Anesth Analg 2005;101:1288-1291
© 2005 International Anesthesia Research Society
doi: 10.1213/01.ANE.0000181339.39448.F0
CARDIOVASCULAR ANESTHESIA
Is the Valve OK or Not? Immediate Evaluation of a Replaced Aortic Valve
Rebecca A. Schroeder, MD, and
Jonathan B. Mark, MD
Department of Anesthesiology Durham Veterans Medical Center Duke University School of Medicine Durham, North Carolina
Address correspondence and reprint requests to Rebecca A. Schroeder, MD, Department of Anesthesiology Durham Veterans Medical Center Duke University School of Medicine VAMC (112C), 508 Fulton St. Durham, NC 27705. Address electronic mail to schro016{at}mc.duke.edu.
 |
Abstract
|
|---|
Transesophageal echocardiography is a crucial tool in intraoperative evaluation of newly implanted/repaired heart valves because suspected valvular malfunction needs to be identified and sometimes surgically corrected. Although color Doppler is often adequate in evaluating the expected regurgitant jets, as well as excluding pathologic paravalvular leaks, spectral Doppler techniques are the most commonly used methods for estimating transvalvular gradients in the operating room. However, these methods are subject to a variety of confounding factors, including subvalvular gradients and pressure recovery. Other methods of valve area estimation should also be used when evaluating a prostethic aortic valve, including the continuity equation and the left ventricular outflow tract/aortic valve velocity ratio.
|
Introduction
|
|---|
Transesophageal echocardiography (TEE) is useful for intraoperative assessment of newly implanted prosthetic heart valves. The TEE evaluation should include documentation of leaflet movement with 2-dimensional imaging, absence of paravalvular regurgitation, and estimation of transvalvular pressure gradients. Abnormalities identified in the operating room (OR) may require immediate surgical correction.
We present a case in which dysfunction of an aortic mechanical valve was suspected because of measurement of unusually high peak velocities on spectral Doppler analysis. This case highlights the dangers of relying on peak transvalvular velocities as the primary indicator of valvular stenosis.
|
Case Report
|
|---|
A 63-yr-old, 84 kg, 179 cm male (body surface area, 2.05) presented for aortic valve (AV) replacement for severe aortic stenosis (AS). An echocardiogram showed severe concentric left ventricular (LV) hypertrophy, preserved systolic LV function, and a bicuspid, stenotic AV (area 0.9 cm2).
Because of extensive calcification of the aortic annulus, the patient underwent AV replacement with a 21-mm Carbomedics bileaflet valve. The initial, pre-bypass, cardiac output (CO) was measured via intermittent thermodilution to be 4.5 L/min. At the time of separation from cardiopulmonary bypass, CO was 6.8 L/min while dopamine and vasopressin were infused. A small, eccentric paravalvular leak was visualized by TEE. One leaflet of the prosthesis moved well; the other was not visible. Continuous wave Doppler measurement of the AV showed a peak velocity of 4.8 m/s, a peak gradient of 91 mm Hg, and a mean gradient of 42 mm Hg. According to the manufacturer’s specifications, the expected mean gradient for this valve is 12.3 mm Hg (1). The surgeon was informed, but as the patient was hemodynamically stable, the chest was closed and the patient was taken to the intensive care unit (ICU) still on the same vasopressor infusions. His hematocrit at that time was 28%.
After 2 h, the patient was returned to the OR because of excessive bleeding. Reexamination of the prosthetic valve showed high peak velocity (5.14 m/s) with a peak gradient of 106 mm Hg and mean gradient of 45 mm Hg, a measurement confirmed 5 times by 2 operators (Fig. 1). The second leaflet still could not be seen. A cardiologist was consulted who concurred that there was an obstruction at the valvular level and strongly recommended surgical intervention. By this time, however, the chest had again been closed. An unsuccessful attempt was made to document motion of the second leaflet with C-arm fluoroscopy. However, given the patient’s clinical improvement (CO, 8.2 L/min), he was taken to the ICU for observation.
View larger version (87K):
[in this window]
[in a new window]
|
Figure 1. A continuous-wave Doppler measurement through the prosthetic aortic valve from the transgastric position. The peak velocity recorded is 5.3 meters/s, which is abnormally high, even for mechanical prosthetic valves.
|
|
The following morning, he was taken to the cardiac catheterization lab, where, under adequate fluoroscopy, both leaflets were seen to be functioning normally. A subsequent follow-up transthoracic echocardiogram (TTE) at 1 yr showed a peak gradient of 25 mm Hg and a mean gradient of 15 mm Hg.
|
Discussion
|
|---|
Use of TEE after valvular procedures is essential in ensuring adequate functioning of the newly implanted or repaired valve (2–6). In our case, 2-dimensional images of the prosthesis suggested a problem with one leaflet, whereas Doppler examination showed an unusually high peak velocity through the new valve, raising clinical suspicion of prosthesis malfunction. However, fluoroscopy in the catheterization laboratory the next day showed normal motion of both leaflets. We believe several factors must be considered for proper interpretation of Doppler data in this confusing clinical setting.
Bernoulli’s Equation, as it is applied to echocardiography, is adapted to give the modified Bernoulli Equation as follows:
where V2 is the peak velocity through the valve, and V1 is the subvalvular velocity; that is, through the LV outflow tract (LVOT) (7). In general practice, V1 is generally ignored, yielding the simplified form of the equation:
However, in cases such as ours, where V1 is significantly more than 1.0 m/s (2.6 m/s in this case), the modified Bernoulli Equation must be used (Fig. 2). Bird et al. (8) showed that subvalvular gradients (41 ± 17 mm Hg) are present in patients with AS but without evidence of anatomic subvalvular obstruction. Disregarding V1 introduces an unacceptable degree of error in these patients.
View larger version (92K):
[in this window]
[in a new window]
|
Figure 2. A high-repetition pulsed-wave Doppler measurement with the sampling volume placed in the left ventricular outflow tract documenting a significant degree of subvalvular obstruction. This invalidates the use of the simplified form of Bernoulli’s equation.
|
|
A second point is the need to calculate valve area using the continuity equation. With a measured LVOT diameter of 0.9 cm, V1 of 2.6 m/s, and V2 of 5.14 m/s, the prosthetic valve area is 1.30 cm2, a value close to 1.54 cm2, the effective area of a 21-mm Carbomedics valve (1). Even more useful is measurement of the peak flow ratio (LVOT/AV), which in this case (2.6/5.14) is approximately 0.5, very near the published normal for this size and type of valve (0.4) (9,10). Both of these calculations confirm normal function of the new valve.
Pressure recovery is responsible for the observed differences between Doppler-derived gradients and catheter-measured gradients and is probably the least recognized factor involved. As flow traverses a narrow orifice, the flow stream contracts (vena contracta) and subsequently expands on reaching a wider passage. Pressure exerted by the flow stream decreases with increasing velocity, and "recovers" as the fluid slows distal to the maximal obstruction. Thus, pressure measured in the LVOT will decrease at the level of the AV and recover in the proximal ascending aorta. Spectral Doppler measurements allow calculation of the maximal pressure gradient between the proximal flow stream (LVOT) and the vena contracta (valve orifice), whereas catheter measurements record gradients between the proximal flow stream (LVOT) and the recovered pressure beyond the vena contracta (proximal ascending aorta).
Pressure recovery may be responsible for a significant portion of our misleading measurements. In St. Jude bileaflet valves, the proportion of the peak laboratory-measured gradient (analogous to the Doppler-derived gradient) attributed to pressure recovery was 53% for the central orifice and 29% for the slightly larger side orifices (11). Interestingly, a smaller proximal aorta may be an important predictor for pressure recovery. In a series of 23 patients with native aortic stenosis, all of those with Doppler-catheter gradient differences more than 20 mm Hg had aortas that measured <3 cm in diameter (12). In our patient, the diameter of the proximal aorta was 2.8 cm.
Several other factors may have been important in our case. The presence of postbypass anemia and an increased CO, as well as factors such as patient/prosthetic mismatch and technical errors, may have contributed. The possibility of measurement or operator error must also be considered. Measurement through the smaller central orifice of a bileaflet prosthesis will yield a higher peak velocity than the larger side orifices, an error decreased by multiple measurements.
Diagnostic evaluation in the OR can use a variety of echocardiographic and other maneuvers depending on the expertise of the anesthesiologist and the surgeon, as well as the stage of the operation. Use of epiaortic scanning in the hands of a skilled surgeon may settle the question definitely by imaging the leaflets more clearly. However, when using Doppler techniques, the same issues would apply as with TEE. If the chest has been closed, or if doubt remains before leaving the OR, a TTE could be performed, which may be able to image the leaflets or give a more superior Doppler signal of the LVOT given its more flexible position on the chest. Various case reports have been published describing similar cases in which gradients have been measured with the use of needles placed directly into the LV and ascending aorta. Unfortunately, these directly-measured gradients are subject to error from similar sources as Doppler-measured gradients (that is, pressure recovery), as well as confounding factors from impact energy interacting with single-holed needles and acoustic shadowing from the prosthesis itself (13). Thus, these data should be interpreted with caution.
In summary, we report a case during which an unusually high peak velocity measured after aortic prosthetic valve implantation, combined with an inability to visualize leaflet motion, raised serious concerns of prosthesis malfunction or stuck leaflet. In this setting, it is imperative to consider the presence of a subvalvular gradient and apply the modified rather than the simplified Bernoulli Equation. Furthermore, effective valve area should be calculated using the continuity equation and, most importantly, by the LVOT/AV velocity ratio to assess AV function (normal, 0.35–0.5 for range of prosthetic valves) (13). The phenomenon of pressure recovery should be considered in evaluating prosthetic valve function when small prostheses (19, 21 mm) are implanted, and especially when the proximal ascending aorta is small. Other echocardiographic techniques may be helpful, especially epiaortic scanning by a skilled surgeon, and TTE, if the chest is already closed.
|
Footnotes
|
|---|
Accepted for publication June 9, 2005.
|
References
|
|---|
- Carbomedics heart valves pressure gradient measurement. Available at: http://www.mitroflow.com/professional_surgeon_pressure.asp. Accessed January 24, 2005.
- Oh JK, Taliercio CP, Holmes DR Jr, et al. Prediction of the severity of aortic stenosis by Doppler aortic valve area determination: prospective Doppler-catheterization correlation in 100 patients. J Am Coll Cardiol 1988;11:1227–34.[Abstract]
- Khandheria BK, Seward JB, Tajik AJ. Transesophageal echocardiography. Mayo Clin Proc 1994;69:856–63.[ISI][Medline]
- Chaliki HP, Click RL, Abel MD. Comparison of intraoperative transesophageal echocardiographic examinations with the operative findings: prospective review of 1918 cases. J Am Soc Echo 1999;12:237–40.[ISI][Medline]
- Nowrangi SK, Connolly HM, Freeman WK, Click RL. Impact of intraoperative transesophageal echocardiography among patients undergoing aortic valve replacement for aortic stenosis. J Am Soc Echo 2001;14:863–6.[ISI][Medline]
- Ionescu AA, West RR, Proudman C, et al. Prospective study of routine perioperative transesophageal echocardiography for elective valve replacement: clinical impact and cost-saving implications. J Am Soc Echocardiogr 2001;14:659–67.[ISI][Medline]
- Hemodynamic assessment. In: Oh JK, Seward JB, Tajik AJ, eds. The echo manual. Philadelphia: Lippincott Williams & Wilkins, 1999:59–72.
- Bird JJ, Murgo JP, Pasipoularides A. Fluid dynamics of aortic stenosis: subvalvular gradients without subvalvular obstruction. Circulation 1982;66:835–40.[Abstract/Free Full Text]
- Chafizadeh ER, Zoghbi WA. Doppler echocardiographic assessment of the St. Jude Medical prosthetic valve in the aortic position using the continuity equation. Circulation 1991;83:213–23.[Abstract/Free Full Text]
- Prosthetic valves. In: Otto CM, ed. Textbook of clinical echocardiography. Philadelphia: WB Saunders, 2004:355–83.
- Bech-Hanssen O, Caidahl K, Wallentin I, et al. Aortic prosthetic valve design and size: relation to Doppler echocardiographic findings and pressure recovery: an in vitro study. J Am Soc Echo 2000;13:39–50.[Medline]
- Baumgartner H, Stefenelli T, Niederberger J, et al. "Overestimation" of catheter gradients by Doppler ultrasound in patients with aortic stenosis: a predictable manifestation of pressure recovery. J Am Coll Cardiol 1999;33:1655–61.[Abstract/Free Full Text]
- Rehfeldt KH, Click RL. Prosthetic valve malfunction masked by intraoperative pressure measurements. Anesth Analg 2002;94:857–8.[Abstract/Free Full Text]
Top
Abstract
Introduction
