(see also Phlebostatic Axis)
Intra-arterial pressure monitoring measures the amount of force exerted by circulating blood over a specific area, also known as arterial tension. It provides a constant beat-to-beat measurement of the systolic, diastolic, and mean arterial blood pressures. The pressure readings are obtained via an arterial cannula connected to pressure tubing, a transducer (connected to an electronic monitor), and a pressurized flush bag. The transducer is an external, disposable, fluid-air interface, which detects changes of pressure within the artery. The transducer interprets the intra-arterial fluctuations and relays the readings and waveform to the bedside monitor.
The normal arterial pressure line tracing has the following characteristics as shown in Figure 2.52.
This test assesses how fast the system vibrates in response to a pressure signal. Squeezing the fast flush allows the transducer to “taste” some of the 300 mm Hg in the pressurized bag. It produces a waveform that rises sharply, hits a plateau, and then drops off sharply when the flush valve is released and is known as the “square wave.” This test is done to ensure the validity and accuracy of arterial line values and is an indicator of the characteristics of the system.
There are three types of waveforms that result from the square wave test: adequate, overdamped, or underdamped.
An adequate damped waveform (produced after the square wave ends) will have a sharp vertical upstroke and a small overshoot, followed by a straight vertical downstroke with one or two oscillations before a quick return to baseline. This is optimal and no intervention is required.
An overdamped waveform (produced after the square wave ends) generates a slightly slurred upstroke and a slurred downstroke with no oscillations above or below the baseline. This waveform results in an underestimation of the systolic pressure and an overestimation of the diastolic pressure while the mean blood pressure remains unchanged. Overdamping is usually related to a clot in the catheter tip, air bubbles in the line, loose connections or kinks in the tubing, arterial spasm, or not enough pressure in the pressure bag.
An underdamped waveform (produced after the square wave ends) is present if there is a sharp vertical upstroke, a sharp vertical downstroke, and many post-flush oscillations (more than three) above and below the baseline. The underdamped tracing will overestimate the systolic pressure and underestimate the diastolic pressure while the mean blood pressure will remain unchanged. Underdamping is related to catheter whip or fling, stiff noncompliant tubing (that may be too long), hypothermia, tachycardia or other dysrhythmia, or increased vascular resistance.
Arterial Line Pressure or Cuff Pressure?
Disparity between arterial line pressure and radial artery cuff pressure is frequently seen in the ICU, leaving the caretaker wondering which to believe. First, remember that the two are not the same. Cuff pressure measurement is based on flow-induced oscillations in the arterial wall. Arterial line pressure is based on exactly that pressure. Thus, the decision which to use should be based on two factors:
Much research has been done comparing the two methods, with as many different outcomes as there are studies. The one fact that remains consistent, however, is that mean arterial blood pressure from both methods can be interpreted in a consistent manner. Thus, many sources suggest that using the MAP as a guide to therapy is the best course. Remember, MAP is not the average between the SBP and the DBP. It is a mean, representing CO × SVR (normal 70 to 110 mm Hg). To calculate, think of 1, 2, 3…. Systolic BP + diastolic BP × 2 ÷ 3.