Thursday, January 25, 2018

NEJM: Acute Myocardial Infarction After Laboratory-Confirmed Influenza Infection


According to The American Heart Association (data for 2009) every year an estimated 785,000 Americans experience their first heart attack, and another 470,000 suffer a recurrent heart attack. They also estimate another 195,000 `silent’ myocardial infarctions occur each year.
Making for just under 1.5 million coronary attacks a year (cite Lloyd-Jones D, Adams RJ, Brown TM, et al. Heart Disease and Stroke Statistics—2010 Update. Circulation. 2010;121:e1-e170) - or about 4000 heart attacks each day.
Ask any paramedic, ER nurse, or doctor and they will tell you that heart attacks are more common during the winter than in the summer.  In fact, a 1998 study looked at the rate of heart attacks in the United States, and found that Acute Myocardial Infarctions (AMIs) run as much 53% higher during the winter months than than during the summer.
While cold weather combined with strenuous physical activity (like clearing snow from sidewalks) have often been blamed for this spike, even in balmy Southern California, studies have shown a 33% increase in heart attacks over the holidays.
In his 2004 article in Circulation  The ''Merry Christmas Coronary'' and ''Happy New Year Heart Attack'' Phenomenon,  Dr Robert Kloner posited the following reasons for this uptick in excess winter cardiac mortality.

While all are likely contributors, in recent years item #5 on his list  - influenza and other respiratory problems - have been increasingly linked to this seasonal increase in heart attacks.
A little over two years ago, in UNSW: Flu Vaccine Provides Significant Protection Against Heart Attacks, we saw another study that found that if you are over 50 - getting the flu vaccine can cut your risk of a heart attack by up to 45%.

And in May 2017, in Int. Med. J.: Triggering Of Acute M.I. By Respiratory Infection we looked at research  from the University of Sydney that found the risk of a heart attack is increased 17-fold in the week following a respiratory infection such as influenza or pneumonia.  

Overnight the NEJM published a report that also finds a `significant association' between recent (lab confirmed) influenza infection and Myocardial Infarction.  In fact, the odds of having a heart attack in the 7 days following influenza diagnosis went up 6-fold among the subjects in this study.

Acute Myocardial Infarction after Laboratory-Confirmed Influenza Infection

Jeffrey C. Kwong, M.D.,Kevin L. Schwartz, M.D.,Michael A. Campitelli, M.P.H.,Hannah Chung, M.P.H.,Natasha S. Crowcroft, M.D.,Timothy Karnauchow, Ph.D.,Kevin Katz, M.D.,Dennis T. Ko, M.D.,Allison J. McGeer, M.D.,Dayre McNally, M.D., Ph.D., David C. Richardson, M.D.,Laura C. Rosella, Ph.D., M.H.Sc.,Andrew Simor, M.D.,Marek Smieja, M.D., Ph.D.,George Zahariadis, M.D.,and Jonathan B. Gubbay, M.B., B.S., M.Med.Sc.


Acute myocardial infarction can be triggered by acute respiratory infections. Previous studies have suggested an association between influenza and acute myocardial infarction, but those studies used nonspecific measures of influenza infection or study designs that were susceptible to bias. We evaluated the association between laboratory-confirmed influenza infection and acute myocardial infarction.


We used the self-controlled case-series design to evaluate the association between laboratory-confirmed influenza infection and hospitalization for acute myocardial infarction. We used various high-specificity laboratory methods to confirm influenza infection in respiratory specimens, and we ascertained hospitalization for acute myocardial infarction from administrative data. We defined the “risk interval” as the first 7 days after respiratory specimen collection and the “control interval” as 1 year before and 1 year after the risk interval.


We identified 364 hospitalizations for acute myocardial infarction that occurred within 1 year before and 1 year after a positive test result for influenza. Of these, 20 (20.0 admissions per week) occurred during the risk interval and 344 (3.3 admissions per week) occurred during the control interval.
The incidence ratio of an admission for acute myocardial infarction during the risk interval as compared with the control interval was 6.05 (95% confidence interval [CI], 3.86 to 9.50). No increased incidence was observed after day 7.
Incidence ratios for acute myocardial infarction within 7 days after detection of influenza B, influenza A, respiratory syncytial virus, and other viruses were 10.11 (95% CI, 4.37 to 23.38), 5.17 (95% CI, 3.02 to 8.84), 3.51 (95% CI, 1.11 to 11.12), and 2.77 (95% CI, 1.23 to 6.24), respectively.

We found a significant association between respiratory infections, especially influenza, and acute myocardial infarction. (Funded by the Canadian Institutes of Health Research and others.)

Interestingly, Influenza B (rather than A) produced the highest coronary risk, although given the size of the research cohort (n=364), this may not hold true across the board. As with all studies, this one has certain limitations, and so its conclusions must be evaluated along side other studies of its kind.
While their methodologies and risk assessments may vary, the trend is pretty obvious. Over the past five years researchers have repeatedly found compelling links between recent severe respiratory infections and coronary events.
For a bit more on this study, the Institute for Clinical Evaluative Sciences (ICES) has the following press release.

Researchers confirm link between flu and heart attack 
January 24, 2018 Toronto

Chances of a heart attack are increased six-fold during the first seven days after detection of laboratory-confirmed influenza infection according to a new study by researchers at the Institute for Clinical Evaluative Sciences (ICES) and Public Health Ontario (PHO).

“Our findings are important because an association between influenza and acute myocardial infarction reinforces the importance of vaccination,” says Dr. Jeff Kwong, a scientist at ICES and PHO and lead author of the study.

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