Anesthesia Basic Concepts

Anesthesia comes in three broad flavors, mixed and matched to the patient and surgery:

• Sedation — medication to reduce anxiety and awareness along a continuum. The ASA depth levels:
• Minimal (anxiolysis) — normal response to voice.
• Moderate ("conscious") — purposeful response to voice/touch.
• Deep — purposeful response only to repeated or painful stimulus.
• General anesthesia — unarousable even to pain.
• General anesthesia (GA) — render the patient insensible to surgical stimulus.
• Regional anesthesia (RA) — local anesthetic placed near nerves to numb a region:
• Neuraxial (spinal, epidural) — a large region.
• Peripheral nerve block — a single nerve / extremity.

Sedation is a continuum, not a fixed state — a "moderate" sedation can drift into a general, so you must be ready to rescue the airway at any depth.

A general anesthetic is assembled from several goals, each handled by different drugs:

1. Anxiolysis — often a pre-op benzodiazepine.
2. Amnesia — no recall of the event.
3. Hypnosis / unconsciousness — the sedation continuum, taken all the way down.
4. Analgesia — pain control (opioids, regional, multimodal).
5. Immobility / muscle relaxation — paralysis when the surgery or airway requires it; plus blunting of the autonomic (sympathetic) response to stimulation.

You rarely get all of these from one drug — a balanced anesthetic combines a hypnotic, an analgesic, and (often) a paralytic.

First described with ether, the classic stages still map onto what you see:

• Mask (inhalational) induction walks the patient through stage 1 → 2 → 3 (you want stage 2 brief).
• IV induction is so fast you don't really see stage 2 — awake → stage 3.
• Emergence runs the stages in reverse. Stage 2 on the way out is dangerous: the patient may move and look awake but is still anesthetized — extubating or stimulating here risks laryngospasm and aspiration.

Minimum Alveolar Concentration (MAC) is the common yardstick for how much volatile a patient is getting. Four things to know:

1. It's a continuum. 1 MAC = the concentration at which 50% of patients don't move to a skin incision; 1.3 MAC covers ~95–99%. MAC-awake ≈ 0.3 MAC; amnesia is lost well below 1 MAC.
2. It's additive. 0.5 MAC of nitrous + 0.5 MAC of a volatile ≈ 1 MAC total.
3. It shifts with the patient. MAC values are for healthy 40-year-olds. Lowered by age, opioids, propofol, acute alcohol, hypothermia, pregnancy. Raised by chronic alcohol, stimulants (cocaine, amphetamines, ephedrine), and youth (peaks ~6 months old).
4. The monitor shows it. Modern machines display a calculated MAC from the gases in use — a helpful training guide for how much you're delivering.

Anesthesia is often compared to flying a plane — both are very safe, and both have their danger at takeoff and landing. The perioperative course:

• Pre-op (boarding): the H&P, optimizing the patient, IV/lines, anxiolysis, and building the plan (plus backup plans).
• Induction (takeoff): induce, secure the airway, position, add monitoring.
• Maintenance (cruise): keep them comfortable and still with vapor/analgesia/relaxation; stay vigilant.
• Emergence (landing): titrate off the anesthetic, reverse paralysis, remove the airway when safe, off to recovery.
• Post-op (to the terminal): PACU or ICU — vitals, pain, and PONV managed until recovered.

Most problems happen going to sleep and waking up — so that's where your attention and preparation concentrate.