Lecture 6: ECGs

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P wave

atrial depolarization

P wave normal duration

0.08-0.10 seconds

QRS complex

ventricular depolarization

QRS complex normal duration

0.06-0.10 seconds

T wave

ventricular repolarization

• duration not normally measured

PR interval

time from atrial depolarization onset to ventricular depolarization onset

• “atrial depolarization + AV nodal”

PR interval normal duration

delay 0.12-0.20 seconds

• 3 to 5 small squares on ECG paper

ST segment

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

• beginning: end of QRS

• end: beginning of the T wave

• elevation/depression of this is IMPORTANT

• duration not normally measured

QT interval

estimates duration of ventricular action potentials

QT interval normal duration

0.20-0.40 seconds

• high heart rates reduce action potential duration and therefore the QT interval

ECG interpretation

• clinician evaluates cardiac rhythms by recording rhythm strip via ECG (usually use LEAD II)

sinus rhythm

when P waves (atrial depolarizations) lead directly to QRS complexes (ventricular depolarizations) in a 1:1 fashion

• normal rate: 60-100bpm

sinus tachycardia

100+ bpm

sinus bradycardia

lower than 60 bpm

QRS interpretation

• amplitude >0.5 mV in at least one standard lead; upper limit of normal amplitude = 2.5-3.0 mV

• small septal Q waves amplitude are less than 1/3 amplitude of the R wave

ST segment interpretation

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 >0.5mm in any lead

T wave interpretation

normally rounded and asymmetrical with a more gradual ascent than descent

• isolated T wave inversion in an asymptomatic adult is generally a normal variant

influences on ECG waveform

1. location of electrodes

2. conduction pathways and conduction speed

3. changes in muscle mass

location of electrodes

changes the viewing angle and distance to the heart, which directly alters the shape, direction, and size of the recorded waveform

conduction pathway and speed

dictate the timing, shape, and duration of every wave and interval

changes in muscle mass

an increase or decrease in the thickness and mass of the heart wall changes the strength and timing of the electrical vector

vectors

represent both the size (magnitude/voltage) and direction of electrical force generated by heart muscle cells

mean electrical axis

represents the average direction of electrical current flow (the QRS vector) during ventricular depolarization

• used diagnostically to identify left and right axis deviations

• “the summative, average vector of all instantaneous vectors combined during ventricular depolarization”

direction of mean electrical vector relative to positive and negative electrodes

determines polarity of waveform and magnitude of waveform

positive deflection on ECG

an electrical wave moving toward a positive electrode

negative deflection on ECG

an electrical wave moving away from a positive electrode

biphasic

perpendicular wave (- and + summed close to 0)

5 rules of interpreting an ECG

1. wave of depolarization traveling toward positive electrode → positive deflection

2. wave of repolarization traveling toward + electrode → negative deflection

3. wave of depolarization/repolarization perpendicular to electrode axis → no NET deflection

4. amplitude of measured potentials depends on orientation of positive electrode relative to mean vector

5. voltage amplitude ± is related to mass of tissue undergoing depolarization/repolarization

lead I

has positive electrode on left arm and negative electrode on right arm

• 0 degrees

• measures potential difference across the chest between the two arms

lead II

positive electrode is on left leg and negative electrode is on right arm

• + 60 degrees

lead III

has positive electrode on left leg and negative electrode on left arm

• + 120 degrees

aVL

left arm: -30 degrees

aVR

right arm: -150 degrees

aVF

foot: +90 degrees

mean electrical axis (w/ axial ref system)

corresponds to axis that is perpendicular to lead axis with smallest net QRS amplitude

• perpendicular to most biphasic lead

• accurately estimate exact axis and determine deviation

ST segment elevation

full wall MI

ST segment depression

myocardial ischemia

tissue hypoxia caused by ischemia

• results in membrane depolarization → as ATP levels decline, there is a net loss of K+ as it leaks out of the cell = decreased activity of the Na+/K+ ATPase pump → alters pacemaker activity, leading to changes in rhythm and ectopic beats

cellular hypoxia

results in accumulation of intracellular calcium

• leads to afterdepolarizations and tachycardia