Hemodynamic predictors for weaning patients from ventricular assist devices (VADs)

Development of a hybrid decision support model for optimal ventricular assist device weaning

BACKGROUND

Despite the small but promising body of evidence for cardiac recovery in patients that have received ventricular assist device (VAD) support, the criteria for identifying and selecting candidates who might be weaned from a VAD have not been established.

METHODS

A clinical decision support system was developed based on a Bayesian Belief Network that combined expert knowledge with multivariate statistical analysis. Expert knowledge was derived from interviews of 11 members of the Artificial Heart Program at the University of Pittsburgh Medical Center. This was supplemented by retrospective clinical data from the 19 VAD patients considered for weaning between 1996 and 2004. Artificial Neural Networks and Natural Language Processing were used to mine these data and extract sensitive variables.

RESULTS

Three decision support models were compared. The model exclusively based on expert-derived knowledge was the least accurate and most conservative. It underestimated the incidence of heart recovery, incorrectly identifying 4 of the successfully weaned patients as transplant candidates. The model derived exclusively from clinical data performed better but misidentified 2 patients: 1 weaned successfully, and 1 that needed a cardiac transplant ultimately. An expert-data hybrid model performed best, with 94.74% accuracy and 75.37% to 99.07% confidence interval, misidentifying only 1 patient weaned from support.

CONCLUSIONS

A clinical decision support system may facilitate and improve the identification of VAD patients who are candidates for cardiac recovery and may benefit from VAD removal. It could be potentially used to translate success of active centers to those less established and thereby expand use of VAD therapy.

Left Ventricle Load Impedance Control by Apical VAD Can Help Heart Recovery and Patient Perfusion: A Numerical Study

The aim of this work is to investigate the dependence between left ventricular load impedance control by an apical ventricular assist device (VAD) and the consequent benefits for pathological heart recovery. A pathological left ventricle with 34% contractility has been simulated in the assisted and nonassisted conditions. By means of an extended Kalman filter, left ventricular pressure-volume loops have been partially estimated and ventricular as well as circulatory quantities inferred. The heart operation mode, based on cardiac energetic criteria, is imposed by controlling the VAD filling phase. In the assisted condition, results show that the left ventricle end-diastolic volume, left atrial pressure, and wall stress all decrease; stroke volume, ejection fraction, ventricular efficiency, aortic pressure, and cardiac output all increase. Benefits are also evident for the right ventricle and systemic and pulmonary circulation. The strategy outlined in this work also shows that good results for heart recovery are achievable and a possible way to improve the functional properties of commercial pulsatile VADs.

Biventricular Assist Device-Induced Right Ventricular Reverse Structural and Functional Remodeling

BACKGROUND

Support with a left ventricular assist device (LVAD) induces LV reverse structural and functional remodeling, evidenced by normalization of passive end-diastolic pressure-volume relationships (EDPVRs) and cardiomyocyte function. These changes are not evident in the right ventricle (RV), which remains dilated during LVAD support. However, studies on whether RV reverse remodeling could be induced by RV or biventricular assist support (BiVAD) have not been published.

METHODS

Whole hearts from 16 patients with end-stage congestive heart failure (CHF) at the time of cardiac transplantation without LVAD support, 16 patients with LVAD support, and 3 patients with BiVAD support were used to study RV EDPVRs, with estimation of chamber stiffness. Perfused isolated myocardial trabeculae were used for functional studies. Furthermore, RV free wall samples were used for histology and collagen determination by hydroxyproline.

RESULTS

The RV size, calculated from the ex vivo RV EDPVRs, RV mass, and myocyte diameter were significantly smaller in BiVAD-supported hearts than in non-supported or LVAD-supported hearts (p < 0.05) and reached normal levels. Furthermore, cardiomyocyte function demonstrated a normalized response to increased stimulation frequency and after perfusion with isoproterenol following BiVAD support. In addition, myocardial collagen content and chamber stiffness increased tremendously after BiVAD support (p < 0.05 vs CHF and LVAD).

CONCLUSION

BiVAD-induced hemodynamic unloading support resulted in significant reverse structural and functional remodeling of the right chamber. The lack of these findings during LVAD support alone provides additional support that diastolic pressure and volume unloading is an important mechanism underlying the process of reverse remodeling.

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.

Ventricular cardiac-assist devices in infants and children: anesthetic considerations

OBJECTIVE

The application of a mechanical cardiac-assist device is now a common procedure in modern cardiac surgery in patients with end-stage failure, whereas in pediatric patients it is still a great challenge. In the recent literature, a broad range of survival and weaning rates have been reported, depending on the variety of mechanical devices and the choice of patients with different conditions before implantation or if the device is used in emergency surgery. In this article, the authors report their experience with pediatric cardiac-assist devices and the perioperative anesthesia management in this group of patients.

PATIENTS AND METHODS

From 1997 to 2001, 11 infants and children were supported with a left and/or right ventricular-assist device. Diagnosis included myocarditis and complex cardiac malformations (hypoplastic left heart syndrome, tetralogy of Fallot with cardiomyopathy, and combined heart defects). The data sets of all patients were recorded using the online anesthesia record-keeping system NarkoData (IMESO GmbH, Hüttenberg, Germany). The program collects all perioperative data during surgery and during a stay in the PACU, including vital signs, administered drugs, as well as the data set of the German Society of Anesthesiology and Intensive Care Medicine.

RESULTS

All patients were divided into 2 groups: group 1 = survivors and group 2 = nonsurvivors. A total of 5 patients were in group 1, and group 2 consisted of 6 patients. The duration of anesthesia in group 1 patients (173.2 +/- 95.1 minutes) was significantly (p < 0.05) shorter than in group 2 (631.1 +/- 258.8 minutes) as well as the amount of packed red cells (group 1= 540.5 +/- 150.3 mL, group 2 = 880.6 +/- 400.3 mL). Cardiopulmonary bypass before implantation of a VAD was necessary only in 2 patients from group 1, whereas 5 patients in group 2 were on pump during the procedure. The rate of aortic cross-clamping was also significantly lower in group 1 than in group 2 (p < 0.05).

CONCLUSIONS

The surgical outcome depends on the patient's condition at the time of surgery. Emergency surgery, preoperative multiorgan failure, and the need for an extracorporeal circulation with aortic cross-clamping seem to predict a negative outcome in this group of patients.

Isolated right ventricular assist for postcardiotomy myocardial infarction

Due to myocardial infarction, profound postcardiotomy right heart failure developed in a 57-year-old man after implantation of an aortic homograft for infective aortic valve endocarditis. Despite maximum medical therapy and intraaortic balloon counterpulsation, signs of endorgan injury developed, and therefore a Thoratec (Pleasanton, CA) right ventricular assist device was implanted. After 17 days of support, myocardial and endorgan function had recovered and the fully mobilized patient was successfully weaned from support and discharged from the hospital.

Successful application of hypothermia combined with intra-aortic balloon pump support to low-cardiac-output state after open heart surgery

The authors report a successful application of hypothermia, along with intra-aortic balloon pump (IABP) support, to postcardiotomy ventricular failure. Surface-cooling hypothermia was applied in 8 patients after open heart surgery. The original cardiac procedure consisted of 3 aortocoronary bypass graftings (ACBGs), 2 aortic valve replacements (AVRs), 1 repair for left ventricular (LV) rupture after mitral valve replacement (MVR), 1 MVR + ACBG, and 1 MVR + AVR + tricuspid valve annuloplasty (TAP). Their ages ranged from fifty-two to sixty-eight years with a mean of sixty-one years. Hemodynamic criteria for induction of hypothermia included cardiac index (CI) less than 2.0 L/min/m2 with left atrial pressure greater than 18 mmHg despite the use of IABP and maximum pharmacologic support. Blood temperature was maintained at around 33 degrees C. By six hours after induction of hypothermia the tissue oxygen consumption decreased significantly with no hemodynamic deterioration as compared with that before cooling. The duration of hypothermia ranged from thirty-six to one hundred fifty-nine hours with a mean of seventy-eight hours. All 8 patients finally discontinued IABP support with a mean driving time of one hundred thirty-two hours. Five of them were ultimately discharged from the hospital and returned to their previous life-style. The authors believe that, from the perspective of monetary and personal resources, the use of hypothermia with IABP support could be a therapeutic option for patients with postcardiotomy ventricular failure.

Linear end-systolic pressure-volume relationship during pulsatile left ventricular bypass represents native heart function

This study assessed whether the end-systolic pressure-volume relationship obtained without any interventions during pulsatile left ventricular bypass adequately represents native heart function. In 11 anesthetized Holstein calves, left ventricular pressure was measured with a micromanometer while left ventricular volume was simultaneously calculated from orthogonal left ventricular diameters measured with ultrasonic dimension transducers. End-systolic pressure and volume data were subjected to linear regression analysis to achieve an end-systolic pressure-volume relationship. Data from both caval occlusions and aortic occlusion were used for the control end-systolic pressure-volume relationship (median r = 0.941, slope = 7.4 +/- 0.8 mm Hg per milliliter per 100 gm left ventricular weight; mean +/- standard error of the mean). During left atrial-aortic bypass with a Pierce-Donachy pneumatic assist pump in the asynchronous mode, the end-systolic pressure-volume relationships were obtained without interventions to change ventricular loading conditions. During maximal ventricular unloading during full to empty pumping, termed 100%, the resulting narrow range of pressure and volume data did not yield highly linear end-systolic pressure-volume relationships (median r = 0.669, slope = 4.9 +/- 0.9 mm Hg per milliliter per 100 gm left ventricular weight). However, at reduced rates off pumping, the end-systolic pressure-volume relationships were considerably linear (80%, median r = 0.819; 60%, median r = 0.868; 40%, median r = 0.899). Slopes did not significantly differ from control values (80%, 6.9 +/- 1.1; 60%, 8.2 +/- 1.1; 40%, 7.8 +/- 1.1). The end-systolic pressure-volume relationship obtained without exogenous load changes during asynchronous, pulsatile left ventricular bypass represents native left ventricular systolic function.