Physiological measurements are the backbone of biomedical engineering. They help us understand how the body works and catch problems early. From heart rate to brain activity, these measurements give us a window into the complex systems that keep us alive.

In this chapter, we'll dive into the tools and techniques used to measure vital signs, electrical activity, and internal structures. You'll learn how engineers design sensors, process signals, and create images that doctors use every day to save lives.

Physiological Measurements in Biomedical Engineering

Principles and Applications

• Physiological measurements involve the quantification and analysis of various biological signals and parameters to assess the functioning of the human body
• These measurements are essential for diagnosing diseases, monitoring patient health, and evaluating the effectiveness of medical treatments
• Biomedical engineers develop and utilize various sensors, transducers, and instrumentation to accurately measure and record physiological data
  • Examples of sensors and transducers include electrodes for ECG, force sensors for gait analysis, and thermistors for temperature monitoring
• Signal processing techniques, such as filtering, amplification, and digitization, are employed to extract meaningful information from the acquired physiological signals
  • Filtering removes unwanted noise and artifacts from the signal
  • Amplification increases the signal strength for better analysis
  • Digitization converts the analog signal into a digital format for computer processing
• Physiological measurements are used in a wide range of applications, including patient monitoring, medical research, sports medicine, and rehabilitation engineering
  • Patient monitoring involves continuous observation of vital signs and physiological parameters in clinical settings (intensive care units)
  • Medical research utilizes physiological measurements to study disease mechanisms and develop new therapies
  • Sports medicine employs physiological monitoring to optimize athletic performance and prevent injuries
  • Rehabilitation engineering uses physiological measurements to assess and improve the function of individuals with disabilities

Signal Processing and Data Analysis

• Raw physiological signals often contain noise, artifacts, and interference that need to be removed or minimized before analysis
• Filtering techniques, such as low-pass, high-pass, and band-pass filters, are used to remove specific frequency components from the signal
  • Low-pass filters remove high-frequency noise (muscle artifacts in ECG)
  • High-pass filters remove low-frequency baseline drift (respiratory variations in ECG)
  • Band-pass filters allow a specific range of frequencies to pass through while attenuating others
• Amplification is necessary to boost the signal strength of weak physiological signals for better visualization and analysis
  • Differential amplifiers are commonly used to amplify the difference between two signals while rejecting common-mode noise
• Analog-to-digital conversion (ADC) is performed to convert the continuous-time physiological signal into a discrete-time digital signal for computer processing
  • The sampling rate and resolution of the ADC determine the quality and accuracy of the digitized signal
• Signal averaging and ensemble averaging techniques are used to improve the signal-to-noise ratio by reducing random noise
  • Signal averaging involves collecting multiple recordings of the same signal and averaging them to reduce noise
  • Ensemble averaging is used for periodic signals, where multiple cycles are averaged to enhance the signal quality
• Advanced signal processing techniques, such as time-frequency analysis (wavelet transform) and adaptive filtering, are employed for more complex physiological signals
  • Wavelet transform provides time-frequency localization, enabling the analysis of non-stationary signals (EEG)
  • Adaptive filtering automatically adjusts filter parameters based on the signal characteristics to optimize noise reduction

Vital Signs Measurement Techniques

Blood Pressure and Heart Rate Monitoring

• Blood pressure measurement techniques include invasive methods, such as arterial line monitoring, and non-invasive methods, like auscultatory and oscillometric techniques
  • Arterial line monitoring involves inserting a catheter into an artery to directly measure blood pressure
  • Auscultatory method uses a sphygmomanometer and a stethoscope to listen for Korotkoff sounds during cuff deflation
  • Oscillometric method measures the oscillations in the cuff pressure to estimate systolic and diastolic blood pressure
• Heart rate can be measured using electrocardiography (ECG), photoplethysmography (PPG), and pulse oximetry, which detect electrical activity or changes in blood volume
  • ECG measures the electrical activity of the heart using electrodes placed on the skin
  • PPG uses an optical sensor to detect changes in blood volume in the microvascular bed of tissue
  • Pulse oximetry measures the heart rate and oxygen saturation by detecting changes in light absorption through the skin
• Continuous monitoring of blood pressure and heart rate is crucial in critical care settings, such as intensive care units (ICUs), to detect any abnormalities or deterioration in a patient's condition
  • Bedside monitors display real-time waveforms and numerical values of blood pressure and heart rate
  • Alarms are set to alert healthcare providers when the values exceed predefined thresholds

Respiratory Rate and Other Vital Signs

• Respiratory rate is typically measured by observing chest wall movements, using impedance pneumography, or through capnography, which measures the concentration of exhaled carbon dioxide
  • Visual observation involves counting the number of chest wall movements per minute
  • Impedance pneumography measures the changes in electrical impedance across the chest during respiration
  • Capnography uses an infrared sensor to measure the concentration of exhaled carbon dioxide, providing a waveform and numerical value of respiratory rate
• Body temperature can be measured using various types of thermometers, such as oral, rectal, tympanic, or skin surface thermometers
  • Oral and rectal thermometers are traditional methods that use mercury or digital sensors
  • Tympanic thermometers measure the infrared radiation emitted by the tympanic membrane in the ear canal
  • Skin surface thermometers use thermistors or infrared sensors to measure the temperature of the skin
• Oxygen saturation (SpO2) is commonly measured using pulse oximetry, which utilizes the principles of light absorption by oxygenated and deoxygenated hemoglobin
  • A pulse oximeter probe is placed on the finger, toe, or earlobe, and it emits red and infrared light
  • The ratio of light absorption at these two wavelengths is used to calculate the oxygen saturation percentage
• Continuous monitoring of respiratory rate, temperature, and oxygen saturation, along with blood pressure and heart rate, provides a comprehensive assessment of a patient's vital signs
  • Integrated monitoring systems display all vital signs on a single screen for easy visualization and trend analysis
  • Early detection of abnormalities in vital signs allows for timely interventions and improved patient outcomes

Electrical Activity Assessment with ECG and EMG

Electrocardiography (ECG)

• ECG is a non-invasive technique that records the electrical activity of the heart using electrodes placed on the skin surface
• The ECG waveform consists of P, QRS, and T waves, which represent the depolarization and repolarization of the atria and ventricles
  • P wave represents atrial depolarization
  • QRS complex represents ventricular depolarization
  • T wave represents ventricular repolarization
• ECG is used to diagnose various cardiac conditions, such as arrhythmias, myocardial infarction, and conduction disorders
  • Arrhythmias are abnormalities in the heart rhythm, such as atrial fibrillation or ventricular tachycardia
  • Myocardial infarction (heart attack) is detected by the presence of ST-segment elevation or depression
  • Conduction disorders, such as bundle branch blocks, are identified by the abnormal morphology of the QRS complex
• ECG signal processing involves filtering, amplification, and digitization to remove noise and artifacts and improve the signal quality
  • High-pass filtering removes baseline wander caused by respiration or motion artifacts
  • Low-pass filtering removes high-frequency noise, such as muscle artifacts or power line interference
  • Notch filtering eliminates the 50/60 Hz power line interference
• Advanced ECG analysis techniques include heart rate variability (HRV) analysis and signal-averaged ECG (SAECG)
  • HRV analysis assesses the variations in the time intervals between consecutive heartbeats, providing information about the autonomic nervous system function
  • SAECG involves averaging multiple ECG beats to enhance the signal-to-noise ratio and detect late potentials, which are indicators of increased risk for ventricular arrhythmias

Electromyography (EMG)

• EMG is a technique that measures the electrical activity of skeletal muscles using surface or needle electrodes
• The EMG signal represents the summation of action potentials generated by the muscle fibers during contraction
  • Surface EMG uses electrodes placed on the skin surface to record the overall activity of a muscle or muscle group
  • Needle EMG involves inserting a fine wire electrode into the muscle to record the activity of individual motor units
• EMG is used to assess neuromuscular disorders, evaluate muscle function, and study the biomechanics of movement
  • Neuromuscular disorders, such as myopathies or neuropathies, can be diagnosed by analyzing the characteristics of the EMG signal
  • Muscle function assessment involves measuring the amplitude and frequency content of the EMG signal during different levels of muscle contraction
  • Biomechanical studies use EMG to investigate muscle activation patterns and coordination during various movements or tasks
• EMG signal processing includes amplification, filtering, and rectification to improve the signal quality and extract relevant features
  • Amplification is necessary due to the low amplitude of the EMG signal (typically in the range of microvolts)
  • Band-pass filtering removes low-frequency motion artifacts and high-frequency noise
  • Rectification converts the bipolar EMG signal into a unipolar signal by taking the absolute value or squaring the signal
• Advanced EMG analysis techniques include time-domain and frequency-domain analysis, as well as muscle synergy analysis
  • Time-domain analysis involves calculating parameters such as root mean square (RMS) amplitude, integrated EMG, and zero crossings
  • Frequency-domain analysis, such as power spectral density (PSD) estimation, provides information about the frequency content of the EMG signal and muscle fatigue
  • Muscle synergy analysis aims to identify the underlying neural control strategies by decomposing the EMG signals into a set of synergistic activation patterns

Imaging Techniques for Anatomical Visualization

X-ray and Computed Tomography (CT)

• X-ray imaging uses ionizing radiation to create two-dimensional projections of the body's internal structures, particularly bones and dense tissues
  • X-rays are generated by an X-ray tube and pass through the body, being attenuated by different tissues based on their density
  • The resulting image is a shadowgram, where denser structures (bones) appear white, and softer tissues appear gray or black
• Computed tomography (CT) employs X-rays and computer reconstruction to generate cross-sectional images of the body, providing detailed visualization of soft tissues and organs
  • CT scanners use a rotating X-ray tube and detector array to acquire multiple projections from different angles
  • The projections are then processed using mathematical algorithms (e.g., filtered back projection) to reconstruct a 3D image of the body
  • CT images provide high spatial resolution and excellent contrast between different soft tissues
• Both X-ray and CT imaging are widely used for diagnostic purposes, such as detecting fractures, lung abnormalities, or tumors
  • Chest X-rays are commonly used to assess the lungs, heart, and major blood vessels
  • CT scans are used for detailed imaging of the brain, chest, abdomen, and pelvis, as well as for guided interventional procedures
• Radiation dose is a concern in X-ray and CT imaging, as exposure to ionizing radiation can increase the risk of cancer
  • Low-dose CT protocols and iterative reconstruction techniques are used to minimize radiation exposure while maintaining image quality
  • Protective measures, such as lead shielding and collimation, are employed to reduce unnecessary radiation exposure to the patient and staff

Magnetic Resonance Imaging (MRI) and Ultrasound

• Magnetic resonance imaging (MRI) utilizes strong magnetic fields and radio waves to create high-resolution images of the body's soft tissues, without the use of ionizing radiation
  • MRI scanners use a powerful magnetic field (typically 1.5 or 3 Tesla) to align the hydrogen nuclei (protons) in the body
  • Radio frequency (RF) pulses are applied to excite the protons, causing them to emit signals that are detected by receiver coils
  • The signals are then processed using Fourier transform and other mathematical techniques to reconstruct images of the body
• MRI provides excellent soft tissue contrast and can be used for functional imaging (fMRI) to study brain activity and connectivity based on changes in blood oxygenation levels
  • Different MRI sequences, such as T1-weighted, T2-weighted, and diffusion-weighted imaging, highlight different tissue characteristics
  • fMRI measures the blood oxygenation level-dependent (BOLD) signal, which reflects changes in neural activity based on the hemodynamic response
  • MRI can also be used for spectroscopy (MRS) to measure the concentration of metabolites in the body
• Ultrasound imaging uses high-frequency sound waves to visualize soft tissues and organs in real-time, making it useful for monitoring fetal development and assessing blood flow
  • Ultrasound transducers emit sound waves that penetrate the body and are reflected back by different tissues
  • The reflected echoes are processed to create real-time images of the body's internal structures
  • Doppler ultrasound is used to measure blood flow velocity by detecting the frequency shift of the reflected sound waves
• MRI and ultrasound are non-invasive imaging modalities that do not involve ionizing radiation, making them safer for repeated examinations
  • MRI is particularly useful for imaging the brain, spinal cord, joints, and soft tissues
  • Ultrasound is commonly used in obstetrics, cardiology, and vascular imaging, as well as for guiding interventional procedures
• Biomedical engineers contribute to the development and optimization of MRI and ultrasound systems, as well as the processing and analysis of the acquired images
  • Advanced techniques, such as parallel imaging, compressed sensing, and machine learning, are used to improve image quality, reduce scan time, and extract quantitative information from the images
  • Image segmentation, registration, and visualization tools are developed to facilitate the interpretation and analysis of the imaging data