Extreme Heat Is a Clinical Risk Factor Now — Here's What NPs Need to Build Into Practice

 

CLIMATE & HEALTH

Extreme Heat Is a Clinical Risk Factor Now — Here's What NPs Need to Build Into Practice

Heat waves are longer, more frequent, and more intense — and the health effects go far beyond heat stroke.

CLINICAL BOTTOM LINE Heat waves in the US are now twice as frequent as in the 1980s, and the heat wave season is more than three times as long as it was in the 1960s. Extreme heat drives measurable increases in cardiovascular events, kidney injury, respiratory exacerbations, adverse pregnancy outcomes, and mental health crises — not just classic heat stroke. Older adults, patients on common cardiovascular and psychiatric medications, and historically marginalized or low-income communities carry disproportionate risk. A heat-risk review belongs in routine care for any patient with cardiovascular, renal, or psychiatric disease, or who is taking a medication that impairs thermoregulation.

 

The Scope of the Problem

Global mean temperature has already risen 1.1°C since the Industrial Revolution, with projections of 2.5–2.9°C by century's end absent drastic emissions reductions, according to a 2024 New England Journal of Medicine review. In the US, the annual number of heat waves is now twice what it was in the 1980s, and the heat wave season is more than three times as long as it was in the 1960s. More than a third of heat-related deaths across 43 countries between 1991 and 2018 have been attributed to anthropogenic greenhouse gas emissions.

This isn't an abstract environmental issue — it's already showing up in emergency department and hospitalization data. Days in the 95th percentile of local warm-season temperatures were associated with a 7.8% increase in the relative risk of an ED visit for any cause in one large US study, and an analysis of roughly 50 million summertime hospitalizations found increased admissions for cardiovascular, respiratory, diabetes-related, fluid/electrolyte, and renal causes as the daily maximum heat index rose.

Beyond Heat Stroke: The Full Range of Health Risks

Heat stroke and heat exhaustion get the attention, but they represent only a fraction of the overall health burden. Substantial epidemiologic evidence links both heat waves and single days of high temperature to a much broader range of outcomes.

System	Associated Risks
Cardiovascular	Acute MI, arrhythmias, CHF exacerbation, ischemic stroke
Respiratory	Asthma and COPD exacerbations
Renal	Acute kidney injury, electrolyte imbalance
Pregnancy	Preterm birth, stillbirth, low birth weight, congenital heart defects
Mental health	Increased anxiety, depression, suicidality, aggression/violence

 

Risks scale with both the absolute temperature and the deviation from local historical norms — a heat index that would be unremarkable in Phoenix can be dangerous in Seattle, because populations acclimate to their typical climate. This matters clinically: don't rely on a single national temperature threshold to gauge risk for a given patient population.

Why Older Adults Are Physiologically Disadvantaged

A 2020 narrative review in Environment International lays out the mechanisms in detail. Maintaining body temperature during heat exposure depends on cutaneous vasodilation (routing blood to the skin) and sweating — both of which decline with age. Older adults show blunted nitric-oxide-dependent vasodilation, reduced cardiac reserve to support the elevated cardiac output heat stress demands, and attenuated sweat gland output even though the number of heat-responsive sweat glands is preserved. The net effect is greater body heat storage and higher core temperature for the same environmental exposure compared to younger adults.

•     Cardiovascular: smaller increases in cardiac output during heat stress, endothelial dysfunction, stiffer central arteries, and impaired baroreflex-mediated blood pressure regulation.

•     Fluid regulation: diminished thirst response to dehydration, impaired renal water conservation, and reduced ability to concentrate urine — all increasing susceptibility to both hyper- and hyponatremia.

•     Renal: basal renal blood flow already declines roughly 10% per decade after age 40; heat-induced renal vasoconstriction combined with dehydration compounds this, raising acute kidney injury risk.

As much as 90% of heat wave-associated mortality is attributable to major adverse cardiovascular events — and critically, the risk is elevated not only in patients with known cardiovascular disease but in those without it as well.

Medications That Compound Heat Risk

Many of the medications NPs prescribe routinely for chronic disease interfere with the body's ability to dissipate heat, dehydrate patients further, or blunt the cardiovascular compensation heat stress requires. This is worth a specific medication-reconciliation pass before summer or ahead of a forecasted heat wave.

Medication Class	Heat-Related Concern
ACE inhibitors / ARBs	AKI and electrolyte imbalance risk; blunted thirst; impaired renal autoregulation
Diuretics (thiazide, loop, spironolactone)	Dehydration, hypovolemia, blunted cardiac output increase, hypotension
Beta-blockers	Blunted chronotropic reserve limits cardiac output response to heat
Anticholinergics	Impaired sweating, increased heat stroke risk, hypotension
Antipsychotics	Impaired sweating; risk of drug-induced hyperthermia
Antidepressants (SSRIs, tricyclics)	Impaired sweat rate, hyponatremia risk (SSRIs), limited cardiac output (tricyclics)
NSAIDs	Nephrotoxic; compounds AKI risk during heat stress and dehydration

 

None of this is a reason to stop these medications — they treat serious conditions. It's a reason to counsel patients on hydration, watch electrolytes more closely during heat events, and have a specific sick-day/heat-day conversation with anyone on this list who also has cardiovascular, renal, or autonomic disease.

CASE FROM PRACTICE An 81-year-old woman with hypertension (lisinopril, HCTZ) and mild cognitive impairment lives alone in an un-air-conditioned second-floor apartment. During a 4-day heat advisory, her daughter calls reporting confusion and reduced oral intake — she "doesn't feel like drinking water." This profile stacks nearly every risk factor discussed above: advanced age, two medications that impair fluid regulation and heat dissipation, cognitive impairment limiting behavioral thermoregulation (she may not recognize or act on thirst or overheating), social isolation, and no air conditioning. A same-day BMP, orthostatic vitals, and a same-day plan for cooling access (a cooling center, a family member's air-conditioned home) are appropriate — this is not a wait-and-see situation.

 

The Disparities Are Not Fully Explained by Heat Alone

A NOTE ON EQUITY Communities where members of marginalized racial and ethnic groups and low-income populations live face disproportionately high heat-related illness risk — and this is not fully explained by socioeconomic status or air conditioning access alone. Historically redlined neighborhoods — a discriminatory mortgage-lending practice from the 1930s — continue to have more paved surface and less green space than other areas, producing measurably higher ambient temperatures ("urban heat islands"). Low-income urban communities can run as much as 5°C hotter than wealthier ones nearby. Contributing factors include more limited access to care, more frequent occupational heat exposure (factory work without air conditioning, outdoor labor), and a higher baseline prevalence of hypertension, diabetes, and kidney disease. For patients in these circumstances, standard advice ("stay in air conditioning") may not be actionable — screening for access to a cooling center or a cooler environment during heat advisories is a more useful clinical question than assuming air conditioning is available.

 

What NPs Can Actually Do

•     Ask about air conditioning access and living situation for any older or high-risk patient, especially before summer months — this single question identifies a major modifiable risk factor.

•     Review medication lists specifically for heat-interacting drugs (table above) in patients with cardiovascular, renal, autonomic, or psychiatric disease, and add heat-specific counseling, not just general "stay hydrated" advice.

•     Identify socially isolated patients before heat season — they are less likely to have someone checking on them during a multi-day heat event, which is when risk accumulates most.

•     Counsel on individual protective measures: limiting outdoor exposure during peak heat, loose light-colored clothing, adequate hydration, and cooling devices — while noting that air conditioning itself has an environmental cost worth naming for patients who ask.

•     Know your local cooling center resources and heat-alert systems so you can give patients a concrete plan, not just a warning, before a forecasted heat wave.

Board & Practice Prep

•     Heat exhaustion presents with normal mental status; heat stroke involves CNS dysfunction with core temperature typically ≥40°C — this distinction determines emergency triage.

•     A heat wave is generally defined (NOAA) as 2 or more continuous days of unusually high heat relative to local historical norms — not an absolute temperature threshold.

•     There is currently no standardized, universally accepted definition of a "heat-related death," which complicates surveillance and ICD coding of indirect heat effects.

•     Heat-induced hemoconcentration and hypercoagulability contribute to elevated cardiovascular event risk independent of classic heat illness — worth remembering when a patient presents with an MI or stroke during a heat event with no other clear trigger.

•     Mortality displacement ("harvesting") — where heat-related deaths occur mainly in already-frail patients who would have died soon regardless — has been studied and found not to fully explain heat-attributable mortality; the excess risk is real.

The Takeaway

Extreme heat is no longer a seasonal inconvenience to mention in passing — it's a modifiable clinical risk factor with mechanisms as well-characterized as many risk factors NPs already screen for routinely. Building a heat-risk review into care for older adults, patients with cardiovascular or renal disease, and anyone on a heat-interacting medication is a low-cost intervention with a clear evidence base behind it, and it may prevent the kind of quiet decompensation that too often shows up only after a multi-day heat event has already done its damage.

References

Bell ML, Gasparrini A, Benjamin GC. Climate change, extreme heat, and health. N Engl J Med. 2024;390(19):1793-1801. doi:10.1056/NEJMra2210769

Meade RD, Akerman AP, Notley SR, McGinn R, Poirier P, Gosselin P, Kenny GP. Physiological factors characterizing heat-vulnerable older adults: a narrative review. Environ Int. 2020;144:105909. doi:10.1016/j.envint.2020.105909