Key Difference – Hypoxia vs Ischemia
Hypoxia and Ischemia are both diseases caused due to the insufficiency of oxygen supply in the body, but there is a difference between hypoxia and ischemia. The key difference between these two diseases is that the Hypoxia is a condition in which the body or a region of the body is deprived of adequate oxygen supply while Ischemia is a reduction of blood supply to tissues, causing a limitation of oxygen and glucose required for the metabolism.
What is Hypoxia?
Hypoxia can be classified as either generalized (affecting the whole body) or localized (affecting a single region of the body). Hypoxia is different from hypoxemia. Hypoxia refers to a state in which the oxygen supply is insufficient for the demand, whereas hypoxemia refers to states that have low arterial oxygen concentration. Complete deprivation of oxygen supply is called “anoxia.”
Generalized hypoxia can occur in healthy people at a high altitude where oxygen concentration of the air is low. It causes altitude sickness causing potentially fatal outcomes such as high altitude pulmonary edema (HAPE) and high altitude cerebral edema (HACE) due to hypoxic damage. Hypoxia can also be occurred in healthy people when breathing mixtures of gasses with a low oxygen concentration, e.g. Diving deep underwater (Deep sea divers). Sometimes mild and non-damaging intermittent hypoxia is used intentionally for altitude training to improve athletic performance by adaptation of both the systemic and cellular bio environments.
Hypoxia can be a serious complication of preterm birth in the neonate due to immature lungs. Lungs of the human fetus mature toward the latter part of the pregnancy. To minimize this complication, infant at risk of hypoxia are often placed inside an incubator which is capable of maintaining continuous positive airway pressure to prevent lung collapse.
What is Ischemia?
Ischemia is caused by problems with the circulatory system causing damage or dysfunction of oxygen sensitive tissues. Most tissues cannot survive more than few minutes without a continuous oxygen supply. Lack of oxygen leads to a process known as Ischemic cascade. The damage is caused by the accumulation of metabolic waste products, damage to cell membranes, mitochondrial (powerhouse of the cell) dysfunction. This leads to leakage of or activation of autolyzing and proteolytic enzymes causing damage to the cell and surrounding tissues. The sudden restoration of blood supply to an ischemic tissue can lead to increased damage by a process called reperfusion injury which can be more harmful than the initial ischemic damage. Reintroduction of blood supply brings more oxygen back to the damaged tissues. It causes a greater production of oxygen free radicals and reactive oxygen species that in turn damage cells and tissues. As a secondary complication, it increases calcium concentration inside cells causing fatal cardiac arrhythmias and also increased cellular damage by activating many proteolytic enzymes. Heart ischemia leads to heart attacks and brain ischemia leads to strokes. Any organ of the body can get ischemia damage due to inadequate oxygen supply to meet the continuous demand for cellular metabolism.
What is the Difference Between Hypoxia and Ischemia?
Definition of Hypoxia and Ischemia
Hypoxia: Hypoxia refers to a state where the oxygen supply is insufficient for the demand.
Ischemia: Ischemia is damage or dysfunction of oxygen sensitive tissues due to reduction of blood supply.
Causes and Complications of Hypoxia and Ischemia
Hypoxia: Causes of Hypoxia can be the high altitude where oxygen concentration of the air is low, breathing mixtures of gasses with a low oxygen concentration, etc.
Ischemia: Ischemia is caused by problems with the circulatory system.
Hypoxia: Hypoxia damage can cause fatal outcomes such as high altitude pulmonary edema and high altitude cerebral edema. It can also cause preterm birth complications.
Ischemia: Complications of Ischemia include heart attacks, strokes, reperfusion injury and the fatal cardiac arrhythmias can be a secondary complication.
Characteristics of Hypoxia and Ischemia
Hypoxia: Hypoxia can be reversed when the oxygen supply is restored.
Ischemia: Ischemia is potentially reversible when the blood supply is restored. However, oxygen sensitive tissues such as brain and heart may not recover unless the blood supply is restored rapidly.
Hypoxia: Hypoxia can be physiological such as in exercise.
Ischemia: Ischemia is almost always pathological.
Hypoxia: Hypoxia can affect the whole body (generalized) or a single region of the body (localized).
Ischemia: Ischemia affects one part of the body (local) in most of the cases.
Image Courtesy: “Cynosis” by James Heilman, MD – Own work. (CC BY-SA 3.0) via Wikimedia Commons “Heart inferior wall infarct” by Patrick J. Lynch, medical illustrator – Patrick J. Lynch, medical illustrator. (CC BY-SA 3.0) via Wikimedia Commons