Intraoperative Monitoring

Intraoperative Monitoring - ASA's Vigilant Five

• Oxygenation:
  • Inspired O2 analyzer (detects hypoxic mixture).
  • Pulse oximetry (SpO2 >92%; detects hypoxemia).
• Ventilation:
  • Capnography (ETCO2 35-45 mmHg): Confirms ETT, assesses ventilation.
  • Clinical signs: chest rise, breath sounds.
• Circulation:
  • ECG (continuous): Detects arrhythmias, ischemia.
  • Blood Pressure (NIBP/IBP): q3-5min; maintain MAP within 20% of baseline.
  • Peripheral pulse palpation/plethysmography.
• Temperature:
  • Core temperature monitoring (e.g., esophageal, nasopharyngeal).
  • Maintain normothermia (36.0-37.5°C).
• Neuromuscular Function (if NMBAs used):
  • Peripheral Nerve Stimulator (PNS): Assesses block depth.
  • Train-of-Four (TOF) count/ratio (target TOF ratio >0.9 for safe extubation).

⭐ Capnography (ETCO2) is the most reliable method to confirm endotracheal tube placement immediately after intubation and continuously monitor ventilation adequacy during general anesthesia an

Intraoperative Monitoring - Heart's Beat & Flow

• ECG: Standard 5-lead (II & V5 for ischemia). Detects arrhythmias, ischemia (ST >1mm), K+.
• Invasive Cardiovascular Monitoring:

Intraoperative Monitoring - Breath by Breath

• Capnography: Gold standard for ETT placement confirmation.
  • Measures $P_{ET}CO_2$; normal range 35-45 mmHg.
  • $P_{a}CO_2 - P_{ET}CO_2$ gradient: normally 2-5 mmHg; ↑ with ↑ dead space (e.g., PE, ↓ cardiac output).
  • Waveform: Phase I (baseline/dead space), II (expiratory upstroke), III (alveolar plateau), 0 (inspiratory downstroke).
  • 📌 Mnemonic: "No Trace, Wrong Place!" (for absent $P_{ET}CO_2$ after intubation).

  ⭐ Sudden ↓ $P_{ET}CO_2$ to 0 or near 0 indicates esophageal intubation, ventilator disconnect, or cardiac arrest.

• Ventilator Parameters & Spirometry:
  • Tidal Volume ($V_T$): Target 6-8 mL/kg Ideal Body Weight.
  • Plateau Pressure ($P_{plat}$): Keep <30 cmH2O (lung protective ventilation).
  • Driving Pressure ($P_{plat}$ - PEEP): Aim for <15 cmH2O.
  • Loops (Volume-Pressure, Flow-Volume): Diagnose obstruction (e.g., bronchospasm), leaks, compliance changes.

Intraoperative Monitoring - Nerve & Muscle Check

Nerve (Depth of Anesthesia):

• Bispectral Index (BIS):
  • Measures hypnotic effect. Range: 0-100.
  • Target for GA: 40-60.
  • < 40: Deep hypnosis; > 60: Risk of awareness.
  • 

Muscle (Neuromuscular Blockade):

• Peripheral Nerve Stimulator (PNS) assesses NMB.
• Stimulation Patterns:
  • Train-of-Four (TOF): T4/T1 ratio for recovery.
  • Post-Tetanic Count (PTC): Deep blocks (TOF=0).
  • Double Burst Stimulation (DBS): Detects residual block.
• Sites: 📌 Ulnar nerve (adductor pollicis), Facial nerve (orbicularis oculi).
• 
• TOF Ratio & Clinical Correlation:

  TOF Ratio (T4/T1)	Clinical Correlation
  > 0.9	Safe extubation
  0.7 - 0.9	Residual weakness; diplopia
  < 0.7	Significant blockade; ↓ resp. function
  0 (No T4)	Deep block

⭐ TOF ratio > 0.9 (adductor pollicis) crucial for safe extubation, prevents post-op respiratory complications.

High‑Yield Points - ⚡ Biggest Takeaways

• Standard ASA monitoring (ECG, NIBP, SpO2, Temperature, EtCO2) is universally mandatory.
• ECG leads II & V5 are vital for detecting arrhythmias & myocardial ischemia.
• Capnography (EtCO2): gold standard for ET tube placement & monitors ventilation adequacy.
• Pulse oximetry (SpO2) monitors oxygenation; <90% signals critical hypoxemia.
• Neuromuscular monitoring (TOF ratio) guides muscle relaxant administration and reversal.
• Core temperature monitoring prevents hypothermia & detects malignant hyperthermia.