Review Question - QID 100101

An increase in blood lactate is one of the primary laboratory findings in shock of any type.

Shock has multiple etiologies, but as a unifying principle the defining feature of shock is an inability to provide metabolic substrate (oxygen) at a rate that meets metabolic demand. Inadequate oxygen delivery to cells impairs the cells' ability to continue oxidative phosphorylation. In the absence of oxidative phosphorylation but continuing--and in some cases increased--cellular metabolic demand, anaerobic respiration is employed to mitigate the ATP deficit producing lactate. As NAD+ is needed in order for further glycolysis to proceed to generate more ATP, NADH must be oxidized. The production of lactate from pyruvate accomplishes this oxidation, thus allowing glycolysis to proceed unabated. The subsequent increase in lactate can be detected in blood and is a reliable marker of the decrease in oxidative phosphorylation seen in shock.

Mizock and Falk report that the severity of lactic acidosis in critically ill patients correlates with overall oxygen debt and survival. Indeed, the trending of lactate levels can be used as an ongoing monitor of perfusion while resuscitative efforts to treat the shock are carried out.

Levy reviews the literature and reports that in the special case of septic shock, relative hypoxia may be an exacerbatory, but not the primary, cause of hyperlactatemia. Recent cellular studies have discovered that lactate increases even in oxygenated tissues if the metabolic demands of the cell are above normal. It is the exaggerated throughput of aerobic glycolysis which stimulates sarcolemmal Na/K ATPases that greatly increase lactate production.

Illustration A depicts anaerobic respiration. Pyruvate generates the NAD+ necessary to continue gylcolysis by accepting electrons from NADH and in the process becomes lactate.

Incorrect Answers:
Answer 1: The accumulation of venous CO2 and lactic acid both decrease--not increase--blood pH.
Answer 2: Increased production of lactic acid creates a metabolic acidosis which decreases--not increases--blood bicarbonate levels.
Answer 3: Ketones increase in the blood due to fatty acid oxidation in response to low blood glucose, not low oxygen levels. Anaerobic respiration, like aerobic respiration utilizes glucose. Fatty acid oxidation requires oxygen and would thus be expected to decrease in states of relative hypoxia.
Answer 4: Total hemoglobin would not be expected to decrease in acute shock, even when do to blood loss, as whole blood is lost and the blood volume has been neither endogenously or exogenously repleted. In fact, in hypovolemic shock, hemoglobin concentration may even be increased.