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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
Measure the QT interval on this lead II strip, in seconds.
lead II, 25 mm/s, 10 mm/mV1 small box = 0.04 s, 1 large box = 0.20 sstrip length 6 s
QT = s
2.ECG rhythm strip interpretationHard
This lead II strip shows a conduction problem between the atria and the ventricles. 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
1third-degree (complete) AV block
2first-degree AV block
3second-degree AV block type I (Wenckebach)
4junctional rhythm
3.Human physiologyMedium
How is the ejection fraction calculated?
ACardiac output ÷ heart rate
BEnd-systolic volume ÷ stroke volume
CEnd-diastolic volume ÷ end-systolic volume
DStroke volume ÷ end-diastolic volume
4.Human physiologyHard
Why does left ventricular failure cause pulmonary oedema?
AThe right ventricle pumps less blood into the lungs
BRaised left atrial pressure increases pulmonary capillary hydrostatic pressure
CThe bronchial arteries become blocked
DPlasma oncotic pressure in the lungs rises sharply
3Solutions
1.ECG rhythm strip interpretation
Answer: QT = 0.4 s
QT and QTc: the QT interval runs from the start of the QRS complex to the end of the T wave. It shortens as the heart rate rises, so it is corrected for rate with the R–R interval in seconds: Bazett QTc = QT/√RR and Fridericia QTc = QT/∛RR. Bazett is the formula most textbooks use; it over-corrects at fast rates, where Fridericia is preferred. Limits differ between references: a QTc above about 0.45 s in men or 0.46 s in women is usually called prolonged, and above 0.50 s markedly prolonged.
Measure
10 small boxes from the start of the QRS to the end of the T wave
Convert
QT = 10 × 0.04 s = 0.4 s
Answer
QT = 0.4 s
A rough rule: at normal rates the QT should be less than half the R–R interval (here R–R = 1.08 s).
2.ECG rhythm strip interpretation
Answer: third-degree (complete) AV block
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.
Five steps
1. Rate: R–R = 33 small boxes, 1500 ÷ 33 ≈ 45 per minute.
2. Rhythm: R–R regular (1.32 s) and P–P regular (0.60 s), but the two are independent.
3. P waves: upright and regular at 100 per minute, but not linked to the QRS complexes; some hide in a QRS or a T wave.
4. PR interval: varies from beat to beat; there is no fixed relation.
5. QRS: 0.08 s, narrow (below 0.12 s).
Interpretation
third-degree (complete) AV block
Answer
third-degree (complete) AV block
Type II and third-degree blocks are the ones that can suddenly progress, so textbooks rank them as more serious than type I.
3.Human physiology
Answer: D · Stroke volume ÷ end-diastolic volume
Why this is the answer
Ejection fraction is the proportion of blood in the full ventricle that is ejected per beat. A normal value is about 55–70%.
Answer
D · Stroke volume ÷ end-diastolic volume
4.Human physiology
Answer: B · Raised left atrial pressure increases pulmonary capillary hydrostatic pressure
Why this is the answer
A failing left ventricle backs up pressure into the left atrium and pulmonary veins. High capillary hydrostatic pressure pushes fluid into the lung interstitium and alveoli.
Answer
B · Raised left atrial pressure increases pulmonary capillary hydrostatic pressure