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Wolff-Parkinson-White (WPW) Syndrome

Definition

Pathophysiology

ECG Features (Classic Triad)

Accessory Pathway Locations

• Left lateral (most common) - negative delta in aVL
• Right free wall - positive delta in V1
• Posteroseptal - negative delta in II, III, aVF (mimics inferior MI)
• Anteroseptal

Associated Conditions

• Hypertrophic cardiomyopathy (most common structural association)
• Ebstein anomaly
• Transposition of great vessels
• Mitral valve prolapse
• Endocardial fibroelastosis
• Tricuspid atresia

Tachyarrhythmias in WPW

1. Orthodromic AV Reentrant Tachycardia (AVRT) - Most Common (~95%)

• Impulse travels antegrade via AV node → ventricles → retrograde via AP → atria
• Narrow QRS tachycardia (no pre-excitation during tachycardia)
• P waves follow QRS (in ST segment), with short RP interval
• Rate: typically 150-250 bpm

2. Antidromic AVRT - Uncommon (~5%)

• Impulse travels antegrade via AP → ventricles → retrograde via AV node
• Wide, bizarre QRS - often indistinguishable from VT
• Pre-excitation is maximal (entire QRS through AP)

3. WPW with Atrial Fibrillation - Most Dangerous

• AF impulses bypass the rate-limiting AV node and conduct rapidly down the AP
• Ventricular rates can reach 250-300+ bpm
• Can degenerate into ventricular fibrillation and sudden cardiac death
• ~25% of APs have minimum R-R intervals <250 ms during AF, conferring higher SCD risk - Harrison's 22E

Variants

• Concealed AP: Only retrograde conduction (ventricle → atrium). No pre-excitation in sinus rhythm, but orthodromic SVT can occur.
• Mahaim fibers (Atriofascicular pathways): Duplicate AV node/His-Purkinje system connecting right atrium to right bundle branch → wide LBBB-pattern tachycardia.
• PJRT (Permanent Junctional Reciprocating Tachycardia): Slowly conducting posteroseptal AP → near-incessant orthodromic SVT → can cause tachycardia-induced cardiomyopathy.

Management

Acute Tachycardia (Orthodromic AVRT - narrow complex)

• Vagal maneuvers (Valsalva, carotid sinus massage)
• IV adenosine (drug of choice - blocks AV node, terminates reentry)
• IV verapamil or beta-blockers if no pre-excitation present

Acute WPW + AF (Wide complex, pre-excited AF) - EMERGENCY

• Hemodynamically unstable: Immediate DC cardioversion
• Hemodynamically stable: IV procainamide or ibutilide (slow AP conduction)
• CONTRAINDICATED: Digoxin, verapamil, diltiazem, beta-blockers, adenosine, IV amiodarone
  • These agents block the AV node but NOT the AP - they can paradoxically accelerate conduction through the AP, precipitating VF and cardiac arrest - Braunwald's Heart Disease; Harrison's 22E

Definitive Treatment

• Radiofrequency catheter ablation of the accessory pathway
  • Success rate: >95% for most AP locations
  • Very low risk of major complications
  • AF typically does not recur after successful ablation (since it was often triggered by AVRT degenerating into AF)
  • Preferred for symptomatic patients and those with high-risk APs - Braunwald's Heart Disease

Risk Stratification (Asymptomatic WPW)

• EP study to measure the shortest pre-excited R-R interval during AF and the AP effective refractory period
• High-risk features: AP refractory period <250 ms, multiple APs, Ebstein anomaly
• Ablation may be considered in high-risk asymptomatic patients (e.g., athletes, competitive sports)

Complete Anatomy of the Large Intestine

Distinguishing Features (Unique to the Large Intestine)

Segments

1. Cecum

• First part of the large intestine, located in the right iliac fossa
• Inferior to the ileocecal opening (ileocecal valve)
• Considered intraperitoneal (no true mesentery, but mobile)
• Receives ileal contents through the ileocecal valve
• Continuous superiorly with the ascending colon

2. Appendix (Vermiform Appendix)

• Narrow, hollow, blind-ended tube attached to the posteromedial wall of the cecum, just inferior to the ileocecal opening
• Contains large aggregations of lymphoid tissue (part of MALT)
• Suspended from terminal ileum by the mesoappendix (contains appendicular vessels)
• Base consistently located where the free taenia leads to it
• Variable positions: retrocecal (most common ~65%), pelvic/descending, subcecal, pre-ileal, post-ileal
• Surface projection of base = McBurney's point (junction of lateral 1/3 and middle 1/3 of line from ASIS to umbilicus)

3. Ascending Colon

• Runs retroperitoneally from cecum upward through the right flank to the right hypochondrium
• Ends at the right colic flexure (hepatic flexure) - just inferior to the right lobe of the liver
• Bends leftward at the hepatic flexure to become the transverse colon

4. Transverse Colon

• Intraperitoneal - suspended by the transverse mesocolon
• Runs from hepatic flexure across the abdomen to the left colic flexure (splenic flexure)
• Splenic flexure is higher, more posterior, and more acute than the hepatic flexure
• Attached to the diaphragm by the phrenicocolic ligament

5. Descending Colon

• Retroperitoneal; runs from splenic flexure downward through the left flank to the left iliac fossa
• Narrower than the ascending colon
• Continues as the sigmoid colon

6. Sigmoid Colon

• S-shaped, intraperitoneal, suspended by the sigmoid mesocolon
• Begins above the pelvic inlet, ends at vertebral level S2 where it becomes the rectum
• Mobile between its fixed ends (descending colon junction and rectum)

7. Rectum

• Begins at S2-S3 (end of sigmoid mesocolon)
• Retroperitoneal (no mesentery)
• ~12-15 cm long; lacks haustra and taeniae coli (longitudinal muscle becomes continuous)
• Contains transverse rectal folds (valves of Houston) - 3 permanent mucosal folds
• Functions as a reservoir for feces prior to evacuation

8. Anal Canal

• 3-4 cm long; distal continuation of the rectum
• Contains anal columns (columns of Morgagni) - longitudinal mucosal folds in the upper anal canal
• Anal sinuses - pockets between columns, each with an anal gland
• Pectinate (dentate) line - marks the embryological junction between hindgut endoderm above and ectodermal proctodeum below; important clinical landmark
• Internal anal sphincter - thickened circular smooth muscle (involuntary)
• External anal sphincter - voluntary striated muscle; coordinates with levator ani for continence

Blood Supply

Arterial Supply

The SMA supplies the right colon up to the mid-transverse colon (midgut derivative); the IMA supplies the left colon, sigmoid, and upper rectum (hindgut derivative). The marginal artery of Drummond is the anastomotic arcade running along the entire colon connecting SMA and IMA branches - a critical collateral pathway.

The watershed zone (Griffith's point at splenic flexure and Sudeck's point at rectosigmoid junction) are areas most vulnerable to ischemia due to poor collateral flow.

Venous Drainage

• Right colon (cecum to mid-transverse) → Superior mesenteric vein → portal vein
• Left colon (splenic flexure to sigmoid) → Inferior mesenteric vein → splenic vein → portal vein
• Rectum (upper) → superior rectal vein → IMV → portal system
• Rectum (lower) / anal canal → middle rectal vein (internal iliac) + inferior rectal vein (internal pudendal) → systemic circulation
• The anal canal is a portosystemic anastomosis - important in portal hypertension (causes anorectal varices)

Lymphatic Drainage

Nerve Supply

• Sympathetic: Pre-ganglionic from T10-L2; post-ganglionic via superior and inferior mesenteric plexuses → inhibits motility, contracts internal anal sphincter
• Parasympathetic:
  • Right colon: via vagus nerve (CN X)
  • Left colon, sigmoid, rectum: via pelvic splanchnic nerves (S2-S4)
  • Parasympathetics stimulate motility and relaxation of internal sphincter
• Somatic: External anal sphincter supplied by the pudendal nerve (S2-S4)

Histological Wall Layers

Peritoneal Relations Summary