Standard Monitors

Standard I — Qualified anesthesia personnel shall be present in the room throughout the conduct of all general anesthetics, regional anesthetics, and monitored anesthesia care.

Standard II — During all anesthetics, the patient's oxygenation, ventilation, circulation, and temperature shall be continually evaluated.

• Oxygenation: Anesthesia machine → inspired FiO₂ analyzer + low-O₂ alarm. All anesthetics → pulse oximetry with variable-pitch tone.
• Ventilation: Capnography with expired tidal volume. Disconnect alarm required if mechanically ventilated.
• Circulation: EKG (minimum 3-lead, 5-lead if cardiac concern); BP cycle q5 min minimum; at least one additional continual assessment (pulse-ox tracing, A-line tracing, palpable pulse, auscultation, doppler).
• Temperature: Probe if clinically significant changes are anticipated.

Continual vs continuous (ITE): Continual = repeated regularly and frequently in steady rapid succession (e.g., BP q5 min). Continuous = prolonged without any interruption (e.g., EKG display, disconnect alarm during mechanical ventilation).

• SaO₂ (fractional) = O₂Hb / (O₂Hb + Hb + MetHb + COHb)
• SpO₂ (functional, what the probe reads) = O₂Hb / (O₂Hb + Hb)

The probe emits at 660 nm (red, for Hb) and 940 nm (infrared, for O₂Hb). Photoplethysmography isolates arterial pulsatile flow (AC) from non-pulsatile background (DC) — the patient is their own control.

The R ratio (AC/DC at 660 ÷ AC/DC at 940) maps to SpO₂. A 1:1 ratio = SpO₂ 85%, which is why a disconnected probe reads ~85%.

Methemoglobin (MetHb) — Absorbs equally at 660 and 940 nm. Pulls SpO₂ toward 85%. - If true SpO₂ > 85% → reads falsely LOW - If true SpO₂ < 85% → reads falsely HIGH - Causes: prilocaine, benzocaine, dapsone, metoclopramide, nitric oxide, nitroglycerin - Treatment: methylene blue (vitamin C in G6PD deficiency)

Carboxyhemoglobin (COHb) — Absorbs similarly to O₂Hb. 50% COHb → SpO₂ ~95% despite low SaO₂. Falsely HIGH reading. - Causes: smoke inhalation, desiccated CO₂ absorbent, volatile degradation - Treatment: 100% FiO₂, hyperbaric O₂

Cyanide toxicity — Cyanosis with HIGH SpO₂. Uncoupling of oxidative phosphorylation → high lactate, similar PO₂ on ABG vs. VBG. - Causes: sodium nitroprusside, smoke inhalation - Treatment: hydroxocobalamin

Other falsely LOW SpO₂: dyes (methylene blue > indocyanine > indigo carmine), blue nail polish, motion, ambient light, low perfusion (cold, anemic, high SVR). No effect on SpO₂: bilirubin, HbF, HbS, acrylic nails, fluorescein. Cyanosis clinically apparent at 5 g/dL desaturated Hb (~SpO₂ < 85%).

Both the number and tracing provide physiologic information: - Bronchospasm → upsloping (shark-fin) trace - Rebreathing / exhausted CO₂ absorber → elevated baseline - Esophageal intubation → no sustained CO₂ trace (may see brief washout from gastric CO₂) - Cardiac oscillations → ripples on the plateau - Spontaneous breaths during mechanical ventilation → curare cleft / notch

EtCO₂ vs PaCO₂: EtCO₂ is typically 2–5 mmHg lower than PaCO₂. Gradient widens with V/Q mismatch, dead space, low CO, PE.

3-lead system — Monitors I, II, or III (one at a time). Lead II is best for P waves and sinus rhythm.

5-lead system — Four limb leads + V5 at left anterior axillary line, 5th ICS. - V5 alone: ~75% sensitive for ischemia - II + V5: ~80% - II + V4 + V5: ~98%

Modified 3-lead: if anterior ischemia is the concern, move L arm lead to V5 position and monitor lead I.

NIBP (oscillometric) — MAP is the most accurate value (largest oscillation). SBP and DBP are proprietary algorithms. Inaccurate in atrial fibrillation, severe PVD, very calcified vessels. Affected by external pressure on the cuff (surgeon leaning on arm).

Cuff sizing: width should be ~40% of arm circumference. Cuff too small → falsely HIGH reading. Cuff too large → falsely LOW.

Invasive arterial line — Gold standard for beat-to-beat BP. Most accurate when zeroed, leveled, and properly damped. Allows ABG sampling and dynamic indices (pulse-pressure variation).

• Depth of anesthesia: BIS, Sedline (processed EEG); target 40–60 for general anesthesia.
• Neuromuscular block: train-of-four at adductor pollicis, ulnar nerve.
• Temperature: core sites (esophageal, nasopharyngeal, tympanic, bladder, PA catheter).
• Cerebral oximetry (NIRS): regional cerebral O₂ saturation; useful in cardiac, neuro, beach-chair shoulder cases.
• Precordial / esophageal stethoscope: classic teaching tool, still useful in peds.