Liver with normal stiffness in the supine position stiffens in the left decubitus position, whereas stiff livers harden or soften.
When 2dSWE was measured for both supine and left decubitus positions, the values revealed a significant positive correlation as shown in Fig. 1a (p < 0.0001, r = 0.68). Because 12 values of 2dSWE in each liver were dispersed on a case-by-case basis, it is reasonable to assume that SWE is substantially affected by changing body positions only when the difference between SpSWE and LdSWE (∆2dSWE, SpSWE - LdSWE) is greater than the dispersion of SpSWE. Among 298 cases, LdSWE increased or decreased from SpSWE over the magnitude of CVR in 81 cases (27.2%). These 81 cases can be classified into four groups based on SpSWE normality and positive/negative ∆2dSWE values. For 37 cases in which SpSWE was slower than the upper normal limit of 1.41 m/sec (see Methods), ∆2dSWE was negative as shown in Fig. 1b in all the cases (NH). On the other hand, in 44 cases with stiff livers in the supine position, ∆2dSWE was negative (HH) or positive (HS) in 27 and 17 cases, respectively. To assess the possibility that ∆2dSWE is determined by cardiac function, the cardio-thorax ratio was compared between cases with negative and positive ∆2dSWE. As shown in Fig. 1c, the cardio-thorax ratio was not significantly different between the two groups (p = 0.51).
IVC shrinks in the left decubitus position as the liver hardens but not as the liver softens.
Next, the effects of the body positions on IVC diameter were evaluated irrespective of whether the ∆2dSWE scale was beyond or within CVR. In the results, the diameter of IVC in the left decubitus position was significantly reduced compared with the supine position in the cases with SpSWE slower than 1.41 m/sec as shown in the left panel of Fig. 2a (p = 0.013). Consistently, the diameter also shortens in cases with SpSWE of 1.41 m/sec or faster and negative ∆2dSWE values (Fig. 2a middle panel, p = 0.0070). On the other hand, IVC diameters in the supine and left decubitus positions were not significantly different in cases with SpSWE of 1.41 m/sec or faster and positive ∆2dSWE (Fig. 2a right panel, p = 0.32).
Liver stiffness is tightly associated with body postural change in cases in which a stiff liver softens in the left decubitus position, especially in the right lobe.
To understand the implications of the pressure connection between the liver and IVC, the correlation between SpSWE and ∆2dSWE was evaluated. As shown in Fig. 2b, a significant correlation was not observed in cases with SpSWE slower than 1.41 m/sec (p = 0.56) or a SpSWE value of 1.41 m/sec or faster and negative ∆2dSWE (p = 0.88). In contrast, SpSWE and ∆2dSWE revealed a significant positive correlation in cases with SpSWE of 1.41 m/sec or faster and positive ∆2dSWE (p < 0.0001, r = 0.38). When the same relation was separately evaluated in the right or left lobe, as shown in Fig. 2c the correlation was clearly tighter in the right lobe (p < 0.0001, r = 0.48) compared with the left lobe (p < 0.0001, r = 0.31).
Gravity unevenly impacts liver architecture between the right and left lobes.
The paradoxical increment/shrinkage of LdSWE/IVC on the left decubitus position indicates that pressure thresholds exist between the hepatic veins and IVC, where outflow blocks would be built under architectural deformation of the liver during postural changes. Given that postural change may not evenly impact liver architecture, ∆2dSWE was separately evaluated in right and left lobes. As shown in Fig. 3, large differences in ∆2dSWE are noted between the right and left lobes in both cases with positive or negative ∆2dSWE in the entire liver. When ∆2dSWE is positive or negative in the entire liver, ∆2dSWE in a single lobe is reciprocally negative or positive, respectively, suggesting that the impact of postural change on liver architecture would be detected much easier in a single lobe compared with the entire liver.
Softening of the stiff liver on left decubitus position suggests fibrous progression of the liver.
To infer the relationship between pathological differences of the liver and ∆2dSWE, FIB4 and its constituents, including platelet count, age, and alanine aminotransferase, were compared among NH, HH, and HS cases. As shown in Fig. 4, FIB4 and platelet counts are significantly increased and decreased, respectively, in HS against NH, especially when HH and HS were not evaluated in the entire liver but in a single lobe of the right or left (judged in the entire liver, right lobe, left lobe; (FIB4) p = 0.04, p = 0.006, p = 0.01; (platelet counts) p = 0.29, p = 0.05, p = 0.05, respectively). In terms of age and alanine aminotransferase, no significant differences are noted between NH and HS even when HH and HS were judged in each lobe (Fig. 5). In addition, no significant differences in factors are noted between NH and HH.