Page 287 - Clinical Application of Mechanical Ventilation

Monitoring in Mechanical Ventilation 253

FLUID BALANCE AND ANION GAP

Mechanical ventilation may affect a patient’s renal function and fluid balance. Since
fluid balance and electrolyte concentration are related, the anion gap may also be af-
anion gap: The difference
between cations (positive ions) fected as a result of positive pressure ventilation. Proper fluid and electrolyte mainte-
and anions (negative ions) in the nance should be an integral part of mechanical ventilation to prevent these adverse
plasma. The normal range is 15 to
1
20 when K is included in the outcomes.
calculation (10 to 14 mEq/L when
1
K is excluded).
Fluid Balance

Positive pressure ventilation reduces cardiac output and thus renal perfusion. Urine
output is decreased due to hypoperfusion of the kidneys. Mechanical ventilation
also reduces urine output as a result of an increase in antidiuretic hormone (ADH)
and a reduction of atrial natriuretic factor (ANF). The end result of these changes
Oliguria may be seen is decreased fluid output and fluid retention.
after bleeding, diarrhea, renal
failure, shock, drug poisoning, For these reasons, the fluid level of a ventilator patient must be monitored closely
deep coma, or hypertrophy of because positive pressure ventilation affects fluid balance (intake and output). Fluid
the prostate.
intake is recorded by adding all fluids received by the patient to include fluids
provided via the intravenous, oral, and nasogastric routes. Fluid output is commonly
oliguria: Below normal urine done by measuring the urine output. Oliguria indicates fluid deficiency and may
output.
occur as a result of decreased renal perfusion, decreased fluid intake, and decreased
cardiac output. Normal urine output is 50 to 60 mL/hour. Urine output of below
20 mL/hour (or 400 mL in a 24-hour period or 160 mL in 8 hours) is indicative of
Urine output of below
20 mL/hour (or 400 mL in a fluid deficiency (Kraus et al., 1993).
24-hour period or 160 mL in Reduction in cardiac output can be directly attributed to decreased venous re-
8 hours) is indicative of fluid
deficiency. turn secondary to positive pressure ventilation and increased intrathoracic pressure.
Positive pressure ventilation also causes an increase in the production of antidiuretic
hormone (ADH) which further reduces the urine output.
Normal urine output is
50 to 60 mL/hour.
Anion Gap

Table 9-6 shows a typical set of electrolyte parameters with their normal results.
Using some of these parameters, the anion gap may be calculated and used to assess
a patient’s overall electrolyte balance. Anion gap is the relationship of the cations
–
(sodium [Na ] and potassium [K ]) to the anions (chloride [Cl ] and bicarbonate
1
1
2
[HCO ]). The anion gap may be determined as follows:
3
+
Anion gap = Na - Cl - - HCO 3 -
Normal range: 10 to 14 mEq/L
or
-
See Appendix 1 for Anion gap = Na + + K + - Cl - - HCO
3
example.
Normal range: 15 to 20 mEq/L

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