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Effect of minute ventilation on central venous-to-arterial carbon dioxide difference
Critical Carevolume 15, Article number: P44 (2011)
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.
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.
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.
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.
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