Unlock the secrets of your heart's electrical activity with this guide to electrocardiography (ECG). Delve into the fundamentals of ECG recording, interpretation, and clinical implications. From understanding the significance of P waves and QRS complexes to decoding intervals like the P-R and Q-T i...
Electrocardiogram (ECG)
When a cardiac impulse passes through
the heart, electrical current also spreads
from the heart to the adjacent tissues
surrounding the heart and a small portion
spreads all the way to the surface of the
body: if electrodes are placed on the skin,
on opposite sides of the heart, electrical
potentials generated by the current can be
recorded. The recording is known as
electrocardiogram.
The waveforms of a normal electrocardiogram are 3:
- The P wave, caused by atrial depolarization, before atrial contraction.
+ atria repolarize 0.2 s after, during the QRS complex depolarization (that precedes ventricular
contraction), therefore the atrial T wave (atrial repolarization wave) is normally obscured by
the QRS complex in the ECG.
- The QRS complex, caused by ventricular depolarization, before ventricular contraction.
- The (ventricular) T wave, caused by repolarization due to potentials generated as the
ventricles recover from depolarization, it can last 0.15 s (very long) but the voltage is considerably
less than the QRS.
Thus, the ECG is composed by both depolarization and repolarization waves that, in the heart can be
described as such: if we take as an example a single muscle fiber, it can be depicted into 4 stages of
depolarization and repolarization.
- Depolarization: the normal negative potential inside the cell
becomes more positive and the situation reverses with
respect to the outside.
- Repolarization: from a more positive to a more negative
environment inside the cell.
Remember!
During propagation of the action potential:
intracellular currents are generated
à in the same direction as AP propagation
corresponding interstitial currents are generated
ß in a direction opposed to AP propagation
A pair of extracellular electrodes records a potential difference
V = RxI positive in the direction of AP propagation.
- à+ AP propagation
+ à - current
The electrocardiogram is based on the ideas that:
• whenever a current flows between two points, we observe a
difference in potential
• current uses any possible path (like water when it infiltrates a roof), so whenever current flows
between two points A and B of extracellular fluids in the body, a fraction of such current (however
small) will flow between any two sites in the body that are not equally distant from points A and B
• when an AP propagates in the myocardial tissue, the AP current that enters the cells travels
intracellularly toward the resting tissue to depolarize it, charges accumulate at the inner
face of the membrane, leave its outer face and travel back extracellularly
• a propagating AP generates an extracellular current from the
resting tissue to the depolarized one; any electrode that “sees”
the depolarizing wavefront approaching will be positive with respect
to electrodes that see the wavefront moving away
• for a repolarization wavefront, the electrode that sees it moving away
will read positive
• No potential is recorded in the ECG when the ventricular muscle is
either completely polarized or completely depolarized.
71 Body At Work II
, Enrico Tiepolo
The principle of the recording is that if the (nominally) positive electrode (the red one) of the measuring
apparatus has a higher potential than the (nominally) negative one the device reads a positive signal,
while it reads negative if the opposite occurs.
- P-R or P-Q interval. It is the time gap between the beginning of the P wave and the beginning
of the Q wave, which normally is 0.16 s. Often, it is addressed as the P-R interval as the Q wave
can (normally is) absent.
o Faster heart rate à shorter P-R interval à increased sympathetic/ decreased
parasympathetic = increase in atrioventricular node conduction speed à shorter delay
between atrial and ventricular depolarization
o Slower heart rate à longer P-R interval à increased parasympathetic/ decreased
sympathetic = decrease of atrioventricular node conduction speed à longer delay
- Q(R)-T interval. The time between the beginning of the Q wave (if present) and the end of the
T wave. It lasts around 0.35 s and it is called Q-T interval.
- R-R interval. Time interval between two heartbeats. It is a very important number as its
reciprocal shows the heart rate itself.
o E.g. if the interval is 1s, then the heart rate will be 60 beats/min
o If the interval is 0.83 s à heart rate 60/0.83 times/min = 72 beats/min
The flow of electrical current in the chest around the heart can be better understood if we imagine the
heart as an organ actually suspended in a conductive medium. The cardiac impulse arrives at the
ventricle firstly at the septum and then spreads to the inner surfaces: current flows from the inner part
to the outer part of the muscle (from endocardium to epicardium, because Purkinje fibers are deep into
the muscle). So, if electrodes are positioned as usual (right negative and left positive) the current is
recorded positive as it goes from the center to the apex and then negative when it goes from the apex to
the base.
- The average current flows with negativity towards the base of the heart
- The current with positivity toward the apex.
Simultaneous contraction of the ventricle is obtained by a longer depolarization state of the cells of the
septum, until when the base cells are depolarized too, then contraction happens.
Instead, repolarization happens from the epicardium to the endocardium and it’s simultaneous in all the
parts of the ventricle. It’s recorded positive since the general direction (the mean vector) is from left to
right, so from the positive electrode towards the negative one
(since it’s a repolarization current [negative current] it will be
recorded as positive if it approaches the negative electrode).
72 Body At Work II
The benefits of buying summaries with Stuvia:
Guaranteed quality through customer reviews
Stuvia customers have reviewed more than 700,000 summaries. This how you know that you are buying the best documents.
Quick and easy check-out
You can quickly pay through credit card or Stuvia-credit for the summaries. There is no membership needed.
Focus on what matters
Your fellow students write the study notes themselves, which is why the documents are always reliable and up-to-date. This ensures you quickly get to the core!
Frequently asked questions
What do I get when I buy this document?
You get a PDF, available immediately after your purchase. The purchased document is accessible anytime, anywhere and indefinitely through your profile.
Satisfaction guarantee: how does it work?
Our satisfaction guarantee ensures that you always find a study document that suits you well. You fill out a form, and our customer service team takes care of the rest.
Who am I buying these notes from?
Stuvia is a marketplace, so you are not buying this document from us, but from seller enricot03. Stuvia facilitates payment to the seller.
Will I be stuck with a subscription?
No, you only buy these notes for $6.11. You're not tied to anything after your purchase.
