HYPOXIA : TYPES, CAUSES AND TREATMENTS;
DEFINITION;
Hypoxia means the lack of oxygen at tissue level. Anoxia means complete absence of oxygen in the tissues.
TYPES OF HYPOXIA;
1. HYPOXIC HYPOXIA
2. ANAEMIC HYPOXIA
3. STAGNANT ( ischaemic) HYPOXIA
4. HISTOTOXIC HYPOXIA
1. HYPOXIC HYPOXIA:
DEFINITION:
It is characterized by the a low arterial pO2 when O2 carrying capacity of blood and rate of blood flow to tissues are normal or elevated.
Characteristic feature :
Thus, characteristic features of hypoxic hypoxia are:
(1) low arterial pO2,
(2) low arterial O2 content,
(3) low arterial % O2-saturation of haemoglobin, and
(4) low A-V pO2 difference.
Causes:
1. Low pO2 in inspired air, for example:
(i) High altitude (more than 10000 feet or 3000 mts above sea level).
(ii) Breathing in a closed space.
(iii) Breathing an artificial gas mixture containing low pO2
2. Decreased pulmonary ventilation as seen in:
(i) Airway obstruction.
(i) Weakness/paralysis of respiratory muscles.
(ii) Depression of respiratory centres by drugs specially morphine.
3. Defect in exchange of gases through alveolar-capillary membrane. This includes all the factors affecting V/P ratio.
4. Venous-arterial shunts i.e. venous blood enters arterial blood without going into the lungs, therefore, arterial pO2 decreases. For example: cyanotic congenital heart disease (Fallot's tetrology).
Pathophysiology:
2. ANAEMIC HYPOXIA:
Definition:
Hypoxia in which arterial pO2 is normal but the amount of haemoglobin available to carry O2 is reduced
Causes:
1. Anaemia
2. Haemorrhage
3. Conversion of Haemoglobin to some abnormal form.
For example:
(i) Methaemoglobin i.e. iron in the haemoglobin is present in ferric (Fe³+) form instead of ferrous (Fe²+) form.
(ii) Carboxy (Carbonmonoxy) Haemoglobin: COHb i.e, haemoglobin combines with 'CO'. The affinity of haemoglobin for 'CO' is 210 times its affinity for O2 and COHb so formed liberates O2 very slowly
COHb is cherry red in colour and visible in the skin,
nail beds and mucous membranes. 'CO' produces severe hypoxia by:
(a) preventing the haemoglobin to combine with O2, and
(b) shifting O2,-haemoglobin dissociation Curve to left, therefore, less release of O2.
Important Note
The symptoms of CO poisoning are those of any type of hypoxia. In addition, chronic exposure to sublethal concentrations of CO produces: progressive brain damage, mental changes and Parkinsonism like state. Death results when 70-80 % of the Circulating haemoglobin is converted to CoHb.
Characteristics features :
1. As haemoglobin content is less than normal or it is converted to some abnormal form, thus, normal haemoglobin content will reduce to less than 50% of normal.
2. Arterial pO2 is normal (100 mmHg), as there is no defect in alveolar ventilation or pulmonary blood flow.
3. Arterial percentage saturation of haemoglobin with Oxygen decreases as its concentration in blood is low (about 50% of normal) due to anaemia.
4. O2 Content of arterial blood is less than normal, approximately 10 mL/dL at arterial pO2 100 mmHg.
Therefore, when this arterial blood reaches the tissues, the tissues will take up 5 mL/dL of O2(O2 utilisation of resting tissues), hence in venous blood:
(i) pO2 : 40 mmHg
(ii) % O2-saturation of haemoglobin: decreases, and
(iii) O2 content: 10-5= 5 mL/dL.
Thus characteristic features of anaemic hypoxia are:
(1) normal arterial pO2
(2) arterial O2 content, moderately reduced
(3) arterial % O2-saturation of haemoglobin: decreases and
(4) A-V pO2 difference is normal, 100-40= 60 mmHg.
Pathophysiology:
In anaemic hypOxia at rest, hypoxia is not severe, because in anaemia there is increased amount of 2,3 DPG in the RBCs which combines with oxyhaemoglobin and results in increased liberation of O2.
HbO2 +2,3 DPG Hb 2,3 DPG + O2
But during exercise when there is increased O, demand by tissues due to increased O2 Consumption, the tissue demand is not met fully and severe hypoxia develops.
3. STAGNANT (ISCHAEMIC) HYPOXIA:
Definition:
Hypoxia in which the blood flow to the tissues is so low that adequate O2 is not delivered to them despite a normal arterial pO2 and haemoglobin concentration.
Causes:
1. Circulatory failure
2. Haemorrhage, via baroreceptors produces reflex vasoconstriction and thus blood flow to tissues decreases,
3. Congenital heart failure. It is associated with pulmonary congestion which produces defect in Oxygenation, therefore, patient also suffers from hypoxic hypoxia in addition to stagnant hypOxia.
Characteristic Features:
1. Since there is no defect in oxygenation of blood, % O2-saturation of haemoglobin; pO2 and O2 content of arterial blood are normal.
2. As blood stays for a longer time at tissue level, therefore,
(i) resting O2 uptake by tissue increases from 5 mL/dL to 10 mL/dL;
(ii) accumulation of CO2 in tissues shifts 'oxygen- haemoglobin dissociation curve to 'Right', more O2 is released from haemoglobin, therefore, venous blood is further reduced.
Hence in venous blood:
(i) pO2 : 25 mmHg
(ii) % O2 saturation of haemoglobin: 45%, and
(iii) O2 Content: 19-10=9 mL/dL.
Thus, characteristic features of stagnant hypoxia are:
1. Normal arterial pO2
2. Normal arterial O2 content
3. Normal arterial % O2-rsaturation of haemoglobin,
and
4. A-V p02 difference: 100-25=75 mmHg, more than normal.
4 .HISTOTOXIC HYPOXIA:
Definition:
Hypoxia in which the amount of oxygen delivered to the tissues is adequate, but because of the action of a toxic agent the tissue cells cannot make use of the O2 supplied to them.
Causes:
Cyanide poisoning: It produces hypoxia at tissue level causing poisoning of cellular enzymes specially cytochrome Oxidase, and also produces tissue oedema.
Characteristic features:
Since tissues cannot utilize O2 due to cyanide poisoning, so practically there is no difference of A-V pO2. If tissues take 0.5 mL/dL of O2 then % O2-saturation of haemoglobin is 96% with O2 content 18.5 mL/dL and pO2 90 mmHg.
Thus, characteristic features of histotoxic hypoxia are:
1. Normal arterial pO2
2. No difference of O2 content of arterial and venous blood,
3. Normal arterial % O2 saturation of haemoglobin
4. A-V PO2 difference: 100-90=10 mmHg i.e. Less than normal.
