Stephen Dubin

Cardiac energy considerations during intraaortic balloon pumping

Cardiac oxygen availability and oxygen consumption were used in a theoretical study as indexes of myocardial energy supply and utilization, respectively. A detailed computer simulation of the closed-loop canine cardiovascular system was utilized to study the dependence of these indexes on timing of the intraaortic balloon pump. Oxygen availability exhibited higher sensitivity to balloon timing than oxygen utilization. While maximum augmentation of oxygen availability was 58%, oxygen consumption could be reduced by only 13%. Animal experiments were initiated to validate the theoretical results. The results of both the animal experiments and the computer simulation suggested that neither balloon timing that maximizes oxygen availability nor timing that minimizes oxygen consumption correlates with timing that minimizes aortic end diastolic pressure. Thus, end diastolic pressure, presently used as a determinant of proper timing in patients undergoing cardiac assistance, was found to be a poor index of ventricular energy consumption.<<ETX>>

Performance optimization of left ventricular assistance. A computer model study.

Performance of temporary parallel left ventricular assistance was investigated and the theoretic conditions leading to optimal behavior of the mechanical system were explored. Computer models of nonpulsatile and pulsatile left ventricular assist devices (LVADs) were incorporated into a previously reported closed-loop simulation of the canine cardiovascular system. Assuming the assisted heart was capable of recovery, LVAD performance was assessed based on both myocardial oxygen balance and cardiac output. With a synchronous LVAD, and operating in a counterpulsation mode, these variables were sensitive to the phasing of pump ejection. Maximum reduction in cardiac oxygen consumption, maximum increase in oxygen availability, and maximum increase in cardiac output with the atrio-aortic device were obtained when pump ejection immediately followed aortic valve closure. These variables were directly proportional to the magnitude of bypass volume. The pulsatile asynchronous and nonpulsatile LVAD models affected oxygen balance in a similar manner, but neither performed so well as the synchronous model when equal bypass volumes were used. Ventricular uptake of blood provided a further 27% decrease in oxygen consumption and further 78% increase in oxygen availability than atrial uptake. In summary, the model predicted that the pulsatile synchronous LVAD, filling from the ventricle during heart systole and ejecting into either the ascending or descending aorta just after ventricular systole, would be most beneficial to both myocardial oxygen balance and cardiac output.

Optimal controller for intraaortic balloon pumping

An optimal control algorithm was adapted to identify and track the optimal deflation time of the intraaortic balloon pump (IABP). Routines for handling physiologically imposed constraints were added to the algorithm, which was implemented in a computer-controlled system. The system was designed to provide real time optimization for the clinical setting. Proper values for the algorithm parameters were determined and the system was tested in animal experiments. The results indicate that the controlling deflation time relative to the R wave, which precedes the next ejection phase, reduces the time required for optimization when the heart rate varies.<<ETX>>

Numerical calculation of the mean electrical axis of electrocardiographic deflections

Formulas are presented for numerical calculation of magnitude and direction of the mean electrical axis of electrocardiographic deflections suitable for use with hand calculators and electronic digital computers.

Porous Cathodes for Implantable Hybrid Cells

The in vivo power output of implantable power sources using porous platinum- or palladium-black cathodes is approximately four to five times greater than ones using commercially available cathodes of platinum black dispersed in Teflon. Porous electrodes produce a much lower foreign-body reaction than electrodes with plastic binders. Comparison of BET surface-area measurements for the powders and compacted electrodes illustrate that there is an approximate 20-percent decrease in surface area following compaction. The particle size, distribution, and shape of the powders were studied with the aid of scanning electromicrographs. In vitro galvanostatic measurements were made on the implantable power sources to study the electrochemical activity before implantation.

Synchronized External Pulsation for Improved Tolerance to Acceleration Stress: Model Studies and Preliminary Experiments

Synchronized external pulsation is proposed as a method to improve tolerance to acceleration stress. This technique uses a modified anti-G suit which is pressurized and depressurized synchronously with the heart cycle. The feasibility of the procedure has been studied using a computer model of the cardiovascular system which includes the effects of Gz stress, and contains simulations of baroreceptor control of heart rate and venous tone. Model predictions indicate that for unprotected subjects, carotid pressure at eye level (ophthalmic artery pressure) decreases to 20 mmHg (beginning of central light loss) at approximately +3.6 Gz. Applying standard anti-G suit pressure to the model increases this level to 5.3 Gz. When synchronized external pulsation of 2 psi is superimposed on the standard anti-G suit pressure, the tolerance to acceleration stress is further augmented by at least 0.9 G above the protection afforded by the standard anti-G suit alone. A set of preliminary experiments on human subjects to test the feasibility of using the technique in the high-G environment has also been carried out. The results under various protection modes compare favorably to the model predictions. Our results suggest that the computer model presented here is a useful tool for studying cardiovascular responses under +GZ stress. It also indicates that using synchronized external pulsation pressure superimposed on the standard anti-G suit pressure may offer extra protection to acceleration stress.

Polarization and Corrosion Studies of Porous and Solid Anodes for Implantable Power-Generating Electrodes

An implantable power source for driving a pacemaker consisting of a hybrid cell utilizing a sacrificial porous aluminum anode and a catalytic cathode for reducing oxygen is considered. Porous aluminum anodes show a significant improvement in polarization over a solid aluminum anode under in vitro transient operating conditions found with certain types of pacemakers. However, the in vivo transient behavior of porous aluminum anodes is only slightly better than solid aluminum anodes. Using in vivo and in vitro linear polarization studies, the corrosion of solid aluminum has been determined to be more than an order of magnitude less than that of solid zinc. The weight loss of aluminum due to corrosion is less than 3.0 percent that of the aluminum needed to supply the faradaic current to the pacemaker. A hybrid cell consisting of an aluminum anode and a porous palladium-black cathode has powered a commercially available pacemaker. Further histopathologic evaluation of tissues surrounding anodes is needed before selecting either zinc or aluminum as an anode material.

Body surface estimation: a critical evaluation

Estimates of body surface area (BSA) are widely used in intensive care, cancer chemotherapy, anesthesia, hemodialysis and other critically important medical computations. Although formulas have been proposed, virtually all BSA estimates use a formula promulgated in 1916 based on a sample of twelve possibly atypical subjects. The inclusion of a computation into a computer program imparts the impression of accuracy which is not always justified. The authors have attempted to examine and compare some direct measurement methods and estimation formulas used for BSA particularly with respect to inter-method consistency and correlation with a variety of published data.

Fiber optic system for in-vivo sizing of proteins in animal eye lenses

A compact fiber optic system, utilizing a lensless backscatter fiber optic probe, and a semiconductor laser is used as a non-invasive tool for in vivo characterization of the proteins in the eye lens of several animals. The system exploits the extremely sensitive technique of dynamic light scattering, which uses a laser beam to probe the temporal characteristics of the proteins present in eye lens fluid. The technique, with appropriate electronics and signal processing provides a rapid means of determining the size of the (alpha) -crystallin in the protein-water system. Changes in the size of the protein molecules can be tracked over the age of the eye lens; an abrupt increase in size is associated with the early cataractous formation. This paper describes the fiber optic system and discusses results obtained from measurements made on sedated rabbits, pigs and cats. A clear difference in the size of the (alpha) -crystallin of normal and cataractous lenses is observed.

Confidence regions of planar cardiac vectors

A method is presented for plotting the 90%, 95%, and 99% confidence regions of planar cardiac vectors based on the bivariate normal distribution.