IHCD generally accounts for nearly half of CHD deaths, but no
previous studies have examined the possible roles of environmental
stimuli on the incidence of IHCD. In contrast to our results
for OHCD, our results for IHCD did not support any association
between air pollution, temperature and IHCD. We speculate that
the following reasons might help explain this null association. First,
as an approximate surrogate of SCD, OHCD typically occurs
immediately after the onset of cardiac symptoms, so it may be more
sensitive to an acute exposure to environmental risk factors. In
contrast, IHCD may have a longer period between the onset of
cardiac symptoms and death, and thus IHCD may be more likely to
be related with a longer-term exposure to environmental hazards.
Second, the decedents of OHCDs are more likely to be stable or nonrecognized
CHD patients (Zipes, 2005), and thus they may spend
more time outdoors than IHCD decedents. Consequently, IHCD may
not be as sensitive to a change in outdoor air pollution and temperature
as OHCD. Third, the convenient medical service at a hospital
may relieve any potential subclinical or clinical effects
provoked by ambient air pollution and temperature. Nevertheless,
further studies are still needed to validate our findings.