Relationship of relevant factors to P(v-a)CO(2)/C(a-v)O(2) ratio in critically ill patients

OBJECTIVE: This study investigated the factors related to the ratio of the venoarterial carbon dioxide tension difference [P(v-a)CO(2)] to the arteriovenous oxygen content difference [C(a-v)O(2)] (hereafter termed “Ratio”). METHODS: We retrospectively studied 1294 pairs of arterial and central venous blood gas measurements in 352 critically ill patients. A high Ratio was defined as > 1.68 based on published literature. Measurements were divided into four groups: Group I [P(v-a)CO(2) ≤ 6 mmHg/central venous oxygen saturation (ScvO(2)) < 70%], Group II [P(v-a)CO(2) ≤ 6 mmHg/ScvO(2) ≥ 70%], Group III [P(v-a)CO(2) > 6 mmHg/ScvO(2) ≥ 70%], and Group IV [P(v-a)CO(2) > 6 mmHg/ScvO(2) < 70%]. RESULTS: The Ratio’s strongest correlation was with P(v-a)CO(2) when compared with ScvO(2) and hemoglobin in all data. The P(v-a)CO(2) and ScvO(2) were significantly higher and the hemoglobin and arterial oxygen saturation were significantly lower in the high Ratio measurements (>1.68) than low Ratio measurements (≤1.68). The P(v-a)CO(2) was best for predicting a high Ratio. A P(v-a)CO(2) threshold of 7 mmHg was associated with a sensitivity of 41.77% and specificity of 90.62% for predicting a high Ratio. CONCLUSIONS: A high P(v-a)CO(2) is the most relevant contributor to a high Ratio among all related factors in critically ill patients.