Pulsatile flow studies of prosthetic aortic valves

Physiological Ventricular Simulator for Valve Surgery Training

Surgical simulation is becoming increasingly important in training cardiac surgeons. However, there are currently no training simulators capable of testing the quality of simulated heart valve procedures under dynamic physiologic conditions. Here we describe a dynamic ventricular simulator, consisting of a 3D printed valve suspension chamber and a model 1423 Harvard apparatus pulsatile pump, which can provide close to physiologic hemodynamic perfusion of porcine aortic roots attached to the valve chamber for education and training in cardiac surgery. The simulator was validated by using it to test aortic valve leaflet repairs (n = 6) and aortic valve replacements (n = 3) that were performed by two trainees. Procedural success could be evaluated by direct visualization of the opening and closing valve, hemodynamic measurements and echocardiography. We conclude that, unlike other methods of simulation, this novel ventricular simulator is able to test the functional efficacy of aortic procedures under dynamic physiologic conditions using clinically relevant echocardiographic and hemodynamic outcomes. While validated for valve surgery, other potential applications include ascending aortic interventions, coronary re-implantation or catheter-based valve replacements.

Natural history of early aortic paraprosthetic regurgitation: a five-year follow-up

OBJECTIVES

To assess the incidence and natural course of paravalvular leaks detected early after aortic valve replacement.

BACKGROUND

Although the use of echocardiography has simplified the postoperative assessment of patients with aortic valve replacement, there are no data regarding the natural history of early detected paravalvular aortic leaks.

METHODS

Eighty-four consecutive patients with aortic valve replacement were prospectively followed clinically every 6 months and by echocardiography early (11 +/- 7 days), at midterm (27 +/- 3 months), and late (63 +/- 4 months) after aortic valve replacement. The competence of artificial valves was assessed by Doppler color flow mapping.

RESULTS

Paraprosthetic leaks were detected in 40 (47.6%) aortic prostheses during the early study; the majority (90%) were small. All leaks remained unchanged during the follow-up period. Left ventricular dimensions and function did not differ between patients with or without paravalvular leak during the follow-up. Left ventricular fractional shortening, however, increased during the intermediate study in both subgroups, indicating improved left ventricular function overall. Three patients had severe paravalvular regurgitation suddenly develop from late infective endocarditis, and 1 patient had a degenerative tissue valve failure 4 years after implantation.

CONCLUSIONS

Paraprosthetic aortic leaks detected early after surgery, in the absence of valve infection, are common, are usually small, and have a benign course. However, the development of new, usually severe, regurgitation should raise the suspicion of prosthetic valve endocarditis or bioprosthetic valve failure.

Experimental analysis of fluid mechanical energy losses in aortic valve stenosis: importance of pressure recovery

Current methods for assessing the severity of aortic stenosis depend primarily on measures of maximum systolic pressure drop at the aortic valve orifice and related calculations such as valve area. It is becoming increasingly obvious, however, that the impact of the obstruction on the left ventricle is equally important in assessing its severity and could potentially be influenced by geometric factors of the valve, causing variable degrees of downstream pressure recovery. The goal of this study was to develop a method for measuring fluid mechanical energy losses in aortic stenosis that could then be directly related to the hemodynamic load placed on the left ventricle. A control volume form of conservation of energy was theoretically analyzed and modified for application to aortic valve stenosis measurements. In vitro physiological pulsatile flow experiments were conducted with different types of aortic stenosis models, including a venturi meter, a nozzle, and 21-mm Medtronic-Hall tilting disc and St. Jude bileaflet mechanical valves. The energy loss created by each model was measured for a wide range of experimental conditions, simulating physiological variation. In all cases, there was more energy lost for the nozzle (mean = 0.27 J) than for any other model for a given stroke volume. The two prosthetic valves generated approximately the same energy losses (mean = 0.18 J), which were not statistically different, whereas the venturi meter had the lowest energy loss for all conditions (mean = 0.037 J). Energy loss correlated poorly with orifice pressure drop (r2 = 0.34) but correlated well with recovered pressure drop (r2 = 0.94). However, when the valves were considered separately, orifice and recovered pressure drop were both strongly correlated with energy loss (r2 = 0.99, 0.96). The results show that recovered pressure drop, not orifice pressure drop, is directly related to the energy loss that determines pump work and therefore is a more accurate measure of the hemodynamic significance of aortic stenosis.

A review of the in vitro evaluation of conduit-mounted cardiac valve prostheses

This review concerns the issues affecting the in vitro evaluation of conduit-mounted prosthetic heart valves at the design development stage, and the question of standardisation of testing at the quality assurance stage. Particular attention is given to areas of conduit valve development and research which have been neglected, ambiguously covered or left to the discretion of the researcher in the current standard for conventional prosthetic valves, ISO 5840.

Dynamic characteristics of prosthetic heart valves

The relation between flow rate (Q) and transvalvular pressure-drop (DP) is of fundamental importance for a prosthetic heart valve tested in steady flow conditions. The Q-DP plot can thus be called the static characteristic of the valve. While in pulsatile flow, with time (t) as a parameter, the instantaneous Q(t)-DP(t) relation can also be obtained. The Q-DP relation forms a phase graph on an X-Y plane during a whole cardiac cycle, and can be regarded as the dynamic characteristic, which to our knowledge has never been systematically explored before. With in vitro experiment the Q(t)-DP(t) relations are presented for five different aortic valves. Properly modelling the characteristics of heart valves is a key link in modelling the interactions between the ventricle and arterial system. Treatments for valves, such as diode analogue and orifice area assumption governed by the Gorlin formula, are found unsatisfactory. A simple one-dimensional flow equation is used to further examine the Q-DP graph, and both the dynamic resistance characteristic and the dynamic flow characteristic can be obtained. It is found that the dynamic characteristic differs from the static one not only in the inertance effect but also in the transient process, which can be quite energy-consuming and therefore important. Geometric relations of these phase graphs with the transvalvular power loss are discussed. The method of dynamic characteristics provides a new way to evaluate the performance of a tested valve.

Doppler echocardiographic assessment of prosthetic aortic valve function. Findings in normal valves

To determine the Doppler-derived aortic flow velocity profiles in relation to type of prosthetic valve and left ventricular function, 70 patients with normal functioning aortic prosthetic valves (group 1 = 44 patients with low-profile mechanical valves and group 2 = 26 patients with high-profile mechanical valves) were evaluated. Peak flow velocity and mean systolic gradient were inversely related to valve size (r = -0.72; r = -0.76) in group 1. On the other hand, aortic flow velocity profiles had significant correlations with left ventricular end-systolic dimension (r = 0.75; r = 0.76) and left ventricular fractional shortening (r = -0.69; r = -0.66) in group 2. Thus, aortic flow velocity profiles in the low-profile mechanical valve were affected by pressure gradient caused by the valve size, whereas the hydromechanical disadvantage of the high profile mechanical valve affected the left ventricular pump function and Doppler-derived flow velocity profiles.

An in vitro study of prosthetic heart valve sound

Patients with an implanted mechanical heart valve sometimes experience the closing sounds of the valve as disturbing. To study the generation of valve sounds in general, a pulse duplicator study was carried out, testing eight commonly used types of prosthetic valves in the aortic position. Pulse rate was set at 70 beats/min, stroke volume at 70 ml and mean 'aortic' pressure at 100 mmHg. Despite the controlled conditions, there was great variability of the closing sound, in both intensity and spectral composition, making noise comparisons and spectral characterization ('sonoprint') difficult. In general, bileaflet mechanical valves produced less noise than did tilting disc valves, particularly those with large opening angles. One small-size (23 mm) tilting disc valve produced 50% less noise than large types. The plastic ball valve, the porcine and the polyurethane trileaflet valve all were very quiet.

Doppler evaluation of bioprosthetic and mechanical aortic valves: data from four models in 107 stable, ambulatory patients

To test the applicability of Doppler ultrasound in the evaluation of prosthetic valve function, 107 patients with normal ejection fractions in whom Starr-Edwards, Björk-Shiley, Carpentier-Edwards, and Hancock models had been implanted in the aortic position were examined. Maximal transvalvular velocity was recorded by non-imaging continuous wave Doppler ultrasound. Means of maximal velocities by model and size ranged from less than 2 to 4 m/sec. The Starr-Edwards valve showed the highest velocities, the Björk-Shiley the lowest, and the bioprosthetic models showed velocities in between. A significant inverse relation between velocity and size, and standard deviations averaging +/- 14% enabled the technique to measure differences between sizes of the same model. Aortic regurgitation was detected in 24% of the patients. This study, conducted in well and stable patients, established values for maximal velocity across normally functioning aortic mechanical and tissue prostheses of different models and sizes. The intersubject variability was relatively small which, together with a previously shown minimal intrasubject variability, was testimony to a methodology that should prove useful in longitudinal postoperative evaluations.

Screening of solid and porous materials for pacemaker electrodes

Several different materials, including one which was porous, were studied to assess their properties as pacemaker electrode tips. Leads were implanted in sheep for periods up to one year. Electrical measurements were made during the implant period and histopathological examination performed after sacrifice. Although titanium vapor-deposited carbon, and silver did not lower the chronic stimulation threshold below that of platinum, their electrical characteristics were within generally acceptable limits. Zinc evoked a severe tissue reactions and a high threshold. Porous titanium alloy electrodes demonstrated reduced dislodgement, more frequent attachment and a lower sensing impedance than other electrodes.

Aortic valve replacement. A randomized study comparing the Björk-Shiley and Lillehei-Kaster disc valves. Transvalvular regurgitation and occurrence of paravalvular fistulas

Three hundred patients were selected at random in order to compare the Björk-Shiley (B-S) and the Lillehei-Kaster (L-K) valves in the aortic position. The transvalvular regurgitation was evaluated by peroperative flow recordings in 97 cases. The leakage was found to be 7.6% and 7.5% of forward flow in the B-S and L-K valves, respectively. The regurgitant flow pattern, however, was different in the two groups, as leakage on closure was significantly larger in the L-K valves, and the leakage after closure of the disc was significantly larger in the B-S valves. Follow-up cine-aortography was carried out two years postoperatively in 91 patients. The contrast leakage through the valves was found to be minimal (Grade I) in about 90% in both groups. In one patient with a L-K valve no leakage could be demonstrated. In the remaining patients the leakage was moderate (Grade II). Paravalvular fistulas were demonstrated in three patients from each group. In one patient the paravalvular regurgitation was marked (Grade III), in the others moderate (Grade II). Cine-aortography was also carried out in a further 9 patients, in whom a paravalvular leakage was suspected according to clinical follow-up examinations. In four of these patients a paravalvular regurgitation was demonstrated. The leakage was moderate in two patients and severe (Grade IV) in another two, who were successfully re-operated on.

Comparison between the in vitro flow dynamics of the standard and the convexo-concave Björk-Shiley tilting disc valve prostheses

In the new convexo-concave model of the Björk-Shiley prosthesis the disc pivots 2.5 mm downstream, creating a space between the disc and the ring when the disc tilts open. Its flow dynamics were compared with those of the standard model under steady flow rates 0--32 1/min of a water-glycerin mixture employed as blood-analogous test fluid. The following results are valid for corresponding sizes of the conventionally used 21, 23, 25, 27, 29 and 31 mm prosthetic valves of the two models. The transprosthetic pressure fall varied directly, but in a parabolic fashion with the steady flow rate for all the partial and full flow orifices. The resistance to flow 0--32 1/min for the full flow orifice of the convexo-concave prosthesis was in average 16% lower than that of the standard model, with highly significant differences for the 21 and 23 mm prostheses (p less than 0.001). There was a redistribution of flow towards the smaller prosthetic opening from 23% (S.D. +/- 1.4) for the standard model to 30% (S.D. +/- 2.4) for the convexo-concave model (p less than 0.001). The minimum steady flow rate required to maintain the convexo-concave disc in completely open position (60 ml/sec) was only half that (37--50%) required for the standard disc (125 ml/sec) (p less than 0.001). Regurgitation through the closed prosthesis varied directly and linearly with the driving pressure and was slightly but definitely lower with the convexo-concave model than with standard model. The main features of the new design are decreased resistance to flow, redistribution of the flow within the prosthesis in order to prevent stasis along its smaller orifice, diminished opening resistance and reduced regurgitation. It is concluded that the flow dynamics of the convexo-concave model Björk-Shiley prosthesis constitute a significant improvement over those of the standard model.

Effects of structural configuration of prosthetic aortic valves upon coronary blood flow

This study was performed in order to determine the effects of the structural configuration of prosthetic aortic valves upon coronary blood flow. A pulse-duplicating system was utilized to which a rigid model of the root of the aorta and the left artery were attached. A stented normal human valve, stented porcine valve, pivoting disc valve, tilting disc value and a caged ball valve were tested at stimulated resting conditions. The results showed that the pulse configuration and proportions of coronary flow that were systolic and diastolic changed with various valves. There was no significant reduction of the mean simulated coronary flow with the porcine valve or disc valves. Coronary flow was augmented with the caged ball valve. These differences were interpreted as being due to several fluid dynamic phenomena. These were piston effects, inertia of reversed blood flow, torque and the effects resulting from the unique pressure distribution on each valve.

Experimental evaluation of mechanical haemolysis with Starr-Edwards, Kay-Shiley and Björk-Shiley valves

Mechanical cell damage was studied in vitro with three types of prostheses: Starr-Edwards, Kay-Shiley and Björk-Shiley valves. Mechanical cell damage was found to be closely related to the flow characteristics in the prosthesis. Considering valves of similar orifice diameter, Björk-Shiley valves produced the lowest rate of haemolysis. This is due to the improved haemodynamic characteristics of the valve which resulted from the laminar type of flow. With Starr-Edwards valves, smaller sizes produced unacceptably high rates of haemolysis. Increasing the mean forward flow across the valve resulted in a disproportionate rise in the energy loss and the rate of haemolysis when compared with Björk-Shiley valves of similar annulus diameters.

Computer visualization of flow patterns for prosthetic heart valves

Flow patterns for commonly used aortic valve prostheses were formed with the aid of fluid dynamics equations, numerical analysis techniques and computer graphics. Simulations of velocities, vorticity, pressure and certain stress values were developed by a computer and displayed for man-machine interaction. The ball, disc and tilting-disc occluders were of interest. The displays comprised both contour and perspective patterns; the latter permitting semi-quantitative evaluation of particular parameters. Comparisons between the computer simulations and those flow visualizations formed by Björk and co-workers were assessed. Although the methodology was of prime concern for presentation, the capability of the tilting-disc prosthesis for presenting a more natural flow regime was shown. Thoughts of future studies were entertained.

Coronary ostial stenosis: a complication of aortic valve replacement of coronary perfusion

Coronary ostial stenosis developing after aortic valve replacement is a clinically well-recognized entity. This non-atheromatous intimal proliferation may be limited to the proximal part of the coronary artery, probably as a complication of intra-operative coronary perfusion. It may also occur in association with widespread intimal thickening in the aortic root, presumedly as a reaction to turbulence around aortic ball valve prostheses. We have encountered this process in 2/508 patients (0.4%), who underwent aortic valve replacement with the Björk-Shiley tilting disc valve prosthesis. The coronary perfusion technique was identical in all the operations.

Aortic valve replacement with frame-supported autologous fascia lata grafts. I. Technical consideration and early results

During the period November 1969 to June 1972, a frame-supported autologous fascia lata graft was implanted in 71 consecutive patients with surgically treated aortic valve disease at the Department of Thoracic & Cardiovascular Surgery, University Hospital, Uppsala. The follow-up period was between 1 and 4 years. Eleven patients died within 28 days of the operation (16%) and 13 after discharge from hospital (18%); the cumulative mortality was thus 34%. Forty-five percent of the patients who died had associated cardiovascular or other diseases. The causes of death were infection (10), myocardial failure (6), myocardial infarction (3), cerebral damage (3), and intraoperative aortic dissection from the cannulation site (2). The majority of the deaths (88%) occurred within 6 months and all within 13 1/2 months after operation. Two fascia lata valves were removed because of endocarditis 23 and 26 months, respectively, after operation. Two valves were also removed on account of mechanical malfunction. The remaining 44 patients with fascia lata valves had returned to work. No embolic complications occurred, despite the fact that only patients with a concomitant prosthetic mitral valve or atrial fibrillation received anticoagulatant treatment. Haemodynamic studies of the valve in vitro and pressure measurements during the operation showed that the valve had a low primary systolic peak gradient of 0-16 mmHg. Certain modifications in the construction aimed at improving the haemodynamic properties of the valve are discussed. Increased stringency in the sterility precautions during the valve construction procedure may have contributed to the fact that early endocarditis, which is a serious complication, did not occur in any of the last 43 patients. As yet the observation time is too short to judge, however, to what extent susceptibility to infection and possible late changes of the valve can affect its function.

Haemolysis with Björk-Shiley and Starr-Edwards prosthetic heart valves: a comparative study

Slater, S. D., Sallam, I. A., Bain, W. H., Turner, M. A., and Lawrie, T. D. V. (1974).Thorax, 29, 624-632. Haemolysis with Björk-Shiley and Starr-Edwards prosthetic heart valves: a comparative study. A comparison was made of the haemolytic complications in 85 patients with two different types of Starr-Edwards cloth-covered ball and cage prosthesis with those in 44 patients with the Björk-Shiley tilting disc valve. Intravascular haemolysis, as detected by the presence of haemosiderinuria, occurred significantly less often with the Björk-Shiley than with the Starr-Edwards valve, the overall incidence with aortic, mitral or multiple replacements being 31%, 15%, and 20% for Björk-Shiley and 94%, 92%, and 88% for Starr-Edwards valves respectively. There was no significant difference in the frequency of haemolysis between each of the two types of Starr-Edwards prosthesis studied at either the aortic (2300 versus 2310 model) or mitral (6300 versus 6310) site.

Haemolytic anaemia developed in only one patient with a Björk-Shiley valve but was common though usually mild with Starr-Edwards prostheses, particularly aortic valve replacements with the 2300 model and in aortic plus mitral (± tricuspid) replacements. The greater severity of haemolysis produced by Starr-Edwards valves, again especially of the latter types, was further demonstrated by higher serum lactate dehydrogenase and 24-hour urinary iron levels.

It is concluded that the Björk-Shiley tilting disc valve represents a significant advance in the amelioration of the haemolytic complications of prosthetic valves.

Systolic time intervals in severe aortic valve disease. Changes with surgery and hemodynamic correlations

Systolic time intervals were recorded in 25 consecutive patients before and after aortic valve surgery. Ten patients with dominant aortic stenosis (AS) and 10 with dominant aortic insufficiency (AI) received a Starr-Edwards prosthesis; five underwent repair procedures for AS or subaortic lesions. Before operation, the mean rate-corrected left ventricular ejection time (LVET c ) was prolonged; postoperatively LVET c decreased significantly. Conversely, preoperative mean rate-corrected preejection period (PEP c ) was short and increased postoperatively. Serial measurement of LVET c in a patient who had two aortic valve replacements suggested its value in quantitating prosthetic malfunction.

In AS, preoperative LVET c correlated closely with cardiac index; normal ejection times were associated with the most severely depressed flows. Preoperative variations in PEP c in AS related inversely to the first derivative of the left ventricular pressure curve (maximal LV dp/dt). In AI, PEP c correlated with the quotient:

[see Equation in PDF File]

where Adbp — LVed = transaortic end-diastolic pressure difference (aortic diastolic pressure minus LV end-diastolic pressure), and dp/dt = maximal LV dp/dt. This study identifies the flow and pressure parameters which correlate with abnormalities of PEP c and LVET c with severe AS and AI. The sensitivity of this technique to the hemodynamic changes of corrective surgery makes it a potentially useful noninvasive means to measure prosthetic valve function.