How does cold weather affect blood flow, heart beat, and heart attack?

Question

When I said cold weather makes the blood not flow as well in the body, I would get the refutation that human body has a constant temperature in warm or cold weather, possibly at 36.7°C (or 37°C), because humans are warm blooded animals.

But in the winter, more people have heart attack and studies have showed that the day after the first snowfall of the year, there are much more heart attacks happening.

(Updated: I can't find the article any more. It said snow shoveling can cause heart attack, but in places where people don't need to shovel snow, the heart attack rate is also higher when it becomes cold.)

How can this be explained with the refutation that "human body has a constant temperature in warm or cold weather" and therefore cold weather should not be the issue?

If we do not consider the issue related to holiday stress or holiday social activities, it seems like some articles indicated it is due to blood vessel constricting in cold weather, causing blood more difficult to reach vital organs in the body. There is also one article that is medically reviewed. Is this the primary cause?

Answer 1

Vascular resistance in the skin is very closely related to temperature of both outside and body core. Vascular resistance in other body organs is not related to outside temp and the body core temp (37C IS correct) does not change much in absence of disease. Studies mentioned above do not imply a cause, only a statistical relationship. Lots of things change when weather gets cold. People cut firewood. Shovel snow. Stay inside and get respiratory diseases. Drink more alcohol. So since these correlational studies don't explain or connect to a cause, you can make up whatever link you want. Maintaining body temperature being hard on the heart doesn't make sense, because core temp is not regulated by the heart, and generating heat is not particularly burdensome to the heart.

Answer 2

The common consensus seems to be that in cold weather the heart must work harder to maintain the body temperature, and that the arteries contract to save energy, which leads to an increased rate of heart attacks.

'In discussing the weather-heart attack connection, study leader David Erlinge, a professor of cardiology, said cold and windy weather “leads to a contraction of blood vessels in the skin to conserve energy,” which increases the workload of the heart.'

From:

https://www.drweil.com/health-wellness/body-mind-spirit/heart/does-cold-weather-cause-heart-attacks/

A comprehensive study has been conducted on the link between the weather and heart attacks:

'We’ve long known that heart attacks are more likely to occur during cold weather. The latest news about this comes from a large study in Sweden that examined every one of the 274,029 heart attacks that occurred in that country between 1998 and 2013 and correlated each one with weather data. This was the most comprehensive study ever performed on weather and heart attacks. The researchers, from Lund University, reported that low temperatures were most strongly associated with the incidence of heart attacks, although they also looked at the effects of other weather factors including the atmospheric pressure, wind velocity, and duration of sunshine on each day of the 16-year study. They reported that heart attack rates were highest in the north of Sweden, which is colder, snowier and windier than other areas. The team also looked at air pollution as a possible risk factor, collecting data on it in Sweden’s three major cities for every day of the study’s duration. They found no evidence that pollution influenced the weather-related findings.'

and the paper referenced:

David Erlinge et al, “Association of Weather with Day-to-Day Incidence of Myocardial Infarction A SWEDEHEART Nationwide Observational Study.” JAMA Cardiology, October 24, 2018, doi:10.1001/jamacardio.2018.3466

A synopsis of the above paper is here:

https://www.hcpfeed.com/2018/10/31/association-of-weather-with-day-to-day-incidence-of-myocardial-infarction/

In 274 029 patients, mean (SD) age was 71.7 (12) years. Incidence of MI increased with lower air temperature, lower atmospheric air pressure, higher wind velocity, and shorter sunshine duration. The most pronounced association was observed for air temperature, where a 1-SD increase in air temperature (7.4°C) was associated with a 2.8% reduction in risk of MI (unadjusted incidence ratio, 0.972; 95% CI, 0.967-0.977; P <.001). Results were consistent for non–ST-elevation MI as well as ST-elevation MI and across a large range of subgroups and health care regions.

Answer 3

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 position.

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).