Defining acidosis in postoperative cardiac patients using Stewart’s method of strong ion difference*

Objective: To define the true incidence and nature of acidosis
in pediatric patients postcardiac surgery, using Stewart’s direct
method of measuring strong ion difference. We also wished to
compare the ability of standard indirect methods (base deficit,
lactate, anion gap, and corrected anion gap) to accurately predict
tissue acidosis.
Design: A single-center prospective observational study.
Setting: A pediatric intensive care unit in a tertiary referral
Patients: Pediatric patients who had undergone cardiac surgery
were studied in the immediate postoperative period. Patients
who had undergone both open and closed cardiac surgery were
Interventions: Routine arterial blood gas analysis and laboratory
electrolyte measurements were made in patients immediately
on admission to the pediatric intensive care unit (PICU) after
cardiac surgery and each morning until discharge from the PICU.
Measurements and Main Results: Figge’s equations were used
to calculate strong ion difference and total tissue acids (unmeasured
acids and lactate). These direct methods then were compared
to indirect measurements: base deficit, lactate anion gap,
and anion gap corrected for albumin. We collected 150 samples
from 44 patients. Tissue acidosis occurred overall in 60 of 150
samples. This was due to raised unmeasured acids alone in 44 of
60 (73.3%), raised lactate alone in six of 60 (10%), and a combination
of the two in ten of 60 (16.6%). Hyperchloremia occurred in
19 of 150 samples overall and 12 of 25 (48%) samples immediately
after cardiopulmonary bypass. Measured base deficit
showed a poor correlation with true tissue acidosis ( p
< .001) and the worst discriminatory ability (area under the
curve, 0.72; 0.62– 0.82). Anion gap corrected for albumin had the
best correlation ( p < .001) and highest area under the
curve (0.90; 0.85– 0.95).
Conclusions: Metabolic acidosis occurs frequently postcardiac
surgery and is largely due to raised unmeasured acids and less
commonly raised lactate. Hyperchloremia is common, particularly
after cardiopulmonary bypass. Base deficit correlates poorly with
true tissue acidosis, and corrected anion gap offers the most
accurate bedside alternative to Stewart’s method of tissue acid
calculation. (Pediatr Crit Care Med 2004; 5:240 –245)


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