Segment 1 is difficult to access, lying between the IVC and the left portal vein; it receives inflow predominantly from the left PV and HA (see Chapter 22). The biliary drainage of segment 1 is variable, and the segment 1 bile duct runs in the hilar plate and enters the posterior aspect of either left or right hepatic duct. Posteriorly, segment 1 rests on the IVC and most of its outflow is directly into the IVC via a series of small veins. Tumors arising in the caudate lobe are closely related to the posterior aspect of the left hepatic vein (LHV) and the middle hepatic vein (MHV).

The anatomy and technical aspects of resection of the caudate lobe are addressed in detail in Chapter 22.

Isolated resection of segments 2 or 3 are possible, although given their frequent small size, they are often resected in combination as a left lateral sectionectomy. However, as more aggressive resections are pursued, particularly in patients with hepatic colorectal metastases, with bilobar lesions in chemotherapy-damaged livers, parenchymal sparing approaches are critical. When either of these segmentectomies is planned, the plane of resection is by either side of the LHV (left and posterior for segment 2 resection, and right and anterior for segment 3). The LHV can be sacrificed during isolated resection of segments 2 or 3, if necessary, provided the fissural or segment 4 vein is preserved; this branch , also referred to as the umbilical vein in Couinaud’s original description, runs in the liver parenchyma in a line delineated by the falciform ligament and is capable of draining the residual liver in the left lateral section. The inflow is approached on the left side of the falciform ligament, by incising the peritoneal reflection onto the liver and identifying the segment 2 (Fig. 24.5) or 3 portal pedicle (Fig. 24.6). Once the inflow is divided, the line of parenchymal transection becomes prominent between the ischemic segment to be resected and the normally perfused residual segment.

The left lateral section is mobilized by division of the falciform ligament and the left triangular ligament (see Chapter 20). The extrahepatic portion of the LHV can be lengthened by dividing the fibrous tissue to the left of the IVC down to the level of the fissure for the ligamentum venosum. The ligamentum venosum is then ligated and divided as it enters the posterior aspect of the LHV allowing encircling of this vein if the point of entry into the MHV is extrahepatic. Dissection of the LHV outside the liver is generally recommended when tumor is in close proximity to its confluence with the IVC; otherwise, the isolation of the LHV is delayed until completion of the parenchymal transection. The inflow to segments 2 and 3 is usually isolated during transection of the parenchyma to the left of the falciform ligament, leaving the portal vein intact to supply segments 4a and 4b. Occasionally, pedicles to segment 2 and 3 can be identified and encircled outside the liver in the left side of the umbilical fissure; in some patients, these pedicles arise from a common trunk. The plane of parenchymal transection follows the falciform ligament. As the liver is divided, the portal structures to segment 2 and 3 will become apparent and can be electively ligated and divided. Once this is done and the parenchyma surrounding the LHV has been dissected, the vein can also be ligated and divided. The transection is completed by dividing the parenchyma anterior to the caudate lobe, in the fissure for the ligamentum venosum.

Segment 4, like the left lateral section, is frequently small in size and is not commonly resected in isolation, but rather as part of a larger procedure, such as central hepatectomy (see below and also Chapter 21) or segmentectomy 4b/5 for gallbladder carcinoma (see below and also Chapter 27). Segment 4a is very rarely resected alone, unlike segment 4b, given its peripheral location and easy access; although a segment 4a/8 resection for a high central tumor may be preferred over a central hepatectomy. Segment 4 is bordered posteriorly by the IVC, by the falciform ligament and umbilical vein on the left, and by the MHV on the right. Inferiorly, the left portal pedicle runs along the base of segment 4, just below the hilar plate (see Figs. 17.5A and 17.6). Inflow and biliary drainage are provided by branches of the main left portal pedicle within the umbilical fissure, directly opposite the pedicle branches to the left lateral section. Venous drainage of segment 4 is predominantly through the MHV, with a contribution from the umbilical vein.

After mobilization of the left liver and division of the falciform (see Chapter 20), the confluence of the LHV and the MHV with the IVC is identified (the left-middle groove identifies the apex of segment 4a), and the inflow pedicles are now approached. By incising the peritoneal reflection on the right side of the umbilical fissure, the segment 4a and 4b pedicles can be identified, encircled, and divided (see Fig. 27.9). Alternatively, these pedicles may be identified within the substance of the liver, just to the right of the falciform, and divided in the course of parenchymal transection (Fig. 24.7) (see also Figs. 19.10 and 21.8, 21.9 and 21.10). The parenchymal demarcation should be seen along the principal resection plane or Cantlie’s line on the right and the divided falciform ligament on the left, and transection of the parenchyma should proceed along these lines. As the resection proceeds toward the base of segment 4, care must be taken to avoid injury to the left portal pedicle, and particularly the left hepatic duct, which lies just below the hilar plate, anterior and cephalad to the left portal vein. As the transection continues superiorly to the level of the suprahepatic IVC, care should be taken to avoid injury to the major hepatic venous branches. The LHV and MHV commonly join within or just outside of the liver to form a common trunk, which then inserts into the IVC. The dissection of the right side is along Cantlie’s line, to the left of the MHV. Several tributaries to the MHV (the 4a and 4b veins) will need to be identified and ligated. If necessary, the MHV may be sacrificed during resection of segment 4, with the bulk of the venous drainage then provided by the LHV (segments 2 and 3) and the RHV (segments 5 and 8). Once the medial border of the hilar dissection has been reached the parenchymal transection is then done in a transverse plane to complete the separation of segment 4.

These segments comprise the right anterior sector or section. The bifurcation of the right portal pedicle into anterior and posterior pedicles is intrahepatic, and primary ligation prior to parenchymal transection can be difficult, especially for segment 8. Therefore some parenchymal transection along Cantlie’s line toward the pedicle is generally performed to expose the pedicles. The segment 5 pedicle emerges anteriorly and runs anteroinferiorly, while the segment 8 pedicle emerges posteriorly and runs superiorly. The axial plane of transection for an isolated segment 5 or segment 8 resection runs through the liver at the level of the bifurcation of the portal vein, with segment 5 below and segment 8 above this line. The medial and lateral limits of the resection are the MHV and the RHV, respectively.

Isolated segment 5 resection begins with division of the liver on the undersurface up the centre of the gallbladder fossa and anteriorly along Cantlie’s line half-way up toward the IVC. Continued parenchymal transection in this plane will ultimately lead to the major right portal pedicles. With careful dissection, the anterior pedicle, and its component branches to segment 5 and 8, are identified (see Fig. 27.13, which illustrates isolation of the segment 5 pedicle during resection of gallbladder cancer). In some
patients, the pedicle to segment 7 arises from the anterior sector; recognition of this anatomical variation is important in order to avoid compromising the integrity of this segment. The parenchyma between the MHV on the left and RHV on the right is then divided, up to the take of the segment 8 pedicle superiorly.

Isolated resection of segment 8 is challenging as there are few external landmarks delimiting its boundaries, which consist of the IVC posteriorly, Cantlie’s line or the MHV medially and the RHV laterally (Fig. 24.8). The segment 8 pedicle corresponds to the ascending division of the right anterior sectorial pedicle and is most commonly identified after initial division of liver parenchyma between segments 4 and 5 in the sagittal plane, along the level of the portal bifurcation. The horizontal transection should proceed in a coronal plane anterior to the RHV to its intersection with the sagittal plane that includes Cantlie’s line and the MHV down to the anterior surface of the IVC. Mobilization of the right liver, as one would do for a major right hepatectomy, is necessary to free the posterior aspect of segment 8 (see Chapter 19 and Figs. 19.5 and 19.6; see Chapter 23 and Figs. 23.12 and 23.13). Care must be taken during the parenchymal division to adhere to the correct transection plane posteriorly. This allows resection of all the liver in segment 8 while avoiding injury to the RHV.

The combined resection of segments 5 and 8 constitutes an anterior sectionectomy, a procedure that can be performed in place of a right hepatectomy, when appropriate, provided the integrity of the procedure is not compromised from an oncologic standpoint. This procedure, and its counterpart posterior sectionectomy (resection of segments 6 and 7), are covered in detail in Chapter 23.

Segments 6 and 7 comprise the posterior section. The right posterior sectorial pedicle can be identified in the incisura dextra of Ganz, which corresponds to the horizontal fissure lateral to the gallbladder fossa. The pedicle to segment 6/7 can be isolated after initial division of the liver parenchyma in this area. Alternatively, the posterior sectoral branches of the hepatic artery and portal vein may be isolated with dissection within the proximal porta hepatis, but this approach is not always possible (see Figs. 23.7 and 23.17). The pedicle for an isolated resection of segment 6 or 7 is therefore usually
approached and controlled from within the liver, during parenchymal transection. It is important to bear in mind that the sagittal transection plane between the anterior and posterior sectors is oblique, approximately half way between the horizontal and vertical planes (Fig. 24.9). A resection of segment 6 or 7 requires complete mobilization of the right lobe of the liver with division of the retrohepatic veins draining into the IVC (Fig. 24.10). A combined resection of segments 6 and 7, or posterior sectionectomy, can be performed with preservation or sacrifice of the RHV.

Isolated segment 6 resection is among the more straightforward of the isolated segmental resections, similar to resection of segment 3, given its very peripheral location. The resection begins with an oblique transection line along the incisura dextra of Ganz on the under surface of the liver and anteriorly approximately half-way toward the IVC, posterior to the plane of the RHV. The axial transection plane is at the level of the portal vein bifurcation. The posterior pedicle will be encountered medially, at the intersection of the transection planes. Care must be taken at this point to fully expose the pedicle branches and avoid injury to the segment 7 inflow.

Isolated segment 7 can be performed with initial transection of the parenchyma in the axial plane, just above the bifurcation of the portal vein, which corresponds to the
horizontal resection line for a segment 6 resection. The resection continues medially to the lateral margin of the RHV; it is during this portion of the transection that the pedicle branches to segment 7 are encountered. The horizontal transection line runs obliquely through the liver lateral and posterior to the RHV (Fig. 24.11A). Resection of segment 7 can be done to include the RHV, provided there is a reasonable sized accessory hepatic vein draining segment 6 directly into the IVC (Fig. 24.11B).

Resection of segments 6 and 7 together constitutes a right posterior sectionectomy, a procedure that can be performed as an alternative to a right hepatectomy when the anterior sector can be spared without compromising is the completeness of the resection, especially in patients with diseased parenchyma. Posterior sectionectomy is described in detail in Chapter 23.

Resection of these two segments is very commonly performed. The vertical plane of transection is along Cantlie’s line between segments 4b and 5, to the right of the MHV from the edge of the liver to half-up toward the IVC, with the axial transection plane being at the level of the portal vein confluence, as it would be for an isolated segment 5 resection. The portal pedicles to segment 5 and 6 are usually divided during the parenchymal transection (Fig. 24.12). This resection requires mobilization of the right lobe from the retroperitoneum and diaphragm with division of the accessory hepatic veins in the anterior surface of the IVC. It is convenient to start the transection in the middle of the gallbladder fossa along Cantlie’s line up to the level of the right portal pedicle. The axial transection plane is then developed from the top of the vertical plane, with the pedicle branches identified and divided in the course of parenchymal transection. Branches of the RHV will be encountered and require division in the course of this resection. Once again, care must be taken to accurately identify the pertinent pedicles for division and avoid injury to adjacent structures.

Segmentectomy 4b and 5 is commonly performed for gallbladder cancer and combines the elements of resecting each segment individually, as described above. This procedure may include en bloc resection of the extrahepatic biliary tree and porta hepatis lymphatic tissue and is discussed in more detail in Chapter 27.

This resection is indicated primarily for large central tumors where an extended right or left hepatectomy risks leaving behind a liver remnant of insufficient volume. The technical aspects of central hepatectomy are described in detail in Chapter 21.