By Philippe Pibarot
Low-flow, low-gradient (LF-LG) aortic stenosis may occur with depressed (classical LF-LG) or preserved (paradoxical LF-LG) left ventricular ejection fraction, and both situations are amongst the most challenging encountered in patients with valvular heart disease. In both cases, the decrease in transvalvular pressure gradient relative to stenotic severity is due to a reduction in transvalvular flow. In patients with low left ventricular ejection fraction, LF-LG aortic stenosis, dobutamine stress echocardiography (DSE) is useful to distinguish between true-severe versus pseudo-severe stenosis and to assess the severity of myocardial impairment. The use of DSE for this purpose has received a Class IIa recommendation in both the American College of Cardiology/American Heart Association guidelines1 and the European Society of Cardiology/ European Association (ESC/EACTS) of Cardiothoracic Surgery guidelines2 for valvular heart disease.
Distinguishing between true-severe and pseudo-severe stenosis
The evaluation of the changes in aortic valve area and gradient during dobutamine infusion is helpful in differentiating true- from pseudo- severe aortic stenosis. Typically, pseudo-severe aortic stenosis shows an increase in valve area and relatively little increase in gradient in response to increasing flow whereas true severe aortic stenosis is characterised by little or no increase in valve area and an increase in gradient, which is congruent with the relative increase in flow3. Several parameters and criteria have been proposed in the literature to identify patients with pseudo-severe stenosis during DSE including: a peak stress mean gradient ≤30 or 1.0 or 1.2cm2, and/or an absolute increase in valve area ≥0.3cm2. The optimal cut-off values thus remain to be determined. The prevalence of pseudo-severe aortic stenosis is reported as being between 20% and 30%.
Several patients may nonetheless have an ambiguous response to DSE (eg. a peak-stress gradient of 29mmHg and a valve area of 0.8cm2) due to variable increases in flow, and interpreting the changes in the valve area and gradients without considering the relative changes in flow may often be problematic. Hence, to overcome this limitation, the investigators of the TOPAS (Truly or pseudo-severe aortic stenosis) study have proposed to rather calculate the projected valve area that would have occurred at a standardised flow rate of 250mL/s, and this new parameter has been shown to be more closely related to actual severity, impairment of myocardial blood flow, and survival than the traditional DSE parameters3. A recent multicenter study4 also reported that the measurement of the projected aortic valve area derived from stress echocardiography is helpful to determine the actual severity of the stenosis and predict risk of adverse events in the patients with paradoxical (ie. preserved left ventricular ejection fraction) LF-LG aortic stenosis and the prevalence of pseudo-severe stenosis in this series was 33%.
Assessing left ventricular contractile/flow reserve
Patients with no left ventricular flow reserve are defined by a percent increase in stroke volume <20% during DSE and have higher operative mortality (22–33%) with surgical aortic valve replacement than those with flow reserve (5–8%); they represent approximately 30 to 40% of patients with low left ventricular ejection fraction, LF-LG aortic stenosis3. In patients with no flow reserve surviving operation, the postoperative improvement in left ventricular ejection fraction as well as the late survival rate were as good as in the patients with flow reserve and much better than in those with no flow reserve treated medically. Hence, the absence of left ventricular flow reserve should not necessarily preclude consideration of valve replacement in these patients.
Therapeutic management
Patients with true-severe stenosis and evidence of left ventricular flow reserve on DSE should be considered for valve replacement (Class IIa indication in 2012 ESC/EACTS guidelines)2. Patients with left ventricular flow reserve and pseudo-severe stenosis should probably be treated medically at first but nonetheless followed up very closely (ie every two to three months) and the therapeutic options reconsidered in case of lack of improvement or deterioration.
Valve replacement can certainly be contemplated in the patients with no left ventricular flow reserve having evidence of a true-severe aortic stenosis on DSE (projected valve area) or CT (valve calcium scoring)3. However, given that operative risk for open heart surgery is generally very high in absence of flow reserve, transcatheter aortic valve implantation (TAVI) could provide a valuable alternative in these patients even though the rates of morbidity and mortality may be higher than in patients with normal flow. Indeed, recent studies report a greater and more rapid improvement of left ventricular ejection fraction in the patients treated by TAVI than those treated by surgery5. This advantage is believed to be related to better myocardial protection as well as to a lesser incidence of prosthesis-patient mismatch. On the other hand, TAVI is associated with higher incidence of paravalvular regurgitation, which may eventually have a negative impact on outcomes. Further randomised studies comparing surgery vs. TAVI in LF-LG aortic stenosis patients are warranted.
Philippe Pibarot, professor, Department of Medicine, Laval University; chair, Canada Research Chair in Valvular Heart Diseases, Québec Heart & Lung Institute, Québec, Canada
References
1. Bonow et al. Circulation. 2006; 114: 450–27
2. Vahanian et al. European Heart Journal 2012; 33, 2451–96
3. Pibarot and Dumesnil. J Am Coll Cardiol 2012;60:1845-53
4. Clavel et al. JACC Cardiovasc Imaging. In press
5. Clavel et al. Circulation 2010;122:1928-36
