Time of Death:
Determination of the time of death, or the interval between
the time of death and when a body is found (i.e., postmortem
interval), can be difficult. A forensic pathologist attempts to
determine the time of death as accurately as possible realizing,
however, that such a determination is only a best estimate.
Unless a death is witnessed, or a watch breaks during a
traumatic incident, the exact time of death cannot be determined.
The longer the time since death, the greater the chance for error
in determining the postmortem interval. There are numerous
individual observations which, when used together, provide the
best estimate of the time of death. These include: rigor mortis,
livor mortis, body temperature, decompositional changes, and
stomach contents. A thorough scene investigation must also be
performed and environmental conditions should be documented.
The environment is the single most important factor in
determining the postmortem interval.
Rigor Mortis:
After death, the muscles of the body initially become flaccid.
Within one to three hours they become increasingly rigid and the
joints freeze by a process named rigor mortis (or postmortem
rigidity or rigor). Although the exact chemical reaction causing
muscles to stiffen has not been determined, rigor mortis is
similar to physiologic muscle contraction and involves calcium,
ATP, and ADP. The difference, however, is the formation of
locking chemical bridges between the muscle proteins actin and
myosin, and lack of muscle shortening in rigor mortis. In
physiologic muscle contraction, actin molecules slide over
myosin and the muscle shortens.
Rigor mortis is affected by body temperature and metabolic
rate: the higher the body temperature, more lactic acid is
produced, and rigor occurs sooner. For example, a person
dying with pneumonia and a fever will develop rigor sooner
than a person with normal body temperature. Similarly, if a
person's muscles were involved in strenuous physical activity
just before death, rigor develops much more quickly. The
process is also retarded in cooler environmental temperatures
and accelerated in warmer ones.
All muscles of the body begin to stiffen at the same time
after death. Muscle groups appear to stiffen at different rates
because of their different sizes. For example, stiffness is
apparent sooner in the jaw than in the knees. Thus, an examiner
must check to see if joints are moveable in the jaws, arms, and
legs.
A body is said to be in complete rigor when the jaw, elbow,
and knee joints are immovable. This takes approximately 10-12
hours at an environmental temperature in the range of 70 - 75 'F.
A body remains rigid for 24-36 hours before the muscles begin
to loosen, apparently in the same order they stiffened.
A body remains rigid until rigor passes or a joint is
physically moved and rigor is broken. Consequently, in addition
to an estimate of the time of death, body position in full rigor
can indicate whether or not a body has been moved after death.
Livor Mortis:
Livor mortis is the discoloration of the body after death by
the settling of blood no longer being pumped through the body
by the heart. Blood settles in vessels by gravity in dependent
areas of the body and colors the skin purple red. Some
dependent areas may not discolor because skin compressess
against a bony surface and prevents blood from settling in
capillaries. For example, if blood settles to the back, pale areas
occur over the scapula and buttocks. Livor mortis is noticeable
approximately one hour after death. The color increases in
intensity, becomes "fixed" in about 8 hours, does not blanche
under pressure, and remains in those areas even if a body is
repositioned. There may be a slight discoloration in new
dependent areas after repositioning even though the blood
remains fixed in the original position. Fixed blood seen in a
nondependent location indicates that a body has been moved
after death. Livor mortis is visible until the body becomes
completely discolored by decomposition.
Variations in color during livor mortis depend on the cause
of death. Carbon monoxide or cyanide poisoning, hypothermia,
and refrigeration cause bright, cherry red livor mortis. People
who die from extensive blood loss have very light
or nonexistent livor mortis due to the small amount of blood in
their system. Livor mortis is more difficult to determine in
dark-skinned individuals.
In some cases, capillaries in dependent areas rupture causing
small hemorrhages or bleeds. These hemorrhages, called
"Tardieu" spots, are particularly common in distal extremities of
hanging victims. If they occur on the head they should not be
mistaken for smaller pinpoint hemorrhages called petechiae.
Petechiae occur when death is due to any sudden increase in
blood pressure, such as a sudden stoppage of the heart in natural
death, or in chest or neck compression in accidents or
strangulation. They are commonly seen on the head, in the eyes,
and internal organs.
Body Cooling (Algor mortis)
After death, the body cools from its normal internal
temperature to the surrounding environmental temperature.
Many studies have examined this decrease in body temperature
to determine formulae which could predict its consistency.
Unfortunately, because of numerous variables, body cooling is
an inaccurate method of predicting postmortem interval. In
general, however, evaluating a decrease in body temperature is
most helpful within the first ten hours after death. During this
time, with a normal body temperature and at an ideal
environmental temperature of 70 - 75°F, the body cools at
approximately 1.5°F per hour.
The problem with using the 1.5°F per hour calculation is the
assumption that the internal and environmental temperatures are
98.6° and 70 - 75°, respectively. If a decedent's body
temperature were higher than normal because of infection or
physical exercise, 98.6° could not be used. Furthermore, the
outside environment is rarely in the 70 - 75° range. For example,
a body may actually gain heat if an individual expires outdoors
in July, when temperatures may be greater than 100°.
Conversely, if a person expires in a 25° environment, rapid
cooling will take place.
Nonetheless, if body temperature is measured at a scene it
should be taken on at least two separate occasions before a body
is moved. A rectal or liver temperature is the most accurate
measurement. The environmental temperature should also be
recorded. If these relatively simple procedures are followed, a
very crude estimate of the postmortem interval can be made.