Following loss of heart function, blood circulation in the body ceases. In addition, relaxation of muscle tone causes the vascular bed to allow movement of fluid within blood vessels along the line of gravity. These fluids move along blood vessels and settle down at dependent parts of the body, which can be visually manifested as livor mortis, post mortem hypostasis or lividity.
In addition, currents occur between warmer and colder areas of the body resulting in redistribution of drugs and chemicals after death.
Hypostasis is not always seen in a body and it may be absent in the young, the old and the clinically anemic or in those who have died from severe hemorrhages. It may also be masked in individuals with dark skin colors, jaundice or some dermatological conditions.
Compression of skin in contact with a firm surface can prevent collection of blood in these dependent parts resulting in areas of pallor within the areas of lividity. This can also be caused by pressure of clothes or any objects in contact with the dead body.
Hypostasis can help the doctor estimate the position(s) of the body at the time of death. The distribution of lividity must be examined meticulously to understand the position at death, as it may sometimes mislead, especially if the position of the body has been changed after death. A good example is of a dead body being flown in running river.
A body suspended during hanging will develop hypostasis over distal parts of upper and lower extremities, whereas a body facing downwards shall present with congested face and conjunctiva which can sometimes be misinterpreted as signs of asphyxial deaths. Similarly, if a body found facing down has fixed hypostasis on the back, it would mean that the body had been lying facing upwards for several hours after death before it was moved to the position it was found in.
In cases where a person is dead for a long period of time (usually more than 6-12 hours), red blood cells collected along inside the capillaries may undergo lysis, releasing hemoglobin. This would result in staining of the surrounding tissue and hypostasis present would not blanch on applying pressure over the overlying body surface.
However, if the body is moved before the hypostatis has become fixed, hypostasis will slowly disappear and will begin to reform at the new dependent location. The appearance of hypostasis is so variable that it has no reliable role in determining the time of death.
Whenever confusion arises in ascertaining whether the discolored area is contusion or hypostasis, one must give a small incision on the surface and pour running water over the cut area. In case the lesion is contusion, the discoloration would not wash off unlike in hypostasis.
The color of hypostasis and its extent also bears forensic significance. Cherry red hypostasis is an indication of carbon monoxide poisoning, brick red of cyanide poisoning and septic deaths due to Clostridium Perfringes results in bronze hypostasis. Similarly, Pinkish hypostasis is seen in bodies preserved in cold temperatures and in individuals dying from hypothermia. However, no decision on cause of death should be made solely based on the color of hypostasis.