[Epidemiological and pathogenic features of a bacillary dysentery outbreak in a boarding school caused by Shigella sonneri].

Objective: To investigate the etiologic and epidemiologic features of an infectious diarrhea outbreak in a boarding school in Fuyang city, Anhui province. Methods: Traceability hypothesis of this study was tested according to the epidemiological characteristics of the cases. Feces, anal swabs, water samples and food residues related to the patients and chefs were collected for pathogen isolation and detection. Biochemical identification, virulence gene detection, drug susceptibility test, PFGE and multilocus sequence typing were performed. Results: The incidence rate (3.41%) of different dormitory buildings within the water supply area by shallow wells was higher than that (0.98%) of the deep wells, with statistical significance (χ(2)=17.215, P<0.001). Sixteen strains belonged to the Shigella Sonneri family were isolated from the patient's samples, and all carrying the ipaH gene. Seven strains belonged to sen and ial genes. Set1 gene that did not appear in all the 16 strains were highly resistant to ampicillin, tetracycline, compound xinnomine, cefazoline, cefotaxime, gentamicin, naphthidinic acid and streptomycin, including 9 strains to doxycycline. The pulse field pattern of the 16 strains of Shigella sonneri appeared the same, with the ST type as ST152. Conclusion: When combined data from the etiological and epidemiological investigation, it was confirmed that Shigella sonneri was the pathogen of this outbreak, and water from the shallow wells might be responsible for the source of infection.

Effect of structural compliance on cavitation threshold measurement of mechanical heart valves.

An in vitro study was designed to evaluate the effect of structural compliance on cavitation threshold measurements of mechanical heart valves. Using a dual channel high speed video image analysis method, the experiment was carried out in a pulse duplicator under a single, simulated physiologic condition. Each valve was mounted on a compliance adjustable fixture. One video camera served as a displacement monitor, while the other monitored cavitation bubbles at the inflow side of the valve. On-line adjustment of the mounting compliance allowed for cavitation threshold detection with respect to compliance without influencing the testing condition. Visible cavitation bubbles disappeared when the compliance was increased. Ten 29 mm mitral valves, including two each of St. Jude Medical, Carbomedics, Edwards-Duromedics (ED), Edwards Tekna (ET) and Medtronic-Hall (MH) were tested. All the bileaflet valves tested in this study showed similar cavitation thresholds at a compliance range of 0.5-0.65 mil/lb at a dp/dt of 4,000 mmHg/sec (averaged dp/dt during valve closure). Under the same dp/dt, MH monoleaflet valves showed a higher compliance range of 3.5-3.8 mil/lb to achieve the cavitation threshold. This study demonstrated that the mounting compliance of the valve must be known and well controlled in order to quantify the cavitation threshold with respect to dp/dt. In addition, the more compliant sewing ring of the ET valve may be responsible for higher cavitation thresholds with respect to dp/dt as observed in previous studies. The present in vitro study suggests that the investigation of the sewing structural compliance may be essential for the evaluation of in vivo cavitation potential.

An interlaboratory comparison of the FDA protocol for the evaluation of cavitation potential of mechanical heart valves.

Five laboratories carried out measurements of cavitation threshold for a common set of six mechanical prosthetic heart valves, two each from three different manufacturers. This study was intended to evaluate to what extent FDA's current guidance for cavitation testing would lead to consistent results in a variety of laboratory settings and to seek areas for improvement in the recommended test protocol. The inter-laboratory study protocol specified: (1) characterization of the test fluid by oxygen content and electrical conductivity, (2) location and frequency response of pressure sensors, (3) determination of ventricular and atrial pressures (P) and loading rates (dP/dt) averaged over the time period of valve closure and over the time periods of 1 ms, 5 ms, and 20 ms prior to video visualization. The protocol did not specify: (1) the fluid pumping equipment to be used to generate cavitation, (2) the pump or fluid parameters adjusted to raise or lower the loading rate, (3) the equipment, technique, or sensitivity used to visualize cavitation, and (4) a specific definition of the threshold for cavitation. Results from the five laboratories are reported. Significant differences in results were observed in dP/dt and in the pressure difference across the valves during closure at cavitation threshold. Specific differences in test systems included a wide range of ventricular compliance and single valved versus double valved test systems. Three single valve systems with compliant ventricles produced results in reasonable agreement with one another. Further similarity in test equipment should be specified to assure adequate interlaboratory reliability for cavitation testing. Areas needing better specification include the design of the valve mount, the design of the cavitation generators, and qualitative criteria for detection of threshold cavitation.

The closing velocity of mechanical heart valve leaflets.

The closing motion of the occluder leaflets in bileaflet type mechanical heart valves (MHV) was monitored with a laser sweeping technique. The angular displacements of the leaflets were registered with precision of 0.2 microsecond steps. Experimental measurements were made using five 29 mm Edwards-Duromedics including three original specification (EDOS) and two modified specification (EDMS), and two 29 mm St Jude Medical MHVs. The testing valve was installed in the mitral position of a physiologic pulsatile mock circulatory flow loop using water-glycerine solution as the testing fluid. Each valve was tested by: (1) direct mounting the valve on metal washers, and (2) mounting the valve with its sewing ring. Experiments were carried out at pulse rates of 70, 90, and 120 beats min-1, with the corresponding cardiac output of 5, 6, and 7.5 litres min-1, and maximum left ventricular pressure gradients (dp/dt) of 1,800, 3,000 and 5,600 mm Hg s-1, respectively. The maximum leaflet closing velocity of each of the tested valve types are presented. The difference in leaflet closing movements between the direct rigid mounting and the sewing ring mounting are discussed. The details of the laser sweeping technique are presented.

Venturi pressure cannot cause cavitation in mechanical heart valve prostheses.

Surface pitting of certain mechanical heart valve (MHV) explants has prompted investigation into possible causes of cavitation during MHV operation. Leaflets of a 29 mm MHV were glued shut with B-datum (BD) gaps fixed at 0.0089, 0.0174, and 0.0219 cm. Each BD gap setting was tested in a steady flow chamber, with leakage flow established at transvalvular pressures (delta P) of 20 to 200 mmHg. Laser Doppler velocimeter (LDV) velocity measurements were recorded 220 microns distal to the BD, along with leakage flowrates. Maximum LDV velocities were compared with those calculated using the mass conservation equation. At identical P, the LDV flow velocities for the three BD settings were found to be approximately equal. This indicates a geometric independence of the leakage flow velocity. At atmospheric pressure, the local velocity necessary to cavitate blood as a liquid is approximately 13 m/sec. These results demonstrate that the leakage velocity is insufficient to cause cavitation. A simplified theoretical model is proposed to illustrate the necessary delta P to produce Venturi related cavitation.

The closing velocity of Baxter Duromedic heart valve prostheses.

The closing velocity of a mechanical heart valve (MHV) leaflet has been conventionally related to the valve sound, and possibly to recently observed MHV cavitation phenomenon. Presently, the MHV leaflet terminal closing velocity has been indirectly assessed either by listening to the valve sound and/or averaging over the closing period. Leaflet motion during the closing phase is not uniform. Using a laser sweeping technique developed in this laboratory, an experimental study was carried out to analyze the closing motion of Baxter Duromedic (DM) 29 mm mitral MHVs. The results were compared with that of a St. Jude Mechanical (SJ) 29 mm mitral MHV, using the same technique. The in vitro experiment was carried out by mounting the testing MHV at the mitral position of a mock circulatory testing facility, with heart rates ranging from 70-120 beats/min, ventricular pressure slopes (dp/dt) from 1800-5600 mmHg/sec, and cardiac outputs from 5.0-7.5 liters/min. This paper introduces the laser sweeping technique developed for precision monitoring of MHV leaflet motion, and presents the detailed leaflet motions within the last 3 degrees before final closure. The experimental results showed that the final closing velocity of DM, which is known to generate a louder valve sound and has a shorter closing period than that of SJ, actually has approximately the same closing velocity within the range of the experiment. Theoretical analysis further confirms that a short closing period may not lead to a higher leaflet final closing velocity.

Laser assessment of leaflet closing motion in prosthetic heart valves.

The dynamics of leaflet motion in heart valve prostheses (HVP), and in particular the closing velocity, is believed to be related to the valve sound and possibly to the phenomenon of valve cavitation. This paper describes a non-intrusive laser sweeping technique enabling the study of leaflet motion. The principle of measurement and the equipment involved are presented, together with the results of two commercially available, 29 mm bileaflet mitral valves, a St. Jude Medical, and an Edwards Duromedic valve. Experiments were carried out in a pulsatile mock flow testing loop designed to mimic physiological pressure waveforms and ventricular contraction. Measurements of heart rate were made in the range 70-120 beats min-1, with a ventricular pressure slope range of 1800-5600 mm Hgs-1 and a cardiac output range of 5.0-7.5 litres min-1. Motion analysis of the measured data focuses on the velocity of the leaflet immediately before closure.

In vitro pulsatile flow study on effective orifice area of prosthetic mechanical heart valves.

The effective orifice area (EOA) of a mechanical heart valve is an index of how well the valve design utilizes its primary orifice area (POA). In vitro measurements of EOA of aortic valves were maintained by means of pressure drop and root mean square flow rate measurements in a pulse duplicator during systole. Edwards-Duromedics, St. Jude Medical, and Carbomedics aortic valves of sizes 19 19 mm, 21 mm, 25 mm, and 27 mm were analyzed over a cardiac output range of 3 to 7 liters/min. The resultant ratios of EOA/POA were in the range of 0.6-0.8. A simplified equation suggested by the FDA was used in this study to calculate EOAs. To agree with original assumptions of the simplification, the entrance flow area (EFA) where upstream pressure is measured, must be large as compared to that of the test valves. If not, the formula can yield questionable results such as implying that the EOA can be larger than the POA (Walker P et al, 1992) [1]. This paper discusses the limitations in using such an equation. In conclusion, we suggest utilizing the parameter square root of 1-(POA/EFA)2 to evaluate the validity of the data processing, before using the equation. The parameter should be close to one, and in this study it was 0.997.

Graft compliance and anastomotic flow patterns.

The oscillatory flow patterns at the venous anastomosis of a hemodialysis angioaccess loop graft system were studied using two new compliant vascular prostheses: a longitudinally compliant polytetrafluoroethylene-composite (Baxter Ultraflex PTFE-Plus) graft (BA) and a radially compliant ultrafine polyester fiber (TORAY-UFPF) graft (TR). A non-compliant Gore-Tex polytetrafluoroethylene graft was used as the control. The experimental grafts were 8 mm inside diameter x 25 cm long. Flow experiments were done in a transparent, elastic bench-top flow model; fabrication was based on silicone rubber casts obtained from femoral-to-femoral arteriovenous loop grafts surgically implanted in dogs. The loop graft constructed in the dog model was made to mimic the branchial-to-cephalic angioaccess loop graft commonly used in hemodialysis patients. The flow model was connected to a pulse generator, an adjustable arterial afterload, and a venous afterload. Under identical input conditions, the pressure and flow waveforms were monitored simultaneously at the proximal and distal ends of both the arterial and venous anastomoses. For each graft studied, the anastomotic flow field was visualized using laser illuminated hydrogen bubbles as tracers. At pulse rates of 60 and 90 beats/min, graft flow rates were 2.2 and 2.5 L/min, respectively. Among the grafts studied, measurable differences in pressure and flow wave attenuation and their respective phase lags resulted in characteristically dissimilar flow patterns at the venous anastomosis. Growth of the separation zone at the toe of the anastomosis, and the pattern of retrograde flow in the distal vein are visibly different in all three grafts.