PISA stands for Proximal Isovelocity Surface Area. As flow passes through a narrowed orifice, it accelerates through it. However, it actually begins accelerating before it approaches that orifice. Blood closest to the orifice is traveling at a higher velocity than blood further away, and we can actually visualise this flow as it progressively accelerates, using colour Doppler.
Although textbooks and guidelines describe the PISA as a hemisphere and some manufacturers’ measuring software (e.g. EchoPac) even insists upon PISA being measured with a hemispherical shaped tool, for a uniformly round orifice (which is assumed to be the case for primary MR), this would be illogical because blood converging towards the orifice from 45 degrees inwards is clearly going to show a lower Doppler shift than blood flowing precisely in line with the Doppler beam. Indeed, one study published by my colleagues at Imperial College showed that a PISA only approaches a hemispherical shape when the orifice is slit-like (Moraldo et al., 2013).
When measuring your PISA, therefore, it is advisable to measure at its maximum point along single plane – this will usually be directly in line with your transducer beam. Instructions to measure the PISA along its radius have been shown to be confusing – encouraging people to look for a semicircular shape – and have poor reproducibility (Moraldo et al., 2o13).
In patients with severe MR, we can often see a PISA forming before even doing anything to our controls! However, by manipulating our baseline or sample rate – the pulse repetition frequency (scale) – we can enlarge and optimise our PISA, making all blood flowing faster than a given velocity appear in our aliasing colour. This is usually an orangey-yellow on transthoracic echocardiography (on transesophageal echo, it will be light blue, because blood will be aliasing in the opposite direction).
The PISA method
- In the 4 chamber or apical long axis view, zoom over the mitral valve.
- Turn on colour, positioning the box over the leaflets (you’re not interested in the jet). Do not have your colour box any larger than necessary because frame rate is important here, particularly in animals with fast heart rates.
- Reduce your colour baseline to around 40cm/s. In patients with really severe MR, you may not even need to reduce this far
- Maximise your scale.
- Save at least three cardiac cycles.
You will then need to go back over your saved loops frame by frame, in order to identify the largest and best-defined PISA. Measure the PISA radius from the vena contracta to the edge of the PISA, where your colour begins to alias. It is good practice to measure several beats and take an average, because beat to beat variability has been shown to approach 15% (Moraldo et al., 2013). This variability would be even greater in patients with irregular heart rhythms, which often accompany significant MR.
You can use your PISA measurement alone as an indicator of MR severity – a PISA radius >1cm at a Nyquist limit of 40cm/s would already be considered severe, for example – but if you wish to calculate the regurgitant orifice area and regurgitant volume, you must also take a continuous wave (CW) Doppler trade through the regurgitant jet.
Obtaining your continuous wave trace
Placing your Doppler beam through the PISA itself often results in the best traces. In highly eccentric jets, it can be difficult to obtain a full Doppler profile.
Once you have acquired a good CW trace, you need to trace around it to obtain the velocity time integral (VTI). In patients with irregular heart rhythms (AF is common with severe MR), it is important (but laborious!) to trace several and average them.
Calculating EROA and regurgitant volumes
While our colour flow Doppler visually displays a sphere and not a hemisphere due to Doppler physics, the true (unseen by Doppler) movement of blood flow is in the shape of hemispherical shells for a circular orifice. Therefore we can assume our maximum PISA distance measured from the sphere to in fact be the same as the radius of a hemisphere, and calculate the surface area of this hemisphere as 2πr².
PISA = 2πr²
Where r is the distance we have measured.
Moraldo, M., Cecaro, F., Shun-Shin, M. et al., 2013. Evidence-based recommendations for PISA measurements in mitral regurgitation: systematic review, clinical and in-vitro study. Int J Cardiology, 168(2):1220-1228.