One required examination for declaration of death by neurological criteria is the apnea challenge test. The apnea challenge test evaluates the medulla brainstem respiratory center response to a rise in CO2. The current guideline requires arterial sampling of CO2 and therefore either arterial puncture or arterial catheter placement for the examination. Arterial catheter is an invasive monitoring device and is not without complications. Recent studies have demonstrated that there is a direct correlation between peripheral venous and arterial blood gas measures and that peripheral venous CO2 measures may be used as alternatives to arterial CO2 measure.The purpose of this study is to evaluate brainstem response during the apnea challenge test to a rise in venous CO2 and correlate it with the rise in arterial measured CO2. The objective of this study is to demonstrate a similar rise between the venous and arterial CO2 during the apnea challenge test and eliminate the necessity of arterial blood sampling for the sole purpose of apnea challenge test in the future.