Friday, February 23, 2018

NEJM: Flu Season A Risk Factor For Developing Post-Cardiac Surgery ARDS












#13,167



ARDS (Acute Respiratory Distress Syndrome) is a rapidly progressing - often life-threatening complication - hallmarked by the leaking of fluid into the small air sacs (alveoli) of the lungs, usually as the result of a direct or indirect lung injury.
ARDS can be caused by a variety of insults to the lung, including aspiration, chemical or smoke inhalation, pneumonia, septic shock, or trauma.  It is often seen in critically ill patients with liver of kidney failure.
With the build up of fluids from ARDS, the patient is unable to pass oxygen efficiently  through their lung's alveoli into the the bloodstream, even when placed on a ventilator.
Low oxygen levels (hypoxia) can lead to further organ damage and even death.
ARDS is most often observed in patients already hospitalized for another serious illness, and while survival rates vary depending on age, the underlying cause, and comorbidities  - some estimates put the mortality rate somewhere between 33%-50%.

As the following excerpt from the 2016 study (Acute respiratory distress syndrome following cardiovascular surgery: current concepts and novel therapeutic approaches) explains, ARDS is a rare, but serious complication following cardiac surgery.

Cardiac surgery is a known risk factor for ARDS, especially when using cardiopulmonary bypass (CPB), because CPB induces a systemic inflammatory response and pulmonary ischemia-reperfusion injury. Today more high-risk patients undergo cardiac surgical interventions and an increasing number of patients is provided with complex procedures [2,3].
To date there are eight clinical studies that analyzed the incidence, risk factors, and mortality of ARDS following cardiac surgery (overview in [4▪]). The incidence of ARDS varied from 0.17 to 2.5% and mortality from 15 to 91.6%.

Yesterday the NEJM published a correspondence which suggests that post-cardiac surgery ARDS complications may occur more frequently during flu season, even when the patient shows no signs of influenza infection.

Influenza Season and ARDS after Cardiac Surgery

February 22, 2018
N Engl J Med 2018; 378:772-773
DOI: 10.1056/NEJMc1712727

To the Editor:

A number of concurrent risk factors are associated with development of the acute respiratory distress syndrome (ARDS). One such risk factor might be asymptomatic respiratory viral infection — for example, influenza — which could prime the lungs for ARDS in patients with another overt risk factor. Patients who undergo cardiac surgery could potentially carry these viruses yet have no clinical signs or symptoms.1
(Continue . . . )

The correspondence, along with a 24-page Supplementary Appendix, continue on to describe a two-year single-center observational cohort study on cardiac patients at the ICU of a tertiary university hospital in the Netherlands.
While patients were not tested for flu, the study found that the incidence of ARDS complications following cardiac surgery doubled during flu season, even though the surgical patients showed no signs of respiratory infection before surgery.
 In the discussion section of the supplemental file, the authors wrote:
Cardiac surgery during influenza season is an independent risk factor for development of postoperative ARDS compared to surgery during seasons with little respiratory virus transmission.

The main finding of the present study is that the risk for the development of ARDS after  cardiac surgery is about twice increased during the influenza season as compared to seasons with low burden of respiratory virus infections.
Moreover, the influenza season did increase the duration of mechanical ventilation. The influenza season was estimated on the basis of weekly reporting of influenza-like illness within the community by sentinel surveillance at general practitioner offices, confirmed by detecting influenza in nasopharyngeal samples.
On multivariate modelling, the influenza season proved to be an independent risk factor for the development of ARDS postoperatively, besides well known factors like EuroSCORE and total time on CPB.

Of note, the 2009 pandemic fell within the study period. 

Since influenza tests (or other respiratory panels) were not conducted, the authors point out that they haven't proven that an underlying viral infection causes increased ARDS in cardiac surgery patients. They wrote:
Our study also has several weaknesses. First and for all, our cohort study shows an association but does not prove a causal relation between viral infection and ARDS in cardiac surgery patients. There are potential confounders that vary by season, such as vitamin D level or ambient temperature, for which we could not adjust.
Still, if confirmed by other studies, this research raises interesting questions about the potential value of pre-surgical testing for viral infection, even when a patient appears to be asymptomatic.

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