Principles of Physiological Measurement

Basic Concepts & Errors - Measure Matters

• Units: Standardized measures (e.g., SI units).
• Accuracy: Closeness to true value.
• Precision: Reproducibility of measurements.
• Resolution: Smallest detectable change.
• Sensitivity: $\frac{\Delta Output}{\Delta Input}$; ratio of change in output to change in input.
• Linearity: Output directly proportional to input.
• Hysteresis: Output depends on input's previous value/direction.

Transducers & Electrodes - Signal Starters

• Transducers: Convert physiological signals (non-electrical) into measurable electrical signals.
  • Types & Principles:

    Transducer Type	Principle	Example Application(s)
    Resistive	Change in resistance	Strain gauge (pressure), Thermistor (temp)
    Capacitive	Change in capacitance	Pressure, Displacement
    Inductive (LVDT)	Change in inductance	Displacement, Flow
    Piezoelectric	Mechanical stress → voltage	Ultrasound, Arterial pulse
    Photoelectric	Light intensity → current/voltage	Pulse oximeter (SpO2)

• Electrodes: Interface between body & recording device to pick up biopotentials (ECG, EEG, EMG).
  • Common Type: Silver-Silver Chloride (Ag/AgCl) - widely used for surface recording.
    • Properties: Non-polarisable, low noise, stable.
  • Other Types: Needle (EMG), Microelectrodes (intracellular).
  • Ideal properties: Low impedance, biocompatibility.

⭐ Ag/AgCl electrodes are preferred for biopotential measurements as they are non-polarisable, meaning they resist changes in potential due to current flow, ensuring stable recordings.

Signal Processing & Recording - Wave Wranglers

• Objective: Convert raw physiological signals to usable data.
• Signal Chain:

• Amplification:
  • Boosts low-amplitude biopotentials (µV-mV).
  • Uses differential amplifier (high gain & input Z, low output Z).
  • Gain: $A = V_{out} / V_{in}$.
  • CMRR: $CMRR = A_d / A_c$. Higher is better.

  ⭐ High Common Mode Rejection Ratio (CMRR) is crucial in bio-amplifiers for rejecting common-mode noise (e.g., power line interference), ensuring signal clarity.

• Filtering: Removes noise/artifacts.
  • Low-pass: Below cutoff (ECG: < 150 Hz).
  • High-pass: Above cutoff (ECG: > 0.05 Hz; EEG: > 0.5 Hz).
  • Band-pass: Specific range (EMG: 10-1000 Hz).
  • Notch: Specific freq. (50/60 Hz line noise).
• Analog-to-Digital Conversion (ADC):
  • Sampling (Nyquist: rate > $2 \times f_{max}$).
  • Quantization (amplitude levels).
• Artifacts: Movement, EMG, line noise. Mitigate: Shielding (metal enclosures), proper grounding, patient stillness.

Data Interpretation & Validity - Trust The Test

• Accuracy: Overall correctness. Formula: $ (TP+TN) / (TP+FP+FN+TN) $.
• Precision (Reliability): Consistency/reproducibility of results on repeat testing.

Key diagnostic test metrics (TP=True Positive, FP=False Positive, FN=False Negative, TN=True Negative):

Predictive values are calculated using:

High‑Yield Points - ⚡ Biggest Takeaways

• Validity reflects accuracy (true value); Reliability reflects precision (reproducibility).
• Sensitivity (True Positive Rate) detects disease; Specificity (True Negative Rate) confirms absence.
• PPV & NPV are critically dependent on disease prevalence.
• Bias (systematic error) impacts accuracy; random error impacts precision.
• Blinding (e.g., double-blind) is crucial to reduce observer bias.
• Instrument calibration is essential for maintaining measurement accuracy.
• Standard Deviation (SD) measures data spread; SEM indicates mean's precision.