On the basis of the physiologic effects elicited on the body, hypoxia can be divided into four different types: hypoxic hypoxia, hypemic hypoxia, stagnant hypoxia, and histotoxic hypoxia.

Hypoxic hypoxia is a deficiency in alveolar oxygen exchange. A reduction in PO ₂ in inspired air or the effective gas exchange area of the lung may cause oxygen deficiency. The result is an inadequate oxygen supply to the arterial blood, which in turn decreases the amount of oxygen available to the tissues. ^3,21,23 Decreased barometric pressure at high altitudes causes a reduction in the alveolar partial pressure of oxygen (PaO ₂). The blood oxygen saturation, which is 98% at sea level, is reduced to 87% at 10,000 ft and 60% at 22,000 ft. This reduction in the amount of oxygen in the blood decreases the availability of the oxygen to the tissues and causes an impairment of body functions. ³⁴ Hypoxic hypoxia is also referred to as altitude hypoxia because its primary cause is exposure to low barometric pressure. Hypoxic hypoxia interferes with gas exchange in two phases of respiration: ventilation and diffusion. During the ventilation phase, a reduction in PaO ₂ may occur. Specific causes include breathing air at reduced barometric pressure, strangulation/respiratory arrest/laryngospasm, severe asthma, breath holding, hypoventilation, breathing gas mixtures with insufficient PO ₂, and malfunctioning oxygen equipment at altitude. Causes of reduction in the gas exchange area include pneumonia, drowning, atelectasis, emphysema (chronic obstructive pulmonary disease), pneumothorax, pulmonary embolism, congenital heart defects, and physiologic shunting. Some causes of diffusion barriers are pneumonia and drowning. ^3,21,23

Hypemic hypoxia is a reduction in the oxygen-carrying capacity of the blood. If the number of red blood cells per unit volume of blood is reduced, as from various types of anemia or from a loss of blood, the oxygen-carrying capacity and thus the oxygen content of the blood are reduced. ³⁴ Even with normal ventilation and diffusion, cellular hypoxia can occur if the rate of delivery of oxygen does not satisfy metabolic requirements. ^1,23,34 Hypemic hypoxia interferes with the transportation phase of respiration and causes a reduction in oxygen-carrying capacity. Specific causes of hypemic hypoxia include anemia, hemorrhage, hemoglobin abnormalities, use of drugs (e.g., sulfanilamides, nitrites), and intake of chemicals (e.g., cyanide, carbon monoxide). ^23,31 Carbon monoxide is significant to air medical crews because it is present in the exhaust fumes of both conventional and jet-engine aircraft. It is also present in cigarette smoke and any fire or smoke situations. Carbon monoxide binds with hemoglobin 200 times more readily than does oxygen and displaces oxygen to form carboxyhemoglobin. ^23,31

Stagnant hypoxia occurs when conditions result in reduced total cardiac output, pooling of the blood within certain regions of the body, a decreased blood flow to the tissues, or restriction of blood flow. ^6,23 Stagnant hypoxia interferes with the transportation phase of respiration by reducing systemic blood flow. Specific causes include heart failure, shock, continuous positive-pressure breathing, acceleration (g forces), and pulmonary embolism. A reduction in regional or local blood flow may be caused by extremes of environmental temperatures, postural changes (prolonged sitting, bed rest, or weightlessness), tourniquets (restrictive clothing, straps), hyperventilation, embolism by clots or gas bubbles, and cerebral vascular accidents. ^23,31

Histotoxic hypoxia (tissue poisoning) occurs when metabolic disorders or poisoning of the cytochrome oxidase enzyme system results in a cell’s inability to use molecular oxygen. ^23,31 Histotoxic hypoxia interferes with the utilization phase of respiration because of metabolic poisoning or dysfunction. Specific causes include respiratory enzyme poisoning or degradation and the intake of carbon monoxide, cyanide, or alcohol. ^23,31 Carbon monoxide can cause both hypemic and histotoxic hypoxia.

These two terms are frequently used synonymously but are not interchangeable. Effective performance time (EPT) denotes the amount of time an individual is able to perform useful flying duties in an environment of inadequate oxygen. ²⁰Time of useful consciousness (TUC) refers to the elapsed time from the point of exposure to an oxygen-deficient environment to the point at which deliberate function is lost. ^6,29 EPT more accurately refers to critical (functional) performance than does TUC. With the loss of effective performance in flight, an individual is no longer capable of taking the proper corrective or protective action. ²³ Thus, for air medical personnel the emphasis is on prevention.

In addition to altitude, factors that influence TUC are rate of ascent and an individual’s physical fitness, physical activity, temperature, individual tolerance, and self-imposed stresses, such as smoking, intake of alcohol and medication, and fatigue. ⁶ Another factor that dramatically reduces both EPT and TUC is rapid decompression, which occurs when a quick loss of cabin pressure occurs in a pressurized aircraft at high altitudes. On decompression at altitudes above 10,058 m (33,000 ft), an immediate reversal of oxygen flow in the alveoli takes place, caused by a higher P O₂ within the pulmonary capillaries, which depletes the blood’s oxygen reserve and reduces the EPT at rest by up to 50%. Exercise also reduces the EPT considerably. ^23,31,34Table 5-2 presents altitude and time of useful consciousness.

Hypoxia has the three following causes: 1, high altitude; 2, hypoventilation; and 3, pathologic condition of the lung.

The onset of hypoxia may be gradual or insidious. Intellectual impairment occurs, demonstrated by slowed thinking, faulty memory of events, lessened immediate recall, delayed reaction time, and a tendency to fixate.

The individual symptoms of hypoxia can be identified in subjects under safe and controlled conditions in an altitude chamber. Once recognized, these symptoms do not vary dramatically in similar time exposures or among subjects. Hypoxia can be classified by objective signs (those perceived by an observer) or subjective symptoms (those perceived by the subject). ²³ Signs and symptoms that appear on both lists in Box 5-1 may be seen by observers and recognized by the hypoxic subject when they occur. ^{3,11,18,21,35}

Hyperventilation at altitude is an important consideration for air medical personnel and also for the air medical patient. Hyperventilation is of concern because it produces changes in cellular respiration. Although the causes are unrelated, the symptoms of hyperventilation and hypoxia are similar and often result in confusion and inappropriate corrective procedures. Despite increased knowledge, training, and improved life-support equipment, both hypoxia and hyperventilation are hazards in flying and diving operations. ²³Hyperventilation is an abnormal increase in the rate and depth of breathing that upsets the chemical balance of the blood³¹; it is commonly caused by psychologic stress (e.g., fear, anxiety, apprehensiveness, and anger) and environmental stress (e.g., hypoxia, pressure breathing, vibration, and heat). Certain drugs such as salicylates and female sex hormones also cause or enhance hyperventilation, and any condition that creates metabolic acidosis results in hyperventilation at high altitudes. ^3,23,31

Boyle’s law states that at a constant temperature, the volume of a gas is inversely proportional to the pressure. On ascent, gases expand; on descent, gases contract. Therefore, trapped or partially trapped gases within certain body cavities (e.g., the gastrointestinal [GI] tract, lungs, skull, middle ear, sinuses, and teeth) expand in direct proportion to the decrease in pressure. ^{3,11,12,15,16,19,20,26,34}