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Ann Thorac Surg 1995;60:1048-1052
© 1995 The Society of Thoracic Surgeons
Departments of Cardiac Surgery and Cardiac Radiology, University of Bristol and Bristol Royal Infirmary, Bristol, United Kingdom
Accepted for publication April 29, 1995.
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Abstract |
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 TopFootnotes Abstract IntroductionMaterial and MethodsResultsCommentAcknowledgmentsReferences |
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Methods. Dobutamine-stress Doppler echocardiography was used to evaluate the hemodynamic performance of small CarboMedics aortic prostheses (19 mm and 21 mm) in 18 patients (16 women; mean age, 64 years) who had undergone aortic valve replacement 23.5 ± 19 months (standard deviation) previously. Dobutamine infusion was started at a rate of 5 µg • kg-1 • min-2 and increased to 10 and 20 µg • kg-1 • min-2 at 15-minute intervals. Pulsed and continuous wave Doppler studies were performed at rest and at the end of each stage. Effective orifice area, performance index, and discharge coefficient of both valves were calculated, and peak and mean velocity and pressure drop across the prostheses were measured.
Results. Heart rate and cardiac output increased by 74% and 94%, respectively, and mean arterial blood pressure decreased by 9% at maximum stress. Effective orifice area, discharge coefficient, and performance index were comparable in both valve sizes at rest and maximum stress. Also, there was no significant difference in mean transvalvular pressure drop (gradient) for 19-mm and 21-mm prostheses at rest (8.1 ± 8.4 and 4.8 ± 3.8 mm Hg) or maximum stress (15.1 ± 14.2 and 8.8 ± 5.8 mm Hg, respectively). No significant correlation could be demonstrated between transvalvular pressure drop and patient's body surface area.
Conclusions. These data show that 19-mm and 21-mm CarboMedics aortic prostheses exhibit equally favorable hemodynamic performance with minimal pressure gradient, both at rest and under stress conditions.
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Introduction |
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TopFootnotesAbstractIntroductionMaterial and MethodsResultsCommentAcknowledgmentsReferences |
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The hemodynamic characteristics of prosthetic heart valves constitute an important part of their functional assessment. The performance of a range of prosthetic valves has been investigated in vitro and in vivo at rest [1, 2]. However, it has been suggested that in vivo resting assessments are inadequate to characterize valve performance fully [3]. Current methods for the in vivo assessment of prosthetic heart valves under high flow conditions are dependent on exercise, and are not entirely suitable for wider clinical use.
The CarboMedics valve prosthesis is an all-pyrolytic carbon bileaflet valve that incorporates several design characteristics that should result in improved performance. The implantation of small mechanical prostheses for aortic valve replacement, however, raises concerns about potential for residual transvalvular gradient and impaired long-term outcome. Therefore, this study was performed to evaluate the in vivo hemodynamic performance of small size (19 and 21 mm) CarboMedics aortic prostheses under high cardiac output (CO) conditions, using dobutamine-stress Doppler echocardiography.
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Material and Methods |
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TopFootnotesAbstractIntroductionMaterial and MethodsResultsCommentAcknowledgmentsReferences |
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. All patients were in sinus rhythm, and on no medication other than anticoagulation. Normal coronary arteries had been documented previously in all subjects by preoperative coronary angiography.
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Using a peripheral venous cannula, a graded infusion of dobutamine was administered intravenously at increments of 5, 10, and 20 µg • kg-1 • min-2 at 15-minute intervals. During the study, patients underwent continuous electrocardiographic monitoring, and blood pressure was recorded at 5-minute intervals with an automated cuff. Criteria for stopping the dobutamine infusion included (1) hypotension (systolic blood pressure less than 100 mm Hg), (2) dyspnea, or (3) significant ventricular or supraventricular arrhythmias. Repeat (stress) Doppler measurements were obtained before each incremental increase in the infusion rate. After the completion of the final assessment at a dose of 20 µg • kg-1 • min-2 (maximum stress), dobutamine infusion was discontinued, and the patient was monitored for a minimum of 20 minutes or until heart rate (HR) had returned to prestress values.
Doppler Measurements and Calculations
All tests were performed by an experienced investigator (M.B.I.) who was blinded as to the size of the prosthesis inserted. Echocardiography was carried out using an Aloka SSD-830 ultrasound system with a 2.5-MHz transducer (Aloka, Japan) with facilities for continuous-wave and pulsed-wave Doppler. Parasternal long-axis views were obtained and the early systolic diameter (D) of the left ventricular outflow tract (LVOT) was measured just below the prosthetic valve using an inner edge-to-inner edge method. For each patient, an average of three diameter measurements was used. The LVOT cross-sectional area (CSA) was calculated as 
. The pulsed-wave Doppler cursor was then placed in the LVOT immediately proximal to the aortic valve, and pulsed-Doppler flow velocity was recorded. Peak and mean velocities in the LVOT were then measured.
Cardiac output was calculated as 
, where VTI is the velocity time integral in the LVOT and HR is the heart rate per minute. This method of noninvasive cardiac output measurement correlates closely with that obtained with both the thermodilution and Fick methods [5].
Systolic valve flow (Q) was also calculated as 
. Flow velocity across the valve was obtained by means of continuous-wave Doppler from the apical view. Great care was taken to orient the transducer so that the angle between the Doppler cursor and the LVOT was as close to 0 degrees as possible, and to obtain the highest possible velocity signal. Peak velocity was measured, averaging from three velocity envelopes, and mean velocity was calculated by on-line averaging of the instantaneous velocities measured throughout the velocity complexes. Measurements were made in triplicate in each stage to ensure reproducibility. The modified Bernoulli equation was used to calculate peak and mean pressure drop (gradient) across the prosthesis as 
, where 
P is pressure drop, and VCW and VPW are the velocities (peak and mean) across the valve (using continuous-wave Doppler) and in the LVOT (using pulsed-wave Doppler), respectively.
Velocity ratio (VR) is the ratio of mean subaortic to mean transaortic velocity, and gives an approximate guide to orifice behavior, independent of measurements of LVOT diameter [6].
The prosthetic valve effective orifice area (EOA) was calculated using the modified continuity equation as 
. This simplified equation has shown an extremely good correlation with that of the original continuity equation [7, 8].
Effective orifice area index (EOAI) is a measure of how well the flow area of the valve matches the body size, and is calculated as 
, where BSA is patient's body surface area. This index is used to detect mismatch between valve size and BSA.
Discharge coefficient (Cd) is a measure of how effectively the valve uses its nominal flow area, and is calculated as 
, where AOA is the actual (nominal) orifice area, as provided by the manufacturer.
Performance index (PI) is a measure of how effectively the external dimension of the valve is used in providing forward flow, and is calculated as 
, where SRA is the sewing ring area of the prosthesis, as provided by the manufacturer.
Statistical Analysis
Parameters were calculated for each patient at each level of dobutamine infusion, and data are presented as mean ± standard deviation unless otherwise stated. Rest and maximum stress results were compared using the Mann-Whitney U test and a p value of less than 0.05 was considered statistically significant. Correlation between two variables was analyzed using Pearson and Spearman correlation tests.
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Results |
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TopFootnotesAbstractIntroductionMaterial and MethodsResultsCommentAcknowledgmentsReferences |
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). Dobutamine infusion protocol was very well tolerated. The test had to be stopped at 10 µg • kg-1 • min-2 in 2 patients (1 from each group) due to dyspnea, otherwise the only side effect was the development of infrequent atrial or ventricular ectopic beats (60% of patients). All patients achieved a significant increase in HR, CO, and systolic valve flow that was comparable between the two groups. Mean blood pressure, however, remained unchanged (Table 2). Although a significant difference in the time after operation was present between the two groups, there was no evidence of a significant difference in the time-related remodeling of the LVOT as indicated by the similar flow velocity across the LVOT in the two groups (Table 2), which also excludes differences in the constriction of the LVOT in response to the beta effect of dobutamine. The contribution of this response to gradients across the LVOT is taken into account in the modified Bernoulli equation (see above).
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). Results of mean transvalvular velocities and gradients are presented in Table 2. No significant correlation was found between patients' BSA and transvalvular pressure drop (r = 0.26 for 19-mm prosthesis and r = 0.11 for 21-mm prosthesis), or between BSA and EOA in either group (r = 0.36 for 19-mm valves and r = 0.23 for 21-mm valves).
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). This remained true when valves were indexed for BSA (EOAI). Because of the smaller nominal orifice area of the 19-mm compared with the 21-mm prosthesis, the former appeared to achieve a better use of its actual flow area, expressed as discharge coefficient and PI, although neither of these values reached statistical significance at rest or stress (Table 2).
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Comment |
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TopFootnotesAbstractIntroductionMaterial and MethodsResultsCommentAcknowledgmentsReferences |
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Recently, we have used dobutamine-stress echocardiography in the evaluation of the hemodynamic performance of aortic valve prostheses [15]. The safety and benign side effects of pharmacologic stress with dobutamine have been confirmed [16], and it has proved to be a valid alternative to treadmill exercise testing with a higher diagnostic yield [17]. With this method, patients remain in the supine position throughout the study period, and body position can be optimized to obtain high-quality echocardiographic images and precise Doppler measurements. It is important to note that invasive studies have shown that bileaflet prostheses produce localized high flow velocity and pressure-drop between the two leaflets, with a significant early pressure recovery downstream. Whereas in catheter studies, gradients are measured a few centimeters from the valve plane where pressure recovery has occurred, continuous-wave Doppler imaging interrogates the area between the valve leaflets when searching for the highest velocities in the clinical situation, hence recording higher gradients [18–20]. It should be borne in mind, therefore, that Doppler studies of bileaflet valves could overestimate catheter-derived gradients.
The CarboMedics prosthesis was first introduced into clinical practice in 1986. It incorporates several design characteristics that should result in improved performance. Among these is the elimination of pivot guards, struts, and orifice projection that are thought to reduce turbulence and leaflet excursion promoting a more rapid and synchronous movement [21, 22]. Considering the possibility of Doppler overestimation of transvalvular gradients, this study indicates that size 19-mm and 21-mm CarboMedics prostheses show efficient use of the orifice area and have equally favorable hemodynamic performance, even under high flow conditions. Our results are consistent with the few studies available in the literature reporting in vivo Doppler assessment of CarboMedics valves, both at rest and during exercise [14, 18, 19]. However, we were unable to demonstrate the increase in EOA for the 19-mm prostheses during exercise reported recently by DePaulis and associates [14].
Higher gradients were said to occur more frequently with a small prosthesis inserted in a patient with a large body surface area [9], and EOA of a specific prosthetic valve corrected for BSA (EOAI) provides a more useful index of its performance in an individual patient. From the hemodynamic data of valve behavior at rest and the expected increase in CO with exercise, an EOAI more than 0.9 cm2/m2 has been predicted as a requirement to minimize the postoperative transvalvular gradient [9, 23]. Although the EOAI at maximum stress in this unselected group of patients were 0.82 cm2/m2 (19 mm) and 0.73 cm2/m2 (21 mm), nevertheless, values less than the theoretic index recommended above were not associated with unacceptably high transvalvular gradients even at high flow conditions. This probably implies that the orifice of the CarboMedics valve is effectively used to provide forward flow with minimal production of gradient.
In conclusion, 19- and 21-mm CarboMedics aortic bileaflet prostheses appear to have equally favorable hemodynamic performance in most patients with only minimal pressure gradient generation across the prostheses, under both rest and stress conditions.
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Acknowledgments |
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TopFootnotesAbstractIntroductionMaterial and MethodsResultsCommentAcknowledgmentsReferences |
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Footnotes |
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TopFootnotesAbstractIntroductionMaterial and MethodsResultsCommentAcknowledgmentsReferences |
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References |
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TopFootnotesAbstractIntroductionMaterial and MethodsResultsCommentAcknowledgmentsReferences |
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