Objective We studied the transport of oxygen in patients with hyperlactatemia (lactate over 2 mmol/L) and lactic acidosis (lactate over 5 mmol/L) in the immediate postoperative period after cardiac surgery with CPB.
Methods The study included 105 patients after CABG with CPB. All patients had blood lactate levels increase of more than 2 mmol/L. We measured oxygen delivery index (DO2I) at the time when the maximal lactate levels are recorded. The patients were divided into 2 groups: group 1 – patients with DO2I less than 300 ml/min/m2 (n = 8), group 2 – the patients with oxygen delivery more than 600 ml/min/m2 (n = 7). We supported parameters at all patients: the haemoglobin more than 90 g/l, SaO2 was higher than 96%. We examined parameters of oxygen delivery, oxygen consumption, acid-base status in the early postoperative period. Quantitative data were presented as median (25th–75th percentile).
Results The cardiac index was 2.10 ± 0.16 l/min/m2 in 1th group and 4.33 ± 0.58 l/min/m2 in the 2nd group (P < 0.05). Patients in the 2nd group had low systemic vascular resistance (SVR): 642.3 ± 180.4 dyn s/cm5, and in group 1 – 1508 ± 312.4 dyn s/cm5 (p = 0.0048). In the first group of patients oxygen extraction ratio was found 26.3 ± 5.2%, in the second one it was 17.4 ± 9.9% (p = 0.044). Hemoglobin oxygen saturation in the mixed venous blood (SvO2) was 72.9 ± 4.8% and 81.9 ± 9.8% (p = 0.038) in the two groups respectively. Unexpectedly we found significantly more severe lactic acidosis was in the second group. The blood lactate levels were 5.44 ± 2.97 mmol/L in the first group and 9.14 ± 2.38 mmol/L in the 2nd group (P < 0.05). Blood lactate over 5 mmol/L was 4/50% in the first group and 7/100% in the second group (P < 0.05). Metabolic acidosis (p < 7.35) was 2/25% in the 1st group and 6/85.7% in the 2nd group (P < 0.05).
Conclusions Lactate can increase at any level of oxygen delivery after cardiac surgery with CPB. Lactic acidosis developed not always, but it developed in all the patients with the high oxygen delivery and the decreased oxygen extraction. This condition is accompanied by severe metabolic acidosis, low oxygen extraction and the venous hyperoxia, decreased SVR. These are the signs of tissue hypoxia, which may be associated with a mitochondrial distress syndrome the same as in the SIRS.