PulmCrit- May 23, 2016 - By Josh Farkas
"Although the term dehydration is often used loosely, the medical definition of dehydration is loss of free water resulting in hypernatremia. This is common in the ICU, usually from failure to replace daily water losses. Normally, hypernatremia triggers thirst and water intake, but this is impossible among intubated patients.
Hypernatremia should generally be aggressively corrected. Hypernatremia may make patients miserable, manifesting as “agitation” and leading to increased sedative administration (imagine being thirstier than you’ve ever been in your life yet unable to drink). I’ve seen some patients with significant agitation on the ventilator which persisted until we fixed their hypernatremia:
To make matters even worse, dehydration does little to improve tissue edema (which is due to excessiveextracellular fluid volume). For example, imagine removing one liter of free water from a patient. Since water diffuses freely between the intracellular and extracellular spaces, water will be removed from both spaces. With an equal rise in osmolarity throughout the body, the amount of water loss from each space will be proportionate to its volume. Therefore, since the intracellular volume is about twice the size of the extracellular volume, removing a liter of free water will only remove one third of a liter from the extracellular space...
- Critically ill patients often avidly retain sodium. This may cause diuresis attempts to fail, if patients excrete dilute urine leading to a loss of water without loss of sodium. Such patients may seem to respond to diuresis, but in fact they are merely becoming progressively dehydrated and hypernatremic (occult diuresis resistance).
- Combining a loop diuretic and thiazide diuretic is proven to increase sodium excretion in the urine (natriuresis). A recent RCT confirms that this combination promotes balanced loss of sodium and water, allowing volume removal without dehydration."