Quantification of Pulsatility as a Function of Vascular Input Impedance: An In Vitro Study

Mock circulatory loop applications for testing cardiovascular assist devices and in vitro studies

The mock circulatory loop (MCL) is an in vitro experimental system that can provide continuous pulsatile flows and simulate different physiological or pathological parameters of the human circulation system. It is of great significance for testing cardiovascular assist device (CAD), which is a type of clinical instrument used to treat cardiovascular disease and alleviate the dilemma of insufficient donor hearts. The MCL installed with different types of CADs can simulate specific conditions of clinical surgery for evaluating the effectiveness and reliability of those CADs under the repeated performance tests and reliability tests. Also, patient-specific cardiovascular models can be employed in the circulation of MCL for targeted pathological study associated with hemodynamics. Therefore, The MCL system has various combinations of different functional units according to its richful applications, which are comprehensively reviewed in the current work. Four types of CADs including prosthetic heart valve (PHV), ventricular assist device (VAD), total artificial heart (TAH) and intra-aortic balloon pump (IABP) applied in MCL experiments are documented and compared in detail. Moreover, MCLs with more complicated structures for achieving advanced functions are further introduced, such as MCL for the pediatric application, MCL with anatomical phantoms and MCL synchronizing multiple circulation systems. By reviewing the constructions and functions of available MCLs, the features of MCLs for different applications are summarized, and directions of developing the MCLs are suggested.

Pumping Rate Study of a Left Ventricular Assist Device in a Mock Circulatory System

The aim of this work was to investigate the hemodynamic influence of the change of pump rate on the cardiovascular system with consideration of heart rate and the resonant characteristics of the arterial system when a reliable synchronous triggering source is unavailable. Hemodynamic waveforms are recorded at baseline conditions and with the pump rate of left ventricular assist device (LVAD) at 55, 60, 66, and 70 beats per minute for four test conditions in a mock circulatory system. The total input work (TIW) and energy equivalent pressure (EEP) are calculated as metrics for evaluating the hemodynamic performance within different test conditions. Experimental results show that TIW and EEP achieve their maximum values, where the pump rate is equal to the heart rate. In addition, it demonstrates that TIW and EEP are significantly affected by changing pump rate of LVAD, especially when the pump rate is closing to the natural frequency of the arterial system. When a reliable synchronous triggering source is not available for LVAD, it is suggested that selecting a pump rate equal to the resonant frequency of the arterial system could achieve better supporting effects.

Left ventricular assist devices and gastrointestinal bleeding: a narrative review of case reports and case series

BACKGROUND

The use of left ventricular assist devices (LVADs) has become a state-of-the-art therapy for advanced cardiac heart failure; however, multiple reports in the literature describe an increased risk for gastrointestinal (GI) bleeding in these patients. We characterized this association by reviewing recent studies on this topic.

HYPOTHESIS

GI bleeding occurs frequently in patients with LVADs, especially with devices with nonpulsatile flow patterns.

METHODS

We performed a comprehensive literature review to identify articles that reported GI bleeding in patients with LVADs. Databases used included PubMed, EMBASE, Scopus, Web of Knowledge, and Ovid. Baseline and outcome data were then ed from these reports.

RESULTS

We identified 10 case reports and 22 case series with 1543 patients. The mean age was 54.2 years. Most patients had nonpulsatile LVADs (1316, 85.3%). Three hundred and seventeen patients (20.5%) developed GI bleeding; this occurred more frequently in patients with nonpulsatile LVADs. Multiple procedures were performed without complications but often did not identify a definite bleeding site. Suspect lesions occurred throughout the GI tract but were more frequent in the upper GI tract. Many patients had arteriovenous malformations. All patients received medical therapy. None of the patients had their LVAD replaced. The use of anticoagulation did not appear to predispose these patients to more GI bleeding episodes.

CONCLUSIONS

Patients with LVADs have frequent GI bleeds, especially from arteriovenous malformations, which can occur throughout the GI tract. Most diagnostic and therapeutic interventions can be used safely in these patients. The pathogenesis of the GI bleeding in these patients may involve the use of anticoagulant medications, the formation of arteriovenous malformations, loss of von Willebrand factor activity, and mucosal ischemia.

Increasing the transmitted flow pulse in a rotary left ventricular assist device

Long-term rotary left ventricular assist devices (LVADs) are increasingly employed to bridge patients with end-stage heart failure to transplant or as a destination therapy. Significant recent device development has increased patient support times, shifting further development focus toward physiologically sensitive control of the pump operation. Sensorless control of these devices would benefit from increased observability of the ventricular volume/preload to the pump, in order to regulate flow based on preload, imitating the native Frank-Starling flow control. Monitoring the transmitted flow pulse through the pump has been used as a surrogate for preload, although means of maximizing its transmission are not clear. However, it is known that a flat hydraulic performance curve of the rotary pump induces high changes in flow for a given change in pressure head. The aim of this study was to determine geometric pump parameters responsible for increasing this flow pulse transmission and to demonstrate this increase in vitro. The sensitivity of the performance gradient to blade angles, blade heights, blade clearance, and channel areas were studied. Resulting pressure head, flow, and hydraulic efficiency were analyzed with respect to textbook designed procedures. Then pumps with comparably "flat" and "steep" performance curves were used to simulate LVAD support in vitro over a range of pump flow rates to observe the transmitted flow pulsatility. It was found that an outlet blade angle of 90°, inlet blade angle between 25 and 45°, and large throat area generated a "flatter" performance curve. The transmitted flow pulsatility through a pump with a flat performance curve was 68% higher than that of a steep performance curve at a flow rate of 5 L/min. Substantial gains in the observability of LVAD preload/resident blood volume in the ventricle exist through the careful selection of specific pump geometries.

Performance evaluation of a pediatric viscous impeller pump for Fontan cavopulmonary assist

OBJECTIVE

The anatomic and physiologic constraints for pediatric cavopulmonary assist differ markedly from adult Fontan circulations owing to smaller vessel sizes and risk of elevated pulmonary resistance. In this study, hemodynamic and hemolysis performance of a catheter-based viscous impeller pump (VIP) to power the Fontan circulation is assessed at a pediatric scale (∼15 kg) and performance range (0-30 mm Hg).

METHODS

Computer simulation and mock circulation studies were conducted to assess the hydraulic performance, acute hemodynamic response to different levels VIP support, and the potential for vena caval collapse. Computational fluid dynamics simulations were used to estimate VIP hydraulic performance, shear rates, and potential for hemolysis. Hemolysis was quantified in a mock loop with fresh bovine blood.

RESULTS

A VIP augmented 4-way total cavopulmonary connection flow at pediatric scales and restored systemic pressures and flows to biventricular values, without causing flow obstruction or suction. VIP generated flows up to 4.1 L/min and pressure heads of up to 38 mm Hg at 11,000 rpm. Maximal shear rate was 160 Pa, predicting low hemolysis risk. Observed hemolysis was low with plasma free hemoglobin of 11.4 mg · dL(-1) · h(-1).

CONCLUSIONS

A VIP will augment Fontan cavopulmonary flow in the proper pressure and flow ranges, with low hemolysis risk under more stringent pediatric scale and physiology compared with adult scale. This technology may be developed to simultaneously reduce systemic venous pressure and improve cardiac output after stage 2 or 3 Fontan repair. It may serve to compress surgical staging, lessening the pathophysiologic burden of repair.

Mechanics and Function of the Pulmonary Vasculature: Implications for Pulmonary Vascular Disease and Right Ventricular Function

The relationship between cardiac function and the afterload against which the heart muscle must work to circulate blood throughout the pulmonary circulation is defined by a complex interaction between many coupled system parameters. These parameters range broadly and incorporate system effects originating primarily from three distinct locations: input power from the heart, hydraulic impedance from the large conduit pulmonary arteries, and hydraulic resistance from the more distal microcirculation. These organ systems are not independent, but rather, form a coupled system in which a change to any individual parameter affects all other system parameters. The result is a highly nonlinear system which requires not only detailed study of each specific component and the effect of disease on their specific function, but also requires study of the interconnected relationship between the microcirculation, the conduit arteries, and the heart in response to age and disease. Here, we investigate systems-level changes associated with pulmonary hypertensive disease progression in an effort to better understand this coupled relationship.

Hemodynamic energy changes after ischemia-reperfusion injury in an aortic cross-clamped rabbit model

Blood flow pulsatility can be quantified using the concept of hemodynamic energy. Because ischemia-reperfusion injury is known to affect microcirculation as well as vascular tone detrimentally, we hypothesized that vascular tone changes after ischemia-reperfusion injury would influence pulsatility of the blood vessels. We investigated the changes in pulsatility after ischemia-reperfusion injury using hemodynamic energy parameters-energy equivalent pressure (EEP) and surplus hemodynamic energy (SHE). Twenty-one New Zealand white male rabbits were divided into three groups. Ischemia group (I group, n = 7) underwent 3 hours of ischemia by clamping the abdominal aorta. Reperfusion group (I/R group, n = 7) underwent 2 hours of ischemia followed by 1 hour of reperfusion. Control group (C group, n = 7) underwent a sham procedure. Observed parameters were mean arterial pressure (MAP), mean blood flow (MBF), pulse pressure (PP), EEP, and SHE, measured at baseline, during ischemia (60 minutes after clamping), and reperfusion 5, 15, 30, and 60 minutes after clamp release. In I group, all parameters, except MBF, were higher during ischemia than at baseline. In I/R group, all except MBF were higher during ischemia. After reperfusion, MBF increased and the other parameters decreased. Interestingly, PP and EEP showed only minor changes during reperfusion (p = NS), whereas SHE decreased abruptly immediately after reperfusion and then gradually recovered to its baseline level. Surplus hemodynamic energy showed more significant changes than PP or EEP during reperfusion period (p < 0.05). In C group, no changes were noted throughout the observation period. Ischemia-reperfusion injury reduces vascular pulsatility. In this study, SHE was found to be a more sensitive hemodynamic energy parameter during ischemia-reperfusion injury than PP or EEP.

Biomechanics

This review covers research achievements in the field of biomechanics that have been obtained in the past few years and provides information to researchers in other fields of study.