Standard Monitoring: ECG, BP, Pulse Oximetry

ECG Monitoring - Heart's Electric Symphony

Continuously monitors cardiac electrical activity for arrhythmias & myocardial ischemia.

• Purpose:
  • Rate & rhythm assessment.
  • Detect myocardial ischemia (ST segment changes).
  • Identify electrolyte imbalances (e.g., K⁺).
  • Monitor pacemaker function.
• Lead Systems:
  • 3-Lead: Standard limb leads (I, II, III). Basic rhythm monitoring.
  • 5-Lead: Adds a precordial lead (usually V5). Better ischemia detection.
    • Lead II: Best for P-wave morphology & arrhythmia diagnosis.
    • Lead V5: Sensitive for anterior/lateral ischemia (LAD/LCx territory).
• Key Intraoperative Changes:
  • Ischemia:
    • ST Depression: >1mm (subendocardial ischemia).
    • ST Elevation: >1mm (limb), >2mm (precordial) (transmural injury).
  • Arrhythmias: Bradycardia, tachycardia, AF, VT, ectopics.
  • Hyperkalemia: Peaked T waves, wide QRS.

⭐ Lead II is optimal for rhythm monitoring and P-wave detection; V5 is superior for detecting intraoperative myocardial ischemia.

BP Monitoring - Pressure Under Watch

• Goal: Maintain tissue perfusion; guide hemodynamics.
• Non-Invasive BP (NIBP)
  • Method: Oscillometry.
  • Cuff size: Width 40%, bladder length 80% arm circumference.
    • Too small → falsely ↑BP; Too large → falsely ↓BP.
  • Placement: Arm at heart level.
• Invasive BP (IBP) / Arterial Line
  • "Gold Standard": Continuous beat-to-beat monitoring.
  • Indications: Major surgery, hemodynamic instability, vasoactive drug infusions.
  • Sites: Radial (check Allen's test), femoral, brachial.
  • Transducer: Level at phlebostatic axis (4th ICS, mid-axillary line).
    • Transducer high → falsely ↓BP; Transducer low → falsely ↑BP.
  • Waveform: Dicrotic notch (aortic valve closure).
• Calculated Pressures
  • Mean Arterial Pressure (MAP): $MAP \approx DP + 1/3(SP-DP)$. Target >65 mmHg.
  • Pulse Pressure (PP): $PP = SP - DP$. Indicates stroke volume.

⭐ In IBP, overdamping (e.g., air bubbles, clots, kinks) causes underestimated Systolic BP, overestimated Diastolic BP, but MAP is least affected.

Pulse Oximetry - Oxygen's Window

• Measures SpO2 (peripheral O2 saturation) & pulse rate. Non-invasive, continuous.

• Principle: Spectrophotometry (Beer-Lambert law).

  • Differentiates HbO2 & Hb by absorption of two light wavelengths:
    • Red light (660 nm): Absorbed by deoxy-Hb (📌 RED D: RED light for Deoxy Hb).
    • Infrared light (940 nm): Absorbed by oxy-Hb (📌 I O: Infrared for Oxy Hb).

• Normal SpO2: 95-100%. Critical: < 90%.

• Placement: Finger, toe, earlobe. Neonates: foot/palm.

• Limitations & Inaccuracies:

  • Motion, poor perfusion (shock, vasoconstriction, hypothermia).
  • Ambient light, dark nail polish (blue, black, green).
  • IV dyes (e.g., methylene blue: transiently falsely ↓ SpO2).
  • Dyshemoglobinemias:
    • Carboxyhemoglobin (COHb): Falsely ↑ SpO2 (COHb mimics HbO2 at 660 nm).
    • Methemoglobin (MetHb): SpO2 tends towards 85% regardless of true SaO2.

⭐ In carbon monoxide poisoning, pulse oximetry is unreliable, showing falsely high SpO2 because COHb absorbs light at 660 nm similarly to oxyhemoglobin (HbO2).

High‑Yield Points - ⚡ Biggest Takeaways

• ECG: Lead II best for arrhythmia detection; V5 for myocardial ischemia.
• NIBP cuff: width 40% of arm circumference, bladder length 80%.
• Pulse oximetry (SpO2): measures O2 saturation via Beer-Lambert law.
• Carboxyhemoglobin falsely ↑ SpO2; Methemoglobin falsely ↓ SpO2 towards 85%.
• MAP = DBP + 1/3 (SBP-DBP); critical for assessing organ perfusion.
• Key SpO2 inaccuracies: motion, poor perfusion, IV dyes (e.g., methylene blue), dark nail polish.
• Oscillometric NIBP directly measures MAP; SBP and DBP are calculated.