Continuous assessment of cardiac function during rotary blood pump support: a contractility index derived from pump flow.

BACKGROUND: The clinical application of rotary blood pumps (RBPs) for bridge-to-recovery and destination therapy has focused interest on the remaining contractile function of the heart and its course. This study reports a method to determine contractility that uses readily measured variables of the RBP. METHOD: The proposed index (I(Q)) is defined as the slope of a linear regression between the maximum derivative of the pump flow and its peak-to-peak value. I(Q) was compared with the maximal derivative of ventricular pressure (dP/dt(max)) vs end-diastolic volume (EDV) and the pre-load-recruitable stroke work. All indices were evaluated using computer simulations and animal experiments. For in vivo studies, a MicroMed-DeBakey ventricular assist device (VAD) was implanted in 7 healthy sheep. Ventricular contractility was examined under normal conditions and after pharmacologic intervention. For the computer simulation, variations of ventricular contractility, ventricular pre-load and after-load, and pump speeds were studied. RESULTS: In vivo and computer simulations showed the I(Q) index to be sensitive to changes of cardiac contractility, similar to other classic indices. For reduced cardiac contractility, it decreased to 9.3 +/- 3.9 (s(-1)) vs 15.3 +/- 4.0 (s(-1)) in the control condition (in vivo experiments). The I(Q) index was only marginally influenced by pre-load and after-load changes: a variation of 7.0% +/- 8.9% and 1.3% +/- 7.1%, respectively, was observed in computer simulations. CONCLUSIONS: The I(Q) index, which can be derived from pump data only, is a useful parameter for continuous monitoring of the cardiac contractility in patients with RBP support.

Coronary hemodynamics and myocardial oxygen consumption during support with rotary blood pumps.

Mechanical support offered by rotary pumps is increasingly used to assist the failing heart, although several questions concerning physiology remain. In this study, we sought to evaluate the effect of left-ventricular assist device (VAD) therapy on coronary hemodynamics, myocardial oxygen consumption, and pulmonary blood flow in sheep. We performed an acute experiment in 10 sheep to obtain invasively measured coronary perfusion data, as well as pressure and flow conditions under cardiovascular assistance. A DeBakey VAD (MicroMed Cardiovascular, Inc., Houston, TX, USA) was implanted, and systemic and coronary hemodynamic measurements were performed at defined baseline conditions and at five levels of assistance. Data were measured when the pump was clamped, as well as under minimum, maximum, and moderate levels of assistance, and in a pump-off condition where backflow occurs. Coronary flow at the different levels of support showed no significant impact of pump activity. The change from baseline ranged from -10.8% to +4.6% (not significant [n.s.]). In the pulmonary artery, we observed a consistent increase in flow up to +4.5% (n.s.) and a decrease in the pulmonary artery pressure down to -14.4% (P = 0.004). Myocardial oxygen consumption fell with increasing pump support down to -34.6% (P = 0.008). Left-ventricular pressure fell about 52.2% (P = 0.016) as support was increased. These results show that blood flow in the coronary arteries is not affected by flow changes imposed by rotary blood pumps. An undiminished coronary perfusion at falling oxygen consumption might contribute to cardiac recovery.

Suction events during left ventricular support and ventricular arrhythmias.

BACKGROUND: Axial blood pumps have very successfully entered the arena of prolonged clinical support. However, they offer only limited inherent load-responsive mechanisms for adjusting pumping performance to venous return and changes in the physiologic requirements of the patient. Therefore, excessive ventricular unloading can be observed in various situations of temporarily reduced venous return. In this study we report severe ventricular arrhythmias closely related to suction events in rotary blood pumps, a phenomenon that has not been described previously. METHODS: Data from a clinical trial intended to prove the feasibility of an automatic speed control system for pump recipients were analyzed with regard to electrocardiographic changes during ventricular collapse. The occurrence of excessive unloading was detected by an automatic suction detection system. Electrocardiograms (ECGs) were classified semi-manually, aided by a graphical use interface. For statistical data analysis, Wilcoxon's signed-rank test was utilized. RESULTS: After automatic suction detection a significant increase in monomorphic ventricular tachycardia was observed, from 0.015 to 0.099 event per second (p < 0.05). Furthermore, it was found that arrhythmic activity in terms of morphologic ventricular tachycardia increased after suction from 0.009 to 0.014 event per second. CONCLUSIONS: Excessive ventricular unloading of the left ventricle during continuous left ventricular support can induce ventricular arrhythmias. We detected evidence supporting an increase in arrhythmic activity after suction. This turned out to be a transient effect, which vanished within 5 minutes after suction. ECG events related to suction have a sudden onset and are severe ventricular arrhythmias, which can consist of even just one extrasystolic beat, and they usually cease after clearance of suction.

Left ventricular pressure-volume loop analysis during continuous cardiac assist in acute animal trials.

For better understanding of the interaction between left ventricle and continuous cardiac assist, the effect of different working conditions and support levels on left ventricular pressure-volume (PV) loop was investigated in acute animal experiments. A MicroMed-DeBakey ventricular assist device (MicroMed Cardiovascular Inc., Houston, TX, USA) was implanted in seven healthy sheep (102 +/- 20 kg). Measurements of hemodynamic variables were taken with clamped graft, on minimum, medium, and maximum support, and in pump-off condition (backflow). Each pump condition was studied for different heart rates, central venous pressures, and under pharmacologically altered contractility. End-systolic and end-diastolic volume normalized by the body surface area (BSA) (end systolic volume index [ESVI] and end diastolic volume index [EDVI]) showed significant correlation both within each sheep and in the pooled data. The linear regression for the pooled data was ESVI = 0.845 x EDVI - 15.21, R(2) = 0.924, P < 0.0001, n = 200. EDVI and stroke volume (SV) normalized by BSA (stroke volume index [SVI]) also showed a lower but significant correlation: SVI = 0.155 x EDVI + 15.21, R(2) = 0.291, P < 0.0001, n = 200. An increase of preload due to infusion caused, in the clamped graft condition, an increase in end diastolic volume of 22%, no significant increase in SV, a decrease both of systemic vascular resistance of 30% and ventricular contractility (maximum elastance [E(max)] and peak rate of rise of ventricular pressure [dP/dt(max)] decreasing 38 and 21%, respectively). PV loop analysis in continuous cardiac assist reveals that the ESVI and the EDVI are strongly correlated and that ESVI varies considerably with preload. SVI becomes slightly dependent on EDVI, which may be due to autoregulatory mechanisms.

Advanced suction detection for an axial flow pump.

An automatic detection system for ventricular collapse was developed and tested in a first clinical trial as part of a physiological speed control concept for axial flow pumps. From this clinical experience, and based on the acquired data during this trial, an optimization of the developed system was performed. An already-existing database of 784 individual cases was extended. For harmonization of this database an additional 412 snap files were extracted from continuous data recordings and classified manually using a standardized procedure. The already-developed and clinically tested algorithms were supplemented by one additional indicator derived from a preexisting criterion. One threshold value was replaced by application of a numerically optimized nonlinear characteristic curve dependent on heart rate. Finally, in a multidimensional optimization process of the entire suction detection system, 7 individual indicators were adjusted by using 17 independent threshold values. The optimization criteria were applied using a three-level hierarchical system. Within the final database consisting of 1196 snap shots the overall amount of maldetections could be reduced to 23 cases including 5 false positive events (0.42%) and 18 false negative decisions (1.5%). By application of the clinical experience from the first clinical trial of a physiologic control system it became possible to optimize the sensitivity and specificity of the suction detection system to unprecedented accuracy.

First clinical experience with an automatic control system for rotary blood pumps during ergometry and right-heart catheterization.

BACKGROUND: At present, most clinically implanted rotary blood pumps are operated at constant speed and adjusted by the physician. It is generally assumed that an adaptation of pump speed to the patient's physiologic requirements would be beneficial. The data provided in this paper, based on hemodynamic and spirometric data during exercise in which a pre-load-sensitive control was used, lend quantitative support to this assumption. METHODS: An automatic speed control was developed and implemented with Matlab on a dSpace controller board. The system uses pump speed, pump power, and pump flow as its only input signals. It was connected to the clinical hardware of the DeBakey VAD System. The control is pre-load-sensitive and uses an expert system to detect excessive unloading and eventual suction. This system was used to quantify the cardiovascular reaction of patients to both automatically controlled and constant pump speed. A sub-group of 5 patients underwent bicycle ergometry with Swan-Ganz catheterization and spiroergometry. RESULTS: The automatic, closed-loop speed control showed robust and stable performance. It provided an increase in pump flow (+0.94 +/- 0.5 liters/min, p < 0.05) compared with constant-speed mode in response to physical activity. Pulmonary arterial (PAP) and capillary wedge pressure (PCWP) clearly decreased (-7.4 +/- 4.1 mm Hg for PAP and -8.3 +/- 4.2 mm Hg for PCWP, p < 0.05), and venous oxygen saturation moderately increased (+5.2%). CONCLUSION: An automatic speed-control system for rotary blood pumps was developed and demonstrated by spiroergometry to be appropriately responsive to physiologic demand.

Development of a reliable automatic speed control system for rotary blood pumps.

BACKGROUND: Axial blood pumps have been very successfully introduced into the arena of prolonged clinical support. However, they do not offer inherent load-responsive mechanisms for adjusting pumping performance to venous return and changes in physiologic requirements of the patient. To provide for these adjustments we developed an algorithm for demand-responsive pump control based on a reliable suction detection system. METHODS: A PC-based system that analyzes pump performance based on available flow, heart rate and short-term performance history was developed. The physician defines levels of "desired flow" at rest and during exercise, depending on heart rate. In case this desired flow cannot be maintained due to limited venous return, the maximal available flow level is determined from an analysis of the actual pump data (flow, speed and power consumption). An expert system continuously checks the flow signal for any indication of suction. Periodic speed variations then adapt pump performance to the patient's condition. RESULTS: First, stability and functionality were proven under various settings in vitro. The algorithms were then tested in 15 patients in intensive care, in the standard ward, and during bicycle exercise. The system reacted properly to demand changes, at exercise level, in response to coughing and at various Valsalva maneuvers. Suction could also be successfully prevented during severe arrhythmia and in patients with critical cardiac geometry. Exercise tests showed decreases in pulmonary arterial pressure (-22 +/- 9.9%) and pulmonary capillary wedge pressure (-42 +/- 18.54%), and an increase in pump flow (19 +/- 9.5%) and workload (8 +/- 6.1%), all when compared with constant-speed pumping. CONCLUSIONS: A closed-loop control system equipped with an expert system for reliable suction detection was developed that improves response to change in venous return for rotary pump recipients. The system was robust, stable and safe under a wide range of everyday living conditions.

Development of a suction detection system for axial blood pumps.

Axial flow blood pumps for cardiac assistance have proven their clinical viability and benefit in recent years. However, the clinical systems to date have no direct mechanism to decrease pump speed when adequate supply is not available. This may lead to ventricular collapse or increase the probability of hemolysis and thrombotic risks. Based on various experiences with left ventricular assist device (LVAD) patients in various states of recovery, at implant, in the intensive care unit, in the standard ward, and during physical exercise, 11 different algorithms were developed for the automatic detection of ventricular suction. These detection algorithms analyze the flow pattern for the presence of distinct suction indicators. For selection and optimization of the algorithms, 1000 records from approximately 100 patients were collected. Each record contains 5 s of pump flow, current, and arterial pressure. Three experts classified these records in terms of suction probability and other abnormalities. The optimization was developed in Matlab, capable of solving a fifth-dimensional optimization problem with 256 different algorithm combinations. The optimization resulted in a set of 6 algorithms, each with specific thresholds. The system detects 100% of the known suction events with 0.28% of false-positive interpretations. If tuned to avoid any false-positive detection, 90.7% of the certain events would be detected. A strategy for the development of a robust suction detection system for axial blood pumps was found. This system will be integrated into an automatic pump speed control system to provide adequate perfusion for the LVAD recipient, without excessive unloading of the ventricle.

Weaning of rotary blood pump recipients after myocardial recovery: a computer study of changes in cardiac energetics.

BACKGROUND: Weaning of patients from mechanical cardiac support after myocardial recovery has always involved multiple, interacting factors, particularly the training of the myocardium during reduction of pump flow. Rotary pumps offer training advantages when support flow is reduced, even to nearly zero. We report a computer analysis that evaluates the work required of the heart during partial unloading and removal of rotary pumps. METHODS AND RESULTS: A computer model of the assisted circulation, previously implemented in MATLAB (The MathWorks Inc, Natick, Mass), has been augmented with a model of the MicroMed DeBakey ventricular assist device (MicroMed Technology, Inc, Houston, Tex). Flow, pressure patterns, and external work (pressure-volume area, calculated as the area of the ventricular pressure-volume loop [external work] plus potential energy) were calculated for nonassisted and various continuously assisted patients. Under low-flow conditions, the heart imposes an oscillating forward-backward flow through the non-occlusive rotary pump, causing an increase in ventricular work. Thus, an assist flow of 1 to 1.5 L/min requires work equivalent to that of the unsupported heart. At 60% contractility, the nonassisted pressure-volume area is 1.10 Ws/beat, and the potential energy is 0.38 Ws/beat. At a Qpump of 1 L/min, the pressure-volume area is 1.21 Ws/beat, and the potential energy is 0.37 Ws/beat. At a Qpump of 3 L/min, the pressure-volume area is 0.93 Ws/beat, and the potential energy is 0.29 Ws/beat. These conditions cannot be achieved with pulsatile systems. CONCLUSION: During weaning and retraining, an implanted rotary pump can provide a workload to the heart like that in the nonassisted situation, thus increasing the predictability of weaning and reducing the risk of reiterating heart failure.

Automatic system for noninvasive blood pressure determination in rotary pump recipients.

In patients with implanted rotary pumps, the arterial pressure pulsatility is usually far lower than in normal individuals. Depending on the remaining degree of pulsatility, cuff-based systems such as the classical Riva-Rocci-determination of arterial blood pressure and correlated sounds or pressure measurements based on cuffpressure oscillations become inaccurate or even impossible. Therefore, a system was developed which evaluates the flow in the radial artery using an ultrasound wristwatch sensor, and this additional information is used for pressure determination. A computerized data acquisition and cuff-control system based on a PC using Matlab software, a wristwatch ultrasound device, and a compressor-driven pressure cuff was set up. The cuff was controlled for automatic inflation and deflation cycles. Cuff pressure and arterial flow was recorded. Several algorithm strategies were developed, which gave data for systolic blood pressure and heart rate together with a reliability index for data quality. Finally, the new algorithms were implemented in a microcontroller system. Comparisons with invasive measurements showed excellent correlation with systolic blood pressure (mean deltaP -0.3 mm Hg, n = 28). During exercise of rotary pump patients and therefore enhanced pulsatility the difference from manual evaluation was -2.1 mm Hg (n = 18). In conclusion, adaptation of the classical cuff-pressure method with ultrasound evaluation of peripheral flow allows reliable determination of blood pressure in patients with low pulsatility resulting from implanted rotary cardiac assist pumps. By development of a wristwatch sensor and an automatic control system a robust method for daily use could be developed.

Interaction of the cardiovascular system with an implanted rotary assist device: simulation study with a refined computer model.

In recent years, implanted rotary pumps have achieved the level of extended clinical application including complete mobilization and physical exercise of the recipients. A computer model was developed to study the interaction between a continuous-flow pump and the recovering cardiovascular system, the effects of changing pre- and afterloads, and the possibilities for indirect estimation of hemodynamic parameters and pump control. A numerical model of the cardiovascular system using Matlab Simulink simulation software was established. Data of circulatory system modules were derived from patients, our own in vitro and in vivo experiments, and the literature. Special care was taken to simulate properly the dynamic pressure-volume characteristics of both left and right ventricle, the Frank-Starling behavior, and the impedance of the proximal vessels. Excellent correlation with measured data was achieved including pressure and flow patterns within the time domain, response to varying loads, and effects of previously observed pressure-flow hysteresis in rotary pumps. Potential energy, external work, pressure-volume area, and other derived heart work parameters could be calculated. The model offers the possibility to perform parameter variations to study the effects of changing patient condition and therapy and to display them with three-dimensional graphics (demonstrated with the effects on right ventricular work and efficiency). The presented model gives an improved understanding of the interaction between the pump and both ventricles. It can be used for the investigation of various clinical and control questions in normal and pathological conditions of the left ventricular assist device recipient.