Discussion
We demonstrated that residence altitude, particularly HF-specific readmission, could be a risk factor for readmission in patients with HF. Also, the relationship might be confounded by the unfavourable sociogeographic conditions at higher altitudes.
Mechanism of the association between high altitude and readmissions
Nearly 70% of Japan is covered with mountains, more in rural areas, that have relatively steep slopes, low temperatures and large temperature changes even though they are not so high above sea level. These geographical disadvantages are frequently hard, particularly for elderly people and those with HF.11–13 In addition, the mountain districts have social disadvantages, such as ageing, underpopulation and limited healthcare services, which might lead to delay in detection of worsening conditions and finally admission.
In fact, in our study, high altitude was significantly associated with unfavourable social and climate conditions as expected. Also, the association between all-cause readmission and residence altitude after adjusting for patient clinical conditions was further weakened after adding the adjustment of sociogeographic parameters. These results could suggest that unfavourable sociogeographic conditions represented by residence altitude could be an important risk factor for readmission in patients with HF.
The cause of strong association between residence altitude and HF-specific readmission after adjustment with clinical and sociogeographic parameters was unknown but might be influenced by unmeasured confounder such as the steepness of slope.
Interestingly, the second and third most frequent causes of all-cause readmission were lung and renal diseases. These diseases could easily worsen during a relatively short period, particularly in elderly people. Therefore, their early detection is important. Possibly, the results of our study might partly explain the delay in detection at a high altitude area.
Several articles have reported the beneficial effects of high altitude exposure for improving anaemia and lung and heart function in elderly patients.20–22 However, this type of high altitude generally is considered >2500 m above sea level.21 In our study, left ventricular ejection fraction and haemoglobin were not associated with altitude. Because the present target places were not so high, altitude might be more associated with unfavourable sociogeographic condition rather than its beneficial effects.
Associations between other sociogeographic and climate parameters and readmissions
In the present study, remoteness from the hospital, fewer healthcare services and the parameters regarding temperature were not significantly associated with readmissions. The distance from the residence to the primary hospital was relatively close compared with distances noted in Western countries. Also, the study location included relatively warm and comfortable places, even at high altitudes. These reasons might explain the difference in our results from those of prior studies.13 17 However, several articles have reported that remoteness was not necessarily associated with adverse outcomes, which was compatible with our results.23 24
Clinical implication
Our study demonstrated that high altitude places in rural Japan were sociogeographically deprived places, which could worsen the conditions in patients with HF and cause readmissions. This information would be important especially for policy makers and hospital staff.
Because medical resources are limited, probably one solution would be to expand the monitoring system for patients using phones and information and communication technology.25 26 Obviously, regular home visits by doctors and medical staff would be efficient, but preparation of financial support, such as long-term care insurance, would be crucial to take this service fairly.27 Furthermore, enlightenment of residents regarding the importance of constructing the cooperation system in the neighbourhood would be effective.
Study limitations
Our data should be interpreted according to the study’s limitations. First, geographic bias may exist, and generalisation might be difficult because this is a single-centre study. However, high altitude places in our study represent poor sociogeographic areas. Therefore, our results may be reproduced in a location with a similar environment. Nonetheless, a much larger multicentre study is essential to confirm our results. Second, this retrospective analysis may have been biased because of potential unmeasured confounders such as steepness of slope, although we tried to correct the confounders regarding altitude. Finally, although most regular patients should be admitted to Kitaishikai Hospital, some might present to other hospitals. However, the possibility would be small considering the circumstances of Kitaishikai Hospital.