The chemistry of autopsy

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When a person dies, their body cycles through four stages of death. First comes pallor mortis. Within 15-30 minutes of death, capillary circulation collapses and the skin goes pale (an effect most prominent in those with light skin). Pallor mortis is quickly followed by algor mortis during which the body cools to match the temperature of the surroundings and as the body cools, the muscles stiffen and the body grows rigid, a phenomenon known as rigor mortis. Six to eight hours later, the final stage of death takes hold of the body- livor mortis. A stage characterised by the pooling of the blood to certain areas of the body due to the force of gravity. These four stages are often quite helpful in determining the time of death, but to gain more insight into how someone has died, autopsies or post-mortem exams are performed. An autopsy involves the examination of the external body, the internal organs and the substances that lie within them.

Let's take a look at this case example:

“A 55 year old male is found deceased in a bed in a secure residence . There is an antidepressant medication on scene next to the bed including the tricyclic antidepressant imipramine. In addition, there is a half full bottle of wine on the floor. At autopsy, the medical examiner can find no immediate anatomical cause of death. The medical examiner submits venous blood, heart blood, vitreous humor and liver to the forensic toxicologist for analysis.”

There are several pieces of important information in this extract, the most prominent of which being the presence of medication and alcohol. This is particularly important as no immediate anatomical cause of death was found by the medical examiner, meaning the cause of death is most likely accessible on a molecular level. The individual’s age and sex will come in useful when interpreting the toxicology results later and as for the submitted specimens, they would be ideally stored between 2-8 degrees celsius to slow distribution and putrefaction (decomposition).

Unfortunately, determining whether the imipramine and/or alcohol was responsible for the death is not as simple as measuring the concentrations of the relevant compounds in the specimens, however several other factors must be taken into account. Most prominently, the phenomenon of post-mortem redistribution (PMR).

Postmortem redistribution involves the changes in drug concentrations after death. Simply put, a drug’s movement around the body from blood to tissue and tissue to blood after death.

The extent to which a drug will display PMR is down to its chemistry. Various drug properties such as volume of distribution, lipophilicity and acid-base

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dissociation are all factors that affect a drug’s post mortem redistribution.

Volume of distribution relates the total amount of drug to the concentration of the drug in the plasma at any given time. Simply put, it’s a proportionality constant represented by the equation below:

Volume of Distribution (L) = Amount of drug in the body (mg) / Plasma concentration of drug (mg/L)

From this, we can tell that a drug with a high volume of distribution tends to leave the plasma and enter the tissues and those with a low Vd tend to stay in the plasma and thus a drug’s Vd is highly relevant to its overall postmortem redistribution.

PMR is also affected by the drug’s lipophilicity, which is defined as the affinity of a drug for a lipid environment. The greater a drug’s lipophilicity, the faster its redistribution.

The final property that dictates a drug’s likelihood to display PMR is its acid-base dissociation constant (Ka). In fact, toxicologists are more often concerned with a drug’s pKa, which is the negative log of Ka useful in giving smaller values, which are often more comparable for analysis. Most drugs are weak acids or bases and the smaller the pKa the stronger the basic properties of the drug and conversely the larger the pKa, the stronger the acidic properties of the drug. The pKa of a drug influences its lipophilicity, solubility, ability to bind to proteins and permeability which together impacts its tendency to distribute. All in all, basic, highly lipophilic drugs with a high volume of distribution are most likely to undergo PMR.

As you may recall, the antidepressant imipramine was found on the scene. This drug belongs to a family of tricyclic compounds, named as such due to their characteristic three ring structure. More specifically, imipramine is a dibenzazepine, which refers to its two benzene rings attached to a single azepine group. As for the wine also found in the room, the primary compound in question is ethanol. Ethanol is not as straightforward a compound as its presence in the body is not all down to the consumption of alcohol. After death, bacteria can metabolise the contents of the body to ethanol altering its concentration meaning determining whether there was excess amounts of alcohol in the body can be quite difficult.

Now let’s say we had two sets of data following the examination of the specimens provided from the initial autopsy.

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The next step is interpreting these results through comparison with reference data. Below are the reference values for the levels of both compounds in a live individual, postmortem levels in a natural death and postmortem levels in death by intoxication fatality. In both scenarios, the imipramine has undergone PMR post mortem. We know this by referencing its central to peripheral blood ratio both times. The reference level i.e. the level if it had not redistributed is 1.8. The ratio in scenario one is 3.5 and scenario two is 2.5. Seeing as both of these are higher than the reference level, we can conclude that the drug has undergone PMR- we need to consider this when deciding on the cause of death. With all of this in mind, we can conclude that with the data from scenario one the death was due to an intoxication and with the data from scenario two, the cause of death was likely natural. An interesting conclusion we can draw with scenario two is that we don’t actually know whether or not the wine had been ingested. This is

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because the ethanol present could have been the product of bacteria metabolism.

Overall, chemistry is hugely significant in post-mortem examinations, particularly in forensic toxicology. Without considering the chemical properties of the compounds within the body, any conclusions we come to would be misinformed.

By Rojin Zahaki

References and further reading: https://www.sciencedirect.com/top ics/ pharmacology-toxicology-and-p harmaceutical-science/volume-of-di stribution#:~:text=Volume%20of % 20distribution%20(Vd)%20is,and% 20unbound%20in%20tissue%20wa ter).

https://www.ncbi.nlm.nih.gov/book s/ NBK545280/

https://emerypharma.com/blog/dr uglipophilicity-and-absorption-a-co ntinuous-challenge-toward-the-goal -of-drug-discovery/#:~:text=Lipoph ilicity%20is%20defined%20as%20t he, %2C%20pharmacokinetic%2C%

20and%20metabolic%20properties. https://pubmed.ncbi.nlm.nih.gov/1 6035199/

https://journals.sagepub.com/doi/f ull/ 10.1177/1177391X0700100003

https://chem.libretexts.org/Booksh elves/General_Chemistry/Map%3A_ A_Molecular_Approach_(Tro)/16%3 A_Acids_and_Bases/16.04%3A_Acid _Strength_and_the_Acid_Dissociatio n_Constant_(Ka)#:~:text=For%20an% 20aqueous%20solution%20of,aK b%3DKw.&text=At%2025%C2%B0 C%2C%20p,%2BpKb%3D14.00.

acs.org/content/acs/en/acs-webina rs/ popular-chemistry/death-chemis try.html

https://www.scienceabc.com/huma ns/ post-mortemstages-of-death-diff erentstages-the-body-goes-through -afterdeath.html

https://www.pathologyoutlines.com / topic/forensicschemistry.html

https://link.springer.com/chapter/ 10.1007%2F978-1-59745-127-7_8

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