Equipment · 7 min
Anesthesia machine + equipment — last-night quick guide
FDA-1993 checkout, vaporizer, circuit, capnography, SGA, ETT.
Rule
FDA-1993 anesthesia machine pre-use checkout
(1) Verify backup ventilation available (AMBU). (2) High-pressure system: cylinder pressures, pipeline 50 psi. (3) Low-pressure system: leak test (negative pressure or modern self-test). (4) O2 analyzer calibrate — 21% room air, 100% O2. (5) Breathing system: leak + flow + valves. (6) Manual + ventilator: ventilator function. (7) Monitors: ETCO2, pulse ox, BP, NMB. (8) Final position: vaporizers off, all controls neutral. ALWAYS DO before every patient — even if assigned room with previous setup.
Vaporizer principles
| Concept | Detail |
|---|---|
| Variable bypass | Splits gas: small portion through vaporizing chamber (saturated), rest bypasses; mixes to dial setting |
| Temperature compensated | Bimetallic strip adjusts splitting ratio as temp drops; otherwise output drops with temp |
| Agent specific | Each vaporizer calibrated for one agent; filling wrong = catastrophic dosing error (use color-coded keyed fillers) |
| Desflurane (Tec-6) | Vapor pressure ~660 mmHg at 20°C — boils at room temp; needs HEATED vaporizer + pressurized; cannot be variable bypass |
Rule
Capnography waveform interpretation
Phase I (baseline) → II (rising — exhalation begins) → III (alveolar plateau, ETCO2) → drop to baseline (inspiration). NORMAL ETCO2 35-45. Pattern abnormalities: SHARK FIN (sloped expiratory) = bronchospasm; CURARE CLEFT = spontaneous breath through paralyzed; SUDDEN ↓ to 0 = ETT dislodged/disconnect/arrest; sudden ↓ but waveform = PE/AGE/sudden ↓ CO; gradual ↑ = hypoventilation/CO2 absorbent exhausted/MH.
Mnemonic — Sudden ↓ vs gradual ↓ vs ↑
ETCO2 patterns — what they mean
ETCO2 patterns
- Sudden ↓ to 0ETT dislodgement, complete circuit disconnect, complete obstruction, cardiac arrest (no flow)
- Sudden ↓ but waveform presentPE, air embolism, sudden ↓ CO, hypovolemia, hyperventilation, decreased sample line flow
- Gradual ↓Hyperventilation (alveolar), ↓ metabolic rate (hypothermia, deep sedation), ↓ pulmonary perfusion gradual
- Gradual ↑Hypoventilation, ↑ metabolism (fever, MH early), CO2 absorbent exhausted, rebreathing
- Sudden ↑Tourniquet release, IV bicarbonate, laparoscopic CO2 absorption, MH onset (sustained ↑ refractory to ↑ MV)
- Steeple/curare cleftSpontaneous breath through paralyzed → notch in plateau (residual paralysis returning)
- Shark finBronchospasm — sloped expiratory phase (asthma, COPD)
Capnography is the single most useful intraop monitor — confirms intubation, ventilation, perfusion, CO2 production. Sudden changes ALWAYS deserve immediate workup.
ETT sizing
| Patient | Cuffed ID (mm) | Depth at lip (cm) |
|---|---|---|
| Adult male | 8.0-8.5 | 21-23 |
| Adult female | 7.0-7.5 | 20-22 |
| Term neonate | 3.0 | 9-10 |
| 6 months | 3.5 | 10-11 |
| 1 year | 4.0 | 11 |
| >2 yr cuffed | (age/4)+3.5 | (age/2)+12 |
Rule
Pulse oximetry physics + pitfalls
Beer-Lambert law: 660 nm (red) + 940 nm (IR) absorbance ratio gives O2 saturation. PULSATILE component → arterial. Normal SpO2 95-100. PITFALLS: low perfusion (vasoconstriction, hypothermia — unreliable reading); methemoglobin (reads ~85%); carboxyhemoglobin (reads ~100% even with severe CO toxicity — co-oximetry needed); fingernail polish (esp blue/black); IV dyes (methylene blue → false low); motion artifact.
Watch out
BIS limitations
BIS is NOT reliable with: ketamine (high BIS despite deep), N2O (no effect), dex (low EEG change), hypothermia (low BIS), pediatric <5 yr (unvalidated), neuro pathology, frontal EMG. Use as ADJUNCT, never replace ETCO2 + agent monitoring + clinical assessment.