PBL 5 - common ECG abnormalities and their physiological basis

Question 1

where do ventricles begin to depolarise from?

Answer 1

the apex

Question 2

what is atrial excitation associated with?

Answer 2

atrial muscle contraction

Question 3

what is the p wave?

Answer 3

atrial depolarisation

Question 4

what is the QRS complex?

Answer 4

ventricular depolarisation

Question 5

what is the T wave?

Answer 5

ventricular repolarisation

Question 6

why can’t you see atrial repolarisation?

Answer 6

hidden by the QRS complex

Question 7

on ECG paper, how long is 1 square?

Answer 7

0.2 seconds

Question 8

when determining ECG abnormalities, what 3 things do you need to look for?

Answer 8

1. does the ECG complex look normal? (accounting for different leads)
2. is the rhythm normal (atrial vs. ventricular rhythm)
3. what underlying physiological mechanism is disturbed? (pacemakers, conduction, contraction)

Question 9

describe this ECG

Answer 9

• clear recording
• taken from lead II
• pretty normal shape
• repeated small deviation
• extended period between p wave and the QRS complex (extended P-R interval) = 1st degree heart block

Question 10

what is 1st degree heart block? what is seen on ECG?

Answer 10

• increased P-R interval
• problem in conduction through the AVN and bundle of His — a slowing in conduction that leads to an increased temporal separation between the depolarisation + contraction in the atria and the depolarisation + contraction in the ventricles

Question 11

describe this ECG

Answer 11

= complete heart block

• each P wave is not associated with a QRST complex
• abnormal shape because the depolarisation gas started in an abnormal position and therefore spread abnormally throughout the ventricles
• QRS rhythm is very slow
• abnormal condition through ventricular muscle

Question 12

what is complete heart block?

Answer 12

no conduction through the AVN and the bundle of His down into the ventricles

Question 13

why is there not a normal cardiac output in complete heart block?

Answer 13

• ventricles contracting much more slowly

- because the ventricle is contracting abnormally, it wont be acting as efficiently as normal therefore not a normal CO

Question 14

in complete heart block why is there still a QRS complex ?

Answer 14

• come about because some portion of the ventricles has now become the pacemaker region for the ventricles
• QRS rhythm is very slow

Question 15

describe this ECG

Answer 15

= sinus arrhythmia

• an irregularity in rhythm
• origin within SAN itself
• one P wave per QRS complex

Question 16

describe the normal fluctuation in the sinus rhythm in healthy individuals associated with breathing

Answer 16

• during inspiration, the parasympathetic drive to the sinus node is reduced — HR speeds up
• during expiration, the parasympathetic drive to the sinus node is increased — HR slows

normal = slow, deep breathing

Question 17

describe this ECG

Answer 17

= ventricular extrasystole

• rhythm strip from one of chest leads
• repeating QRS complex and associated T wave
• very large abnormal complex from one of ventricles
• not a cause for concern
• can come about because for some reason an area of the ventricular muscle has become more excitable and has generated an extra contraction for this beat in the ventricle

Question 18

what is a wobbly baseline related to?

Answer 18

the atria

Question 19

describe this ECG

Answer 19

= atrial fibrillation

• multiple depolarisation across atria in an uncoordinated way
• creating lots of p like waves
• has a relatively small effect on ventricular filling and function
• can produce some stagnant areas of blood within the atria
• atria doesn’t empty normally and so the stagnant blood can be associated with blood clotting — concern in addition to potential effect on heart pumping

Question 20

a myocyte that has just depolarised is in a state of what?

Answer 20

refractoriness — can’t depolarise again immediately after depolarisation

Question 21

what does the refractoriness allow?

Answer 21

allows electrical activity to spread out across the heart muscle

Question 22

how does depolarisation change in damaged myocytes?

Answer 22

• depolarisation spreads more slowly

- slowed conduction

Question 23

describe this ECG

Answer 23

= ventricular fibrillation

• seen in heart attack
• significant change in electrical activity
• normal waves at the start
• uneven contraction -> uneven pressure -> abnormal stroke volume

Question 24

why is the size of electrical activity during ventricular fibrillation much larger than in atrial fibrillation?

Answer 24

muscle mass is larger in the ventricles

Question 25

what is ST segment elevation/depression an effect of and why?

Answer 25

myocardial hypoxia on repolarisation (=marker for local effect if hypoxia on myocardium)

hypoxia interferes with ion channels that would be mediating the repolarisation of the ventricles

ST elevation = K+ channels opening

Question 26

what are leads I, II and III?

Answer 26

recordings from the primary 3 limb leads

Question 27

what are aVR, aVL and aVF?

Answer 27

augmented limb leads

Question 28

what is the right panel?

Answer 28

electrodes placed directly on the chest wall = V1, V2, V3, V4, V5, V6

Question 29

what is this?

Answer 29

rhythm strip — longer period recording taken from a single lead (by convention lead II) — shows rhythm, across a longer period of time of the cardiac cycle

Question 30

describe this ECG

Answer 30

AF - indicative of a patient with mitral valve disease

Question 31

describe this ECG

Answer 31

1st degree AV block due to delay in conduction through the AVN

Question 32

describe this ECG

Answer 32

sinus tachycardia

Question 33

describe this ECG

Answer 33

ST elevation indicative of an MI, a patient with a recent MI would also have raised cardiac troponin levels