Electrocardiogram

Question 1

define an Electrocardiogram

Answer 1

Summation of all the depolarizations and repolarizations in a cardiac cycle

Question 2

Define the P wave

Answer 2

Summation of:

• lots of atrial myocytes depolarization
• Positive charge moving towards electrode causes Upward deflection
• generation of SA node it not detectable
• 1-2 Small boxes in length

Question 3

Define the QRS complex

Answer 3

Summation of lots of ventricular myocytes depolarizing

• Q = slight negative deflection - small due to correspondence with septal depolarization
• R = positive (upward) deflection - ventricular muscle depolarizing spreading towards electrode
• S = negative (downward) deflection - ventricular muscles depolarizing spreading AWAY from electrode.
• -> 1-3 small boxes in length

Question 4

What is occurring between P wave and QRS complex? (P-R interval)

Answer 4

Plateau of action potential caused by prolonged opening of Calcium channels results in no net movement of ions and thus no current is measured on ECG between P and QRS waves
–> 1-3 small boxes

Question 5

Define T wave

Answer 5

Summation of:

• lots of ventricular cells REPOLARIZING
• reversal of charge movement makes direction of QRS and T waves similar.
• -> variable in length

Question 6

What is occuring between S and T waves (S-T segment)

Answer 6

Corresponds to all the PHase 2’s of ventricular muscle

• plateau in AP caused by calcium channels
• -> variable in length (roughly half of R-R interval)

Question 7

Define Q-T interval

Answer 7

ventricular action potential duration (initiation of ventricular depolarization to end of ventricular repolarization
–> variable (with HR) (gets shorter with high HR)

Question 8

Define the six Frontal plane leads (limb leads)

Answer 8

Standard Leads (bipolar leads)
- I = negative electrode on right wrist with positive electrode on left wrist
- II = Negative electrode on right wrist with positive electrode on left ankle
- III = Negative electrode on left wrist with positive electrode on left Ankle
Unipolar Limb Leads:
- aVR = augmented voltage - right arm (+)
- aVL = augmented voltage - left arm (+)
- aVF = augmented voltage - left foot (+)

Question 9

Describe the (Transverse/Horizontal plane) chest lead placement

Answer 9

V1: 4th intercostal space, R sternal border (Right atria)
V2: 4th intercostal space, L sternal border
V4: 5th L intercostal space in midclavicular line
V3: Between V2 and V4
V5: in line with V4, in anterior axillary line (L. Ventricle)
V6: in line with V4 and V5, in midaxillary line (L. ventricle)

Question 10

What to look for in a ECG

Answer 10

• Rate and regularity
• Mean QRS axis in the frontal plane
• Rhythm (intervals, waveform morphologies)

Question 11

How is rate determined and normal value.

Answer 11

determined from R-R interval generally. (#large boxes divided by 300)

• Bradycardia = RESTING HR under 60 bpm
• Tachycardia = RESTING HR over 100bpm

Question 12

What are some key points about vector analysis

Answer 12

• voltage recorded in the ECG will be GREATEST when the vector is PARALLEL to lead
• voltage recorded in the ECG will be LEAST when vector is PERPENDICULAR to lead

Question 13

What would an increased QRS voltage be due too?

Answer 13

• Hypertrophy (more muscle that is depolarizing)

- Thin chest (can detect voltages better

Question 14

What would a decrease QRS voltage be due to?

Answer 14

• Previous MI (less muscle, also wide QRS)
• Fluid in pericardium or pleural space (dampens signal that reaches outside surface of body)
• Large chest size, pulmonary emphysema insulate charge and reduce what reaches the surface

Question 15

Describe how to find the approximation of axis of the QRS

Answer 15

1) look at lead I and aVF
2) “normal” vector was 60 degrees which would make leads I and aVF normally positive
- Right axis deviation = I neg; aVF positive
- Left axis deviation = I positive; aVF negative
- Indeterminant axis = Both negative
* *How do you check your approximation??**

Question 16

Define Lead I

Answer 16

• Lead I runs from RA to LA, positive pole on LA
• Atrial depolarization will yield small positive deflection in lead I due to the vector pointing down and to the left
• Ventricular depolarization will lead a larger positive deflection in lead I due to vector point down and to left.

Question 17

Define Lead II

Answer 17

• Runs from RA to LL, Positive pole on LL
• SAME atrial depolarization vector will yield a small positive in lead II
• SAME ventricular depolarization vector will yield a larger positive deflection in lead II (since vector is MORE Parallel to lead II)

Question 18

Define Lead III

Answer 18

• lead III runs from the LA to the LL, positive pole on the LL
• Vectors can be almost perpendicular to the axis of this lead so the deflections of the superimposed vectors are small

Question 19

Define Lead aVR

Answer 19

• lead aVR is a positive axis to RA
• “negative” pole is the grounding axis perpendicular to RA axis (looks a lot like lead III)
• SAME atrial depolarization will yield a small NEGATIVE deflection in lead aVR
• SAME ventricular depolarization will yield a BIGGER NEGATIVE deflection in lead aVR

Question 20

Define Lead aVF

Answer 20

• lead aVF is a positive axis to LL (foot)
• Negative axis looks like Lead I
• SAME atrial depolarization will yield a small POSITIVE deflection in lead aVF (positive charge moving towards positive pole)
• SAME ventricular depolarization will yield a POSITIVE deflection in lead aVF

Question 21

Define Lead AVL

Answer 21

• Lead aVL is a positive axis to LA
• NEGATIVE axis looks like lead II
• SAME atrial depolarization will yield small positive deflection in lead aVL (axis of atrial depolarization almost perpendicular)
• SAME ventricular depolarization will yield a small positive deflection (almost perpendicular)