Page 211 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

!
(complete atrial excitation) and the ST seg- myocardium should be depolarizing (first 0.04 s of
ment (complete ventricular excitation) lie ap- QRS). The so-called “0.04-sec vector” is therefore
prox. on the isoelectric line (0 mV). The PQ (or said to point away from the infarction. Anterior MI is
PR) interval (! 0.2 s) is measured from the detected as highly negative Q waves (with smaller R
beginning of the P wave to the beginning of the waves) mainly in leads V5, V6, I and aVL. Q wave ab-
normalities can persist for years after MI (! I 2/3), so
Q wave (or to the R wave if Q wave is absent) they may not necessarily be indicative of an acute in-
and corresponds to the time required for atrio- farction. ST elevation points to ischemic but not (yet)
ventricular conduction (! B). The QT interval is necrotic parts of the myocardium. This can be ob-
measured from the start of the Q wave to the served: (1) in myocardial ischemia (angina pectoris),
end of the T wave. It represents the overall (2) in the initial phase of transmural MI, (3) in non-
time required for depolarization and repolari- transmural MI, and (4) along the margins of a trans-
zation of the ventricles and is dependent on mural MI that occurred a few hours to a few days
prior (! I 4). The ST segment normalizes within 1 to
the heart rate (0.35 to 0.40 s at a heart rate of 2 days of MI, but the T wave remains inverted for a
Cardiovascular System (Einthoven and Goldberger leads) on the Excitation in Electrolyte Disturbances
75 min ).
–1
couple of weeks (! I 5 and 2).
Figure E illustrates the six frontal leads
Cabrera circle. Synchronous measurement of
causes
the amplitude of Q, R and S from two or more
hyperkalemia
Hyperkalemia.
Mild
leads can be used to determine any integral
various changes, like elevation of the MDP
vector in the frontal plane (! G). The direction
(! p. 192) in the SA node. It can sometimes
axis (! C3 and G, red arrows). If the excitation
B3c). In severe hyperkalemia, the more positive
8 of the largest mean QRS vector is called the QRS have positive chronotropic effects (! p. 193
+
spreads normally, the QRS axis roughly corre-
MDP leads to the inactivation of Na channels
sponds to the anatomic longitudinal axis of the (! p. 46) and to a reduction in the slope and
heart. amplitude of APs in the AV node (negative dro-
The mean QRS axis (“electrical axis”) of the motropic effect; ! p. 193 B4). Moreover, the K +
heart, which normally lies between + 90 conductance (g K) rises, and the PP slope be-
degrees to –30 degrees in adults (! G, H). Right comes flatter due to a negative chronotropic
type (α = + 120" to + 90") is not unusual in effect (! p. 193 B3a). Faster myocardial re-
children, but is often a sign of abnormality in polarization decreases the cytosolic Ca 2+ conc.
adults. Mean QRS axes ranging from + 90 In extreme cases, the pacemaker is also
degrees to + 60 degrees are described as the brought to a standstill (cardiac paralysis). Hy-
vertical type (! G1), and those ranging from pokalemia (moderate) has positive
+ 60 degrees to + 30 degrees are classified as chronotropic and inotropic effects (! p. 193
the intermediate type (! G2). Left type occurs B3a), whereas hypercalcemia is thought to
when α = + 30 degrees to –30 degrees (! G3). raise the g K and thereby shortens the duration
Abnormal deviation: Right axis deviation of the myocardial AP.
+
(# + 120") can develop due right ventricular ECG. Changes in serum K and Ca 2+ induce
hypertrophy, while left axis deviation (more characteristic changes in myocardial excita-
negative than –30") can occur due to left tion.
ventricular hypertrophy. ! Hyperkalemia (# 6.5 mmol/L): tall, peaked T
waves and conduction disturbances associated
An extensive myocardial infarction (MI) can shift
the electrical axis of the heart. Marked Q wave abnor- with an increased PQ interval and a widened
mality (! I 1) is typical in transmural myocardial in- QRS. Cardiac arrest can occur in extreme cases.
farction (involving entire thickness of ventricular ! Hypokalemia (! 2.5 mmol/L): ST depres-
wall): Q wave duration # 0.04 s and Q wave ampli- sion, biphasic T wave (first positive, then nega-
tude # 25% of total amplitude of the QRS complex. tive) followed by a positive U wave.
These changes appear within 24 hours of MI and are ! Hypercalcemia (# 2.75 mmol/L total cal-
caused by failure of the dead myocardium to con- cium): shortened QT interval due to a short-
duct electrical impulses. Preponderance of the exci- ened ST segment.
198 tatory vector in the healthy contralateral side of the
heart therefore occurs while the affected part of the ! Hypocalcemia (! 2.25 mmol/L total cal-
cium): prolonged QT interval.
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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