ECG Interpretation: cram pack

Formula sheets, the key method steps for every topic and 4 practice problems, with full solutions at the back.

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1Formulas and methods

ECG rhythm strip interpretation

Paper (25 mm/s, 10 mm/mV): 1 small box = 0.04 s, 1 large box = 0.20 s, 10 mm = 1 mV
Heart rate, regular rhythm: 300 ÷ large boxes = 1500 ÷ small boxes = 60 ÷ RR (s)
Heart rate, irregular rhythm: QRS complexes in 6 s × 10
Normal PR interval: 0.12–0.20 s
Normal QRS duration: < 0.12 s
QTc (Bazett): QT/√RR
QTc (Fridericia): QT/∛RR
QTc (Framingham): QT + 154·(1 − RR)
Sinus rates: bradycardia < 60, normal 60–100, tachycardia > 100 /min
Escape rates: junctional 40–60, ventricular 20–40 /min
Fast rhythms: SVT 150–250 /min (narrow), VT > 100 /min (wide)

Method

Rate: for a regular rhythm divide 300 by the large boxes or 1500 by the small boxes between two R waves; for an irregular rhythm count the QRS complexes in 6 seconds and multiply by 10.
Rhythm: compare the R–R intervals. Equal is regular; a repeating pattern is regularly irregular; no pattern at all is irregularly irregular.
P waves: is there an upright P wave in lead II before every QRS, all alike? Look for fibrillatory or sawtooth waves, inverted P waves and P waves without a QRS.
PR interval: from the start of P to the start of the QRS, normally 0.12–0.20 s and constant. Watch whether it lengthens, stays fixed before a dropped beat, or varies at random.
QRS duration: below 0.12 s is narrow; 0.12 s or more is wide. Then name the rhythm, and for the QT use QTc = QT/√RR (Bazett) with times in seconds.

2Practice problems

1.ECG rhythm strip interpretationMedium

Interpret this lead II rhythm strip. Which rhythm is it? Click your answer.

lead II, 25 mm/s, 10 mm/mV1 small box = 0.04 s, 1 large box = 0.20 sstrip length 6 s

1atrial fibrillation
2sinus rhythm with PVCs
3junctional rhythm
4atrial flutter, 4:1 conduction

2.ECG rhythm strip interpretationHard

This lead II strip shows second-degree AV block type I (Wenckebach). Measure the PR interval of the last conducted beat before a non-conducted P wave (the longest PR), in seconds.

lead II, 25 mm/s, 10 mm/mV1 small box = 0.04 s, 1 large box = 0.20 sstrip length 6 s

longest PR = s

3.Human physiologyMedium

Why can cardiac muscle not be tetanised like skeletal muscle?

AIt has no calcium in the sarcoplasmic reticulum
BIt is not striated
CIts long refractory period lasts almost as long as the contraction
DIt is controlled only by hormones

4.Human physiologyHard

After several days of persistent vomiting of stomach contents, which acid–base disturbance is most likely?

AMetabolic acidosis
BRespiratory acidosis
CRespiratory alkalosis
DMetabolic alkalosis

3Solutions

1.ECG rhythm strip interpretation

Answer: atrial fibrillation

Rhythm criteria (lead II): sinus rhythm has an upright P wave before every QRS, a constant PR of 0.12–0.20 s and a narrow QRS: normal sinus rhythm at 60–100/min, sinus bradycardia below 60, sinus tachycardia above 100. Atrial fibrillation: no P waves, a fibrillatory baseline and an irregularly irregular R–R. Atrial flutter: sawtooth flutter waves at about 250–350/min with a fixed conduction ratio such as 2:1 or 4:1. First-degree AV block: sinus rhythm with a constant PR above 0.20 s. Junctional rhythm: narrow QRS at 40–60/min with absent or inverted P waves (PR below 0.12 s, or P after the QRS). A PVC is an early, wide (0.12 s or more), bizarre QRS without a P wave, with the T wave in the opposite direction and usually a full compensatory pause; in bigeminy every second beat is a PVC.

Five steps

1. Rate: 8 QRS complexes in 6 s, × 10 = 80 per minute.

2. Rhythm: irregularly irregular, R–R from 0.48 to 1.00 s with no pattern.

3. P waves: none; the baseline shows small irregular fibrillatory waves.

4. PR interval: not measurable.

5. QRS: 0.10 s, narrow (below 0.12 s).

Interpretation

atrial fibrillation

Answer

atrial fibrillation

2.ECG rhythm strip interpretation

Answer: longest PR = 0.36 s

AV blocks: march out the P waves and check which ones are followed by a QRS. First-degree: every P conducted, PR constant and above 0.20 s. Second-degree type I (Mobitz I, Wenckebach): the PR lengthens from beat to beat until one P wave is not conducted, then the cycle starts again (group beating). Second-degree type II (Mobitz II): the PR of the conducted beats stays constant, and P waves are suddenly not conducted; the QRS is often wide. Third-degree (complete) block: P waves and QRS complexes are each regular but independent, the PR varies at random and the atrial rate is faster than the escape rate (junctional 40–60/min with a narrow QRS, ventricular 20–40/min with a wide QRS). A 2:1 block cannot be called type I or type II from one strip.

March out the P waves

P–P = 1.00 s, regular; one P wave in every 4 is not conducted

PR in each group

PR₁ = 0.16 s, PR₂ = 0.30 s, PR₃ = 0.36 s

Answer

longest PR = 0.36 s

The biggest step in PR comes between the first and second beats of a group; later steps are smaller, which is why the R–R intervals shorten before the pause.

3.Human physiology

Answer: C · Its long refractory period lasts almost as long as the contraction

Why this is the answer

The plateau of the cardiac action potential gives a long refractory period, so the muscle relaxes before it can be excited again. This allows the heart to refill.

Answer

C · Its long refractory period lasts almost as long as the contraction

4.Human physiology

Answer: D · Metabolic alkalosis

Why this is the answer

Vomiting removes HCl, and the stomach's acid secretion leaves bicarbonate in the blood. Volume loss and low potassium help keep the alkalosis going.

Answer

D · Metabolic alkalosis

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