Higher Environmental Heat Associated with Risk of Acute Kidney Injury in a Northern Climate

Worldwide, increases in carbon dioxide emissions is leading to global warming, resulting in more frequent, more intense, and longer lasting heat waves. Underlying the pathogenesis of heat-related morbidity is multiple organ dysfunction; the kidneys appear to be particularly vulnerable. It is thought that acute kidney injury (AKI) during heat waves occurs primarily from volume contraction. Some ability to thermoregulate may be lost in older adults, making them more likely to experience heat-induced AKI than their younger counterparts.

In 10 previous studies reporting on the effects of heat exposure on AKI, higher risks for AKI were associated with a variety of heat indicators. However, there were limitations to those studies: only two of the studies were specific to renal outcomes and all 10 were restricted to either the United States or Australia, limiting the generalizability to more varied northern climates.

Researchers, led by Rebecca K. McTavish, MSc, recently conducted a matched case-control study to examine the association between heat periods and hospital encounters with AKI among older adults in the northern climate of Ontario, Canada. Heat periods were defined as 3 consecutive days exceeding the 95th percentile of area-specific maximum temperature. Study results were reported in the American Journal of Kidney Diseases [2018;71(2):200-208].

The researchers utilized Ontario’s linked healthcare administrative databases to match 52,913 older adults who had a hospital encounter with AKI in April through September 2005 to 2012 with 174,222 controls for exact date, age, sex, rural residence, income, and history of chronic kidney disease (CKD). Case patients and controls were selected without knowledge of their heat exposure. The majority of case patients (69%, n=36,424) were matched with four controls.

Median age of the total cohort (case patients and matched controls) was 80 years, 49% were women, 8% lived in rural areas with population <10,000, 24% lived in lowest-income-quintile neighborhoods, and 16% had a history of CKD. Case patients presented to 180 different hospitals across Ontario; 2.7% of case patients (n=1440) received dialysis for their AKI. Median 3-day government payer healthcare cost for hospital encounters related to AKI was $13,877 compared with $33 for controls who did not have a hospital encounter with AKI. Most case patients (69%; n=36,515) presented to the hospital after 12 noon, providing reassurance that high-heat exposure tended to precede the AKI event.

Compared with the absence of high heat periods, high heat periods were associated with greater risk for a hospital encounter with AKI (adjusted odds ratio [OR], 1.11; 95% confidence interval [CI], 1.00-1.23); statistical significance was borderline (P=.05). When all dates that met the definition of a heat period during the warm season of 2012 were excluded, the researchers identified 16,071 AKI events over 355,467,493 person-days at risk, corresponding to an incidence rate of 1650 AKI events per 100,000 person-years, in the absence of a heat period in older adults. The relative increase in the risk of ~11% translated to an additional 182 heat-associated AKI events per 100,000 person-years during April to September.

The researchers performed additional analyses to examine high humidex versus high temperature periods. The two exposure definitions did not always coincide and were only moderately correlated. Compared with the absence of periods of high humidex, there was no association between high humidex periods and greater risk for a hospital encounter with AKI (OR, 1.04; 95% CI, 0.93-1.16).

There was no difference in the association between heat periods and hospital encounters with AKI between individuals 66 to 79 years of age compared with individuals >79 years of age. The association between high humidex periods and hospital encounters with AKI did differ across age groups; the association was greater in those >79 years of age (P=.01).

The researchers also repeated the analyses matching AKI case patients and controls in geographic location rather than time. Results were similar to those of the primary analyses: there was an association between high heat periods and greater risk for a hospital encounter with AKI (adjusted OR, 1.11; 95% CI, 1.00-1.23). In the geography-matched analysis, there was also an association between high humidex periods and greater risk of a hospital encounter with AKI (adjusted OR, 1.20-95% CI, 1.09-1.33).

Study limitations cited by the researchers included focusing on older adults, potentially reducing the generalizability of the findings to younger age groups; lack of data on how long individuals were outside or had access to air conditioning; the possibility of residual confounding; and lack of consideration of the risk for less severe AKI that did not require a hospital encounter.

The researchers concluded, “We found in older adults in a northern climate that exposure to periods of higher environmental heat is associated with a modestly higher risk for hospital encounter with AKI.”

Takeaway Points

  1. Previous studies in warm climates have linked high environmental heat and the risk of acute kidney injury (AKI); researchers in Ontario, Canada conducted a matched case-control study to determine whether this association generalizes to a northern climate with more variable temperatures.
  2. The study matched 52,913 patients with a hospital encounter with AKI in April through September 2005 to 2012 with 174,222 controls for exact date, age, sex, rural residence, income, and history of chronic kidney disease.
  3. Heat periods, defined as 3 consecutive days exceeding the 95% percentile of area-specific maximum temperature, were associated with significantly higher risk for AKI (adjusted odds ratio, 1.11; 95% confidence interval, 1.00-1.23).