Structures

• Femoral triangle
  
• Popliteal fossa

Circulation

• Arterial supply
  
• Venous drainage

Nervous System

• Dermatomes and nerve distribution
  
• Lumbar plexus, femoral, saphenous and sciatic nerves
  
• Blocks
  
  
  
  • Lumbar plexus block
    
  • Femoral nerve block
    
  • 3-in-1 block
    
  • Fascia iliaca block
    
  • Adductor canal block
    
  • Sciatic nerve block
    
  • Popliteal nerve block
    
  • Ankle block

Structures

• Femoral triangle
  
• Popliteal fossa

Femoral Triangle

The femoral triangle is a hollow area in the anterior thigh providing relatively easy access to the femoral neurovascular bundle (Figure 6.1).

Boundaries

• Superior – inguinal ligament (attachments are ASIS and pubic tubercle)
  
• Lateral – sartorius
  
• Medial – adductor longus
  
• Floor – iliacus, pectineus, psoas muscles
  
• Roof – areolar tissue, fascia lata, subcutaneous tissue, skin

Contents (lateral to medial)

• Femoral nerve – invested in fascia iliaca
  
• Femoral branch of the genitofemoral nerve
  
• Femoral sheath – a prolongation of the transversalis fascia containing
  
  
  
  • Femoral artery
    
  • Femoral vein
    
  • Femoral canal – the medial component of the sheath containing lymphatics and possibly inguinal node (Cloquet’s node). The femoral ring is the abdominal opening of the canal.

Why is the femoral triangle important to the anaesthetist?

It is an important anatomical landmark for anaesthetists to be able to access the neurovascular bundle.

• Femoral nerve: for femoral nerve block, 3-in-1 block, fascia iliaca block
  
• Femoral artery: for arterial cannulation (continuous BP, cardiac catheterisation, intra-aortic balloon pump)
  
• Femoral vein: for central venous catheters, vascaths, IVC filters

How do you locate the femoral artery on an actor? Where do the nerve and vein lie in relation to this?

The femoral artery is palpable at the mid-inguinal point, which is situated halfway between the pubic symphysis and the ASIS. The nerve lies lateral to the artery, and the femoral vein lies medial to the artery.

Popliteal Fossa

The popliteal fossa is a diamond-shaped area posterior to the knee joint. This is the site for popliteal nerve block for providing analgesia for procedures performed in the lower leg (Figure 6.2).

Boundaries

• Superomedial – semimembranosus
  
• Superolateral – biceps femoris
  
• Inferomedial – medial head of gastrocnemius
  
• Inferolateral – lateral head of gastrocnemius

• Floor – posterior capsule of the knee joint and posterior surface of the femur
  
• Roof – skin and popliteal fascia (continuous with the fascia lata of the leg)

Contents (medial to lateral)

• Popliteal artery, vein and their genicular branches
  
• Popliteal lymph nodes
  
• Tibial nerve
  
• Common peroneal nerve: both nerves are branches of the sciatic nerve, which commonly divides around the fossa. Subject to anatomical variation this occurs approximately 5–10 cm above the popliteal skin crease.

Circulation

• Arterial supply
  
• Venous drainage

Arterial Supply of Lower Limb

The main blood supply to the lower limb is by the femoral artery, which is the continuation of the external iliac artery. The femoral artery continues as the popliteal artery in the lower leg (Table 6.1 and Figure 6.3).

Table 6.1 Arterial Supply of Lower Limb

See Femoral Triangle and Popliteal Fossa respectively for the relations of femoral artery and the popliteal artery.

What are the causes of limb ischaemia?

Emboli rank first for being the culprit for acute limb ischaemia and most commonly arise from the heart, a proximal arterial aneurysm or atherosclerosis. Thrombosis may be induced by any of the three factors of Virchow’s triad: static or turbulent blood flow, hypercoagulability and endothelial injury.

Other causes include vasculitis, trauma, compartment syndrome, fibrodysplasia and iatrogenic interventions (e.g. cannulation of vessels).

What are the features of an ischaemic limb?

• Acute: 6 Ps – pain, pallor, paraesthesia, paralysis, pulselessness, perishing cold
  
• Chronic: may be asymptomatic, intermittent claudication, ischaemic rest pain, ulceration and gangrene

What are the management options for an acutely ischaemic limb?

The management of a patient presenting with an acutely ischaemic limb begins with taking a thorough history and examination. Acute ischaemia caused by complete arterial blockage can cause irreversible damage in 6 hours.

• History with particular mention to
  
  
  
  • Onset of symptoms – sudden onset over the last few hours suggests an acute event whereas a slow onset over days/weeks suggests a more chronic problem
    
  • Comorbidities – MI, AF, aneurysm would be suggestive of an embolism
    
  • Use (or discontinuation) of anticoagulants
  
  
• Examination
  
  
  
  • Assess the viability of the limb (irreversible signs such as fixed staining, mottling, gangrene – surgical interest)
    
  • Airway and targeted examination of other systems
  
  
• Investigations
  
  
  
  • FBC, clotting, creatine kinase, group and save, doppler, magnetic resonance angiography or computed tomographic angiography or intra-arterial angiography
  
  
• General management
  
  
  
  • ABC measures
    
  • Analgesia
  
  
• Specific management
  
  
  
  • Immediate involvement of the vascular team
    
  • Anticoagulation with heparin
    
  • Surgical – embolectomy +/– fasciotomy, bypass procedures
    
  • Endovascular – angioplasty, thrombectomy, local intra-arterial thrombolysis, etc.

Anaesthetic management

Patients are usually systemically ill, and the surgical procedure is often urgent, but a thorough anaesthetic and medical history and examination is carried out and system optimisation should be considered within the time available. Surgeons may choose to perform an embolectomy under local anaesthesia but given the high perioperative risks, anaesthetic presence is usually necessary. If more invasive surgery is planned, general anaesthesia is the preferred choice for various reasons (use of therapeutic doses of anticoagulant drug, non-fasted state, etc.)

Points to remember

• General measures of management of an acutely ill patient with the use of invasive monitoring, regular acid-base analysis, electrolyte corrections, fluid resuscitation and inotropic support.
  
• Reperfusion injury – hyperkalaemia, myocardial depression, arrhythmias, cardiac arrest, myoglobinaemia and acute renal failure can happen due to reperfusion of the ischaemic limb. In the case of muscle necrosis or irreversible ischaemia, these risks may be overwhelming and primary amputation may be indicated.

Venous Drainage of Lower Limb

The venous drainage of the lower limb can be divided into two groups

• Deep venous system
  
• The deep veins accompany the arteries in the leg. The peroneal vein drains into the posterior tibial vein which, along with the anterior tibial vein, empties into the popliteal vein. The popliteal vein continues as the femoral vein and accepts the profunda femoris vein and becomes the external iliac vein.
  
• Superficial venous system
  
• The superficial veins form the dorsal venous arch which continue as the great saphenous vein on the medial side of the leg (drains into the femoral vein) and small saphenous vein laterally (drains into the popliteal vein).

What might be the indications for cannulating the femoral vein?

• Central venous access – which can be used to administer drugs/infusions
  
• Central venous pressure monitoring
  
• Dialysis catheters (vascaths)
  
• IVC filter insertion
  
• Port of access for interventional angiography (e.g. coronary angiogram)

What are the possible complications of femoral vein cannulation?

Immediate

• Arterial injury
  
• Haematoma
  
• Damage to the femoral nerve
  
• Bowel/bladder injury

Delayed

• Pseudoaneurysm
  
• Venous thrombosis
  
• Infection
  
• Septic arthritis (usually following the puncture of the hip capsule in infants)

What clinical features would make you suspect IVC thrombosis in a patient with a femoral vein catheter?

Features of local obstruction

• Lower limb pain and oedema
  
• Scrotal swelling

Features of clot migration

• Pulmonary embolism
  
• Budd Chiari syndrome – ascites, portal hypertension, collateral vein enlargement, hepatic fibrosis due to extension or migration of the thrombus to the hepatic veins
  
• Renal failure

Features of venous hypertension

• Bilateral lower-extremity oedema
  
• Varicose veins and non-healing venous ulcers
  
• Caput medusae (visibly dilated superficial abdominal veins from collateral drainage)

Nervous System

• Dermatomes and nerve distribution
  
• Lumbar plexus, femoral, saphenous and sciatic nerves
  
• Blocks
  
  
  
  • Lumbar plexus block
    
  • Femoral nerve block
    
  • 3-in-1 block
    
  • Fascia iliaca block
    
  • Adductor canal block
    
  • Sciatic nerve block
    
  • Popliteal nerve block
    
  • Ankle block

Dermatomes and Nerve Distribution

Figures 6.4 and 6.5 show the dermatomal and peripheral nerve distribution of the anterior and posterior parts of the lower limb.

• Thigh
  
• Cutaneous supply of the thigh from the groin to the knee is from the lumbar plexus. The femoral nerve is responsible for the anterolateral aspect of the skin of the thigh with an important branch being the lateral cutaneous nerve of the thigh. The obturator is responsible for the posteromedial aspect, with the posterior cutaneous nerve of the thigh being an important branch. Superiorly, iliohypogastric and subcostal nerves supply the skin directly over the anterior superior iliac spine (ASIS) and beneath the inguinal region.
  
• Knee
  
• Cutaneous innervation is derived from the lumbar plexus and the sacral plexus, more specifically, the femoral nerve and the sciatic nerve. The femoral nerve is responsible for the anteromedial aspect, whereas the sciatic nerve innervates the posterolateral aspect of the knee.
  
• Lower leg
  
• Like the knee, the femoral and sciatic nerves are responsible for the cutaneous innervation of the lower leg. The saphenous nerve (branch of the femoral nerve) supplies the medial aspect of the leg, including the medial malleolus. The sciatic nerve and its branches supply the rest of the leg and foot. This is covered in more detail under Ankle Block.

The Lumbar Plexus

The lumbar plexus is a network of nerve fibres that provide motor and sensory innervation to the lower limb. They are formed of the anterior rami of the lumbar spinal nerves from L1–L4 with 50% of cases receiving contribution from T12. The plexus is found anterior to the transverse processes of lumbar vertebrae within the psoas major muscle compartment. These spinal nerves divide into cords and combine to form six major peripheral nerves (Table 6.2 and Figure 6.6).

L1 and, in 50% of cases, a branch of T12 splits into two divisions – upper and lower. Upper division gives rise to iliohypogastric and ilio-inguinal nerves. The lower division forms the genitofemoral nerve after joining with a branch from L2 anterior rami.

The rest of L2–L3 and some branches of L4 rami divide into two divisions – dorsal and ventral. Dorsal divisions of L2–L3 form the lateral cutaneous nerve of the thigh and that of L2–L4 form the femoral nerve. The ventral divisions of L2–L4 join to form the obturator nerve.

All nerves apart from the obturator emerge between the quadratus lumborum and the psoas muscles. The obturator nerve passes medially and travels under the iliac vessels to the lower limbs.

Table 6.2 Summary of Lumbar Plexus

What do the branches of the lumbar plexus supply and what (if any) are their functions?

Table 6.3 Summary of Function of the Branches of the Lumbar Plexus

Femoral Nerve

The femoral nerve arises from primary rami of L2–L4 (dorsal divisions) within the lumbar plexus. It supplies

• Skin overlying medial aspect of anterior thigh and medial leg below the knee
  
• Pectineus, sartorius, quadriceps femoris muscles
  
• Femoral shaft, hip and knee joint

Course of the nerve

The nerve emerges from the lateral margin of the psoas muscle, passing inferiorly between the psoas and iliacus muscles and crosses beneath the inguinal ligament to enter the thigh. The nerve resides in the femoral triangle, lateral to the femoral artery and is invested in the fascia of iliacus muscle which separates it from the femoral sheath. It divides into its terminal branches at the base of the triangle.

Terminal branches

These stem from anterior and posterior divisions.

Anterior division

• Muscular branches which innervate sartorius and pectineus.
  
• Cutaneous branches which supply skin overlying medial aspect of anterolateral thigh – intermediate and medial cutaneous nerve of the thigh.

Posterior division

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3 ABDOMEN 4 SPINE 2 THORAX 5 UPPER LIMB 1 HEAD AND NECK 7 MISCELLANEOUS

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