Imaging is essential to provide the diagnosis of clinically unsuspected acute arterial complications during the postoperative period of liver transplantation. Routine Doppler US is the established imaging method of choice to monitor the liver graft during the first days after transplantation in children and adults [23, 24]. Arterial PSV and RI have been proven to be effective detecting arterial complications [3]. Hemodynamic differences between children and adults are reflected by the Doppler parameters. However, the majority of findings and reference values of Doppler US in liver transplantation are based on US parameters established for adults. Most acute arterial complications occur at the anastomosis. Nevertheless, the evaluation of velocities at the anastomotic site or proximal to the anastomosis during the postoperative period is technically difficult to achieve because of their deep location, small caliber, discontinuous flow and presence of anatomical reconstruction variants, especially in children. Post anastomotic Doppler parameters are indirect indicators of anastomosis [4, 5] and easier to depict. Some studies evaluated RI and systolic acceleration parameters at the anastomosis [25–27], while others did it on the post anastomotic main artery or the intrahepatic arteries [23, 28, 29]. Several studies detected that critical stenosis and imminent thrombosis have to be carefully evaluated when hepatic artery velocities are less than 50cm/s in both children and adults [3, 5, 18]. However, there are a limited number of investigations performed only in children regarding reference values of arterial Doppler US parameters during the acute postoperative period. Moreover, there are some controversies about the abnormal values of PSV measured at or distal to the anastomosis ranging from 30 to 50 cm/s. [30–32].
As the assessment of the Doppler parameters at the anastomotic site is especially difficult in children and infants, we evaluated arterial Doppler parameters distal to the anastomosis, near the hepatic hilum. The hepatic hilum is almost always clearly depicted on pediatric US, and measurements at this location become easily reproducible. Therefore, in order to diagnose acute arterial complications as easy and early as possible, we aimed for evaluate the utility of RI and PSV of the hepatic artery distal to the anastomosis and determine a cut-off value for PSV that may urge further imaging investigations in children. We noticed that the arterial PSV less than 50cm/s at hepatic hilum distal to the anastomosis at 3 and 4 days after transplantation correlated with normal findings at CT, suggesting that false-positive rate tended to occur. We found that the median velocity at 3 days after surgery was 46 cm/s among the non-complications group, whereas it was 26cm/s in patients with acute arterial stenosis and 19cm/s in patients with thrombosis. At 4 days after surgery, the median velocity among the non-complications group was 53 cm/s; and in patients with acute arterial complications was 31cm/s in stenosis and 13cm/s in thrombosis. Variation between the three groups was statistically significant (p < .025). At ROC analysis, we found that the optimal PSV threshold value to discriminate pediatric acute hepatic arterial complications was less than 30 cm/s. These velocities are congruent with the study of Someda et al in children [33] and slightly lower than those detected by Park et al. in 374 adults where 48cm/s was the optimal cut-off value [5]. None of the patients with acute arterial thrombosis or stenosis had abnormal liver function, so we succeeded in detecting them before hypoperfusion had any clinical manifestation. In our pediatric cohort, normal RI values and normal wave morphology showed statistically significant association with normal graft status. However, unlike other studies such as Ahmad et al [3], RI nor parvus tardus pattern did not attempt to predict acute arterial complications during the first 5 days of the postoperative period. Our study did not confirm that RI less than 0.5 was statistically associated with acute arterial complications during the first days after surgery. However, our Doppler analysis showed that low PSV distal to the anastomosis the first 5 days after liver transplantation, preceded a parvus tardus waveform in some cases. This is congruent with the study by Choi et al [12] where Systolic Acceleration Time (SAT) and parvus-tardus wave morphology were reliable predictors of hepatic artery stenosis whereas RI was not and with the study of Dodd et al [4] where the combination of SAT, peak velocity, RI and no flow had a 97% sensitivity and 64% specificity for the diagnosis of arterial complications. An optimal cut-off PSV of less than 30 cm/s at 3 days after surgery had 81.6% sensitivity and 86.8% specificity for detecting acute arterial complications, with a false positive rate of 13.2%. Similar values were detected at 4 and 5 days after surgery. Moreover, from these preliminary results, we observed that PSV less than 50cm/s in children could also be found during the first days after transplantation in normal grafts. A cut-off PSV of less than 50 cm/sec at 3 days after surgery had 100% sensitivity and 34.2% specificity for assessing acute arterial complications, with a false positive rate of 65.8%. Specificity tended to increase progressively at 4 and 5 days after surgery obtaining a slightly lower false positive rate. Therefore, in order to avoid unnecessary radiation exposure and invasive examinations, further imaging with CT or angiography is recommended in children with PSV values between 30-50cm/s only along with constellation of other US, laboratory or clinical findings. In order to detect maintained lower velocities, which may indicate hypoperfusion, or progressively reduced velocities indicating progressive thrombosis, we propose a strict Doppler US evaluation each 12 hours for PSV values ranging from 30-50cm/s with no other associated anomalies. In addition, and specially in patients in whom may urge an invasive procedure, it is important to point out that clinical assessment of transplant patients must be include into the decision-making process during the whole postoperative period.
Two main limitations associated with our study deserve further explanation, the retrospective design and the small sample size. During the 5 years that US data were recorded, different radiologists performed the US studies. Doppler US tends to be dependent of the operator's experience and skills. However, all parameters were collected using a strict protocol that was already established and included measurements of the PSV and RI distal to the anastomosis. The other limitation resides in the sample size. Although the number of pediatric patients with liver transplantation is considered substantial, the analysis of differences in age, disease prevalence and liver graft types are limited from a statistical point of view. In addition, several confounders may determine graft status as a complex multifactorial condition and several factors may explain low velocities. Although we did not find any statistical relation, enteral feeding, severe allograft edema, portal venous hyperperfusion in reduced-size grafts, dehydration and intravascular volume depletion have been described as possible contributing factors [10]. However, the diversity of the study population can be defended as strength of the study measuring velocities rather than a limitation, and the described values are of clinical utility in guiding the complete radiological interpretation.
To our knowledge, there is no prior consensus defining an optimal cut-off value of the PSV, neither a monitoring protocol during the postoperative period in pediatric liver transplantation. Our results argue in favor of close monitoring the hepatic artery with routine Doppler US daily at least the first 5 days after surgery. Other authors also suggested serial Doppler US for the detection of clinically unsuspected vascular complications, with different time intervals [9, 23, 34, 35]. Further studies would help clarify the long-term outcome of pediatric liver recipients with PSV less than 50cm/s without evidence of arterial complications.