notes by dr Claudio Italiano
The term cyanosis (from the Greek kyànosis, formed on kyànos, blue, livid) is a bluish coloration of the skin and mucous membranes, due to the presence of more than 5 g / 100 ml of reduced (non-oxygenated) hemoglobin in the blood or abnormal hemoglobin compounds such as methaemoglobin or sulfohemoglobin. It is generally more pronounced in the lips, nail bed, ears and cheekbones. The bright complexion of the skin, characteristic of polycythemia vera, should be distinguished from true cyanosis, discussed in this section. A cherry red color, rather than due to cyanosis, is due to carboxyhemoglobin (cf. carbon monoxide poisoning).
Cyanosis is divided into central and peripheral:
Reduced arterial oxygen saturation
Decreased atmospheric pressure (high altitudes)
Alterations of pulmonary function
Alveolar hypoventilation
Impaired ventilation-perfusion ratio (perfusion of hypo-ventilated alveoli)
Altered diffusion of anatomical Shunt Oxygen
Some types of congenital heart disease
Arteriovenous pulmonary fistulae
Small multiple intrapulmonary shunts
Low affinity hemoglobin for oxygen
Abnormal hemoglobin
Methaemoglobinaemia (hereditary, acquired)
Solfohemoglobinemia (acquired)
Carboxyhemoglobinemia (cyanosis not true)
Reduction of cardiac output
Exposure to cold
Redistribution of blood flow from the extremities
Arterial obstruction
Venous obstruction
The degree of cyanosis is influenced by the quality of the skin pigment, the color of the plasma, the thickness of the skin and the state of the skin capillaries. Identifying and defining the degree of cyanosis requires an accurate clinical evaluation and is however difficult, as shown by the oximetry studies. Sometimes cyanosis of the central type can be recognized when the arterial saturation drops to 85%; other times it is not highlighted until the saturation has decreased to 75%, especially in dark-skinned subjects.
Two may be the causes of increased hemoglobin, resulting in cyanosis:
1) an increased quantity of venous blood in the skin, due to dilation of the
venules or venous terminations of the capillaries;
2) a reduced oxygen saturation at the level of the capillaries. Usually,
cyanosis is evident when the average concentration of hemoglobin reduced at
capillary level exceeds 5 g / dl (50 g / l).
The critical factor in the development of cyanosis is the absolute concentration of reduced hemoglobin and
not the relative concentration. In a severely anemic patient, the relative
concentration of reduced hemoglobin in venous blood may be very high when
considered in relation to the total amount of hemoglobin present, but since the
latter is significantly decreased, the absolute concentration of reduced
hemoglobin may still be low; it follows that in severe anemia, even when there
is a marked arterial desaturation, cyanosis does not occur.
On the contrary, the
greater the quantity of total hemoglobin, the greater the tendency to develop
cyanosis; therefore patients with marked polycythemia become cyanotic at higher
arterial oxygen saturation levels than patients with normal hematocrit values.
Likewise, a district stasis condition, which causes an increase in the total
amount of reduced hemoglobin locally, causes the appearance of cyanosis. Finally,
cyanosis may also appear when a form of functionally inactive hemoglobin is
present in the circulation, such as methaemoglobin or sulfohemoglobin.
Cyanosis is divided into two types
a) central
b) peripheral.
The central type is due either to an arterial desaturation or to the presence of
an abnormal hemoglobin derivative and is characterized by both cutaneous and
mucosal involvement. In the peripheral type, however, the arterial saturation is
normal and the cyanosis is caused by a slowing of the district circulation and
by an excessive supply of oxygen. These phenomena occur by vasoconstriction and
reduction of peripheral blood flow, such as in exposure to cold, shock,
congestive heart failure and peripheral vasculopathies.
Often, in these cases,
the mucosa of the oral or sublingual cavity is spared. The clinical distinction
between central and peripheral cyanosis is not always easy and in some
situations, such as cardiogenic shock with pulmonary edema, both types of
cyanosis may be present.
In the correct etiological definition of cyanosis some aspects are important:
1. The history, with particular reference to the period of onset of cyanosis (a
cyanosis present since birth is, generally, the expression of a congenital heart
disease) and the possible exposure to drugs or chemical agents responsible for
the production of abnormal hemoglobins.
2. Clinical differentiation between central and peripheral cyanosis. On the one
hand, the presence of respiratory or cardio-circulatory diseases can be
demonstrated by physical or radiographic examination; on the other hand,
maneuvers that induce an increase in the district flow (massage or bland heating
of a cyanotic end) are able to regress peripheral cyanosis, but do not affect
the central type. The presence or absence of digital hippocratism (digital
hippocratism without cyanosis is a frequent finding in patients with bacterial
endocarditis and in those with ulcerative colitis; it is occasional in healthy
subjects and sometimes can be the expression of occupational diseases, such as
for example workers who use a pneumatic hammer.
A modest cyanosis of the lips and
cheeks without digital hippocratism is a frequent finding in patients with
mitral stenosis and is likely to be due to minimal arterial hypoxia (secondary
to chronic stasis fibrotic pulmonary changes ) and the reduction of cardiac
output.The associated presence of the two phenomena, cyanosis and digital
hippocratism, is a frequent finding in some types of congenital and occasional
cardiopathies in some pulmonary diseases, such as pulmonary abscess or pulmonary
arteriovenous shunts. Digital Hippocratism is never present in peripheral
cyanosis or in a central type cuta. The determination of the oxygen tension in
the arterial blood or of the arterial oxygen saturation and the spectroscopic
analysis and other hematochemical investigations for the identification of
abnormal hemoglobins.
The decrease in arterial oxygen saturation results from a marked reduction in
the oxygen tension in the arterial blood. This can happen by reducing the oxygen
tension in the inspired air without compensating alveolar hyperventilation that
maintains the alveolar oxygen tension. If you climb to an altitude of 2500 m,
cyanosis is not significantly highlighted, but if you proceed further up to an
altitude of 5000 m it becomes very marked.
The explanation of the phenomenon is clear when analyzing the S-shaped morphology of the dissociation curve of
hemoglobin. At 2500 m the oxygen tension in the inspired air is about 120 mmHg,
the alveolar one of about 80 mmHg and the hemoglobin is almost completely
saturated, while at 5000 m the atmospheric oxygen and alveolar tensions are
respectively equal to 85 and 50 mmHg and at these levels the dissociation curve
of hemoglobin shows that the arterial blood is saturated for only about 75%.
This means that 25% of hemoglobin is in a reduced form, a quantity that is
clearly sufficient to give rise to cyanosis in the absence of anemia. Similarly,
an abnormal hemoglobin with low oxygen affinity (eg, Kansas hemoglobin) causes a
decrease in arterial oxygen saturation resulting in central cyanosis.
Also a marked impairment of pulmonary function by alveolar hypoventilation or by
perfusion of hypoventilated or non-ventilated lung areas is a frequent cause of
cyanosis of the central type. The phenomenon can occur acutely, as in massive
pneumonia or pulmonary edema, or it may have a chronic character, as is the case
with chronic lung diseases (eg, emphysema). In the latter case, polycythemia is
usually present and sometimes digital hippocratism may appear. However, in many
chronic pulmonary diseases with fibrosis and obliteration of the pulmonary
capillaries, cyanosis does not occur, as the hypoventilated areas are relatively
poorly perfused. Another cause of reduced arterial oxygen saturation is the
systemic venous blood shunt in the arterial circulation.
Cyanosis is present in some types of congenital heart disease. Since the direction of blood flow is
generally determined by the pressure gradient, in order for a right-to-left
shunt to occur in congenital heart disease with arteriovenous communication,
coexistence or an obstructive lesion in the venous system downstream of the
defect is usually necessary or of high pulmonary vascular resistance. The most
frequently associated congenital heart disease associated with cyanosis in
adults is the combination of interventricular septal defect with right
ventricular outflow tract stenosis (Fallot tetralogy).
Arteriovenous pulmonary fistulas may be congenital or acquired, solitary or multiple, microscopic or
massive; the degree of cyanosis that results is based on the number and width of
the fistulae. They are present, with a certain frequency, in hereditary
hemorrhagic telangiectasia. Also in some cirrhotic patients there is an arterial
oxygen desaturation, probably due to the presence of arterovenous pulmonary
fistulas or anastomosis between the veins of the portal system and the pulmonary
ones.
Probably the most frequent cause of peripheral cyanosis is the generalized
vasoconstriction produced by exposure to cold (air or water). This type of
answer is physiological. When the cardiac output is lowered, as in congestive
heart failure and in severe shock conditions, the cutaneous vasoconstriction
allows, as a compensatory mechanism, the deviation of blood towards the most
vital districts such as the central nervous system or the heart, to the
detriment some extremities that become cold and intensely cyanotic.
Although arterial oxygen saturation is normal, cyanosis appears equally due to the
combined effect of decreased flow at the cutaneous level and reduced oxygen
tension at the venous ends of the capillaries. In arterial thrombosis of a limb,
which can occur due to embolic phenomena, as well as in arteriolar
vasoconstriction following a cold-induced vasospasm (Raynaud's phenomenon), the
skin is generally pale and cold, but it can also be slightly cyanotic. In case
of venous obstruction with congestion and stasis of blood flow of the limb,
cyanosis is also present. District venous hypertension (observed in
thrombophlebitis) or generalized (as in tricuspid valvulopathy or constrictive
pericarditis) causes dilatation of subpapillary venous plexuses, intensifying
cyanosis.