Reading the heartbeat line

Science · The Human Body · ages 19-20

Name ______________________ Date ____________

Why is slow conduction through the AV node useful during a normal beat?
- It allows atrial emptying before ventricular contraction
- It stops atrial emptying until ventricular contraction ends
- It starts ventricular contraction before atrial emptying begins
Which pairing correctly names the electrical events represented by P, QRS and T in a normal ECG?
- P: ventricular depolarization; QRS: atrial depolarization; T: ventricular repolarization
- P: atrial depolarization; QRS: ventricular depolarization; T: ventricular repolarization
- P: atrial repolarization; QRS: ventricular repolarization; T: ventricular depolarization
The main upstroke of an SA node pacemaker action potential is driven by rapid sodium entry, like the neuronal spike taught earlier.
Circle one: True False
A regular ECG recorded at 25 mm/s has 5 large squares between successive R peaks. What is the ventricular rate in beats per minute?
Answer: ______________
Which sequence gives the normal route of excitation from the pacemaker to ventricular muscle?
- SA node, ventricles, Purkinje fibres, bundle branches, His bundle, AV node, atria
- AV node, atria, SA node, His bundle, bundle branches, Purkinje fibres, ventricles
- SA node, atria, AV node, His bundle, bundle branches, Purkinje fibres, ventricles
On a reliable ECG, successive R-R intervals are 0.7 s, 1.1 s and 0.8 s. What do these intervals show?
- An irregular ventricular rhythm
- A regular but rapid ventricular rhythm
- A regular but slow ventricular rhythm
A friend says a standard 12-lead ECG needs 12 skin electrodes and chest placement does not matter. Which correction is accurate?
- Use 12 skin electrodes; move chest contacts wherever they attach.
- Use 10 skin electrodes; chest contacts may go anywhere on the chest.
- Use 10 skin electrodes; chest contacts use standard landmarks.
In a model beat, early ventricular contraction reduces the blood volume before contraction. Heart rate and the volume left after contraction stay unchanged. What happens to blood ejected per beat?
- More blood is ejected per beat
- Less blood is ejected per beat
- The same blood volume is ejected per beat

Answer key

For grown-ups. Fold this page away before handing over the rest.

Reading the heartbeat line W1-mt_D-WKFc7BZm-s1

It allows atrial emptying before ventricular contraction · AV nodal delay gives atrial contraction time to add blood to the ventricles before ventricular contraction. It does not stop all blood flow.
P: atrial depolarization; QRS: ventricular depolarization; T: ventricular repolarization · P represents atrial depolarization, QRS ventricular depolarization, and T ventricular repolarization. These are electrical events, not direct measurements of contraction.
False · Calcium entry drives the main upstroke in SA node pacemaker cells. The neuronal sodium-driven upstroke cannot be transferred unchanged to these cells.
60 · For a regular ventricular rhythm at 25 mm/s, divide 300 by the large-square count. 300 / 5 = 60 beats per minute.
SA node, atria, AV node, His bundle, bundle branches, Purkinje fibres, ventricles · Excitation normally starts at the SA node, crosses the atria and passes through the AV node. The His bundle, bundle branches and Purkinje fibres then carry it to ventricular muscle.
An irregular ventricular rhythm · The R-R intervals have unequal durations, so ventricular activation is irregular. The intervals alone do not identify the cause.
Use 10 skin electrodes; chest contacts use standard landmarks. · A standard 12-lead ECG uses ten electrodes to form twelve electrical views. Chest contacts use standard landmarks because moving them can change the recorded wave pattern.
Less blood is ejected per beat · Stroke volume is volume before contraction minus volume left afterwards. A smaller starting volume minus the same remaining volume gives less blood ejected per beat.