoxical QT response to low-dose epinephrine (baseline, 450-609 milliseconds) followed by a 15-beat run of nonsustained VT during administration of 0.2 µg kg 1 min 1 of epinephrine., -induced macrovoltage T-wave alternans (TW). n epinephrine QT stress test from a 37-year-old woman having a previously known splicing mutation denoted SP/344/g>a in KVLQT1. Once again, prolongation of the QT interval is evident (506-556 milliseconds) during low-dose epinephrine. t 0.2 µg kg 1 min 1 of epinephrine, there is visible TW. In addition, the inverted P waves during epinephrine returned to a normal sinus P wave during the recovery period. sium channels (I Ks ) in the heart that is responsible for phase 3 repolarization. 2 The I Ks provides approximately 4 times the potassium current compared with the other predominant phase 3 repolarizing potassium current (I Kr encoded by HERG). The KVLQT1-encoded I Ks potassium channel is phosphorylated during sympathetic activation. Patients with LQTS who have KVLQT1 mutations are more prone to experience cardiac events under physically or mentally stressful situations and benefit the most from β-blocker therapy. 10,23 Sympathetic stimulation has more effects in patients with LQT1 than in patients with other types of LQTS and healthy individuals. 24-26 perturbation in the I Ks channel signaling complex could disrupt the balance of several currents, including I Ks, I Cl, and I Na-Ca, that are under sympathetic control and lead to failure of β-adrenergic stimulation to abbreviate the action potential duration and hence the QT interval in patients with LQT1. 26 -Mediated QT Prolongation: n In Vivo ssay for I Ks Perturbations Our data suggest that epinephrine provocation is indeed an in vivo functional assay capable of clinically evaluating the integrity of the I Ks channel signaling complex. 27 Every LQT1 patient, regardless of the underlying KVLQT1 channel mutation, displayed the paradoxical response of epinephrine-mediated QT interval prolongation, whereas LQTS patients with an intact I Ks pathway (ie, LQT2 and LQT3 patients) demonstrated shortening of the QT interval during epinephrine consistent with previous in vitro cellular mechanistic hypotheses. This strikingly divergent genotype-specific response explains why epinephrine challenges conducted in the decade preceding the molecular revelations in LQTS yielded such heterogeneous responses. 19 Moreover, the vast majority of putative KVLQT1 mutations have not been functionally characterized by site-directed mutagenesis molecular/cellular electrophysiologic studies. Thus far, we have observed this paradoxical response to epinephrine uniformly in LQT1 patients whether or not the patient had an N- terminal deletion, a missense mutation in the pore, or an illdefined splicing error (Figure 1). challenge testing may help provide objective evidence indicating whether a putative KVLQT1 mutation is functionally apparent in the whole organism (the patient) or perhaps simply an otherwise innocuous, dormant polymorphism. Provocation and Strategic LQTS Genotyping When a diagnostic QTc is present on the 12-lead ECG, the epinephrine challenge may foster strategic genotyping similar to the association of gene-specific arrhythmogenic triggers, like auditory stimuli in LQT2 and swimming in LQT1 patients. 10-12 If epinephrine-mediated QT prolongation is manifest, one should screen for KVLQT1 first. Indeed, this paradoxical response might be anticipated in patients with LQT5 because of a defect in the β-subunit (mink) of the I Ks channel. However, we have not yet con-
420 Infusion QT Stress Testing in LQTS Mayo Clin Proc, May 2002, Vol 77 ducted an epinephrine study on a known genotyped LQT5 patient to confirm this expectation. Nonetheless, LQT1 is at least 25 times more common than LQT5; thus, epinephrine-triggered prolongation can be considered pathognomonic for LQT1. In contrast, if there is epinephrine-mediated QT shortening (ie, a normal response to epinephrine), mutational analysis should begin with HERG unless other genotype-phenotype clues are present. Exposing Concealed LQTS With Use of The QT interval is a surface marker of cardiac electrical activity, specifically cellular repolarization. It is generally accepted that the absolute QT interval provides a surface rendering of the underlying cellular action potential durations. Despite overlap of the resting QTc between healthy persons and patients with LQTS, the 12-lead ECG remains one of the principal tools in the LQTS evaluation, and the baseline QTc is still one of the most important diagnostic criteria. 28 In our study, although the baseline QTc was significantly greater in LQTS patients than in controls, we found that 47% of genotyped LQT1 patients had a nondiagnostic resting QTc (<460 milliseconds), whereas 41% of healthy controls had borderline prolonged QTc (>440 milliseconds). In contrast, every LQT2/3 patient had a diagnostic resting QTc (>460 milliseconds). In this setting, the current gold standard clinical test, the 12-lead ECG, failed to distinguish nearly half of the LQT1 patients and controls. Perhaps more important than facilitating strategic genotyping, the epinephrine challenge may allow one to unmask patients harboring concealed LQT1. lthough some controls showed significant paradoxical QT prolongation at higher doses of epinephrine, low-dose epinephrine (0.05 µg kg 1 min 1 ) completely discriminated LQT1 patients from controls. It is possible that the QT prolongation seen at higher doses of epinephrine in controls was secondary to epinephrine-mediated hypokalemia, as previously shown. 21 However, we did not determine serum potassium levels in this study. Thus, the epinephrine challenge may be of diagnostic value in the clinical evaluation of unexplained syncope or sudden death in which there is a clinical index of suspicion for LQTS but a nondiagnostic ECG. Indeed, we have demonstrated how this epinephrine challenge revealed concealed LQT1 in the mother of a 17-year-old female adolescent found dead in bed and fostered a successful molecular autopsy confirming a KVLQT1 mutation in the deceased person. 20 rrhythmogenicity and Safety of Provocation studies can be conducted safely in patients with LQTS. None of our LQTS patients or controls required defibrillation. Only 1 of the 37 LQTS patients had a prolonged episode of NSVT. ecause epinephrine-induced polymorphic VT or ventricular fibrillation remains a theoretical concern, we continue to connect patients to a bedside cardioverter-defibrillator system at the start of each epinephrine challenge. Importantly, infusion of epinephrine does not appear to have a role in risk stratification. induced PVCs were observed as commonly in controls as in LQTS patients, and their presence does not indicate clinical risk. ecause no diagnostic information was apparent at the highest dose of epinephrine (0.3 µg kg 1 min 1 ), we have discontinued this dose, and our current maximum infusion is 0.2 µg kg 1 min 1. In this study, more than 95% of the LQTS patients and controls would have completed the entire protocol without adverse effects, complications, or reaching end point criteria with this revised ceiling dose. Study Limitations This study has 3 main limitations. First, it was not blinded. The principal investigator (M.J..) who measured the various parameters of repolarization knew which study participants were controls and which had clinical LQTS at the start of the epinephrine provocation. However, another investigator (J..H.) independently confirmed the QT measurements in each study and was blinded to the patient s underlying genotype. Moreover, nearly half of the LQTS patients were not successfully genotyped until several months after the epinephrine study. In fact, the genotype of 8 of the 19 LQT1 patients was unknown at the time of the study. Thus, it is most unlikely that bias was introduced because of the unblinded study design. Second, only 3 LQT3 patients, from the same family harboring obviously the same sodium channel mutation, were studied, and each LQT3 patient had shortening of their QT interval during epinephrine infusion similar to the LQT2 patients. However, it remains to be shown whether epinephrine-induced QT shortening is universal for LQT3 patients. In addition, our LQTS cohort contained no LQT2 patient with an equivocal, nondiagnostic resting QTc. Whether the apparent accentuated shortening of the QT interval with epinephrine seen in the genotyped LQT2 patients compared with controls could be used to unmask concealed LQT2 remains to be shown. Finally and most importantly, this epinephrine study was conducted in the absence of the potential confounding effect of β-blocker therapy, except for 2 patients (both LQT2) who were taking β-blockers at the time of the study. The remaining LQTS patients were not taking β-blockers because of initial clinical evaluation, elective noncompliance, implantable cardioverter-defibrillator as monotherapy, or 48-hour washout of β-blocker. Thus, we do not know whether β- blockers would obscure the observed genotype-specific re-
Mayo Clin Proc, May 2002, Vol 77 Infusion QT Stress Testing in LQTS 421 sponse to epinephrine or the ability to unmask an LQT1 patient with an equivocal QTc at rest. We urge caution in attempting to generalize these findings to the patient taking β-blockers at the time of study and recommend admission to the hospital for monitored β-blocker washout if this test is deemed appropriate for clinical evaluation. CONCLUSIONS There appears to be an essential role for epinephrine infusion QT stress testing in evaluating LQTS and unexplained sudden cardiac death. The previous disparate responses to epinephrine are now understood in light of the profound molecular heterogeneity underlying LQTS. In addition, the diagnostic superiority of epinephrine-mediated changes in QT interval rather than the effect on QTc are clearly evident. Provocation with epinephrine may provide an in vivo functional assay to interrogate putative KVLQT1 mutations. 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