Monitoring & normal values
Connect every monitor to a question about the patient — then check that the signal can actually answer it.
ASA standard monitors
ASA Standards for Basic Anesthetic Monitoring, amended October 15, 2025 — summarized; read the standard for exact scope and exceptions.
Oxygenation
Inspired O₂ analyzer with low-concentration alarm (general anesthesia with a machine). Pulse oximetry with variable-pitch tone and alarms.
Ventilation
Continuous quantitative ETCO₂ (capnography) per the standard; expired CO₂ identification to confirm airway device position; disconnect alarm on mechanical ventilation.
Circulation
Continuous ECG; BP and HR at least every 5 minutes; plus one of pulse palpation, auscultation, pleth, arterial trace or ultrasound.
Temperature
When clinically significant changes are intended, anticipated or suspected. Confirm the method (esophageal, nasopharyngeal, bladder, skin) with the team.
Plus: qualified anesthesia personnel present throughout. Read the ASA standard →
Typical intra-op targets
| Parameter | Typical target / norm | Notes |
|---|---|---|
| SpO₂ | ≥ 92–94% | Team-specific; waveform quality matters |
| ETCO₂ | 35–45 mmHg | PaCO₂–ETCO₂ gradient normally ≈ 2–5 mmHg |
| MAP | ≥ 65 mmHg or within ≈ 20% of baseline | Intra-op MAP < 65 is associated with AKI and myocardial injury |
| Tidal volume | 6–8 mL/kg predicted body weight | PBW calculator |
| PEEP | ≈ 5 cmH₂O to start | Individualized |
| Plateau / driving pressure | < 30 / < 15 cmH₂O | Lung-protective ventilation |
| Temperature | ≥ 36 °C | Hypothermia → bleeding, infection, delayed emergence |
| BIS (if used) | ≈ 40–60 | General anesthesia range |
| Urine output | ≈ 0.5 mL/kg/h | Context-dependent |
5-lead ECG placement
Figure: ECG Electrode Placement — Open Critical Care & World Health Organization, CC BY 4.0, via Wikimedia Commons. Converted to WebP.
Lead II + V5 is the classic intra-op combination: II for rhythm (P waves), V5 for ischemia.
Room-specific choices
- A cycling BP cuff can interrupt pulse oximetry on the same arm — discuss placement and signal quality with your resident (not an absolute rule).
- Arterial lines and processed EEG (BIS) depend on the case.
- Quantitative neuromuscular monitoring when a paralytic is used — see NMB monitoring.
The oxygen–hemoglobin curve
Figure: Oxygen-Haemoglobin dissociation curves — Peter Southwood, CC0 1.0, via Wikimedia Commons.
- SpO₂ 90% ≈ PaO₂ 60 mmHg — the edge of the cliff.
- Right shift (↓ affinity, unloads O₂): ↑CO₂, ↑H⁺ (acidosis), ↑temperature, ↑2,3-BPG.
- Left shift: the opposite, plus fetal hemoglobin and carbon monoxide.