Impedance to retrograde and forward flow in chronic mitral regurgitation and the physiology of a double outlet ventricle.

OBJECTIVE

Mitral regurgitation (MR) is generally characterised as exhibiting a 'low impedance leak into the left atrium'. This notion is widely accepted without measured impedance data. The aim of this study was to define the impedance to retrograde and forward blood flow and to examine hydraulic (pressure-volume) and mechanical (stress-shortening) function in chronic severe MR.

METHODS

A mathematical model of a double outlet ventricle was developed and the ratio of retrograde to forward impedance was plotted over a wide range of regurgitant fraction (RF). The model predicts that an impedance ratio >1 indicates that the impedance to retrograde flow exceeds that of forward flow. Left ventricular (LV) systolic pressure/flow rate was used as an index of impedance (mm Hg/mL/s). Data from 10 patients with severe MR were used to assess the clinical applicability of the model. All patients had degenerative valve disease with partial flail leaflet, an RF >50% and an ejection fraction (EF) >0.60. There were seven males and three females, aged 59±10. LV volumes as well as retrograde and forward flow rates were determined with echocardiographic and Doppler techniques.

RESULTS

The model indicates that the impedance ratio is >1 when the RF ranges from zero to 57%. Clinical data: end-diastolic volume=184±47 mL; EF=0.63±3%; RF=53±4%. Values for retrograde and forward impedance were 0.77±0.17 and 0.63±0.12 (p=0.003); the impedance ratio was 1.22±0.19. Total impedance to LV emptying was low (0.35±0.06). The ratio of systolic wall stress to EF (580±81 g/cm2) was normal. Data are mean±SD.

CONCLUSIONS

The model, supported by clinical data, indicates that the impedance to retrograde flow exceeds the impedance to forward flow in chronic severe MR. These findings refute the notion of a low impedance leak into the left atrium. The double outlet of an enlarged ventricle provides a mechanism for low total impedance to ejection in the presence of a normal stress-shortening relation.