Can We Cool Patients with CKD without Heating the Planet?

Human activities, specifically the use of fossil fuels since the Industrial Revolution, have contributed to the rise in mean global surface temperature by 1.1°C since 1880 and projected to rise by up to 2.9°C by 2100. This rise cannot be explained solely by natural activities such as solar irradiance and volcanic activity.¹ A major climate tipping point is estimated to occur when mean global temperature increases by ≥1.5°C, leading to potentially irreversible, catastrophic effects on major ecosystems. However, we are already experiencing threats to human health from rising temperatures, including more frequent and intense heat waves.

Thermoregulation is a critical evolutionary adaptation that allows humans to survive acute and long-term temperature changes. However, extreme temperatures are associated with excess mortality and morbidity. In the general population, exposure to heat waves and higher ambient temperatures are associated with increased risk of cardiovascular death and hospital admissions.² Heat exposure increases risk of developing CKD, AKI, nephrolithiasis, and electrolyte abnormalities.² Yet few studies have investigated health outcomes of patients with CKD during extreme heat events despite long-standing assumptions that this group is at high risk.

Patients with CKD have impaired thermoregulatory mechanisms and are often on medications (i.e., diuretics and sodium-glucose cotransporter-2 inhibitors) that can exacerbate their ability to maintain salt and water homeostasis during heat waves. They often have comorbid conditions, such as heart disease, diabetes, hypertension, and advanced age, which put them at further risk from rising temperatures. A major limitation of epidemiological studies to date is a lack of data on patients with CKD and their risk for heat-related morbidity or mortality. Research has shown the risk of heat to patients with ESKD on dialysis, but not the nondialysis CKD population specifically. One study of patients on dialysis in the northeastern United States showed that extreme heat was associated with increased risk of same-day hospital admissions and same-day mortality.³ A larger nationwide study of patients on dialysis similarly observed an increased risk of death during heat waves in urban areas.⁴ To our knowledge, there have been no high-quality epidemiological studies investigating heat-related morbidity or mortality in the nondialysis CKD population until now. The recent study by Romine et al. offers critical insight into medical utilization among patients with nondialysis CKD after extreme heat.⁵

Romine et al. analyzed a nationwide cohort of individuals with CKD stages 3–5 who had commercial or Medicare insurance, leveraging a substantial dataset of medical claims and weekly approximated heat indices across 8 years. They used panel regression and linear probability models with individual and year fixed effects to investigate how exposure to heat (approximated heat indices binned every −12.2°C, where <−1.11°C and >37.8°C were defined as extreme) was associated with weekly health care utilization, including emergency department (ED), outpatient, and inpatient settings. The study found that exposure to one additional day with a heat index of 32.2°C–37.8°C (versus 15.6°C–21.1°C) was associated with increased ED and outpatient utilization, and a higher proportion of those ED/outpatient visits were attributable to a kidney-related primary diagnosis. At 32.2°C–37.8°C, while all-cause inpatient admissions did not increase, the proportion of those admissions attributable to a kidney-related primary diagnosis did. The study also examined the relationship between medical utilization and likelihood of access to air conditioning (AC) using previously published data⁶ (primarily in urban settings). Compared with those most likely to have AC access, patients least likely to have AC access were more likely to visit the ED with a kidney-related primary diagnosis. Notably, there was no statistically significant difference in ED utilization by CKD stage, which is likely due to smaller sample sizes in CKD stages 4 and 5.

There are several notable strengths of this study. First, this is a very large, nationwide dataset that covers multiple geographic locations and over several years, thereby improving generalizability of the results. Second, it is the first high-quality epidemiological assessment of medical utilization within the CKD population after exposure to extreme heat. Third, the full spectrum of temperature extremes (i.e., extreme cold) are also included in their analysis. Finally, it is notable that >90% of patients included in the study are CKD stage 3, <10% of patients have CKD stage 4 or 5, and <1% are on dialysis. This is an important step forward in recognizing that patients with earlier CKD are at risk of heat-related complications.

Several limitations of the study warrant attention. The authors used approximated heat indices, which is less accurate than measured heat indices, and they did not assess nighttime temperatures. They also excluded patients who are underinsured, uninsured, or on Medicaid. Medical utilization data were aggregated by census block group, preventing accurate tracing of individuals who are using health care multiple times in a week or traveled elsewhere during or after a heat wave. It is possible that some of the medical utilization after the heat events was displacement and would have occurred regardless of heat exposure. Subgroup analysis of medical utilization by CKD stage 3A versus 3B was not performed, thus limiting conclusions on whether patients with CKD stage 3B benefit from AC more than those with CKD stage 3A. In addition, AC access data were only available for 66% of the study population—most of whom reside in large metropolitan/urban areas—limiting specific conclusions on the relationship of AC access and medical utilization for those living in rural areas. Finally, several confounding factors may further influence health outcomes. Densely populated urban areas have lower prevalence of AC compared with suburban areas.⁶ These urban environments may be more harmful to human health as a consequence of heat-absorbing surfaces, increased air pollution, and higher nighttime temperatures (often referred to as the urban heat island effect).⁷ Urban environments with higher levels of air pollution, often home to marginalized and low-income communities, may have a greater effect on health independent from access to AC.

This study contributes to the body of literature supporting the use of AC as a preventive measure during heat waves.^2,8 However, indiscriminate increase in AC consumption is not a sustainable intervention to manage the public health challenge from rising ambient temperatures and extreme heat events. Currently, AC access reflects a wealth gap,^6,9 and the challenge will be to increase access to cooling centers and cooler environments without increasing AC consumption. AC draws an exceptional amount of electricity, which further exacerbates short-term and long-term temperature rise. AC systems absorb heat inside buildings; that heat is released outside and raises outdoor temperatures.¹⁰ Davis and Gertler estimated that if up to 80% of households adopted AC, residential electricity consumption would increase by 83%, costing up to $4 billion in annual electricity expenditure and emitting 30 million tons of annual carbon dioxide by 2100.⁹ Widespread AC consumption in the United States for mitigation of health-related complications would increase air pollution that would affect health and carbon emissions that further exacerbate climate change and climate-related illnesses. This is an example of burden shifting, whereby a change in technology use results in changes to the type, timing, or location of pollution rather than achieving a global reduction in pollution.

With this study, Romine et al. have shown the need for interventions that will mitigate heat-related illnesses for patients with CKD. We recommend that practitioners create a heat action plan for their patients, analogous to an asthma or seizure action plan. The heat action plan should include appropriate medication adjustments, locations of nearby cooling centers, and what symptoms to look for during intense heat.⁸ On a community level, many programs can be implemented including public education and awareness of heat stress and other heat-related illnesses. Although widespread adoption of AC is not an environmentally responsible solution, increasing access to cooling centers (which can be established in public areas, such as libraries, shopping centers, and sports arenas) and use of AC are important interventions to provide temporary relief and shelter during heat waves, especially if powered by sustainable energy sources. Finally, green infrastructure—e.g., increased vegetation near roads, on building rooftops, and in public spaces—is a well-documented method to cool urban areas.² Several other key advantages of green infrastructure include beautification, improved biodiversity, and carbon sequestration.

Extreme heat events pose a significant threat to public health, including individuals with CKD. The study by Romine et al. provides valuable insight into increased medical utilization associated with heat exposure in this vulnerable population, emphasizing the need for targeted strategies and further research to identify effective interventions. While access to AC and cooling centers can provide temporary relief, long-term solutions must prioritize equitable and sustainable strategies, including public education, individualized heat plans, and investment in green infrastructure. Addressing the intersection of extreme heat and health requires a multifaceted approach that balances immediate adaptation with long-term mitigation efforts in a sustainable yet resilient fashion to reduce the burden of heat-related morbidity and mortality in the CKD population.

Supplementary Material

cjasn-20-757-s001.pdf ^{(1.3MB, pdf)}

Disclosures

Disclosure forms, as provided by each author, are available with the online version of the article at http://links.lww.com/CJN/C270.

Funding

None.

Author Contributions

Conceptualization: Amy S. Li, Sarah E. Young.

Supervision: Sarah E. Young.

Visualization: Amy S. Li, Sarah E. Young.

Writing – original draft: Amy S. Li.

Writing – review & editing: Sherri M. Cook, Amy S. Li, Sarah E. Young.