Cath Lab Digest

The measurement of translesional pressure gradients was an integral part of the development of coronary angioplasty. Gruentzig demonstrated a relationship between post-PTCA translesional gradient and the occurrence of both early complications and late restenosis in the 1980s.¹ At that time, only fluid-filled catheter systems were used, and were limited by their bulk, lack of fidelity, and pressure damping in small catheter lumens. Intracoronary gradient assessment has been repopularized by the availability of 0.014” guidewire mounted pressure sensors. These devices have been available for a number of years, but their utility for pressure gradient measurement in a variety of extra-coronary settings is not widely appreciated. This paper will review some of the applications of gradient measurement with pressure wires for valvular, peripheral, and congenital cardiovascular diseases.

Renal Artery Stenosis
Renal artery translesional pressure gradients are used in practice for decision making about intervention for renal artery stenosis. A 20mm peak-to-peak pressure gradient between the aorta and distal renal artery is typically considered physiologically significant. There are no clear data to support this “magic number” for the determination of clinically important renal artery lesions, or to support this gradient “cut-off” as an indication intervention for renal artery stenosis. Despite the lack of evidence for a threshold value for important renal artery stenosis, pressure gradient measurement still has great utility in these cases. The absence of a gradient is useful in many cases. Monitoring changes in gradient during intervention is also useful.

Methods for Measurement: Figure 1 shows the most common method used to measure a renal artery origin lesion to obtain a translesional gradient. There is a 5 French diagnostic catheter placed into the renal orifice and then pulled back. This approach is limited by pressure damping, which may create the appearance of a gradient when none is present. It is easy to angle the catheter so it is either wedged in the lesion or damped against the wall of the vessel. A moderate lesion may appear hemodynamically severe.

Damping is only one of the artifacts to which pressure tracings are subject (Table 1). Most translesional gradient measurements are done using two fluid-filled systems. Many of the disposable pressure transducers in use today are subject to more drift than most of us realize. It is critical to zero and match both pressure systems in the aorta prior to measuring a gradient. For borderline measurements, reversing the transducers to verify a consistent gradient remains an important way to double-check the accuracy of the measurements.
Figure 2 shows the traditional method for double-pressure renal artery translesional pressure gradient assessment. A 6 Fr guide catheter is used to place a 4 Fr exchange type catheter distally in the renal artery. A distal contrast injection (lower right panel) confirms that the 4F catheter is well positioned. There is an acute angle where the guide meets the renal artery orifice, which may contribute to an unreliable distal pressure measurement.

As an alternative to two fluid-filled catheter systems, a pressure wire may be used. Figure 3 shows a case using a pressure wire to measure the translesional gradient. The stenosis is a non subtotal renal artery lesion. The patient is hypertensive, with a systemic systolic pressure that ranges between 170–200 mmHg. With the pressure wire passed through a diagnostic catheter, the peak-to-peak gradient is 15 to 20 mm Hg. The lesion is completely resolved with placement of a stent (lower panel).

Figure 4 shows a restenosis lesion in a previously placed renal origin stent. The severity of the lesion based on angiography alone is uncertain. The second angiographic panel shows a pressure wire in the distal renal artery placed via a 6F diagnostic catheter. The 0.014” pressure wire can be used with 4–6F diagnostic catheters. The simultaneous proximal and distal pressures in this case are equal. This is clearly not a hemodynamically important restenotic lesion.