This is how exposure to cold can result in heart attack:
- Cold triggers the release of norepinephrine (noradrenaline), which results in constriction of blood vessels in the skin and thus in an increase of blood pressure.
- The heart needs to work harder to pump blood against the increased blood pressure, so it uses more oxygen.
- In individuals with coronary atherosclerosis, the limited blood flow may not be able to deliver enough oxygen to meet increased heart oxygen demand, which results in pain (angina pectoris) or in ischemic damage of the heart muscle (myocardial infarction).
The study below has shown that the described mechanism is harmful only for "cold-intolerant" patients with prolonged high blood pressure response, while in "cold-tolerant" patients, the heart rate can quickly decrease and thus normalize the blood pressure.
Mechanisms of cold intolerances in patients with angina (Journal of the American College of Cardiology, 1994):
Seven cold-intolerant and seven cold-tolerant patients with angina
underwent exercise treadmill testing at 6 and 25 °C with measurement
of catecholamines. Baroreceptor function was assessed by the decrease
in systolic blood pressure after patients stood up from the supine
Norepinephrine levels increased by 139% in the cold environment, but there were no differences between cold-intolerant and
cold-tolerant patients. Consequently, blood pressure was higher in
the cold environment in all patients, but the heart rate was
similar. However, cold-intolerant patients had a steeper heart rate
response in the cold and developed ischemia.
Exposure to cold causes an increase in blood pressure with an
associated increase in myocardial oxygen demand in all patients. In
cold-tolerant patients, this increase may be offset by a reduction in
heart rate if baroreceptor function is normal. If baroceptor function
is abnormal, heart rate may not decrease in response to a cold-induced
increased in pressure.
In conclusion, exposure to cold can cause heart attack because of increased blood pressure, not thermogenesis. It's unlikely that cold exposure, as expected in everyday environment, would cause heart attack in otherwise healthy individuals, and even in those with coronary atherosclerosis it would more likely cause pain (angina pectoris) rather than the actual heart attack (myocardial infarction).