Continuous central venous saturation monitoring in critically ill patients

Introduction

Central venous oxygen saturation (ScvO₂) is a useful therapeutic target in septic shock. ScvO₂ is an indirect index of the balance between oxygen supply and demand, thus in critically ill patients a fall in ScvO₂ reflects a decrease in tissue oxygenation. ScvO₂ depends on arterial oxygen saturation, oxygen consumption, cardiac output and hemoglobin. The aim of the study was to evaluate events of tissue oxygenation impairment that could be unrecognized by simple blood gas analysis, by continuously monitoring ScvO₂ and to establish whether peripheral oxygen saturation (SpO₂), mean arterial pressure (MAP), heart rate (HR), and central venous pressure (CVP) could predict LowScvO₂ events.

Methods

Ventilated critically ill patients requiring a central venous catheter (CVC) for clinical use were enrolled. Continuous ScvO₂ monitoring was obtained by a fiberoptic sensor inserted in the CVC and recorded for 72 hours with SpO₂, HR, MAP and CVP. LowScvO₂ events were defined as ScvO₂ < 65% maintained for at least 5 minutes.

Results

Thirty-seven patients (24 males) were enrolled. The mean clinical characteristics at admission to intensive care were: age 59 ± 16 years, BMI 26.1 ± 4.5 kg/m², SAPS II 40 ± 13 (on 33 patients), PaO₂/FiO₂ 206 ± 79, MAP 80 ± 13 mmHg, HR 92 ± 21 bpm, CVP 12 ± 3 mmHg, Hb 10.6 ± 1.9 g/dl. Continuous monitoring analysis detected 147 LowScvO₂ events in 15 patients; while central venous blood gas analysis identified only nine LowScvO₂ events in eight patients (6%). Table 1 summarizes patients' variables according to three ScvO₂ ranges. SpO₂, HR, MAP and CVP were not correlated with LowScvO₂ events. Most patients had long periods of ScvO₂ > 75 (supranormal ScvO₂).

Table 1 Patients' variables according to ScvO₂ range

Conclusions

Continuous ScvO₂ monitoring showed that most events of poor tissue oxygenation are relatively common, are not recognized by extemporary central venous blood gas analysis and are not mirrored by changes in SpO₂, HR, MAP or CVP.