Getting From A to C: From Pressure Waves to Graphic Display to Interpretation

Highlights

• •Understanding the mechanics of the visual representation of cardiac pressures is fundamental to assessment of cardiovascular hemodynamics.
• •Without that understanding, one can neither adequately assess waveforms, nor problem-solve potential pitfalls of interpretation.
• •Acquiring an appropriate pressure waveform is critical to accurate interpretation for patient care and justifies the risk to the patient of the invasive procedure.

Editors' introductory note

In this ongoing series, “Pearls in Hemodynamics” we hope to provide brief vignettes of unique or important fundamental hemodynamic teaching points. We plan to expand on these concepts with case studies that further amplify our understanding of invasive hemodynamics. If you have illustrative case examples that you feel would be of value, we invite you to submit them to us so that others can learn as we all strive to provide the best care for our patients.

From A to B: pressure waves

Since the earliest description of direct measurements of cardiac pressure in a horse by Hales,¹ there has been intense and continuing advancement to the field.2, 3, 4, 5, 6 However, in today's modern catheterization lab, digitized pressure waveforms have become so routine that much of the underlying principles supporting their accuracy have essentially been lost. Understanding fundamental hemodynamic principles is important since they form the underpinnings of not only invasive but also noninvasive hemodynamic assessment by techniques such as echocardiography. Invasive hemodynamics represents the gold standard by which noninvasive hemodynamic assessments were developed and adjudicated.

Pressure waves originate in a cardiac chamber from mechanical contraction and, in the case of the arterial system, produce phasic (systolic/diastolic) blood pressure. To record pressure waves, a pressure sensor or transducer (converting a mechanical impulse to electrical signal) must have the sensitivity to detect the specific pressure wave frequency. This sensitivity or system response producing a useful signal is a function of the associated cardiac catheter and tubing connected to the pressure transducer. The system should have a frequency response that reflects the physiologic frequency range of the pressure wave; a frequency response too high produces excess resonance (under damped also known as “ringing”), and too low a frequency response produces insufficient resonance (over damped) signals. An inadequate frequency response (either underdamped or overdamped) will lead to “lost” data and, if not appreciated, to potential signal misinterpretation. In general, the following factors impact the frequency response of any pressure system:

• •The length of the catheter and associated tubing
• •Catheter compliance
• •Lumen diameter of the catheter
• •Density or viscosity of the fluid in the cardiac catheter and associated tubing

Therefore, the ideal pressure recording system would use short, noncompliant catheter/tubing with a larger diameter lumen filled with a lower density fluid, typically saline, resulting in a relatively undamped system (see Fig. 1, panel C). While combinations of saline with air, contrast, or blood will dampen an underdamped system, their impact can also lead to an overdamped system. The use of catheters with the pressure manometer incorporated within the catheter tip or those using fiber optics address the shortcomings of fluid filled systems but are not widely available.

Fig. 1.

(A) Proper zeroing to mid chest. (B) Zeroing 2 catheters: left upper correctly and upper right, incorrectly. Note the possible impact to the measured pressure, bottom panel. (C) Overdamped LV on the left and underdamped on the right after air bubbles were removed from the LV catheter. Note the diastolic over shooting (in red below the zero line) as well as systolic overshoot in the underdamped system. (D) The impact of averaging of the pulmonary capillary wedge pressure over a respiratory cycle (blue line) vs. the pressure measured at end expiration (red line). LV, left ventricle.

From B to C: pressure waves to graphic display

Now that we have the appropriate catheter setup, how do you convert a pressure wave to something that can be assessed visually? The wave must be converted into an electronic signal. This conversion occurs via a transducer which uses the displacement of a membrane by pressure into an electrical signal. This signal is graphically displayed on a video monitor allowing both visual assessment and computer quantitation of the pressure waveform. Accuracy of the signal also requires that the transducer (like the tubing) have a physiologically appropriate frequency response or data could be lost or misinterpreted as noted earlier.

Pitfalls of pressure wave acquisition and interpretation

• •Inaccurate calibration
• •Zeroing catheters at different anatomic locations⁷
• •Using an inappropriately underdamped or overdamped system
• •Catheters/tubing of different diameter and lengths or of different compliance. As an example, using a compliant pulmonary artery catheter with a small lumen for evaluation of complex hemodynamics such as pericardial constriction
• •Use of pressures in a location affected by pressure amplification. For example, use of femoral artery pressure to represent central systemic arterial pressure.
• •Using a catheter with side holes partially across a cardiac obstruction: for example, a pigtail with side holes partially in the left ventricle and ascending aorta in a patient with aortic stenosis
• •Averaging waveform pressures obtained over the respiratory cycle, rather than at end expiration or suspended respiration. See Figure, panel D.

Pearls in brief

• Pearl #1

  Pressure is generated by mechanical (muscular) contraction in the heart.

• Pearl #2

  Pressure is measured by converting the mechanical impulse into an electrical signal.

• Pearl #3

  The pressure recording system requires appropriate fidelity (correctly damped) with attention to catheters, tubing, and transducer fidelity.

• Pearl #4

  The transducer electrical signal is displayed on a video monitor for analysis and review.

• Pearl #5

  Errors in signal acquisition lead to errors of data interpretation which lead to erroneous conclusions that can negatively impact patient care.