Assessment of a handy-type blood gas analyzer for measuring intramucosal pH (pHi)

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Introduction

pHi is important in the evaluation of critically ill patients. When pHi data is needed quickly, a handy-type blood gas analyzer may be useful but it must be accurate. We therefore assessed the accuracy of a new handy-type blood gas analyzer (Opti, AVL, Switzerland; hereafter called V-O) in measuring pHi by comparing it with three other analyzers: (1) ABL520 (B-5), (2) ABL300 (B-3), (both from Radiometer, Holland), and (3) Compact 2 (AVL; V-C).

Methods

We infused normal saline into the balloon of a gastrointestinal tonometer (Instrumentarium Corp, Finland) located in the patient's stomach. After a 60 min equilibration time, the saline was withdrawn and the CO₂ (PrCO₂) measured by the four gas analyzers (the analysis of each sample of normal saline by the four analyzers were completed within 5 min). At the same time, we withdrew and analyzed arterial blood. Then, pHi was calculated from the Henderson-Hasselbalch equation, and the CO₂ gap (PrCO₂–PaCO₂) was also calculated.

Results

Twenty-five pHi and CO₂ gap values were obtained for each machine. The CO₂ gaps for the various machines were as follows: B-5: +4.5± 2.6 (mean ± SD: mmHg); B-3: +1.5± 3.1; V-C: +2.9± 2.3; and V-O: -13.2± 4.1. The calculated pHi values were 7.336± 0.056 (mean ± SD), 7.332± 0.051; 7.330± 0.051, and 7.542± 0.089, respectively. With respect to the (B-5) results for CO₂ gap, the values for correlation coefficient (r), precision (P) and bias (B) were as follows: (B-3) r, 0.900; P, 1.3; B, -3.1; (V-C) r, 0.911; P, 1.4; B, -1.5; and (V-O) r, 0.790; P, 2.9; B, -18.3. A statistically significant relationship with B-5 was observed for each of these three groups, but the discrepancy between V-O and B-5 was large.

Discussion

We can now use blood gas analyzers at the bedside and get results quickly. However, the value of pHi obtained differs according to the kind of analyzer used, a point that needs to be kept in mind. Our important finding is that the data obtained with the handy analyzer tested here differed quite considerably from those obtained using the other three. We conclude that although the new handy analyzer is convenient, it does not allow accurate calculations of pHi and CO₂ gap.