Dependent lividity, also known as livor mortis, is one of the earliest visible changes that occurs after death. This post-mortem sign is the discoloration of the skin appearing on the lowest, or dependent, parts of the body. It is caused entirely by the force of gravity acting on the blood components inside the vessels. The presence, distribution, and characteristics of this discoloration provide significant data for death investigators.
The Mechanism of Blood Pooling
The formation of dependent lividity begins immediately upon the cessation of the heart’s pumping action and the failure of the circulatory system. With the loss of blood pressure, the blood inside the veins and capillaries loses the force that previously kept it in constant motion. The heavy components of the blood, primarily the red blood cells, begin to sink.
Gravity pulls this static blood downward, causing it to settle in the small vessels and capillaries of the body parts closest to the ground. This pooling, or hypostasis, results in the characteristic reddish-purple discoloration visible on the skin’s surface. Areas of the body pressed against a hard surface, such as the floor or a mattress, remain pale. This occurs because the pressure squeezes the capillaries shut, preventing the blood from settling in those specific locations.
Observable Stages and Timelines
Dependent lividity progresses through distinct phases that allow forensic experts to estimate the time since death. The first stage, known as non-fixed or blanching lividity, begins to appear within 20 minutes to 3 hours after death. During this early phase, the discoloration will temporarily disappear, or “blanch,” if pressure is applied to the skin. This temporary fading happens because the pooled blood is still liquid and can be pushed out of the vessels under external force.
As time passes, the blood components begin to coagulate and break down within the vessels, leading to the second stage: fixed or non-blanching lividity. This fixation process begins around 6 to 8 hours after death and is complete within 8 to 12 hours. Once fixed, the discoloration will no longer fade when pressed because the blood cells have leaked into the surrounding tissues, making the staining permanent.
The color of the lividity can also offer clues about the circumstances of death. The typical purplish-red hue is caused by deoxygenated hemoglobin accumulating in the dependent vessels. A bright cherry-red color suggests a high concentration of oxygenated hemoglobin, which is often a sign of carbon monoxide or cyanide poisoning. This variation in color provides an important detail that can assist in determining the cause of death.
Forensic Importance of Lividity
The two primary forensic applications of dependent lividity are determining the body’s position after death and estimating the post-mortem interval. The distribution of fixed lividity confirms the position the body was in when the blood settled and became locked in place. For instance, if a body is discovered lying face up, but the lividity is fixed across the chest and abdomen, investigators can conclude the body was originally face down for the fixation period before being moved.
If lividity is present in two separate areas, this dual pattern indicates the body was moved before the lividity became fully fixed, allowing the blood to pool in a new dependent area. This observation can be used to challenge or corroborate witness statements regarding the events surrounding the death.
The stage of lividity—whether non-fixed or fixed—provides a general time frame for the post-mortem interval. Blanching lividity suggests death occurred recently. Non-blanching lividity narrows the time of death to a window of approximately 8 to 12 hours after the event. While dependent lividity is not a precise clock, its predictable progression is a valuable tool for establishing a scientifically grounded timeline.