Pulse Oximeter 

1. Principle of Pulse Oximetry

Pulse oximetry is based on two core principles:

A. Spectrophotometry

• Uses differential absorption of light by:
• Oxyhemoglobin (HbO₂) → absorbs more infrared light (940 nm)
• Deoxyhemoglobin (Hb) → absorbs more red light (660 nm)

B. Beer–Lambert Law

Relates light absorption to concentration of Hb.

C. Pulsatile Flow Detection (Photoplethysmography)

• Differentiates:
• AC component → arterial pulsatile blood
• DC component → venous blood, tissue, bone

 Only arterial component used → gives SpO₂

2. What Does Pulse Oximeter Measure?

• SpO₂ (Peripheral oxygen saturation)
  → % of Hb bound to oxygen
• Pulse rate

3. Components of Pulse Oximeter

A. Light Source

• Two LEDs:
• Red (660 nm)
• Infrared (940 nm)

B. Photodetector

• Detects transmitted/reflected light

C. Microprocessor

• Calculates ratio → SpO₂

D. Display Unit

• Shows SpO₂, pulse rate, waveform

4. Types of Pulse Oximeters

A. Based on Technique

1. Transmission Type
• Probe on opposite sides (finger, ear)
• Most common

2. Reflectance Type

• Same side emitter + detector
• Used in:
• Forehead
• Neonates

B. Based on Clinical Use

• Fingertip (portable)
• Bedside ICU monitor
• Handheld
• Wearable

5. Oxygen Dissociation Curve & SpO₂ Interpretation

Flat upper part (>90%)

• Large PaO₂ change → minimal SpO₂ change

Steep part (<90%)

• Small drop in PaO₂ → large fall in SpO₂ 

6. Accuracy & Limitations 

Normal Accuracy

• Reliable between 70–100%
• Error ±2%

Factors Causing False Readings

A. False LOW SpO₂

• Motion artifact
• Poor perfusion (shock, hypothermia)
• Nail polish (dark colors)
• Ambient light interference
• Venous pulsations

B. False NORMAL / HIGH SpO₂ (DANGEROUS)

1. Carbon monoxide poisoning
• COHb absorbs like HbO₂
• → SpO₂ falsely normal
• Actual hypoxia present

2. Methemoglobinemia

• Fixed reading ~85% regardless of true saturation

C. Other Limitations

• Severe anemia → unreliable oxygen delivery assessment
• Dyshemoglobinemias not detected
• Cannot measure:
• PaO₂
• CO₂
• Acid-base status

7. Plethysmographic Waveform 

Waveform Components

• Reflects arterial pulsations
• Can assess:
• Perfusion
• Volume status (dynamic variation)

Pleth Variability Index (PVI)

• Indicates fluid responsiveness
• Useful in:
• Mechanically ventilated patients

8. Advanced Pulse Oximetry

A. Co-oximetry

• Measures:
• HbO₂
• Hb
• COHb
• MetHb

B. Masimo SET Technology

• Reduces motion artifact
• Better accuracy in ICU

9. ICU Pitfalls 

SpO₂ ≠ Oxygenation always

• Example:
• SpO₂ 100% but severe anemia → poor oxygen delivery

SpO₂ lag behind PaO₂

• Especially in rapid deterioration

Never rely in shock alone

• Use:
• ABG
• Lactate
• Clinical context

10. SpO₂ vs SaO₂ vs PaO₂ 

11. Pleth Variability Index (PVI) 

Definition

• PVI = Dynamic variation in pleth amplitude during respiratory cycle

PVI=PImax (PImax −PImin ) ×100

Where:

• PI = Perfusion Index (pulse strength)

A. Perfusion Assessment

B. Respiratory Variation 

In mechanically ventilated patients:

• Inspiration → ↓ venous return → ↓ stroke volume → ↓ pleth amplitude
• Expiration → ↑ venous return → ↑ amplitude

 This cyclic variation = dynamic preload indicator

 Physiological Basis of PVI

Based on heart–lung interaction

During positive pressure ventilation:

• Inspiration:
• ↑ intrathoracic pressure
• ↓ venous return
• ↓ stroke volume
• Expiration:
• Opposite effect

➡️ Greater variation = preload dependent patient

Interpretation of PVI 

Conditions for Accurate PVI 

Must have:

• Controlled mechanical ventilation
• Regular rhythm (no AF)
• Tidal volume ≥ 8 ml/kg
• No spontaneous breathing
• Adequate perfusion

Perfusion Index (PI) 

Perfusion Index (PI) is a numerical value derived from the pulse oximeter that reflects the strength of peripheral perfusion.

 It represents the ratio of:

• Pulsatile blood flow (arterial)
  to
• Non-pulsatile components (venous + tissue)

• High PI → large pulsations → good perfusion
• Low PI → small pulsations → poor perfusion

Normal Values of PI

Wide variability depending on:

• Site (finger vs ear)
• Temperature
• Patient condition

Factors Affecting PI

A. Physiological Factors

Increase PI

• Vasodilation (sepsis early phase, anesthesia)
• Warm environment
• Good cardiac output

Decrease PI

• Shock (hypovolemic, cardiogenic)
• Vasoconstriction (cold, vasopressors)
• Hypothermia

B. Technical Factors

• Probe position
• Motion artifact
• External pressure on probe

Clinical Applications of PI 

A. Assessment of Peripheral Perfusion

• Tissue perfusion
• Shock severity

Example:

• Septic shock:
• Early → high PI (vasodilation)
• Late → low PI (vasoconstriction)

B. Early Shock Detection

• PI falls before BP drops 

Useful in:Trauma,ICU monitoring

C. Monitoring Response to Therapy

• Fluid resuscitation → ↑ PI
• Vasopressors → ↓ PI (due to vasoconstriction)

D. Regional Anesthesia 

Successful block → ↑ PI

Mechanism:

• Sympathetic blockade → vasodilation → ↑ perfusion

Used in:

• Spinal anesthesia
• Peripheral nerve block success

• PI = amplitude
• PVI = variation of amplitude

Limitations of PI 

A. Site-specific

• Reflects only local perfusion, not global

B. Affected by External Factors

• Temperature
• Vasopressors
• Probe issues

C. Not a Direct Measure of Cardiac Output

• Cannot replace advanced monitoring

ICU Pitfalls

Normal PI ≠ Adequate perfusion globally

• Example:
• Septic shock (vasodilated) → high PI but poor perfusion

Low PI may be due to vasoconstriction, not hypovolemia

• Example:
• Noradrenaline use

Advanced Concept: PI & Microcirculation

• PI indirectly reflects microvascular blood flow
• Correlates with:
• Peripheral perfusion index (clinical)
• Tissue oxygenation