A 53-year-old woman is postbleed day 8 from a subarachnoid hemorrhage (SAH) from a, now secured, right middle cerebral artery (MCA) aneurysm. Since admission, the patient has been closely watched and the data from her external ventricular drain (EVD), brain tissue oxygenation monitor, and microdialysis catheter are all monitored. Recordings from the previous day shows PbtO2 (partial pressure of brain tissue O2 ) to be consistently greater than 25 and lactate/pyruvate (L/P) ratio less than 35, while her most recent readings from this morning are noted in the table that follows:
What is the best next step in management to reduce the patient’s risk of delayed cerebral ischemia?a. Continue to maintain increased cerebral perfusion pressure (CPP)
Correct Answer: A
All SAH patients are at risk for delayed cerebral ischemia and cerebral infarction during the acute and subacute stages of their disease. Some centers use multimodality monitoring for evaluation of delayed cerebral ischemia as well as evaluation of interventions attempting to prevent or treat this disease. This particular patient has a brain tissue sensor which measures the partial pressure of oxygen in the brain interstitial cortical tissue. PbtO2 is a balance between oxygen delivery and oxygen consumption in brain cells. It can be affected by a number of parameters, such as cerebral metabolism, cerebral blood flow, sedation, low inspired oxygen, ICP and CPP changes, and other traumatic changes in the cellular environment. The microdialysis catheter is a semipermeable membrane that allows diffusion of water and solutes down the concentration gradient, and is used to measure the concentrations of these solutes. This particular example has both glucose, lactate, and pyruvate as markers of cerebral metabolism. During times of oligemia, glucose level will drop and lactate levels will rise, marking a shift to anaerobic metabolism.
In this scenario, PbtO2 and lactate-pyruvate ratio (LPR) were within goal the day before and changed significantly this morning to show low brain oxygenation and increased LPR as shown in the table. The goal with PbtO2 monitoring is to assess which change will have the greatest effect on PbtO2 trends. In this case, increasing CPPs (as shown in the table) reversed the changes in PbtO2 and LPR (this morning) and improved brain oxygenation (PbtO2) and metabolism.