PBL 5 - common ECG abnormalities and their physiological basis Flashcards
where do ventricles begin to depolarise from?
the apex
what is atrial excitation associated with?
atrial muscle contraction
what is the p wave?
atrial depolarisation
what is the QRS complex?
ventricular depolarisation
what is the T wave?
ventricular repolarisation
why can’t you see atrial repolarisation?
hidden by the QRS complex
on ECG paper, how long is 1 square?
0.2 seconds
when determining ECG abnormalities, what 3 things do you need to look for?
- does the ECG complex look normal? (accounting for different leads)
- is the rhythm normal (atrial vs. ventricular rhythm)
- what underlying physiological mechanism is disturbed? (pacemakers, conduction, contraction)
describe this ECG
- 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
what is 1st degree heart block? what is seen on ECG?
- 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
describe this ECG
= 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
what is complete heart block?
no conduction through the AVN and the bundle of His down into the ventricles
why is there not a normal cardiac output in complete heart block?
- ventricles contracting much more slowly
- because the ventricle is contracting abnormally, it wont be acting as efficiently as normal therefore not a normal CO
in complete heart block why is there still a QRS complex ?
- come about because some portion of the ventricles has now become the pacemaker region for the ventricles
- QRS rhythm is very slow
describe this ECG
= sinus arrhythmia
- an irregularity in rhythm
- origin within SAN itself
- one P wave per QRS complex
describe the normal fluctuation in the sinus rhythm in healthy individuals associated with breathing
- 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
describe this ECG
= 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
what is a wobbly baseline related to?
the atria
describe this ECG
= 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
a myocyte that has just depolarised is in a state of what?
refractoriness — can’t depolarise again immediately after depolarisation
what does the refractoriness allow?
allows electrical activity to spread out across the heart muscle
how does depolarisation change in damaged myocytes?
- depolarisation spreads more slowly
- slowed conduction
describe this ECG
= ventricular fibrillation
- seen in heart attack
- significant change in electrical activity
- normal waves at the start
- uneven contraction -> uneven pressure -> abnormal stroke volume
why is the size of electrical activity during ventricular fibrillation much larger than in atrial fibrillation?
muscle mass is larger in the ventricles
what is ST segment elevation/depression an effect of and why?
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
what are leads I, II and III?
recordings from the primary 3 limb leads
what are aVR, aVL and aVF?
augmented limb leads
what is the right panel?
electrodes placed directly on the chest wall = V1, V2, V3, V4, V5, V6
what is this?
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
describe this ECG
AF - indicative of a patient with mitral valve disease
describe this ECG
1st degree AV block due to delay in conduction through the AVN
describe this ECG
sinus tachycardia
describe this ECG
ST elevation indicative of an MI, a patient with a recent MI would also have raised cardiac troponin levels
