The high ratio of "central venous to arterial carbon dioxide" to "arterial to central venous oxygen content " is associated with elevated lactate in patients with septic shock.

So, the aim of the present study is to evaluate the ratio of "central venous-to-arterial CO2 tension or content" to "arterial-to-venous O2 content'' as an indicator of anaerobic metabolism in septic shock.

This is a prospective single center observational study that will be conducted in adult intensive care unit of Ain Shams University After being approved by local institutional ethic committee (Ain Shams University, Egypt). Informed consent was obtained from each participant's next of kin. The investigator will study mechanically ventilated participants. the attending physician's decision is to give IV fluids to participants due to the presence of at least one clinical sign of inadequate tissue perfusion due to septic shock : (a) systolic arterial pressure <90 mmHg, mean arterial pressure <65 mmHg, or the need for vasopressor infusion; (b) skin mottling; (c) lactate level >2 mmo/L; or (d) urinary output <0.5 mL/kg/h for ≥2 h. Jugular central venous and arterial catheters will be in place by the attending physician. Investigator will confirm the position of the tip of the central venous catheter in the superior vena cava or upper part of the right atrium on chest radiograph. Participants will be monitored by Flo Trac device as a part of routine management in local ICU.

A first set of measurements will be performed, including hemodynamic and tissue oxygenation variables (heart rate, mean arterial pressure, cardiac index (CI), oxygen delivery (DO2), oxygen consumption (VO2), Central venous oxygen saturation (ScvO2), arterial lactic acid level, central venous to arterial carbon dioxide tension difference (∆PCO2), central venous to arterial CO2 content difference (∆ContCO2), arterial-to-central venous oxygen content difference (∆ContO2), ∆ContCO2/∆ContO2 ratio and ∆PCO2/∆ContO2 ratio. 500 ml of isotonic saline 0.9% will be infused to the participants via a specific venous line over 15 minutes. Immediately after volume expansion, a second set of the previous measurements will be recorded. Ventilation parameters, Norepinephrine dose and sedation drugs will be kept constant during the fluid challenge.

After fluid challenge, Participants with an increase in cardiac index ≥15% will be defined as fluid responders. Fluid responders will be divided into 2 groups based on increase in oxygen consumption (VO2) ( < or ≥15%).

Inclusion Criteria:

1. Age >18 years.
2. Suspected infection focus
3. Mean blood pressure <65 mmHg, or required vasopressor infusion.
4. Lactic acid level >2 mmol/l.
5. Mechanically ventilated patients.

Exclusion Criteria:

1. Age equal or less than 18 years.
2. Pregnant women.
3. Liver cirrhosis Child-Pugh C.
4. Chronic obstructive pulmonary disease.
5. Previous episode of septic shock within the last 3 months.
6. Patients with a contraindication to volume expansion.

Critically ill patients who will be admitted to intensive care units and eligible for the study.