504 SECTION VICardiovascular Physiology
appearance of tall peaked T waves, a manifestation of altered re-
polarization (Figure 30–18). At higher K+ levels, paralysis of the
atria and prolongation of the QRS complexes occur. Ventricular
arrhythmias may develop. The resting membrane potential of the
muscle fibers decreases as the extracellular K+ concentration in-
creases. The fibers eventually become unexcitable, and the heart
stops in diastole. Conversely, a decrease in the plasma K+ level
causes prolongation of the PR interval, prominent U waves, and,
occasionally, late T wave inversion in the precardial leads. If the T
and U waves merge, the apparent QT interval is often prolonged;
if the T and U waves are separated, the true QT interval is seen to
be of normal duration. Hypokalemia is a serious condition, but it
is not as rapidly fatal as hyperkalemia.
Increases in extracellular Ca2+ concentration enhance myo-
cardial contractility. When large amounts of Ca2+ are infused
into experimental animals, the heart relaxes less during diastole
and eventually stops in systole (calcium rigor). However, in
clinical conditions associated with hypercalcemia, the plasma
calcium level is rarely if ever high enough to affect the heart.
Hypocalcemia causes prolongation of the ST segment and con-
sequently of the QT interval, a change that is also produced by
phenothiazines and tricyclic antidepressant drugs and by vari-
ous diseases of the central nervous system.
CHAPTER SUMMARY
■ Contractions in the heart are controlled via a well-regulated
electrical signaling cascade that originates in pacemaker cells in
the sinoatrial (SA) node and is passed via internodal atrial path-
ways to the atrioventrical (AV) node, the bundle of His, the
Purkinje system, and to all parts of the ventricle.
■ Most cardiac cells have an action potential that includes a rapid
depolarization, an initial rapid repolarization, a plateau, and a
slow repolarization process to return to resting potential. These
changes are defined by sequential activation and inactivation of
Na+, Ca2+, and K+ channels.
■ Pacemaker cells have a slightly different sequence of events.
After repolarization to the resting potential, there is a slow de-
polarization that occurs due to a channel that can pass both Na+
and K+. As this “funny” current continues to depolarize the cell,
Ca2+ channels are activated to rapidly depolarize the cell. The
hyperpolarization phase is again dominated by K+ current.
■ Spread of the electrical signal from cell to cell is via gap junc-
tions. The rate of spread is dependent on anatomical features,
but also can be altered (to a certain extent) via neural input.
■ The electrocardiogram (ECG) is an algebraic sum of the electri-
cal activity in the heart. The normal ECG includes well-defined
waves and segments, including the P wave (atrial depolarization),FIGURE 30–18 Correlation of plasma K+ level and the ECG, assuming that the plasma Ca2+ level is normal. The diagrammed com-
plexes are left ventricular epicardial leads. (Reproduced with permission from Goldman MJ: Principles of Clinical Electrocardiography, 12th ed. Originally published by
Appleton & Lange. Copyright © 1986 by McGraw-Hill.)
RTUTUPNormal tracing (plasma K+ 4–5.5 meq/L). PR interval = 0.16 s; QRS interval = 0.06 s;
QT interval = 0.4 s (normal for an assumed heart rate of 60).Hyperkalemia (plasma K+±7.0 meq/L). The PR and QRS intervals are within normal
limits. Very tall, slender peaked T waves are now present.Hyperkalemia (plasma K+±8.5 meq/L). There is no evidence of atrial activity; the QRS
complex is broad and slurred and the QRS interval has widened to 0.2 s. The T waves
remain tall and slender. Further elevation of the plasma K+ level may result in ventricular
tachycardia and ventricular fibrillation.Hypokalemia (plasma K+±3.5 meq/L). PR interval = 0.2 s; QRS interval = 0.06 s; ST
segment depression. A prominent U wave is now present immediately following the T. The
actual QT interval remains 0.4 s. If the U wave is erroneously considered a part of the T, a
falsely prolonged QT interval of 0.6 s will be measured.Hypokalemia (plasma K+±2.5 meq/L). The PR interval is lengthened to 0.32 s; the ST
segment is depressed; the T wave is inverted; a prominent U wave is seen. The true QT
interval remains normal.