Diagnosing Dehydration: What Would Osler Do?

November 3, 2009

Astrict definition of “dehydration” is a relative absence of water as manifested by hypernatremia. However, patients who are dehydrated are usually also volume-contracted; I prefer the latter term for a patient who has experienced vomiting, diarrhea, or decreased intake. Volume contraction denotes the need for replacement with saline equivalents, whereas dehydration indicates the need for water-and only possibly saline as well.

How is dehydration best diagnosed? I believe that if a patient has produced scant urine over 6 to 8 hours because of vomiting, diarrhea, or reduced intake-from any cause-then the patient is dehydrated and needs intravenous fluids. However, emergency physicians and hospitalists seem to think that dehydration can be diagnosed only on the basis of elevated blood urea nitrogen (BUN) and creatinine levels. Your thoughts?
-- Gary Freeman, MD
         Liverpool, NY

A strict definition of “dehydration” is a relative absence of water as manifested by hypernatremia. However, patients who are dehydrated are usually also volume-contracted; I prefer the latter term for a patient who has experienced vomiting, diarrhea, or decreased intake. Volume contraction denotes the need for replacement with saline equivalents, whereas dehydration indicates the need for water-and only possibly saline as well.

A clinician can definitely suspect volume contraction or dehydration on the basis of the history and physical findings before the results of laboratory tests, such as BUN and creatinine measurements, are available. Patients with suspected dehydration often have a history of vomiting, diarrhea, or decreased intake accompanied by volume-depleting medications (eg, diuretics).

A physical examination of such a patient may demonstrate any or all of the following:

•Tachycardia.
•Absence of jugular venous distention (JVD).
•Hypotension.
•Orthostatic changes in blood pressure and heart rate.

Unfortunately, other physical findings that are presumed to be “typical” in volume depletion-such as dry mucous membranes-lack specificity and are not reliable.

I sympathize with your frustration. What we are seeing is a generation that no longer relies on the physical findings for diagnosis but instead defers to laboratory and imaging results. Determinations of orthostatic changes are often not made. Careful evaluation for jugular pulsations is the exception rather than rule. In fact, nearly every contemporary examination is rubberstamped “RRR,” with “no JVD, murmurs, rubs, or gallops.” Because the patient’s posture is not varied during the examination, in most instances the jugular pulse is not appreciated at all. Distention is only absent to the eye of that examiner. It is no wonder that laboratory results are the default diagnostic bottom line.

For those who still carefully examine their patients, the inability to appreciate a jugular pulse in a supine patient is a more efficient way to diagnose volume depletion than waiting for BUN and creatinine levels. However, fewer and fewer clinicians will be able, or will even attempt, to elicit that finding.

William Osler would be very disappointed.

-- Gregory W. Rutecki, MD
        Professor of Medicine
        University of South Alabama College of Medicine
        Mobile

More . . .
Read Dr Henry Schneiderman's further caveats about relying on lab data to diagnose hypovolemia.