A glance at recent pollen counts could help clinicians predict short-term outcomes following acute coronary syndrome (ACS), Australian research suggests.
A retrospective study of 15,379 patients with acute coronary syndrome (ACS) undergoing revascularisation found that high grass and total pollen exposure in the days to hospital admission for a cardiac event was “significantly related” to in-hospital mortality. It also saw high grass pollen exposure within two days of hospitalisation trend toward 30-day major adverse cardiac and cerebrovascular events (MACCE).
The findings “underscore a potential biological link between pollen exposure and clinical outcomes”, the authors from the Western Health Department of Cardiology, Melbourne, wrote in the Journal of the American Heart Association.
Atmospheric changes in pollen concentration can trigger allergic inflammatory responses and inflammation is known to play a “key role” in acute and chronic coronary syndromes, they noted.
Therefore, they assessed whether these changes were associated with different ACS subtypes and short-term clinical outcomes.
Despite their rationale, the retrospective review of unstable angina, non-ST-segment-elevation infarction and ST-segment-elevation infarction patients across Victoria’s public and private hospitals revealed no correlation between daily or seasonal grass and total pollen concentrations, and frequency of ACS subtype presentation.
However, patients exposed to high grass pollen concentrations (average ≥ 50 grains/m3 air/day) in the two days before ACS hospitalisation were more likely to face in-hospital mortality (odds ratio [OR]: 2.17, 95% CI: 1.12–4.21, P = 0.021) and, possibly, 30-day major adverse cardiac and cerebrovascular events (OR: 1.50, 95% CI: 0.97–2.32, P = 0.66) than those encountering lower concentrations.
Total pollen exposure averaging above 111 grains/m3 air/day in the seven days prior to ACS was also linked to in-hospital mortality (OR: 2.78, 95% CI: 1.00–7.74, P = 0.05).
While the authors suggested a pollen-induced inflammatory response may influence clinical outcomes and other studies have found weak associations between pollen exposure and cardio- and cerebrovascular outcomes, the study failed to identify and adjust for potential confounders.
Additionally, patients’ allergy, asthma, antihistamine- or steroid-use status were left out, since they weren’t captured in the data source (Victorian Cardiac Outcomes Registry), and there was no information on patients’ socioeconomic status which “could impact their general cardiovascular health and the clinical outcomes after ACS”.
Further, given average pollen counts are measured at fixed stations, separated by large distances, it couldn’t track individuals’ pollen exposure.
Nevertheless, they felt the association between pollen exposure and post-ACS clinical outcomes was plausible, though contentious and poorly defined.
The biological link between pollen and clinical outcomes
“A possible explanation of pre-ACS exposure to pollen grains and subsequent in-hospital mortality is the development of heightened inflammatory response adversely impacting different organ function, thereby leading to a clinical adverse event,” they wrote.
“Increased inflammation is a hallmark of ACS, and raised inflammatory markers in patients with ACS treated with PCI were associated with increased risk of poor cardiovascular outcomes.”
Precise mechanisms remain elusive, but previous studies have linked higher blood eosinophil counts with increased risk of all cardiovascular and ischaemic heart disease mortality.
“Eosinophils are thought to actively participate in the inflammatory process associated with myocardial infarction along with their traditional role in allergic reactions,” they wrote.
Further, “myocardial damage and pathologic remodelling because of inflammatory processes, with subsequent development of severe left ventricular dysfunction, heart failure and malignant arrhythmias have been suggested to be a potential link to increased risk of adverse cardiovascular outcomes.”
Studies into pollen exposure and cardio- and cerebrovascular risk have varied results, which “could be explained by the complex interaction between geographical and patient-related factors that lead to different [sensitisation] and symptom development thresholds after exposure to pollen”, they wrote.
The latest results need validating and the relationship between pollen and post-ACS adverse events could be better defined with studies of individuals in predefined locations.
If the association holds up, however, it could help clinicians better predict patient outcomes and inform treatment.