Hemodynamic Monitoring: Principles to Practice – M. L. Cheatham ...

Hemodynamic Monitoring: Principles to Practice – M. L. Cheatham, MD, FACS, FCCMMEASURING PRESSURE VARIABLES• The hydraulic system is much more subject topotential errors and artifacts than is the electronicsystem– Learning to troubleshoot the hydraulic portion of ainvasive pressure monitoring system is essential» Confirms the validity of the data» Helps avoid inappropriate therapeuticinterventions based upon erroneous dataHYDRAULIC SYSTEM COMPONENTS• Intravascular catheter– Tubing to access the desired blood vessel orcompartment– Detects the pressure waves generated by cardiaccontraction• Connecting tubing and stopcocks– Transmits pressure wave from patient tomeasuring device– Allows control of blood vessel to avoidhemorrhage, introduction of air, etc…PRESSURE TRANSDUCER• Converts the mechanical impulse of a pressure waveinto an electrical signal through movement of a “straingauge”– Modern transducers utilize a displaceable siliconsensing diaphragm and a “Wheatstone Bridge”Transducer DomeSensing DiaphragmWHEATSTONE BRIDGE• An electrical circuit forcomparison of resistances• Consists of a power source andfour resistors, three of whichhave a known resistance• To determine the unknownresistance, the resistance of theother three are balanced until thecurrent passing through bothsides of the parallel “bridge”decreases to zeroPressure TubingPressure WaveWHEATSTONE BRIDGE• The Wheatstone Bridge is used to measure theresistance change in a strain gauge– The resistance change is proportional to thechanging physiologic variable (i.e. pressure) ormechanical strain applied to the transducer– “Zeroing” a transducer is simply determining thevalue of the unknown resistance at rest» “Balancing the bridge”– As the physiologic variable changes, theresistance varies proportionally, a current isinduced across the bridge, and the voltmetervalue is converted to a pressure measurementHYDRAULIC SYSTEM COMPONENTS• Continuous flush device– Flushes the tubing with fluidat a rate of 1-3 mL/hr andhelps prevent blood fromclotting off the catheter– The “fast flush” featureincreases the flow to 30mL/min and can be used totest the system’s complianceRevised 01/13/20092