Know Your A-Lines (Part 2)

    NextPrevious

    Know Your A-Lines (Part 2)

    Once you understand the basic functions of the components of your arterial line set-up and know how to position it properly the next level of mastery involves spotting other errors and optimizing them.
    If you notice that your pulse pressure is very wide your patient may be severely volume depleted, they may have profound aortic insufficiency, or your arterial line may be lying to you.  Alternatively, a very narrow pulse pressure can relay cardiac tamponade or an error in measurement.  Here’s how, why, and how to fix it.
    Why:
    The fluid pressure wave the is transmitted to the arterial catheter and hydraulic tubing to displace the transducer and register as a blood pressure has a frequency which is expressed in cycles per unit time (beats per minute) and an amplitude of energy is expressed per beat until the wave reflects against the transducer.  At certain lengths the wave is reflected back upon itself such the peaks and troughs of the waves (heartbeats) align exactly.  This causes the amplitudes to be additive.  The shortest distance that provokes this is described as the first harmonic and can be determined by the inverse of the wave’s frequency. As the wave travels along your tubing it expresses force against the wall of the tubing and expends energy.  This causes the wave to “wind down” and is termed dampening.  The analogy to describe this process is dropping a ball onto the ground: each time the ball bounces it expends more energy and each bounce is smaller until it comes to rest.  How long it takes the ball to come to rest depends on the “hardness” of the ground, and this is termed the dampening constant.  In your arterial line set-up this hardness constant is derived from the elasticity of the patient’s vasculature and of the IV tubing and stopcock system you use to conduct the pressure wave.
     A figure demonstrating sign waves and harmonics.
    How:
    If the system is underdampened, harmonic waves develop and generate a blood pressure with a falsely extreme amplitudes (higher systolic and lower diastolic pressures than the patient is experiencing).  Underdampending can be caused by excessively long and stiff IV tubing.  You can detect underdampending on your waveform by looking for beats with a very rapid upstroke and a sharp, peaked appearance on your monitor.  If the system is overdampened then excess friction leads the pressure wave to expend its energy early and promotes falsely narrow pulse pressures.  Overdampending can be caused by bubbles within the tubing, kinks within the catheter, blood clots on or within the catheter, low volumes of fluid within the flush system, low pressure applied to the pressure bag.  The good news is that the MAP (described by the midpoint in the sine wave of the heartbeat) is unchanged by these artifacts.  However your systolic and diastolic blood pressures still yield clinical data and having gone through the trouble of placing an invasive monitoring system, you want them to be accurate.
    The fix:
    You can test to determine if your system is overdampened or underdampened by performing a square flush test.  Do this by opening the continuous flus hvalve to create a square wave and observing the subsequent oscillations.  If the system returns to baseline after one or two oscillations then the dampening of the system has been optimized.  If there are more than one or two oscillations then the system is underdampened.  If there are no oscillations then the system is overdampened.
    Tubing should be exactly 4 feet to promote the correct length.  Longer lengths of tubing can promote harmonics.
    Use dedicate arterial pressure tubing when possible to maintain the appropriate compliance (“hardness”) of the walls of the system.  Avoid large diameter tubing as this will lead to overdampening.
    Remove air bubbles by flushing the line (flush to air rather than to the patient) or by aspirating them.
    Tighten all connections.
    Remove any excessive stopcocks.
    Cheatham ML. Hemodynamic Monitoring: Principles to Practice.  http://www.surgicalcriticalcare.net/Lectures/PDF/hemodynamic%20monitoring%20principles%20to%20practice.pdf.  Updated 1/13/2009.  Accessed 5/2/2016.
    Nickson C.  Life in the Fast Lane.  http://lifeinthefastlane.com/ccc/arterial-line/. Updated 6/14/2015.  Accessed 5/3/2016.
    • Welcome! This is the website for the Mount Sinai Emergency Ultrasound Division. It serves as an information resource for residents, fellows, medical students and others seeking information about point-of-care ultrasound. There is a lot ofRead more

    • renal handling of water

      If you were on a tea & toast diet, how much water would you need to drink before you develop hyponatremia? I haven’t seen anyone work out the numbers before so here are my calculations. AndRead more

    • acute acidemia physiology

      As alluded to in the first post, don’t be fooled by a “normal” potassium in the setting of DKA because osmotic diuresis and H+/K+ exchange means that total body potassium is actually LOW. You all knowRead more

    • renal handling of potassium

      the first symptom of hyperkalemia is death Earlier post covered temporizing measures to counter hyperkalemia — namely, intracellular shift, increasing cardiac myocyte threshold potential. Give furosemide if the patient still urinates and consider dialysis, but then askRead more

    • bicarbonate revisited

      Previous post reviewed the safety of balanced crystalloids in hyper K. But what was up with serum bicarbonate decreasing with saline administration? This post introduces a new way of looking at the anion gap to possiblyRead more

    • hyperkalemia and balanced crystalloids

      Is it safe to give LR or plasmalyte to a hyperkalemic patient (these balanced crystalloids have 4-5 mEq/L K as opposed to 0 mEq/L K in normal saline)? Postponing the discussion of renal handling of potassium toRead more

    • hyperkalemia physiology

      You’ll likely encounter hyperkalemia on your next Resus / Cardiac shift, and you’ll instinctively treat it. But take a moment to review the fascinating physiology behind the “cocktail”! First, consider how K+ is buffered byRead more

    • Slow down your tachycardia (but not really)

      You’re sitting in resus bemoaning the departure of your most beloved attending when suddenly a patient wheels in without warning. The patient looks relatively stable but the triage RN tells you her heart rate wasRead more

    • Otitis externa: use the ear wick!

      Acute otitis externa (AOE) is a common complaint seen in pediatric as well as adult emergency departments. AOE is typically not accompanied by acute otitis media, although concurrent cases are possible. Also called “swimmer’s ear”Read more

    NextPrevious