Machine Check & Safety Systems
Bailey Freeman, DNP, CRNA and Angela Mordecai, DNP, CRNA
What Are Anesthesia Machine Safety Systems?
Anesthesia machines are equipped with multiple built-in safety systems designed to protect the patient from gas delivery errors, hypoxia, and equipment malfunction. These include:
- Fail-safe valves to stop or limit the flow of non-oxygen gases when O₂ pressure is lost.
- Hypoxic guards to maintain a minimum oxygen concentration in the gas mixture.
- CO₂ absorbers to remove carbon dioxide from exhaled gases in rebreathing systems.
- Oxygen alarms to alert the provider to falling pipeline or cylinder pressure.
How Do They Work?
Fail-Safe Systems
- Fail-Safe Valve: Shuts off or reduces the flow of nitrous oxide and other gases if oxygen pressure drops below ~30 psi. Why? Loss of O₂ pressure is a common failure mode; limiting other gases helps prevent delivering a hypoxic mixture.
- Hypoxic Guard: Mechanically or electronically prevents oxygen concentration from falling below ~25% in the delivered mixture. Why? Provides a “floor” for FiO₂, but does not replace continuous FiO₂ monitoring.
CO₂ Absorber
- Uses a chemical absorbent (e.g., soda lime) to eliminate CO₂ from exhaled gas in circle systems. Why? Prevents CO₂ rebreathing and hypercarbia when using low flows or closed circuits.
- Monitor for exhaustion via color change or rising FiCO₂ / ETCO₂ levels. Why? Color can revert at rest; physiologic monitoring confirms absorber performance.
- Replace the absorber when color change appears or based on facility protocol. Why? Delayed replacement risks hypercapnia and acidosis.
Oxygen Alarms
- Alert the user when O₂ supply pressure drops below a safe threshold (typically ~30 psi). Why? Early warning allows time to switch to backup cylinder.
- Must be audible, functional, and tested daily before use. Why? Muted or failed alarms remove a critical safety net.
- O₂ analyzers monitor inspired oxygen concentration continuously throughout the case. Why? Pipeline pressure ≠ inspired O₂; measure what the patient actually receives.
How Do I Check Them?
Pre-Use Checklist (First Case of the Day)
- Turn power on; verify machine is plugged into a red outlet (emergency power). Why? Ensures function during a power failure.
- O₂ analyzer two-point calibration:
- Expose sensor to room air for ~3 minutes; calibrate to 21%. Why? Zeroes the sensor at ambient FiO₂.
- Reinsert in machine to 100% O₂ exposure for ~3 minutes; calibrate to 100%. Why? Confirms accurate high-end reading.
- Backup O₂ cylinder: Crack open each tank for ~2 seconds, then close; check pressure (full ≈ 1900 psi; replace ≤ 500 psi). Leave the wrench on the O₂ tank. Why? Confirms a ready backup and prevents hidden empty tanks.
- Verify pipeline pressures are 45–55 psi for all gases. Why? Too low risks hypoxia; too high can damage regulators.
- Breathing circuit positive pressure leak test: APL closed (≈70 cmH₂O), occlude patient end, pressurize to 30–40 cmH₂O with O₂ flush, hold 10 s without drop; squeeze bag to stress test; cycle each vaporizer on/off (no pressure drop); then open APL to 0 to confirm patency; finally squeeze bag empty and watch scavenger bag inflate/deflate. Why? Detects circuit and vaporizer leaks; verifies APL and scavenging function.
- Ventilator check: Attach reservoir bag to distal circuit as a test lung; APL open; FGF 6–10 L/min; switch to ventilator mode; confirm bellows descend on inspiration and fully ascend on expiration; confirm test lung inflates/deflates and inspiratory/expiratory flutter valves lift; return to manual; restore mask on circuit; FGF off; set your patient‑specific vent settings; pause apnea alarm until induction. Why? Ensures safe mechanical ventilation before connecting to a patient.
- Verify CO₂ absorber is white (not purple). Replace if purple. Why? Purple indicates exhaustion under load.
- Confirm vaporizer fill level and secure mounting; top up as needed. Why? Prevents anesthetic interruption and leaks.
- Monitors present and functional (ECG + electrodes, pulse oximeter, NIBP with multiple sizes, ETCO₂, temperature, nerve stimulator as needed); set audible alarms to appropriate limits. Why? Alarms without sensors—or sensors without alarms—don’t protect patients.
Quick Start: SAMMTIDE
Use SAMMTIDE as your fast, full‑room setup mnemonic before every case. Click each item below for the why and how.
SUCTION
- Turn on wall suction to “Line”; verify strong suction at the head of bed. Why? Immediate airway clearance during induction or laryngospasm.
- Yankauer + tubing reach the head of bed. Why? Short or kinked tubing delays suction when seconds matter.
AIRWAY
- Laryngoscopes: Two different blades and working handles (e.g., Miller and Mac); check lights are steady/bright. Why? Backup size/type improves first‑pass success.
- ETT x2: Appropriate sizes (adult male ~8.0 mm ID; adult female ~7.0 mm ID). Stylet in each; tip not beyond Murphy eye. Shape into a “smiley face”; check cuff integrity with ~10 mL air then deflate. Why? Pre‑shaped, leak‑tested tubes streamline intubation.
- Adjuncts: Two oral airway sizes; tongue blade ×1; verify bag‑mask (ambu), LMAs, and an intubating bougie are present. Why? Plan A/B/C reduces hypoxemia risk.
MONITOR
- Monitors on, alarms audible; set limits for patient and procedure. Why? Human factors: alarms save attention when workload is high.
- Equipment present: ECG + electrodes, pulse oximeter, BP cuff (more than one size), ETCO₂, temperature probe, nerve stimulator. Why? Standard ASA monitoring + case‑specific needs.
MACHINE
Do the full pre‑use check above for the first case; for subsequent cases perform an abbreviated check:
- New circuit leak test; confirm absorber color; verify vaporizer levels; verify vent settings match the next patient. Why? Turnover changes introduce new failure points.
TAPE / TABLE
- Tape for eyes, ETT, and devices. Why? Prevents corneal abrasion and dislodgement.
- Know table controls; confirm “low & locked.” Why? Fall prevention and efficient positioning.
IV SUPPLIES
Not the patient’s current IV—keep a backup set ready.
- IV start kit: catheter(s), tape, gauze; IV fluids; primary/secondary/blood tubing. Why? Immediate access if the existing line infiltrates or fails.
- Arterial line kit + transducer; central line kit + transducer if appropriate. Why? Avoids intra‑case delays for invasive monitoring.
DRUGS
- Aseptic discipline: One needle, one syringe, one use. Label immediately after draw‑up (drug, concentration, date/time, initials). Why? Medication safety and traceability.
- Triple‑check: When drawing up; before leaving the OR; before administering. Why? Independent checks reduce look‑alike/sound‑alike errors.
- Secure meds before leaving the OR. Why? Prevents diversion or tampering.
- Basic induction kit (titrate to patient):
- Midazolam 1–2 mg IV (anxiolysis).
- Fentanyl 1–2 mcg/kg IV (analgesia).
- Lidocaine 1–1.5 mg/kg IV (max 100 mg) to blunt laryngoscopy response and reduce propofol “sting.”
- Propofol ~2 mg/kg IV (adjust for comorbidities/hemodynamics).
- Rocuronium 0.6 mg/kg IV (NDMR intubating dose).
- Succinylcholine 1.5 mg/kg IV if needed (depolarizing intubating dose).
Why? A prepared, labeled, right‑sized set of meds supports smooth, timely induction and safer rescue if plans change.
EVERYTHING ELSE
- Mask straps; forced‑air warmer + blanket; fluid warmer. Why? Normothermia and normovolemia improve outcomes.
- NG/OG tube; stethoscope; IV poles + drape clamps; positioning devices. Why? Anticipate surgical needs and protect pressure points.
What Should I Document?
- Completion of machine checks (full or abbreviated), including O₂ analyzer calibration and leak tests.
- Backup O₂ cylinder pressure; pipeline pressures; absorber status; vaporizer agent/type, level, and secure mounting.
- Monitor setup and alarm parameters; any malfunctions and corrective actions taken.
- Drug preparation and labeling per policy (as applicable to your record system).
Scope Guide
Strategies
- Build the pre‑use checklist into your daily rhythm—pair it with SAMMTIDE to cover the room, the patient, and the machine end‑to‑end.
- Practice blade/ETT selection on mannequins; pre‑shape ETTs and rehearse bougie use to improve first‑pass success.
- Keep a standardized “induction tray” layout to reduce cognitive load under stress.
Clinical Optimization
- Continuously monitor FiO₂ and ETCO₂; rising ETCO₂ with normal ventilation suggests absorber exhaustion or circuit issues.
- For long/low‑flow cases, have spare CO₂ canisters ready; schedule mid‑case checks.
- Set alarm limits deliberately (not defaults) based on patient age, pathology, and surgical plan.
Pearls
- Never silence an alarm without troubleshooting the cause immediately.
- Fail‑safe and hypoxic guards reduce risk but don’t guarantee safe FiO₂—your O₂ analyzer is the truth source.
- Do the same steps in the same order every time; reliability comes from routine.
References
- StatPearls. Anesthesia Machine Pre-use Check. NCBI Bookshelf. https://www.ncbi.nlm.nih.gov/books/NBK554538/
- OpenAnesthesia. CO₂ Absorbers, Fail-Safe Devices, and O₂ Alarms. https://www.openanesthesia.org/
- How Equipment Works. Safety Systems in Anesthesia Machines. https://www.howequipmentworks.com/
