In vitro evaluation of an axial flow pump: mock-pump interaction and an approach to control.

Continuous flow pump support has emerged as an alternative therapy in patients with congestive heart failure. For long-term applications, it is important to have a control system that changes the pump function according to the physiological conditions of the patient, thereby preventing risk situations. In the early stages of development, the evaluation of control algorithms for artificial blood pumps can be done in vitro using cardiovascular mock systems. A systemic cardiovascular mock loop was constructed and an axial flow pump was connected to it. The level of pump assistance was estimated using a pulsatility index (IPAo) obtained from the aortic pressure wave. An IPAo proportional-integral control system was implemented and its responses to peripheral resistance and systemic compliance changes were evaluated. IPAo is an indicator of the assistance level of a continuous flow pump operated at constant speed. The IPAo control algorithm responds by increasing the pump speed when peripheral resistance or systemic compliance is reduced. Control system operation around an IPAo fixed value provides a safety point for pump operation by maintaining aortic pressure pulsatility and avoiding ventricular suction. In vitro experimental results show that the IPAo can be taken into consideration in multiobjective control algorithm designs.

Detection of left ventricular regional myocardial ischaemia in dogs by intraventricular conductance catheter.

Nine adult mongrel dogs were instrumented with ultrasonic microcrystals to measure left ventricular basal anteroposterior diameter and midwall myocardial segment length near the cardiac apex. Pneumatic cuff occluders were positioned around the left circumflex coronary artery near its origin and around the left anterior descending coronary artery two thirds of the way along its length. A pressure microtransducer was implanted into the left ventricle. Ten days after instrumentation the animals were anaesthetised with morphine chlorhydrate and pentobarbital sodium. An eight electrode catheter was advanced into the left ventricle to measure ventricular apical and basal regional and total electrical conductance. Minor ischaemia caused by occlusion of the left anterior descending artery was detected only by the electrode pair located near the apex, as decreased local ejection fraction. Major ischaemia caused by left circumflex artery occlusion was detected by both apical and basal electrode pairs and by total conductance, the three conductance signals indicating reduced ejection fractions compared with control values. The basal diameter signal indicated that basal regional motility changed only during major ischaemia, thus confirming the specificity of the changes in the basal conductance signals. The apical segment length signal confirmed the altered motility indicated by the apical conductance signal. These results suggest that regional wall motion abnormalities may be detected by the use of a multielectrode conductance catheter.

Animal model of acute pulmonary thromboembolism treated by local recirculation of streptokinase through the lung.

Massive acute pulmonary thromboembolism has a high mortality within the first few hours. Surgical intervention can remove only larger thrombi. Systemic fibrinolytic administration requires many hours for adequate treatment. We describe an anesthetized dog model of acute, massive, disseminated pulmonary thromboembolism achieved by injection of 1.7 ml/kg of 1-hour-old thrombi directly into the pulmonary artery. The emboli were lysed with 50,000 IU streptokinase recirculated for 20 minutes through the isolated pulmonary vascular bed by use of a roller pump while the systemic bed was supported by conventional cardiopulmonary bypass. On reestablishing natural circulation all hemodynamic parameters returned to preembolism values. Success of lysis was histologically confirmed. Perfusion of the pulmonary vascular bed without inclusion of streptokinase in the perfusate worsened the hemodynamic state of the animals compared with an untreated nonperfused control group.

Dynamic abdominal and thoracic aortomyoplasty in heart failure: assessment of counterpulsation.

BACKGROUND: Aortic counterpulsation, either biologic or mechanical, is a useful technique to support circulation during left ventricular dysfunction. METHODS: In this study we used an induced cardiac failure model in acute open chest sheep to compare hemodynamic improvements between thoracic and abdominal aortic counterpulsation. This was achieved with left latissimus dorsi and left hemidiaphragm muscle flaps. RESULTS: Thoracic and abdominal aortic counterpulsation in heart failure resulted in a significant improvement of hemodynamic parameters. Subendocardial viability index, defined as diastolic pressure-time index to systolic tension-time index, in thoracic and abdominal aortomyoplasty showed significant improvement (p<0.05) when cardiac assistance was performed by electrical stimulation of each muscle flap. A new counterpulsation index derived from diastolic and systolic areas beneath the aortic pressure curve was tested, obtaining a correlation coefficient with the subendocardial viability index of 0.758 (p<0.001). Values of subendocardial viability index and counterpulsation index showed minimal variability. CONCLUSIONS: Treatment of experimentally induced cardiac failure with dynamic abdominal aortic counterpulsation allows an effective hemodynamic improvement in open chest sheep. Furthermore, this diastolic arterial pressure augmentation could be evaluated through a new counterpulsation index derived from diastolic and systolic areas beneath the aortic pressure curve.

Merging medical humanism and technology.

Dr. René Gerónimo Favaloro (12 July 1923 to 29 July 2000) was an Argentinian cardiovascular surgeon who revolutionized the field with his pioneering contributions, the most remarkable being the standardization of the coronary artery bypass surgery, performed for the first time in 1967. This technique, today one of the most commonly performed major operations, improved and saved the lives of countless patients with ischemic heart disease throughout the world and gained him international recognition.

Adventitia-dependent mechanical properties of brachiocephalic ovine arteries in in vivo and in vitro studies.

AIM: An adventitia dependent regulation of the vascular smooth muscle tone has been described. However, if the adventitia plays an active role on arterial wall biomechanical behaviour and functions remains to be established. Our aim was to characterize the influence of adventitia on arterial wall mechanical properties and the arterial conduit and buffer functions. METHODS: Ovine brachiocephalic arteries were studied in vivo (n = 8) and in vitro (with null tone) in a circulation mock (n = 8). Isobaric, isoflow and isofrequency studies were performed. In each segment, pressure and diameter waves were assessed before and after adventitia removal. From the arterial stress-strain relationship, we derived the elastic and the viscous modulus. The buffering and conduit functions were calculated using the Kelvin-Voigt's time constant and the inverse of the characteristic impedance, respectively. RESULTS: In in vivo studies arterial diameter decreased after adventitia removal (P < 0.05). Elastic and viscous modulus in in vivo studies were significantly higher in adventitia-removed arteries, compared with values in intact vessels (P < 0.05). This behaviour was not observed in in vitro experiments. An impairment of buffer and conduit functions was observed in vivo after adventitia removal (P < 0.05), while both functions remain unchanged in in vitro studies (P > 0.05). CONCLUSIONS: Arterial wall viscosity and elasticity were influenced by adventitia removal in in vivo studies, possibly by a smooth muscle-dependent mechanism, since it was not present in in vitro experiments. Adventitia would be involved in a physiological mechanism of arterial wall viscous and elastic properties regulation, that could influence arterial buffering and conduit functions.

Upgrading BME studies in Buenos Aires, Argentine. Favaloro University.

The Favaloro University initiated the academic path in biomedical sciences in Buenos Aires, Argentina. As a very promising area, the biomedical world offers the integration between several domains, complementing medicine with engineering topics. The profile of the graduated students proved to be very versatile, allowing their instantaneous incorporation to the competitive market in this vast field. The university strategy is to concentrate basic science in the first 3 years and add the specific biological/medical concepts in the last 2 years. The students achieve a bachelor degree that integrates their knowledge in basic sciences. Afterwards, they attend a professional 2 year cycle, choosing between biomedical, physics and medical computing branches. The results proved to be effective in terms of professional and academic quality, market insertion and even the creation of promising seeds for future enterprises.

Effects of lidocaine on Vmax and conduction velocity in uniform anisotropic canine ventricular muscle: possible role of its binding-rate constants.

We evaluated the effect of lidocaine (5 micrograms/ml) on Vmax and conduction velocity during longitudinal and transverse propagation to fiber orientation in strips of uniform anisotropic ventricular muscle from adult canine hearts. Tissues were markedly anisotropic, with conduction velocities 3.2 times faster during longitudinal propagation than during transverse propagation to the long axis of the fibers, and with the greatest values of Vmax associated with the slowest conduction velocities. After addition of lidocaine, Vmax (normalized values with respect to control for each cycle length, expressed as mean +/- SEM) decreased from 0.99 +/- 0.02 at a cycle length of 1,000 ms to 0.86 +/- 0.02 at a cycle length of 300 ms during longitudinal propagation. During transverse propagation, Vmax decreased from 0.99 +/- 0.03 at a cycle length of 1,000 ms to 0.87 +/- 0.03 at a cycle length of 300 ms. The differences in the relative changes between both directions at these cycle lengths, as well as with intermediate values of 500, 400, and 350 ms, were not significant. Similar results were obtained for conduction velocity. We conclude from these findings that under these experimental conditions the effects of lidocaine are characterized by a relative change both in Vmax and conduction velocity that is almost the same during longitudinal and transverse propagation at all cycle lengths explored. Moreover, the rapid binding-rate constants reported for lidocaine may play a significant role in determining the characteristics of Vmax and conduction velocity depressions in both directions.

Relationship between Vmax and conduction velocity in uniform anisotropic canine ventricular muscle: differences between the effects of lidocaine and amiodarone.

The purpose of these experiments was to determine if Vmax in anisotropic myocardium varies approximately as the square of the conduction velocity (theta) after the addition of lidocaine (6.5 micrograms/ml) and amiodarone (20 micrograms/ml). We measured Vmax and theta in 16 epicardial strips of uniform anisotropic ventricular muscles, over a wide range of stimulation frequencies. The relationship of Vmax to theta 2 was evaluated by linear regression analysis. We found that the decrease in Vmax was proportional to the square of the decrease in theta in the presence of lidocaine both during longitudinal (LP) and transverse (TP) propagation (mean slope +/- SEM: 0.961 +/- 0.047 and 0.918 +/- 0.068, respectively). The changes in Vmax, in the presence of amiodarone, were not predicted by the quadratic changes in theta during TP. However, during LP, the changes in Vmax and theta were well fitted by the predicted relationship. The slope was significantly different from that of lidocaine (2.399 +/- 0.673 vs. 0.961 +/- 0.047, p less than 0.05). On the other hand, the predicted values of theta, assuming theta = square root of Vmax x k, were significantly more depressed than the measured values. We conclude that in uniform anisotropic ventricular muscle, when the changes in Vmax and theta are solely due to a decrease in sodium conductance, a quadratic relationship between the changes in both variables is seen and the slope of the regression line should be 1, such as we have shown for lidocaine.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential effects of amiodarone on Vmax and conduction velocity in anisotropic myocardium.

Despite its widespread clinical use, the precise mechanism of action of amiodarone (AMI) has not been completely defined. We examined the effects of AMI (20 micrograms/ml) on Vmax and on conduction velocity (theta) during longitudinal (LP) and transverse (TP) propagation with respect to fiber orientation, in 10 strips of uniform anisotropic epicardial muscle obtained from the left ventricle of adult canine hearts. Mean values +/- SEM (standard error of the mean) were calculated as normalized values (beat 50/beat 1) after 4 h of AMI superfusion at five different basic cycle lengths (BCL). Vmax decreased from 0.99 +/- 0.01 at a BCL of 5,000 ms to 0.43 +/- 0.03 at a BCL of 300 ms during LP. During TP, Vmax decreased from 0.99 +/- 0.01 at a BCL of 5,000 ms to 0.54 +/- 0.05 at a BCL of 300 ms. The differences in the relative changes between both directions at a BCL of 300 ms, as well as at intermediate values of 1,000, 500, and 400, were significant (p less than 0.01). theta during LP (theta L) was depressed from 0.99 +/- 0.01 at a BCL of 5,000 ms to 0.80 +/- 0.04 at a BCL of 300 ms. In contrast, theta during TP (theta T) did not change as the BCL was decreased. In consequence, theta L was significantly more depressed than theta T at BCLs shorter than 1,000 ms (p less than 0.05). Moreover, theta T after AMI was not statistically different from control at any BCL studied. The lack of depression of theta T associated with a marked depression of Vmax during either LP or TP suggests that in addition to its sodium channel blocking properties, AMI could produce a decrease in the effective axial resistivity.

Real-time pressure-volume diagrams for the evaluation of ventricular function.

Using three intraventricular diameter signals obtained from ultrasonic distance gauges and applying the general ellipsoid model to the left ventricle, it was possible to obtain the left ventricular volume signal. Implanting a miniature transducer in the left ventricle the pressure signal was attained. With these two signals the pressure-volume diagrams were constructed on line, and ventricular function during load manoeuvres could be studied from them. Because the whole process was done on line, using a microcomputer, the performance of the left ventricle to load manoeuvres in different conditions could be seen instantly.