🩺Biomedical Instrumentation Unit 5 – Bioelectric Signals and ECG

Fundamentals of Bioelectric Signals

• Bioelectric signals originate from the electrical activity of cells and tissues in the body
• Generated by the movement of ions across cell membranes, creating potential differences
• Cells involved in bioelectric signal generation include neurons, muscle cells, and endocrine cells
• Action potentials are the primary mechanism for signal transmission in excitable cells
  • Occur when the cell membrane depolarizes and repolarizes rapidly
  • Triggered by stimuli that exceed the cell's threshold potential
• Bioelectric signals can be measured using electrodes placed on the skin surface (surface electrodes) or within the body (intracellular or extracellular electrodes)
• Signal characteristics depend on the type of tissue, location, and physiological state
• Bioelectric signals are typically low in amplitude (microvolts to millivolts) and require amplification for analysis
• Signal processing techniques are used to filter, analyze, and interpret bioelectric signals (Fourier analysis, wavelet analysis)

Physiological Basis of ECG

• ECG (electrocardiogram) records the electrical activity of the heart over time
• Cardiac cells generate action potentials that propagate through the heart, causing contraction
• The conduction system of the heart coordinates the sequence of electrical activation
  • Sinoatrial (SA) node initiates the heartbeat, acting as the natural pacemaker
  • Atrioventricular (AV) node delays the impulse, allowing time for ventricular filling
  • His-Purkinje system rapidly distributes the impulse to the ventricles
• Each phase of the cardiac cycle produces a characteristic waveform on the ECG
  • P wave represents atrial depolarization
  • QRS complex represents ventricular depolarization
  • T wave represents ventricular repolarization
• Changes in ECG waveforms can indicate various cardiac abnormalities (arrhythmias, ischemia, infarction)
• ECG provides valuable information about heart rate, rhythm, and conduction

ECG Signal Characteristics

• ECG signal consists of a series of waves and intervals that represent different phases of the cardiac cycle
• Key components of the ECG signal include P wave, QRS complex, T wave, and U wave (not always visible)
• Amplitude of ECG waveforms is typically measured in millivolts (mV)
  • P wave amplitude: 0.1-0.2 mV
  • QRS complex amplitude: 1-2 mV
  • T wave amplitude: 0.2-0.5 mV
• Duration of ECG waveforms and intervals is measured in milliseconds (ms)
  • P wave duration: 60-120 ms
  • QRS complex duration: 60-100 ms
  • T wave duration: 100-250 ms
• ECG signal frequency content ranges from 0.05 to 100 Hz, with most diagnostic information in the 0.5-50 Hz range
• Signal-to-noise ratio (SNR) is an important consideration in ECG signal quality
• Artifacts and noise can distort the ECG signal (muscle activity, electrode movement, power line interference)

ECG Measurement Techniques

• Standard 12-lead ECG is the most common measurement technique
  • Uses 10 electrodes placed on the limbs and chest to record 12 different views of the heart's electrical activity
  • Leads I, II, and III are bipolar limb leads that measure potential differences between electrodes
  • Leads aVR, aVL, and aVF are augmented unipolar limb leads that measure potential differences relative to a reference point
  • Leads V1-V6 are unipolar chest leads that measure potential differences relative to a reference point
• Holter monitoring is used for continuous ECG recording over an extended period (24-48 hours)
  • Portable device worn by the patient to record ECG during daily activities
  • Useful for detecting intermittent arrhythmias or ischemic events
• Stress ECG (exercise stress test) measures the heart's response to physical exertion
  • Patient undergoes graded exercise on a treadmill or stationary bike while ECG is recorded
  • Helps diagnose coronary artery disease and assess exercise capacity
• Intracardiac electrogram (EGM) records electrical activity directly from the heart using catheters or implantable devices
  • Provides more detailed and localized information than surface ECG
  • Used during electrophysiology studies and ablation procedures

ECG Instrumentation and Equipment

• ECG instrumentation consists of electrodes, lead wires, amplifiers, filters, and a display or recording device
• Electrodes transduce ionic currents in the body into electrical currents in the measurement circuit
  • Types of electrodes include disposable adhesive electrodes, reusable disc electrodes, and needle electrodes
  • Electrode-skin interface is a critical factor in signal quality and requires proper skin preparation and electrode placement
• Lead wires connect the electrodes to the ECG amplifier and should be shielded to minimize interference
• ECG amplifiers boost the low-amplitude bioelectric signals to a level suitable for further processing and display
  • Differential amplifiers are used to reject common-mode noise and amplify the difference between two input signals
  • Typical ECG amplifier gain is 1000-5000, with a bandwidth of 0.05-100 Hz
• Filters remove unwanted frequency components from the ECG signal
  • High-pass filters remove low-frequency baseline wander (0.5-1 Hz cutoff)
  • Low-pass filters remove high-frequency noise and artifacts (50-100 Hz cutoff)
  • Notch filters remove power line interference (50 or 60 Hz)
• Analog-to-digital converters (ADCs) sample and digitize the analog ECG signal for digital storage, processing, and analysis
  • Sampling rates of 250-1000 Hz are commonly used in ECG acquisition
  • Resolution of 12-16 bits is typical for ECG ADCs

Signal Processing in ECG

• Signal processing techniques are used to improve the quality and interpretability of ECG signals
• Filtering is used to remove noise and artifacts from the ECG signal
  • Digital filters (FIR, IIR) are implemented in software for more flexibility and control
  • Adaptive filters can adjust their parameters based on signal characteristics
• Baseline wander correction removes low-frequency drift in the ECG signal
  • High-pass filtering, polynomial fitting, or cubic spline interpolation can be used
• Powerline interference removal eliminates 50 or 60 Hz noise from the ECG signal
  • Notch filtering, adaptive filtering, or subtraction of a noise template can be used
• QRS detection identifies the location and timing of QRS complexes in the ECG signal
  • Algorithms based on amplitude, slope, or wavelet transforms are commonly used
  • Enables the calculation of heart rate and the analysis of rhythm and morphology
• Signal averaging improves the signal-to-noise ratio by combining multiple ECG beats
  • Reduces random noise and enhances small, consistent signal components (P wave, late potentials)
• Time-frequency analysis (short-time Fourier transform, wavelet transform) provides information about the time-varying frequency content of the ECG signal
  • Useful for detecting and characterizing transient events and abnormalities

Clinical Applications of ECG

• Diagnosis of cardiac arrhythmias
  • Identification of abnormal heart rhythms (bradycardia, tachycardia, atrial fibrillation, ventricular tachycardia)
  • Assessment of conduction disorders (heart block, bundle branch block)
• Detection of myocardial ischemia and infarction
  • ST segment changes (elevation or depression) indicate reduced blood flow to the heart muscle
  • Q waves and T wave inversions can suggest previous myocardial infarction
• Evaluation of electrolyte imbalances
  • Hyperkalemia can cause tall, peaked T waves and a prolonged PR interval
  • Hypokalemia can cause flattened or inverted T waves and a prolonged QT interval
• Monitoring of drug effects on the heart
  • Antiarrhythmic drugs can prolong the QT interval and increase the risk of torsades de pointes
  • Digitalis toxicity can cause characteristic "scooped" ST segment depression
• Risk stratification and prognosis
  • Left ventricular hypertrophy, as indicated by increased QRS voltage, is associated with increased cardiovascular risk
  • Prolonged QT interval is a risk factor for ventricular arrhythmias and sudden cardiac death
• Guidance of therapy
  • Pacemaker and implantable cardioverter-defibrillator (ICD) placement and programming
  • Evaluation of the effectiveness of antiarrhythmic medications
  • Monitoring of cardiac resynchronization therapy (CRT) in heart failure patients

Challenges and Future Developments

• Improving signal quality and reducing noise in ambulatory and long-term ECG monitoring
  • Development of more comfortable and less obtrusive wearable ECG devices
  • Use of active electrodes and shielding to minimize motion artifacts and interference
• Enhancing the accuracy and reliability of automated ECG analysis algorithms
  • Machine learning and deep learning approaches for arrhythmia detection and classification
  • Incorporation of patient-specific information and context into analysis algorithms
• Integrating ECG with other physiological signals and data sources
  • Multimodal monitoring that combines ECG with blood pressure, respiration, and activity data
  • Fusion of ECG with imaging data (echocardiography, cardiac CT, MRI) for comprehensive assessment
• Developing personalized and predictive models for cardiovascular risk assessment
  • Using ECG features and other clinical data to stratify patients based on their risk profiles
  • Identifying early warning signs of impending cardiac events and guiding preventive interventions
• Advancing telemedicine and remote ECG monitoring
  • Enabling real-time transmission and interpretation of ECG data from patients to healthcare providers
  • Facilitating timely diagnosis and management of cardiac conditions in underserved or remote areas
• Exploring novel applications of ECG beyond the cardiovascular system
  • Using ECG-derived respiration (EDR) for respiratory monitoring
  • Analyzing heart rate variability (HRV) as a marker of autonomic function and stress
  • Investigating the potential of ECG in biometric identification and authentication