Heart attack

A myocardial infarction, more commonly known as a heart attack is a medical emergency often resulting in fatality or disability.

Medical Information
Heart attacks occur when blood is not flowing properly to the heart, resulting in injury to the heart due to oxygen deprivation, generally due to blockage in one of the coronary arteries.

Being at risk of having a heart attack can be increased for variety of reasons, including but not limited to, chronic high stress levels (this is seen once in the series, having occurred to Soichiro Yagami, though it was not fatal and didn't leave any permanent damage; aside from use of a Death Note, this is the only cause of a heart attack exhibited in the series), heart diseases, old age, smoking, high cholesterol or other high lipids in the blood, lack of physical activity, high blood pressure, obesity, chronic kidney disease, excessive alcohol consumption, and cocaine and amphetamines use. Symptoms include abnormal heart beat, shortness of breath, sweating, nausea, vomiting, though symptoms seem to be more common in men than in women. Another common symptom includes sudden pain, usually felt behind the breast pone, and sometimes traveling to the left arm and/or the left side of the neck. Sudden pain, especially chest pain, is exhibited very commonly in the Death Note series, as can be seen when individuals who are suffering heart attacks often clutch at their chest or neck.

Types

 * Acute myocardial infarction: An acute myocardial infarction is a sudden and serious heart attack, as opposed to one that gradually develops over time
 * "Silent" myocardial infarction: a fairly common type of heart attack, one which occurs with without any warning symptoms. It occurs in almost 64% of all heart attack cases

Death Notes
When a name is written in a Death Note but the cause of death is left unspecified, the default death is a heart attack. Acute myocardial infarction is the default type of heart attack, as the heart attack which occurs after writing a name in the Death Note is to occur forty seconds after the name is written down, meaning that the heart attack would no doubt have to be sudden.

Plot
When Light Yagami gains ownership of the Death Note, he writes down the names of criminals. The causes of death are left unspecified, meaning that all the criminals die of heart attacks. This is intentional on Light's part, as he wants to show that there was some sort of higher power behind all of it, which will be obvious if only criminals are dying of only heart attacks. However, Light uses various other methods to kill people with the Death Note throughout the series, though this is generally in order to be discreet, as is the case with Naomi Misora and Kiichiro Osoreda. There are a few other exceptions, such as with Takuo Shibuimaru, who is a test subject of sorts, various criminals in order to test out the capabilities of the Death Note, and Wedy, for some unknown reason, who dies in a motor vehicle accident.

Light's plan of letting people know that there was a higher authority committing these murders ultimately succeeds, and heart attacks became known as the signature of Kira, a figure who punishes criminals with god-like powers.

Other Kira use only heart attacks as well, such as Kyosuke Higuchi, who presumably wants to be thought of as Kira despite his killings being targeted at competing business executives rather than criminals and C-Kira, who also wants to be thought of as the original Kira. Kira who work with Light Yagami, such as Misa Amane, Teru Mikami, and Kiyomi Takada also do this in order to make it look like Kira is only one being and so that the deaths of criminals will still be attributed to him.

Heart attacks as a default death are also used when the person writing the name has a limited amount of time to write the name. This is seen to be the case with the Shinigami Gelus, who had to act quickly, before Misa Amane's stalker killed her. Rem is another example, since the Japanese Task Force knew that she was up to something and was desperately searching for her before she could kill anyone else. Mafia member Jack Neylon also does this, presumably due to convenience and, later on, limited time when the mafia's base is being invaded.