Ch20: Serous Membrane Effusion & Thoracentesis（浆膜腔积液与胸腔穿刺）

Preparatory Mindset

Serous membrane effusions (pleural, pericardial, peritoneal) accumulate when fluid formation exceeds lymphatic drainage. The central exam question is always: transudate or exudate? Master Light's criteria, the SAAG for ascites, the key biochemical discriminators (protein, LDH, glucose, ADA), and the procedures — thoracentesis and abdominocentesis — with their indications, contraindications, and complications. This is a high-yield, heavily tested topic with clear tables.

Illustrations（图解速览）

Core Concepts — Serous Fluid Formation

Anatomy & normal physiology

• Serous cavities: pleural (lungs), pericardial (heart), peritoneal (abdomen/pelvis)
• Lined by two mesothelial membranes: parietal (lines cavity wall) + visceral (covers organ)
• Serous fluid = ultrafiltrate of plasma — formation depends on hydrostatic vs colloid (oncotic) pressure balance; normally small amount (lubrication), production = reabsorption
• Effusion develops when formation > lymphatic removal

Transudate vs Exudate — mechanism

• Transudate: altered systemic factors — ↑hydrostatic pressure (heart failure), ↓oncotic pressure (nephrotic syndrome, cirrhosis, hypoalbuminemia) — pleura itself normal
• Exudate: altered pleural surface — inflammation/tumor ↑ capillary permeability or ↓lymphatic drainage (pleurisy, malignancy, PE, SLE, TB)

Core Concepts — Pleural Fluid Analysis (胸水)

Light's criteria (exudate if any one) — MUST KNOW

1. Pleural fluid protein / serum protein >0.5
2. Pleural fluid LDH / serum LDH >0.6
3. Pleural fluid LDH >2/3 upper limit of normal serum (or >200 IU/L per some sources)

Transudate vs Exudate — comparison table

Specific markers — HIGH YIELD

• pH <7.0 = pyothorax or esophageal rupture (consider chest tube); pH <7.3 + glucose <3.3 = complicated parapneumonic (drainage indication)
• Exudate causes: pneumonia (parapneumonic), TB, malignancy (metastatic, mesothelioma), PE, SLE/RA, pancreatitis
• Transudate causes: CHF (most common overall), cirrhosis/hepatic hydrothorax, nephrotic syndrome, hypoalbuminemia, myxedema, constrictive pericarditis, SVC obstruction

Core Concepts — Ascites Analysis (腹水)

SAAG (serum-ascites albumin gradient) — MUST KNOW

• SAAG = serum albumin − ascitic albumin
• High SAAG (≥11 g/L): portal hypertension — cirrhosis, heart failure, Budd-Chiari, massive liver metastases
• Low SAAG (<11 g/L): non-portal — peritoneal carcinomatosis, TB peritonitis, nephrotic syndrome (hypoalbuminemia), pancreatic ascites, biliary ascites
• Note: total protein is NOT reliable for ascites (SAAG replaces it); serum-ascites protein gradient useful in some contexts

Ascitic fluid tests

• Cell count/differential: PMN ≥250/mm³ = spontaneous bacterial peritonitis (SBP) — culture + antibiotics
• Protein, albumin (SAAG), LDH, glucose, ADA (TB), amylase (pancreatic), cytology (malignancy), Gram stain/culture, triglyceride (chylous)

Causes of ascites

• Portal hypertension (cirrhosis, HF, Budd-Chiari, constrictive pericarditis)
• Peritoneal disease (carcinomatosis, TB, SBP, pancreatic)
• Hypoalbuminemia (nephrotic, malnutrition)

Core Concepts — Thoracentesis (胸腔穿刺术)

Indications

• Diagnostic: any pleural effusion of unknown cause (clinically significant — >10 mm on USS/lateral decubitus)
• Therapeutic: relieve dyspnea from pneumothorax/large effusion; drain + rinse pyothorax; instill drugs (antibiotics, chemotherapy)

Contraindications (relative — no absolute)

Coagulation disorder, anticoagulant therapy, chest wall cellulitis at site, uncertain fluid location, minimal fluid, uncontrolled cough, altered chest wall anatomy, uncooperative patient

Preparation

• Informed consent; comfort patient; sterile technique
• Equipment: lidocaine 1–2% (10 ml), gauze, 5-ml syringe, antiseptic, drapes, gloves, thoracentesis bag (needle + catheter), 3-way stopcock, 30–50 ml syringe, dressings, collection tubes (red/purple top, blood culture), vacuum bottle (large volume), USS (if guided), epinephrine (rescue)

Positioning & site

• Sitting with arms supported on bedside table + footstool; lateral recumbent; semi-Fowler's
• Pleural effusion: 7th–9th ICS, scapular line or posterior axillary line (confirmed by dullness/↓fremitus + USS/CXR)
• Pneumothorax: 2nd–3rd ICS, midclavicular line
• Insert on superior edge of rib — the intercostal neurovascular bundle runs along the lower edge of each rib
• Be careful of liver (right, up to 5th ICS on expiration) and spleen (left, up to 9th ICS)

Procedure & amounts

1. Sterile prep + drape; anesthetize skin with lidocaine (wheal), advance with aspiration, inject 0.1–0.2 ml lidocaine every 1–2 mm
2. Insert 22-gauge needle attached to 30-ml syringe, above the rib, aspirate while advancing; a "pop" may be felt entering the pleura; note depth
3. Diagnostic: 50–100 ml; Therapeutic: first time ≤600 ml, then ≤1000 ml per session; Pneumothorax: ≤1000 ml at a time
4. If >500 ml withdrawn, monitor symptoms/BP — stop if chest pain, dyspnea, or hypotension
5. Remove needle, sterile dressing; process fluid (transudate vs exudate)

Complications

• Pneumothorax (most common — including tension pneumothorax: cyanosis, dyspnea, sweating, hypotension → urgent decompression), bleeding/hemothorax, re-expansion pulmonary edema (large rapid drainage), infection, organ puncture (liver/spleen/diaphragm), vasovagal reaction

Core Concepts — Abdominocentesis (腹腔穿刺术)

Indications

• Diagnostic: new-onset ascites of unknown etiology; diagnose SBP, TB, metastatic cancer, blood in peritoneal cavity (trauma)
• Therapeutic: relieve pressure (large ascites with severe distension, chest tightness, SOB, oliguria); intraperitoneal drugs/chemotherapy (hyperthermic perfusion); ascites concentration recycling; artificial pneumoperitoneum

Contraindications

• Absolute: acute abdomen requiring surgery
• Relative: PT >21 s / INR >1.6 / platelets <50,000 (increased bleeding); late pregnancy, ovarian cyst, hydatid, distended bladder, abdominal wall cellulitis, severe distended bowel, severe adhesions, early hepatic encephalopathy/restless patient

Procedure

1. Consent + education; empty bladder; supine, head elevated 20–30°
2. Sites: ①outer 1/3 of line between umbilicus and ASIS; ②umbilicus horizontal line × anterior/mid-axillary (lateral decubitus); ③1 cm above umbilicus-pubis midpoint, 1.5 cm lateral; ④USS-guided; avoid scars
3. Sterile prep (15 cm diameter, 3×), gloves; lidocaine infiltration down to peritoneum (aspirate periodically)
4. Z-tracking (advance 1–2 cm after piercing skin, then redirect) to prevent fluid leak
5. Slow advance with aspiration; peritoneal "give" felt; diagnostic = 50 ml
6. Large-volume: direct drainage to vacuum containers; limit ≤3 L (cirrhosis); albumin 1–2 units after 4–5 L
7. Remove, sterilize, pressure dressing; supine 2–4 h

Complications & management

High-Yield Points

• Light's criteria (exudate if ≥1): PF/Serum protein >0.5; PF/Serum LDH >0.6; PF LDH >2/3 upper normal
• Transudate: protein <30, LDH <200, SG <1.018, Rivalta negative — CHF/cirrhosis/nephrotic
• Exudate: protein >30, LDH >200, Rivalta positive — infection/malignancy/TB/PE/SLE
• ADA >45 U/L = TB pleurisy; CEA >20 = malignant; glucose <3.3 = pyothorax/SLE/TB/RA; chylous (TRIG >1.21) = chylothorax
• pH <7.0 = pyothorax/esophageal rupture (chest tube)
• SAAG ≥11 = portal hypertension ascites; <11 = carcinomatosis/TB/nephrotic
• SBP: ascitic PMN ≥250/mm³ → antibiotics (no need for culture wait)
• Thoracentesis: above the rib (neurovascular bundle below); effusion 7th–9th ICS scapular/posterior axillary; pneumothorax 2nd–3rd ICS MCL
• Therapeutic thoracentesis: first ≤600 ml then ≤1000 ml; pneumothorax ≤1000 ml — risk re-expansion pulmonary edema
• Tension pneumothorax after thoracentesis: cyanosis + dyspnea + hypotension → immediate decompression
• Abdominocentesis: Z-tracking, empty bladder, ≤3 L (albumin after 4–5 L), SBP diagnosis
• Pericardial effusion/tamponade: fluid in pericardial sac compresses heart — Beck's triad (hypotension, muffled sounds, JVD)

Topic Summary

Serous effusions: transudate (systemic — HF/cirrhosis/nephrotic) vs exudate (surface — infection/malignancy/TB/PE); classify with Light's criteria + protein/LDH/SG/Rivalta. High-yield markers: ADA (TB), CEA (malignant), glucose/pH (complicated parapneumonic), chylothorax (TRIG). Ascites: SAAG (≥11 portal HTN) + PMN ≥250 = SBP. Thoracentesis: above rib, effusion 7th–9th ICS, pneumothorax 2nd–3rd ICS, ≤600 ml first then 1000 ml, tension pneumothorax complication. Abdominocentesis: Z-tracking, ≤3 L, albumin after large volume. Master the tables — this chapter is guaranteed exam and OSCE material.