How can CO2-derived indices guide resuscitation in critically ill patients?

Abstract

Assessing the adequacy of oxygen delivery with oxygen requirements is one of the key-goal of haemodynamic resuscitation. Clinical examination, lactate and central or mixed venous oxygen saturation (S_vO₂ and S_cvO₂, respectively) all have their limitations. Many of them may be overcome by the use of the carbon dioxide (CO₂)-derived variables. The venoarterial difference in CO₂ tension ("ΔPCO₂" or "PCO₂ gap") is not an indicator of anaerobic metabolism since it is influenced by the oxygen consumption. By contrast, it reliably indicates whether blood flow is sufficient to carry CO₂ from the peripheral tissue to the lungs in view of its clearance: it, thus, reflects the adequacy of cardiac output with the metabolic condition. The ratio of the PCO₂ gap with the arteriovenous difference of oxygen content (PCO₂ gap/C_a-vO₂) might be a marker of anaerobiosis. Conversely to S_vO₂ and S_cvO₂, it remains interpretable if the oxygen extraction is impaired as it is in case of sepsis. Compared to lactate, it has the main advantage to change without delay and to provide a real-time monitoring of tissue hypoxia.

Keywords: PCO2 gap; cardiac output; lactate; respiratory quotient; tissue hypoxia.

Figure 1

Relationship between content (CCO₂) and partial pressure (PCO₂) of carbon dioxide.

Figure 2

Illustration of the influence of cardiac output on the amplitude of the venoarterial difference of carbon dioxide partial pressure. P_aCO₂, arterial partial pressure in carbon dioxide; P_vCO₂, venous partial pressure in carbon dioxide; Qc, cardiac output; ∆PCO₂, venoarterial difference of carbon dioxide partial pressure.

Figure 3

Interpretation of indices of tissue oxygenation. Hb, haemoglobin; S_vO₂, venous oxygen saturation; S_aO₂, arterial oxygen saturation; C_a-vO₂, arteriovenous difference in oxygen content; ∆PCO₂, venoarterial difference in carbon dioxide partial pressure.

Figure 4

Estimation of the respiratory quotient from the ratio between venoarterial difference in carbon dioxide partial pressure and arteriovenous difference in oxygen content. C_a-vO₂, arteriovenous difference in oxygen content; CO₂, carbon dioxide; C_v-aCO₂, venoarterial difference in carbon dioxide content; P_v-aCO₂, venoarterial difference in carbon dioxide partial pressure.