Subspecialty Basic Sciences Appendix

Ultrasound-guided (primary modality): - High-frequency (10–15 MHz) for superficial structures (better resolution, less penetration) - Low-frequency (2–5 MHz) for deep structures (more penetration, less resolution) - Hypoechoic = appears dark (fluid, nerves often) - Hyperechoic = appears white (fascia, bone, needle, pleura)

Nerve stimulation: - < 0.2 mA + muscle contraction → intraneural (DO NOT INJECT) - < 0.5 mA + muscle contraction → proximity to motor nerve (acceptable)

Field block: terminal cutaneous nerves (intercostobrachial, superficial cervical plexus, ankle).

Roots C5–T1 form the plexus. Intercostobrachial nerve (T2) is spared by all brachial blocks — supplement with subcutaneous infiltration along axillary crease for upper-arm procedures.

Interscalene: - Phrenic nerve (C3–5) is anterior to anterior scalene - Dorsal scapular and long thoracic nerves traverse middle scalene - Vertebral artery medial and deep to anterior scalene - Nerve roots appear hypoechoic — "traffic light sign"

Supraclavicular: - Subclavian artery sits on first rib - Watch for pleura deep to ribs

Infraclavicular: - Axillary artery cephalad to axillary vein - Cords hyperechoic on US - Pec major and pec minor superficial to vessels

Axillary: - Musculocutaneous (most lateral) often missed — between biceps and coracobrachialis - Median (~10 o'clock) lateral and superficial - Ulnar (~2 o'clock) superficial and medial - Radial (~6 o'clock) posterior to artery - Axillary artery lateral to axillary vein

Lumbar plexus (L1–L4 ± T12): - Lateral femoral cutaneous (L1–L3) — sensory only - Femoral (L2–L4) — sensory + motor; branches into saphenous (sensory) - Obturator (L2–L4) — sensory + motor of medial thigh; adductors

Sacral plexus (L5–S4): - Sciatic (L5–S4) — branches into tibial and peroneal nerves proximal to popliteal crease - Posterior femoral cutaneous (S1–S3) — sensory only

• Paravertebral — borders: costotransverse ligament posterior, parietal pleura anterolateral, vertebrae medial, ribs sup/inf. Pneumothorax, hypotension, bradycardia from sympathectomy. Anticoag per epidural guidelines.
• Intercostal — dorsal/ventral rami; high vascular uptake → LAST risk; pneumothorax risk
• TAP — subcostal n (T12), ilioinguinal (L1), iliohypogastric (L1); fascial plane between internal oblique and transversus abdominis
• Erector Spinae — paraspinal fascial plane; safer than paravertebral (distance from pleura/cord); great for rib fractures, catheter-friendly
• PECS I/II, serratus plane — breast and chest wall surgery

Definitions (IASP): - Pain: unpleasant sensory and emotional experience associated with actual or potential tissue damage - Allodynia: pain from a non-painful stimulus - Hyperalgesia: ↑ pain from a normally painful stimulus - Dysesthesia: unpleasant abnormal sensation - Paresthesia: abnormal sensation (not necessarily unpleasant)

Pain types: - Nociceptive: from tissue damage (somatic — sharp, localizable via A-δ/C fibers; visceral — dull, diffuse via sympathetic afferents) - Neuropathic: PNS or CNS damage; burning, shooting; less responsive to opioids

Pain pathway: - 1st-order neuron (dorsal root ganglion) → 2nd-order (crosses midline, ascends in contralateral spinothalamic tract) → 3rd-order (thalamus to postcentral gyrus) - A-δ: thin myelinated, fast = sharp localized - C fibers: thin unmyelinated, slow = dull diffuse - Dorsal horn — Rexed laminae I, II, III, V; excitatory (glutamate, substance P); inhibitory (glycine, GABA) - Descending modulation: PAG and ventromedial medulla via NE, serotonin, endogenous opioids

Tolerance vs dependence: - Tolerance — need ↑ dose for same effect; constipation does NOT develop tolerance - Dependence — withdrawal on cessation; precipitated by antagonists

CNS: - MAC ↓ 40% (returns to normal by day 3 postpartum) - ↑ sensitivity to local anesthetics (MLAC decreased) - ↑ epidural blood volume (IVC obstruction by gravid uterus) - ↓ CSF volume - ↑ epidural space pressure

Respiratory: - ↓ FRC (sharp ↓ ERV); rapid desaturation - ↑ minute ventilation 50% (↑ TV and ↑ RR) - ↓ PaCO₂ → compensatory ↓ HCO₃⁻ - Elevated diaphragm but larger AP chest diameter - VC and closing capacity unchanged - Upper airway edema → smaller ETT (6.0–6.5)

Cardiovascular: - ↑ CO 40%, ↑ SV 30%, ↑ HR 20% - ↓ SVR - Peak CO during active labor and immediately after delivery; CO returns to baseline at ~2 weeks - Aortocaval compression > 20 weeks → maintain left uterine displacement ≥ 15°

Hematologic: - ↑ plasma volume > ↑ RBC volume → dilutional anemia - Hypercoagulable (↑ factors VII, VIII, X, XII, fibrinogen) - Mild thrombocytopenia

GI: ↓ LES tone, ↑ intragastric pressure → aspiration risk after 16–20 weeks.

Airway anatomy: - Large occiput → roll under shoulders - Anterior, more cephalad larynx (C3–C4 vs C5–C6 in adult) - Funnel-shaped subglottic region — narrowest at the cricoid in < 8 yo (clinically) - Large tongue, floppy epiglottis - Brisk vagal response → pre-treat with atropine 0.02 mg/kg in infants for sux

ETT sizing: - Cuffed: age/4 + 3.5 - Uncuffed: age/4 + 4 - Depth (cm at lip): age/2 + 12 or 3× ETT size

Pediatric breathing circuits (Mapleson): - Lack unidirectional valves, no CO₂ absorber - Lower airway resistance, increased venting - Spontaneous: Mapleson A most efficient (1× MV) - Controlled: Mapleson D most efficient (2× MV)

Fetal circulation — 3 shunts: 1. Ductus venosus (umbilical vein → IVC) 2. Foramen ovale (RA → LA) 3. Ductus arteriosus (pulmonary artery → aorta)

• CO = HR × SV
• SV = EDV − ESV
• EF = SV / EDV (normal LV ~60%)
• CI = CO / BSA (normal 2.6–4.2 L/min/m²)
• MAP ≈ DBP + 1/3 (SBP − DBP) (also ≈ DBP + PP/3)
• CPP = DBP − LVEDP (or DBP − PCWP)
• SVR = (MAP − CVP) × 80 / CO (normal 800–1200 dyn·s/cm⁵)
• PVR = (MPAP − PCWP) × 80 / CO (normal < 250)
• PP = SBP − DBP
• DO₂ = CO × CaO₂ × 10 (normal ~1000 mL/min)
• VO₂ = CO × (CaO₂ − CvO₂) × 10 (normal ~250 mL/min)
• O₂ ER = VO₂ / DO₂ (normal ~25%)