Lect. 17
Fainting (syncope)
Objectives:
1. Define the term syncope.
2. List the types of syncope……. measurement of systemic arterial
pressure.
3. State the influence of decreased aortic distensibility, increase in
heart rate, and increase or decrease in peripheral resistance on
systolic and diastolic systemic arterial pressures.
syncope
is a sudden transient loss of consciousness. It usually results
from cerebral ischemia; it is often due to cardiovascular abnormalities
including either venous pooling or reduced cardiac output. A person who
has fainted typically exhibits shallow breathing, a weak pulse, and low
blood pressure.
Etiology or Types:
1) Vasovagal syncope:
fainting is due to hypotension produced as a result of sudden
vasodilation associated with bradycardia. This is commonly occurs in
strong emotions (Psychophysiological syncope). Higher brain centers
involved with emotion inhibit sympathetic activity to the
cardiovascular centers and enhance parasympathetic activity to the
heart resulting in markedly reduced blood pressure and brain blood
flow. Anther common example of vasovagal syncope is in case of
blood donation.
2) Syncope due to reduction in venous return; by the increased
intrathoracic pressure, this reduces the cardiac output leading to
hypotension. Types:
1. Micturition syncope: hypotension; in addition to reflex bradycardia
induced by voiding urine.
2. Defecation syncope.
3. Cough syncope.
4. Valsalva maneuver.
3) Orthostatic syncope or hypotension:
It means a rapid hypotension that occurs on sudden standing. It is
accompanied by dimness of vision, dizziness and even fainting. It
results from failure of the baroreflex to compensate for the sudden
downward gravitational pull on the blood. On standing from supine
position, the effect of gravity leads to pooling of blood in the lower
part of the body→ decrease in venous return→ decrease in cardiac
output →fall in arterial blood pressure (Orthostatic hypotension) →
decreased cerebral blood flow→ brain ischemia→ syncope
(Orthostatic syncope or fainting).. It may occur in normal persons, in
hypovolemia, idiopathic, and in patients with diseases that damage the
sympathetic nervous system e.g., diabetes and syphilis, and in primary
autonomic insufficiency (decreased production of catecholamines due
to Dopamine B-hydroxylase enzyme defficiency.
4) Carotid sinus syncope; carotid sinus syndrome.
An external pressure on the carotid sinus area might produce syncope
due to reflex slowing of the heart rate and peripheral vasodilation. The
applied external pressure stimulate the baroreceptors in the carotid
sinus which leads to reflex increase in the vagal tone to the heart
(bradycardia) and decrease in the sympathetic vasoconstrictor tone
(vasodilation). On the same basis, an attack of paroxysmal atrial
tachycardia can be terminated by initiating a carotid sinus massage. A
strong blow on the carotid sinus area could lead to complete cardiac
arrest. Some individuals pathologically have an abnormal
hypersensitivity of the carotid sinus. Thus, mild pressure on the
carotid sinus area e.g. during shaving or by a tight collar, produces
bradycardia and generalized vasodilation, which may markedly
decrease the cardiac output and arterial blood pressure resulting in
brain ischemia and fainting.
5) Neurocardiogenic syncope; reduced cardiac output:
Syncope due to heart block or sinus arrest.
syncope may also occur in severe arrhythmias (tachycardia more
than 160 beat/minute or in bradycardia less than 40 beat/minute),
Myocardial infarction with pump failure.
Valve diseases (aortic stenosis, mitral stenosis).
Long QT syndrome.
Cardiomyopathy (HOCM).
Congestive heart failure.
5) Deglutition syncope: due to VD and bradycardia induced by
swallowing.
6) Effort syncope: common in patients having aortic or pulmonary valve
stenosis.