We read with interest the recent paper 'Impedance to retrograde and forward flow in chronic mitral regurgitation and the physiology of a double outlet ventricle' by Gaash, Shah, Labib and Meyer; particularly their conclusion that 'the impedance to retrograde flow exceeds the impedance to forward flow in chronic severe MR'.\cite{27683406} After careful reading of the paper we believe they have made a fundamental fluid mechanical error which brings this conclusion into question.
It is a fundamental tenet of fluid mechanics that flow depends on the pressure gradient, not the pressure, in the fluid.* In the paper the authors say that 'resistances to forward and retrograde blood flow were determined using standard formulae' with a citation to a paper by Ford, et al.\cite{2295132} Equation (1) of this paper correctly defines resistance \(R\ =\ \frac{\Delta P}{F}\) where \(R\) is the resistance, \(\ \Delta P\) is the mean systolic pressure gradient and \(F\) is the flow. In the body of the paper, however, there is no mention of either aortic or left atrial pressures, which would be necessary to calculate the \(\Delta P\) for the forward and backward flows. This leads us to believe that their calculations were based on the left ventricular pressure rather than the appropriate pressure gradients. This could explain their non-intuitive conclusion that the impedance (resistance) to backward flow was higher than the impedance (resistance) to forward flow.
\(\)It is important that this question be resolved because of the clinical implications of their results.
* We note that the term 'gradient' in general use in cardiology to denote the pressure difference between pressures measured at two different locations differs from its meaning in mechanics. 'Gradient' in mechanics refers to the slope of the pressure  with distance. To align the two terms it would be necessary to specify the distance between the two measurement sites in cardiology.