Do Doppler gradients correlate with catheterization gradients for aortic valve stenosis? If so, how accurately and precisely does the echocardiogram predict a catheter-based gradient? We have been faced with this question since the arrival of Doppler echocardiography. In this issue of JASE , Schlingmann et al . report on the use of the pressure recovery–corrected peak Doppler gradient to improve agreement with the catheterization gradient for congenital aortic valve stenosis. In this study, the investigators compared 100 Doppler and catheterization gradients in 80 children. They then concluded that use of the pressure recovery–corrected peak Doppler gradient improves agreement with the subsequently measured catheterization gradient, thereby enhancing the role of echocardiography in the decision to proceed with catheterization.

In the clinical setting, the severity of aortic valve stenosis is generally first evaluated noninvasively using Doppler echocardiography. On the basis of this noninvasive assessment, a decision is made to send patients to catheterization to confirm the severity of stenosis and for potential balloon aortic valvuloplasty. As Schlingmann et al . properly note, echocardiography acts as a gatekeeper to catheterization. It is not a surprise to find a significant difference between Doppler and catheterization gradients. We agree with the investigators regarding the two reasons for the discrepancy: (1) Doppler and catheterization gradients measure two temporally different phenomena, and (2) the phenomenon of pressure recovery plays a role.

However, we are concerned that there may be more fundamental reasons to cause this discrepancy. The effects of sedation, general anesthesia, and analgesic medications on cardiac physiology cannot be ignored. Those universally affect blood pressure, heart rate, systemic vascular resistance, and cardiac output, with a varying degree of response in each individual. Furthermore, it is common for small infants and children to get agitated during echocardiographic studies, thereby increasing blood pressure and heart rate. Ultimately, we often measure the Doppler and catheterization gradients in two completely different physiologic states. Schlingmann et al . report the results of a retrospectively conducted study in which echocardiography and catheterization were not performed simultaneously. Our group has previously compared 23 Doppler and catheterization gradients acquired simultaneously during catheterization studies in 14 children. We reached the same conclusion as in Schlingmann et al .’s study, although with a different implication. In the study conducted by Schlingmann et al ., the SD of mean differences between the Doppler-derived and catheterization gradients remains relatively large despite correcting for pressure recovery (peak, 15.9 mm Hg; mean, 15.6 mm Hg; and pressure recovery–corrected peak, 14.1 mm Hg). This indicates that the pressure recovery–corrected peak Doppler gradient improves accuracy but is still not very precise. We suspect that this low precision resulted partially from their study design (nonsimultaneous measurements). Our previous study showed that the pressure recovery–corrected peak Doppler gradient improves not only accuracy but also precision, despite a smaller sample size (SDs of mean difference: peak, 20.1 mm Hg; mean, 10.9 mm Hg; and pressure recovery–corrected peak, 7.1 mm Hg). Nonsimultaneous measurements of two tests during different physiologic states may in fact represent a more common “real world” scenario, but it is uncertain whether this provides a firm scientific basis for comparing the accuracy and precision of the pressure recovery correction.

What is the best echocardiographic window to obtain the Doppler gradient for congenital aortic valve stenosis? Does an eccentric jet affect pressure recovery? Where in the aorta is the pressure recovery completed? Heterogeneous valve morphology and an angled aorta may produce an eccentric jet. It is a common practice to use multiple windows (apical, right parasternal, and suprasternal notch) to identify the highest velocity, presumably reflecting the most coaxial Doppler window. Thaden et al . emphasized the importance of multiple imaging windows to determine the severity of aortic stenosis. In the study of Schlingmann et al ., only the apical window was used to obtain the Doppler gradients, because the previous study reported from the same institution (Boston Children’s Hospital) showed that the apical window had less magnitude of overestimation of gradient on the basis of the nonsimultaneous comparison of Doppler and catheterization gradients. However, this observation does not mean that the apical window is best for every patient. Future studies may be warranted to evaluate the pressure recovery correction using the highest Doppler gradients obtained from the multiple windows. Niederberger et al . studied the effect of an eccentric jet on pressure recovery in an in vitro pulsatile flow model. They found that the effect of pressure recovery was significantly less with increasing jet eccentricity (>30°). This is explained by the theory that an eccentric jet hits the aortic wall, losing reconvertable energy. VanAuker et al . assessed the effect of jet eccentricity on the computational demonstration of aortic stenosis. Their results suggested that the higher degree of jet eccentricity increased the distance to complete pressure recovery and the maximal pressure gradient and decreased the effective valve area. The presence of an eccentric jet likely complicates the use of pressure recovery and obscures its utility.

The location of ascending aortic measurement for the pressure recovery correction is another concern in relation to an eccentric jet, which delays and diminishes pressure recovery as described above. Schlingmann et al . calculated pressure recovery at four aortic locations and selected the level of the right pulmonary artery because it had the best agreement with the catheterization gradient. However, the optimal level of ascending aortic measurement may differ considerably in each patient, depending on the degree of jet eccentricity. Further study may be pursued to evaluate the effect of pressure recovery depending on the degree of jet eccentricity in congenital aortic stenosis.

The severity of aortic stenosis should not be determined without assessment of resultant cardiac remodeling. Even with severe aortic stenosis, aortic valve gradient can be low because of left ventricular systolic dysfunction. Left ventricular hypertrophy generally signifies a more chronic burden of aortic valve stenosis on the left ventricle. When left ventricular hypertrophy is out of proportion compared with the Doppler-derived gradient, further evaluation is usually warranted. Schlingmann et al . did not include data on left ventricular hypertrophy or systolic dysfunction. Their lowest reported peak-to-peak catheterization gradient was 0 mm Hg. Their Figure 2 shows that seven samples had peak-to-peak catheterization gradients < 10 mm Hg. This raises the concern that there may have been some patients in their cohort with significant left ventricular systolic dysfunction.

Aortic size is another of the major determinants of pressure recovery. In the in vitro pulsatile flow model, Niederberger et al . demonstrated that relevant pressure recovery can occur with an aortic diameter <30 mm. Schöbel et al . proved this at catheterization in 37 patients with aortic valve stenosis and concluded that pressure recovery can be clinically relevant in those patients with small ascending aortas. More recently, in a study of 1,563 patients with asymptomatic aortic stenosis, clinically important pressure recovery was more frequently seen in patients with a smaller aorta. Schlingmann et al . included patients with wide ranges of age (8.6 months to 24.6 years) and weight (8.7–119 kg) in their cohort. Although data on ascending aortic measurement are not shown, there should be a wide range of aortic size. The impact of pressure recovery may be lesser in young adult patients who have dilated ascending aortas in association with bicuspid aortic valves. Pressure recovery may be a more clinically significant phenomenon in the small aortas of young children.

The study by Schlingmann et al . adds to the discussion surrounding the comparison between echocardiography and catheterization gradient measurements in patients with aortic stenosis. Yet challenges in the clinical decision-making process remain to be investigated.