The major findings of the present study were: 1) contrast TTE showed large RL shunt more frequently than contrast TEE, and 2) large RL shunt evaluated by contrast TTE had greater accuracy for the association with CS compared to that evaluated by contrast TEE. To the best of our knowledge, this is the first study to show the efficacy of contrast TTE for evaluating large RL shunt associated with CS.
Since randomized trials, such as the RESPECT, REDUCE, and CLOSE trials, have demonstrated the benefits of transcatheter closure for the reduction of stroke [7–9], transcatheter closure of PFO is expected to increase as a therapeutic option. PFO is common, but not all PFOs are the same. It is necessary to diagnose high-risk PFO, which is more likely to be linked to CS. Larger RL shunt carries a greater potential for transseptal passage of thrombus. Large RL shunt is a particularly important risk factor for an increased likelihood of CS [14–16]. Furthermore, the RESPECT and REDUCE trials revealed that the effect of transcatheter PFO closure for preventing CS was increased in patients with large RL shunt [7, 8]. The CLOSE trial included patients with large RL shunt or atrial septal aneurysm, and demonstrated that transcatheter closure was superior, with a lower rate of stroke (in fact, no stroke at all), compared to medical therapy . With the advent of transcatheter closure of PFO, patient selection has become important. Large RL shunt is useful to identify patients who obtain greater benefit from transcatheter closure. Therefore, accurate assessment of large RL shunt is essential.
Contrast TEE is used for the assessment of RL shunt. The amount of RL shunt evaluated by contrast TEE has reportedly been associated with CS [14, 15], however several studies have shown that the distribution of RL shunt is not linked to CS [11, 17, 18]. These conflicting results could be explained by differences in the performance of the Valsalva maneuver. The Valsalva maneuver increases intrathoracic pressure and decreases venous return and left atrial pressure during the strain phase. Rebound venous return after the release increases right atrial pressure and the gradient between right and left atrial pressures, leading to opening of the flap-like foramen ovale and facilitating RL shunt. Thus, an adequate Valsalva maneuver is essential to assess the severity of RL shunt in patients with PFO.
Contrast TEE has technical limitations in the accurate assessment of RL shunt. The Valsalva maneuver is insufficient due to probe insertion. Under sedation, the Valsalva maneuver itself is difficult to perform. The fasting state causes a lower right atrial pressure, leading to a reduction in the right and left atrial pressure gradient with the Valsalva maneuver. Thus, contrast TEE could result in an underestimation of the severity of RL shunt . Whereas, contrast TTE has increasingly been utilized for the detection of PFO. The diagnosis of PFO has greatly improved with high sensitivity and specificity [13, 20]. Contrast TTE has an advantage in that patients are able to adequately perform the Valsalva maneuver. On the basis of these findings, contrast TTE can be reliable for evaluating RL shunt. Additionally, TTE is simple to use, easily available, and low cost [21–23]. In clinical practice, contrast TTE could be used to assess RL shunt with an increased risk of CS and to select patients for transcatheter closure.
The present study had some limitations. First, the number of patients was small. Second, the severity of RL shunt depended on the degree of Valsalva maneuver. However, the assessment of RL shunt was uniform because contrast TTE and TEE with the Valsalva maneuver were performed at a single institution. Finally, the efficacy of large RL shunt of contrast TTE was not assessed in an independent population. The accuracy in this study represented a best-case scenario.
In conclusion, contrast TTE can identify large RL shunt more frequently than contrast TEE. Large RL shunt evaluated by contrast TTE provides greater accuracy for the association with CS. Contrast TTE can be valuable for evaluating large RL shunt associated with CS in patients with PFO, and may be effective for stratifying patients who should undergo transcatheter PFO closure.