Non Invasive Blood Pressure 

1. Definition 

Non-invasive blood pressure (NIBP) refers to the measurement of arterial blood pressure without arterial cannulation, typically using an oscillometric cuff-based device.

Why NIBP is critical in ICU

  • First-line hemodynamic monitoring in:
    • All ward and ICU patients (baseline)
    • Emergency/triage settings
  • Used for:
    • Trend monitoring
    • Initial shock assessment
    • Guiding fluids, vasopressors (when invasive BP unavailable)

2. Basic Principle (Oscillometric Method)

Mechanism

  1. Cuff inflates → occludes artery
  2. Gradual deflation → blood starts flowing
  3. Oscillations appear due to arterial pulsations

Key points

  • Maximum oscillation amplitude = Mean Arterial Pressure (MAP)
  • SBP & DBP are derived using algorithms (not directly measured)

3. Types of NIBP Methods

Method

Principle

Use

Oscillometric

Cuff oscillations

ICU standard

Auscultatory

Korotkoff sounds

Manual BP

Palpatory

Pulse disappearance

Emergency

Doppler-assisted

Flow detection

Low-flow states

4. Cuff Size & Placement 

Correct cuff size

  • Width = 40% of arm circumference
  • Length = 80% of arm circumference

Errors

Situation

Effect

Small cuff

False high BP

Large cuff

False low BP

Placement

  • Preferred: Upper arm(Measured over brachial artery)
  • Alternatives:
    • Forearm-SBP: ↑ slightly (≈ +5–10 mmHg) but MAP: Usually similar (most reliable)
    • Thigh (shock/trauma)-SBP: ↑ slightly (≈ +10–40 mmHg)  but MAP: Usually similar (most reliable)
  • Must be at heart level
    • Below heart → falsely high
    • Above heart → falsely low

Coarctation of Aorta

  • Arm BP > Leg BP (REVERSE of normal)

5. Accuracy & Limitations

Accuracy hierarchy

Invasive arterial BP > NIBP

Situations where NIBP is unreliable

  • Shock states (low perfusion)
  • Severe hypotension (MAP < 60 mmHg)
  • Arrhythmias (e.g., AF)
  • Severe vasoconstriction (e.g., high-dose norepinephrine)
  • Movement artifacts
  • Obesity / improper cuff size

6. ICU Interpretation

MAP is most reliable parameter

  • Used for perfusion assessment

MAP=SBP+2/3(DBP)

Why DBP is weighted more?

  • Heart spends ~2/3 time in diastole

7. Sources of Error 

Technical errors

  • Wrong cuff size
  • Loose cuff
  • Air leak in tubing
  • Calibration issues

Physiological errors

  • Arrhythmia → irregular oscillations
  • Tremors/shivering
  • Low cardiac output

8. Complications of NIBP

Usually safe, but in ICU:

  • Skin injury / pressure necrosis
  • Nerve compression (rare)
  • Petechiae/ecchymosis
  • Compartment syndrome (very rare, prolonged cycling)