TTP represents the time until the maximal concentration of contrast within the region of interest and compromised blood flow due to stenosis or true-lumen collapse is likely to lead to prolonged TTP values. In this study we aimed to investigate TTP as a suitable parameter of technical success for the improvement of hemodynamics in the compromised vessel lumen after dissection membrane fenestration. This hypothesis is supported by the results of this study as treatment led to significant dTTP values.
The hemodynamic features prevailing in aortic dissections are difficult to study and remain only partially understood, to date [12]. Unlike other endovascular therapy approaches, for example, in the treatment of peripheral arteries, the diagnostic usability of monochromatic DSA series must be questioned when assessing the technical success of aortic dissection flap fenestration [13]. Intraluminal manometry, on the other hand, is an intraprocedural technique that is favorably reported in the setting of aortic dissection flap fenestration as a potential solution for this dilemma. Nevertheless, based on our experiences, we doubt the reliability of this tool when assessing the severity of malperfusion syndromes in aortic dissections or the success of the chosen treatment, respectively. For example, even if preexisting imaging, laboratory studies and clinical presentation clearly indicated a significant true lumen collapse and compromise of perfusion to aortic branch vessels, manometry revealed no relevant transluminal pressure gradients in some cases. On the other hand, in some cases of technically feasible balloon fenestrations an improvement of pressure gradients or even worsening of those parameters was not documented. This leads us to the presumption that measurement of transluminal blood pressure gradients may be reliable only to a limited extent. The observation that manometry results and hemodynamic parameters obtained from color-coded images only showed a moderate correlation might further emphasize this limitation.
Interestingly, transluminal blood pressure gradients were differently influenced by the treatment in patients with Stanford type A and B dissections. While significant differences were documented in patients with type B dissection, patients with type A dissections presented with differences below the level of significance. An explanation for this result might be that all patients with type A dissection had undergone surgical repair treatment of the thoracic aorta prior to fenestration. Surgical repair of type A dissection seals the proximal entry of the aortic dissection, which is likely to result in alterations of hemodynamics prevailing in the distal aortic lumina. In this situation, the patency of the false lumen as well as the persisting extent of transluminal blood pressure gradient may highly depend on further existing entries and re-entries. Based on our results, we hypothesize that the parameters gained from color-coded DSA, including TTP, offer more stable and representative results in the evaluation of aortic lumen malperfusion, especially in type B dissections. All in all, both methods, blood pressure measurements as well as color-coded DSA, have weaknesses and it is unclear whether manometry is the correct gold standard at all.
As shown in another study, the presented algorithm may serve as a periprocedural success control and might also influence the treatment strategy [11]. For example, the optimal position for dissection flap fenestration remains a matter of debate. The creation of the fenestration tear at the level of the compromised vessel has been performed with remarkable success [6]. In contrast, fenestration in the proximal aorta seems to mimic an additional entry tear and may intensify the true lumen collapse. Beside the postinterventional success control, color-coded summation images might help to define the proper location and the required number of fenestrations more precisely.
To date, the utilization of quantitative color-coded DSA images has only been reported in a limited number of studies, predominantly in the field of neuroendovascular and peripheral artery procedures [8, 9, 14, 15]. Only two other studies have addressed the application of color-coding DSA in aortic dissection so far. Tinelli et al. [16] utilized parametric imaging for the assessment of chronic thoracic aortic dissections during thoracic endovascular aortic repair (TEVAR). The authors reported increased angiographic accuracy with sufficient identification of the true and false lumen in 72.7%, and identification of the entry tear in almost half of the cases. This might be particularly beneficial in the emergency setting with insufficient preoperative diagnostic management and may contribute to an optimal procedural management. Fang et al. [17] investigated the application of color-coded DSA to analyze hemodynamic changes in renal artery perfusion before and after thoracic endovascular aortic repair in type B aortic dissections. Time-intensity curve parameters, including average peak ratio, average delayed time to peak, and average area under the curve ratio, changed significantly after the procedure, suggesting the method to be beneficial in the intraprocedural evaluation of renal blood flow.
Preinterventional cross-sectional studies should be available in most cases and support the investigation of hemodynamic and morphologic features in aortic dissections. Beside conventional CTA and MRA, more novel imaging techniques, such as four-dimensional flow MRI offer more precise characterization of blood flow while also enabling the color-coding of flow velocity or other parameters [18,19,20]. Nevertheless, the clinical urgency of malperfusion syndromes might impede the acquisition of current imaging studies prior to the procedure, especially when being time consuming. These techniques might thus be more practicable in non-symptomatic intervals or in the setting of follow-up examinations.
Color-coded imaging gained from DSA series is an implemented algorithm, neither necessitating additional amounts of contrast media nor the use of further radiation exposure. Its application is fast and simple and thus might be feasible to be employed intraprocedurally, even within the angiography suite [21]. Therefore, the implementation of this technique in a prospective study setting would be possible. When applying color-coding to DSA, different aspects and method restrictions should be considered: Since the selection of ROIs for TTP calculation is conducted manually, ROI placement might vary among different readers, resulting in a limited interrater reliability [22]. Nonetheless, this effect might be minor as typically the same user within one procedure performs the ROI placement. Also, relevant influence of the color gradient might result from different catheter placements and the location of pixel labelling in relation to the catheter. Moreover, the amount and concentration of contrast medium as well as the imaging frame rate should be the same with regard to all DSA series. Finally, the extent of true lumen compromise might fluctuate due to dynamic flap obstruction and be furthermore influenced by different parameters, such as the aortic diameter, patients´ heart rate and blood pressure.
The presented study underlies different limitations. First, the study was conducted in a retrospective design and the number of included patients is limited. Therefore, it must be recognized that although this study showed significant TTP changes before and after fenestration, the relationship between TTP and actual hemodynamic parameters is still unclear. Second, intraprocedural manometric measurements were introduced after the start of the study, and corresponding pressure gradients were available in 19 of 25 interventions. Third, catheter placement for manometry was not necessarily the same as placement of ROIs for hemodynamic analysis, which possibly might have influenced the measurement results.
In conclusion, hemodynamic parameters extracted from color-coded DSA images may help to evaluate the procedures´ technical success in percutaneous fenestration of dissection flaps. Color-coded DSA images may offer a stable diagnostic tool, especially in cases of technically unfeasible or inconclusive manometry. However, the correlation with results from transluminal blood pressure measurements is limited. It is important to note, that due to the underlying technique, changes in cardiac output or blood pressure as well as the injection rate of contrast administration may lead to deviations.