see also: [Cardiac output monitoring](bookends://sonnysoftware.com/ref/DL/55924) > Do note that if you don't have or lose your art line, the most accurate measurement of oscillometric BP measurement is the MAP. Unfortunately, the accuracy falls apart in patients with circulatory shock, which is the time you want to know the blood pressure. # Zeroing 1. turn the dial off to the patient, open to air 2. open cap to air 3. hit zero on the machine 4. turn dial back off to air this will zero pressure to atmospheric pressure, can occur at any height of transducer) # Physics ## transducer position - A column of water in an incompressible tube interfaces with the transducer - This transducer needs to be at the level of the heart - if it is too low below the person, the column of water pressing down would make blood pressure **incorrectly high** - if the transducer is too far above the heart, there is less pressure to transducer, making it record incorrectly low 10m H2O = 760 mmHg = 1atm ∴ 1mH2O = 76 mmHg - if the transducer were 1m too low (eg on the floor from a bed 3 feet high), BP would be incorrectly recorded as 76 mmHg higher than the patient's actual BP) - Every cm too low makes BP 0.76 too high (10 cm would make it 7.6 too high) ## Wheatstone's bridge Can review [Propofology - Wheatstone's Bridge](https://www.propofology.com/tutorials/resistor-circuit-analysis-wheatstones-bridge-frca-topic) article and video for a comprehensive explanation of the pressure transducer; this is an interesting subject that will never come up on an ED exam, but is a topic for the ICU and Anaesthetics exams. The gist is that a Wheatstone bridge is a special kind of electrical circuit that, in general, is used to measure an unknown electrical resistance. In an arterial line transducer, the "unknown" resistor in the wheatstone bridge is coupled to a pressure gauge attached to the column of water, such that as the pressure changes, the resistance changes, and the pressure is inferred from measuring the resistance. # Arterial wave form > even in an over-dampened or under-dampened arterial line, the MAP is relatively accurate, even though the values are not. ![[Pasted image 20241127184201.png]] ## arterial wave form and cardiac output ![[Pasted image 20241127174729.png]] ![[Pasted image 20241127174752.png]]