BIOMEDICAL ELECTRONICS (EC-604 (B)) - Unit 1 Short Notes

UNIT 1: BIOMEDICAL INSTRUMENTATION AND MEASUREMENTS

1. Physiological Foundations

Cell Structure & Function:

• Basic Unit: The cell is the fundamental structural and functional unit of life.

• Key Organelles:

  • Nucleus: Contains genetic material (DNA).

  • Mitochondria: Powerhouse; site of aerobic respiration (ATP production).

  • Endoplasmic Reticulum (ER): Rough ER (protein synthesis), Smooth ER (lipid synthesis, detox).

  • Golgi Apparatus: Modifies, sorts, and packages proteins.

  • Lysosomes: Digestive enzymes for waste breakdown.

• Cell Membrane (Plasma Membrane):

  • Structure: Phospholipid bilayer with embedded proteins (fluid mosaic model).

  • Functions: Selective permeability, cell signaling, adhesion, maintains cell potential.

[!TIP] Exam often asks for a labeled diagram of a cell. Focus on the nucleus, mitochondria, and the phospholipid bilayer structure of the membrane.

Resting Membrane Potential (RMP) & Action Potential (AP):

• Resting Membrane Potential (~ -70 mV in neurons):

  • Cause: Unequal distribution of ions (high K⁺ inside, high Na⁺/Cl⁻ outside) and selective permeability (more K⁺ leak channels).

  • Key Ion: Potassium (K⁺) diffusion is the primary determinant.

  • Established by: Na⁺/K⁺ ATPase pump (3 Na⁺ out, 2 K⁺ in; electrogenic).

• Action Potential (Nerve Impulse):

  • Phases:

    1. Depolarization: Stimulus opens voltage-gated Na⁺ channels → Na⁺ influx → membrane potential becomes positive.

    2. Repolarization: Na⁺ channels inactivate; voltage-gated K⁺ channels open → K⁺ efflux → returns to negative.

    3. Hyperpolarization (Afterpotential): K⁺ channels close slowly → potential becomes more negative than RMP.

  • Propagation: Local current flow depolarizes adjacent membrane segments. Myelinated axons use saltatory conduction (jumps between Nodes of Ranvier).

  • All-or-None Law: Once threshold is reached, AP amplitude is constant; stimulus intensity is coded by frequency of APs.

Major Physiological Systems Overview:

• Cardiovascular: Heart (pump), blood vessels (conduits). Generates electrical (ECG) and mechanical (pulse, pressure) signals.

• Respiratory: Lungs (gas exchange), airways. Measures volume/flow (spirometry) and blood gases (PO₂, PCO₂).

• Nervous: Brain, spinal cord, nerves. Generates bioelectric signals (EEG, EMG, EOG).

2. Electrodes and Transducers (High-frequency topic)

Electrode Theory:

• Half-Cell Potential: Potential developed at the metal-electrolyte interface due to ion exchange. It is a source of DC drift in recordings.

• Polarization: Accumulation of reaction products at the electrode-electrolyte interface, increasing impedance and distorting signals. Non-polarizable electrodes (e.g., Ag/AgCl) minimize this.

• Impedance (Z): Total opposition to AC current flow (Z = R + jX). Low, stable impedance is crucial for good signal pickup.

• Stability: Resistance to changes in half-cell potential and impedance over time and with motion.

Electrode Selection Criteria:

1. Biocompatibility: Non-toxic, non-allergenic, non-irritating.

2. Low Noise & Polarization: Minimizes signal distortion.

3. Appropriate Frequency Response: Must pass the signal's frequency range (e.g., DC for pH, 0.05-100 Hz for ECG).

4. Low Motion Artifact: Insensitive to cable movement or electrode shift.

5. Ease of Use & Patient Comfort.

Classification of Electrodes:

Transducer Principles:

Specific Biomedical Transducers:

• Pulse Oximeter (Finger-tip):

  • Principle: Spectrophotometry + Photoplethysmography (PPG).

  • Method: Two LEDs (Red ~660nm, IR ~940nm) emit light through finger. Photodetector measures transmitted light.

  • SpO₂ Calculation: Uses Beer-Lambert Law. Ratio of pulsatile (AC) to non-pulsatile (DC) components at both wavelengths is correlated to arterial oxygen saturation via an empirical calibration curve.

  \boxed{SpO_2 = f\left(\frac{AC_{Red}/DC_{Red}}{AC_{IR}/DC_{IR}}\right)}

• Photo Spirometer:

  • Principle: Optical measurement of respiratory flow/volume.

  • Method: A light beam (often IR) is interrupted by a vane or fan in the airflow path. The frequency of interruption is proportional to flow rate. Integrating flow over time gives volume.

  • Advantages: No moving seals (unlike mechanical spirometers), low resistance, hygienic.

• Partial Pressure of Oxygen (PO₂) - Clark Electrode:

  • Principle: Polarography (amperometric).

  • Construction: Platinum (cathode) + Ag/AgCl (anode) in KCl electrolyte, covered by O₂-permeable Teflon membrane.

  • Working: O₂ diffuses through membrane → reduced at cathode (O₂ + 2H₂O + 4e⁻ → 4OH⁻). Current is proportional to PO₂ (at fixed voltage, -0.6V vs. Ag/AgCl).

  • Limitation: Consumes O₂; requires stirring (in blood) to avoid boundary layer effect.

• Temperature Sensors:

  • Thermocouple: Two dissimilar metals joined → Seebeck effect (ΔT → ΔV). Output is small (µV/°C), requires amplification. Good for point measurement.

  • RTD (Resistance Temperature Detector): Pure metal (Pt, Ni) → resistance increases linearly with T. High accuracy & stability. Requires current source.

  • Thermistor: Semiconductor (NTC: resistance ↓ with T↑). High sensitivity (non-linear), limited range.

3. Cardiovascular Measurements (High-frequency topic)

Electrocardiogram (ECG):

• Definition: Recording of the heart's electrical activity via surface electrodes.

• Waveform Components (Lead II typical):

  • P wave: Atrial depolarization.

  • QRS complex: Ventricular depolarization. Most amplitude.

  • T wave: Ventricular repolarization.

  • U wave (optional): Possible late ventricular repolarization or afterdepolarization.

• Lead Systems:

  • Bipolar (Einthoven): I, II, III (limb leads).

  • Unipolar (Goldberger): aVR, aVL, aVF (augmented limb leads).

  • Chest (Precordial): V1-V6.

• Key Intervals:

  • PR Interval: AV nodal conduction time (0.12-0.20 s).

  • QRS Duration: Ventricular depolarization (<0.12 s).

  • QT Interval: Total ventricular activity (rate-corrected: QTc).

Heart Sounds & Phonocardiography:

• S1 ("Lub"): AV valve closure. Start of systole.

• S2 ("Dub"): Semilunar valve closure. End of systole/start of diastole.

• S3: Ventricular gallop (rapid filling). Pathological in adults.

• S4: Atrial gallop (atrial kick). Always pathological.

• Murmurs: Turbulent blood flow (stenosis/regurgitation). Timing (systolic/diastolic) indicates affected valve.

Blood Pressure Measurement:

• Non-Invasive:

  • Auscultatory (Korotkoff): Cuff inflation → deflation. Sounds: Phase I (systolic), Phase V (diastolic). Gold standard but requires training.

  • Oscillometric: Cuff pressure oscillations detected. Algorithm determines systolic/diastolic from oscillation envelope. Used in automated monitors.

• Invasive (Direct Arterial):

  • Method: Catheter with fluid-filled system or catheter-tip pressure transducer inserted into artery (e.g., radial).

  • Advantage: Continuous real-time waveform, accurate for rapid changes.

Photoplethysmography (PPG):

• Principle: Detects blood volume changes in microvascular bed (e.g., finger, ear) using transmittance or reflectance of light (usually IR).

• Signal: AC component (pulsatile) superimposed on DC component (venous/arterial/tissue).

• Applications: Pulse rate, SpO₂ (as above), arterial stiffness, blood volume estimation.

4. Respiratory Measurements (High-frequency topic)

Spirometry:

• Definition: Measurement of lung volumes and flow rates during forced breathing maneuvers.

• Key Parameters:

  • Tidal Volume (TV): Normal breath volume.

  • Vital Capacity (VC): Max volume exhaled after max inhalation.

  • Forced Vital Capacity (FVC): VC exhaled as fast as possible.

  • Forced Expiratory Volume in 1 sec (FEV₁): Volume exhaled in first second of FVC.

  • FEV₁/FVC Ratio: < 70% indicates obstructive disease (e.g., asthma, COPD).

• Types of Spirometers:

  • Water-sealed (volume-displacement): Gold standard, mechanical.

  • Pneumotachograph: Measures flow (pressure drop across known resistance) → integrates for volume. Low resistance.

  • Ultrasonic: Measures flow via ultrasound transit time change (no flow obstruction).

Photo Spirometer: (See Transducers section)

Blood Gas Analysis (PO₂ & PCO₂):

• PO₂: Measured by Clark electrode (polarographic, see above).

• PCO₂:

  • Principle: Severinghaus electrode (potentiometric). PCO₂ affects pH of bicarbonate buffer behind a CO₂-permeable membrane. pH change → potential change (measured vs. reference).

  • \boxed{PCO_2 \propto \Delta pH}

5. Neurological Measurements (High-frequency topic)

Electroencephalogram (EEG):

• Definition: Recording of cortical neuronal activity (post-synaptic potentials) via scalp electrodes.

• Frequency Bands & Conditions:

  | Band | Frequency (Hz) | Typical State / Clinical Significance | | :--- | :--- | :--- | | Delta (δ) | 0.5 - 4 | Deep sleep (adults), infants, brain damage | | Theta (θ) | 4 - 8 | Drowsiness, early sleep, meditation | | Alpha (α) | 8 - 13 | Relaxed wakefulness, eyes closed (attenuates with eye open/attention) | | Beta (β) | 13 - 30 | Active thinking, anxiety, muscle artifact | | Gamma (γ) | >30 | High-level cognitive processing, perception |

• Clinical Uses: Epilepsy diagnosis/sleep staging, encephalopathies, coma assessment, brain death.

6. Medical Imaging (High-frequency topic)

Magnetic Resonance Imaging (MRI):

• Basic Principle: Nuclear Magnetic Resonance (NMR).

  1. Alignment: Protons (¹H in water) align with strong static B₀ field.

  2. Excitation: RF pulse at Larmor frequency (ω₀ = γB₀) tips magnetization into transverse plane.

  3. Relaxation & Signal:

     • T1 (Spin-Lattice): Recovery of longitudinal magnetization.

     • T2 (Spin-Spin): Decay of transverse magnetization (dephasing).

  4. Spatial Encoding: Gradient fields (Gx, Gy, Gz) encode position.

• System Components: Superconducting magnet (B₀), RF coils (transmit/receive), Gradient coils, Computer system.

• Advantages: Excellent soft-tissue contrast, no ionizing radiation, multi-planar imaging.

• Safety Concerns:

  • Ferromagnetic objects: Projectile risk.

  • Specific Absorption Rate (SAR): RF energy → tissue heating.

  • Acoustic noise: Gradient coil vibration.

Endoscopy:

• Types:

  • Rigid: Simple tubes (e.g., cystoscope, laparoscope). Good illumination, channel for instruments.

  • Flexible Fiberoptic: Uses coherent fiber bundle for image transmission. Image quality degrades with fiber breakage.

  • Video (Chip): CCD/CMOS at tip → electronic signal → monitor. Superior image quality, can record/teach.

• Components: Insertion tube, control section (angulation wires), light source (xenon/LED), eyepiece/video connector, working channel.

• Applications: Diagnostic (visual inspection, biopsy) and surgical (laparoscopy, arthroscopy).

Thermography:

• Principle: Infrared (IR) imaging (7-14 µm wavelength) detects skin surface temperature patterns.

• Physics: All objects emit IR radiation proportional to temperature (Stefan-Boltzmann law).

• Applications:

  • Inflammation: Increased blood flow → higher temperature.

  • Vascular disorders: Asymmetry in limb temperature.

  • Breast screening: Detect abnormal angiogenesis (controversial adjunct, not primary screening).

  • Neurological: Sympathetic dysfunction (e.g., CRPS).

7. Therapeutic and Life Support Equipment (High-frequency topic)

Defibrillators:

• Principle: Delivery of a controlled DC shock to depolarize a critical mass of myocardial cells, allowing the natural pacemaker (SA node) to regain control.

• Classification:

  • External: Paddles on chest wall (manual or AED).

  • Internal: Paddles applied directly to heart (open-chest surgery).

  • Implantable (ICD): Monitors and automatically shocks for ventricular fibrillation.

• Energy Dosing: Typically 200-360 Joules (monophasic) or 120-200 J (biphasic) for adults.

Pacemakers:

• Components: Pulse generator (battery + circuitry), leads (electrodes).

• Types:

  • Fixed-rate: Constant pacing rate (no sensing).

  • On-demand (Demand): Senses intrinsic activity; inhibits pacing if heartbeat is adequate.

• Pacing Modes (NBG Code): e.g., VVI = Ventricular pacing, Ventricular sensing, Inhibited response. DDD = Dual chamber (Atrial & Ventricular) pacing & sensing, Dual response (tracking).

Heart-Lung Machine (Cardiopulmonary Bypass - CPB):

• Function: Temporarily takes over heart and lung function during open-heart surgery.

• Components:

  1. Venous Cannula: Drains deoxygenated blood.

  2. Blood Pump (Roller/centrifugal): Provides arterial flow.

  3. Oxygenator: Bubble (direct gas-liquid contact) or Membrane (semi-permeable, gas exchange across membrane; preferred).

  4. Heat Exchanger: Controls patient temperature (hypothermia).

  5. Arterial Filter: Removes debris/air.

  6. Cardioplegia Delivery: Stops the heart.

Hemodialysis (Dialyzers):

• Principle: Diffusion across a semi-permeable membrane (dialyzer/"artificial kidney").

• Process: Blood flows on one side, dialysate fluid on the other. Waste solutes (urea, creatinine) and excess electrolytes move down concentration gradients.

• System Components: Vascular access, blood pump, dialyzer (membrane + housing), dialysate concentrate & mixing system, ultrafiltration control, safety monitors.

8. Biotelemetry and Remote Monitoring (High-frequency topic)

Bio-Telemetry System:

• Block Diagram:

  Transducer → Signal Conditioner → Transmitter → Channel (Air/IR) → Receiver → Display/Recorder

• Applications: Monitoring ambulatory patients (Holter ECG), astronauts, athletes, wildlife.

• Advantages: Mobility, continuous monitoring, reduces cable artifacts.

• Disadvantages: Limited bandwidth, power constraints (battery), signal interference/security issues.

Biometric Systems:

• Physiological (Static): Based on inherent physical traits.

  • Examples: Fingerprint, Iris scan, Face recognition, ECG pattern.

• Behavioral (Dynamic): Based on patterns of behavior.

  • Examples: Gait analysis, Voice recognition, Keystroke dynamics, Signature.

• Use: Security (access control), health monitoring (continuous authentication, fall detection).

Critical Patient Telemetry:

• Wireless transmission of vital signs (ECG, SpO₂, NIBP, Temp) from patient to central nurse station.

• Integration with Hospital Information Systems (HIS) for electronic health records (EHR).

• Enables early warning scores (e.g., NEWS) and rapid response teams.

9. Signal Conditioning and Data Acquisition

Biomedical Amplifiers:

• Key Requirements:

  • High Input Impedance (>1 MΩ): Prevents loading the high-impedance bio-signal source (e.g., skin-electrode interface).

  • Low Noise: Especially 1/f (flicker) noise for DC/low-freq signals.

  • High Common-Mode Rejection Ratio (CMRR): Rejects 50/60 Hz power line interference. \boxed{CMRR (dB) = 20 \log_{10} \left( \frac{A_d}{A_{cm}} \right)} where A_d = differential gain, A_cm = common-mode gain.

  • Safety: Electrical isolation (patient protection from mains shock).

  • Right Leg Drive (RLD): Feedback circuit to reduce common-mode voltage.

• Instrumentation Amplifier (IA): The standard front-end. Provides high CMRR, high input impedance, and settable gain (often via single resistor).

Filtering:

• Low-Pass (LPF): Removes high-frequency noise (EMG, power line harmonics). Cutoff ~0.5-100 Hz for most bio-signals.

• High-Pass (HPF): Removes baseline wander (respiration, body movement). Cutoff ~0.05 Hz (ECG).

• Notch (Band-Stop): Rejects specific interference (50/60 Hz power line). Often a twin-T or active notch filter.

Analog-to-Digital Conversion (ADC):

• Sampling Theorem (Nyquist): \boxed{f_s > 2 f_{max}} (Sampling frequency > twice highest signal frequency) to avoid aliasing.

• Resolution: Number of bits (n). Smallest change = \frac{V_{ref}}{2^n}.

• Quantization: Mapping continuous amplitude to discrete levels → introduces quantization error/noise.

Microcontroller-based Data Acquisition System:

• General Architecture:

  Sensor → Signal Conditioning (Amp, Filter) → ADC → Microcontroller (Process, Store) → Output (Display, Comm)

• Role of MCU: Controls ADC timing, performs digital filtering (e.g., moving average), computes parameters (HR, SpO₂), manages communication (UART, USB, Bluetooth).

• Applications: Portable ECG monitors, pulse oximeters, wearable fitness trackers.