SINUS
AND ATRIAL ARRHYTHMIAS
Certain arrhythmias are caused by a disturbance at the sinus node, including sinus bradycardia, sinus tachycardia, sinus arrhythmia, and wandering pacemaker. The sinus node is under the control of the parasympathetic and sympathetic nerves, and altered function of these nerves may influence cardiac activity. The SA node is depressed by parasympathetic (vagus) functions or stimulated by sympathetic activity.
In sinus
bradycardia the sinus node originates impulses at a slow rate, less than 60
beats/min (see Plate 2-24). Sinus bradycardia is
common in patients with high vagal tone, hypothyroidism, and increased
intracranial tension; during athletic training; and during treatment with
digitalis and/or reserpine. Usually the slow rate is caused, at least in part,
by vagal inhibition of the sinus node.
Sympathetic-nerve
stimulation or the blocking of vagus nerves can produce sinus tachycardia. The
sinus node originates impulses at a rate greater than 100 beats/min, and close
inspection of these curves shows some variation in the R-R interval (see Plate 2-24). It is important to observe this variation
to differentiate sinus tachycardia from atrial tachycardia, in which
there is no significant variation between the R-R intervals. Sinus tachycardia
is found in patients after exercise or smoking; in hyperthyroidism, anxiety,
toxic states, fever, anemia, and diseases involving the heart or lungs; and
from other causes. Sinus tachycardia is characterized by a slowing of the pulse
rate during carotid sinus pressure, then by the gradual return of the rate to
its basic level on release of pressure. This is in contrast to the reaction to
carotid pressure in atrial tachycardia, which may cause the rhythm to change
abruptly to a sinus rhythm.
Sinus arrhythmia
is a variation in cardiac rate during breathing or sometimes with other organ
function, such as contraction of the spleen. The arrhythmia is typically found in children or in patients with
Cheyne-Stokes respiration. Usually, afferent impulses from the lungs travel to
the cardiac center, with efferent impulses traveling over the vagus nerve to
the sinus node. The pacemaker activity at the node varies reflexively with
respiration. Generally, there are about five cardiac beats to each respiratory
cycle. With expiration, the cardiac rate is slow; with inspiration, it is more
rapid (see Plate 2-24).
WANDERING
PACEMAKER
A wandering
pacemaker is present when, with each beat, the pacemaker changes its position
in the atrium, often traveling down to and into the A-V node and back to the
sinus node again. This occurs when there is a variation in the vagal tone at
the sinus node or there are changes in sympathetic stimulation. In the ECG the
P-R interval becomes progressively shorter, and the P waves often disappear
within the QRS complexes or may even appear after the QRS complexes. In
lead II, at times the P waves may be inverted because the atria are
depolarized from the A-V node to the sinus node, instead of in the usual direction.
A wandering pacemaker is not a serious irregularity; it is often transient and
may be stopped by a ticholinergic agents such as atropine (see Plate 2-24).
