Pressure transducers are a cornerstone for measuring arterial, central venous, pulmonary artery, and even intracranial pressures. These transducers operate through a Wheatstone bridge, a circuit with an arrangement of resistors of known resistance except for one. This unknown resistor is a strain gauge which is coupled to your pressure of interest (A-line, CVP, etc.) via incompressible tubing usually filled with (heparinized) saline. As the force on the gauge changes due to fluctuations in pressure, the intrinsic resistance within the gauge also changes causing a shift in the current flowing through the Wheatstone circuit. An algorithm is used to interpret this flux in current as a change in the pressure magnitude – this is the number we ultimately read on the monitor. 🙂
The waveform produced by the monitor is dependent on what is actually being measured. For example, the morphology of a CVP tracing looks very different than that of an arterial line. For the purposes of this post, I’ll talk about arterial lines since they’re fairly common in the intensive care unit (ICU) and operating room (OR).
First of all, unlike an automated blood pressure cuff, an arterial line (A-line) directly measures the systolic pressure, diastolic pressure, and mean arterial pressure. In the waveform above, the nadir corresponds to the diastolic blood pressure (DBP). The upstroke immediately after represents ventricular contraction and culminates at the systolic peak which represents the systolic blood pressure (SBP).
As the ventricle relaxes, its pressure falls below that of the aorta resulting in closure of the aortic valve which creates a reflected pressure wave represented by the dicrotic notch. As one moves distally in the body (think about an A-line in the foot versus in the radial artery), the dicrotic notch will appear more delayed. Also, in severely vasodilated patients (septic shock, neurogenic shock), the dicrotic notch will occur at a much lower pressure. In this case, you may want to consider afterload enhancement with agents like norepinephrine and/or vasopressin.
Now how does the overall morphology of the arterial line change as you move more distally? Again, let’s consider a patient who has two arterial lines: one in the radial artery and one in the dorsalis pedis artery (foot). The mean arterial pressure (MAP) between the two should be similar, but the more distal foot A-line will have a higher SBP and lower DBP compared to the radial A-line. This is a consequence of reflected pressure waves at all the branch points blood encounters as it exits the heart and barrels down the arterial tree.
Drop me a line with questions below! I’ll be addending this post with more information in the coming days.