The man with three days of diarrhoea: reading the volume deficit
A 62-year-old man is admitted after 3 days of profuse watery diarrhoea. On arrival: temperature 38.2 °C, pulse 112/min regular, blood pressure 96/60 mmHg lying and 84/55 mmHg sitting, respirations 22/min, weight 67 kg (his usual weight is 70 kg). He has passed only 40 mL of dark urine in the last 4 hours. He is drowsy but oriented, with dry mucous membranes and cool hands.
Results: haematocrit 48% (42% recorded a year ago), sodium 134 mmol/L, potassium 3.1 mmol/L, creatinine 1.5 mg/dL (baseline 0.9), urine specific gravity 1.030, urine sodium 12 mmol/L.
He takes no diuretics and has no known kidney disease.
1. Using the haematocrit, the urine sodium and the urine specific gravity together, what route of fluid loss is most likely, and approximately what fraction of his plasma volume has been lost? State the limits of the estimate.
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Route of loss. Urine sodium of 12 mmol/L with a specific gravity of 1.030 shows the kidney is conserving sodium and water avidly. A renal cause of loss (diuretic excess, tubular injury, adrenal problem) would typically give urine sodium above 40 mmol/L. Avid conservation with a concentrated urine therefore points to an extrarenal (gastrointestinal) loss that the kidney is trying to defend against - consistent with the diarrhoea history.
Plasma fraction. Red cell mass is assumed unchanged over days. Plasma occupies (100 − Hct) of blood volume, so:
- old plasma fraction = 1 − 0.42 = 0.58
- new plasma fraction = 1 − 0.48 = 0.52
- plasma volume ratio = (0.52 / 0.48) ÷ (0.58 / 0.42) = 1.083 ÷ 1.381 ≈ 0.78
So about 22% of plasma volume is missing. With plasma volume near 5% of body weight (≈ 3.3 L), that is roughly 0.7 L. The 3 kg weight loss is about 3 L total, so most of the deficit is interstitial rather than intravascular.
Limits. Haematocrit is a crude index: sepsis, capillary leak, a pre-existing anaemia or the timing of the sample can all move it independently of volume, and a single value cannot separate red cell change from plasma change. The figure is an estimate of direction and order of magnitude, not a measurement.
2. Classify the deficit as mild, moderate or severe, and identify which of his findings is the earliest reliable compensation and which is a late or lagging sign.
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Classification: moderate. Three lines of evidence agree:
- Weight loss 3 kg of 70 kg = 4.3% of body weight.
- Postural fall in systolic pressure of 12 mmHg (96 → 84) with a pulse of 112 - orthostatic change plus tachycardia is the classic moderate-deficit picture.
- Urine output 40 mL in 4 h = 10 mL/h, against a target of about 0.5 mL/kg/h ≈ 33 mL/h - established oliguria.
A mild deficit (< 5%) usually produces thirst and concentrated urine with a normal supine blood pressure; severe (> 10%) produces supine hypotension, marked obtundation and unrecordable urine output. He sits between the two, closer to moderate.
Earliest compensation. Baroreceptor-driven tachycardia and peripheral vasoconstriction - hence the cool hands and the postural drop before any fall in lying blood pressure. Lagging signs are oliguria (the kidney only shuts down after several hours of reduced perfusion) and the rising creatinine and haematocrit. Weight is the single most reliable measure available here, because it is not confounded by vasomotor compensation.
3. He weighs 67 kg. Set out the arithmetic for a 24-hour fluid plan covering maintenance plus the estimated deficit, and give one physiological reason not to give the whole deficit as one rapid bolus.
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Maintenance (4-2-1 rule).
- first 10 kg × 4 = 40 mL/h
- next 10 kg × 2 = 20 mL/h
- remaining 47 kg × 1 = 47 mL/h
- total ≈ 107 mL/h ≈ 2.6 L/day
Deficit. A moderate deficit by weight is roughly 5–8% of 70 kg = 3.5–5.5 L. Because he is still losing fluid and is febrile, ongoing losses of perhaps 0.5–1 L/day must be added, not ignored.
Plan arithmetic. For example, replace half the deficit in the first 8 h and the remainder over the next 16 h, on top of maintenance and ongoing losses:
- hours 0–8: 107 mL/h + (0.5 × 4.5 L)/8 h ≈ 107 + 280 ≈ 387 mL/h
- hours 8–24: 107 mL/h + (2.25 L)/16 h ≈ 107 + 140 ≈ 247 mL/h
Why not the whole deficit at once. Rapid replacement of a largely interstitial deficit expands the interstitial and intravascular compartments together; in an older, febrile, mildly drowsy patient this risks pulmonary oedema, a dilutional fall in haemoglobin and sodium, and a precipitous potassium shift. His potassium is already 3.1 mmol/L and his creatinine raised, so rate and composition must be reassessed after each bolus rather than fixed in advance.