Q: Gastric-arterial pCO2 gradient continues to remain a controversial parameter to measure in septic shock. Given, if it is use in clinical practice, what is the cutoff level to designate patient at high risk of dying from septic shock? 

 Answer: 15 mm Hg

Gastric-arterial pCO2 gradient has been suggested as a prognostic marker in patients with septic shock, as it may measure the end organ ischemia. Gastric PCO2 is measured by gastric tonometry, which indirectly measures the perfusion to the gut. Theoretically,  trending gastric to arterial PCO2 gap should reflect the end organ ischemia, but evidence based literature failed to show its clinical relevance. 

On similar line, attempts have been made to calculate and trend  gastric PCO2-(End-tidal) PCO2 gap, but so far there is no real success.

 
Reference: 

1. Gutierrez G, Palizas F, Doglio G, et al. Gastric intramucosal pH as a therapeutic index of tissue oxygenation in critically ill patients. Lancet 1992; 339:195. 

2. Uusaro A, Russell JA, Walley KR, Takala J - Gastric-arterial PCO2 gradient does not reflect systemic and splanchnic hemodynamics or oxygen transport after cardiac surgery - Shock. 2000 Jul;14(1):13-7.