Page 288 - Clinical Application of Mechanical Ventilation

254 Chapter 9

TABLE 9-6 Normal Serum Electrolytes

Cation Concentration (mEq/L) Anion Concentration (mEq/L)

Na 1 140 (138 to 142) Cl – 103 (101 to 105)

K 1 4 (3 to 5) HCO 3 - 25 (23 to 27)
Ca 11 5 (4.5 to 5.5) Protein 16 (14 to 18)

-
Mg 11 2 (1.5 to 2.5) HPO , H PO 4 - 2 (1.5 to 2.5)
2
4
SO 4 - 1 (0.8 to 1.2)
Organic acids 4 (3.5 to 4.5)

Total cations 151 Total anions 151
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Metabolic Acidosis and Anion Gap. Metabolic acidosis in the presence of a normal anion
Metabolic acidosis in the
presence of a normal anion gap is usually caused by a loss of base. This condition is called hyperchloremic metabolic
gap is usually caused by a loss acidosis because it is usually related to excessive chloride ions in the plasma.
of base.
Metabolic acidosis in the presence of an increased anion gap is usually due to
increased fixed acids. These acids may be produced biologically (e.g., renal failure,
diabetic ketoacidosis, lactic acidosis), or they may be added from an external source
Metabolic acidosis in
the presence of an increased (e.g., poisoning by salicylates and alcohol) (Chang, 2012).
anion gap is usually due to
increased fixed acids. Respiratory Compensation for Metabolic Acidosis. In mechanically ventilated patients
with metabolic acidosis, hyperventilation (TPaCO ) may occur as a compensa-
2
tory mechanism for metabolic acidosis. One should not assume that respiratory
insufficiency ( primary alveolar hyperventilation) is present. The cause of meta-
bolic acidosis must be identified and corrected. More importantly, the ventilator
frequency must not be reduced due to an abnormally low PaCO . Otherwise,
2
persistent hyperventilation and worsening of the work of breathing will continue
due to a sudden decrease of ventilator frequency (Rooth, 1974).
Metabolic Alkalosis. It is also important to monitor a patient’s potassium level dur-
+
Severe K depletion +
can lead to metabolic ing mechanical ventilation. Severe K depletion can lead to metabolic alkalosis and
alkalosis and compensatory compensatory hypoventilation (Adams et al., 1982). This may prolong the weaning
hypoventilation.
process when mechanical ventilation is needed to supplement the decreasing spon-
taneous ventilation.

ARTERIAL BLOOD GASES

Arterial blood gas (ABG) analysis provides useful information about a patient’s ven-
tilation (PaCO ), oxygenation (PaO ), and acid-base (pH) status. It is therefore an
2
2
essential monitoring tool for patients receiving mechanical ventilation as these patients
often have gas exchange and acid-base abnormalities. Table 9-7 shows the normal ABG

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