Invasive Hemodynamic Monitoring: Troubleshooting A-Lines
Background:
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Pressure = Force applied over a Cross-Sectional Area
Why measure BP? Brain, Heart, Kidney, Gut…
NIBP: Oscillometric systolic blood pressure is recorded at the point where cuff pressure oscillations begin to
increase, mean pressure corresponds to the point of maximal oscillations, and diastolic pressure is measured at the
point where the oscillations become attenuated. Systolic and diastolic values are less reliable than MAP values.
Systolic pressure is typically identified as the pressure at which pulsations are increasing and are at 25% to 50% of
maximum. Diastolic pressure is the most unreliable oscillometric measurement and is commonly recorded when the
pulse amplitude has declined to a small fraction of its peak value.
Complications: Pain, Limb edema, Petechiae and ecchymoses, Venous stasis and thrombophlebitis, Peripheral
neuropathy, Compartment syndrome
“Caution should be exercised when using these monitors in patients with depressed consciousness, preexisting
peripheral neuropathies, arterial or venous insufficiency, or irregular cardiac rhythms, as well as in those receiving
anticoagulant or thrombolytic therapy.”
Arterial Pressure Monitoring
Indications:
-Continuous, real-time blood pressure monitoring
-Planned pharmacologic or mechanical cardiovascular manipulation
-Repeated blood sampling
-Determination of volume responsiveness from systolic pressure or pulse -pressure
variation
-Supplementary diagnostic information from the arterial waveform
-Failure of indirect arterial blood pressure measurement
Complications:
-Distal ischemia, pseudoaneurysm, arteriovenous fistula
-Hemorrhage, hematoma
-Arterial embolization
-Local infection, sepsis
-Peripheral neuropathy
-Misuse of equipment
What pressure does the system read? A, B: 20. C: 0. D: 40 cm H2O
-Misinterpretation of data
Zeroing, Leveling, and Calibrating
-Zero = subtracting out atmospheric pressure (so 120/80 not 920/860)
-Level = Midpoint of Right Atrium = preload/venous return of the heart = 5 cm vertically below sternal angle (in ANY position up to 60°!)
Natural Frequency, Damping Coefficient, and Dynamic Response of Pressure Monitoring Systems
- Optimize by short lengths of stiff pressure tubing and limiting the number of stopcocks
- Higher-frequency sine waves (and more harmonics) are needed to provide faithful reconstruction
of the original pressure waveform (intrinsic characteristics of the system)
Overdampening
NORMAL
Underdampening = Overshoot/Ringing/Resonance/Whip
-Displays a falsely narrowed pulse pressure; MAP may be OK
-Caused by friction, air bubbles in the transducer or tubing, clotting, loose connection, catheter against vessel wall
-Prevention: keeping pressure bag inflated (prevents clotting of catheter)
-Treatment: Aspirate, Flush, Flick, Fix (tighten tubing/align stopcocks/pull back catheter); replace transducer/tubing
Tidbits:
-Rapid Flush Test / Fast Flush – less than 3 oscillations indicates adequate dampening
-Can always quickly recheck zero (no drift when reopening stopcock to air)
-When removing an a-line: possibly decrease embolization risk by occluding proximally and distally and aspirating while d/c-ing
-“Burp the bag!” = removing all air from flush bag when setting up tubing (ensure pt does not receive an air bolus)
-Allen’s Test is not accurate
-Pulse pressure variation is predictive of volume responsiveness
-In Contrast to NIBP, A-lines measure SBP and DBP but calculate the MAP
-MAP = DBP + 1/3 (SBP-DBP) although at faster HRs, less time is spent in diastole…
-21% of patients in peripheral vascular surgery had a blood pressure difference between the two arms that exceeded 20 mm Hg
- During hypothermia, thermoregulatory vasoconstriction causes radial artery systolic pressure to exceed femoral artery pressure
◄- Lateral Decubitus Position
MILLER Anesthesia, Chapter 40