Question from resident
Hello, A quick question. I usually level the transducer for an arterial line to the heart. Usually this is also about the position of the brain based on the supine or prone position. Yesterday I did a long case where towards the end they wanted the patient sat up. Previously MAPs had been mid 70’s, UOP 1-1.5 cc/kg/hr. I moved the transducer up to heart level with pt sitting up, and MAPs were low 70’s. 5 minutes later my attending came in the room and moved the transducer at the level of the head, at which time the patient’s MAP was measured as 58-60 which lead to a different perception of what adequate pressure in a patient is. Clearly, a good pressure does not automaticaly mean good flow. I also know the conversion is about 1 mmhg for every 1.4 cm H20. I have had a similar discussion for beach chair position for ortho, but have never done a sitting neurosurg case. I will read about this, but could you give me a broader perspective on your practice.
Nobody really knows the definitive answer to your good question about where to place the transducer when the patient is not flat, but the issue revolves around whether the circulation above the heart functions as a siphon system or as a waterfall system. The two best sources, besides Dr. Jaffe our esteemed neurosurgical anesthesia faculty, that provide a balanced analysis are:
Until actual data of some sort sort all this out, my own practice is to measure BP at the level of the most vulnerable tissue–the brain and then aim to maintain MAP within the patients unanesthetized preop BP.