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Ann Thorac Surg 1996;61:48-57
© 1996 The Society of Thoracic Surgeons

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Original Articles: Cardiovascular

Pressure Gradients Across Bileaflet Aortic Valves by Direct Measurement and Echocardiography

Clinic for Cardiac Surgery and Clinic of Internal Medicine, Cardiology, Triemli Hospital, and Department of Internal Medicine, Cardiology, University Hospital, Zürich, Switzerland

	Abstract

Top Footnotes Abstract Introduction Methods Results Comment Acknowledgments References

 
Background. Pressure gradients calculated from echocardiography after aortic valve replacement are commonly much higher than would be expected from in vitro measurements.

Methods. The mean, peak-to-peak, and maximal gradients across bileaflet aortic prostheses (St. Jude Medical) were measured invasively in 52 patients at high and low heart rate, cardiac index, and stroke volume. One week after operation the gradients were calculated from a standard transthoracic echocardiogram (p = 4v₂²). In a second study 3 to 12 months later, gradients were calculated using the standard, simplified Bernoulli equation, and with the equation considering subvalvular flow velocities (p = 4(v₂^2-v₁²)). Invasive and echocardiographic measurements were matched and compared.

Results. Invasively measured mean gradients for 21 to 29-mm valves ranged from 7.4 ± 4.9 to 4.3 ± 1.6 mm Hg at systolic flow rates from 11.3 ± 0.7 to 16.2 ± 1.8 L•min^-1•m^-2. Mean echocardiographic gradients were 15.1 ± 4.5 to 7.5 ± 2.2 mm Hg (p < 0.001) with the standard method, and 10.5 ± 1.9 to 5.6 ± 1.5 mm Hg when considering the subvalvular flow velocity (p < 0.001).

Conclusions. Mean gradients across bileaflet prostheses are generally low, even in small valves and with high systolic flow. The correlation of the invasive in vivo with in vitro gradients is good. Standard echocardiography overestimates gradients across bileaflet heart valves and high gradients are not due to valve dysfunction. Gradients obtained by echocardiography considering the subvalvular flow velocity correlate better to invasively measured and in vitro gradients.

	Introduction

Top Footnotes Abstract Introduction Methods Results Comment Acknowledgments References

 
At the end of the 1970s the development of bileaflet heart valves made of pyrolytic carbon led to a reliable device with a low rate of valve-related complications. In vitro studies in pulsatile flow models, with direct measurement of the pressure gradients across the St. Jude Medical valves, show mean transvalvular pressure gradients from 0.7 mm Hg for the 27-mm valve to 15.0 mm Hg for 19-mm valves at systolic flow rates ranging from 8.6 to 14.7 L/min, corresponding to a cardiac output between 3.0 and 5.25 L/min (Table 1) [1–8]. Similar data were obtained for other bileaflet valve prostheses [4–6]. The long-term clinical results after aortic valve replacement (AVR) with these valves are excellent [9, 10]. One of the main diagnostic procedures in native heart valve disease is Doppler echocardiography, transthoracic or transesophageal. The method is well established to assess the severity of aortic stenosis by measurement of the maximal and mean systolic flow velocity through the stenotic valve. Maximal and mean systolic pressure gradients across the valve can be calculated, using the simplified Bernoulli equation p = 4v², and these match very well with catheter gradients [11]. Besides clinical evaluation the follow-up after AVR is mainly based on echocardiography, due to its noninvasiveness, reliability, and widely accepted use. However, most echocardiographic studies show much higher pressure gradients as measured in vitro for the corresponding valves [5, 12–14], only few show gradients similar to in vitro results [15, 16]. Conflicting data are available regarding the accuracy of the Doppler measurements of transvalvular gradients in patients after AVR [15, 17–19]. Very few data are available comparing pressure gradients obtained invasively in vivo and their relation to gradients obtained by echocardiography in the same patients after AVR [15, 18].

	Methods

Top Footnotes Abstract Introduction Methods Results Comment Acknowledgments References

 
The study was performed on 52 consecutive patients undergoing elective AVR by the same surgeon. The largest possible St. Jude Medical valve was implanted in 52 patients: 21-mm (2 patients); 21-mm high performance (8); 23-mm (15); 25-mm (13); 27-mm (7); and 29-mm valve (7 patients). The average age of the 34 men and 18 women was 63.5 ± 11.2 years (range, 27 to 80 years); 71% were 60 years or older and 33% 70 years or older. All patients gave informed consent for intraoperative hemodynamic evaluation and postoperative repeat echocardiography. Sixty-five percent of the patients had a predominant aortic stenosis; the preoperative mean pressure gradient ranged from 42 to 96 mm Hg; 19% had pure aortic incompetence and 16% had a combined vitium with at least moderate stenosis and regurgitation. Malfunction of a degenerated bioprosthesis necessitated replacement in 2 patients and 1 patient had undergone a previous coronary artery bypass grafting. Preoperative atrioventricular block and hence pacemaker implantation at the operation was necessary in 2 patients, the rest were in stable sinus rhythm. A combined procedure with mitral valve replacement or reconstruction or bypass grafting was performed in 29% of patients. Other additional surgical procedures were performed in 9 patients (17%), including enlargement or reduction plasty of the ascending aorta (4/9), resection of asymmetric left ventricular hypertrophy (2/9), pacemaker implantation (2/9), and carotid endarterectomy (1/9). Preoperative functional status was class II for 35% and class III or IV for 65% of the patients (New York Heart Association functional class). The patient population showed significant differences between patients with an aortic annulus of 23 mm or less, and patients with a larger annulus (Table 2). Patients with a narrow annulus were mainly women, older (annulus of 21 mm, 69.7 ± 4.2 years; annulus more than 21 mm, 62.1 ± 11.8 years; p = 0.002), shorter, had a smaller body surface area, had predominantly aortic stenoses, and were more symptomatic. Follow-up for the first echocardiography was 100%, 3 patients were lost to follow-up for the second echocardiogram: A 78-year-old man with a 23-mm valve died of congestive heart failure 2 months after AVR. The preoperative left ventricular ejection fraction was 0.30 and the cardiac index 1.3 L•min^-1•m^-2. The invasive gradients after AVR were mean 6.3, peak-to-peak 0, and maximal 15.1 mm Hg at a cardiac index of 3.8 L•min^-1•m^-2. Two asymptomatic patients with a clinically excellent result withdrew their consent for the second echocardiogram. At 3 to 12 months all surviving patients (n = 51) were in functional class I or II and had no evidence of valve dysfunction. No paravalvular leak or thrombus formation was detected and all valves had a normal leaflet motion.

View larger version (20K): [in this window] [in a new window]   Fig 1. . Intraoperative direct pressure tracings of a patient with severe aortic stenosis (A) and after aortic valve replacement with a 21HP valve (B). The maximal instantaneous gradient is now at very early systole and not at mid-systole as before valve replacement. (pl = maximal instantaneous pressure drop; pM = mean systolic gradient; pP = peak-to-peak gradient; Ts = systolic ejection period.)

First Echocardiogram
Six days after operation the first Doppler ultrasound examination was performed on resting patients at spontaneous sinus rhythm, and at paced heart rates of 110 to 130 beats/min. At this time all but 2 patients with implanted DDD pacemakers were in stable sinus rhythm and atrial pacing was possible using the temporary pacing wires implanted at operation. The Doppler ultrasound examination was performed and recorded on an Ultramark (UM9 HDI) system (Advanced Technology Laboratories, Bothel, WA) with a 2.5-MHz dedicated continuous wave Doppler-transducer. Mean and maximal pressure gradients were calculated with the simplified Bernoulli equation p = 4v₂².

Second Echocardiogram
Three to 12 months after operation the second Doppler ultrasound examination on resting patients was performed by a second examiner. The patients were in sinus rhythm. The examinations were performed and recorded on a Hewlett-Packard Sonos 1500 phased array imaging system (Hewlett-Packard) with an integrated 1.9-MHz continuous wave and a 2.5-MHz pulsed wave transducer. Cardiac output was measured with the pulsed wave Dopper from apex, and was used to match the Doppler echocardiographic gradients with the gradients from the direct measurement. In addition to the aortic valve flow velocity (v₂), the subvalvular flow velocity (v₁) was determined from the continuous-wave spectrum (Fig 2). The maximal pressure gradient was calculated with the simplified Bernoulli equation p = 4v₂², and accounting for the subvalvular flow velocity with the less simplified Bernoulli equation p = 4 (v₂² - v₁²). In the same way, the mean systolic pressure gradient was calculated by applying the Bernoulli equation (with and without respect to the subvalvular flow velocity) to the instantaneous velocities of aortic flow throughout systole and averaging the values. The applied formulas were

View larger version (136K): [in this window] [in a new window]   Fig 2. . Recording of the continuous wave Doppler echocardiographic envelope 4 months after operation from the same patient as in Figure 1. The maximal instantaneous flow velocity and hence pressure gradient is before mid-systole, but markedly later as the maximal instantaneous pressure gradient in the pressure tracings in Fig 1B. (v1 = subvalvular flow velocity; v2 = valvular flow velocity.)

for p mean = 4v₂²

and

for p mean = 4(v₂² - v₁²), with v_{1 ... 1} to v_{1 ... n} = instantaneous systolic subvalvular flow velocities, v_{2 ... 1} to v_{2 ... n} = instantaneous systolic valvular flow velocities, and n = number of samples achieved during systolic forward flow.

Matching
The measurements of direct and Doppler echocardiographic derived gradients were not taken simultaneously. Direct measurements obtained at a cardiac index of 2.5 to 3.5 L/min were matched with the first (1 week) echocardiogram recorded at the most similar heart rate. Matching of the direct measured gradients with the gradients derived from the second echocardiogram was done by systolic flow rates.

Statistics
The parametric data were expressed as mean values ± standard deviation. The age, height, and other patient data were compared using unpaired Student's t test, relative frequencies with the Fisher's exact test. Correlation of mean pressure gradients with systolic flow and valve size was analyzed separately for the three measurements with the analysis of covariance. We performed multiple analyses with dummy-variables for the valve sizes with the Statview program (Statview, Abacus Concepts, Inc, Berkeley, CA) on a Macintosh computer. The paired Student's t test was used to compare direct gradients with gradients derived from echocardiographic calculations, and to compare hemodynamic data during measurements of the gradients. Correlations between echocardiographic gradients were calculated with the least squares method of linear regression. A p value of 0.05 or less was considered statistically significant.

	Results

Top Footnotes Abstract Introduction Methods Results Comment Acknowledgments References

 
Comparing the left ventricular and aortic pressure tracings with the flow velocity envelope in continuous wave Doppler echocardiography it is evident that the maximal instantaneous pressure gradient from direct measurement is not comparable with the maximal pressure gradient in echocardiography. The maximal gradient in direct pressure tracings is at very early systole, whereas the highest valvular flow velocity (v₂) in echocardiography is later in systole (Figs 1B and 2). In time, the maximal echocardiographic gradient would correspond to the invasive peak-to-peak gradient.

Invasive Gradients by Direct Measurement
For each valve size mean gradients increase with flow. At a given systolic flow the pressure gradients are lower with larger valves (Fig 3). At systolic flow rates of less than 14 L/min the 21- and 21HP-mm valves have mean gradients of less than 10 mm Hg, the 25-mm, 27-mm, and 29-mm valves had mean gradients of less than 5 mm Hg. The pressure drop through the 21-mm to 29-mm aortic valves ranges from 7.4 ± 2.3 to 4.3 ± 1.7 mm Hg, measured at systolic flow rates from 10.7 to 15.4 L/min (Table 3). The maximal instantaneous gradients are usually quite high and at the moment of valve opening. Peak-to-peak gradients are in the same range as mean gradients, but show a greater variability (Table 3).

View larger version (31K): [in this window] [in a new window]   Fig 3. . Mean pressure gradients by direct measurement of 52 St. Jude Medical aortic valves in vivo. The pressure gradients are plotted for each valve size individually. With higher systolic flow across the valves the gradients increase and at a given systolic flow, the gradients decrease from small to larger valve sizes.

View larger version (80K): [in this window] [in a new window]   Fig 4. . Mean pressure gradients by direct measurement and echocardiography. First bar: mean pressure gradient by direct measurement; second bar: mean pressure gradient in second echocardiogram (p = 4(v22 - v12)); third bar: mean pressure gradient in second echocardiogram (p = 4v22); fourth bar: mean pressure gradient in first echocardiogram (p = 4v22).

View larger version (21K): [in this window] [in a new window]   Fig 5. . Mean and maximal pressure gradients in echocardiography. Correlation of gradients calculated with the simplified Bernoulli equation (p = 4v22) with gradients calculated with respect to the subvalvular flow velocity (p = 4(v22 - v12)). The correlation equations are: Mean gradient: p(4v22) = 1.107 p(4(v22 - v12)) + 1.779; r2 = 0.858 and p(4(v22 - v12) = 0.775 p(4v22) - 0.268; r2 = 0.858. Maximal gradient: p(4v22) = 1.139 p(4(v22 - v12)) + 2.096; r2 = 0.885 and p(4(v22 - v12) = 0.777 p(4v22) + 0.08; r2 = 0.885.

	Comment

Top Footnotes Abstract Introduction Methods Results Comment Acknowledgments References

 
We could not perform simultaneous echocardiographic and direct measurement of pressure gradients on the open chest. Intraoperative transesophageal echocardiography does not permit alignment of the ultrasound beam with the direction of the maximal systolic jet through the aortic valve. Consequently, echocardiographic pressure gradients can be measured only with the transthoracic technique. For this reason, echocardiograms were not performed until a transthoracic echocardiogram of good quality could be obtained. In the simultaneous pressure tracings from the left ventricle and the aortic root the maximal instantaneous gradient is at very early systole. It does not match in time with the maximal instantaneous gradient obtained by echocardiography, which is calculated from the maximal flow velocity. The maximal flow velocity through the valve is later in systole. In pulsatile flow the velocity is flow dependent for prosthetic heart valves with a constant anatomic valve orifice area. The maximal flow velocity reflects the maximal systolic flow through the valve. Hence, the maximal instantaneous pressure gradient obtained by echocardiography is the pressure gradient at maximal systolic flow. The early maximal gradient in pressure tracings indicates a leaflet inertia that is overcome in early systole when the valve opens fully. The different timing of maximal pressure gradients demonstrates that maximal gradients obtained from different methods are not comparable. In relation to the large number of implanted valves, only limited data on direct measurements of pressure gradients in vivo are available. When an intraoperative direct measurement is not performed, a dual catheter technique with simultaneous recording of left ventricle and ascending aortic pressures is necessary. To pass a catheter through a prosthetic heart valve may damage it and lead, especially in mechanical valves, to substantial regurgitation. In this condition the actual systolic flow through the valve is higher than the flow measured with thermodilution. Proper in vivo gradients in relation to the systolic flow can only be obtained by transseptal catheterization or transthoracic puncture [15] of the left ventricle. Because these techniques for direct measurement of pressure gradients are invasive and have associated complications, they cannot be performed routinely in patients after AVR. Lillehei [20] reported the results of invasively obtained peak-to-peak gradients in 33 patients after AVR with the St. Jude valve, from different centers. At cardiac indices more than 2.8 L•min^-1•m^-2 the gradients were 9.0 to 11.0 mm Hg for the 21-mm and 19-mm valves and less than 5 mm Hg for the larger valves. Identical results were published from the Minneapolis group [21] on 18 and 22 patients (probably the same). Higher gradients were reported by Albes and colleagues [22] with peak-to-peak gradients from 3 to 13 mm Hg (cardiac output 6 to 8 L/min) in 25-mm valves (n = 6) and Burstow and co-workers [15] in two 21-mm valves. In a series of 28 valves Chaux and colleagues [23] showed a twofold increase of the mean gradients (5.2, 3.2, 3.4 mm Hg for the 21-, 23-, and 25-mm valve, respectively) with isoproterenol infusion and increasing the cardiac output from 4 to 7 L/min. Much higher gradients were found in 19-mm valves (n = 6), when the mean gradient rose from 22 to 40 mm Hg, and the peak-to-peak gradient rose from 16.7 to 32.4 mm Hg after exercise [24]. Our own measurements match very well with these low gradients. Correlation of the in vivo and in vitro gradients measured by different institutions (Table 1) is good for the larger valves (25 to 29 mm). Our data clearly show the limitations of direct gradient measurement. For the small valves (19 and 21 mm) the direct measurement may overestimate the gradients across the valve. The tip of the needle or catheter in the left ventricle cannot be placed precisely beyond the valve and therefore, subvalvular and localized intraventricular gradients are included in the measurement. The small, hypertrophic hearts of short elderly women with severe aortic stenosis tend to collapse at the end of systole, especially in hypovolemia. Mainly these were the patients with a small aortic annulus, forcing the implantation of smaller valves (Table 2). With pacing to a higher heart rate and thus reduction of the stroke volume or even inotropic support the gradients occur or increase. This is demonstrated by the unexpected increase of the gradients in the 21-mm and 21HP-mm valves with inotropic stimulation (Fig 4). These dynamic left intraventricular gradients even occur after clearance of a fixed outflow tract obstruction and were observed in 15 of 51 patients after AVR [25].

The routine echocardiographic follow-up after AVR includes calculation of the transvalvular pressure drop and the valve function is often judged by these gradients. But a review of the literature [5, 12–19, 26–28] for reported mean and maximal gradients of St. Jude Medical and other valves in aortic position shows a large variability in the reported gradients for the different valve types and sizes. Cooper and colleagues [12] and Labovitz [29] found a relatively large range of ``normal values'' among peak and mean echocardiography-derived gradients obtained in prosthetic valves of the same size. This ``normal range'' is usually much higher than the in vitro gradients for the corresponding valves. They explain this variability with the wide variation of hemodynamic parameters as heart rate, cardiac output, stroke volume, and flow period in patients with clinically normal functioning valves. Our data show a substantial variability of the gradients obtained from different observers and at different times. A very localized and narrow high velocity jet [30] between and just downstream of the staight edges of the two leaflets, not representative for the flow velocity of the rest of the valve orifice profile may lead to essential overestimation of the gradients when the ultrasound beam is focused on this jet. Measurement either in this jet or in the large side orifices of the valve explain the variability of gradients between observers and examinations in the same patients under comparable hemodynamic conditions. Moreover in vitro studies show that the phenomenon of pressure recovery [30–32] is another reason for systematic overestimation of pressure gradients by echocardiography. In severe native aortic stenosis its influence is not important, but as the gradients get lower the discrepancy between echocardiography and invasive gradients increases [33]. Especially in bileaflet prostheses the localized gradients and pressure recovery [1, 30–32] are responsible for high echocardiographic gradients compared to directly measured in vitro and in vivo gradients. The systematic overestimation of the gradients by echocardiography is not only due to pressure recovery and localized gradients, but also to application of the incomplete Bernoulli equation. The equation is p = 4(v₂²-v₁²), when viscous friction is neglected. In our series the average v₁ max and v₁ mean were 0.98 and 0.77 m/s, respectively. In severe aortic stenosis, with maximal gradients of 50, 75, and 100 mm Hg, calculated with the simplified equation (p = 4v₂²) the v₂ are 3.54, 4.33, and 5.00 m/s. With v₁ assumed to be 1.00 m/s the pressure gradients calculated with the complete equation would be 46, 71, and 96 mm Hg; the difference is 4 mm Hg and the relative error drops from 7.8% for p = 50 mm Hg to 4.2% for p = 100 mm Hg. Therefore, it is reasonable to neglect the subvalvular flow velocity for calculation of gradients in moderate or severe aortic stenosis. However, with lower valvular flow velocities, as seen in mild aortic stenosis or after AVR, the influence of the subvalvular flow velocity is more important and the relative error increases. The average maximal valvular and subvalvular flow velocities in our patients were v₂ = 2.14 ± 0.36 m/s and v₁ = 0.98 ± 0.23 m/s, respectively. The discrepancy between the two calculations reaches 28%. Therefore, the subvalvular flow is not negligible after AVR.

In the second echocardiographic study we could show a very good correlation between the two different calculations of pressure gradients (Fig 5), and we believe that the gradients may be calculated by applying the simplified Bernoulli equation. There are outliners in this correlation and we recommend recalculation of selected high pressure gradients, according for the subvalvular flow velocity. Baumgartner and colleagues [34] could show in vitro that in malfunctioning bileaflet valves, the difference between direct and echocardiographic gradients decreases as the valve obstruction increases. Therefore, echocardiographic gradients increase less than the real gradients when a valve gets progressively obstructed.

The follow-up after AVR should be based mainly on clinical findings. If there is no mismatch of valve size and body surface (more than 1 cm² effective orifice area per meter square body surface) the pressure drop through the valve can be supposed to be small, even in small valves and with exercise. The high performance valve series offers a good opportunity to implant a valve with a larger orifice area in a small aortic annulus. Doppler echocardiography is an important examination for assessment of paravalvular leakage, proper opening and closing of the leaflets, thrombus formation, subvalvular stenoses, left ventricular function, and regional wall motion. Pressure gradients are less important and should be interpreted in combination with other findings.

Some investigators compare different valve types by pressure gradients derived from echocardiography and recommend implantation of valves with lower gradients [26]. Echocardiography systematically overestimates the overall gradient, probably in a variable degree, dependent on the valve design and its influence on localized flow velocity profiles. Therefore, the performance of different types of mechanical heart valves should not be compared by echocardiography. In vitro measurements under standard conditions provide the only reliable data to compare the performance of different valve types.

In conclusion the bileaflet St. Jude valve prostheses have an excellent hemodynamic performance in the aortic position. The mean gradients are less than 10 mm Hg under resting conditions and increase with exercise, but remain less than 13 mm Hg even in small valves with high systolic flow. Standard echocardiography overestimates the transvalvular pressure drop. High gradients do not necessarily indicate valve dysfunction. Gradients obtained under consideration of the subvalvular flow velocity correlate better to direct measurements and in vitro gradients.

	Acknowledgments

Top Footnotes Abstract Introduction Methods Results Comment Acknowledgments References

 
We would gratefully acknowledge the biostatistical support of Dr Burkhardt Seifert from the Department of Biostatistics, University Zürich.

	Footnotes

Top Footnotes Abstract Introduction Methods Results Comment Acknowledgments References

 
Presented at the Poster Session of the Thirty-first Annual Meeting of The Society of Thoracic Surgeons, Palm Springs, CA, Jan 30-Feb 1, 1995.

Address reprint requests to Dr Laske, Herzzentrum Hirslanden, Witellikerstr. 36, CH 8008 Zürich, Switzerland.

	References

Top Footnotes Abstract Introduction Methods Results Comment Acknowledgments References

 

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This Article Abstract Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to Personal Folders Download to citation manager Author home page(s): Andreas Laske Marko I. Turina Permission Requests Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Laske, A. Articles by Turina, M. I. Search for Related Content PubMed PubMed Citation Articles by Laske, A. Articles by Turina, M. I.