The Anesthesia Machine & Ventilation Modes

Strip away the screens and the anesthesia machine is three systems bolted together:

1. Gas delivery — takes O₂, air, and N₂O (from pipeline ~50 psi or backup cylinders), lets you dial an exact fresh gas flow and FiO₂, then passes it through a vaporizer that adds a precise % of volatile agent.
2. The breathing (circle) system — a loop with one-way valves, a CO₂ absorber, and a ventilator (bellows or piston) that moves gas in and out of the patient and recirculates it to save agent.
3. Scavenging — captures exhaled waste anesthetic so it doesn't end up in the room.

Everything you touch on the machine is tuning one of those three jobs.

Two dials set the gas the patient breathes: total fresh gas flow (FGF) and the O₂ mixture (FiO₂).

• High flow (≥ 2 L/min or more): changes reach the patient fast. Use it at induction and emergence, when you're rapidly changing agent concentration, or to denitrogenate/preoxygenate. Wasteful of agent and O₂.
• Low flow (~0.5–1 L/min): once you're at a steady state, turn the flows down. Saves volatile agent and money, warms and humidifies the circuit, less pollution. The trade-off: changes happen slowly, and you must watch that FiO₂ doesn't drift down (the circuit consumes O₂).
• Minimum FiO₂: modern machines won't let you deliver a hypoxic mixture — there's a hard O₂/N₂O proportioning limit (≥ 21–25% O₂).
• O₂ flush (35–75 L/min) bypasses the vaporizer — pure O₂, no agent. Great to refill a bag fast; dangerous if you lean on it (dilutes your anesthetic → awareness, and can cause barotrauma if pressed during inspiration).

Every mode answers two questions: who triggers the breath (machine on a timer, or the patient), and what the machine controls (a set volume, or a set pressure).

The classic mode. You set a tidal volume and rate; the machine delivers exactly that volume each breath, and the pressure rises to whatever it takes to get there.

You set: tidal volume (~6–8 mL/kg ideal body weight), respiratory rate, I:E ratio, PEEP, and a pressure limit (Pmax) as a safety ceiling.

When to use it: - The default for most routine paralyzed cases. - When you want a guaranteed minute ventilation and stable CO₂. - Best when lung compliance is normal and stable.

Watch out: because volume is fixed, a drop in compliance (Trendelenburg, pneumoperitoneum, bronchospasm, light anesthesia/coughing) makes peak pressures climb — the machine will push until it hits Pmax and then alarms/dumps volume.

You set an inspiratory pressure; the machine holds that pressure for the inspiratory time, and the delivered volume varies with the patient's compliance and resistance. Flow is decelerating, which gives a lower peak pressure and often better gas distribution.

You set: inspiratory pressure (Pinsp, titrate to a target TV), rate, inspiratory time / I:E, PEEP.

When to use it: - Stiff lungs / high pressures — ARDS, morbid obesity, laparoscopy, one-lung ventilation, peds — where you want to cap pressure and protect the lung. - Leaky systems — uncuffed peds tubes or an LMA — where a pressure target tolerates the leak better than a volume target.

Watch out: the flip side of VCV — if compliance worsens, your tidal volume falls (and CO₂ rises) silently. You must watch the exhaled TV, not just the pressure.

The hybrid that's become many people's default. It looks and breathes like PCV (decelerating flow, a pressure plateau) but the machine automatically adjusts the pressure breath-to-breath to hit a tidal volume you set — using the lowest pressure that achieves it.

You set: target tidal volume, rate, I:E, PEEP, and a pressure ceiling.

When to use it: - A great all-rounder — you get the guaranteed volume of VCV and the lower, lung-protective pressures of PCV. - Cases where compliance changes during the case (laparoscopy up/down, positioning) — it tracks the changes for you.

Watch out: if there's a big leak the algorithm chases the volume by raising pressure (it can't tell leak from low compliance). And it needs a few breaths to settle after a sudden compliance change.

A mixed mode: the machine guarantees a set number of mandatory breaths (volume- or pressure-controlled) but synchronizes them to the patient's own efforts, and any extra breaths the patient takes are pressure-supported.

When to use it: - Weaning / transitioning a patient from full mechanical ventilation toward spontaneous breathing. - A patient who is starting to breathe over the ventilator but isn't ready to do all the work.

In modern OR practice SIMV has largely been superseded by PSV/PSVPro and volume-guaranteed modes, but you'll still see it, especially carried over from the ICU.

Pressure Support Ventilation (PSV) is fully patient-triggered: every time the patient starts to inhale, the machine delivers a set pressure boost above PEEP to help overcome the resistance of the tube/LMA and circuit, then cycles off to exhalation when inspiratory flow falls to a set percentage. The patient sets their own rate and largely their own volume — the machine just assists.

PSVPro (GE's "Pressure Support Ventilation Pro") is PSV plus an automatic apnea backup. If the patient stops triggering for longer than a set apnea time, the machine automatically switches to mandatory (pressure-control) backup ventilation and alarms — then hands control back when the patient resumes breathing. That safety net is the whole point of the "Pro."

What you set in PSVPro: - Pressure support (above PEEP) — typically 5–15 cmH₂O; enough to give a comfortable tidal volume (start ~10 and titrate to TV/RR). - PEEP — usually 5 cmH₂O. - Trigger sensitivity — how hard the patient must "ask" for a breath (flow trigger, ~2 L/min). Too sensitive → auto-triggering; too dull → missed efforts / work of breathing. - ETS (Expiratory Trigger Sensitivity) — the % of peak inspiratory flow at which the breath cycles to exhalation (default ~25%). Lower ETS = longer inspiration. - Apnea backup — backup rate, pressure/volume, and the apnea time (e.g., ~20–30 s) before backup kicks in. Set this deliberately — it's your safety net.

When to use PSVPro: - Spontaneously breathing GA on an LMA — supports each breath, overcomes LMA/circuit resistance, keeps tidal volumes up without paralyzing. - Emergence / weaning — as the patient starts breathing at the end of the case, switch from a mandatory mode to PSVPro for a smooth, patient-paced wake-up with a backup if they're still apneic from opioids. - Any time you want the patient breathing spontaneously but with a guaranteed floor.

These settings cut across every mode and are how you tailor ventilation to the patient:

• PEEP (positive end-expiratory pressure) — keeps alveoli open at end-expiration. Default ~5 cmH₂O; raise it for obesity, laparoscopy, atelectasis, hypoxia. Too much → ↓ venous return / ↓ BP and overdistension.
• Recruitment maneuver — a brief sustained inflation (e.g., 30 cmH₂O for ~30 s, or stepwise) to re-open collapsed lung, then set adequate PEEP to keep it open. Watch the blood pressure — it transiently drops.
• I:E ratio — default 1:2. Prolong expiration (1:3–1:4) for obstructive/air-trapping patients (asthma, COPD) to avoid breath-stacking. Inverse ratio (>1:1) is a rare rescue.
• Inspiratory pause / plateau — holds the breath briefly; improves gas distribution and lets you read the plateau pressure (a truer measure of alveolar pressure than peak).
• Pmax / pressure limit — a safety ceiling; in VCV it caps how hard the machine pushes.
• Trigger sensitivity — how easily the patient initiates a breath in supported modes (see PSVPro above).
• Spirometry loops (pressure–volume, flow–volume) — watch them to spot leaks, bronchospasm, and changing compliance at a glance.

Newer machines (e.g., GE Aisys) add automation worth knowing:

• End-tidal Control (Et Control) — you set a target end-tidal O₂ and a target end-tidal agent (≈ MAC), and the machine automatically titrates fresh gas flow and the vaporizer to hit and hold them. It naturally drives you to efficient low-flow anesthesia without babysitting the dials.
• Electronic vaporizer (Aladin cassette) — agent delivery is digital and very precise, including at low flows.
• Fresh-gas decoupling / compensation — the delivered tidal volume stays accurate regardless of fresh gas flow (older machines added FGF to the TV).
• Auxiliary O₂ flowmeter / common gas outlet — a separate O₂ source for nasal cannula or to drive a circuit off the machine.
• Standby mode — parks the machine between cases without a full power-down.

The ventilator is constantly guarding a few failure modes. Know what each alarm means so you react instead of silencing:

• Apnea / disconnect (low airway pressure or lost EtCO₂) — circuit disconnected, leak, or the patient stopped breathing in a spontaneous mode. #1 thing to rule out fast.
• High airway pressure — kink, bronchospasm, mainstem, coughing/light, mucus plug, insufflation.
• Sustained / continuous high pressure — a stuck valve or obstructed scavenging; risk of barotrauma — relieve it.
• Low FiO₂ — hypoxic mixture or supply problem.
• Low / high minute volume / tidal volume — under- or over-ventilation; common first sign that compliance changed in a pressure mode.
• Subatmospheric / negative pressure — patient inspiring against an empty circuit or active scavenging.

An alarm is information, not a nuisance. Find the cause before you reach for the silence button — and if you can't, hand-ventilate to reestablish a known, safe state.

There's rarely one 'right' mode — pick the one whose guaranteed variable matches what you care about most (a volume, or a pressure ceiling), then fine-tune with the knobs.