Defibrillation thresholds with right pectoral implantable cardioverter defibrillators and impact of waveform tuning (the Tilt and Tune trial).

AIMS: Assess defibrillation thresholds (DFTs) with right active pectoral implantable cardioverter defibrillator (RICDs). Defibrillation thresholds in patients receiving RICDs are regarded as 'high' and potentially improved by waveform optimization (tuning). However, this has not been systematically tested. METHODS AND RESULTS: Patients receiving RICDs [Single chamber (VVI) = 16, DDD = 32, cardiac resynchronization therapy (CRT) = 43] were randomized to either 50/50% fixed tilt (FT) or tuned waveform (3.5 ms time constant based). Defibrillation threshold was tested with a binary search protocol in single coil anodal configuration. Then RICDs were compared with left-sided placements. Baseline patient characteristics in FT (n = 54) and tuned (n = 37) were similar (65 ± 14 years, 71% male, Left ventricular ejection fraction 31 ± 13%; and proportions VVI/DDD/Cardiac resynchronization therapy defibrillator). Tuning reduced Phase 1 by 15% and Phase 2 by 45%. For FT vs. tuned: high voltage impedance was 61.9 ± 13.2 vs. 64.5 ± 12.7 Ω (P = 0.33) and mean DFT 14.2 ± 8.8 vs. 14.9 ± 9.2 J (P = 0.8). When high voltage impedance was >62 Ω (mean 73.6 ± 8.6 Ω), DFT was identical [FT 13.0 ± 7.9 J vs. tuned 12.0 ± 5.9 J (P= 0.7)]. Defibrillation thresholds exceeded 20 J (600 V) in >20% of patients [FT 11/54 (20.4%) vs. tuned 12/37 (32%) patients]. Defibrillation threshold with RICD was greater and exhibited wider dispersion compared with left ICDs (n = 54) under similar conditions. CONCLUSION: This first randomized trial investigating DFTs with right ICDs confirms relatively higher DFTs with RICDs than reported for left pectoral ICDs. However, DFTs were generally unaffected by 3.5 ms time constant-based waveform tuning compared with a 50% tilt waveform. Implant testing may be preferred with RICDs. CLINICAL TRIAL NUMBER: NCT00873691.

Efficacy of tuned waveforms based on different membrane time constants on defibrillation thresholds: primary results from the POWER trial.

BACKGROUND: The efficacy of tuned defibrillation waveforms versus the nominal fixed-tilt waveform has been previously studied. However, the optimal membrane time constant for tuning was not known. The POWER (Pulsewidth Optimized Waveform Evaluation tRial) trial was designed to determine the optimal membrane time constant for programming "tuned" biphasic waveforms. METHODS: This acute, multicenter study included 121 implantable cardioverter-defibrillator/cardiac resynchronization therapy defibrillator patients who were randomized at implant to any two of the three membrane time constant waveforms (2.5, 3.5, and 4.5 ms). Fixed pulse widths were programmed using the measured high voltage shock impedance. The defibrillation threshold (DFT) estimates were obtained using a hybrid protocol starting with an upper limit of vulnerability estimate followed by a step-up/step-down ventricular fibrillation induction process. RESULTS: DFT voltage was significantly lower using 3.5- and 4.5-ms waveforms as compared to the 2.5-ms waveform (P = 0.004 and 0.035, respectively). DFT voltage with both 3.5- and 4.5-ms waveforms was ≤ that obtained with the 2.5-ms waveform in 78.5% of the cases. The mean difference in DFT voltage using the 3.5-ms waveform and the 4.5-ms waveform was not significant (P = 0.4). However, the 3.5-ms waveform gave a lower DFT than the 4.5-ms waveform in 19 patients although the reverse was true in only nine (P = 0.02 not significant for multiple comparisons). CONCLUSIONS: The use of a 3.5- or 4.5-ms time constant-based waveforms had lower DFTs when compared to the 2.5-ms waveform. This study suggests that the first defibrillation attempt at implantation should be with 3.5- or 4.5-ms time constant-based waveforms. The 3.5-ms-based waveform trended toward the best choice.

Temporal stability of defibrillation thresholds with cardiac resynchronization therapy.

BACKGROUND: Defibrillation thresholds (DFTs) are typically stable over time among patients with implantable cardioverter-defibrillators (ICDs). However, the impact of cardiac resynchronization therapy (CRT) on DFTs has not been studied systematically. OBJECTIVE: This study prospectively evaluated the effect of CRT and left ventricular (LV) chamber reverse remodeling on DFTs. METHODS: This prospective, multicenter study evaluated 54 cardiac resynchronization therapy defibrillator (CRT-D) patients. Echocardiography and DFTs were performed both at implantation and at 6 months after implantation. All patients received dual-coil leads and a CRT-D pulse generator. DFTs were measured using a binary search method and tuned biphasic waveforms, where the shock pulse widths were determined by the measured shock impedance. Echocardiograms were analyzed by an independent core laboratory with a responder defined as a decrease of left ventricular end systolic volume >15%. RESULTS: The study cohort was 74% male, with a mean age of 68.7 ± 10.9 years. The baseline ejection fraction was 0.245 ± 0.076, and the mean New York Heart Association class was 2.9 ± 0.4. In CRT responders (n = 32) the mean DFT was 415.6 ± 108.1 V at implantation vs. 415.6 ± 124.7 V at 6 months (P = .9), and in nonresponders (n = 19) the mean DFT was 452.6 ± 102 V at implantation vs. 447.4 ± 112.4 V at 6 months (P = .8). There was no significant change in DFT peak voltage, delivered energy, or shock impedance over time. CONCLUSION: DFTs were unchanged at 6 months in CRT patients with or without LV chamber reverse remodeling.

Does defibrillation threshold increase as left ventricular ejection fraction decreases?

AIMS: Advanced cardiac disease, entailing more hypertrophy, fibrosis, scarring, dilatation and conduction delays, poses the question of whether defibrillation thresholds (DFTs) increase as left ventricular ejection fraction (LVEF) decreases. This question has been approached indirectly or insufficiently in previous studies. In this study we add and expand on our previous work, stratifying DFT for various LVEF ranges. METHODS AND RESULTS: This retrospective analysis included DFT data from three acute, multicentre, randomized studies that included 230 ICD/CRT-D patients. All DFTs were obtained with the SVC coil turned ON and with pulse-width optimized waveforms based on a 3.5 ms membrane time constant. As the LVEF decreased, DFT estimates increased from 395.2 +/- 115 V for LVEF > or = 46% to 425.8 +/- 117.6 V for LVEF < or = 25%. However, these changes in DFT estimates were very minor and not statistically significant. Only 3% of the patients in this population had an elevated DFT of >20 J. CONCLUSION: This analysis shows that over a very broad range of LVEF, DFT changes minimally (approximately 1 J), if at all. Our results are consistent with previous studies that demonstrated no difference in the DFT estimates: (a) between patient groups receiving ICD (typically higher LVEF) vs. CRT-D (typically lower LVEF) and (b) between patient groups receiving a device for primary prevention indications (typically lower LVEF) vs. secondary prevention indications (typically higher LVEF).

Performance of dedicated versus integrated bipolar defibrillator leads with CRT-defibrillators: results from a Prospective Multicenter Study.

INTRODUCTION: Right ventricular (RV) anodal stimulation may occur in cardiac resynchronization therapy defibrillators (CRT-D) when left ventricular (LV) pacing is configured between the LV lead and an electrode on the RV defibrillator lead. RV defibrillator leads can have a dedicated proximal pacing ring electrode (dedicated bipolar) or utilize the distal shocking coil as the proximal pacing electrode (integrated bipolar). This study compares the performance of integrated versus dedicated leads with respect to anodal stimulation incidence, sensing, and inappropriate ventricular tachyarrhythmia detection in patients implanted with CRT-D. METHODS: Two hundred ninety-two patients were randomly assigned to receive dedicated or integrated bipolar RV leads at the time of CRT-D implantation. Patients were followed for 6 months. RESULTS: Patients with dedicated bipolar RV leads exhibited markedly higher rates of anodal stimulation than did patients with integrated leads. The incidence of anodal stimulation was 64% at implant for dedicated bipolar RV leads compared to 1% for integrated bipolar RV leads. The likelihood of anodal stimulation in patients with dedicated leads fell progressively during the 6-month follow-up (51.5%), but always exceeded the incidence of anodal stimulation in patients with integrated leads (5%). Clinically detectable undersensing and oversensing were very unusual and did not differ significantly between lead designs. There were no inappropriate ventricular tachyarrhythmia detections for either lead type. CONCLUSION: Integrated bipolar RV defibrillator leads had a significantly lower incidence of RV anodal stimulation when compared to dedicated bipolar RV defibrillation leads, with no clinically detectable oversensing or undersensing, and with no inappropriate ventricular tachyarrhythmia detections for either lead type.

Optimization of superior vena cava coil position and usage for transvenous defibrillation.

BACKGROUND: Prior studies of active pectoral implantable defibrillator (ICD) lead systems demonstrated a lowering of defibrillation thresholds (DFTs) with the addition of a superior vena cava (SVC) coil. These studies were done on fixed-tilt waveforms where a large reduction in impedance leads to large phase duration changes. OBJECTIVE: The present study was designed to evaluate the SVC coil benefit and intercoil spacing on DFTs with a "tuned" waveform. METHODS: This prospective, multicenter study included 113 patients randomized at implant to a 17-cm and a 21-cm intercoil spacing ICD lead. DFTs were measured with SVC coil turned ON versus OFF in a random order, using an optimized binary search method. RESULTS: DFT voltage (423 +/- 120 vs. 438 +/- 118 V; P = .042) and stored energy (9.8 +/- 5.6 vs. 10.2 +/- 5.8 J; P = .043) were significantly reduced with the SVC coil ON. However, intercoil distance had no significant effect on DFT voltage (437.3 +/- 115.1 vs. 407.7 +/- 123.8 V; P = .19) or stored energy (10.3 +/- 5.4 vs. 9.2 +/- 5.8 J; P = .31). Subgroup analyses showed that the dual-coil leads were most effective when placed in the high position (innominate vein-SVC junction) or when the single-coil shock impedance was > or =58 Omega, regardless of intercoil spacing. CONCLUSION: With a tuned waveform, the addition of an SVC coil to the shocking pathway reduces DFTs, although this difference was smaller than reported previously. Intercoil distance had no significant effect on the defibrillation parameters.

Thermal injury with contemporary cast-application techniques and methods to circumvent morbidity.

BACKGROUND: Thermal injuries caused by application of casts continue to occur despite the development of newer cast materials. We studied the risk of these injuries with contemporary methods of immobilization. METHODS: Using cylindrical and L-shaped limb models, we recorded the internal and external temperature changes that occurred during cast application. Variables that we assessed included the thickness of the cast or splint, dip-water temperature, limb diameter and shape, cast type (plaster, fiberglass, or composite), padding type, and placement of the curing cast on a pillow. These data were then plotted on known time-versus-temperature graphs to assess the potential for thermal injury. RESULTS: The external temperature of the plaster casts was an average (and standard deviation) of 2.7 degrees +/- 1.9 degrees C cooler than the internal temperature. The external temperature of twenty-four-ply casts peaked at an average of 84 +/- 42 seconds prior to the peak in the internal temperature. The average difference between the internal and external temperatures of the thicker (twenty-four-ply) casts (4.9 degrees +/- 1.3 degrees C) was significantly larger than that of the thinner (six and twelve-ply) casts (1.5 degrees +/- 1 degrees C) (p < 0.05). Use of dip water with a temperature of <24 degrees C avoided cast temperatures that can cause thermal injury regardless of the thickness of the plaster cast. A dip-water temperature of 50 degrees C combined with a twenty-four-ply cast thickness consistently yielded temperatures high enough to cause burns. Use of splinting material that was folded back on itself was associated with a significant risk of thermal injury. Likewise, placing a cast on a pillow during curing resulted in temperatures in the area of pillow contact that were high enough to cause thermal damage, as did overwrapping of a curing plaster cast with fiberglass. Attempts to decrease internal temperatures with the application of isopropyl alcohol to the exterior of the cast did not decrease the risk of thermal injury. CONCLUSIONS: Excessively thick plaster and a dip-water temperature of >24 degrees C should be avoided. Splints should be cut to a proper length and not folded over. Placing the limb on a pillow during the curing process puts the limb at risk. Overwrapping of plaster in fiberglass should be delayed until the plaster is fully cured and cooled.

A method to measure cervical spine motion over extended periods of time.

STUDY DESIGN: System validation study. OBJECTIVE: To develop and validate a motion sensor system for measuring cervical spine motion over extended time periods. SUMMARY OF BACKGROUND DATA: Many studies using different methodologies have tried to estimate cervical spine motion. These have mostly been carried out in a laboratory setting performing active/passive range of motion or activities of daily living. However, cervical spine performance over extended periods of time in natural environments remains unknown. METHODS: A novel motion sensor system, Wisconsin Analysis of Spine Motion Performance (WASP), was validated using 2 benchmarks: a materials testing machine (MTS) and optical motion tracking laboratory. Parameters tested included drift, frequency response, accuracy, effect of sensor orientation, and coupled motions. Applied motions from the MTS and measured motions in subject volunteers under various conditions were compared with WASP using correlation coefficients. Intersubject and intrasubject variability analyses for WASP were also performed. RESULTS: The average WASP slopes for accuracy (compared with MTS) in flexion-extension, lateral bending, and axial rotation were 0.89, 0.93, and 0.38, respectively. The correlation coefficient was 0.99 in all cases. Compared with optical motion tracking, the WASP regression slopes were 1.1, 1.02, and 0.4 and the correlation coefficients were 0.98, 0.92, and 0.93 in the 3 axes of motion. Coupled motion was noted during all subject motions. WASP peak detection algorithm had a 0% error discounting boundary conditions. CONCLUSION: WASP was accurate in flexion-extension and lateral bending. In axial rotation, WASP was less accurate. However, the system was highly reliable with low intersubject and intrasubject variability. WASP can be used in estimating cervical spine motion with high reliability while keeping in mind the decreased accuracy in measuring axial rotation.

"Tuned" defibrillation waveforms outperform 50/50% tilt defibrillation waveforms: a randomized multi-center study.

INTRODUCTION: A superior performance of a tuned waveform based on duration using an assumed cardiac membrane time constant of 3.5 ms and of a 50/50% tilt waveform over a standard 65/65% tilt waveform has been documented before. However, there has been no direct comparison of the tuned versus the 50/50% tilt waveforms. METHODS: In 34 patients, defibrillation thresholds (DFTs) for tuned versus 50/50% tilt waveforms in a random order were measured by using the optimized binary search method. High voltage lead impedance was measured and used to select the pulse widths for tuned and 50/50% tilt defibrillation waveforms. RESULTS: Delivered energy (7.3 +/- 4.6 J vs 8.7 +/- 5.3 J, P = 0.01), stored energy (8.2 +/- 5.1 J vs 9.7 +/- 5.6 J, P = 0.01), and delivered voltage (405.9 +/- 121.7 V vs 445.0 +/- 122.6 V, P = 0.008) were significantly lower for the tuned than for the 50/50% tilt waveform. In four patients with DFT >/= 15 J, the tuned waveform lowered the mean energy DFT by 2.8 J and mean voltage DFT by 45 V. For all patients, the mean peak delivered energy DFT was reduced from 29 J to 22 J (24% decrease). Multiple regression analysis showed that a left ventricular ejection fraction < 20% is a significant predictor of this advantage. CONCLUSION: Energy and voltage DFTs are lowered with an implantable cardioverter defibrillator that uses a tuned waveform compared to a standard 50% tilt biphasic waveform.

The effect of stretch rate and activation state on skeletal muscle force in the anatomical range.

BACKGROUND: The effects of stretch rate and activation state on muscle mechanics require further clarification. This subject is of particular interest because of the role of skeletal muscle undergoing eccentric contractions in musculoskeletal injuries. METHODS: The present study investigated the force-displacement behavior of rabbit tibialis anterior muscle at three stretch rates (2.5, 10, 25 cm/s) and three activation states (passive, tetanic, denervated). A phenomenological power law model and a dynamic systems model were used to describe the mechanical responses. FINDINGS: The power law model showed excellent agreement with the passive and denervated responses to stretch (R(mean)=0.97). Repeated measures analysis of variance found a difference (P=0.042) in peak force between the passive and denervated states at a stretch rate of 2.5 cm/s. The dynamic systems model closely fit the tetanized muscle responses (R(mean)=0.95). There was no difference in the displacement at yield (P=0.83) for the three stretch rates of the tetanized muscle undergoing stretch. INTERPRETATION: Differences between the passive and denervated responses suggest that mechanoreceptors may play a role in stimulating the muscle as it is stretched through the anatomical range. The displacement at yield did not change significantly over a decade range of stretch velocities, suggesting that a strain threshold exists beyond which cross bridges cannot remain bound. The power law and dynamic systems models presented offer mathematically tractable approaches to interpret the response of lengthening skeletal muscle. These findings on active, passive, and denervated muscle point to a possible role of the muscle spindle to tissue mechanical behavior that should be accounted for in future studies of force-elongation behavior of skeletal muscle.

Stiffness of occipital-cervical constructs: beam theory.

OBJECTIVE: The purpose of this study is to show that stiffness of an occipital-cervical construct can be predicted based on rod geometry and material. MATERIALS AND METHODS: Various rod-plate implants were tested as previously reported biomechanical studies of occipital-cervical fixation with the exception that no spine was used. A testing frame that holds paired contoured rods and plates to the same position as in the biomechanical testing protocol for occipital-cervical fixation was tested in the flexion-extension direction on a servo-hydraulic testing machine. Stiffness was determined from the plots of applied moment versus angular displacement. The occipital-cervical constructs were then modeled as a curved beam in pure bending in the sagittal plane to calculate the moment of inertia and theoretical stiffness. The Pearson correlation coefficient was used to assess the correlation of the experimental to the theoretical calculated stiffness. Product of inertia and material stiffness were determined for implants from previously published studies and the predicted rank order of this product was compared with the rank order of the observed biomechanical results in each study. RESULTS: A strong correlation was observed between the experimental and theoretical stiffness (R = 0.85). A strong influence of the inertia was also found on the experimental construct stiffness (R = 0.77). In five of six previously published studies, the best experimental performance was predicted using simple mechanical calculations. CONCLUSION: This study shows that both the theoretical stiffness and the calculated area moment of inertia are strongly correlated with the experimental stiffness of tested occipital-cervical fixation constructs.

Biomechanics of occipitocervical fixation.

STUDY DESIGN: A human cadaveric biomechanical study comparing occipital fixation techniques. OBJECTIVES: To compare ranges of motion between midline and lateral occipital fixation and between rigid and nonrigid occipital fixation of an unstable craniocervical spine. SUMMARY OF BACKGROUND DATA: New fixation techniques using rods and screws increase surgical choice on where fixation is placed onto the occiput. Lateral fixation theoretically gives improved resistance to deformation because of its increased effective moment arm and bilateral purchase. Midline fixation allows significantly longer screw purchase. This study compares these two fixation location. METHODS: Cadaveric occipital cervical spine specimens were tested biomechanically intact and under six different fixation techniques. Range of motion between the skull and C2 at 1.5 N-m and 2 N-m bending moments was measured in flexion-extension, lateral bending, and axial rotation. Mechanical testing of different rod diameters and a reconstruction plate was performed and compared with biomechanical testing. Results were compared between the intact condition and all fixations, between the medial and lateral fixations, and between the rigid and nonrigid fixations by analysis of variance. RESULTS: The range of motion of all constructs was significantly reduced compared with intact. Significant differences between groups were only seen in lateral bending in fixation placed laterally. Mechanical testing demonstrated that construct stiffness was predicted by area moment of inertia of the rod and plate to a greater degree than variation in placement of occipital screws or locking of the implant. CONCLUSION: The choice of location of occipital fixation should be based more on the ease of use and instability pattern. The decreased stiffness of the newer small rod systems should be considered.

Biomechanical comparison of four C1 to C2 rigid fixative techniques: anterior transarticular, posterior transarticular, C1 to C2 pedicle, and C1 to C2 intralaminar screws.

OBJECTIVE: We performed a biomechanical comparison of several C1 to C2 fixation techniques including crossed laminar (intralaminar) screw fixation, anterior C1 to C2 transarticular screw fixation, C1 to 2 pedicle screw fixation, and posterior C1 to C2 transarticular screw fixation. METHODS: Eight cadaveric cervical spines were tested intact and after dens fracture. Four different C1 to C2 screw fixation techniques were tested. Posterior transarticular and pedicle screw constructs were tested twice, once with supplemental sublaminar cables and once without cables. The specimens were tested in three modes of loading: flexion-extension, lateral bending, and axial rotation. All tests were performed in load and torque control. Pure bending moments of 2 nm were applied in flexion-extension and lateral bending, whereas a 1 nm moment was applied in axial rotation. Linear displacements were recorded from extensometers rigidly affixed to the C1 and C2 vertebrae. Linear displacements were reduced to angular displacements using trigonometry. RESULTS: Adding cable fixation results in a stiffer construct for posterior transarticular screws. The addition of cables did not affect the stiffness of C1 to C2 pedicle screw constructs. There were no significant differences in stiffness between anterior and posterior transarticular screw techniques, unless cable fixation was added to the posterior construct. All three posterior screw constructs with supplemental cable fixation provide equal stiffness with regard to flexion-extension and axial rotation. C1 lateral mass-C2 intralaminar screw fixation restored resistance to lateral bending but not to the same degree as the other screw fixation techniques. CONCLUSION: All four screw fixation techniques limit motion at the C1 to 2 articulation. The addition of cable fixation improves resistance to flexion and extension for posterior transarticular screw fixation.