HYDRATION DURING HEATWAVES - A COIN WITH TWO SIDES - Filenews 1/8
Hydration is a fundamental measure to prevent complications of heat stress, where water loss through sweating increases significantly. However, the “more water, the better” approach does not correspond to current scientific knowledge. Inadequate fluid intake increases the risk of dehydration, acute kidney injury and heat stress, while excessive consumption of free water, especially in vulnerable groups, can cause hyponatremia, a potentially life-threatening electrolyte disorder.
Dehydration during heat stress: mechanisms, vulnerable groups and pharmacological risk factors
Dehydration is the most common disturbance of water balance during heat stress and is caused when water losses exceed intake. The decrease in intravascular volume and the increase in plasma osmolality activate compensatory mechanisms, mainly through the secretion of antidiuretic hormone (ADH) and the activation of the renin–angiotensin–aldosterone system (RAAS). ADH increases renal water reabsorption via aquaporin-2, while the RAAS promotes sodium and water retention, with the aim of maintaining circulating volume and blood pressure.
When fluid losses are prolonged or their replacement is inadequate, these mechanisms are not sufficient, resulting in the appearance of clinical manifestations such as thirst, dry mouth, fatigue, decreased diuresis, dizziness, orthostatic hypotension and tachycardia. In severe cases, prolonged hypovolemia can lead to acute kidney injury, heat exhaustion and even heat stroke.
Elderly patients are particularly vulnerable, as their thirst sensation and renal urine concentration are reduced. Patients with heart failure, chronic kidney disease, or diabetes mellitus have a reduced ability to adapt to changes in intravascular volume.
Medication is another factor that can affect the body's response to dehydration. Diuretics increase renal excretion, while angiotensin-converting enzyme inhibitors (ACE inhibitors), angiotensin II receptor blockers (ARBs), and aldosterone antagonists can reduce the ability to maintain renal perfusion in hypovolemic conditions. In addition, nonsteroidal anti-inflammatory drugs (NSAIDs) can worsen renal dysfunction in dehydrated patients.
Hyponatremia in heat: when excessive water consumption becomes dangerous
While dehydration is the most common complication of high temperatures, excessive water consumption can lead to an equally serious disorder: hyponatremia. Hyponatremia is defined as a serum sodium concentration below 135 mmol/L and is caused when the amount of water in the body exceeds the ability of the kidneys to excrete it, resulting in a dilution of sodium in the plasma. During heat stress, increased secretion of antidiuretic hormone (ADH), due to heat stress, hypovolemia, intense exercise or other stimuli, can limit the excretion of free water and increase the risk of its occurrence.
Of particular importance are patients taking thiazide diuretics, such as hydrochlorothiazide or indapamide, drugs widely used to treat arterial hypertension and heart failure. These drugs inhibit the sodium-chloride cotransporter in the distal convoluted tubule, reducing sodium reabsorption. At the same time, they limit the ability to dilute urine, making it more difficult to eliminate free water. The combination of these mechanisms with increased fluid intake and increased ADH activity during hot flashes can lead to a significant drop in serum sodium.
Thiazide-induced hyponatremia occurs more frequently in elderly patients, people with chronic kidney disease or heart failure. In addition, there is an increased risk in patients taking drugs that enhance the action of ADH, such as selective serotonin reuptake inhibitors (SSRIs) and some antiepileptic drugs.
Clinically, hyponatremia may initially manifest with nonspecific symptoms, such as fatigue, nausea, headache, muscle cramps and unsteadiness in gait. In severe or rapidly progressive cases, it can cause confusion, seizures, impaired level of consciousness, and coma due to cerebral edema.
Conclusions
Proper hydration during heatwaves requires a balance between adequate fluid replacement and avoidance of excessive water intake, as both dehydration and hyponatremia can lead to serious complications, especially in vulnerable patients.
Prevention requires an individualized approach, taking into account age, underlying diseases, renal function, medication, and actual fluid losses. Proper patient education about the risks of both insufficient and excessive water intake are particularly important during the summer months and can contribute substantially to reducing hospitalizations and serious complications. Any modification of medication during periods of heat should always be carried out in collaboration with the attending physician, after an individualized assessment of the patient's clinical condition.
In conclusion, the general instruction to drink plenty of fluids during the summer months is not always completely correct.
Written by Dr. Maria Kyprianou
Internal Physician, Department of Internal Medicine, Apollonius Private Hospital
