Medicine use, heat and thermoregulation in Australian patients

To the Editor: Australia is already a hot country and the frequency of hot spells looks set to increase over coming decades.1 Health care professionals will therefore need to consider the effect that medications can have on people’s capacity to respond to hot conditions.

Medications can directly increase body temperature by altering thermoregulatory mechanisms, pharmacological action, idiosyncratic reactions, or hypersensitivity reactions.2-4 Uncommon side effects — including toxic states, anticholinergic poisoning syndrome, serotonin syndrome and neuroleptic malignant syndrome — can also generate heat.

Indirect impairment of physiological response to thermal stress may occur by preventing adjustment to hypotension, or by interfering with behavioural responses. Despite these risks, hyperthermia and reduced sweating each rate only one mention in the 2010 Australian medicines handbook (in relation to anticholinergics).5 The handbook does not list heat stress as an adverse reaction to dexamphetamine,5 which can impair thermoregulation by increasing metabolic rate.

Medications can affect thermoregulation by:2-5

• impairing sweating triggered by the parasympathetic nervous system — eg, anticholinergics, phenothiazines;

• impairing sweating by carbonic anhydrase inhibition — eg, topiramate, zonisamide;

• impairing cardiac output or causing hypovolaemia — eg, antihypertensives, alpha-blockers, diuretics;

• impairing behavioural responses to heat (such as drinking or taking cooling action) — eg, sedatives and hypnotics, anxiolytics, phenothiazines, illicit drugs; or

• rarer effects, such as increasing metabolic rate — eg, monoamine oxidase inhibitors, thyroxine, amphetamines, cimetidine.

Elderly people with impaired thermoregulation, the socially disadvantaged and people with mental illness have been identified as being most at risk from heat stress.6 Although the risk of medications directly impairing physiological responses to heat is low, thermoregulatory effects can be cumulative.

People in certain occupations (eg, outdoor labourers and tradespeople, indoor workers in hot industries such as metal manufacturing) may be at increased risk of heat exposure, and there are anecdotal reports of tourists suffering heat stress in climates that are hotter than those to which they are accustomed.

There is a need to educate workers, the public and health practitioners about the relationship between certain medications and heat. Professional associations and trade unions have a responsibility to warn and educate their members. It may be helpful for the Therapeutic Drugs Administration to actively solicit reports of adverse events related to medicine use, heat and thermoregulation via the Australian Adverse Drug Reaction Reporting System.

Carers of elderly and chronically ill people might flag patients on certain medications for extra care during hot spells. Additionally, health practitioners can ensure that these people are on appropriate heatwave notification and emergency support lists, which are being developed by Australian state and territory health departments and emergency service agencies.