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The RVEIO and RV function: More, please

In Charles Dickens' Oliver Twist (1838), Oliver, poor and hungry, brazenly asks for another serving of food at his orphanage—Please, sir, I want some more. We found the article by Kahyaoglu M et al. on the right ventricular early inflow-outflow (RVEIO) index intriguing, but accompanied by only a meager amount of other publications on this topic. The study evaluated the RVEIO index with COVID-19 pneumonia severity, and we, like Oliver, are left wanting more. The concept of RVEIO came from the recently-derived left ventricular early inflow-outflow (LVEIO) index, described in 2015 as a simplified construct relating to regurgitant fraction in severe mitral regurgitation (MR). Conventionally, mitral regurgitant volume can be calculated as the difference between the total inflow volume crossing the mitral valve in diastole from the smaller outflow volume that exits the LVOT in systole. In the LVEIO numerator, the peak velocity of the inflow wave, E (“early” diastolic), is a surrogate for total inflow and reflects the sum of the pulmonary venous flow and the prior beat's regurgitant volume that initially crosses the mitral orifice and relates to the severity of MR. However, importantly, the E wave value is a velocity, not a volume, and is determined by the driving left atrial pressure and the accepting relaxation and compliance characteristics of the left ventricle. In the denominator of the LVEIO, the LVOT velocity-time integral (VTI) is derived by the area under the Doppler outflow waveform and represents the forward stroke distance traveled by a theoretical “circular” area of blood in the outflow tract with each ventricular contraction, creating a cylinder of stroke volume. By removing the diameter measurements in conventional calculations, the simplified LVEIO index has eliminated the volumetric errors associated with assumptions of a circular valve annulus and outflow tract—both having been shown to actually be oval. Simplistically, in MR, the LVEIO ratio loosely reflects regurgitant volume when normalized to stroke volume. Stepping back, the concept of comparing ventricular inflow to outflow as a ratio could potentially be a powerful and expedient method to predict mortality in echo referral populations, since it is driven by such bad actors as low stroke volume, elevated filling pressures, and regurgitant volume. Studies can easily report the LVEIO by simply dividing two routinely-collected Doppler parameters of standard echocardiograms that are typically stored in vast echo lab databases, the mitral E wave velocity and the LVOT VTI. In n = 6264 echocardiograms, LVEIO rose with increasing regurgitant fraction, and when LVEIO ≥8, there was a likelihood ratio of 26.5 for severe MR. In a study of n = 18 692 echo studies, an LVEIO ≥5.4 had an 85% sensitivity and 90% specificity in detecting severe MR and was related to patient mortality. Since the E wave rises with left atrial pressures, it can be particularly difficult to disentangle the causes of the higher E wave velocity in patients with both CHF and functional MR, as evidenced by a significant fall in AUC of 0.92 to 0.80 when LVEIO was used in those patients with EF < 50%. Further complicating the outcome analyses, left atrial compliance likely plays a role in accommodating some of the regurgitant volume and pressure, thereby lessening the degree and impact of resultant pulmonary edema. In 2017, the right heart's RVEIO was similarly validated for severe tricuspid regurgitation (TR) from the tricuspid E wave and RVOT VTI in n = 1251 echo studies, with a higher diagnostic threshold ratio of >10, and, similar to the LV and LVEIO, losing some discriminating ability in the setting of RV dysfunction (AUC significantly fell from 0.91 to 0.84). However, the pressure-volume and Starling curves of the RV are clearly different than those of the LV, and, if PA pressures acutely rise due to pulmonary afterload, the right heart's lesser contractile force becomes readily apparent. As the RV fails, the RVEIO increases markedly with afterload mismatch, as exemplified by values of >14 for severe pulmonary embolism. Akin to LV dysfunction in LVEIO, intrinsic RV dysfunction can also result in elevated right atrial filling pressures and a higher RVEIO, even in the absence of severe TR. In COVID-19, RV abnormalities can be noted in 66%, and moderate or severe TR in 21% of patients, both of which could elevate RVEIO, the former being related to COVID-19 mortality. Unlike MR, TV regurgitant volume itself may have less of a direct mortality effect, as TR volumes are more easily tolerated. It is therefore possible that an elevated RVEIO in COVID-19 may be a distress signal not from tricuspid regurgitation, but from RV dysfunction, due to increased afterload or primary infection. All three etiologies, RV dysfunction, either from myocarditis or due to increased afterload, and functional TR may co-exist in COVID-19, creating varying RVEIO Received: 15 September 2021 Accepted: 16 September 2021