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 and the R–R interval on this regular lead II strip, then calculate the rate-corrected QTc with Bazett’s formula QTc = QT/√RR (both in seconds). Give the answer in seconds to three decimals.
lead II, 25 mm/s, 10 mm/mV1 small box = 0.04 s, 1 large box = 0.20 sstrip length 6 s
QTc = s
2.ECG rhythm strip interpretationHard
This lead II strip shows second-degree AV block type I (Wenckebach). By how much does the PR interval increase from the first to the second beat of a group (ΔPR), in seconds?
lead II, 25 mm/s, 10 mm/mV1 small box = 0.04 s, 1 large box = 0.20 sstrip length 6 s
ΔPR = s
3.Human physiologyMedium
Since action potentials are all-or-none, how does a neuron signal a stronger stimulus?
- ABy firing action potentials more often
- BBy making each action potential longer
- CBy changing the neurotransmitter it releases
- DBy making each action potential larger
4.Human physiologyHard
Why does a person with a high spinal cord injury above T1 risk low blood pressure?
- AThe phrenic nerve can no longer control the tone of the arteries
- BThe vagus nerve is cut, so the heart rate rises
- CRenin release from the kidney stops completely
- DThe sympathetic outflow to the vessels and heart is cut off
3Solutions
1.ECG rhythm strip interpretation
Answer: QTc = 0.402 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
QT = 9 × 0.04 = 0.36 s
RR = 20 × 0.04 = 0.80 s
Bazett’s formula
QTc = 0.36/√0.80 = 0.36/0.8944 = 0.402 s
Within the usual limits (about 0.45 s for men, 0.46 s for women). (402 ms)
2.ECG rhythm strip interpretation
Answer: ΔPR = 0.12 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 = 0.92 s, regular; one P wave in every 4 is not conducted
PR in each group
PR₁ = 0.18 s, PR₂ = 0.30 s, PR₃ = 0.38 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: A · By firing action potentials more often
Why this is the answer
Stimulus strength is coded by the frequency of action potentials and by how many neurons are recruited, not by their amplitude.
A · By firing action potentials more often
4.Human physiology
Answer: D · The sympathetic outflow to the vessels and heart is cut off
Why this is the answer
Sympathetic outflow leaves the cord at T1–L2. A lesion above it removes vasomotor tone and cardiac sympathetic drive while the vagus still works, causing hypotension and bradycardia (neurogenic shock).
D · The sympathetic outflow to the vessels and heart is cut off