In all cows, the liver was clearly visible by ultrasonography between the right flank and the right 8th
ICS, as previously described [1
], except in one cow in which it was only visible from the 11th
ICS. None of the cows examined had any B-mode ultrasonographic abnormalities in the liver or adjacent structures. The portal vein and its portal branches were ventrolateral to the hepatic venous system and caudal vena cava, as previously described [1
Doppler measurements were made between the right flank and the 9th ICS. No further cranial Doppler signals could be obtained because of the absence of liver parenchyma, small vessel size, or the parenchyma being hidden by the lung.
The mean velocity can be obtained by different methods [17
]: directly with the ultrasound software, including the whole vessel diameter within the sample size (uniform insonation method); or by estimation, determining the maximum velocity (by one-off measurement or from a trace, as in our case) of the Doppler spectrum and multiplied by an arithmetic factor of 0.57 to correctly estimate mean velocity [17
]. We applied the average maximum velocity method to our measurements to identify whether turbulence was attributable to a disturbed flow or tissue noise (main problem with the uniform insonation method). Different authors have used a non-corrected average maximum velocity profile method, and a correction factor of 0.57 must thus be applied to the values reported [2
] for both Vmean and F values (which depend on the first factor). In the maximum velocity method, the Doppler spectrum is measured over a small region (referred to as “sample volume”) in the center of the vessel. However, as a result of respiratory movements, which slightly alter the position of the abdominal structures, the sample is not always in the center of the vessel. This can lead to artifactual turbulence due to tissue motion, as in the insonation method. However, in our experience, tissue motion does not greatly affect the Doppler spectrum obtained, as the vessel diameter is large enough to ensure that the sample remains within the vessel lumen most of the time.
As with venous flow, measurements must be adequate to depict the type of flow (laminar vs. turbulent) and phasicity, if it exists. For this purpose, some authors have used pulsatility indices (venous pulsatility index (VPI)) as described for humans [2
], under the assumption that the variations encountered in portal flow are cardiac in origin. As we found no significant cardiac influence but a respiratory influence in Doppler waveform, we considered that VPI was not appropriate for characterizing the portal vein Doppler spectrum. We obtained Vmax and Vrev in addition to F and Vmean to characterize the portal and hepatic blood flow, similarly to other authors [7
], and represent differences between laminar and turbulent flow (difference between Vmax, Vrev, and Vmean).
As age and weight did not influence any of the variables, all cows were included in a single group. The results are presented as arithmetic means, 95% confidence interval (CI), median, minimum, and maximum values. As measurements can vary depending on the access point, we considered those measures taken through the ICS with the highest number of valid observations obtained (most frequent ICS for each vessel) in order to establish reference values.
3.2. Portal System
In order to characterize the portal system, we considered the splenic and cranial mesenteric veins, which are the main vessels conforming the portal vein (not previously described used ultrasound). They have a characteristic typical identifiable inverted T-shape image (Figure 2
), which can be useful for identifying these vessels in the ultrasound examination. This image is obtained in an oblique transverse plane, where the splenic vein comes from the medial side of the abdomen joining the cranial mesenteric vein that travels with a caudolateral to craniomedial direction, to conform a short thick main portal vein in an almost perpendicular orientation from the aforementioned two vessels.
The cranial mesenteric (Table 1
) and splenic veins (Table 2
) were mainly visualized in the 12th
ICS. Some difficulties may be encountered when trying to align the cranial mesenteric vein appropriately for Doppler examination between the ribs, because of its parallel position in relation to the abdominal wall. The transducer must therefore be angled to obtain adequate insonation angles (<60°). Although the splenic vein always has a suitable angle of insonation, its deep location can sometimes decrease the Doppler signal. These vessels have a typical laminar hepatopetal flow that gradually turns into a turbulent flow when they fuse to form the main portal vein, where the turbulence is maximal (Figure 3
Considering the values obtained at the most frequent ICS for each vessel, the splenic vein (12th ICS) had higher values of Vmean and Vmax than and similar F and Vrev values to the cranial mesenteric vein (11th ICS), indicating a comparable contribution to the portal blood flow.
The portal vein and its portal branches could not be observed in the flank ( Table 3
; Table 4
). The main portal vein can be explored longitudinally in a craniocaudal orientation mostly in the 11th
ICS (12/16). As already mentioned, it is sometimes difficult to align the ultrasound probe within the intercostal space for Doppler examination due to a small window for ultrasound recording between the ribs. The portal vein rapidly ramifies in many portal branches of different size that travel in multiple directions, observed cranially to the 12th
ICS until the 9th
ICS. The right and caudal lobe branches are multiple, variable, and small, emerging directly from the main portal vein or from a single/multiple right trunk and, as lobe separation is not evident in the bovine liver, these branches cannot be confidently labelled by ultrasound examination. This finding is consistent with descriptions in the anatomical literature [18
] and is referred to occasionally as “stellate branches” [1
] (Figure 4
A). The largest branch that can be identified travelling deep ventromedially corresponds to the proximal part of the left portal vein or pars transversa
] (Figure 4
Regarding blood flow (Table 3
and Table 4
), turbulence and F are higher in the main portal vein (characterized by high peak Vmax and high Vrev values) and gradually decrease in the portal branches towards the periphery as the vessels taper, reaching their lowest velocity at the 9th
ICS. The results indicate an increase in velocity and flow from the afferent vessels, the splenic and cranial mesenteric veins, to the confluent main portal vein, which distributes blood throughout the liver parenchyma with a consequently gradually slower speed and flow as the portal veins decrease in size. Similarly, turbulence varies from minimum in the splenic and cranial mesenteric veins to maximum in the main portal vein, and then decreases gradually.
Our findings are consistent with those of an invasive experiment [5
], in which it was found that the turbulent nature of the portal vein flow was due to a high angle of insertion between splenic and cranial mesenteric veins. We hypothesized that turbulence in the main portal vein will not be a consequence only of the opposite orientation of the splenic and mesenteric veins, but also of a large increase in the diameter of the vein from these to the main portal vein that also generates turbulence in longitudinal vessels, not only in ramifications [19
Although the portal vein Doppler spectrum is characterized by a marked turbulent hepatopetal flow, some phasicity can be observed and correlated with respiratory movements that consequently change abdominal pressure and portal flow (Figure 5
), as seen in dogs [17
] and also in cows after invasive examination [5
] considered the variations cardiac in origin and, as in this study, no ECG correlation was obtained, as the Doppler spectra could not be directly correlated with cardiac cycle. Only 3 of the 16 cows presented reverse flow in systemic veins (Table 5
and Table 6
), showing that pulsatility due to cardiac cycle is much less likely to cause variation in the portal vein blood flow. We believe that if these variations were cardiac in origin, phasicity in the hepatic veins would be expected with synchronous cardiac influence as in humans or dogs; however, this was not observed. Therefore, we should not refer to “pulsatility” of the portal vein, as the flow is not pulsatile, and the variations must be caused by changes in abdominal pressure (i.e., respiratory cycle) and in the amount of blood emitted by the gastrointestinal tract and spleen. Furthermore, based on studies in humans [22
], sheep [23
], and cattle [2
], the portal blood flow depends on posture, physical activity, feed intake, age, and milk yield, which also influence the size and thickness of the liver as body weight does. The correlation between stage of pregnancy and diameter of the portal vein may also influence the portal blood flow [25
]. In this study, as all cows were given the same diet and were neither pregnant nor lactating at the time of investigation, these factors were not investigated and must be considered in future research. Body weight and age did not significantly influence the results, which could therefore be taken as reference values to evaluate other factors such as diet, pregnancy, and milk yield.
3.3. Caudal Vena Cava and Hepatic Veins
Hepatic veins and caudal vena cava were best visualized between the flank and the 11th
ICS, but could be examined until the 9th
ICS for smaller and distal hepatic branches. The best approach to the caudal vena cava (Table 5
) was obtained from the right flank, where the ultrasound probe could be best aligned (craniocaudal direction), although it was achievable from the 12th
(8/16) and the 11th
The right and middle hepatic veins and their branches are anatomically variable [18
] but consistently visualized with an easy alignment for Doppler investigation because of their lateromedial orientation, which allows insonation angles close to 0° in many instances (Figure 6
Hepatic veins are mainly found between the 12th
(11/16) and the 10th
ICS (12/16), but are also found between the flank (4/16) and the 9th
ICS (8/16) in some individuals (Table 6
The blood flow between the caudal vena cava and its hepatic veins had a laminar Doppler profile without reverse flow (Figure 6
), with variations due to respiratory changes in abdominal pressure without cardiac phasicity as in the portal system, except for some measurements in three cows (Table 5
and Table 6
). Vrev was inconsistently detected in three animals. Reverse flow was inconsistently found and we hypothesized that it could be caused by excessive abdominal pressure due to respiration effort. These three animals had normal levels of the other individual portal system Doppler values, indicating that cardiac influence is not transmitted to the portal system by the hepatic sinusoids. In healthy dogs and humans, reverse flow in the caudal vena cava is correlated with atrial contraction during atrial systole [10
]. We did not detect any cardiac pathology in these individuals by auscultation or basic echocardiography. ECG correlation with the Doppler spectrum could explain the cardiac influence on the caudal vena cava and hepatic veins in the three cows in which Vrev was detected.
The main difficulty in obtaining Doppler measurements in the caudal vena cava was the caudocranial orientation of this vessel. Fortunately, a flank approach provided easy access with acceptable Doppler angles. However, a high variability in the diameter of the caudal vena cava diameter was observed because of its triangular shape within the liver in cross-section. In order to calculate F, the vessel diameter is measured in the sagittal plane (to ensure vessel orientation and Doppler angle), where it cannot be confirmed if this measurement correctly estimates vessel area, as the diameter is transformed into area assuming that it is basically round. We recommend that an additional transverse image be obtained in future studies to measure the caudal vena cava with a trace tool in cross-section. Although this would extend the time of ultrasound examination, more accurate results would be obtained. As this method may also overestimate the vessel area due to an oblique angle of approach (form round to oval), care must be taken according to the method applied.
As in portal branches, all parameters in hepatic veins depend on vessel diameter and distance from the caudal vena cava, related to peripheral tapering of these vessels. An anecdotally similar Vmean between the intrahepatic portal and hepatic vessels can be observed, reflecting some correspondence in liver tension/liver sinusoids.