Effect of minute ventilation on central venous-to-arterial carbon dioxide difference

Introduction

The central venous-to-arterial carbon dioxide difference (P(cv-a)CO₂, dPCO₂) is a global index of tissue perfusion. A normal dPCO₂ indicates cardiac output (CO) is high enough to wash out CO₂ production from peripheral tissues. An increased dPCO₂ suggests that CO is not high enough with respect to global metabolic conditions. PCO₂ depends on alveolar ventilation. We hypothesized that minute ventilation (MV) has an effect on dPCO₂.

Methods

A prospective experimental, pilot study was performed on 19 patients admitted to a medical ICU with septic shock between August 2010 and November 2010. All patients were intubated and on a mechanical ventilator with continuously monitoring end-tidal CO₂, central venous pressure (CVP), blood pressure (BP), and CO. Mechanical ventilator was set consecutively in three steps every 30 minutes (T0, T30, T60) by increasing the respiratory rate (RR) for MV of 8 l, 15 l, and 8 l, respectively. Tidal volume, RR, MV, auto-PEEP, CO and dPCO₂ were recorded at each step of MV changed for all patients.

Results

Patients' age and APACHE II scores were 67.3 ± 13.2 years and 24.4 ± 6.6, respectively. There was a significant difference between the dPCO₂ between T0 and T30 (3.5 ± 3.5 vs. 5.9 ± 2.0, P = 0.04) (Table 1). Moreover, there was significantly decreased CO from T0 to T30 (5.1 ± 1.4 vs. 4.5 ± 1.11, P = 0.002) and, also, T30 and T60 (4.5 ± 1.1 vs. 5.0 ± 1.3, P = 0.009). Auto-PEEP values were inversely correlated with decreased CO (P < 0.001) at T30.

Table 1

Conclusions

Minute ventilation had an effect on dPCO₂ by reduced CO due to development of auto-PEEP. The dPCO₂ should be measured during normal minute ventilation without auto-PEEP.