Basic Electrophysiology Chapter 2 Basic Electrophysiology

Cardiac Cells Types of cardiac cells Myocardial cells (mechanical cells) contain contractile filaments. When electrically stimulated, the contractile filaments slide together and the myocardial cell contracts. Pacemaker cells are specialized cells responsible for spontaneously generating and conducting electrical impulses.

Cardiac Cells Properties of Cardiac cells Automaticity is the ability of cardiac pacemaker cells to create an electrical impulse without being stimulated from another source. Excitability (irritability) refers to the ability of cardiac muscle cells to respond to an outside stimulus. Conductivity refers to the ability of a cardiac cell to receive an electrical impulse and conduct it to an adjoining cardiac cell. Contractility refers to the ability of myocardial cells to shorten in response to an impulse.

Cardiac Action Potential Current is the flow of electrical charge from one point to another. Voltage is the measurement of potential energy and measure between 2 points. Electrolytes are elements or compounds that break into charged particals (ions) when melted or dissolved in water or another solvent. The main electrolytes that affect the function of the heart are Na+, K+, Ca++, and C1-.

Cardiac Action Potential The action potential is a 5-phase cycle that reflects the difference in the concentration of these charged particles across the cell membrane at any given time.

Cardiac Action Potential Polarization (Resting phase) Polarized state is when the inside of a cell is more negative than the outside. The voltage (difference in electrical charges) across the cell membrane is the membrane potential.

Cardiac Action Potential Depolarization (Electrical event) Permeability refers to the ability of a membrane channel to allow passage of electrolytes once it is open. The movement of charged particles across a cell membrane causing the inside of the cell to become positive is called depolarization. Depolarization must take place before the heart can mechanically contract and pump blood. It occurs because of the movement of Na+ into the cells. The P wave represents atrial depolarization. The QRS complex represents ventricular depolarization.

Cardiac Action Potential Repolarization (Recovery phase) After the cell depolarizes, it quickly begins to recover and restore its electrical charges to normal. The movement of charged particles across a cell membrane in which the inside of the cell is restored to its negative charge is repolarization.

Refractory Periods Refractoriness describes the period of recovery that cells need after being discharged before they are able to respond to a stimulus. Absolute refractory period is when cells won’t respond to further stimulation. During relative refractory period some cardiac cells have repolarized to their threshold potential & can be stimulated to respond to a stronger than normal stimulus. During the supernormal period, a weaker than normal stimulus can cause cardiac cells to depolarize during this period.

Conduction System Sinoatrial (SA) node The heart’s pacemaker cells have a built-in rate that becomes slower & slower from the SA node down to the end of the His-Purkinje system. The SA node is the primary pacemaker of the heart because it has the fastest firing rate of all of the heart’s normal pacemaker sites. Other areas of the heart can take over if: The SA node fails to fire The SA node fires too slowly The SA node fails to activate surrounding atrial myocardium As the impulse leaves the SA node, it is spread from cell to cell in wavelike form across the atrial muscle. As it spreads, it stimulates the right atrium, interatrial septum, & the left atrium.

Conduction System Atrioventricular (AV) Junction The AV junction is the AV node and the nonbranching portion of the bundle of His. This area consists of specialized conduction tissue that provides the electrical links between the atria and ventricles.

Conduction System Atrioventricular (AV) node The AV node is a group of cells located in the floor of the right atrium behind the tricuspid valve & near the opening of the coronary sinus. As the impuse from the atria enters the AV node, there is a delay in conduction of the impulse to the ventricles. The delay in conduction allows the atria to empty blood into the ventricles before the next ventricular contraction begins. When atrial rates are very fast, the AV node helps to regulate the # of impulses reaching the ventricles to protect them from dangerously fast rates.

Conduction System Bundle of His (common bundle or AV bundle) The bundle of His conducts the electrical impulse to the right and left bundle branches. Receives a dual blood supply from branches of the left anterior & posterior descending coronary arteries.

Conduction System Right and Left Bundle Branches The right bundle branch innervates the right ventricle The left bundle branch spreads the electrical impulse to the interventricular septum and left ventricle. Fascicles are small bundles of nerve fibers.

Conduction System Purkinje Fibers The right and left bundle branches divide into smaller and smaller branches and then into a special network of fibers called the Purkinje Fibers.

Causes of Dysrhythmias Enhanced Automaticity Cardiac cells that aren’t normally associated with a pacemaker function begin to depolarize spontaneously A pacemaker site other than the SA node increases its firing rate beyond that which is considered normal

Causes of Dysrhythmias Triggered Activity Results from abnormal electrical impulses that sometimes occur during repolarization, when cells are normally quiet. It occurs when pacemaker cells from a site other than the SA node & myocardial working cells depolarize more than once after being stimulated by a single impulse Ectopic refers to an impulse originating from a source other than the SA node.

Causes of Dysrhythmias Reentry Reentry is the spread of an impulse through tissue already stimulated by that same impulse. Reentry requires the following three conditions: Potential conduction circuit or circular conduction pathway A block within part of the circuit Delayed conduction with the remainder of the circuit Escape Beats or Rhythms Escape beats or rhythms are protective mechanisms to maintain cardiac output.

The Electrocardiogram The electrocardiogram (ECG) records the electrical activity of a large mass of atrial and ventricular cells as specific waveforms or complexes. Electrodes Electrode refers to the paper, plastic, or metal device that contains conductive media and is applied to the patient’s skin.

The Electrocardiogram ECG monitoring may be used for the following: Monitor a patient’s heart rate Evaluate the effects of disease or injury on heart function Evaluate pacemaker function Evaluate the response to medications Obtain a baseline recording before, during, and after a medical procedure.

The Electrocardiogram Leads Each lead records the average current flow at a specific time in a portion of the heart. A 12-lead ECG provides views of the heart in both the frontal and horizontal planes and views the surfaces of the left ventricle from 12 different angles.

The Electrocardiogram Frontal Plane Leads Directions in the frontal plane are superior, inferior, right and left. Six lead views: 3 bipolar and 3 unipolar leads. A bipolar lead has a positive and negative electrode. Leads I, II, and III are called bipolar leads or standard limb leads. A lead that consists of a single positive electrode and a reference point is called a unipolar lead. Leads aVR, aVL, and aVF are called unipolar or augmented limb leads.

Standard Limb Leads Lead Positive electrode Negative electrode Surface viewed I Left arm Right arm Lateral II Left leg Inferior III

Augmented Leads Lead Positive electrode Surface viewed aVR Right arm None aVL Left arm Lateral aVF Left leg Inferior

Positive electrode position Chest Leads Lead Positive electrode position Surface viewed V1 Right side of sternum, 4th intercostal space Septum V2 Left side of sternum, 4th intercostal space V3 Midway between V2 & V4 Anterior V4 Left midclavicular line, 5th intercostal space V5 Left anterior axillary line at same level as V4 Lateral V6 Left midaxillary line at same level as V4

What Each Lead “sees” Leads Surface viewed II, III, aVF Inferior V1, V2 Septal V3, V4 Anterior I, aVL, V5, V6 Lateral

The Electrocardiogram Terminology Baseline a straight line recorded when electrical activity isn’t detected. Waveform movement away from baseline in either a positive or negative direction. Segment a line between waveforms; named by the waveform that follows it. Interval a waveform and a segment. Complex several waveforms.

Waveforms P wave The 1st waveform in the cardiac cycle is the P wave Normal characteristics of the P wave: Smooth and rounded No more than 2.5 mm in height No more than 0.11 seconds in duration Positive in leads I, II, aVF, and V2-V6

Waveforms QRS Complex Normal characteristics of the QRS complex: Normal duration of QRS complex is 0.10 seconds or less Normal Q wave in limb leads is less than 0.04 seconds in duration A Q wave is ALWAYS a negative waveform. An R wave is ALWAYS a positive waveform. An S wave is ALWAYS a negative waveform.

Waveforms T Wave Normal characteristics of the T wave: Slightly asymmetric Usually 5 mm or less in height in any limb lead or 10 mm or less in any chest lead Usually 0.5 mm or more in height in leads I and II

Waveforms U Wave A U wave Is a small waveform that, when seen, follows the T wave. Characteristics of the U wave Rounded and symmetric Usually less than 1.5 mm in height and smaller than preceding T wave A U wave more than 1.5 mm in height in any lead is considered abnormal

Waveforms PR segment ST segment PR Interval QT Interval Horizontal line between the end of the P wave and beginning of QRS complex. ST segment Between the QRS complex and the T wave is the ST segment. PR Interval The P wave plus the PR segment equals the PR interval. QT Interval Is measured from the beginning of the QRS complex to the end of the T wave.

Analyzing a Rhythm Strip Ventricular Rhythm To determine if regular or irregular, measure distance between the two R-R intervals. Atrial Rhythm To determine if regular or irregular, measure distance between the P-P intervals.

Analyzing a Rhythm Strip Assess the Rate Tachycardia exists if the rate is more than 100 beats/min Bradycardia exists if the rate is less than 60 beats/min

Analyzing the Rhythm Strip Identify and Examine P waves Look to the left of each QRS complex; if there is a P wave, they occur regularly; and they look similar in size, shape, and position.

Analyzing a Rhythm Strip Assess Intervals (Evaluate Conduction) PR Interval Normal PR interval is 0.12 to 0.20 seconds QRS Duration The QRS is considered narrow (normal) if it measures 0.10 seconds or less The QRS is considered wide if it measures more than 0.10 seconds