442 Chapter 13
First-degree AV node block occurs when the rate of
impulse conduction through the AV node (as reflected by the
P-R interval) exceeds 0.20 second. Second-degree AV node
block occurs when the AV node is damaged so severely that
only one out of every two, three, or four atrial electrical waves
can pass through to the ventricles. This is indicated in an ECG
by the presence of P waves without associated QRS waves.
In third-degree, or complete, AV node block, none of the
atrial waves can pass through the AV node to the ventricles.
The atria are paced by the SA node (follow a normal “sinus
rhythm”), but in complete AV node block a secondary pace-
maker in the Purkinje fibers paces the ventricles. The SA node
is the normal pacemaker because it has the fastest cycle of
spontaneous depolarization, but in complete AV node block the
action potentials from the atria cannot reach the Purkinje fibers
to suppress their pacemaker activity. The pacemaker rate of
the Purkinje fibers (generally about 20 to 40 beats per minute,
depending on location) is abnormally slow, and the brady car-
dia that results is usually corrected by insertion of an artificial
pacemaker.Figure 13.35 Atrioventricular (AV) node block. In
first-degree block, the P-R interval is greater than 0.20 second
(in the example here, the P-R interval is 0.26–0.28 second). In
second-degree block, P waves are seen that are not accompanied
by QRS waves. In this example, the atria are beating 90 times per
minute (as represented by the P waves), while the ventricles are
beating 50 times per minute (as represented by the QRS waves).
In third-degree block, the ventricles are paced independently
of the atria by an ectopic pacemaker. Ventricular depolarization
(QRS) and repolarization (T) therefore have a variable position
in the electrocardiogram relative to the P waves (atrial
depolarization).
QRS
P
QRS
P
T TRR
PPPP
QSTQSTR
PP
QSTR
PP
QSTR
PP
QSTPPPQRST QRST QRST QRST
P P P P P P P PFirst-degree AV blockSecond-degree AV blockThird-degree AV block| CHECKPOINT
- Explain how cholesterol is carried in the plasma and
how the concentrations of cholesterol carriers are
related to the risk for developing atherosclerosis. - Explain how angina pectoris is produced and discuss
the significance of this symptom.
16a. Identify normal and pathological causes of
bradycardia and tachycardia and describe how these
affect the ECG. Also, identify flutter and fibrillation
and describe how these appear in the ECG.
16b. Explain the effects of first-, second-, and third-
degree AV node block on the electrocardiogram.
13.8 Lymphatic System
Lymphatic vessels absorb excess interstitial fluid and
transport this fluid—now called lymph—to ducts that drain
into veins. Lymph nodes, and lymphoid tissue in the thy-
mus, spleen, and tonsils, produce lymphocytes, which are
white blood cells involved in immunity.LEARNING OUTCOMESAfter studying this section, you should be able to:- Explain how the lymph and lymphatic system relate
to the blood and cardiovascular system. - Describe the function of lymph nodes and lymphatic
organs.
The interstitial space, or interstitium, is the space between
blood vessels and the tissue cells of an organ. It contains
interstitial fluid and the extracellular matrix. Interstitial
fluid—an aqueous solution containing salts, nutrients, waste
products of cell metabolism, and plasma proteins—is formed
by filtration out of blood capillaries (chapter 14, section 14.2).
The extracellular matrix consists of a fiber scaffolding formed
predominantly of collagen proteins and a gel formed of
glycosaminoglycans.
The lymphatic system has three basic functions: (1) it
transports interstitial (tissue) fluid, initially formed as a blood
filtrate, back to the blood; (2) it transports absorbed fat from
the small intestine to the blood; and (3) its cells—called
lymphocytes —help provide immunological defenses against
disease-causing agents (pathogens).