Chapter 11 - Guyton Flashcards
P wave
< 2.5 mm tall and < 0.12 sec long, immediately precedes atrial contraction (wider would signify longer duration for atria to depolarize)
PR interval
0.12 - 0.20 sec long (normal value of 0.16 sec), this time is needed for the ventricles to fill with blood!
QRS complex
up to 0.10 sec, immediately precedes ventricular contraction
T wave
positive when QRS positive, ventricles recover from depolarization (.25-.35 seconds after depolarization), ventricular repolarization
Why can atrial repolarization not be seen on the ECG?
masked by the QRS complex
QT interval
0.37 sec for men and 0.40 for women, this represents the time of ventricular contraction, heart rate can be determined with the reciprocal of the time interval between each heartbeat
How can you calculate HR with the ECG?
HR = 60 sec / R-R interval = BPM, usually take average from 3 cycles
Explain the flow of electrical current in the heart?
ventricular depolarization starts at the ventricular septum and the endocardial surfaces of the heart, average current flows positively from the base of the heart to the apex, at the end of depolarization the current reverses from 1/100 of a second and flows toward the outer walls of the ventricles near the base (S wave)
Lead I of Bipolar Limb Leads
negative terminal of the ECG is connected to the right arm and the positive terminal is connected to the left arm
Lead II of the Bipolar Limb Lead
negative terminal of the ECG is connected to the right arm and the positive terminal is connected to the left leg
Lead III of the Bipolar Limb Lead
negative terminal of the ECG is connected to the left arm and the positive terminal is connected to the left leg
Q wave
when initial inflection is negative
R wave
first positive deflection
S wave
negative deflection following the R wave
QS
all negative
R prime
second positive inflection that occurs after the S wave, only in abnormal ECGs
Use of lower case in ECG?
to notate an inflection that is not as strong
Einthoven’s Law
electrical potential of any limb equals the sum of the other two (I + III = II)
Chest (Precordial) Leads
V1 - V6, very sensitive to electrical potential changes underneath the skin
Augmented Unipolar Limb Leads
aVR (+ electrode right arm, - electrode left arm), aVL (+ electrode left arm), aVF (+ electrode left leg)
P pulmonale
Right atrial enlargement/abnormality - we would expect a large P wave > or = 2.5mm tall (no change in duration) in II, III, AVF, V1