Right ventricular pressure during ventricular arrhythmias in humans: potential implications for implantable antitachycardia devices

Effects of mechano-electric feedback on scroll wave stability in human ventricular fibrillation

Recruitment of stretch-activated channels, one of the mechanisms of mechano-electric feedback, has been shown to influence the stability of scroll waves, the waves that underlie reentrant arrhythmias. However, a comprehensive study to examine the effects of recruitment of stretch-activated channels with different reversal potentials and conductances on scroll wave stability has not been undertaken; the mechanisms by which stretch-activated channel opening alters scroll wave stability are also not well understood. The goals of this study were to test the hypothesis that recruitment of stretch-activated channels affects scroll wave stability differently depending on stretch-activated channel reversal potential and channel conductance, and to uncover the relevant mechanisms underlying the observed behaviors. We developed a strongly-coupled model of human ventricular electromechanics that incorporated human ventricular geometry and fiber and sheet orientation reconstructed from MR and diffusion tensor MR images. Since a wide variety of reversal potentials and channel conductances have been reported for stretch-activated channels, two reversal potentials, −60 mV and −10 mV, and a range of channel conductances (0 to 0.07 mS/µF) were implemented. Opening of stretch-activated channels with a reversal potential of −60 mV diminished scroll wave breakup for all values of conductances by flattening heterogeneously the action potential duration restitution curve. Opening of stretch-activated channels with a reversal potential of −10 mV inhibited partially scroll wave breakup at low conductance values (from 0.02 to 0.04 mS/µF) by flattening heterogeneously the conduction velocity restitution relation. For large conductance values (>0.05 mS/µF), recruitment of stretch-activated channels with a reversal potential of −10 mV did not reduce the likelihood of scroll wave breakup because Na channel inactivation in regions of large stretch led to conduction block, which counteracted the increased scroll wave stability due to an overall flatter conduction velocity restitution.

Development of Implantable Devices for Continuous Ambulatory Monitoring of Central Hemodynamic Values in Heart Failure Patients

BACKGROUND

Care and management of patients with congestive heart failure (CHF) is a major health-care challenge. The value of acute hemodynamic data in assessing heart failure has been questioned in some studies, while more intensive hemodynamic monitoring has been reported to improve patient care in others. A series of patient studies are reported here that were conducted to identify device requirements and verify the feasibility of continuous hemodynamic monitoring in CHF patients and devices for remote transfer and use of these data.

METHODS AND RESULTS

The results of four separate studies in 68 CHF patients who received systems for chronic hemodynamic monitoring between 1992 and the present are reviewed. One early study was with five patients followed for 7-16 months and another study was with nine patients followed for 4-22 months. A third study included 21 patients followed up to 39 months, and the fourth study included 32 patients implanted in 1998-99 with many of them still in follow-up. These studies support the technical feasibility of implanted devices and the external instrumentation required to transfer and manage the collected data. They also support the long-term stability and accuracy of these systems. Three additional acute studies conducted with 30 patients and chronic data from 53 of the 68 patients with the implanted systems are presented that support the feature included in the newer monitors--the ability to reliably estimate pulmonary artery diastolic pressures from the right ventricular pressure signal.

CONCLUSIONS

Development of implantable technology to measure several hemodynamic variables in ambulatory CHF patients is feasible. External instrumentation needed to remotely acquire data from the implanted devices has been verified. The potential to eliminate the uncertainties associated with the use of acute, invasive hemodynamics and the ability to evaluate long-term ambulatory hemodynamic patterns is provided. These findings set the stage for determining the potential clinical value of these systems in impacting the care of chronic CHF patients.

Antiarrhythmic therapy--future trends and forecast for the 21st century

This article discusses recent changes in antiarrhythmic therapy, with a focus on nonpharmacologic therapy (electrode catheter ablation, implantable cardioverter-defibrillators [ICDs]), and puts them into perspective for the coming years. The treatment of supraventricular tachycardias and tachycardia involving accessory pathways is likely to remain the domain of catheter ablation. With promising new techniques under investigation, the spectrum of arrhythmias that can be cured will probably be expanded. Treatment of life-threatening ventricular arrhythmias is likely to remain the domain of the ICD in the foreseeable future. With the safety net of the ICD in place, new antiarrhythmic drugs or other forms of antiarrhythmic therapy can be developed and tested.

Right and left ventricular hemodynamic performance during sustained ventricular tachycardia

Several factors may influence hemodynamic tolerance of a ventricular tachycardia (VT) episode but, to date, only VT rate has been used as a major detection criterion in selecting implantable cardioverter-defibrillator therapy algorithms. We examined hemodynamic changes during VT in humans and a possible correlation between left and right ventricular hemodynamic indexes. Right ventricular hemodynamic indexes could reflect systemic hemodynamics but previous studies have been inconclusive. Patients with coronary artery disease and a history of recurrent, sustained VT were studied. Aortic pressure and right and left ventricular pressures were simultaneously recorded with 2 dual micromanometer-tipped high-fidelity pressure catheters during sinus rhythm and during induced sustained monomorphic VT. Beat-to-beat analysis was performed using custom-made software. Nine patients (7 men, mean age 60 +/- 8 years, mean ejection fraction 24 +/- 8%) with 11 VT episodes (mean cycle length 283 +/- 48 ms) were studied. During VT, left and right ventricular systolic pressures showed a mean decrease of 57% and 26%, respectively, with weak correlation (r = 0.67, p = 0.06) between both values. There was also an increase in mean left and right ventricular end-diastolic pressures of 26% and 74%, respectively, and no correlation was seen (r = -0.2, p = 0.6). A significant correlation was found between changes in left and right ventricular maximal positive dP/dt (55% and 28% decrease, respectively (r = 0.69, p = 0.03) and between changes in left and right ventricular maximal negative dP/dt (64% vs 39% decrease, r = 0.71, p = 0.02). Most ventricular time parameters in both ventricles differed significantly during VT compared with sinus rhythm; however, only the decrease in right ventricular time to end-diastolic pressure correlated with the decrease in left ventricular systolic pressure, at the 10th VT beat (r = 0.8, p = 0.01). We conclude that left and right ventricles are hemodynamically unequally affected during rapid VT. Although right ventricular pressures cannot be reliably used to assess changes in the hemodynamic status of the left ventricle, additional parameters, such as dP/dt or changes in ventricular time intervals, should be further evaluated for inclusion in implantable cardioverter-defibrillator algorithms.

Nonpharmacological therapy for malignant ventricular arrhythmias: implantable defibrillator trials

Implantable cardioverter-defibrillators (ICDs) are an important nonpharmacological option in the treatment of malignant ventricular arrhythmias. Technological advances in current devices permit nonthoracotomy implantation with transvenous lead systems using biphasic shocks. Decreasing device size has resulted in pectoral implantation. Battery longevity is still short in comparison with that of pacemakers. Lead failure rates as well as pacing thresholds are significantly higher than those for cardiac pacing lead systems. Other complications of ICD systems include infection, perforation, and thrombosis. The long-term performance of nonthoracotomy lead systems for ICD devices has now been extensively studied. Sudden death recurrence rates for these systems are less than 2% in 3 years and less than 5% at 5 years. Clinical trials with both monophasic and biphasic systems show a high degree of prevention of sudden death. Comparison of ICD outcome with that of drug therapy in three large retrospective studies and two small prospective randomized trials favors improved survival and sudden death prevention with device therapy. However, these studies need corroboration from large prospective trials. Two large prospective trials, CIDS and the AVID study, are now in progress to address this issue.

Arrhythmia recognition strategies and hardware decisions for the implantable cardioverter-defibrillator--a review

The avoidance of inappropriate shocks from the implantable cardioverter-defibrillator (ICD), together with its need to apply antitachycardia pacing to either atria or ventricles, demands considerable sophistication in the design of algorithms to interpret electrical or other cardiac signals in real-time. Methods based on rate and using single short-gap bipolar leads lack discrimination. Right ventricular electrogram morphology algorithms offer improvement but no universal algorithm exists; however, for any given patient an optimum algorithm of this type might be found. One improvement would be to provide atrial information in addition, by employing more than one electrode or a long-gap single bipolar lead. Alternatively, transducer signals could be included, once their efficacy and reliability have been improved. A different approach would be to use the much more sophisticated algorithms at present being tried with surface electrocardiograms. Integrated Circuit technology is reaching the point where this could be done but the requirement for exceptionally high reliability means that special system structures, such as a Memory Intensive Computer Architecture, may be required. When decisions on these approaches are to be made, it must also be remembered that ICDs will soon be implanted and programmed as a routine rather than a highly specialized procedure.

Use of telemetry functions in the assessment of implanted antitachycardia device efficacy

Twenty patients (aged 50 +/- 21 years and mean left ventricular ejection fraction 37 +/- 17%) with recurrent ventricular arrhythmias were treated with an investigational, implantable combined antitachycardia-pacing cardioverter defibrillator. The device's telemetry capabilities include both stored (1-second snapshots) and real-time display of endocardial and device-circuit signals. The device can store these before, during and after up to 50 tachycardia and antitachycardia pacing episodes. All stored events are indexed to a 24-hour internal clock. During 10.1 +/- 5.1 months of follow-up, the device was used in 11 of 20 patients. In the entire group, antitachycardia pacing was activated on 44 +/- 14 occasions per patient (total 874) and shock delivery occurred on 8 +/- 14 occasions per patient (total 156). Reconstruction by stored telemetry of all device-therapy episodes was possible. Twenty-six percent of all shocks delivered were not appropriate and were due to atrial arrhythmias in 2 patients and dysfunction of the sensing lead in 3. The absence of a relation between symptoms and appropriate shock delivery was documented in 1 patient. Antitachycardia pace acceleration occurred in 5.3% of cases; 7% of attempts at pacing were unsuccessful and needed shock therapy. It is concluded that the enhanced telemetry available in newer antitachycardia devices enables more accurate assessment of device use and enhances diagnosis of inappropriate therapy delivery.

The range of sensors and algorithms used in rate adaptive cardiac pacing

Implantable sensors play an important role in physiological cardiac pacing. Sensors can be classified according to the technical methods in which sensing is achieved: the sensing of the evoked ventricular response, intrathoracic impedance and body acceleration forces, and the incorporation of special sensors on pacing electrodes. These sensors differ in their relative merits in terms of speed, proportionality, sensitivity, and specificity of rate response. The efficacy of a sensor can be significantly modified by the algorithm used in relating sensor signal to a pacing rate change. The currently available types of sensors and algorithms are summarized and compared in this review article. The relative merits of these sensors and algorithms form the basis for designing a multisensor pacing system.

Short-term reproducibility over time of right ventricular pulse pressure as a potential hemodynamic sensor for ventricular tachyarrhythmias

The implantable cardioverter defibrillator (ICD) has been shown to effectively terminate episodes of ventricular tachyarrhythmias. Multiple investigators have suggested that the incorporation of hemodynamic sensors may allow ICDs to differentiate between hemodynamically unstable and stable ventricular tachyarrhythmias (VT), as well as differentiate ventricular from supraventricular tachycardias. Right ventricular (RV) pulse pressure has been shown to possess acceptable characteristics as a sensor for incorporation in ICDs. We sought to determine the short-term reproducibility of RV pulse pressure measurements by comparing RV pulse pressure measured during two separate episodes of VT in each of ten study patients. The mean VT cycle length for VT episode 1 was 293 +/- 15 msec, and was 298 +/- 15 msec for VT episode 2 (P = NS). The decrease in mean arterial pressure was 40 +/- 7 mmHg in episode 1 and 37 +/- 7 mmHg in episode 2 (P = NS). The decrease in RV pulse pressure during episode 1 was -13 +/- 2 mmHg, and -12 +/- 2 during episode 2 (P = NS). The decrease in RV pulse pressure during episodes of VT at two different times during a single electrophysiology study is highly reproducible, suggesting that RV pulse pressure may be a reliable sensor over time.

Implantable cardioverter defibrillators: current status and future directions

Sudden cardiac death remains the most common mode of mortality in the United States, accounting for up to 450,000 deaths per year. Survivors of cardiac arrest and patients who have recurrent ventricular tachycardia have a high mortality rate with or without antiarrhythmic therapy. The implantable cardioverter defibrillator (ICD) was introduced in 1980 by Mirowski as a potential treatment for these patients. There are presently over 24,000 implants worldwide and the device has proved to be an effective means of preventing sudden death. The components of an ICD include a generator, defibrillation patches or leads, and pacing/sensing leads. The devices can be implanted with acceptable mortality and morbidity either by median sternotomy, left anterior thoracotomy, subxiphoid, or left subcostal approaches. The long-term results have been excellent with an actuarial incidence of sudden cardiac death of 3% at 5 years. Improvements in battery and capacitor technology, lead design, and tachycardia recognition, combined with the addition of hemodynamic sensors and a better understanding of the science of defibrillation, should lead to further improvements over the next several years in the ICD.

Development of a Rate Adaptive Pacemaker Based on the Maximum Rate-of-Rise of Right Ventricular Pressure (RV dP/dtmax)

The maximum rate of rise of right ventricular pressure (RV dP/dtmax) may change in response to physiological stress and thereby provide an appropriate parameter upon which to base rate adaptive pacing. Initial feasibility testing was carried out in six patients using externally closed loop rate adaptive pacing with a pressure sensing lead (Model 6220) and an investigational VVI pulse generator (Medtronic, Model 2451). During exercise, maximum positive RV dP/dtmax increased from 223 +/- 55 to 405 +/- 181 mmHg.sec.1 (P less than 0.05). Based on these results, rate adaptive pulse generators using maximum positive RV dP/dt were implanted in 12 patients (Medtronic, Model 2503). Exercise treadmill testing in the VVI mode resulted in heart rates ranging from 69 +/- 6 beats/min at rest to 79 +/- 14 beats/min (n = 12; P greater than 0.05). In contrast, VVIR mode pacing rates ranged from 71 +/- 11 beats/min to 115 +/- 24 beats/min (n = 17; P less than 0.05). Holter recording showed heart rates ranging from 51 +/- 6 to 110 +/- 22 beats/min during activities of normal daily living (n = 9; P less than 0.05). Passive postural tilt resulted in rates of 69 +/- 8 beats/min in the supine position increasing to 74 +/- 14 beats/min with 60 degrees upright tilt (n = 16; P greater than 0.05). With up to 5-year follow-up data, there have been no late failures of pacing but one lead showed insulation failure with over- and undersensing after 4.5 years. A number of deficiencies were identified in the prototypes leading to modifications of a subsequent generation of rate responsive pacemaker based on RV dP/dtmax. These initial data demonstrate that rate adaptive pacing based on RV dP/dtmax responds in a physiological manner. This rate responsive system is of particular interest as it is based on a beat-to-beat parameter of cardiac mechanical function.

Antitachycardia devices: realities and promises

Nonpharmacologic therapy for ventricular arrhythmias has gained growing attention with the development of the implantable cardioverter-defibrillator. In addition, the reports of adverse effects of drug therapy from several studies, including the Cardiac Arrhythmia Suppression Trial (CAST), have supported the need for these devices. The development of new implantable cardioverter-defibrillators that have the capability of antitachycardia pacing, bradycardia pacing, cardioversion and defibrillation has enhanced their clinical utility. The currently available implantable cardioverter-defibrillators have been shown to significantly improve survival after sudden cardiac arrest in patients with life-threatening ventricular arrhythmias. Newer devices with expanded capabilities may reduce mortality even further. In this report the features of currently available antitachycardia devices and implantable cardioverter-defibrillators are reviewed and the features and current implant data on newer antitachycardia devices are discussed.

Tachycardia recognition and diagnosis from changes in right atrial pressure waveform--a feasibility study

Implantable defibrillators either monitor heart rate or use a probability density function to detect ventricular fibrillation/tachycardia. As a result, they are unable to discriminate sinus tachycardia and atrial arrhythmias from malignant ventricular rhythms. We have assessed high fidelity fiber-optic pressure recordings in the right atrium during cardiac arrhythmias in 23 patients (mean age 44 years, 11 females) undergoing electrophysiological study. The unfiltered pressure signal was amplified and recorded on paper. During sinus rhythm, a constant amplitude deflection occurred during atrial systole (a wave). A characteristic waveform pattern was observed during each of the studied tachyarrhythmias, which included atrial flutter and fibrillation, atrioventricular nodal reentrant tachycardia, atrioventricular reentrant tachycardia, and ventricular tachycardia with and without ventriculoatrial conduction. The waveform pattern allowed clear visual discrimination of the underlying arrhythmia. Mean atrial pressure was increased during all arrhythmias and did not allow discrimination of the nature of the tachycardia. High fidelity pressure recordings produced characteristic appearances for pattern recognition of each arrhythmia studied. They allowed determination of the temporal relation between electrical and mechanical cardiac events and may have potential in the detection and recognition of cardiac arrhythmias.

Mixed venous oxygen saturation for differentiating stable from unstable tachycardias

Current antitachycardia systems are incapable of adequately distinguishing stable from unstable tachycardias. Previously, integration of a pressure sensor or an impedance sensor, together with electrogram analysis, has been investigated as an improved method of identifying unstable arrhythmias. A mixed venous oxygen saturation sensor was investigated for differentiating stable from unstable paced and induced tachycardias in 10 patients. During rapid pacing at 600, 500, 400, 350, 300, and 250 msec cycle lengths, mixed venous oxygen saturation decreased as cycle length decreased. For any given cycle length, rapid ventricular pacing tended to result in greater mixed venous oxygen desaturation compared with atrial pacing. Mixed venous oxygen saturation decreased similarly during induced ventricular tachycardias at cycle lengths greater than 230 msec. However, ventricular tachycardias at cycle lengths less than or equal to 230 msec and ventricular fibrillation had no effect on mixed venous oxygen saturation until after termination. Subsequently, a mixed venous oxygen saturation-tiered therapy algorithm (cycle length less than or equal to 230 msec = unstable; cycle length greater than 230 msec and MVO2 greater than or equal to 6% over 30 seconds = unstable) was developed and was tested retrospectively in 113 paced and induced tachyarrhythmias in these 10 patients for detecting unstable tachycardias (defined as a decrease from baseline systolic arterial pressure of greater than or equal to 50 mm Hg at 15 seconds). The mixed venous oxygen algorithm had 93% sensitivity and 96% specificity compared with rate-only (rate greater than or equal to 170 beats/min) detection with 93% sensitivity and 71% specificity.(ABSTRACT TRUNCATED AT 250 WORDS)

Reduction in defibrillator shocks with an implantable device combining antitachycardia pacing and shock therapy

Implantable defibrillators reduce the risk of sudden death in patients with malignant ventricular arrhythmias, but significant restriction in quality of life can occur as a result of frequent device activation. To determine if a device that provides both antitachycardia pacing and shock therapy can safely reduce the frequency of shocks after implantation, 46 consecutive patients undergoing initial implantation of a defibrillator were studied. In all patients, the implanted device provided antitachycardia pacing and shock therapy. Detected tachycardia characteristics and the results of therapy were stored in the device's memory. There were 42 men and 4 women, aged 26 to 71 years (mean 58.7 +/- 13.5). Left ventricular ejection fraction ranged from 13% to 67% (mean 32.2 +/- 13.4%) and 31 patients had experienced one or more episodes of cardiac arrest. Induced arrhythmias included sustained monomorphic ventricular tachycardia in 38 patients, nonsustained polymorphic ventricular tachycardia in 2 and ventricular fibrillation in 4. Over a total follow-up period of 255 patient-months (range 1 to 13, mean 6.1), 25 patients experienced spontaneous arrhythmic events. In 22 patients, 909 episodes of tachycardia were treated by antitachycardia pacing, which was successful on 840 occasions (92.4%). Acceleration of ventricular tachycardia by pacing therapy was estimated to have occurred 39 times. Syncope occurred once during pacing-induced acceleration of ventricular tachycardia. Forty-four episodes of tachycardia in seven patients were treated directly by shocks because of short tachycardia cycle length; 88% of all detected tachycardias were treated without the need for shocks. Four patients died from cardiorespiratory failure and one patient died suddenly without any detected tachyarrhythmia.(ABSTRACT TRUNCATED AT 250 WORDS)

A prospective study of changes in right ventricular dP/dt during ventricular tachycardia

The automatic implantable cardioverter defibrillator (AICD) has significantly decreased mortality in high risk ventricular tachycardia (VT) patients. The AICD provides treatment based on ventricular rate, sometimes leading to high energy shocks in conscious patients with stable VT, or patients with sinus or supraventricular tachycardia. Other physiological parameters, such as maximal positive and negative systolic right ventricular (RV) dP/dt (RV + dP/dtmax, RV - dP/dtmax, respectively), may be included in detection algorithms for future implantable defibrillators. We studied frequency band limited positive and negative RV dP/dtmax before, during, and after 13 episodes of VT lasting at least 40 beats in duration in nine male patients. The mean (+/- SEM) RV + dP/dtmax, dropped by 120 +/- 28 mmHg/sec (P less than 0.001) during the first five beats of VT. RV + dP/dtmax then slowly rose toward baseline levels until a significant overshoot occurred during the first ten beats following VT termination (delta = 234 +/- 58 mmHg/second, P less than 0.002). RV + dP/dtmax correlated poorly with mean arterial pressure (r = 0.32, P greater than 0.1), systolic blood pressure (r = 0.19, P greater than 0.1), and VT cycle length (r = 0.34, P greater than 0.1). Conversely, RV - dP/dtmax rose during the first ten beats of VT (74 +/- 27 mmHg/sec, P greater than 0.05) and then slowly drifted back toward baseline levels. Like RV + dP/dtmax, RV - dP/dtmax overshot baseline levels during the recovery phase (-108 +/- 48 mmHg/sec, P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Biosensor applications to antitachycardia devices

Current arrhythmia detection algorithms are unable to adequately distinguish stable from unstable tachycardias; therefore application of a biosensor to antitachycardia devices has been proposed to improve their performance. Right heart pressures and impedance have been investigated for incorporation into these systems. Integration of other parameters (oxygen saturation, preejection period, pH, cardiac output, flow, and temperature) into these devices might also prove useful. The status of these biosensor arrhythmia detection algorithms and their application to antitachycardia devices are described below.

A prospective study of right ventricular pulse pressure and dP/dt to discriminant-induced ventricular tachycardia from supraventricular and sinus tachycardia in man

In the future, automatic implantable cardioverter defibrillators (AICD) may incorporate sensors to differentiate hemodynamically stable from unstable ventricular tachycardias (VT). These sensors should also discriminate between ventricular and supraventricular tachycardias to avoid inappropriate responses from the device. Right ventricular pulse pressure (RVPP) and maximal systolic right ventricular dP/dt (dP/dt) were measured before, during and after 91 episodes of hemodynamically stable VT (VTs), hemodynamically unstable VT (VTus), supraventricular tachycardia (SVT) and sinus tachycardia (ST) induced in 49 male patients. The mean percent changes (mean +/- S.E.M.) in RVPP from baseline (% delta RVPP) during VTs and VTus were -35 +/- 3% and -72 +/- 3%, respectively (both P less than 0.001). The % delta RVPP during ST was +56 +/- 11% (P less than 0.01) and % delta RVPP was unchanged from baseline during SVT (+2 +/- 9%; P greater than 0.01). Mean % change in RV dP/dt from baseline was -20 +/- 3% during VTs (P less than 0.001), -36 +/- 5% during VTus (P less than 0.001), +15 +/- 13% during SVT (P less than 0.01), and +85 +/- 23% during ST (P greater than 0.01). The mean percent changes in RVPP were significantly different between each arrhythmia group (P less than 0.01). The mean % changes in RV dP/dt were significantly different only between ST and VTs or VTus and between SVT and VTus. The range of values for % delta RVPP during VTs overlapped considerably with the ranges of % delta RVPP during VTus and SVT. The ranges of % delta RVPP overlapped minimally between VTus and SVT. Percent change RVPP separated each episode of VTs and VTus from those of ST. The range of common values for % delta dP/dt between all four groups was extensive. It is concluded that % delta RVPP from baseline is significantly different between groups of patients during VTs, VTus, SVT, and ST, but that a large degree of overlap in the range of values for % delta RVPP and RV dP/dt between different arrhythmias groups may limit the specificity of these hemodynamic variables in separating different arrhythmias.