Basic Electrophysiology - Triton College



Basic Electrophysiology

-Types of cardiac cells

-Myocardial cells- working or mechanical cells

-Pacemaker cells- responsible for spontaneous generation and conduction of impulses

-Electrical impulses are the result of brief but rapid flow of ions back and forth across the cell membrane

-The major electrolytes that that affect cardiac function are sodium, potassium, and calcium

-Membrane potential- difference in electrical charge across the cell membrane

-Threshold- membrane potential at which cell membrane will depolarize and generate an action potential

-Action potential- reflection of the difference in the concentration of ions across a cell membrane at any given time

-Types of action potentials

-Fast-response- cells of the atria, ventricles, and purkinje fibers, occurs because of the presence of many voltage-sensitive sodium channels that allow a rapid influx of sodium when these channels are open and prevent influx when they are closed

-slow-response- cells of the SA and AV node have slow calcium and sodium channels, can also abnormally occur anywhere in the heart (Usually secondary to ischemia, injury, or an electrolyte imbalance)

-Polarization- resting state during which no electrical activity occurs, the cell is negatively charged

-Depolarization- when the cardiac cell is stimulated, the cell is positively charged

-Repolarization- the returning back to a resting state, the cell is returned back to a negative charge

Phases of the cardiac action potential

Phase 0- rapid depolarization; sodium moves rapidly into the cells, potassium leaves the cells, calcium slowly moves into the cells, the cell depolarizes and cardiac contraction begins

Phase 1- early repolarization; the flow of sodium into the cell is slowed down

Phase 2- repolarization (plateau phase); there is still a slow inward movement of calcium into the cells and a continued outward movement of potassium from the cells

Phase 3- rapid repolarization; the flow of calcium, potassium, and sodium has stopped

Phase 4- resting membrane potential; the sodium-potassium pump actively transports sodium out of the cell and potassium back in the cell

-Refractoriness- extent to which a cell is able to respond to a stimulus

-Absolute refractory period- cardiac cells cannot be stimulated to conduct an electrical impulse; corresponds with the onset of the QRS to the mid point of the T wave

-Relative refractory period- some cardiac cells have repolarized to their threshold potential and can be stimulated to respond to a stronger than normal stimulus; corresponds with the mid point of the T wave to the end of the T wave

-Properties of cardiac cells

-Automaticity- the ability for a cardiac cell to spontaneously create an impulse

-Excitability- the ability for a cardiac cell to respond to an external stimulus

-Conductivity- the ability for a cardiac cell to receive an impulse and conduct that impulse to an adjacent cardiac cell

-Contractility- the ability for cardiac cells to contract to electrical stimulus

-Conduction System

-Sinoatrial node (SA node) - is the normally the primary pacemaker of the heart, initiating impulse at a regular rate of 60-100 beats a minute; it is located in the upper posterior portion of the right atrium at the junction of the superior vena cava and the right atrium and it receives it blood supply from the right coronary artery

-Atrioventricular junction

-AV node- is located in the posterior septal wall of the right atrium and it receives it blood supply from the left circumflex artery

-Bundle of His- it is located in the upper potion of the interventricular septum and connects the AV node with the two bundle branches; it receives it blood supply from left anterior and posterior descending coronary arteries; has the capability of discharging an electrical impulse of 40-60 beats a minute

-Right and left bundle branches- conduct the impulse to the purkinje fibers in the ventricles

-Purkinje Fibers- an elaborate web of fibers distributed throughout the ventricular myocardium

Causes of dysrhythmias

-Enhanced automaticity- an abnormal condition in which cardiac cells not normally associated with the property of automaticity begin to depolarize spontaneously or when escape pacemaker sites increase their firing rate beyond that which is considered normal

-Reentry- Propagation of an impulse through tissue already activated by the same impulse

-Escape beats or rhythms- when the sinus node slows down or fails to initiate depolarization and a lower pacemaker site spontaneously produces electrical impulses, assuming responsibility for pacing the heart

-Conduction disturbances-can result from trauma, drug toxicity, electrolyte disturbances, myocardial ischemia, or infarction

-Electrodes- are used for surface (skin) electrocardiography

-Leads- is a record of electrical activity between two electrodes

-Frontal Plane leads- view the heart from the front of the body; there are six of these leads (I, II, III, aVR, aVL, and aVF)

-Horizontal plane leads- view the heart as if the body were sliced in half horizontally at the heart; there are six of these leads (V1, V2, V3, V4, V5, and V6)

-each lead has a negative, positive, and a pole

-if the electrical impulse moves toward the positive electrode, the waveform would be upright, if the electrical impulse moves away from the positive electrode, the waveform would be inverted

-there are three types of leads

-Standard limb leads- if an electrode is placed on the right arm, left arm, and left leg, three leads are formed. An imaginary line joining the positive and negative electrodes of a lead is called the axis of the lead. The axes of these three limb leads form an equilateral triangle with the hear at the center (Einthovens’s triangle)

-Lead I- records the electrical potential between the left arm (+) and the right arm (-)

-Lead II- records the electrical potential between the left leg (+) and the right arm (-)

-Lead III- records the electrical potential between the left leg (+) and the left arm (-)

Augmented limb leads- are augmented limb leads that are unipolar consisting of only one electrode

-Lead aVR- views the heart from the right shoulder

-Lead aVL- views the heart form the left shoulder

-Lead aVF- views the heart from the left foot

Precordial (chest) leads- are unipolar leads that view the heart in the horizontal plane

-V1- located in the fourth intercostals space, just to the right of the sternum

-V2- located in the fourth intercostals space, just to the left of the sternum

-V3- located on a line midway between V2 and V4

-V4- located in the left midclavicular line in the fifth intercostals space

-V5- located in the anterior midaxillary line in the fifth intercostals space

-V6- located in the midaxillary line in the fifth intercostals space

Right precordial leads

-V1R- V2

-V2R- V1

-V3R- midway between V2r and V4R

-V4R- located in the right midclavicular line in the fifth intercostals space

-V5R- located in the anterior midaxillary line in the fifth intercostals space

-V6R- located in the midaxillary line in the fifth intercostals space

-ECG paper -the horizontal axis on the ecg paper corresponds with time, each 1mm box equals 0.04 seconds; the lines between every 5 small boxes equal 0.20 seconds; every 5 large boxes equal 1 second; 15 large boxes equal 3 seconds; 30 large boxes equal 6 seconds

-the vertical axis represents voltage or amplitude, the vertical axis of one small box represents 1 mm

-Waveforms

-P wave- represents depolarization and the spread of the electrical impulse throughout the right left atria; a normal P wave appear smooth, rounded, and upright in lead II

-PR segment- represents the conduction of the impulse through the AV node, bundle of His, right and left bundle branches, and the purkinje fibers

-PR interval- represents the depolarization and the spread of the electrical impulse throughout the right left atria and the spread of the impulse through the AV node, bundle of His, right and left bundle branches, and the purkinje fibers

-QRS complex- represents depolarization of the ventricles; it consists of the Q wave, R wave, and the S wave

-Q wave- begins as a downward deflection; the Q wave is always a negative waveform

-R waves- the QRS complex continues as a large, upright, triangular waveform known as the R wave; the R wave is always positive

-S wave- the S wave is the negative waveform following the R wave; the S wave is always negative

Variations of the QRS complex- although the term QRS complex is used, not every QRS complex contains Q wave, R wave, and S wave

-ST segment- represents the early part of repolarization of the ventricles. The term is regardless weather the QRS ends in a Q, R, or S wave.

-normal characteristics are for the ST segment to be within 1 mm above or below the PR segment

-the point where the QRS complex and the ST segment meet is called the junction or J point

-T wave- represents ventricular repolarization. The Lead II the T wave is positive and bigger than the P wave

-QT interval- represents total ventricular activity

-U wave- small waveform when seen follows the T wave, the mechanism is unknown

-Artifact- distortion of an ECG tracing by electrical activity that is noncardiac in origin

-Loose electrode

-Patient movement/muscle activity

-60-cycle (AC) interference

Rate measurement

-Six second count- count the number of QRS complexes within a 6 second period, then multiply that number by 10

-Large boxes- count the number of large boxes between two consecutive QRS complexes, and divide that number by 300

-Small boxes- count the number of small boxes between two consecutive QRS complexes, and divide that number by 1500

-Sequence method- select a QRS complex that falls on a dark vertical line. Number the next consecutive dark vertical lines as follows: 300, 150, 100, 75, 60, and 50. Note were the next QRS complex falls in relation to the 6 dark vertical lines already marked.

-Rhythm/ regularity- are used to describe the regularity or irregularity of a waveform. Primarily we are concerned with the regularity of the QRS complexes “Ventricular rhythm”. If the rhythm is regular, the R-R intervals are the same. The atrial rhythm can be checked for regularity also.

Analyzing a rhythm strip

-Assess rate

-Assess rhythm/regularity

-Identify and examine P waves

-Assess intervals

-PR interval

-Qt interval

-Evaluate overall appearance of the rhythm

-Interpret the rhythm and evaluate its clinical significance

-noting wide QRS complexes, abnormal appearing Q waves, ST depression or elevation, abnormal appearing P waves, and premature beats

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