Abstract
The main goal with arterial or venous gas measurement is to ensure adequate cerebral blood flow, thereby decreasing the incidence of cerebral ischemia (Eastwood et al., 2015). A mixed venous blood gas sample helps the team determine the amount of oxygen left in the blood before the blood undergoing reoxygenation. Basically, we are able to determine whether there is enough oxygen in the blood for every organ or whether an organ has a higher oxygen demand than expected.
Very little literature has been published in the past 5 years looking at the effect of therapeutic hypothermia on mixed venous gas. During hypothermia, the cerebral metabolic demand decreases that will decrease oxygen consumption. One could assume that the mixed venous oxygen saturation (SvO2) would be higher during hypothermia because there is less consumption from the brain. When rewarming occurs, the patient's cerebral oxygen consumption will start to increase, thereby decreasing the SvO2. A decrease in mixed venous oxygenation indicates a possible mismatch in oxygen supply and demand (Lindholm et al., 2002). Hu et al. published a pediatric study in 2016 looking at the correlation between oxygen saturation, jugular bulb oxygenation, and mixed venous gas oxygenation during cardiopulmonary bypass. What they found was that the SvO2 always read higher than the jugular bulb oxygenation and had a positive correlation with jugular bulb oxygenation and temperature (Hu et al., 2016).
Lindholm et al. (2002) looked at the correlation between mixed venous gas and regional oxygen levels with a focus on the hepatic system. He discovered that the mixed venous gas was showing a normal-to-high oxygenation level; however, when samples were taken directly from the hepatic system, the oxygen level was much lower.
What are the clinical implications of this finding? As the patient's temperature decreases, so will his or her SvO2; conversely, as the patient is rewarmed, the SvO2 will increase but this does not mean the cerebral oxygenation is increasing. Monitoring cerebral oxygenation directly during rewarming should take place to avoid hypoxic injury.