Proven Accuracy in Clinical & Experimental Flow Measurement.
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G. Pantalos, PhD during a NASA K-135 flight.
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"Hearts in Space"
University of Louisville's Dr. George Pantalos' research has
culminated with "Art Heart" an artificial left ventricle for the "Hearts in Space" experiment program. The article below was
written to accompany the first orbital experiments for "Art Heart."
NASA Get Away Special Payload [G-572]
After 12 years of development, the Hearts in Space experiment had its first orbital flight opportunity on board the Space Shuttle
Discovery during a 12-day flight of the STS-85 mission in August, 1997. The experiment, a collaborative effort between students
and faculty at the University of Utah, Bellarmine College in Louisville, KY, and Utah State University was a qualified success
as some, but not all, of the mission objectives were achieved. All of the temperature surveillance data, vital to monitoring the
environment and function of the experiment, was collected but a connection failure limited the amount of blood pressure and blood flow data acquired.
Additional pressure and flow data were acquired during a successful follow-up spce flight in October, 1998.
Background
Cardiovascular adaptation to space flight investigations have documented an initial
increase in astronaut cardiac function followed by a progressive reduction in both left
ventricular volume index and stroke volume index with a compensatory increase in heart
rate to maintain cardiac output. The reduced cardiac size and stroke volume have been
presumed to result from a reduction in circulating fluid volume within a few days after orbital insertion, but no specific mechanism for the reduced stroke volume has been
identified. The investigation used a hydraulic simulator of the cardiovascular system, "Art
Heart," to examine the possibility that the observed reduction in stroke volume may, in
part, be related to fluid physics effects (i.e. hydrostatic pressure) on heart function.
Factors which influence the filling of the heart during diastole include: (1) the atrial
pressure, (2) the inertia of the blood as it enters the ventricle, (3) the transmural
pressure difference, (4) the myocardial compliance including myofibril passive, elastic
recoil, and (5) the gravitational acceleration-dependent hydrostatic pressure that exists
in the ventricle due to its size and anatomic orientation. The pressure gradient, which can be estimated to be 6660 dynes/cm2(¥ 5 mm Hg) in an average adult, acts to
augment the diastolic filling of the heart. Investigators hypothesize that the absence of
this contribution to the ventricular filling process in the microgravity environment of
space flight may account, in part, for physiological factors that would act to reduce cardiac filling and consequently, result in a reduced stroke volume.
Preparations for Orbital Flight and Launch
Fourteen experiments on board the NASA KC-135 aircraft and numerous ground tests,
including a shake table test, demonstrated correct experiment function. The parabolic
flight experiments also provided early support for the proposed hypothesis, although the 20 second periods of weightlessness created on the aircraft were not sufficient to
examine the fully regulated response to weightlessness.
Extraordinary efforts on the part of team members and supporting organizations overcame critical pre-launch technical problems. Family and friends from across the
country, including Transonic Systems' engineer Gary Thomas and his family, convened at the NASA Kennedy Space Center in Florida in August 1997 to watch the launch of the
Space Shuttle Discovery. The space shuttle emerged from the haze and clouds over Launch Complex 39A in an arc and hurtled toward orbit. Eight minutes later when the
shuttle entered orbit, all breathed a collective sigh of relief and exclaimed, "Art's in Space!"
Experimental Description
The experimental apparatus consisted of a pneumatically actuated elliptical artificial
ventricle (UTAH-100 human version left ventricle) connected to a closed-loop vascular simulator circuit with adjustable compliance and resistance elements to create
physiologic pressure and flow conditions. A 40% glycerin in water solution was used in
the simulator circuit to simulate the viscosity of whole blood. The ventricle was powered
by a miniaturized controller originally developed for use with clinical artificial heart
recipients. Ventricular instrumentation included high-fidelity, acceleration-insensitive,
catheter-tip pressure transducers in the apex and base to determine the instantaneous ventricular pressures and LVP across the left ventricle LVPapex - LVP base).The
ventricle was also instrumented with pressure transducers immediately upstream of the inflow valve in an artificial atrium and downstream of the outflow valve, and an
ultrasonic transit time flow probe (Transonic Systems Inc.) downstream of the outflow valve. Acceleration of the experiment was sensed by a miniature accelerometer and the
temperature at three locations in the experiment was sensed and recorded by an ambient temperature recording unit. A heating element was incorporated into the
hydraulic circuit to establish and maintain the temperature in the operational range of
20º to 40º C. The experiment was wrapped in a thermal insulating blanket of Nomex®
felt with a Beta cloth cover. A bi-directional roller rump was used to inject or withdraw
fluid from the hydraulic circuit to create different preload conditions for the artificial
ventricle. The experiment was microprocessor controlled with analog signals stored on a
seven channel FM data tape-recorder which can provide up to three hours of continuous
recording. Power for the experiment was provided by an array of alkaline batteries in a sealed battery box.
A baroswitch activated the temperature monitoring portion of the experiment during the launch phase at an altitude of 70,000 feet about two minutes into the launch; the
temperature monitoring being continued until just prior to planned reentry. Orbital operation of the experiment was initiated one day into the mission by the activation of
an aft flight deck switch by a crew member. Initially, the temperature of the experiment was measured. The temperature at that time, 16º C, was below the minimum operating
temperature of 20º C, so a heating cycle was started using the heating elements in the
fluid circuit. The artificial heart was periodically turned on every ten minutes during the
heating cycle for a few beats to circulate the fluid and distribute the heat around the
simulator fluid circuit. Performance of the experimental protocol was initiated once the minimum operational temperature had been established a few hours later. By
experimentally varying the circulating fluid volume in the hydraulic circuit, ventricular
function can be determined for varying preload pressures at a regulated afterload (aortic
) pressure of 95 mm Hg. This variation in preload condition will permit the construction of
a ventricular function curve for the microgravity environment to be compared to a ventricular function curve constructed from data recorded preflight in the 1-G
environment. If the proposed hypothesis is true, there will be a parallel shift to the right
of the ventricular function curve of approximately 2 mm Hg for the microgravity condition.
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