Our investigation focused on the assessment of the IVCc (and of its diameters) collected at two different sites (sagittal SC, and coronal TH) with different measuring modality (M-Mode vs AI). Our study resembles a 2x2 factorial design, intertwining evaluations of the IVC from different technical and anatomical perspectives. Therefore, the discussion of our findings is divided in the one on the clinical role of AI in IVC assessment, and then in the differences between imaging in sagittal or coronal approach.
Looking at the role of the automated border detection (AI) for the IVC assessment, which is the more original and innovative point of the study, our results suggest that the introduction of AI may be great help for clinicians. Indeed, considering the accuracy of this method, AI has the potentialities of saving time for bedside assessment of FR; moreover, it could allow a greater number of calculations of IVCc (or IVCd, according to the ventilation mode), which could be averaged especially in case of borderline results. From practical perspectives, when performing the IVCc calculation in standard M-mode, the operator needs to freeze the image, to calculate the diameters and then to apply the formula. Conversely, with the help of automated border detection the operator can hold his/her hand on the probe and watch the screen whilst the machine keeps calculating values of IVCc (or IVCd). In this regard, we found very good accuracy for the AI calculation as compared to the standard M-mode approach, with a bias of -0.7% (good accuracy) for the SC view and of 3.7% for the TH imaging. However, it must be noted that in both cases the LOA were relatively wide, indicating low precision. Nonetheless, as shown by the violet dotted lines in the Bland-Altman plots (Figs. 1 and 2), in both cases (SC and TH) there was a clear trend in such dispersion, which was greater for the higher IVCc values (over ~ 30% for the SC, and over ~ 20% for the TH). This finding derives probably from the challenge in estimating the IVC when it is almost fully collapsible, with the IVCmin becoming lower than 0.5 mm. In such cases, the evaluation in M-mode using the touch screen (as for the machine used for our study) may be prone to small errors, and the LOA (precision) could be narrower when approaching mechanically ventilated patients with distended IVC. As mentioned, machine learning generated models have been developed for the prediction of FR[33], with encouraging (comparable or superior) results when compared to the hemodynamic response to passive leg raising. Blaivas et al. conducted pioneer studies on this topic using deep learning algorithm capable of video classification for the estimation of FR with the IVC imaging; the authors showed that the trained deep learning algorithm performed moderately well (area under the curve 0.70; 95% CI: 0.43-1.00)[34]. Moreover, the same group of authors showed that performances of this validated deep learning algorithm were dependent on the image quality (much worse on images from a lower quality device)[39]. In summary, the findings of our study on the value of introducing AI for the calculation of the IVC indexes of FR seem encouraging, and may be supportive of its introduction in clinical practice. However, it is of utmost importance to validate these findings in populations of mechanically ventilated patients where theoretically the results could be even more interchangeable due to the greater vessel size with potentially larger agreement between AI and standard M-mode measurements.
Regarding the second part of the study, results of IVCc obtained in SC were compared with those recorded in TH approach. As suggested by a recent systematic review that included seven studies in different cohorts of volunteers or patients (spontaneously breathing, mechanically ventilated, hybrid)[23], the results of SC and TH imaging for IVCc may be significantly different and not truly interchangeable. Our study points in the same direction as we found a mean bias of ~ 14% for the M-mode assessment of the IVCc taken at the two different anatomical sites; moreover, we found significant dispersion with a very large LOA range of ~ 64%. In such case, there was not a clear trend, indicating that dispersion and differences is constant in all ranges of IVCc estimates. In concordance with the above-mentioned systematic review[23], the present results suggest that IVCc obtained in M-mode from SC view overestimates the M-mode TH finding. Therefore, it seems that the IVC collapses more in antero-posterior direction rather than in the latero-lateral one. Surprisingly, the use of AI in comparing results fof SC and TH imaging showed with borderline bias (~ 8%), but still with a low precision (LOA range ~ 54%). However, these results should not discourage research for the introduction of cut-offs for the prediction of FR with the IVC imaged in coronal view (TH); conversely these findings should foster investigations looking at the best cut-off in predicting FR when using TH-derived parameters. Importantly, the use of TH IVCc can be very valuable in patients where the imaging in the SC region is not achievable for clinical reasons as obesity, or for the presence of laparotomy wound or mediastinal drains. It must be kept in mind that the TH imaging is not always feasible and in our population of healthy volunteers, five out of 60 (8.3%) did not have a TH view (and two of them did not have the SC either).
Strengths and Limitations
The main strength of our study was the originality in investigating with AI the differences in SC and TH imaging. Also, our study was conducted in a homogeneous population of healthy and young volunteers. Moreover, considering the mean results for IVCc index in SC and TH imaging (33.3%±12.6 and 19.7%±11.5, respectively) a post hoc sample size of 26 volunteers would have been appropriate, therefore suggesting that the study was appropriately powered.
Our study has several limitations. First, although the sample size was bigger than previous studies[35, 36] and considering the results our study seems well-powered, it was conducted in healthy and relatively young volunteers; therefore, its results could be different in spontaneously breathing patients that could be older than our population and are likely to present associate comorbidities. Second, a single experienced operator collected the images and performed the M-mode calculations, and results may be different in less experienced hands. Third, we did not assess the inter-observer variability. Fourth, the image acquisition followed a schematic pattern to avoid mistakes but an ideal study design would have provided randomization for the order of image acquisition. Nonetheless, we believe this is unlikely to influence results but it remains fair to acknowledge such item.