Rossi, Riccardo MD; De-Giorgio, Fabio MD, PhD; Grassi, Vincenzo M. MD; Pascali, Vincenzo L. MD, PhD; Lancia, Massimo MD
Asphyxial deaths can be described using descriptive terms1 or may be categorized according to different classification schemes.2 A recent publication3 emphasized the importance of a unified system of classification and proposed to classify asphyxia into 4 categories: suffocation, strangulation, mechanical asphyxia, and drowning. Deaths from exposure to gas are generally accepted as a subcategory of suffocation.
In general, asphyxia refers to the inability to receive or use oxygen at cellular level. Substances such as carbon monoxide, hydrogen cyanide, and hydrogen sulfide act as toxic gases, blocking the use of oxygen at cellular level, whereas methane (CH4), carbon dioxide, and nitrogen physically displace oxygen in inspired air, thus preventing the cells from receiving oxygen.
Hypoxia can be classified in 4 categories: 1, hypoxemic hypoxia (low PaO2); 2, anemic hypoxia (low O2 content, hemoglobin disorders); 3, ischemic hypoxia (failure to deliver O2 to tissues); and 4, histotoxic hypoxia (cells unable to use O2). Hence, carbon monoxide, hydrogen cyanide, and hydrogen sulfide produce histotoxic hypoxia, whereas methane, carbon dioxide, and nitrogen cause hypoxemic hypoxia.
A review of the English-language medical literature revealed only accidental deaths after methane gas exposure.
We describe a fatal case due to asphyxia caused by methane gas exposure in a 35-year-old man found dead at home and discuss possible mechanisms of the event.
A 35-year-old single man with unremarkable medical history was found dead at home. He was lying on his back on the sofa next to the kitchen, with no external signs of trauma. In his mouth, sealed with packing tape, there was a hose connected to the gas wall outlet (Fig. 1).
Written suicide notes were found on a table.
Postmortem hypostasis was present on the back of the body. Postmortem rigidity was incomplete. Environmental temperature was 34.5°C, and rectal temperature was 25°C.
A forensic autopsy was performed 48 hours after death. No trauma or signs of violence were observed. The internal organs were moderately congested. Lungs and brain were also edematous. Pleural and epicardial petechiae were present.
MATERIALS AND METHODS
Qualitative and quantitative analysis was performed on 1 mL of blood and on 1 g of brain, heart, kidneys, liver, and lungs to determine the presence and concentration of methane and other toxic gases. All the samples were kept separately in hermetically sealed vials with a Teflon lid and metal ring and were then incubated in a water bath at 60°C for approximately 30 minutes.
In addition, a standard solution containing a known methane concentration was prepared, using a MicroMAT™-10 single component gas standard provided by Alltech Associates Inc. USA at the concentration of 1000 ppm of methane in helium atmosphere.
Finally, a gas chromatographic technique (head space) analysis was carried out using an automatic sampling system connected to a 6850 series GC System gas chromatograph by Agilent Technologies Inc. USA and Hewlett-Packard Inc. USA, equipped with a capillary column from Alltech, Associates Inc. USA type AT-624 30 m × 0.53 mm × 3.0 μm Heliflex Capillary. The column was used in the following operating conditions: column temperature, 35°C for 5 minutes, increasing with a rate of 5°C per minute until 140°C were reached; injector temperature, 150°C; detector temperature, 150°C; carrier gas, helium 5 mL/min; detector, flame ionization detector.
RESULTS AND DISCUSSION
Toxicologic analysis of the samples showed the presence of methane at the following concentration: peripheral blood, 16 μL/g; brain, 14 μL/g; heart, 17 μL/g; kidneys, 19 μL/g; liver, 14 μL/g; and lungs, 56 μL/g. Our original results in parts per million have been converted in microliters per gram to make easier the comparison between the data herein reported with those available in the literature. Neither other gases nor therapeutic or illegal drugs were detected.
Autopsy findings revealed moderate congestion of the internal organs, edema of lungs and brain, and pleural and epicardial petechiae.
Methane is a nontoxic, odorless natural gas used for cooking purposes and domestic heating. Odorants such as butanethiol or tetrahydrothiophene are added to domestic natural gas to assist in leak detection.
Methane is potentially lethal because it can cause explosions4 and it displaces oxygen in the air.5 The atmosphere is composed of nitrogen (78%), oxygen (21%), and other gases. If an inert gas displaces oxygen in the air and reduces it to less than 25% of normal values (5%–6% by volume of oxygen),2 this will cause unconsciousness in a few seconds and death in a few minutes.
Methane is nontoxic, and thus, although postmortem identification of methane in blood demonstrates exposure to the gas, it does not, on its own, confirm the mechanism of death. However, if methane is detected in cardiac blood at concentrations higher than 6.0 μL/g, it is possible to diagnose asphyxia due to oxygen displacement as the cause of death as even without an autopsy.5 This can be considered a small step toward the routine use of “toxopsy” together with classic postmortem investigations.6
The diagnosis of the cause of death, in case of oxygen consumption, is based on the analysis of the death scene and on the exclusion of other possible causes. In fact, typical signs that can be revealed during autopsy, such as petechial hemorrhages,7 facial congestion, edema, and cyanosis, may be seen in many asphyxial deaths.1 In our case, autopsy findings are substantially the same as those mentioned previously.
During inspiration, oxygen diffuses from the alveoli to the bloodstream through the alveolar-capillary membrane. Methane concentration in ambient air is approximately 0.000179%. In our case, the cause of death was ascribed to asphyxia resulting from oxygen displacement. In fact, the victim inspired air containing approximately 95% of methane.2 As a consequence, O2 rapidly decreased causing acute hypoxemic hypoxia and finally death. This physiopathologic mechanism has been previously described both in humans and in animals, and any kind of inert gas that is able to displace oxygen can be the cause of this process.8 Inspiring air containing low oxygen concentration induces a hypoxic state, brain damage, and loss of consciousness. Hence, victims are unable to move away from the gas-filled location and, if they are not rescued or adequate oxygen levels are not restored, respiration arrest occurs, followed by cardiac arrest.
Most deaths from inhalation are accidental, and they usually occur in places such as mines,5 sewers,9 drainage pits,10 or manure pits,11,12 where the gas is naturally present or is produced by putrefying bacteria. Furthermore, these accidents often occur in the workplace, and it is not uncommon for the rescuers, who are unaware of the cause of the accident, to become unconscious and possibly die.
Suicides by carbon dioxide13 and propane14,15 asphyxiation have been reported in the past, but up to now, the review of the literature did not reveal a previous report of a suicide by methane asphyxiation. In the article analyzing the causes of death among Major League Baseball players,16 4 cases of death after methane gas exposure are reported, but the circumstances were not specified (unintentional, suicidal, or homicidal).
In the study conducted by Watanabe and Morita,17 rats were divided into 3 experimental groups: rapid asphyxia, prolonged asphyxia, and asphyxia by the inhalation of gases saturated with a critical gas concentration (maintaining oxygen at 20%). Among these groups, the distribution pattern of methane in the analyzed tissues was not different (except for the lungs) because of the low solubility of methane in water. Thus, in just a few minutes (rapid asphyxia group), the concentration of methane in the tissues reaches equilibrium with that in the inspired air, and this equilibrium remains stable even for prolonged exposure to methane. Moreover, the gas is detectable even after a relative long postmortem interval. According to the criteria of Watanabe and Morita,17 because the man had inspired methane directly from the gas outlet, the case we reported could be classified in the rapid asphyxia group.
Terazawa et al5 suggested that methane easily accumulates in the brain through blood circulation because it is an aliphatic gas. To corroborate this statement, they reported 2 cases with different methane concentrations in the lungs (163.6 μL/g and 231.3 μL/g) and similar concentrations in the brain (23.5 μL/g and 19.3 μL/g). Moreover, the gas spreads in the tissues according to its partial pressure, so that highly vascularized organs reached higher methane concentration and in shorter exposure times. In our case, methane concentration in the lungs and in brain tissue was 56 μL/g and 14 μL/g, respectively. Our data appear consistent with this hypothesis of gas accumulation in the brain.
Cases of asphyxial death due to oxygen displacement in which ambient air investigation and determination of gas concentrations were essential to identify the gas involved have been previously reported.9,10,18 Instead, in our case, the unusual way in which the man was found emphasizes the importance of the scene investigation and is a clear example of why it is not always necessary to determine ambient gas concentrations.
On the basis of the evidence at the scene, autopsy findings, and toxicologic results, the death was classified as a suicide, and the cause of death was attributed to asphyxia by methane inhalation.
The authors thank Dr Sergio Scalise Pantuso for his technical support.
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