Veno-arterial carbon dioxide and pH gradients and survival in critical illness

Abstract. Recent studies have shown that the veno-arterial gradient for carbon dioxide (dVVApCO₂) is increased in the case of low cardiac index (CI). In order to further investigate this matter 220 haemody-namic measurements from 34 patients with septic shock and from 28 patients in the post-operative state without sepsis were evaluated. The arterio-venous gradient for pH (dAVVpH) was also evaluated. CI was found to be inversely correlated to both dVVApCO₂ and dAVVpH when the two groups of patients were analysed separately ( r = -0.76 and r = -0.78, P < 0.001 for the relationship between CI and dVVApCO₂, r = -0.58 and r = -0.69 P <0.001 for the relationship between CI and dAVVpH). When volume loading over 2 h was used to increase CI, 58–66% of_the patients in the two groups showed a decline in dVVApCO₂ > 10% when CI was increased by > 10%. The corresponding values for a reduction in dAVVpH> 10% during volume loading was 36–52%. While dAVVpH was found to be increased in non-survivors ( n = 27) when compared to survivors (0.32 ± 0.01 vs. 0.24±0.1, P <0.05) at the second day in the IUC, dVVApCO₂ was not a significant predictor of mortality despite the fact that CI was found to be decreased in the non-survivors (3.5 ±0.94 vs. 4.3±1.0 L min^-1 m^-2, P <0.01). In conclusion, the veno-arterial carbon dioxide gradient was found to be inversely correlated to cardiac performance in patients both in patients with septic shock and in non-septic post-operative patients. The arterio-venous gradient for pH was increased in low flow states, but this association was weaker. However of these two indices evaluated, only a raised dAVVpH was a significant predictor of later mortality.