Test 1 (Part 4) Flashcards
Electrocardiogram
- 12 Leads placed on skin surface at various locations on the torso
- Each records the VOLTAGE DIFFERENCE between itself and another location (s) on the Torso
- If there is a DIFFERENCE in VOLTAGE then there will be a DEFLECTION
- If there is NO DIFFERENCE then NO DEFLECTION
Characteristics of the ECG
- Illustrates changes of Electrical Activity of Cardiac Tissue produced by regions of DEPOLARIZATION or DEPOLARIZATION
- “Measures” EXTRACELLULAR Potential
- Only causes a DEFLECTION when:
A) Part of the CARDIAC EXCITABLE TISSUE is at a DIFFERENT Membrane Potential than the REST of the HEARTB) Current flow can OCCUR between those REGIONS - Does not cause a DEFLECTION when only the ATRIA and VENTRICLES are DIFFERING POTENTIALS
Characteristics of the ECG
- The ELECTRODES report Voltage differences in regions of EITHER the VENTRICLES of ARTIA
- Does NOT report differences between the ATRIA and VENTRICLES. WHY?
- We do not see a single based on current flow between the Atria and Ventricles - Plots changes in VOLTAGE DIFFERENCE with TIME
ECG Electrode Placements
- Each electrode looks at VOLTAGE CHANGES in the HEART from a different Direction
- One direction, referred to in the example is LEAD I, essentially looks at the HEART from LEFT to RIGHT
Voltmeter
1) Voltmeter Zero:
- Both cells Polarized with INSIDE Negative and OUTSIDE Positive. Electrodes (lead) see Outside
2) Voltmeter Charge:
- One cell DEPOLARIZE with POSITIVE inside and NEGATIVE outside. Positive electrodes sees Positive versus Negative on OPPOSITE Side
Cardiac Depolarization Path
- SA Node causes ATRIA to DEPOLARIZE from RIGHT to LEFT—-> P WAVE!!!!!!
- AV Node delays signal —-> PR INTERVAL!!!!!
- Ventricles Depolarize generally from Right to Left and from Apex to Base —-> QRS COMPLEX!!!!!!
- Action Potential PHASE 2 delays Depolarization of Ventricles —–> ST SEGMENT!!!!!!!
- Ventricles Depolarize generally from LEFT to RIGHT and BASE to APEX —–> T WAVE!!!!!
P Wave
- The P Wave represents PHASE 0 on the AP’s SPREADING through the ATRIAL Muscle
QRS Complex
- The QRS Complex represents PHASE 0 of the AP SPREADING throughout the VENTRICLES
T Wave
- The T WAVE represents PHASE 3 of DEPOLARIZATION of VENTRICULAR Muscle fibers spreading through the VENTRICLES!!!
Type of Information you Should Know
- Where are the Timed K+ gates opening in the Ventricles? —–> T WAVE!!!!
- When is Na+ MOST Permeable? —> P Wave and QRS
- Sketch an AP for SA and also AV Nodal Tissue
- **3 Types of K+ Channels:
1) Help Keep resting membrane Potential
2) Voltage Gated, causing PHASE 1 to occur
3) Close during DEPOLARIZATION and stay CLOSED for a FINITE period of time (TIMED)
- Closed during PHASE 2 and open during PHASE 3
Intervals and Segments
1) Segment represent she duration of a SINGLE EVENT on the ECG
2) An INTERVAL represents the duration of SEVERAL EVENTS
Segments
1) PR SEGMENT:
- End of ATRIAL DEPOLARIZATION until BEGINNING of QRS
2) ST SEGMENT:
- End of QRS until BEGINNING of T WAVE
Intervals
1) PR INTERVAL:
- Beginning of P Wave until BEGINNING of QRS
2) QT INTERVAL:
- Beginning of QRS until end of T WAVE
Why 12 Leads?
- Each LEAD displays VOLTAGE DIFFERENCES in the MYOCARDIUM seen from different perspectives by DIFFERENT ELECTRODES
AP Spreading
- As AP spreads through the HEART it is viewed by EACH LEAD from a DIFFERENT ANGLE
- If the AP is spreading towards the POSITIVE LEAD of the electrode set is PRODUCES a POSITIVE DEFLECTION. If going away, it produces a NEGATIVE DEFLECTION!!!
Labeling of Leads and Electrode Placements
1) STANDARD (Bipolar) Limb Leads use:
A) 1: Right Arm (RA) to Left Arm (LA)
B) 2: RA to Leg (LL)
C) 3: LA to LL
2) AUGMENTED Limb Leads:
A) aVF: Augmented Vector, Foot
- (RA+LA) to LL
B) aVR: Augmented Vector, Right
- (LL +LA) to RA
C) aVL: Augmented Vector, Left
- (LL + RA) to LA
3) Chest or Precordial Leads
- V1-6 (Vectors 1 through 6)
Standard Limb Lead Axis (EINTHOVEN’s TRIANGLE)
- Each lead is assigned a POSITIVE and NEGATIVE Polarity and the Voltage between them measured!!!
Augmented (a) Limb Leads
- One of the limb leads is assigned a POSITIVE POLARITY and the remaining 2 Electrodes are AVERAGED and ASSIGNED a NEGATIVE POLARITY
Chest or Precordial Leads
- Precordial leads are not assigned an AXIS
- Instead assigned REGIONS of the HEART
- Each lead acts as the POSITIVE ELECTRODE
Lead Assignments (IMPORTANT!!!!!!)
- Lead electrode placements are denoted to Predominantly represent certain regions of the Heart
1) INFERIOR
- II, III, aVF
2) SEPTAL
- V1, V2
3) ANTERIOR:
- V2, V3, and V4
4) LATERAL
- I, aVL, V4, V5, and V6
Axis Deviation
- If general direction of AP spreads to UPPER LEFT then LEFT AXIS DEVIATION!!!!!!!
- If general direction is to RIGHT or Lower RIGHT then RIGHT AXIS DEVIATION
- Shifted by HYPERTROPHY, MI, Physical Placement of Heart
- Bundle Branch Block
Axis:
1) EXTREME RIGHT AXIS DEVIATION: Top Left Quadrant
2) LEFT AXIS DEVIATION: Top Right Quadrant
3) NORMAL: Bottom Right Quadrant
4) RIGHT AXIS DEVIATION: Bottom Right Quadrant
P Wave
- Anything changing NORMAL origin or PATH of AP through ATRIA will alter P WAVE such as:
1) ECTOPIC Focus/ Pacemaker in either ATRIUM or VENTRICLES2) ENLARGED ATRIA
P Wave:
- ATRIAL DEPOLARIZATION upright in 1,2, V4-V6, AVF inverted in AVR, Variable in 3, AVL, other Chest Leads
PR Interval
- Dependent on mainly HOW LONG AP takes to travel through AV Node
Can be altered by:
1) Autonomic Stimulation
2) Ischemia or Infarct
3) Structural Defect
4) Drugs Altering Conduction
QRS Interval and Complex
- Normal QRS means normal Distribution by bundles PURKINJE FIBERS and NORMAL PATH (Direction and Length) through MYOCARDIUM
Altered by:
1) If not originated from or near AV node
2) Blockage (Infarct) of BUNDLE
3) Cardiac Ischemia or Infarct
4) HYPERTROPHY
- Thicker Wall
- Dilated Ventricle