Evidence-based recommendations for PISA measurements in mitral regurgitation: systematic review, clinical and in-vitro study

Abstract

Background: Guidelines for quantifying mitral regurgitation (MR) using "proximal isovelocity surface area" (PISA) instruct operators to measure the PISA radius from valve orifice to Doppler flow convergence "hemisphere". Using clinical data and a physically-constructed MR model we (A) analyse the actually-observed colour Doppler PISA shape and (B) test whether instructions to measure a "hemisphere" are helpful.

Methods and results: In part A, the true shape of PISA shells was investigated using three separate approaches. First, a systematic review of published examples consistently showed non-hemispherical, "urchinoid" shapes. Second, our clinical data confirmed that the Doppler-visualized surface is non-hemispherical. Third, in-vitro experiments showed that round orifices never produce a colour Doppler hemisphere. In part B, six observers were instructed to measure hemisphere radius rh and (on a second viewing) urchinoid distance (du) in 11 clinical PISA datasets; 6 established experts also measured PISA distance as the gold standard. rh measurements, generated using the hemisphere instruction significantly underestimated expert values (-28%, p<0.0005), meaning r(h)(2) was underestimated by approximately 2-fold. du measurements, generated using the non-hemisphere instruction were less biased (+7%, p=0.03). Finally, frame-to-frame variability in PISA distance was found to have a coefficient of variation (CV) of 25% in patients and 9% in in-vitro data. Beat-to-beat variability had a CV of 15% in patients.

Conclusions: Doppler-visualized PISA shells are not hemispherical: we should avoid advising observers to measure a hemispherical radius because it encourages underestimation of orifice area by approximately two-fold. If precision is needed (e.g. to detect changes reliably) multi-frame averaging is essential.

Keywords: Echocardiography; Mitral regurgitation; PISA method.

Fig. 1

Difference between speed and velocity. The motion of an object can be described in two exactly equivalent ways: speed-and-direction or velocity components. The left panel (A) shows speed and direction. The right panel (B) shows the same motion decomposed into horizontal velocity (v_horizontal) and vertical velocity (v_vertical). For movement in a plane, speed² = v_horizontal² + v_vertical². Doppler shows only movement towards/away from the probe, which in this figure is approximately vertical.

Fig. 2

Isospeed hemisphere (outer curve) versus isovelocity urchinoid (inner curve). The consequence of the difference between speed (Fig. 1A) and vertical Doppler velocity component (Fig. 1B) is that when flow convergences hemispherically, Doppler imaging correctly shows not a hemisphere but a more completely round shape. For blood moving directly away from the ultrasound probe, i.e. vertically on this figure, (a), speed and velocity are practically synonymous, so the isospeed hemisphere (grey) is at the same position as the Doppler isovelocity surface (blue). For blood moving at 45° (b) the vertical velocity is reduced to cos 45° × speed, i.e. is substantially smaller. Therefore only closer to the orifice (label b on right), where the speed is even higher, is the Doppler velocity component high enough to match its value at (a). For blood moving almost perpendicularly to the ultrasound beam (c), the vertical Doppler velocity component is very much smaller than the speed for example by a factor of ~ 10 at 84° (cos 84° ≈ 0.10). Therefore the Doppler isovelocity surface is ~ 10 times closer to the orifice at that angle.

Fig. 3

Two possible measurements of PISA from the same frame of a 2D colour Doppler image. On the left the operator has measured the radius of a hemisphere, denoted as distance r_h. On the right the operator has measured the distance from the valve orifice to the aliasing point of the Doppler signal, in the direction of the probe beam, denoted as distance d_u (urchinoid distance, when we assume the PISA shape to be a flattened sphere or urchinoid).

Fig. 4

Examples taken from the systematic review of the published literature showing the terms used to describe the shape and measurement distance of the PISA flow convergence zone and the published images. Specific examples manifesting controversy (A, B and C).

Fig. 5

Clinical data used to assess the shape of the PISA flow convergence zone and for measurement of PISA distance. Individual frames from colour Doppler loops visualizing PISA flow convergence zone in 11 patients with MR. In most of the frames the shape of PISA is more similar to an urchinoid rather than a hemisphere.

Fig. 6

Physical model data showing PISA flow convergence zones obtained by averaging 30 frames of 2D colour Doppler video loops of the round orifice sample using Matlab software, to minimise ambiguity. Each image shows the PISA shell obtained in experimental in-vitro condition of stillness at a different aliasing velocity. The shape of the PISA shell is closer to an urchinoid or flattened sphere than a true hemisphere in all frames.

Fig. 7

Physical model data showing PISA flow convergence zones obtained by averaging 30 frames of 2D colour Doppler video loops of the slit-like orifice sample using Matlab software, to minimise ambiguity. Each image shows the PISA shell obtained in experimental in-vitro condition of stillness at a different aliasing velocity. In all the frames it is possible to visualize a hemispherical or a hemiellipsoidal shape of PISA shell.

Fig. 8

Measured PISA distance for the 11 clinical patients, as measured by experts and observers instructed to measure either the radius of a hemisphere, r_h, or the urchinoid distance, d_u. The distance was consistently smaller when operators were measuring r_h than either when they measured d_u or when experts measured PISA distance according to their usual clinical practice. In addition the spread of values for each individual patient for each of the measurement techniques was smaller when observers measured d_u than for the other groups. Lines represent 95% confidence intervals.

Fig. 9

Frame-to-frame and beat-to-beat data for assessing the variability of PISA shape and PISA distance. Top panel: example data from patient 1. Nine systolic frames for each of the 3 beats of colour Doppler PISA data showing the inter and intra beat variability in measuring PISA distance. Bottom panel: variability of measuring PISA distance using the in-vitro model. 30 frames of a 2D colour Doppler video loop showing intraobserver variability in measuring the distance of the flow convergence zone of the round orifice.