Electrocardiogram (ECG) - PDF Free Download

Vectors and ECG s

Vectors and ECG s 2

Electrocardiogram (ECG) Depolarization wave passes through the heart and the electrical currents pass into surrounding tissues. Small part of the extracellular current reaches the surface of the body. The electric potential generated can be recorded from electrodes placed on the skin An EKG is a comparison of two vectors It compares the heart vector showing current flow on the heart with the reference, recording lead vector on the body. 3

Vector diagrams Vectors are used to describe depolarization and repolarization events Vectors are arrows which show two things: Direction or pathway (of charge spread) Magnitude or size (amount of charge) Vector analysis explains the waves on an EKG Q S 4

EKG is Extracellular Recording Depolarization Repolarization Vectors will always be positioned so that head of vector is in area of positive charge; tail is in area of negative charge. i.e a Dipole 5

Electrophysiology

P wave represents depolarization of atria which causes atrial contraction Repolarization of atria not normally detectable on an ECG Excitation of bundle of His and bundle branches occur in middle of PR interval QRS complex reflects depolarization of ventricles T wave reflects repolarization of muscle fibers in ventricles

Rest No current flow, no vector. The following vectors represent the spread of negative charge during depolarization; Then the spread of positive charge during repolarization 10

SA: Sino-atrial node = depol SA nodal fibers, spread of neg charge over atria 11

The atria would start to depolarize to the right - + 12

The atria would start to depolarize to the right, depolarization continue and spreading. + 13

The atria would start to depolarize to the right, depolarization continue and spreading. + 14

The atria would start to repolarize down and to the left, as the current continues downward to the ventricles + 15

+ 16

Atria now have repolarized and now have positive surface charge again. 17

Meanwhile, as the atria are repolarizing... We turn to the Depolarizing AV node AV: Atrioventricular Node These are small diameter fibers with few gap junctions; little or no detectable current flow 18

Septal Depolarization Moving down bundle of His; 19

Apex then Lateral walls 22

Through the thickness of the heart, from endo-, to myo-, to epicardium 24

Ventricles completely depolarized, negative surface charge No current No vector 26

Begin Ventricular Repolarization Spread of positive charge + 27

Rest End of cycle; No current flow, no vector. 35

Recording from Lead II Standard limb lead II 36

The Rules of Vectors Analysis An EKG is a comparison of two vectors It compares the heart vector with the reference recording lead vector on the body. If the vectors run parallel (same direction) the pen moves upward from baseline If the vectors run antiparallel (opposite direction) then the pen moves downward from baseline. If the vectors are perpendicular, the pen remains on baseline. If there is no current flow, the pen remains on baseline. Each lead consists of two electrodes placed on the skin, with a voltmeter between them. The voltmeter is attached to a pen, which travels over paper running at 25 mm/sec. This produces waves called an electrocardiogram. 37

- I + - - II III Einthoven s Triangle + + Bipolar Limb Leads 38

Atrial depolarization Pen here II V T The heart vector is parallel to the lead, but how can you confirm? 39

- Atrial depolarization II Av = A Cos θ Av is the projection of the vector on lead 2. A is the amplitude of the vector + 1. Draw a perpendicular line to the lead vector 2. Draw a line toward from the perpendicular vector toward your cardiac vector 40

Atrial depolarization II 41

AV nodal depolarization II 42

IV septal depol, from L to R II Anti-parallel! Pen deflects down Draw it! 43

IV septal depol, from base to apex II 44

Lateral walls depolarization II Draw it! 45

Depolarization complete; no current flow; pen returns to baseline II 46

Waiting to begin repolarization; no current flow II 47

Ventricular Repolarization begins II 48

Ventricular Repolarization II 49

Ventricular Repolarization complete; no current flow; pen on baseline II 50

Ventricular Repolarization complete; waiting to start all over again II End of one cardiac cycle 51

What does that tell you about the recording you obtain from each lead? Each lead describes the events on the heart from it s own point of view Reading from several leads gives you different points of view about the same set of repeating events (depol, repol) 52

A standard arrangement of electrodes for an ECG is called Einthoven triangle. They are bipolar leads. Einthoven Triangle

Summary: Bipolar Leads and Einthoven s Law Lead I - The negative terminal of the electrocardiograph is connected to the right arm, and the positive terminal is connected to the left arm. Lead II - The negative terminal of the electrocardiograph is connected to the right arm, and the positive terminal is connected to the left leg. Lead III - The negative terminal of the electrocardiograph is connected to the left arm, and the positive terminal is connected to the left leg. Einthoven s Law states that the electrical potential of any limb equals the sum of the other two (+ and - signs of leads must be observed). Lead I Lead III Lead II LA RA LL- LA LL- RA 55

Einthoven limb leads (standard leads) and Einthoven triangle. The Einthoven triangle is an approximate description of the lead vectors associated with the limb leads.

Augmented Limb Lead

6 Leads- bipolar and augmented Lead augmented vector right (avr) Lead augmented vector left (avl) Lead augmented vector foot (avf) The avr, avl, and avf leads can also be represented using the I and II limb leads

Unipolar Recordings Frank Norman Wilson (1890-1952) investigated how electrocardiographic unipolar potentials could be defined. Ideally, those are measured with respect to a remote reference (infinity). But how is one to achieve this in the volume conductor of the size of the human body with electrodes already placed at the extremities? They suggested the use of the central terminal as this reference. This was formed by connecting a 5 kω resistor from each terminal of the limb leads to a common point called the central terminal.

The Wilson central terminal (CT) is formed by connecting a 5 k resistance to each limb electrode and interconnecting the free wires; the CT is the common point. The Wilson central terminal represents the average of the limb potentials. Because no current flows through a high-impedance voltmeter, Kirchhoff's law requires that I R + I L + I F = 0.

(A) The circuit of the Wilson central terminal (CT). (B) The location of the Wilson central terminal in the image space (CT'). It is located in the center of the Einthoven triangle.

Wilson s Central Terminal VR + + VL + VF An equivalent reference electrode The average of the voltages on the three limb electrodes Minimize loading: Use three equal-valued resistors ( > 5 M-ohm ) Resulting electrode voltages are VL, VR, and VF

Simplified Electrocardiographic Recording System Electrodes Z 2 v ecg Z body Z 1 60-Hz ac magnetic field + V cc + Differential amplifier - v o Displacement currents -V cc Two possible interfering inputs are stray magnetic fields and capacitively coupled noise. Orientation of patient cables and changes in electrode-skin impedance are two possible modifying inputs. Z 1 and Z 2 represent the electrode-skin interface impedances.

Basic Biopotential Amplifier Requirements Purpose: To provide voltage and/or current gain to increase the amplitude of weak electric signals of biological origin Features: High input impedance (minimize the loading effects of the amplifier inputs) Protection circuitry (limit the possibility of introducing dangerous microshocks or macroshocks at the input terminals of the amplifier) Low output impedance (low with respect to the load being driven) Adequate output current (to supply the power needed to drive the load) Bandlimited frequency response (match the frequency response of the signal being measured to eliminate out-of-band noise) Quick calibration (include a signal source and a number of selectable fixed gains settings) High common-mode rejection for differential amplifiers (common mode signals are frequently larger than the biopotentials being measured) Additional specific requirements for each application

Generalized Static Characteristics of Instrumentation System Accuracy Difference between the true value and the measured value divided by the true value Precision Number of distinguishable alternatives (eg., a meter with a five digit readout) Resolution Smallest incremental quantity that can be measured with certainty Reproducibility Produces the same output for a given input over a period of time Statistical Control Systematic and bias error can be removed by calibration Random errors can be removed by taking multiple measurements and averaging the result.

Frequent Problems Frequency distortion High-frequency loss rounds the sharp edges of the QRS complex. Low-frequency loss can distort the baseline (no longer horizontal) or cause monophasic waveforms to appear biphasic. Saturation/cutoff distortion Combination of input amplitude & offset voltage drives amplifier into saturation Positive case: clips off the top of the R wave Negative case: clips off the Q, S, P and T waves Ground loops Patients are connected to multiple pieces of equipment; each has a ground (power line or common room ground wire) If more than one instrument has a ground electrode connected to the patient, a ground loop exists. Power line ground can be different for each item of equipment, sending current through the patient and introducing common-mode noise. Open lead wires Can be detected by impedance monitoring.

Artifacts Unwanted voltage transients Patient movement Electrical stimulation signals, like defibrillation Amplifier saturates First-order recovery to baseline Recovery time set by lowfrequency corner of the bandpass amplifier Effect of a voltage transient on an ECG recorded on an electrocardiograph in which the transient causes the amplifier to saturate, and a finite period of time is required for the charge to bleed off enough to bring the ECG back into the amplifier s active region of operation.

Artifacts Upper figure: coupling of 60 Hz power line noise Electric-field coupling between power grid, instrument, patient, and wiring. Lower figure: coupling of electromyographic (EMG) noise Example of tensing chest muscles while ECG is being recorded.

Power-Line Capacitive Coupling to body Power line is coupled into the body,small ac displacement current I db is generated, which produces a common mode voltage C b i db Power line 220 V Electrocardiograph v cm = i db Z G = (0.2 µa) (50 K W) = 10 mv At the amplifier inputs: suppose Z 2 Z 1 = 20k- W and Zin =5M- W v A - v B = v cm (Z 2 Z 1 )/ Z in = (10 mv) (20 KW / 5 MW) = 40 µv Remedies: Reduce or match the electrode skin impedances (minimize Z 1 - Z 2 ) Increase Z in u cm Z 2 u cm Z G i db u cm Z 1 Current flows from the power line through the body and ground impedance, thus creating a common-mode voltage everywhere on the body. Z in G Z in A B

Cure for power line capacitive coupling Easy cure: make Zg = 0, however it is not ALLOWED due to electric safety of the patient (short circuit prevention) Good Cure: Increase input impedance of the amplifier lowering skin-electrode impedance Driven Right Leg Circuit

Power-Line Coupling to cable and Amplifier Small parasitic capacitors connect the power line to the RA and LA leads, and the grounded instrument case and Small ac displacement currents I d1 and I d2 are generated The body impedance is about 500 W and can be neglected v A - v B = i d1 Z 1 - i d2 Z 2 If I d1 and I d2 are approximately equal: v A - v B = i d1 (Z 1 - Z 2 ) Remedies = (6 na) (20 K W) = 120 µv Shield electrodes to reduce i d Reduce or match the electrode skin impedances (minimize Z 1 - Z 2 ) Z 2 Z G Z 1 I d1+ I d2 Power line C 2 I d2 I d1 Electrocardiograph A mechanism of electric-field pickup of an electrocardiograph resulting from the power line. Coupling capacitance between live power line and lead wires causes current to flow through skin-electrode impedances on its way to ground. C 1 A B 220 V G C 3

Magnetic Field Coupling Magnetic-field pickup by the elctrocardiograph (a) Lead wires make a closed loop (shaded area) when patient and electrocardiograph are considered in the circuit. The change in magnetic field passing through this area induces a current in the loop. (b) This effect can be minimized by twisting the lead wires together and keeping them close to the body in order to subtend a much smaller area. Sources Power lines Transformers and ballasts in fluorescent lights Remedies Shielding Route leads away from potential sources twist the lead wires

Other Noise Sources Electromagnetic radiation Patient leads become antennas, especially if detached. Sources Radio Television Radar Research equipment Electrosurgical devices Arching fluorescent lights (needing replacement) Remedy Employ capacitors shunting the inputs to ground (eg., 200 pf). Do not lower the input impedance of the amplifier.