Normal Maintenance Needs

Water. Regardless of the disease process, water and electrolyte losses occur in urine and as evaporative losses from skin and lungs. It is evident from Figure 39.3 that a normothermic 70 kg patient with a normal metabolic rate may lose 2500 ml of water per day. Allowing for a gain of 400 ml from water of metabolism, this hypothetical patient needs 2000 ml day-1 of water. As a rule of thumb, a volume of 30-35 ml kg-1 day-1 of water is a useful estimate for daily maintenance needs.

Sodium. The normal requirement is 1 mmol kg-1 day-1 (50-80 mmol day-1) for adults.

Potassium. The normal requirement is 1 mmol kg-1 day-1 (50-80 mmol day-1) for adults.

Table 39.1 Electrolyte contents of commonly used intravenous fluids

Solution

Electrolyte content (mmol L-l)

Osmolality {mosmol kg-l)

Saline 0.9%

('normal saline')

Na+ 154

cr

154

('half-normal saline')

Na+ 77

cr

77

154

G lac ose 4%/saline 0.18% (glucose-saline)

NV 31

cr

31

284

Glucose 5%

Nil

278

Compound sodium lactate (Hartmann's solution)

Na" 131

lactate)

281

Thus, a 70 kg patient requires daily provision of2000-2500 ml of water and approximately 70 mmol each of Na+ and K+. This could be administered as one of the following:

• 2500 ml of glucose 4%/saline 0.18%; plus potassium as KC1, 1 g (13 mmol) added to each 500 ml of fluid.

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