3 ABDOMEN


Abdomen


3


Structures



  • Spleen
  • Liver

Circulation



  • Mesenteric circulation

Nervous System



  • Anterior abdominal wall
  • Abdominal wall blocks

    • Transversus abdominis plane block
    • Rectus sheath block
    • Ilioinguinal/iliohypogastric nerve blocks
    • Quadratus lumborum block
    • Erector spinae block
    • Penile block

Structures



  • Spleen
  • Liver

Spleen


The spleen is the largest of the lymphatic organs situated in the left hypochondrium against the 9th to the 11th ribs. In health, it weighs anywhere between 50–200 g and is non-palpable. Characteristic examination findings of a palpable spleen are the splenic notch and movement with respiration (Figure 3.1).


The spleen is related to stomach anteromedially, left kidney and splenic flexure inferiorly, pancreas medially, left ribs 9–11 posterolaterally and diaphragm superiorly and posteriorly.


Internal structure



  • Fibrous capsule (outermost layer)
  • Red pulp, which acts as a filter and storage for red blood cells. Blood flows through sinusoids, supported by a framework of trabeculae containing smooth muscle which helps expel blood into the circulation.
  • Marginal zone, containing phagocytic macrophages
  • White pulp (innermost layer), which is the immunologic layer as it is a site of antibody synthesis

Blood supply



  • Splenic artery – the largest branch of the coeliac trunk

    • travels in a characteristically tortuous course along the superior border of the pancreas to reach the splenic hilum where it divides into multiple branches
    • gives off branches to the pancreas, 5–7 short gastric branches, and the left gastro-omental (gastroepiploic) artery

  • Splenic vein

    • the splenic vein drains the spleen and meets the superior mesenteric vein behind the head of the pancreas, forming the portal vein
    • also receives the inferior mesenteric vein
Figure 3.1 Structure of spleen.

Nerve supply



  • Sympathetic fibres are derived from the coeliac plexus.

What are the functions of the spleen?



  • Immune function
  • The spleen is a component of the reticulo-endothelial system. It produces plasma cells and lymphocytes, therefore contributing to both humoral and cell-mediated immunity. It is also the major site of IgM production. The spleen produces opsonins, which facilitate the phagocytosis of encapsulated organisms.
  • Filtration, storage and phagocytosis of blood cells
  • The spleen receives 5% of cardiac output, stores 10% of total body red blood cells and 30% of total body platelets, along with iron storage. The spleen contains phagocytic cells which destroy aged erythrocytes and platelets.
  • Extramedullary haematopoiesis
  • In the fetus, the spleen is a site for myelopoiesis (the production of all types of blood cells). After birth, only lymphopoiesis is maintained but abnormal haematopoiesis can be reactivated in myeloproliferative disorders.

What causes splenomegaly?



  • Infection – viral (e.g. infectious mononucleosis), bacterial (e.g. syphilis, infective endocarditis) or parasitic (e.g. malaria, visceral leishmaniasis)
  • Malignancy – leukaemia, lymphoma, metastases
  • Haematological – sickle cell disease, spherocytosis, thalassaemia, polycythaemia
  • Metabolic – Gaucher disease, Niemann-Pick disease
  • Other – Felty’s syndrome, portal hypertension, collagen vascular diseases

What are the indications for a splenectomy?



  • Trauma – commonest organ injured in blunt abdominal trauma
  • Diagnostic – for histological diagnosis in idiopathic hypersplenism or splenomegaly
  • Hypersplenism – hereditary spherocytosis, idiopathic thrombocytopenic purpura
  • Tumour – lymphoma, leukaemia
  • Surgical – en bloc with distal pancreatectomy and gastrectomy
  • Others – splenic cysts, hydatid cysts, abscesses

What is overwhelming post-splenectomy infection (OPSI)?



  • This is a rare, but frequently fatal, complication of infection with an encapsulated organism and occurs post splenectomy in 4% of patients without prophylaxis.
  • It clinically presents as septic shock, bilateral adrenal haemorrhages and disseminated intravascular coagulopathy and is a medical emergency carrying a 50% mortality.
  • It happens in the absence of splenic function and opsonisation (and therefore bacterial destruction) in a patient who is anatomically or functionally asplenic.
  • The most frequent causative bacterium is Streptococcus pneumoniae. Other pathogens include Neisseria meningitidis, Haemophilus influenza type B, Listeria monocytogenes, Escherichia coli and Klebsiella sp.

What measures are undertaken to minimise the risks of infection in asplenic patients and prevent OPSI?



  • Vaccination: pneumococcus, meningococcus, haemophilus type B and annual influenza vaccine. In the elective setting, these should be administered two weeks preoperatively or in the immediate postoperative period for emergency cases. These are ideally repeated every 5–10 years.
  • Antibiotic prophylaxis: lifelong penicillin V or amoxicillin
  • Education and awareness: patients should be educated on early signs and symptoms of infection/sepsis. They should carry an alert card and will need specialist advice on travel, especially on avoidance of malaria, animal bites and tick bites.

What is functional asplenia?


Functional asplenia occurs when splenic tissue is present but does not function properly and is characterised by the loss of phagocytosis. It occurs in autoimmune diseases, inflammatory bowel disease, sickle cell disease, beta thalassemia, chronic graft-versus-host disease and can be caused by splenic tissue infiltration by tumour cells, sickled erythrocytes, etc.


What abnormalities may be seen in the full blood count of an asplenic patient?



  • Blood count: poikilocytosis, siderocytosis, leucocytosis, thrombocytosis and increased NK cells
  • Blood smear: erythrocyte inclusions like Howell-Jolly bodies (remnants of DNA) and Heinz bodies (inclusions composed of denatured haemoglobin)

Liver


The liver is the second largest organ (second to skin) weighing around 1500 g, which accounts for 2.5% of body weight.


It contains



  • Hepatocytes which are polyhedral epithelial cells arranged in sheets separated from each other by spaces filled with hepatic sinusoids
  • Hepatic sinusoids which are vessels that arise at the portal triad and run between sheets of hepatocytes receiving blood from the portal triad to deliver to central vein

What is the significance of various types of divisions of the liver?



  1. 1. Surgical divisions (Corinaud’s classification)

    • Total of eight segments with independent blood supply and biliary drainage, so they can be resected without damage to the adjacent segments

  2. 2. Functional classification. Liver lobule is the structural unit of liver.

    • Classic lobule

      • Based on direction of blood flow
      • Hexagonal structure with the central vein in the middle and portal triad (branches of portal vein, hepatic artery and bile duct) in the six corners. The hepatic arterial and portal venous blood flows from portal triad to the central vein.

    • Portal lobule

      • Based on direction of bile flow
      • The portal triad is in the middle and the central veins form the corners of the triangle.

    • Hepatic acinus

      • Based on changes in oxygen and nutrient content as blood flows from the portal triad to the central vein
      • It is a rhomboid tissue containing two triangles of adjacent classic lobule, with central veins at the apices.
      • Hepatocytes in the acinus are divided into three zones.

        1. 1. Zone 1 or periportal zone, where the blood supply is the highest and is susceptible to damage by blood-borne toxins and infection
        2. 2. Zone 2 or intermediate zone
        3. 3. Zone 3 or centrilobular zone is closer to the central vein and is higher in CYP 450 levels but gets the least blood supply and hence is susceptible to ischaemia (Figure 3.2)
        Figure 3.2 Structure of liver lobule.

What is special about the blood supply of the liver?


The liver has a dual blood supply and contains 10–15% of the total blood volume, and thereby acts as a powerful reservoir (Table 3.1).


Table 3.1 Hepatic Blood Supply























Hepatic artery


Portal vein


High-pressure/high-resistance system


Low pressure/low resistance


Branch of the coeliac trunk (branch of abdominal aorta)


Formed by the union of superior mesenteric vein and splenic vein


Carries oxygenated blood


Carries oxygen-poor but nutrient-rich blood from the abdominal viscera


20–30% of total blood supply


70–80% of total blood supply


40–50% of total oxygen supply


50–60% of total oxygen supply






Total liver blood flow = 1200 1400 mL/min = 25 % of cardiac output

What factors determine the hepatic blood flow?


Like any other ‘factors affecting blood flow’ question, there is a general classification of factors. The factors listed below are in no order of importance. (It is best discussed using the Hagen-Poiseuille’s equation with circulation-specific factors.)



  • Myogenic autoregulation

    • applicable only in metabolically active liver

  • Metabolic/chemical control

    • CO2, O2 and pH changes can alter the hepatic blood flow
    • Postprandial hyperosmolarity increases the hepatic arterial and portal venous blood flow

  • Neural control

    • Autonomic nervous system via the vagus and splanchnic nerves have control of the hepatic blood flow
    • An important example is the stimulation of the sympathetic system in haemorrhage resulting in constriction of arterioles and expulsion of blood into the general circulation, thus acting as a major reservoir of blood

  • Humoral control

    • Adrenaline, angiotensin II and vasopressin are the main vasoconstrictors

  • Hepatic arterial buffer response (HABR)

    • Phenomenon where decrease in portal venous blood flow increases the hepatic arterial blood flow and vice versa so that a constant oxygen supply and total blood flow are maintained
    • The mechanism of HABR is unknown, but the local production of adenosine is predicted to be one of the causative factors

Other questions…



  • What do you understand by ‘T10 level’ and what structures are present at the level of T10?

Circulation



  • Mesenteric circulation

Mesenteric Circulation


Blood supply to the bowel comes from three main branches of the aorta – coeliac axis, superior and inferior mesenteric arteries.



  • Coeliac axis and branches – left gastric, common hepatic, splenic artery
  • The coeliac axis comes off at the T12 level and supplies the foregut structures such as liver, stomach, spleen, pancreas and the duodenum.
  • Superior mesenteric artery (SMA) and branches – ileocolic artery, right colic artery, middle colic artery
  • The SMA originates at L1, and supplies the embryonic midgut structures such as the duodenum, pancreas, small bowel, caecum, ascending colon and two-thirds of the transverse colon until the splenic flexure.
  • Inferior mesenteric artery (IMA) and branches – left colic artery, sigmoid artery, superior rectal artery
  • The IMA comes off at L3 and supplies the hindgut structures, such as the remaining one third of the transverse colon, descending colon, sigmoid and rectum (Figures 3.3 and 3.4).
Figure 3.3 Arterial supply – small bowel.
Figure 3.4 Arterial supply – large bowel.

What factors affect mesenteric blood flow?


The different variables that have an effect on blood flow are cardiac output, compliance of blood vessels, blood volume, blood viscosity, length and diameter of blood vessels.


This is best defined by Poiseuille’s law which states that the flow of fluid is related to the viscosity of the fluid, the pressure gradient across the tubing and the length and diameter of the tubing.


What are the points of watershed blood flow?


A ‘watershed’ area is an area that has blood supply from the most distal branches of two large arteries. The two watershed areas in the colon are the splenic flexure and the rectosigmoid junction.


Presence of two different blood supplies is expected to maintain the blood supply to the area if there is atherosclerotic disease in one of the arteries. However, in the event of systemic hypoperfusion secondary to sepsis, heart failure, etc., these watershed areas are susceptible to ischaemia as the distal branches are least likely to receive sufficient blood.


What are the causes of mesenteric ischaemia?


Acute mesenteric ischaemia can happen because of the following reasons



  • Embolus – following MI, atrial fibrillation or a mural thrombus
  • Thrombus – usually results from diffuse atherosclerotic disease, previous hypercoagulable states, vasculitis
  • Non-occlusive thrombus – could result from hypovolaemia, spasm which result in a low flow state ultimately resulting in reduced cardiac output and decreased perfusion to the bowel

What are the symptoms and signs of ischaemic bowel?

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Nov 27, 2021 | Posted by in ANESTHESIA | Comments Off on 3 ABDOMEN
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