Anoxia can be histotoxic, i.e. due to tissue damage, or consequent to a severe decrease in the blood supply to the tissue concerned (anoxia): these situations result in a lack of oxygen in the tissue due to insufficient blood supply or a deficiency in oxygen utilisation despite adequate blood supply.
Anoxia is a medical emergency that if not resolved quickly leads to the death of tissues, especially those particularly sensitive to oxygen deficiency, such as nerve tissue.
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Causes of anoxia
Anoxia can be caused, promoted or related to various diseases and conditions, including:
- pulmonary embolism;
- cerebral stroke;
- acute myocardial infarction;
- heart failure;
- angina pectoris;
- Sleep apnoea;
- lung cancer;
- cirrhosis of the liver;
- carbon monoxide intoxication;
- heart failure;
- septic shock;
- severe burns.
The term ‘cerebral anoxia’ refers to the lack of oxygen to the brain cells, which, in the absence of it, die within no more than 5 minutes, leading to often irreversible damage (or only partly reversible thanks to lengthy rehabilitation) that can range from motor and/or sensory deficits, behavioural alterations, deficits in cognitive capacity and/or memory, to – in the most severe cases – coma and death of the patient, if oxygenation is not quickly restored.
In medicine, ‘ischaemia’ refers to a partial or total deficiency in the blood supply to a tissue, caused by various causes, e.g. a thrombus (as is often the case in myocardial infarction) or haemorrhage (as is often the case in a haemorrhagic brain stroke).
Obviously, as the blood supply to a given tissue is lacking in ischaemia and as blood is the vehicle of oxygen, that tissue will undergo anoxia and, if the blood flow is not soon restored, necrosis (death).
Occurs in all circumstances where there is a drop in the partial pressure of circulating oxygen.
It may be caused by inadequate breathing or otherwise insufficient gas exchange, as in pneumothorax, bronchial asthma, an occlusion of the respiratory airways, the presence of liquids in the respiratory spaces such as blood, mucus, exudates.
Venous-arterial shunt due to cardiac or vascular malformation, resulting in mixing of the two types of blood at different oxygen concentrations.
Occurs in all those circumstances where there is a slowing of the blood flow due to heart failure or pathological events related to blood reflux.
In all these cases, where there is a decrease in the partial pressure of oxygen in the venous blood, hypoxia of the apparatuses occurs.
This is a consequence of the fact that although the contact time between blood and tissues is increased, it is always inadequate to supplement the necessary supply.
In such cases, haemoglobin will also be completely desaturated.
This is due either to a decrease in erythrocytes as a result of anaemia of any kind, or to a decrease in their transport power.
The latter circumstance is linked to all forms of methaemoglobin, or to cases of CO poisoning: this shows a significantly higher affinity for chromoprotide than oxygen, and also shifts the dissociation curve to the left, inducing acute anoxia.
Cyanides and hydrogen cyanide are responsible for this last category of deficiency: they have the characteristic of blocking cytochrome oxidase at the tissue level.
It should not normally be counted among the anoxias since oxygen arrives but is not utilised.
Methylene blue is an excellent antidote, in fact by inducing the formation of methaemoglobin it favours the release of toxic agents that show a high affinity for the oxidised form of the chromoprotide.
Neonatal anoxia refers to the depletion of oxygen to the infant’s brain, which can lead to serious and irreversible damage.
It is important to remember that during childbirth, temporary anoxia always occurs. Its function is to act as a stimulus to activate breathing by acting on the respiratory centres located in the bulb.
However, if the anoxia is prolonged, it puts the life of the newborn at risk.
Causes of neonatal anoxia can be
- functional damage to the bulbar respiratory centres;
- airway malformations;
- airway obstruction.
A risk factor for neonatal anoxia is pre-term delivery.
Symptoms and signs
Anoxia itself is a sign of an underlying disease or condition that causes it.
Different symptoms may be associated with anoxia depending on the pathology or condition that caused it.
Symptoms and signs commonly associated with anoxia are:
- general malaise;
- dyspnoea (difficulty breathing);
- blurred vision;
- hearing difficulties;
- confusional state;
- tachycardia (increased heart rate);
- tachypnoea (increased respiratory rate);
- high blood pressure;
- loss of motor coordination;
- difficulty concentrating;
- coma and death in the most severe cases.
The treatment of anoxia is dependent on the cause.
Pending a definitive diagnosis, which is necessary to set up a specific therapy for the aetiological agent, oxygen therapy may be recommended in some cases.
If symptoms are severe, mechanical help with breathing (mechanical ventilation) may be required.
Risks and complications
Anoxia can irreversibly damage vital organs such as the brain and heart in less than five minutes.
The consequences can be convulsions, coma or even death of the patient.