Biomedical Measurement Techniques: Pressure, Flow, and ECG
Direct Blood Pressure Measurement
This method involves inserting a tube or catheter directly into the blood vessel. The catheter is connected to a blood pressure transducer, which generates a corresponding electrical signal. This method is classified into two types:
- Extra-vascular pressure sensor
- Intra-vascular pressure sensor
Indirect Blood Pressure Measurement
In this method, external pressure is applied to the artery using an instrument called a sphygmomanometer. There are two main techniques:
- Palpatory method: Using touch.
- Auscultatory method: Using a stethoscope.
Extra-Vascular Pressure Sensor
This technique couples the vascular pressure to an external sensor via a liquid-filled catheter. The catheter is connected to a pressure sensor through a 3-way stopcock. The system is filled with a Saline-heparin solution (an anti-coagulant agent) and must be flushed every few minutes to prevent blood from clotting. The catheter is inserted into the human body by surgical cut-down or percutaneous insertion.
Blood pressure information is transmitted via the catheter fluid to the sensor diaphragm. A thin, flexible metal diaphragm is stretched across the opening of the transducer top. This diaphragm is connected to an inductive bridge or a Wheatstone bridge strain gauge. The strain gauge will move an amount proportional to the applied pressure.
Intra-Vascular Pressure Sensor
Limitations of extra-vascular systems include:
- The frequency response of the system is limited by the hydraulic properties.
- The tubing system exhibits a low-pass filter effect.
The intra-vascular system eliminates the time delay introduced by the tubing system. It allows high-fidelity measurement of the high-frequency components of the blood pressure signal.
Indirect Blood Pressure Measurement Details
Indirect measurement of blood pressure is an attempt to measure intra-arterial pressures non-invasively. The two main techniques are the Palpatory and Auscultatory methods.
Palpatory Method
- Seat the subject comfortably with their right arm resting on a table.
- Legs should not be crossed, as this may raise systolic blood pressure (ventricular closure) by 2-8 mmHg.
- Wrap the pressure cuff around the upper arm above the elbow.
- Ensure the inflatable bag inside can exert pressure on the brachial artery.
- Wrap the cloth strap flat outside the inflatable bag for external support when inflated.
- The cuff should be loose enough to avoid interfering with venous return from the forearm.
- Close the valve on the bulb by turning it clockwise.
- With one hand, palpate (feel) the radial pulse in the wrist.
Auscultatory Method
- Wrap the pressure cuff around the upper arm above the elbow.
- With one hand, place the stethoscope head over the brachial artery (the major blood vessel in the upper arm) and listen for heartbeat sounds.
- Inflate the cuff 20 to 30 mmHg above the estimated systolic pressure.
- Release the pressure slowly, no greater than 3 mmHg per second.
- The pressure level at which you consistently hear heartbeats is the systolic pressure. The sounds heard through the stethoscope are called Korotkoff sounds.
- These sounds decrease in pitch during the next 10-15 mmHg drop in pressure.
Oscillometric Measurement
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Measurement of Blood Flow and Cardiac Output
Blood flow meters are used to monitor blood flow in various blood vessels and to measure cardiac output. Common types include:
- Electromagnetic blood flow meters
- Ultrasonic blood flow meters
Electromagnetic Blood Flow Meter
An electromagnetic flow meter is a volumetric flow meter with no moving parts. The flowing liquid (blood) must be conductive. The operation of electromagnetic flow meters, or ‘magmeters’, is based upon Faraday’s law, which states: The voltage induced across a conductor as it moves at right angles through a magnetic field is proportional to the velocity of that conductor.
Ultrasonic Blood Flow Meters
Like electromagnetic flow meters, these can measure the instantaneous flow of blood. Ultrasound is generated above the human hearing range (above 20 kHz). At the heart of each ultrasonic flow meter transducer is a piezo-electric crystal, typically glass disks about the size of a coin. These crystals are polarized or expand when electrical energy is applied to their surface electrodes.
There are two main types of ultrasonic flowmeters:
- Transit Time Ultrasound flow meters
- Doppler Ultrasound flow meters
Plethysmography
A plethysmograph is an instrument for measuring changes in volume within an organ or the whole body, usually resulting from fluctuations in the amount of blood or air it contains. Plethysmography measures these volume changes using blood pressure cuffs or other sensors attached to a machine called a plethysmograph.
Impedance Plethysmograph (IPG)
The IPG is based on measuring the electrical impedance (resistance) of a selected body segment. Blood has a much lower impedance compared to other tissues like muscle or bone. Therefore, blood volume variations correspond with measurable changes in electrical impedance, where an increase in blood volume results in lower impedance.
Photoelectric Plethysmograph (PPG)
Photoelectric plethysmography, or simply photoplethysmography (PPG), is a simple optical technique used to detect volumetric changes in blood in peripheral circulation. It is a low-cost and non-invasive method that makes measurements at the skin surface. The technique provides valuable information related to the cardiovascular system. Recent technological advances have revived interest in this technique, which is widely used in clinical physiological measurement and monitoring.
PPG uses low-intensity infrared (IR) light. When light travels through biological tissues, it is absorbed by bones, skin pigments, and both venous and arterial blood. Since light is more strongly absorbed by blood than surrounding tissues, changes in blood flow can be detected by PPG sensors as changes in light intensity.
Measurement of Heart Sounds
Factors involved in the production of heart sounds include:
- The movement of blood through the chambers of the heart.
- The movement of cardiac muscles.
- The movement of the heart valves.
First Heart Sound (S1)
Produced during isometric contraction and the earlier part of the ejection period. It resembles the spoken word “LUBB”.
Second Heart Sound (S2)
Produced at the onset of diastole. It resembles the spoken word “DUBB”.
Third Heart Sound (S3)
Produced during the rapid filling period of the cardiac cycle.
Fourth Heart Sound (S4)
Produced during atrial systole and considered a physiologic heart sound.
Phonocardiography
Phonocardiography is the technique of creating a high-fidelity recording (a phonocardiogram) of the sounds and murmurs made by the heart using a machine called a Phonocardiograph. It records all sounds made by the heart during a cardiac cycle. These sounds result from vibrations created by the closure of the heart valves. There are two main closure sounds:
- Atrioventricular valves (Tricuspid & Bicuspid) close at the beginning of systole (S1).
- Semilunar valves (Aortic & Pulmonary) close at the end of systole (S2).
Biomedical Electrodes
Skin Surface Electrodes
Types include:
- Metal-plate electrodes
- Suction electrodes
- Floating electrodes
Needle Electrodes
A basic needle electrode consists of a solid needle, usually made of stainless steel, with a sharp point. The shank of the needle is insulated with a coating, such as insulating varnish; only the tip is left exposed.
Microelectrodes
When studying the electrophysiology of excitable cells, it is often important to measure potential differences across the cell membrane. To do this, an electrode must be placed within the cell. Microelectrodes have diameters ranging from approximately 0.05 to 10 micrometers.
Transducers for Biomedical Applications
Different types of active transducers include:
- Magnetic induction type
- Piezoelectric type
- Photovoltaic type
- Thermoelectric type
Electro-Conduction System of the Heart
The impulse for cardiac contraction starts at the Sinoatrial node (SA node), located at the opening of the superior vena cava into the right atrium. The SA node is called the pacemaker of the heart. The impulse then passes through the atrial muscle to the Atrioventricular node (AV node). From the AV node, nervous muscle tissue splits into left and right sides, collectively called the ‘Bundle of His’. The Bundle of His further divides into branches called Purkinje fibers, causing the ventricles to contract.
Electrocardiography (ECG/EKG)
Electrocardiography is a method of creating a graphic tracing (electrocardiogram; ECG or EKG) of the electric current generated by the heart muscle during a heartbeat. The tracing is recorded with an electrocardiograph and provides information on the condition and performance of the heart.
Electrodes that record the electrical activity of the heart are placed at 10 different locations: one on each of the four limbs and six at different locations on the anterior surface of the chest. After the electrodes are in place, a millivolt signal from an external source is introduced so that the instrument can be calibrated.
Electrodes and Leads for ECG
To record an ECG, normally five electrodes are affixed to the patient’s body. These electrodes are connected to the ECG machine by the same number of electrical wires. These wires, or the electrodes they connect to, are called leads. The placement and color code used to identify each electrode vary but are standardized. Experimentally, it is found advantageous to record the electrocardiogram from electrodes placed vertically as well as horizontally on the body.
Problems in Biomedical Measurements
Common challenges include:
- Inaccessibility of variables: Difficulty in accessing the physiological variable to be measured.
- Variability of data: Biological signals naturally vary between individuals and over time.
- Lack of knowledge: Incomplete understanding of the interrelationships between physiological variables.
- Interaction among systems: Physiological systems often interact, making isolated measurements difficult.
- Transducer effects: The measurement device itself can alter the physiological variable being measured.
- Artifacts: Unwanted signals interfering with the desired measurement (e.g., muscle noise in ECG).
- Energy limitations: Constraints on the power that can be safely applied to or drawn from the body.
- Safety considerations: Ensuring patient safety during measurement procedures.