Indirect bronchial shunt flow measurements in AbioCor implantable replacement heart recipients.

Bronchial shunt flows in the recipients of the electrohydraulic AbioCor implantable replacement heart have been measured indirectly. A built-in compliance chamber accommodates the differential flow output required of the two ventricles of the AbioCor. An occluder mechanism regulates the flow differential. For a thoracic unit, given a beat rate, an occluder setting, and the pressure differentials across the replacement heart ventricles, the atrial pressure difference depends only on the level of shunt flow present in the vasculature. For a replacement heart recipient, the bronchial shunt is the dominant shunt flow. For patients implanted with the AbioCor, the beat rates and the occluder settings are known and the pressure differentials across the ventricles are estimated. Atrial pressures were measured using catheters. The bronchial shunt flow was deduced from in vitro characterization data based on these parameters. Available data from five patients in the ongoing clinical trial of AbioCor showed 0-1.4 L/minute bronchial shunt flows. Maximum variation for any individual patient was 1.1 L/minute.

Concentrated autologous plasma protein: a biochemically neutral solder for tissue welding.

BACKGROUND AND OBJECTIVE: Xenographic or allographic serum protein solders used for laser welding may have immunologic and/or pathogenic complications. The objective of these studies was to develop a safe, autologous solder. STUDY DESIGN/MATERIALS AND METHODS: Five methods of preparing concentrated autologous plasma protein solder (CAPPS) were evaluated. Next, the CAPPS was evaluated via (1) thermal denaturation studies using differential scanning calorimetry, (2) tissue welding studies to characterize both acute and healing properties. RESULTS: The optimal concentration method to produce CAPPS rapidly was a dialysis method using chemical (osmotic) forces. The CAPPS showed similar denaturation profiles to serum albumin (SA) solders. Acutely, CAPPS provided comparable breaking strengths to SA solders. At 7 days, there was no significant difference in breaking strength or histology between 50% human SA solder and CAPPS (using a porcine skin model). CONCLUSIONS: These studies demonstrate that the CAPPS system provides acceptable acute and chronic properties for laser welding.

Differential scanning calorimetry of albumin solders: interspecies differences and fatty acid binding effects on protein denaturation.

BACKGROUND AND OBJECTIVE: Understanding albumin solder denaturation is important for laser tissue soldering. Human (HSA), bovine (BSA), porcine (PSA), and canine (CSA) albumin both fatty acid containing (FAC) and fatty acid free (FAF) were evaluated by using differential scanning calorimetry (DSC). STUDY DESIGN/MATERIALS AND METHODS: DSC was used to measure difference thermograms to determine the irreversible thermal denaturation profile for 50% albumin solutions. The denaturation transition's onset, end and peak temperatures, and enthalpy were measured. RESULTS: All FAC species, except BSA, exhibited twin peaked endotherms. Single endotherms were observed for all FAF species and BSA-FAC. Onset and end temperatures were significantly [P < 0.001] lower for all FAF species (except BSA's end temperature). There was a 30% decrease in the denaturation enthalpy between FAF and FAC groups. CONCLUSION: FAF albumin solders were found to denature at significantly lower temperatures, while also having a 30% reduction in enthalpy when compared with their FAC counterparts.

Heart booster: a pericardial support device.

BACKGROUND: Congestive heart failure is a pervasive disease afflicting millions of people. For many, their quality of life can be significantly improved by a pericardial device that can enhance cardiac output without the added risk of thromboembolism associated with direct blood contact. METHODS: A cardiac assist device with tubular elements is wrapped around the heart. Fluid is pumped into and out of the wrap causing contraction and dilation of its circumference. This contracting and relaxing action generates cardiac assistance without the need to contact blood. In vitro characterization and in vivo studies in calves were conducted to demonstrate the characteristics of the device. RESULTS: In vitro characterization with the device wrapped around one-half of a ventricle to simulate left ventricular support demonstrated outputs of 6.5 L/min at physiological afterloads. In vivo studies in calves demonstrated both cardiac output and afterload enhancements when the device is activated. CONCLUSIONS: This study provides a first demonstration of a device that provides cardiac support by a contraction and relaxation scheme with the device wrapped around the epicardium of the heart. The main feature of this actuation method is the potential for building a small device for implantation without blood contact.

Long-term mechanical circulatory support system reliability recommendation: American Society for Artificial Internal Organs and Society of Thoracic Surgeons: long-term mechanical circulatory support system reliability recommendation.

Jointly developed by members of the American Society for Artificial Internal Organs and the Society of Thoracic Surgeons along with staff from the Food and Drug Administration, the National Heart, Lung and Blood Institute and other experts, this recommendation describes the reliability considerations and goals for Investigational Device Exemption and Premarket Approval submissions for long-term, mechanical circulatory support systems. The recommendation includes a definition of system failure, a discussion of an appropriate reliability model, a suggested in vitro reliability test plan, reliability considerations for animal implantation tests, in vitro and animal in vivo performance goals, the qualification of design changes during the Investigational Device Exemption clinical trial, the development of a Failure Modes Effects and Criticality Analysis, and the reliability information for surgeons and patient candidates. The document will be periodically reviewed to assess its timeliness and appropriateness within five years.

Long-term mechanical circulatory support system reliability recommendation: American Society for Artificial Internal Organs and The Society of Thoracic Surgeons: long-term mechanical circulatory support system reliability recommendation.

Jointly developed by members of the American Society for Artificial Internal Organs and the Society of Thoracic Surgeons along with staff from the Food and Drug Administration, the National Heart, Lung and Blood Institute and other experts, this recommendation describes the reliability considerations and goals for Investigational Device Exemption and Premarket Approval submissions for long-term, mechanical circulatory support systems. The recommendation includes a definition of system failure, a discussion of an appropriate reliability model, a suggested in vitro reliability test plan, reliability considerations for animal implantation tests, in vitro and animal in vivo performance goals, the qualification of design changes during the Investigational Device Exemption clinical trial, the development of a Failure Modes Effects and Criticality Analysis, and the reliability information for surgeons and patient candidates. The document will be periodically reviewed to assess its timeliness and appropriateness within five years.

Design considerations for bearingless rotary pumps.

The designs of rotary blood pumps have shown substantial technical progress over recent years, especially contact bearing designs. However, the concern for potential thromboembolism remains and can only be eliminated by the use of bearingless pumps. Bearingless designs can be achieved through the application of magnetic, hydrodynamic, and hydrostatic forces or a proper combination of these forces. Although a purely magnetically suspended, actively controlled system can be designed, judicious use of hydraulic forces can allow simplification of device configuration and control. In this study, bearingless designs were evaluated for both axial and centrifugal pump configurations. Trade-offs between shear rates and bearing leak rates were considered based upon constraints imposed by hemolysis and residence time. These principles were used for determining the design feasibility of a rotary pump using combined magnetic and hydraulic stabilizing forces.

A tubular pediatric ventricular assist device. Design considerations and system characteristics.

A clinical need exists for the short-term use of pediatric ventricular assist device in children and small infants who are critically ill with heart failure unresponsive to pharmacologic support or, in the case of irreversible heart failure, as a bridge to a transplant device. The design considerations and device characteristics of a tubular pump are presented. The device consists of an integrally formed inflow valve, pump chamber, and outflow valve in a tubular construction. This design approach was selected due to its simplicity of fabrication, which can result in a reliable and low cost device. The inflow valve and pump are actuated pneumatically through a single drive line. The outflow valve can be either actively actuated or operated passively. A 5 ml stroke volume device was built and characterized in vitro and in vivo. This pump can generate 0.5 L/min at 100 beats per minute. Larger stroke volume devices can be and have been fabricated using the same principle.

A magnetically suspended and hydrostatically stabilized centrifugal blood pump.

A magnetically suspended centrifugal blood pump intended for application as a long-term implantable ventricular assist device has been built and tested. The rotor is freely suspended in the blood by magnetic and hydrostatic restoring forces. This design obviates the need for bearings and shaft seals, and eliminates the problems of reliability and thrombogenicity associated with them. The positional stability and hydrodynamic performance of the pump has been characterized in vitro at flows of up to 10 L/min at physiologic pressures. Radial position control is realized by an analog electronic feedback control system. The pressure distribution in the fluid surrounding the rotor provides dynamic control in the axial direction with no active feedback. Rotor excursion is less than 50 microns (mu) when the housing receives an impulse peaking at an acceleration of 40 g or upon sudden blockage of the flow. In vitro blood measurements indicate an acceptable level of hemolysis compared with that of a standard centrifugal pump.

Temperature-controlled laser photocoagulation of soft tissue: in vivo evaluation using a tissue welding model.

BACKGROUND AND OBJECTIVES: Laser surgical procedures involving photocoagulation of soft tissue have relied on subjective visual endpoints. The thermal damage to the denatured tissue in these procedures is highly dependent on the tissue temperatures achieved during laser irradiation. Therefore, a system capable of real time temperature monitoring and closed loop feedback was used to provide temperature controlled photocoagulation (TCPC). STUDY DESIGN/MATERIALS AND METHODS: The TCPC system consisted of a 1.32 microns Nd:YAG laser, an infrared thermometer, and a microprocessor for data acquisition and feedback control. A porcine skin model was used. Tissue welds were completed to evaluate the photocoagulation effects at different predetermined temperatures. A quantitative measurement of tissue photocoagulation was obtained by tensile strength measurements of the laser repairs. Histology of the irradiated tissue was used to determine the extent of thermal injury associated with different photocoagulation temperatures. RESULTS: The TCPC system was capable of maintaining a relatively constant temperatures (+/- 4 degrees C) during laser irradiation. The tensile strengths of acute repairs increased with temperature over the range studied (65-95 degrees C). Tensile measurements made after several days of healing showed that higher temperature (95 degrees C) welds had lower strengths than repairs completed at lower (65 degrees C or 75 degrees C) temperatures and were significantly lower at 3 days. Acute histology showed that the amount thermal damage was strongly dependent on the tissue temperature and increased both in tissue depth and lateral to the repair with temperature. The histologic results suggest that the increase in the acute repair tensile strength as the weld temperature increased was due to an increase in the depth of tissue photocoagulation. The increase in the lateral tissue injury measured histologically for higher temperature welds likely resulted in the decreased chronic tensile strengths, as a healing response to excessive thermal damage. CONCLUSION: Tissue temperatures can be controlled during laser photocoagulation of skin. The degree of acute and chronic tissue damage is highly dependent on the temperature during welding. By controlling the tissue temperature during laser procedures, the surgical outcome can be more reliably predicted and reproduced, as compared to the conventional open loop methods. In addition, the use of a TCPC system should significantly reduce the learning curve for photothermal surgical procedures.

Laser assisted vascular welding with real time temperature control.

BACKGROUND AND OBJECTIVE: Previous studies in laser assisted vascular welding have been limited by the lack of a reliable end point for tissue fusion. As a means of improving the reproductibility of laser assisted repairs, a system incorporating real time temperature monitoring and closed loop feedback was used. STUDY DESIGN/MATERIALS AND METHODS: The system consisted of a direct view infrared thermometer for monitoring the laser heated spot, a 1.9 microns diode laser, and a microprocessor for data acquisition and feedback control of the laser power to maintain a constant tissue temperature. Rat aortas were welded under constant surface temperature conditions. RESULTS: In vivo temperature stability of +/- 2 degrees C was achieved over a temperature range of 70-90 degrees C pertinent to welding small vessels. When welds were completed using the feedback system to maintain the tissue temperature at 80 degrees C, the acute success rate was 100% and the burst pressure was 290 +/- 70 mmHg. CONCLUSION: These studies demonstrate that the use of real time monitoring and feedback control results in improved consistency for vascular tissue welding.

Progress in the development of the ABIOMED total artificial heart.

The ABIOMED implantable total artificial hearts in the final phase of engineering development. The system has a compact electrohydraulically driven energy converter sandwiched between two blood pumps, an internal electronics pack, an internal battery, a transcutaneous energy transmission coil for power transmission, and external wearable electronics pack and battery. The current effort is to complete development of the system during 1996 in preparation for formal pre clinical testing of the device. In vivo studies with the current thoracic unit (ABH II) have achieved 108 days of survival verifying the thermal, physiologic, and hematologic compatibility of the system. The abdominal implantable electronics pack showed no thermal dissipation problem. System improvements include scaling down the size of the thoracic unit, and efficiency enhancement in the power and hybrid electronics. The new system (ABH III) retains the flow capacity of greater than 10 L/min. Size reduction results in an atrial to sternal dimension that would fit 98% and 75% of men and women, respectively.

Chronic in vivo evaluation of an electrohydraulic total artificial heart.

Development of the Abiomed total artificial heart (TAH) designed for human use is progressing. Implant durations of longer than 60 days have been achieved in calves. The device consists of blood pumps, valves, and a hydraulic atrial flow balancing chamber fabricated from polyetherurethane. The energy converter, a centrifugal hydraulic pump with a rotary fluid switching valve, is positioned between the blood pumps. In two consecutive chronic in vivo studies (47 days and longer than 60 days), cardiac output was maintained in excess of 8 l/min. The atrial flow balancing chamber maintained a mean right-to-left pressure gradient of 7.5 and -1.4 mmHg in each respective study. There were no pulmonary complications. Platelet counts, fibrinogen concentrations, and hematocrit values returned to baseline levels within 20 days, whereas bilirubin, serum glutamic-oxaloacetic transaminase, blood urea nitrogen, and creatinine levels returned to normal within 1 week of implant. After the first post-operative day, plasma free hemoglobin levels of less than 10 mg/dl indicated no device-related hemolysis throughout the duration of the studies. At explant (47 day study), pathologic analysis showed no renal infarcts, no tissue necrosis, and no thermal damage. The device was fully encapsulated by 2-4 mm thick fibrous connective tissue. A newly designed textured-to-smooth surface inflow showed no signs of pannus ingrowth or thrombotic complications. These studies demonstrate that this TAH is suitable for long-term implantation.

An atrial hydraulic shunt in a total artificial heart. A balance mechanism for the bronchial shunt.

An implantable TAH must be able to maintain physiologic atrial pressures over a range of left side shunt flows, which in humans can range up to 5% of the total cardiac output (CO). The authors describe the characteristics of an atrial hydraulic shunt in an electrohydraulic TAH. A small (10 cc stroke) chamber placed in line between the left atrial cuff and the inflow valve is shunted to the right hydraulic chamber. High left atrial pressure increases the volume displaced by this chamber; this concurrently reduces the right chamber stroke volume and flow relative to the left side. For left atrial pressures (LAP) between 0 and 15 mmHg, CO increased from approximately 3 to > 9 L/min (Starling-like behavior). This was attainable with bronchial flow settings between 0.2 to 0.7 L/min, satisfying physiologic needs. Left atrial pressure and RAP (right atrial pressure) tracked each other. The mean difference, LAP-RAP, remained within 5 mmHg at low filling pressures and approached 0 mmHg difference at high filling pressures. The results showed that the atrial hydraulic shunt method can adequately compensate for and manage physiologic left-right flow differences.

A compact and noise free electrohydraulic total artificial heart.

The ABIOMED total artificial heart (TAH) is designed for long-term tether-free use in patients with end-stage heart disease. Blood pumping is achieved through hydraulic fluid motion across flexing diaphragms. The hydraulic power is derived from a miniature centrifugal pump (50% efficiency). Flow directional change needed for alternate left and right filling and ejection is achieved with a rotary valve. With no mechanical contact with the flexing membrane, the wedge angle between the two pumps sandwiching the energy convertor can be easily optimized for anatomic fit. The blood pumps (80 ml strokes) are fitted with trileaflet polyetherurethane valves (24 mm). The TAH is implanted using twist-lock stepless quick connectors to the inflow cuffs and outflow grafts. Left-right flow balance is achieved with an atrial hydraulic shunt placed between the left cuff and inflow valve. Animal studies show that the TAH fits very well in Long Horn calves weighing 90 to 100 kg and can provide cardiac output in excess of 10 L/min. A cadaver (85 kg) study showed the TAH fits within the pericardial region.

Life testing of implantable batteries for a total artificial heart.

Although lithium cells may promise to be ideal as a rechargeable internal battery for a TAH, NiCd cells remain the most easily accessible off the shelf energy source. Twelve 1.2 A.hr prismatic NiCd (Sanyo, San Diego, CA) cells in series are being tested under the load condition of our TAH. The load consisted of a 1.5 A DC current with 1 A pulses of 40 msec duration at 3.33 Hz (100 bpm), a condition that can generate up to 8 L/min of cardiac output at physiologic pressures. Cells were tested at 37 degrees C. Cell voltages and temperatures were monitored. Testing was accelerated to five charge/discharge cycles per day. Discharge was terminated when any one cell dropped below 1.1 V. Charging (C/4) was continued until the battery voltage indicated a change in slope. Cell temperatures remained below 42 degrees C throughout the charge/discharge cycle. The battery pack settled to a nearly constant capacity of over 25 min after 10 cycles and has accumulated more than 1,000 cycles. Voltage differences among cells were small (SD < 25 mV), indicating consistency among cells. NiCd cells can serve as a reliable interim for TAH internal battery application.