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Haemodynamic performance of aortic pericardial bioprostheses and bileaflet prostheses at rest and during exercise: implications for the surgical management of patients with small aortic roots

Abstract

OBJECTIVE To determine the haemodynamic behaviour, at rest and during exercise, of aortic valve pericardial bioprostheses and different sizes of bileaflet prosthesis.

DESIGN Observational study.

SETTING Tertiary medical centre.

PATIENTS AND INTERVENTIONS 74 patients (33 women, 41 men; mean age 64 years) in whom 40 pericardial bioprostheses and 34 bileaflet prostheses sized 19, 21, or 23 mm had been implanted to replace aortic valves.

MAIN OUTCOME MEASURES Doppler echocardiography at rest and at peak exercise, between 12 and 47 months after surgery.

RESULTS All patients achieved a significant increase in heart rate, systolic blood pressure, and cardiac output with exercise. Transvalvar pressure fall, valve area, and left ventricular systolic and diastolic function indices also underwent significant changes with exercise. Reductions in peak and mean transvalvar pressure, at rest and at peak exercise, were greater in patients with small valves (p < 0.05). Valve areas and effective area index were greater in the patients with larger valves (p < 0.001). There were no significant differences between patients with mechanical and biological prostheses with regard to transvalvar pressure fall and valve areas at rest and at peak exercise.

CONCLUSIONS 19 mm and 21 mm aortic prostheses and bioprostheses continue to create significant obstruction, particularly with exercise.

  • small aortic prostheses
  • haemodynamic variables
  • exercise

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The surgical management of patients with small aortic annuli remains controversial. In a previous study we found that patients in whom 19 or 21 mm bioprostheses or mechanical prostheses had been implanted because of aortic stenosis underwent significantly less postoperative regression of left ventricular hypertrophy than patients receiving larger valves,1 and similar results have recently been reported by other groups.2 3 Furthermore, there are data suggesting that patients with small aortic valve prostheses have a worse prognosis than those with larger replacement valves.4 5 The cause of these differences is probably the suboptimal haemodynamic behaviour of even the most advanced small valve prostheses, whether mechanical or bioprosthetic.5-11

Although numerous studies have characterised the haemodynamics of small aortic valve prostheses at rest, there is very little information on their behaviour under high flow conditions during exercise,12 which is not necessarily analogous. In view of the otherwise healthy, active condition of most patients with valve prostheses, this may be a very important factor in deciding whether to implant a small prosthesis or to perform a root enlargement procedure to allow a larger prosthesis to be used.

In this study we determined the haemodynamic behaviour, at rest and during exercise, of 19 mm, 21 mm, and 23 mm bileaflet prostheses and pericardial bioprostheses in the aortic position.

Methods

We studied retrospectively all patients who had received an aortic valve replacement in our centre since 1989 and who complied with the following conditions:

  • only the aortic valve had been replaced

  • at the latest check up the patient was in sinus rhythm and in New York Heart Association (NYHA) functional class I or II

  • left ventricular systolic function was normal (ejection fraction > 55%)

  • the prosthesis implanted was functioning normally by clinical and echocardiographic criteria

  • no non-cardiological cause prevented the patient from performing a cycle ergometer exercise test in the supine position

  • Doppler echocardiograms good enough to evaluate left ventricular structure and function, and aortic valve function had been determined at the most recent check up.

Of the 74 patients meeting the above criteria, 20 (12 with pure aortic stenosis and eight with mixed lesions) had received 19 mm valves (five MitroFlow pericardial bioprostheses (MitroFlow International Inc, Richmond, Ontario, Canada), six Labcor-Santiago pericardial bioprostheses (Labcor Laboratories, Belo Horizonte, Brazil), three Carpentier-Edwards pericardial bioprostheses (Baxter Healthcare Corporation, Edwards Division, Santa Ana, California, USA), and six CarboMedics bileaflet prostheses (CarboMedics Inc, Austin, Texas, USA)); 28 (16 with pure aortic stenosis and 12 with mixed lesions) had received 21 mm valves (seven MitroFlow pericardial bioprostheses, seven Labcor-Santiago bioprostheses, two Carpentier-Edwards bioprostheses, and 12 CarboMedics valves); and 26 (16 with pure aortic stenosis and 10 with mixed lesions) had received 23 mm valves (six Labcor-Santiago bioprostheses, four MitroFlow bioprostheses, and 16 CarboMedics valves).

None of these 74 patients had subaortic obstruction of the outflow tract from ventricular hypertrophy, none had undergone any aortic root enlargement procedure, and none had more than very slight aortic insufficiency. Of the 51 other patients in whom replacement aortic valves had been implanted since 1989, 41 were excluded from the study because they had also undergone surgery for associated cardiac lesions, eight because they were unable to perform the cycle ergometer exercise test for reasons not directly related to their cardiac complaint, and two because their echocardiograms were of substandard quality.

Echocardiography was performed using a Hewlett-Packard Sonos 1000 apparatus (Hewlett-Packard Inc, Andover, Massachusetts, USA) with 3.5 MHz transducers for images and 2.5 MHz transducers for Doppler effect measurements. Conventional echocardiography, and Doppler echocardiography at rest and during maximum effort were all performed in a single session. M mode echocardiograms were taken for measurement (in mm) of telediastolic interventricular septum thickness (IVST), left ventricular posterior wall thickness (PWT), and telediastolic (LVEDD) and telesystolic (LVESD) left ventricular diameters. All measurements were repeated for four to six successive heartbeats, and the values accepted for each patient were the means of these four to six values.

Left ventricular mass (LVM) was calculated from Devereux and Reichek’s formula.13 Left ventricular mass index (LVMI, in g/m2) was defined as LVM/body surface area (BSA) in m2. Left ventricular fractional shortening (FS, %), left ventricular circumferential shortening velocity (Vcf, circ/s), and left ventricular ejection fraction (EF, %) were also calculated.

Transmitral flow was assessed by Doppler echocardiography using an apical two or four chamber approach. The following variables were calculated: the maximum velocities of E and A waves (VmaxE, VmaxA); the deceleration time of E (DTE); the ratio of the velocities of A and E waves (A/E); and isovolumic relaxation time (IRT).

Guided by colour flow images, systolic velocities in the outflow tract near the valve (Vlvot) were measured in pulsed Doppler mode, and velocities through prosthetic aortic valves (Vvalv) in continuous Doppler mode.

Peak and mean systolic transvalvar pressure drops calculated from peak and mean flow velocities, using the modified Bernouilli equation, are denoted PSG and MSG, respectively, in this paper. PSGc denotes peak pressure drops calculated using the Bernouilli equation with correction for prevalvar velocities:

PSGc = 4(Vvalv 2 − Vlvot 2)14

Cardiac output (CO) was calculated as previously described,15 and cardiac index (CI) as CO/BSA.

Effective valve areas were calculated using both the standard continuity equation:

Vastan = SV/VTIvalv,

where SV is stroke volume and VTIvalv the time integral of the velocity through the valve, and the simplified equation:

Vasimp = A × Vlvot/Vvalv,

where A is the area of the left ventricular outflow tract16 17; A was calculated as p(D/2)2, D being the diameter of the outflow tract when viewed from the longitudinal parasternal direction at the beginning of systole, and SV as A × VTIlvot, VTIlvot being the time integral of the velocity through the tract.

The effective area index (EAI) was calculated as VAstan/BSA.

All patients underwent a symptom limited cycle ergometer effort test in the supine position. Doppler echocardiograms were recorded at peak effort using the same technique as at rest.

The values of intralaboratory variability for measurements of left ventricular outflow tract diameter, peak velocity, mean gradient, and effective orifice area were respectively 5%, 8%, 10%, and 11%.

STATISTICS

Data are expressed as mean (SD). The significance of differences in measurements among valve sizes was evaluated by one way analysis of variance. Multivariate analysis of variance and covariance was used to evaluate the effects of valve size and valve type (mechanical or biological) on the variables measured echocardiographically at rest and during exercise, and on the difference between resting and exercise values. Student’s t test for paired data was used to examine changes in measured variables caused by exercise. All distributions were tested for normality using the Kolmogorov-Smirnov test, and Box’s M test was used to assess the homogeneity of dispersion matrices. Correlations were investigated using Pearson correlation analysis. Statistical significance was accepted at p < 0.05.

Results

Table 1 lists the relevant characteristics of the patients studied. Of the 48 receiving 19 or 21 mm valves, 31 (64.6%) were women, as against only two (7.7%) of those receiving a 23 mm valve. The patients receiving bioprostheses were significantly older than those receiving a mechanical prosthesis. In all groups, blood pressures and heart rates were significantly higher after exercise than before.

Table 1

Patient characteristics

Table 2 lists peak and mean transvalvar pressure drops and other haemodynamic data. Both at rest and during exercise, the pressure drops across small valves were greater than across larger valves (fig 1); there were no significant differences between bioprostheses and mechanical prostheses. The increase in peak transvalvar pressure drop caused by exercise was larger for 19 mm and 21 mm valves than for 23 mm valves (p < 0.05).

Table 2

Ultrasonic haemodynamic data, at rest and during exercise, for 19 mm, 21 mm, and 23 mm pericardial bioprosthetic valves and bileaflet prostheses in the aortic position

Figure 1

Box plot of the distribution of mean systolic gradient, at rest and during exercise, for 19 mm, 21 mm, and 23 mm pericardial bioprosthetic valves and bileaflet prostheses in the aortic position.

Left ventricular outflow diameters correlated with valve size (r = 0.64; p < 0.001), cardiac output (r = 0.45; p < 0.001), and stroke volume (r = 0.46; p < 0.001).

Cardiac output and stroke volume were greater in patients with larger valves than in those with smaller valves; there were no significant differences between bioprostheses and mechanical prostheses (table 2). Cardiac output, stroke volume, and cardiac index were all increased by exercise, but the extent of the increase did not depend on either the size or the type of valve.

Among the patients with 21 mm and 23 mm valves, neither cardiac output nor body surface area correlated significantly with transvalvar pressure drop, either at rest or during exercise. Among the patients with 19 mm valves, however, body surface area and cardiac output during exercise were both significantly correlated with resting peak transvalvar pressure drop (BSA,r = 0.5066, p < 0.03; CO,r = 0.5188, p < 0.03), with resting mean transvalvar pressure drop (BSA,r = 0.4675, p < 0.04; CO,r = 0.5006, p < 0.03), with peak transvalvar pressure drop during exercise (BSA,r = 0.7335, p < 0.001; CO,r = 0.5437, p < 0.02), and with mean transvalvar pressure drop during exercise (BSA,r = 0.4781, p < 0.04; CO,r = 0.6129, p < 0.008).

Both at rest and during exercise, VAstan, VAsimp, and effective area index were all greater in patients with larger valves than in those with smaller valves (table 2; fig 2). For the whole group of patients, exercise caused a significant increase in VAstan (p < 0.01) but not in VAsimp. Resting VAstan was greater for mechanical prostheses than for bioprostheses, but there were no significant differences between these two valve types as regards VAsimp or effective area index, either at rest or during exercise.

Figure 2

Box plot of the distribution of effective valve areas calculated by means the standard continuity equation, at rest and during exercise, for 19 mm, 21 mm, and 23 mm pericardial bioprosthetic valves and bileaflet prostheses in the aortic position.

Both at rest and during exercise, patients with larger valves had greater left ventricular end diastolic diameter, and smaller interventricular septal thickness and posterior wall thickness, than those with smaller valves, but the different valve sizes did not result in differences in left ventricular mass or left ventricular mass index (table 3). Table 4, which lists indices of diastolic function, shows that ventricular relaxation improved during exercise.

Table 3

Echocardiographically measured indices of left ventricular structure and systolic function, at rest and during exercise, for 19 mm, 21 mm, and 23 mm pericardial bioprosthetic valves and bileaflet prostheses in the aortic position

Table 4

Echocardiographically measured indices of left ventricular diastolic function, at rest and during exercise, for 19 mm, 21 mm, and 23 mm pericardial bioprosthetic valves and bileaflet prostheses in the aortic position

Discussion

Evaluation of cardiac valve prosthesis function is usually carried out with the patient supine and at rest, and most of the few previous studies of valve prostheses during exercise have involved patient groups that were both small and heterogeneous as to valve size.6 12 18 It is, however, clearly desirable to know how prostheses perform under stress, and therefore to carry out measurements during some challenging situations such as exercise, at least in the case of patients like those included in this study, who had good functional capacity. Such information is of particular interest in thecase of pericardial bioprostheses because of their reputation for good resting haemodynamics.9 10 19 20 In this study we evaluated the performance of aortic valve prostheses at rest and during exercise in patients chosen using selection criteria to ensure that exercise would not prove detrimental.

We found that in the aortic valve position the latest types of mechanical prosthesis and pericardial bioprosthesis had very similar haemodynamic performance, both at rest and during exercise. Since the haemodynamic similarity of the two types of prosthesis has been shown in other studies, the absence of significant differences in resting haemodynamics between our mechanical prosthesis and bioprosthesis groups suggests that the differences between the age and sex distributions in the two groups did not preclude meaningful comparisons. In spite of the age and sex differences, our finding that the two types of prosthesis performed similarly during exercise seems likely to reflect a true equivalence, which would be borne out by a study of a more balanced patient sample. For both types of prosthesis, the salient findings of our study were that the effective valve area, calculated using the standard continuity equation, was significantly increased by exercise; and that 19 mm and 21 mm prostheses showed suboptimal resting haemodynamics and an alarming rise in transvalvar pressure drop during exercise.

Although all our patients were in NYHA classes I or II, and none had a resting fall in transvalvar pressure indicating the need for a further valve replacement, the finding that small valves had a greater fall in transvalvar pressure than larger valves must be viewed in the light of reports that patients with small aortic valve prostheses have less postoperative regression of left ventricular hypertrophy and a worse prognosis than patients given larger prostheses.1-5 Taken together, these observations undoubtedly suggest the possibility that small valves are too inefficient to allow adequate regression of left ventricular hypertrophy, and that this will eventually prove detrimental to the patient. This possibility should be investigated more fully. Although our study groups did not differ significantly in LVM or LVMI, which reflect gross left ventricular hypertrophy, they did differ with regard to ventricular cavity size and, most importantly, wall thickness. Wall thickness was greatest among the patients with the smallest valves, suggesting less regression of ventricular remodelling in these patients despite the lack of difference in terms of gross left ventricular hypertrophy. The significant difference between small and large valves with regard to the effort induced increase in peak transvalvar pressure drop should also be considered from the point of view of long term effects and prognosis. Although the level of effort achieved in our study is maintained at most during only a small proportion of the day, even by active patients, the episodic increase in pressure drop in patients with small valves may imply an increased long term risk of cardiovascular accidents that cannot be ignored.

It is more than a decade since it was first suggested that the effective area of an aortic bioprostheses might increase with stroke volume.21 It was not expected that mechanical prostheses could behave similarly, but our results in this study appear to show that bileaflet prostheses resemble the latest pericardial prostheses in this respect. Hachioda et al have reported an effort induced increase from 1.59 to 1.87 cm2(p < 0.01) in the effective area of St Jude Medical and St Jude Medical Hemodynamic Plus valves implanted in patients subjected to root enlargement during implant, but observed no such increase in patients with 19 mm valves who were not subjected to root enlargement.22 De Paulis et alreported that exercise significantly increased the effective valve area of 19 mm CarboMedics valves, from 1.02 (0.2) to 1.20 (0.3) cm2 (p < 0.05), but not that of 21 mm valves,12 and Izzat et al found that dobutamine induced stress caused no significant increase in the areas of either 21 mm CarboMedics or 21 mm St Jude Medical valves.23 Thus the few previous reports have suggested that an exercise induced increase in valve area may affect 19 mm prostheses but not larger valves (as in the present study).

It is possible that the apparent discrepancy between our findings and those of the investigators cited above is because they all used the simplified continuity equation to calculate valve area. In our study, significant effort induced increases in valve area were revealed only when valve area was calculated using the standard equation instead of the simplified equation. There are also, however, other reasons for questioning the values published by Izzat et al and De Paulis et al.

In De Paulis’s study, an interval of two minutes between cessation of exercise and Doppler echocardiography meant that the “exercise” figures in that paper cannot represent peak effort, an interpretation supported by the fact that the post-exercise cardiac outputs reported (7.6 l/min for both 19 mm and 21 mm valves) are much smaller than those observed in our present study, despite the fact that the values for resting cardiac output in the two studies were similar (4.8 and 5.6 l/min for 19 mm and 21 mm valves, respectively). More strikingly, even though the resting transvalvar pressure drop was similar in the two studies, De Paulis et al found that exercise caused no significant increase for 19 mm valves (peak transvalvar pressure drop rose from 33.4 (13.2) to 34.3 (14.5) mm Hg and mean pressure drop rose from 20.1 (9.0) to 21.3 (9.7) mm Hg), and only a very slight increase for 21 mm valves (25.4 (5.2) to 34.9 (8.1) mm Hg peak, and 12.3 (3.4) to 15.9 (3.9) mm Hg mean, respectively). Note in particular that the group mean for post-exercise peak pressure drop was apparently the same for both 19 and 21 mm valves, despite the two groups having the same average cardiac output (7.6 l/min) but significantly different effective valve areas (1.20 (0.30) and 1.39 (0.30) cm2).

In the study by Izzat et al,23dobutamine induced stress was found to cause significant increases in heart rate, stroke volume, and cardiac output (butnot transvalvar pressure fall) in patients with 21 mm St Jude Medical and CarboMedics prostheses in the aortic position. However, in the study the stated cardiac outputs under stress were so low (6.1 and 6.8 l/min for St. Jude Medical and CarboMedics valves, respectively) that it seems apparent that dobutamine is not a satisfactory substitute for physical exercise. Second, even if one accepts and takes into account these low cardiac outputs, the mean transvalvar pressure fall was also extraordinarily low, both at rest (3.12 (3.6) and 4.87 (3.8) mm Hg for St Jude Medical and CarboMedics valves, respectively) and under stress (9.66 (13.3) and 8.81 (5.8) mm Hg); the resting figures are less than one third of those observed in our own study and in numerous others, despite similar resting valve areas, and the increase due to stress is surprisingly small in view of the reported increase in cardiac output.6 7 12 24 25

In contrast to our findings, both De Paulis and Izzat reported that stress induced little or no increase in transvalvar pressure fall.12 23 Our findings are, however, supported by those of Cochran et al, who found that exercise increased the mean pressure drop from 25 to 32 mm Hg for standard 19 and 21 mm St Jude Medical valves and from 14 to 23 mm Hg for Hemodynamic Plus valves of the same sizes.26

CONCLUSIONS

Despite undeniable recent improvements in their design, small mechanical prostheses and stented bioprostheses continue to obstruct the left ventricular outflow tract to a significant extent, especially during exercise. This finding is keeping with reports that patients with small valves may have a worse prognosis than those with larger valves, and we shall be monitoring our aortic valve replacement patients to clarify this issue. For the moment, we believe that small prostheses should probably not be implanted in the aortic position in patients who are young, physically active, or have body surface areas greater than 1.7 m2. Instead, aortic valve replacement in patients with these characteristics and small aortic annuli should probably be effected using a homograft (if available) or by implantation of a larger valve following aortic root enlargement (in spite of the slight additional risk inherent in the more complex surgical procedure).

References

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