Clinical problem of cyanosis
1.Develop hypotheses to explain the clinical sign of cyanosis
Clinical/Physical Sign
Cyanosis is a clinical sign characterized by a blue colour of the mucous membranes, nail beds,
and skin. It results from the presence of deoxygenated haemoglobin in the blood at a
concentration of at least 0.5 g per mL. Cyanosis is usually undetectable until the oxygen
saturation of hemoglobin falls below 80%
Note: Colour of the Blood
Haemoglobin that is fully saturated with oxygen is bright red, while haemoglobin that has lost
one or more oxygen molecules (deoxygenated haemoglobin) is darker in appearance. When it
has lost most of its oxygen, haemoglobin becomes deep purple in colour. As the blood passes
through the tissues it gives up its oxygen and the proportion of deoxygenated haemoglobin
increases.
CAUSES OF CENTRAL CYANOSIS
A Systemic Excess of Deoxygenated Haemoglobin can occur at various stages in the circulation.
At the Lungs (Pulmonary)
There is a problem with the gas exchange in the lungs and the Oxygen contained
within the lung alveoli is not attaching to the Haemoglobin in the blood contained
within the lung capillaries. Important note: the clinical sign is an excess of deoxygenated
haemoglobin (central cyanosis) but the underlying problem is decreased arterial
haemoglobin oxygen saturation
At the Heart (Cyanotic Congenital Heart Defects)
There is a structural problem with the heart or around the heart in the great
vessels that is not allowing the functional distribution of the blood. Basically, somehow
the deoxygenated haemoglobin in the systemic venous system is finding its way into the
systemic arterial system. Important note: the clinical sign is an excess of deoxygenated
haemoglobin (central cyanosis) but the underlying problem is decreased arterial
haemoglobin oxygen saturation
In the Blood itself
2 forms of haematological problems are associated with central cyanosis.
Structural issues with haemoglobin will cause failure to bind oxygen
(Methemoglobinaemia). Whilst an excess of RBCs will provide excess unneeded and
unused haemoglobin which will be blue/dark purple (Polycythemia)
CAUSES OF PERIPHERAL CYANOSIS
A Local Excess of Deoxygenated Haemoglobin or Peripheral cyanosis reflects poor perfusion of
the peripheries caused by vasoconstriction, reduced cardiac output, or vascular occlusion (It
may also be caused by central cyanosis). It may be widespread or may affect only one extremity;
however, it doesn’t affect mucous membranes. Typically, peripheral cyanosis appears on
exposed areas, such as the fingers, nail beds, feet.
Poor Perfusion of the peripheries caused by numerous interrelated factors
Low temperatures (Cold Baby)
Low cardiac output
Shock
Sepsis
DVT
Painted fingernails
Eating Blue Lollies
2. Understand the underlying pathophysiology of cyanosis.
RESPIRATORY CAUSES (There are more than listed here)
Obstruction to the intake of oxygen – Structural (Not enough air can get to the alveoli)
Acute Laryngotracheitis
Blast Lung Injury
Chronic Bronchial Asthma
Chronic Bronchitis
COPD (Chronic Obstructive Pulmonary Disease)
Emphysema
Foreign Body (common with children; peanuts)
Tension Pneumothorax
Decreased oxygen absorption of oxygen – as in conditions with alveolar-capillary block
Sarcoidosis
Pulmonary Fibrosis
Pneumonia
Pulmonary Oedema
Proteinosis
Lung Cancer
Decreased Perfusion of the Lung
Pulmonary Embolism
Pulmonary Vascular Shunts (arteriovenous fistulas)
Pulmonary Hemangiomas
CARDIOVASCULAR CAUSES
Left-to-Right Shunts – Lungs-to-Lungs Shunts – Blood that is returning to the heart from the
lungs is recirculated back to the lungs without going to the rest of the body. If the blood was
going from the left side of the heart to the right side of the heart then the deoxygenated blood
is not getting pumped around the body and there would be no sign of central cyanosis.
Therefore L – R shunts are called acyanotic. However, Most of the time there is never a
complete Left to right movement of blood and cyanotic symptoms can develop later in life.
(cause cyanosis several months or years after birth)
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Atrial septal defects
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ASD is an abnormal opening in the atrial septum that allows communication
of blood between the left and right atria. (It is usually asymptomatic until
adulthood)
Ventricular septal defects
o Incomplete closure of the ventricular septum allowing free communication
and thus a shunt from L to R ventricles. (It may produce problems from birth
or may not be recognized until later in life) (Only about 30% of cases exist in
isolation; most present with associated structural defects)
Patent or persistent ductus arteriosus (PDA)
o PDA results when the ductus arteriosus remains open after birth (the ductus
arteriosus is the passage from the pulmonary trunk to the aorta which
bypasses the lungs in the foetus) 90% of PDAs occur in isolation..
Atrioventricular septal defects (AVSD)
o Abnormal development of the Atrioventricular canal resulting in incomplete
closure of the AV septum and inadequate formation of the tricuspid and
mitral valves. This can be complete or partial: in the complete form all four
cardiac chambers communicate (1/3 of patients with the complete form
have Downs Syndrome)
Right-to-Left Shunts – Body-to-Body Shunts – Blood that is returning to the heart from the body
is recirculated directly back to the body without going to the lungs to be oxygenated.
(cause cyanosis early in postnatal life)
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Tetralogy of Fallot
o 1. Ventricular Septal Defect (VSD)
o 2. Obstruction to the right ventricular outflow tract (subpulmonary stenosis)
o 3. An aorta that overrides the VSD
o 4. Right ventricular hypertrophy
Transposition of the Great Arteries (TGA) Not Viable without a VSD
o Implies ventriculoarterial discordance, such that the aorta arises from the
right ventricle and the pulmonary artery arises from the left ventricle. This
condition is incompatible with postnatal life unless a shunt exists for
adequate mixing of blood (causing cyanosis)
Truncus Arteriosus
o Normally during embryo development the truncus arteriosus separates to
form the aorta and the pulmonary aorta. Thus without this separation blood
from the R and L ventricles mixes.
Tricuspid Atresia Not Viable without an ASD and VSD
Total Anomalous Pulmonary Venous Connection (TAPVC) Not viable without an ASD
HAEMOTOLOGICAL CAUSES
 Polycythemia is an excess of RBCs. This means that when the blood filters through
the lung capillaries the normal amount of Haemoglobin picks up the normal amount
of oxygen and distributes it: all is good. Except that there is an excess in
Deoxygenated Haemoglobin circulating in the blood which are blue/deep purple
thus causing Cyanosis. This represents an excess of deoxygenated haemoglobin but
not decreased arterial haemoglobin oxygen saturation
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Methemoglobinaemia occurs when iron exits in the oxidized form (ferric) in greater
than 1% of cells. This occurs in congenital and acquired forms (usually due to toxic
exposures). Patients with congenital methemoglobinemia are cyanotic, which is
primarily of cosmetic importance. It generally follows a benign course. In acquired
states, cyanosis reflects acute alteration of tissue oxygen delivery. Methemoglobin
has a very high affinity for oxygen and does not deliver oxygen well to tissues.
Methemoglobin levels greater than 50% can be rapidly fatal.
Central cyanosis may also result from reduced Pao2 at high altitudes.
3. Describe the use of arterial blood gas as a clinical measurement in cyanosis.
Hyperoxia Test
In general, infants with a cardiac origin for their cyanosis exhibit a worsening of their cyanosis
with crying. Administration of 100% oxygen (also known as the Hyperoxia test) can be used as a
bedside diagnostic tool to help differentiate between cardiac and pulmonary causes of central
cyanosis. This test consists of assessing the rise in arterial oxygenation with the administration
of 100% oxygen. The response to 100% oxygen can either be determined by the pulse oximeter
or by actually measuring an arterial blood gas. According to one reference, if the patient's
oxygen saturation increases by more than 10% or the PaO2 rises by more than 20% to 30%, then
the most likely origin for the cyanosis is pulmonary rather than cardiac. If the oxygen saturation
does not increase and the cyanosis does not improve with supplemental oxygen, then a cardiac
origin for the patient's cyanosis should be suspected. The measured PaO2 in patients with a
pulmonary origin for cyanosis should rise well above 200 mm Hg unless the degree of
pulmonary disease that is present is severe. PaO2 values that do not rise above 100 mm Hg in
cyanotic patients are highly suggestive of a cardiac defect. A PaO2 that remains lower than 100
mm Hg despite 100% oxygen is suggestive of a congenital heart defect with decreased
pulmonary blood flow and/or right-to-left shunting. Those congenital heart defects with an
increased pulmonary blood flow may exhibit a rise in their PaO2 (up to 150 mm Hg) in response
to 100% oxygen. Prolonged administration of 100% oxygen may cause some theoretical
problems, such as closing the ductus arteriosus in those infants with critical left heart
obstructions or by causing pulmonary vasodilation (which would potentially worsen pulmonary
vascular congestion).
4. Outline how arterial blood gas measurements relate to the biochemistry and physiology of
gas exchange.
ABGs - Normal Values
pH
PaO2
PaCO2
HCO3-
7.35 – 7.45
80 – 110
36 -44
22 -26
(mmHg)
(mmHg)
(mmol/L)
Gas exchange occurs at two places in the circulatory system: at the lungs and at the tissues
Arterial Blood gases are taken from the Systemic Arterial Circulation which is one point of the
circuit. The measurement is directly downstream from the gas exchange occurring at the lungs
and gives us a good idea of lung function.
pH ; H+ indicates if a patient is acidotic (pH < 7.35) or alkalotic (pH > 7.45).
PaO2 ; A low O2 indicates that the patient is not respiring (absorbing oxygen into the blood
stream) properly. At a PaO2 of less than 60 mm Hg (and a Sa O2 of < 90%) a patient is
hypoxaemic and oxygen should be administered.
PaCO2 ; The carbon dioxide and partial pressure (PaCO2) indicates a respiratory problem: for a
constant metabolic rate, the PaCO2 is determined entirely by ventilation. A high PaCO2
(respiratory acidosis) indicates hypoventilation, a low PaCO2 (respiratory alkalosis)
hyperventilation. PaCO2 levels can also become abnormal when the respiratory system is
working to compensate for a metabolic issue so as to normalize the blood pH. An elevated
PaCO2 level is desired in some disorders associated with respiratory failure; this is known as
permissive hypercapnia.
HCO3- ; The HCO3- ion indicates whether a metabolic problem is present (such as ketoacidosis). A
low HCO3- indicates metabolic acidosis, a high HCO3- indicates metabolic alkalosis. HCO3- levels
can also become abnormal when the kidneys are working to compensate for a respiratory issue
so as to normalize the blood pH.