Hypoxia

Hypoxia literally means “deficient in oxygen”. Hypoxia is a shortage of oxygen in the body. Hypoxia is a pathological condition in which the body as a whole (generalized hypoxia) or a region of the body (tissue hypoxia) is deprived of adequate oxygen supply. Variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise. A mismatch between oxygen supply and its demand at the cellular level may result in a hypoxic condition. Hypoxia in which there is complete deprivation of oxygen supply is referred to as anoxia.

Types of Hypoxia

Hypoxic hypoxia is a generalized hypoxia, an inadequate supply of oxygen to the body as a whole. The term “hypoxic hypoxia” specifies hypoxia caused by low partial pressure of oxygen in arterial blood. In the other causes of hypoxia that follow, the partial pressure of oxygen in arterial blood is normal.

Hypoxic hypoxia may be due to:

• Low partial pressure of atmospheric oxygen such as found at high altitude or by replacement of oxygen in the breathing mix either accidentally as in the modified atmosphere of a sewer or intentionally as in the recreational use of nitrous oxide.
• Low partial pressure of oxygen in the lungs when switching from inhaled anaesthesia to atmospheric air, due to the Fink effect, or diffusion hypoxia.
• A decrease in oxygen saturation of the blood caused by sleep apnea or hypopnea Inadequate pulmonary ventilation (e.g., in chronic obstructive pulmonary disease or respiratory arrest).
• Shunts in the pulmonary circulation or a right-to-left shunt in the heart. Shunts can be caused by collapsed alveoli that are still perfused or a block in ventilation to an area of the lung. Whatever the mechanism, blood meant for the pulmonary system is not ventilated and so no gas exchange occurs (the ventilation/perfusion ratio is zero). Normal anatomical shunt occurs in everyone, because of the Thebesian vessels which empty into the left ventricle and the bronchial circulation which supplies the bronchi with oxygen. hypemic hypoxia in which arterial oxygen pressure is normal, but total oxygen content of the blood is reduced.

Hypoxia when the blood fails to deliver oxygen to target tissues.

• Carbon monoxide poisoning which inhibits the ability of hemoglobin to release the oxygen bound to it.
• Methaemoglobinaemia in which an abnormal version of hemoglobin accumulates in the blood Histotoxic hypoxia in which quantity of oxygen reaching the cells is normal, but the cells are unable to effectively use the oxygen due to disabled oxidative phosphorylation enzymes. The effects of drinking alcoholic beverages is a common example.

Ischemic, or stagnant hypoxia in which there is a local restriction in the flow of otherwise well-oxygenated blood. The oxygen supplied to the region of the body is then insufficient for its needs. Examples are cerebral ischemia, ischemic heart disease and Intrauterine hypoxia, which is an unchallenged cause of perinatal death.

What are the Signs and Symptoms of Hypoxia

You may not even recognize the symptoms of hypoxia because hypoxia is insidious in its onset. The signs and symptoms can be different for every person and may not occur in the same progression as listed below. Therefore, it is important to be aware of all the signs and symptoms.

Signs:

• Rapid Breathing
• Cyanosis
• Poor Coordination
• Lethargy/Lassitude
• Executing Poor Judgment

Symptoms:

• Air Hunger
• Dizziness
• Headache
• Mental and Muscle Fatigue
• Nausea
• Hot and Cold Flashes
• Tingling
• Visual Impairment
• Euphoria
• Degradation or loss of night vision
• Loss of Consciousness

Factors which can increase the risk of hypoxia

The following factors can increas the effects and cause the onset of hypoxia at lower altitudes.

• Being Overweight;
• Drinking too much alcohol;
• Suffering from an illness;
• Being tired or unfit;
• Being cold;
• Being a smoker. (carbon monoxide displaces oxygen in the blood to the extent that a smoker could already be at the equivalent of 8000 feet while still on the ground.

Time of Useful Consciousness

In the event of a rapid decompression of an aircraft cabin, the occupants often lose consciousness. The time available to recognise the development of hypoxia and to do something about it is termed the ‘time of useful consciousness’. This time decreases rapidly with increased altitude. As a guide the following times would be applicable to an average person who is not suffering from any additional risk factor.

Technical data content credited to Mr Steve Griffin