Physiological basis for ECG

Question 1

what does the ECG measure

Answer 1

Potential DIFFERENCES between recording electrodes generated by electrical currents projecting from the heart to body surface during depolarization and repolarization

measures the sum of electrical potentials generated by the conduction of electrical depolarization and repolarization in the heart as projected to the body surface

Question 2

what do different EKG leads do?

Answer 2

view electircal vectors of the heart from different angles, providing information about electrical activity in different regions

Question 3

what info can EKG’s give

Answer 3

LOTS

Hypertrophy and Enlargement
Rate and Rhythm Abnormalities 
Conduction Abnormalities
Ischemia and Infarction
Electrolyte Disturbances, Drug Effects, etc.

Question 4

what 3 things contribute to reproducible rate and rhythm observed on the ECG

Answer 4

automaticity

conduction system

functional synctium (cell to cell via gap junctions)

Question 5

what is the p wave

Answer 5

atrial depolarization

begins at the SA node and spreads toward the AV node

right atrium begins to depolarize before the left

***immediately precedes atrial contraction

Question 6

what is the QRS complex

Answer 6

ventricular depolarization (atria repolarizing simultaneously)

IMMEDIATELy precedes ventricular contraction

atria remain contracted until repolarization

ventricles remain contracted until after the T wave of repolarization

Question 7

what is the T wave

Answer 7

ventricular repolarization

Question 8

if the first deflection is downward

Answer 8

Q wave

Question 9

the first upward deflection is

Answer 9

the R wave

Question 10

second upward deflection

Answer 10

R primary

Question 11

the first downward deflection following an upward deflection

Answer 11

S wave

Question 12

if the entire configuration consists solely of one downward deflection

Answer 12

QS wave

Question 13

if the first wave of the complex is an R wave, the following downward deflection is ….

Answer 13

an S wave NOT a Q wave

Question 14

if there is a downward deflection only then it is a

Answer 14

Q wave if is the first wave of the complex

Question 15

in which direction does repolarization normally travel?

Answer 15

begins at the last area that was depolarized, travels toward the base of the heart in the opposite direction of depolarization

slower than ventricular depolarization

Question 16

5 large squares =

Answer 16

1 second

so one little square is 0.2 seconds

Question 17

2 large squares vertically

Answer 17

1 mV

Question 18

what is the PR interval? why is it important?

Answer 18

represents the time from the start of atrial depolarization to the start of ventricular depolarization

includes AV node delay time for filling

normally 0.12-0.2 seconds (just over 2 small squares)

important for identifying conduction blocks

Question 19

what is the ST segment and why is it important clinically?

Answer 19

Represents time from end of ventricular depolarization to start of ventricular repolarization
Generally horizontal or gently up-sloping in all leads

***Deviation from isoelectric line indicates myocardial ischemia or injury

Question 20

what does ST elevation indicate

Answer 20

MI
acute pericarditis
LVH
hyperkalemia , etc.

Question 21

what is the QT interval

Answer 21

beginning of ventricular depolarization to end of ventricular repolarization

includes all electrical events occurring in the ventricles

HR dependent

more time in repolarization phase (t wave is wider than QRS)

Question 22

what are the causes of long QT syndrome?

Answer 22

Congenital:
-mutant Na (failure to inactivate)
K+ (Defective repolarization)

Acquired:
-electrolyte imbalances, pharmacological, etc.

Question 23

what is the length of the QT interval

Answer 23

less than or equal to 0.44 seconds

Question 24

what are the two ways of determining heart rate

Answer 24

counting boxes 300–> 150–> 100 –> 75–> 60 –> 50–> 43–> 38–> 33–> 30)

or by estimating HR by number of R-R intervals in 6 secs x 10

(#R-R/6 sec x 10) = bpm

(3 seconds between each tick)