Electrocardiography and rhythm disorders

Question 1

What can an ECG be used for?

Answer 1

Can be used to detect:
- Conduction abnormalities
- Structural abnormalities
- Perfusion abnormalities

Question 2

What are the advantages of an ECG?

Answer 2

Relatively cheap and easy to undertake
Reproducible between people & centres (procedure is internationally standardised irrespective of who conducts it- decreases subjectivity in diagnostics)
Quick turnaround on results/ report (no specialist interpretation is needed)

Question 3

What are electrodes?

Answer 3

“sticky bits”
- The interface b/t the body and the electricity coursing through the sensor

Question 4

What are cables/ wires?

Answer 4

Communicate from the electrodes to the device

Question 5

What are the “leads”?

Answer 5

“lines on the paper”
there are 12 leads- views of the heart represented by those lines on the ECG

Question 6

What is a vector?

Answer 6

Cardiac vectors are what we’re measuring on the ECG trace

Question 7

A defibrillater is used if a patient is asystolic (“flat-lines”) and needs their electrical activity restored, true or false?

Answer 7

False: When a patient is asystolic- they have “non-shockable rhythm”
- There is no electrical activity
- So the defibrillator will not issue a shock
- Administer adrenaline instead

Question 8

What is “sinus rhythm”?

Answer 8

Each P-wave is followed by a QRS wave (1:1)
Rate is regular (even R-R intervals) and normal (83 bpm) [gap between each complex is the same]
Otherwise unremarkable

Question 9

What is “Sinus bradycardia”?

Answer 9

• Each P-wave is followed by a QRS wave (1:1) [so it is still a sinus rhythm- generated by the sinoatrial node]
• Rate is regular (even R-R intervals) and slow (56 bpm)
• distance between rhythm is prolonged
• Can be healthy, caused by medication or vagal stimulation; you need to understand the patient/ context

Question 10

What is “sinus tachycardia”?

Answer 10

• Each P-wave is followed by a QRS wave (1:1)
• Rate is regular (even R-R intervals) and fast (107 bpm)
• distance between the rhythm is shortened
• Often a physiological response (i.e. secondary) e.g. excitation of the SNS or response to medication

Question 11

What is “sinus arrhythmia”?

Answer 11

• Each P-wave is followed by a QRS wave; still sinus generated
• Rate is irregular (variable R-R intervals) and normal-ish (65-100 bpm)
• R-R interval varies with breathing cycle:
• Usually the SAN wants to depolarise at 110 bpm but now it is depolarising at 70 bpm
• The parasympathetic NS slows it down via the vagus nerve (the PNS can slow/ speed it up depending on what stage of breathing we’re at)

Question 12

How do the waves change on an ECG between expiration and inspiration?

Answer 12

Expiration:
- lower HR
- longer R-R interval
- there is no innervation of the respiratory musculature; during this, the PNS slows down the SAN/ electrical conduction

Inspiration:
- Higher HR
- Shortened R-R interval
- During inspiration the PNS allows the SAN to send the signals (not slowed down)

Question 13

How can you calculate Heart rate from an ECG?

Answer 13

300/ x
x= number of large squares on the ECG paper

Question 14

What is “Atrial fibrillation”?

Answer 14

• Problems that originate above the AV node
• Oscillating baseline – atria contracting asynchronously
• Muscle is not really contracting; NO P WAVES
• but everything in the ventricles are working; QRS COMPLEX IS NORMAL
• Rhythm can be irregular and rate may be slow: AVN generates the rhythm; if AVN is lost, HR slows down= increased risk of clot formation
• Turbulent flow pattern increases clot risk
• Atria not essential for cardiac cycle: gravity (blood pressure) does the work of getting the blood to the ventricles (contractions are just more efficient)

Question 15

What is “Atrial flutter”?

Answer 15

• Regular saw-tooth pattern in baseline (II, III, aVF):
• there are lots of P waves
• but not every P wave is followed by a QRS complex
• P waves are too fast (can not relax and depolarise again)
• Atrial to ventricular beats at a 2:1 ratio, 3:1 ratio or higher
• Saw-tooth not always visible in all leads

Question 16

What is “First degree heart attack”?

Answer 16

• Prolonged PR segment/interval caused by slower AV conduction
• Regular rhythm: 1:1 ratio of P-waves to QRS complexes (every P results in a QRS complex)
• Most benign heart block, but a progressive disease of ageing

Question 17

What is “second degree heart block (Mobitz I)”

Answer 17

• Gradual prolongation of the PR interval until beat skipped (beat skipped as there is no conduction through the AV node)
• Most P-waves followed by QRS; but some P-waves are not
• Regularly irregular: caused by a diseased AV node (where we have conduction= the R-R intervals are regular, where we don’t have conduction= extended period of time without an R wave)
• Also called Wenckebach

Question 18

What is “second degree heart block (Mobitz II)?

Answer 18

• P-waves are regular, but only some are followed by QRS
• No P-R prolongation (The P-R interval, when you see it, is consistent/ normal length)
• Regularly irregular: successes to failures (e.g. 2:1) or random
• Can rapidly deteriorate into third degree heart block (absolutely no communication b/t P and R waves)

Question 19

What is “Third degree (complete) heart block”?

Answer 19

• Complete dissociation b/t the P waves and the R waves
• P-waves are regular, QRS are regular, but no relationship (P-P waves are regularly spaced, R-R waves are regularly spaced, but they are not paced at the same rate)
• P waves can be hidden within bigger vectors
• A truly non-sinus rhythm (the rhythm of the ventricles pumping is caused by the ventricles themselves) – back-up pacemaker in action when the SAN/ AVN fail, the ventricles are able to create the electrical signal required for mechanical contraction

Question 20

What is “ventricular tachycardia”?

Answer 20

• P-waves hidden (difficult to spot)– dissociated atrial rhythm: the rhythm the ventricles create is causing the P waves to be obscured by these much higher vectors; the P waves are atrial muscle, not very thick, but the QRS complex is ventricles, much thicker
• Rate is regular and fast (100-200 bpm)
• At high risk of deteriorating into fibrillation (cardiac arrest
• Shockable rhythm – defibrillators widely available

Question 21

What is ventricular fibrillation?

Answer 21

• Heart rate irregular and 250 bpm and above
• Heart unable to generate an output: signals are irregular
• Shockable rhythm – defibrillators widely available
• NOTE: it is not about the ventricles ‘beating too fast’ but they’re not contracting in a coordinated manner

Question 22

What is “ST elevation”?

Answer 22

• P waves visible and always followed by QRS (1:1)
• Rhythm is regular and rate is normal (85 bpm)
• ST-segment is elevated >2mm above the isoelectric line
• Caused by infarction (tissue death caused by hypoperfusion)

Question 23

What is “ST depression”?

Answer 23

• P waves visible and always followed by QRS
• Rhythm is regular and rate is normal (95 bpm)
• ST-segment is depressed >2mm below the isoelectric line
• Caused by myocardial ischaemia, not caused by tissue damage (coronary insufficiency)