Cardiovascular magnetic resonance evaluation of aortic stenosis severity using single plane measurement of effective orifice area

Background

Transthoracic echocardiography (TTE) is the standard method for the evaluation of the severity of aortic stenosis (AS). Valve effective orifice area (EOA) measured by the continuity equation is one of the most frequently used stenotic indices. However, TTE measurement of aortic valve EOA is not feasible or not reliable in a significant proportion of patients. Cardiovascular magnetic resonance (CMR) has emerged as a non-invasive alternative to evaluate EOA using velocity measurements. The objectives of this study were: 1) to validate a new CMR method using jet shear layer detection (JSLD) based on acoustical source term (AST) concept to estimate the valve EOA; 2) to introduce a simplified JSLD method not requiring vorticity field derivation.

Methods and results

We performed an in vitro study where EOA was measured by CMR in 4 fixed stenoses (EOA = 0.48, 1.00, 1.38 and 2.11 cm²) under the same steady flow conditions (4-20 L/min). The in vivo study included eight (8) healthy subjects and 37 patients with mild to severe AS (0.72 cm² ≤ EOA ≤ 1.71 cm²). All subjects underwent TTE and CMR examinations. EOA was determinated by TTE with the use of continuity equation method (TTE_CONT). For CMR estimation of EOA, we used 3 methods: 1) Continuity equation (CMR_CONT); 2) Shear layer detection (CMR_JSLD), which was computed from the velocity field of a single CMR velocity profile at the peak systolic phase; 3) Single plane velocity truncation (CMR_SPVT), which is a simplified version of CMR_JSLD method. There was a good agreement between the EOAs obtained in vitro by the different CMR methods and the EOA predicted from the potential flow theory. In the in vivo study, there was good correlation and concordance between the EOA measured by the TTE_CONT method versus those measured by each of the CMR methods: CMR_CONT (r = 0.88), CMR_JSLD (r = 0.93) and CMR_SPVT (r = 0.93). The intra- and inter- observer variability of EOA measurements was 5 ± 5% and 9 ± 5% for TTE_CONT, 2 ± 1% and 7 ± 5% for CMR_CONT, 7 ± 5% and 8 ± 7% for CMR_JSLD, 1 ± 2% and 3 ± 2% for CMR_SPVT. When repeating image acquisition, reproducibility of measurements was 10 ± 8% and 12 ± 5% for TTE_CONT, 9 ± 9% and 8 ± 8% for CMR_CONT, 6 ± 5% and 7 ± 4% for CMR_JSLD and 3 ± 2% and 2 ± 2% for CMR_SPVT.

Conclusion

There was an excellent agreement between the EOA estimated by the CMR_JSLD or CMR_SPVT methods and: 1) the theoretical EOA in vitro, and 2) the TTE_CONT EOA in vivo. The CMR_SPVT method was superior to the TTE and other CMR methods in terms of measurement variability. The novel CMR-based methods proposed in this study may be helpful to corroborate stenosis severity in patients for whom Doppler-echocardiography exam is inconclusive.     

Adaptation-induced plasticity and spike waveforms in cat visual cortex

Orientation-selective neurons shift their preferred orientation after being adapted to a nonpreferred orientation. These shifts of the peaks of tuning curves may be in the attractive or repulsive direction in relation to the adapter orientation. In anesthetized cats, we recorded evoked electrical responses from the visual cortex in a conventional manner. The recorded spikes in cortex may present two typical waveforms: regular spikes or fast spikes. However, there is no evidence whether the shapes of spikes are related to the attractive or repulsive shifts of orientation tuning curves of cells. Our results show that after adaptation the recorded cells with both attractive and repulsive shifts display one or the other shape of spike. However, the magnitude of shifts is systematically higher for regular spikes, which is attributed to putative pyramidal cells, whereas tuning curves for fast spikes have smaller magnitudes and are evoked by putative interneurons.     

Value and limitations of peak-to-peak gradient for evaluation of aortic stenosis

BACKGROUND AND AIM OF THE STUDY: In patients with aortic stenosis (AS), it has been reported that the transvalvular pressure gradients (APs) may be reduced or even abolished in the presence of concomitant arterial hypertension, but the mechanisms underlying this phenomenon remain unclear. The study aim was to: (i) examine the relationship between systemic arterial hemodynamics and the peak-to-peak (deltaP(PtoP)), peak deltaP and mean deltaP; and (ii) propose and validate a new formula for the non-invasive estimation of the deltaP(PtoP) and of the peak left ventricular systolic pressure (LVSP) using Doppler echocardiography. METHODS: Two fixed stenoses (geometric orifice area 1.0 and 1.35 cm2) and one bioprosthesis (effective orifice area (EOA) 1.2 cm2) were tested in a mock flow circulation model. Systemic vascular resistance (R) was increased from 1,500 to 3,300 dyne.s/cm5, and systemic arterial compliance (C) was decreased from 2.9 to 0.9 ml/mmHg, while transvalvular flow was held constant. RESULTS: Neither C nor R had any significant impact on EOA, peak deltaP and mean deltaP. deltaP(PtoP) was decreased markedly, however, when C was reduced (bioprosthesis: -15 mmHg (-69%); orifice 1.35 cm2: -24 mmHg (-30%); cm2: (-13%)). Subsequently, an equation was proposed to predict deltaP(PtoP) from EOA, mean deltaP, and C measured by Doppler echocardiography. LVSP calculated by adding the predicted deltaP(Ptop) to systolic arterial pressure (SAP) was compared with LVSP measured directly in a dataset of 24 pigs with experimentally induced AS. There was a strong agreement between the estimated and measured LVSP (r = 0.97; mean absolute error 5 +/- 5 mmHg). CONCLUSION: deltaP(Ptop) should not be used to evaluate AS severity because, as opposed to peak and mean deltaPs, it is highly influenced by C. The new non-invasive method proposed in this study to estimate the LVSP may be useful for obtaining a more accurate estimate of global LV afterload in patients with AS.     

Discrepancies between catheter and Doppler estimates of valve effective orifice area can be predicted from the pressure recovery phenomenon: practical implications with regard to quantification of aortic stenosis severity

OBJECTIVES: We sought to obtain more coherent evaluations of aortic stenosis severity. BACKGROUND: The valve effective orifice area (EOA) is routinely used to assess aortic stenosis severity. However, there are often discrepancies between measurements of EOA by Doppler echocardiography (EOA(Dop)) and those by a catheter (EOA(cath)). We hypothesized that these discrepancies might be due to the influence of pressure recovery. METHODS: The relationship between EOA(cath) and EOA(Dop) was studied as follows: 1) in an in vitro model measuring the effects of different flow rates and aortic diameters on two fixed stenoses and seven bioprostheses; 2) in an animal model of supravalvular aortic stenosis (14 pigs); and 3) based on catheterization data from 37 patients studied by Sch?bel et al. RESULTS: Pooling of in vitro, animal, and patient data showed a good correlation (r = 0.97) between EOA(cath) (range 0.3 to 2.3 cm(2)) and EOA(Dop) (range 0.2 to 1.7 cm(2)), but EOA(cath) systematically overestimated EOA(Dop) (24 +/- 17% [mean +/- SD]). However, when the energy loss coefficient (ELCo) was calculated from EOA(Dop) and aortic cross-sectional area (A(A)) to account for pressure recovery, a similar correlation (r = 0.97) with EOA(cath) was observed, but the previously noted overestimation was no longer present. CONCLUSIONS: Discrepancies between EOA(cath) and EOA(Dop) are largely due to the pressure recovery phenomenon and can be reconciled by calculating ELCo from the echocardiogram. Thus, ELCo and EOA(cath) are equivalent indexes representing the net energy loss due to stenosis and probably are the most appropriate for quantifying aortic stenosis severity.     

Mitral Prosthesis Opening and Flow Dynamics in a Model of Left Ventricle: An In Vitro Study on a Monoleaflet Mechanical Valve

A complete understanding of the flow past a mitral valve prosthesis require a new generation of pulse duplicators and more realistic flow conditions. The objective of this study is to describe the opening kinetics of a monoleaflet Medtronic Hall 27-mm mechanical valve in mitral position and to determine the flow pattern within the left ventricle using particle image velocimetry (PIV) for different instants during the cardiac cycle. At the onset of diastolic phase, the flow goes through the major orifice and then through the minor orifice. The two jets generated induce two counterclockwise vortices within the ventricle, which can lead to a partial closure of the valve during mitral E wave deceleration. During diastasis and mitral A wave, only one vortex persists in the ventricle and pushes the valve disk backward at the end of the diastolic phase. The valve disc never reached its maximum opening (65 vs.75). This study underscores that the flow past a monoleaflet valve in mitral position is highly dependent on the complex interaction between the inflow, the vortices development within the left ventricle, and the gravity. Such a complex interaction can only be highlighted using new generation of pulse duplicators.     

Stimulus‐dependent augmented gamma oscillatory activity between the functionally connected cortical neurons in the primary visual cortex

Neuronal assemblies typically synchronise within the gamma oscillatory band (30–80 Hz) and are fundamental to information processing. Despite numerous investigations, the exact mechanisms and origins of gamma oscillations are yet to be known. Here, through multiunit recordings in the primary visual cortex of cats, we show that the strength of gamma power (20–40 and 60–80 Hz) is significantly stronger between the functionally connected units than between the unconnected units within an assembly. Furthermore, there is increased frequency coherence in the gamma band between the connected units than between the unconnected units. Finally, the higher gamma rhythms (60–80 Hz) are mostly linked to the fast‐spiking neurons. These results led us to postulate that gamma oscillations are intrinsically generated between the connected units within cell assemblies (microcircuits) in relation to the stimulus within an emergent ‘50‐ms temporal window of opportunity’.     

Comparative analysis of orientation maps in areas 17 and 18 of the cat primary visual cortex following adaptation

Object orientations in the visual field are columned into specific orientation domains in the primary visual cortex [area 17 (A17) and area 18 (A18)] of cats. At the single‐cell level, adapting A17 neurons to a non‐preferred orientation (adaptor) shifts their preferred orientation either towards the adaptor (attractive shift) or away from it (repulsive shift). As A17 and A18 are reciprocally connected, we sought to determine how changes in preferred orientations in A18 neurons are correlated with changes recorded in A17 anesthetised cats. To this end, we simultaneously traced populations of neurons in A17 and A18, using intrinsic optical imaging, before and after long (12 min) and short (3 min) adaptations. The comparison of A17 and A18 maps pre‐adaptation and post‐adaptation showed that variance in shift amplitudes is greater in A18 than A17 for short adaptations. Our results indicate a rapid reconfiguration of functional maps that may spread to many cortical areas.