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EHPLE Anesthesia study materials
EHPLE Anesthesia study resources

EHPLE Anesthesia: Basic Sciences Relevant to Anesthesia: Anatomy, Physiology, Pharmacology, and Physics

EHPLE Anesthesia · Work through your exam, one topic at a time.

All guide sections

Basic Sciences Relevant to Anesthesia: Anatomy, Physiology, Pharmacology, and Physics

What this domain tests

This domain rewards a working model that connects anatomy, physiology, pharmacology, and physics to bedside decisions. Expect calculation items (oxygen delivery, local anesthetic ceiling, clearance), waveform and pressure-volume interpretation, and questions asking why one drug or maneuver is preferred.

Core rules

  • Oxygen delivery: DO2 = CO × CaO2 × 10; CaO2 = (1.34 × Hb × SaO2) + (0.003 × PaO2). Content, not PaO2, drives delivery.
  • Myocardial oxygen supply depends on coronary perfusion (diastolic pressure minus LVEDP); demand depends on heart rate, contractility, and wall stress.
  • Ventilation: alveolar gas equation, dead space (anatomical plus alveolar), and closing capacity. Closing capacity rises with age and exceeds FRC in supine elderly patients, promoting atelectasis.
  • Pharmacology: pKa determines ionization; ion trapping explains fetal and abscess acidosis; context-sensitive half-time reflects redistribution and clearance; zero-order drugs (phenytoin, ethanol, high-dose aspirin) saturate metabolism.
  • Physics: Poiseuille flow (Q ∝ r⁴/ηL) explains why a small ETT raises resistance; turbulent flow appears at high Reynolds number; Laplace law applies to alveoli and aneurysms; Henry law governs volatile solubility.

Worked example

A 70 kg patient has Hb 10 g/dL, SaO2 98%, PaO2 100 mmHg, and CO 5 L/min. CaO2 = 1.34 × 10 × 0.98 + 0.003 × 100 = 13.13 + 0.30 = 13.43 mL/dL. DO2 = 5 L/min × 13.43 mL/dL × 10 = 671 mL/min. If Hb falls to 7 g/dL at the same saturation, CaO2 falls to about 9.5 mL/dL and DO2 to 475 mL/min; a normal PaO2 cannot compensate for anemia. This is why transfusion thresholds and mixed venous saturation are reasoned physiologically rather than by pulse oximetry alone.

Traps

Equating potency (EC50 or MAC) with efficacy; using PaO2 in the oxygen-content equation; forgetting that pregnancy and acidosis shift the oxyhemoglobin curve; confusing pKa with pH; treating HCO3- as a ventilation marker rather than a metabolic one.

Key points

  • Oxygen content depends chiefly on hemoglobin and saturation.
  • Closing capacity exceeds FRC in supine elderly patients.
  • pKa and pH determine ion trapping.
  • Poiseuille law: small radius changes cause large resistance changes.
  • Zero-order kinetics cause toxicity with small dose increases.