Shocks and How to Avoid Them
M. BORGGREFE1, C. WOLPERT1, C. GIUSTETTO2, F. GAITA2, U. BAUERSFELD3, R. SCHIMPF1
The short QT syndrome is a new congenital entity associated with familial atrial fibrillation and/or sudden death or syncope occurring in all age groups, even in newborns. The syndrome is characterised electrocardio- graphically by a shortened QTc interval less than 320 ms, shortened atrial and ventricular effective refractory periods, and a high percentage of inducible ventricular tachyarrhythmias during programmed electrical stim- ulation (Fig. 1) [1, 2]. In this genetically heterogeneous disease three differ- ent gain-of-function mutations in genes encoding for cardiac potassium channels (KCNH2, KCNQ1, and KCNJ2) have been identified so far [3–5]. The initial and long-term follow-up of the five initial patients with a short QT syndrome supplied with an implantable cardioverter defibrillator (ICD) will be reported below.
In two unrelated families with a short QT syndrome (SQT1) ICD devices were implanted for primary and secondary prevention [2]. The mean QT intervals were 252 ± 13 ms (QTc 287 ± 13 ms); there were two male and three female patients, mean age 38 ± 19 years. One patient, a 16-year-old adolescent, received a Marquis VR 7230 (Medtronic Inc., Minneapolis, Minn., USA) in the left subpectoral region and a tripolar pace/sense/shock elec- trode in the right ventricular apex with true bipolar pacing and sensing capabilities (6943 Sprint, Medtronic Inc.). The other four patients received an Atlas VR V-199 and three Photon Micro VR-194 systems (St. Jude Medical Inc., St. Paul, Minn., USA) in the left subpectoral region with true bipolar pacing and sensing (TVL-ADX 1559, RIATA 158) and two SPL leads with integrated bipolar sensing (St. Jude Medical Inc.). Each implantation covered
11stDepartment of Medicine-Cardiology, University Hospital Mannheim, Germany;
2Division of Cardiology, Ospedale Civile di Asti, Italy;3Division of Cardiology, University Children’s Hospital Zurich, Switzerland
a thorough measurement of lead impedances, ventricular pacing thresholds, and the verification of stable R wave signals. In addition, detailed testing of the lowest and highest ventricular sensitivity was performed, including induction of ventricular fibrillation at lowest ventricular sensitivity, which was 1.2 mV for the Medtronic device and 1.0 mV for the St. Jude Medical systems. After inappropriate shock delivery extensive manoeuvres were test- ed in all patients during real-time electrogram recording. These covered breathing manoeuvres, posture changes, exercise testing, and fluoroscopic exclusion of an altered lead position.
Four of the five patients received inappropriate shock deliveries 54 ± 52 days after implantation at a mean sinus rhythm cycle length of 460 ± 120 ms. The post-ventricular sensing refractory periods were 120 ms in the first patient and 125 ms in the other patients [6].
In all patients, measured standard parameters during implant such as ven- tricular shock impedance (54 ± 7.2 Ω), pacing threshold (0.7 ± 0.3 V/0.4 ± 0.13 ms), ventricular sensing (12 ± 4 mV) were within the normal range.
However, the first patient received two inappropriate shock deliveries only 45 min after termination of the operation due to T wave oversensing. The T wave was detected at a maximal ventricular sensitivity of 0.3 mV and double counting of the R and T wave led to inappropriate therapy. After an initial R/T wave signal ratio of 2.4:1, a reduction of the ratio to 2:1 due to slight reduction of the R wave signal was found. For future prevention of inappropriate thera- Fig. 1.ECG from a 71-year-old patient with a short QT syndrome and atrial fibrillation (QT interval 240 ms, QTc 290 ms, paper speed 25 mm/s)
pies, it is necessary to assess both the calculation of the signal and algorithm and the capabilities of specific programming of the sensitivity. The sensing decay after detection of the R wave in the Medtronic algorithm is exponential.
Each detected R wave induces an exponential decay starting at an amplitude of 75% of the R wave. The maximum amplitude of the decay start is the eight- fold maximum programmed sensitivity. The only option to react to T wave oversensing by reprogramming is to reduce the maximum ventricular sensitiv- ity given a stable and high R wave signal, thus elevating the start of the expo- nential decay. The ventricular refractory period with complete suppression of ventricular sensing is permanently programmed at 120 ms. Therefore ventric- ular sensitivity was reduced to 0.9 mV.
During the subsequent months further episodes of inappropriate shocks occurred (Fig. 2). The ventricular sensitivity had to be programmed to 1.2 mV, which is the maximum programmable sensitivity in the Medtronic device. To guarantee correct sensing of low-amplitude ventricular tach- yarrhythmias, undersensing was excluded by an ICD test with induction of ventricular fibrillation.
Fig. 2.Stored endocardial electrogram derived from the ventricular pace/sense/shock electrode of the patient from Fig. 1 (Marquis VR 7230; Medtronic Inc.). First line:
Bipolar near-field electrogram between distal tip and ring of electrode; second line: far- field electrogram between defibrillation coil of ventricular electrode and device can;
third line: corresponding annotation of the detected events by the ICD. VS Sensed ven- tricular events; continuous sensing of R wave and T wave before charging end of con- denser (CE) and charge delivery (CD). Fourth line: Cycle lengths between each sensed event in numbers; double counting of R wave and T wave signals with RT time interval of 140 ms
Another patient, a 67-year-old woman, experienced one inappropriate shock delivery 4 weeks after implantation due to double counting of the R and T wave. As in the first patient, the amplitude of the T wave has increased during follow-up. The algorithms of the Atlas and Photon devices act differ- ently to the Marquis algorithm. The algorithm integrates the QRS signal and starts a sensing decay from a programmable start threshold value (nominal 50% of the maximum amplitude of the R wave, measured in the refractory period, up to 100% of the maximum R wave amplitude). The following decay is linear, in contrast to the exponential decay from Medtronic, and the sensi- tivity is increased every 312 ms by 1 mV. A delay for the linear decay is pro- grammable from 0 to 220 ms. As a third step, the maximum ventricular sen- sitivity can be reduced, and eventually the ventricular refractory period may be increased from 125 to 157 ms. However, the risk of underdetection or prolongation of arrhythmia detection has to be thoroughly evaluated. In this patient the maximum ventricular sensitivity was reduced to 0.5 mV. The R wave signal was stable at 17.7 mV. As a second step the start threshold value was increased from 50% to 62.5% and a decay delay of 60 ms was pro- grammed. All manoeuvres (see above) documented correct R wave sensing.
During a follow-up of 2 years so far, no further inappropriate therapies and double sensing were documented with a stable R and T wave signal.
The daughter of the previous patient, a 40-year-old woman with a short QT syndrome, experienced two inappropriate shocks due to T wave over- sensing. The R wave signal was reduced to 4 mV. The patient was treated with quinidine, which previously in patients with a SQT1 syndrome showed prolongation of the QT interval into the normal range [7, 8]. The first young adolescent also received quinidine at a dosage of 1 g/day after he experi- enced both an episode of primary ventricular fibrillation and multiple inap- propriate shocks [9]. In both patients the drug prolonged the QT interval, and although the amplitude of the T wave was unaltered after drug therapy no more inappropriate episodes occurred. One explanation could be that under drug treatment the frequency content of the T wave signal, which is calculated by the sense amplifier of the ICD, changed and the signal was no longer detected.
Finally, the last patient, a 35-year-old man, received an inappropriate dis- charge 53 days after implantation due to T wave oversensing. Again, a post- operative increase of the T wave amplitude was documented. As in the sec- ond reported case, an adaptation of the programming with a decay delay of 60 ms and a start threshold of 62.5% was initiated. During a follow-up of 2.5 years no inappropriate shock was documented. The other two patients did not receive inappropriate shock therapies. However, both of them were pro- phylactically programmed with a decay delay of 60 ms and start threshold of 62.5% as a reaction to the inappropriate therapies of the first three patients.
Although infrequent, T wave oversensing still constitutes a substantial pro-arrhythmic risk [10]. The most common causes are atrial fibrillation, sinus tachycardia, other supraventricular tachycardias, and R wave oversens- ing [11–16]. However, in patients with a short QT syndrome T wave over- sensing may constitute a significant and inherent risk for inappropriate ther- apies. Shortened QT intervals and significantly elevated T wave amplitude represent a constant phenomenon. A postoperative reduction in the R wave amplitude, and in two cases increase of the T wave signal, was the reason for oversensing despite the fact that no abnormalities were observed at implant or prehospital discharge testing in any of the affected patients. During close follow-up, amplitude changes of the intracardiac R and T wave signals must be assessed. In contrast to patients with long QT syndrome, double sensing of the R and T wave should be less likely in patients with short QT syndrome as the T wave occurs early after beginning of the RR interval and the sensi- tivity is lowest in the early phase of the sensing algorithms after the detec- tion of the R wave. However, the abnormal amplitude of the T wave signal and potentially the frequency content of the T wave may vary in comparison to those of normal patients and thus could pass the sense amplifier of the ICD [17].
There are different algorithms currently available which address T wave oversensing, and which vary between the manufacturers. The programming has to be adjusted based upon the specific sensing algorithm involved. A multi-programmable algorithm appears to be most suitable for preventing oversensing of short coupled high-amplitude T wave signals in patients with short QT syndrome. However, this needs to be evaluated in larger patient numbers. Another three patients who received an ICD did not experience inappropriate therapies during short-term follow-up (Guidant, Indianapolis, Ind., USA) [18, 19].
Nevertheless, irrespective of the different sensing algorithms, a prerequi- site for individual adaptation of sensing parameters is a lead position which guarantees a constant and high R wave signal. This has to be balanced against correct arrhythmia detection and discrimination.
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