Medical Electronics Dr. Neil Townsend Michaelmas Term 2001 (www.robots.ox.ac.uk/~neil/teaching/lectures/med_elec) The story so far The heart pumps blood around the body. It has four chambers which contact in a carefully controlled order (two pairs of contractions) to achieve this effect. Lecture 2 The story so far The heart pumps blood around the body using a carefully controlled order of contractions of its four chambers. The contraction of a muscle cell is a result of a depolarisation and repolarisation cycle called an action potential during which the potential difference between the inside and the outside of the cell changes. The story so far The heart pumps blood around the body using a carefully controlled order of contractions of its four chambers. The contraction of a muscle cell is a result of a depolarisation and repolarisation cycle called an action potential during which the potential difference between the inside and the outside of the cell changes. Considering all of the cardiac cells together we can view the heart as an electrical generator which drives current into a passive resistive medium, the thorax. By taking voltage differences at different points on the thorax the electrical activity of the heart can be observed. The Electrocardiocgram (ECG) Here is an example of an ECG signal Recording the ECG To record the ECG we need a transducer capable of converting the ionic potentials generated within the body into electronic potentials Such a transducer is a pair of electrodes Polarisable (which behave as capacitors) Non polarisable (behave as resistors) Common electrodes lie between these two extremes
Recording the ECG To record the ECG we need a transducer capable of converting the ionic potentials generated within the body into electronic potentials Such a transducer is a pair of electrodes Polarisable (which behave as capacitors) Non polarisable (behave as resistors) Common electrodes lie between these two extremes The electrode most commonly used for ECG signals, the silver silver chloride electrode is closer to a non polarisable electrode. Electrode placement There exists a convention prescribing electrode placement. "12 lead" ECG placement is a well developed tools for recording 12 different ECG traces from an individual For simple ECG recording, however, a three "lead" combination is possible in which electrodes are placed on the right arm (RA), the left arm (LA) and the left leg (LL) Electrode placement In this context, "lead" means a pair of electrodes: The three electrodes above result in three possible differences: (Potential at LA) (Potential at RA) (Potential at LL) (Potential at RA) (Potential at LL) (Potential at LA) Silver silver chloride electrode Electrodes are usually metal disks and a salt of that metal. A paste is applied between the electrode and the skin. This results in a local solution of the metal in the paste at the electrode skin interface. Ionic equilibrium takes place when the electrical field is balanced by the concentration gradient and a layer of Ag + ions is adjacent to a layer of Cl ions. This gives a potential drop E called the half cell potential (normally 0.8 V for an Ag AgClAgCl electrode) Silver silver chloride electrode This double layer of charges will also have a capacitative effect Since the Ag AgCl AgCl electrode is primarily non polarisable there is a large resistive effect. This gives a simple model for the electrode. Silver silver chloride electrode This double layer of charges will also have a capacitative effect Since the Ag AgCl AgCl electrode is primarily non polarisable there is a large resistive effect. This gives a simple model for the electrode. However, the impedance is not infinite at d.c. and so a resistor must be added in parallel with the capacitor.
Movement artefact Movement disturbs the physical positioning of the ions in the ionic equilibrium. The half cell potential will therefore momentarily change. The potential difference between two electrodes will therefore vary. This variation, which is unrelated to the underlying signal we wish to observe, is known as movement artefact It can be a serious cause of interference in the measurement of ECG Overall Equivalent circuit Using the simple model we used earlier for the thorax we can build up an overall circuit for the heart, the body and the electrodes. The resistors and capacitors may not be exactly equal. E and E should be very similar. Hence V should represent the actual different of ionic potential between the two points on the body where the electrodes have been placed. ECG Amplifiers The peak output voltage V of our equivalent circuit is around 1 mv. Therefore, amplification is required if we are going to store or display this information in some way. There are three problems which must be overcome which we will now consider Electrical Field Interference (Problem 1) To put it in hand waving terms: the human body is a good aerial and so the electrical signal at the electrodes is not pure, unadulterated, ECG More specifically, capacitance between power lines and and the system couples current into the patient, wires and machine. Electrical Field Interference This capacitance varies but it is of the order of 50pF. This corresponds to 64MΩ at 50Hz. If the right leg is connected to the common ground of the amplifier with a contact impedance of 5k, the mains potential will appear as a 20mV noise input. The solution The key is to remember that the ECG is a differential signal. The 50Hz noise, however, is common to all the electrodes. It appears equally at the Right Arm and Left Arm terminals. Rejection therefore depends on the use of a differential amplifier in the input stage of the ECG machine. The amount of rejection depends on the ability of the amplifier to reject common mode voltages.
Differential Amplifiers There have been covered in the core course. Let s have a look at the standard circuit. Differential Amplifiers So, the standard circuit gives a gain of: v 2 v 1 v 2 v 1 1 2 R 2 ie a differential gain of A d 1 2 R 2 Differential Amplifiers So, the standard circuit gives a gain of: v 2 v 1 v 2 v 1 1 2 R 2 ie a differential gain of Differential Amplifiers The overall common mode rejection ratio is given by: CMRR = A d1 A d2 A cm1 A cm2 A d 1 2 R 2 and a common mode gain of A cm 1 Magnetic Induction (problem 2) Current in magnetic fields induces voltage in the loop formed by the patient leads Either: Lower the magnetic field strength (rather hard) Minimise the coil area (eg twist the wires together) Source Impedance Unbalance (problem 3) If these impedances are not balanced (ie( the same) then the common mode voltage of the body will be higher at one input to the amplifier than the other. Hence, a fraction of the common mode voltage will be seen as a differential signal. Therefore, make sure the electrodes are on correctly!
The signal So, the signal at the input to the amplifier will have three components: 1. The desired differential ECG signal 2. An unwanted common mode signal 3. Unwanted common mode signal appearing as a differential input The signal The output of the amplifier will therefore consist of three components: 1. The desired output (ECG) 2. Unwanted common mode signal because the common mode rejection is not infinite 3. Unwanted common mode signal due to source imbalance A Modification There is a better approach than setting the system ground to the common mode voltage. The common mode voltage can actually be controlled using a Driven right leg circuit. A small current (<1µA) is injected into the patient to equal the displacement currents flowing in the body. The body sums these currents and the common mode voltage is driven to a low value. A Modification There is a better approach than setting the system ground to the common mode voltage. The common mode voltage can actually be controlled using a Driven right leg circuit. A small current (<1µA) is injected into the patient to equal the displacement currents flowing in the body. The body sums these currents and the common mode voltage is driven to a low value. Also improves patient safety. Diagnostic use of the ECG As has been seen, the ECG can provide diagnostic information to clinicians. Ectopic beats originate somewhere other than the SA node and often have different shapes (morphologies( morphologies). Abnormal heart rates (arrhythmias( arrhythmias) ) can treated. Diagnostic use of the ECG Post heart attack (Myocardial( Infarct) ) the ECG is highly informative. Cardiac muscle damage (infarcts( infarcts) ) generally correspond to loss of amplitude. Insufficient blood supply to cardiac cells (Ischemic( heart condition) ) changes the S T level.
Acquiring ECG for Diagnosis Two methods are in common use: Exercise Stress ECGs Ambulatory monitoring Acquiring ECG for Diagnosis Two methods are in common use: Exercise Stress ECGs Ambulatory monitoring Acquiring ECG for Diagnosis Ambulatory monitoring: ECG monitored for 24 hours. Each beat analysed and either kept for records or ignored (because too normal!) Results printed out: 1 page summary of conditions detected. 24 hour summary detailing the heart rate and ST segment changes over the period of the test. Findings pages with detailed information for each finding or symptom event. Foetal Monitoring It can be helpful to clinicians and midwives to have information about both the maternal and foetal ECG during childbirth Although this may add stress, the behaviour of the foetal ECG (and heart rate) is known to give information about potential foetal distress