Hyperventilation literally means to 'overbreathe' – both too deeply and/or too fast. When someone hyperventilates, they breathe deeply and excessively to the point where noticeable symptoms develop.
This sustained deep breathing causes a reduction of carbon dioxide levels in the blood. This in turn reduces the acidity of the blood in other words the pH of the blood increases and the bicarbonate in the blood is reduced. The resulting lower calcium levels in the blood cause the sensation of 'pins and needles' in the lips and fingertips that sometimes makes the diagnosis of hyperventilation easy.
Someone who is hyperventilating could experience muscle spasms, tremors and sweating, often creating the impression that they are about to have a heart attack.