Achey Breaky Heart- The Cardiac Conduction System

Posted by & filed under Biology, Science, STEM.

The Cardiac Conduction System

Sino-Atrial [SA] node or primary pacemaker – cells that generate an electrical current.

Atrioventricular [AV] node or secondary pacemaker- receives the electrical impulse.

Cardiac muscle action potentials– SA-AV electrical activity causes the right atrium to contract.

Heartbeat -the result of cardiac muscle action potential, travelling throughout the heart.

Intrinsic heart rate: SA & AV nodes send out an electrical impulse at set interval of 100 beats per minute [bpm].

Normal resting heart rate: In a healthy heart, hormones down-regulate heart rate to 60- 80bpm.

Normal active heart rate: Increased physical activity, can increase heart rate up to 100bpm

The conduction system of the heart


View an animation of how the conducting system of the heart works and answer the practice questions.

Trouble viewing the animation? Click to download Adobe flash player





The EKG or ECG [Electro-Kardiogram or Electro Cardiogram]

The EKG tracing of one heart beat

Normal EKG –The EKG describes the Heart Rhythm or Sinus rhythm which is the normal beating of the heart.The components of a normal ECG tracing, consists of waveform components [labeled P, Q, R, S, T and U] which indicate electrical events during one heartbeat.

P wave- indicates that the atria are contracting, pumping blood into the ventricles.
QRS complex – begins with a downward deflection, and represents ventricular depolarization and contraction.
PR interval – indicates the transit time for the electrical signal to travel from the sinus node to the ventricles.
T wave – represents ventricular repolarization.

EKG interpretation – performed using a standard eight-step procedure.

Rhythm, Rate, P Wave, PR Interval, QRS Interval, T Wave, QT Interval & ST Segment

Click on this link to see the EKG video.

Click on this link to see a heart beating in real time. [Caution! Very graphic]

Arryhthmia – Irregular heart rhythm.

Either: Bradychardic [slower rhythm] or Tachycardic [faster rhythm].

Normal Sinus Rhythm – Regular

Normal Sinus Rhythm

Rate – (60-99 bpm)

QRS Duration – Normal

P Wave – Visible before each QRS complex

P-R Interval – Normal



Sinus Bradychardia Rhythm – Regular

Sinus Bradychardia

Rate – less than 60 bpm

QRS Duration – Normal

P Wave – Visible before each QRS complex

P-R Interval – Normal

Note: A heart rate less than 60 bpm is normal in a healthy athletic person at rest.

Sinus Tachychardia Rhythm – Regular

Rate – More than 100 bpm

Sinus Tachycardia

QRS Duration – Normal

P Wave – Visible before each QRS complex

P-R Interval – Normal
Note:The impulse generating the heart beats are occurring at a faster pace than normal.
A heart rate more than 100 bpm is normal in a healthy athletic person while exercising.

Arryhthmias may be:


-SYMPTOMATIC in healthy heart

Frequency of 1-2 times/year or rare.
Symptoms may include: Palpitations or “skipped beats”, heartthrobs or “fluttering” and a sensation of the heart racing.
Examples: Premature  atrial contractions, Premature ventricular contractions (PVCs), Atrial fibrillation.

SYMPTOMATIC in diseased heart

Frequency greater than 1-2 times/year AND accompanied by the following: feeling faint, tired, light-headed, passing out (syncope), shortness of breath, chest pain or discomfort.
Examples: Atrial flutter, Paroxysmal supraventricular tachycardia (PSVT), PSVT Type 1-Accessory pathway tachycardias, PSVT Type 2- AV nodal reentrant tachycardia, Ventricular tachycardia (V-tach), Ventricular fibrillation, Long QT syndrome, Bradyarrhythmias, Sinus node dysfunction, and Heart block.

Visit this website to view different ECG rhythms and descriptions.

Watch this video to see a Doctor’s dilemma in diagnosing an arrhythmic patient.

Your Research/Creative Writing assignment [in APA format]:

For your assigned arrhythmia, write an essay describing a Patient and the following:
– his/her lifestyle,
– symptoms occurring before the event,
– symptoms occurring during the event,
– what the EKG looked like when compared to normal,
– the patho-physiology of the event,
– what factors may have contributed to the onset,
– how the condition was treated and
– what the patient’s prognosis is.
The essay may be as long or as short as you need, in order to address each of the points listed above.

Please cite sources for factual data. Click here to link to the Library


Leave a Reply