ECG Interpretation: Waves, Intervals, and Cardiac Axis Analysis

What does the P wave indicate?

atrial depolarization

What does the QRS Complex indicate?

ventricular depolarization

What does the T wave indicate?

ventricular repolarization

What does the PR Interval indicate?

the time from atrial depolarization onset to ventricular depolarization onset

What does the ST Segment represent?

the period at which the entire ventricle is depolarized & corresponds to the plateau phase of the ventricular action potential

Where is the Beginning of the ST segment?

@ end of the QRS

Where is the End of the ST segment?

@ beginning of the T

Elevation/depression of ___ is IMPORTANT

ST segment

What does the QT Interval indicate?

estimates the duration of ventricular action potentials

What is the duration of P wave?

.08-.1 s

What is the duration of QRS complex?

.06-.1 s

What is the duration of PR interval?

delay .12-.2 s

What is the duration of QT interval?

.2-.4 s

How can a clinician can evaluate cardiac rhythms?

by recording arhythm strip using an ECG, usually using Lead II

What is sinus rhythm?

If P waves (atrial depolarizations) lead directly to QRS complexes (ventricular depolarizations) in a 1 to 1 fashion, the heart is in sinus rhythm.

What is a normal HR?

Normal: 60-100 bpm

What is sinus tachycardia? Sinus bradycardia?

Sinus Tachycardia: 100+ bpm

Sinus Bradycardia: < 60 bpm

What are characteristics of the QRS complex?

amplitude greater than 0.5 mV in at least one standard lead, upper limit of normal amplitude is 2.5 - 3.0 mV, small septal Q waves amplitude less than 1/3 of the amplitude of the R wave.

What are characteristics of the ST segment?

isoelectric, slanting upwards to the T wave in the normal ECG, can be slightly elevated (up to 2.0 mm in some precordial leads), never normally depressed greater than 0.5 mm in any lead

What are characteristics of the T wave?

normally rounded and asymmetrical, with a more gradual ascent than descent, isolated T wave inversion in an asymptomatic adult is generally a normal variant

3 things can influence an ECG waveform, what are they?

1. Location of electrodes

2. Conduction pathways & conduction speed

3. Changes in muscle mass

What is a vector?

When the SA node fires, a wave of depolarization spreads in several directions

Each individual wave represents an individual electrical vector

By summing each of these together, you can obtain a mean electrical vector

The direction of the mean electrical vector relative to the positive and negative electrodes determines what?

polarity of the waveform and the magnitude of the waveform

Summing the mean vectors from several different ECG leads determines what? This can then be used to do what?

the mean electrical axis of the heart

This is used diagnostically to identify left and right axis deviations

These could be due to: bundle branch blocks or ventricular hypertrophy, etc.

What are the 5 rules to interpret an ECG?

A wave of depolarization traveling toward a + electrode results in a + deflection

A wave of repolarization traveling toward a + electrode results in a - deflection

A wave of depolarization/repolarization perpendicular to an electrode axis produces no net deflection

The amplitude of the measured potentials depends on the orientation of the + electrode relative to the mean vector

Voltage amplitude +/- is related to the mass of tissue undergoing depolarization/repolarization

What is the placement of recording electrodes?

Limb leads: Einthoven's triangle

Where is Lead I?

Lead I has the positive electrode on the left arm and the negative electrode on the right arm

Where is Lead II?

Lead II has the positive electrode is on the left leg and the negative electrode is on the right arm

Where is Lead III?

Lead III has the positive electrode on the left leg and the negative electrode on the left arm

What is the axial reference system?

The axial reference system: In this system, the three limbs of Einthoven triangle are broken apart, collapsed, and superimposed over the heart

Lead I = 0º

Lead II = +60º

Lead III = +120

What are the augmented limb leads?

Augmented Limb Leads:

3 leads in addition to the 3 bipolar limb leads of Einthoven's triangle

aVL - Left arm: -30º

aVR - Right arm: -150º

aVF - Foot: +90º

Using the aforementioned 6 leads, you can estimate what?

the mean electrical axis using the axial reference system

The mean electrical axis corresponds to what?

the axis that is perpendicular to the lead axis with the smallest net QRS amplitude a.k.a- perpendicular to the most biphasic lead

Using this method, you can accurately estimate what?

the exact axis & determine if there is a deviation.

With a shortcut to the 6 leads, you can quickly determine what characteristic?

if someone has an axis deviation. You will not be able to determine the exact axis using this method

tissue hypoxia caused by ischemia results in what?

membrane depolarization

As ATP levels decline during hypoxia, there is a net loss of __ as it leaks out of cells, causing decreased activity of the Na+/K+ -ATPase pump

K+

Membrane depolarization also alters pacemaker activity, leading to changes in what?

rhythm and ectopic beats

Cellular hypoxia results in what?

the accumulation of intracellular calcium and leads to afterdepolarizations and tachycardia