Overview

Hypoxia and ischaemia are often interlinked and form an integral part of our understanding of human pathophysiology. However, the two terms describe different processes. In this article, we will explore the meanings of hypoxia and ischaemia and how they are associated.

What is Hypoxia?

Around the early 20th century, the term hypoxia was introduced. “Hypo” in Greek means ‘under’ and the latter part of the term refers to oxygen. Thus the term simply refers to low oxygen levels in the body.¹ This may be confused with hypoxaemia, which sounds very similar but describes low oxygen levels in the circulating blood, specifically within the arteries.² In medical terminology, words ending in -emia, are related to the blood. For example, hypocalcaemia refers to low levels of calcium in the blood.

Inspired air, enters the airways and travels to the lungs. Gas exchange occurs in the alveoli across a thin membrane - this allows for oxygen to diffuse into the blood. Oxygenated blood then flows to the left side of the heart where it can be pumped to the rest of the body. Respiratory failure and issues with gas exchange can lead to hypoxia, as the oxygen saturation in the blood (and eventually other tissues and organs) will be low. It’s also possible for individuals to experience hypoxia at high altitudes (altitude sickness). To understand some of the other causes and effects of hypoxia, it will be important to consider the meaning of ischaemia. 

What is ischaemia?

As mentioned earlier the suffix -emia, is used to describe something related to the blood. The first part of the term comes from the Greek word “ischein”, which means to suppress or to restrain. Altogether, the word ischaemia describes inadequate blood supply or flow. Ischaemia can occur due to blockage, compression or damage to the blood vessels, supplying organs and tissues. For example, if a blood clot is lodged in the arteries supplying the heart during a heart attack, there will be ischaemia. 

What is the relationship between ischaemia and hypoxia?

One of the main roles of blood is to carry oxygen (via red blood cells). When blood flow is compromised to a part of the body, it means that less oxygen (or none) will be delivered to those tissues/ organs. Therefore, ischaemia can lead to hypoxia. Hypoxia which is secondary to ischaemia can be caused by three main processes: 

• Formation of a thrombus/embolus (clots) in an artery - leads to acute disease, e.g., acute limb ischaemia.
• Atherosclerosis leads to the narrowing of the vessels
• Vasospasms of the arteries, e.g., Prinzmetal angina (coronary vasospasm)

It is important to note that hypoxia can also occur in the absence of ischaemia. Anaemia and respiratory disorders can lead to hypoxaemia and eventually hypoxia. In these situations, blood flow itself is not affected, but the saturation of oxygen in the blood is lower due to other disease mechanisms. 

Complications of ischaemia and hypoxia

The major concern with ischaemia and hypoxia is infarction/necrosis. These terms refer to tissue death, as our cells require oxygen to function. Infarction and necrosis mean different things in different parts of the body. For example, in myocardial infarctions (heart attack), the heart’s muscle cells die, due to inadequate blood flow and prolonged hypoxia. Depending on the extent, some patients may have more damage to their heart than others and this means that they can have dysfunctions, such as arrhythmias, ventricular ruptures and valve disorders. In another example, acute limb ischaemia could lead to gangrenes, sensory issues and potentially the need to amputate the necrotic limb. 

Ischaemia can be associated with pain that is often exacerbated by increased oxygen demands. For example, angina or intermittent claudication is made worse by exercise and physical exertion. Ischaemia and hypoxia can also lead to pale or blue discolourations of the skin. When ischaemia affects the extremities, patients may notice that their extremities (e.g., feet) are cold.

The body is capable of dealing with some levels of hypoxia, especially if it’s not prolonged⁽⁷⁾. One way of doing so is by increasing the rate of your breathing when oxygen demands are high. In a state of hypoxia the affinity that our haemoglobin has to oxygen, will also be increased. This ensures that maximal oxygen can be bound to haemoglobin and that oxygen saturations are optimised. 

Summary

Ischaemia describes a reduction in blood flow and supply, whereas hypoxia refers to low oxygen levels in tissues. Hypoxia can occur with or without ischaemia. Despite the body’s compensatory mechanisms for hypoxia, if not addressed appropriately and promptly, tissue death can occur, leading to damage to the body.