Myocardial work still reflect function while strain simply measure deformation when afterload increases

Introduction

Global longitudinal strain is recommended by the European Society of Cardiology to detect subclinical left ventricular (LV) dysfunction, but is markedly load-dependent. Myocardial work was recently introduced as a clinical tool to study LV function by pressure-strain analysis. Since myocardial work incorporates afterload, it is assumed to be less afterload-dependent than strain, but the relationship with afterload is incompletely understood.

Hypothesis

Myocardial work is a better tool than strain, to measure myocardial function during elevated afterload.

Methods

In eleven anesthetized dogs, LV volume and longitudinal strain were measured by sonomicrometry, and pressure by micromanometry. Myocardial work was calculated by pressure-strain analysis. Additionally, stroke work was calculated as the area of the pressure-volume loop. Afterload was instantly increased by aortic constriction using a pneumatic cuff around the ascending aorta. Measurements were performed at baseline, during moderate- and marked afterload elevations.

Results

Table 1 summarizes the results. LV pressure (LVP) successively increased with moderate and marked afterload elevation, while longitudinal strain was successively reduced. Myocardial work and stroke work, on the other hand, increased with moderate afterload elevation, but was then reduced at marked afterload increase (Figure 1 and Table 1). Stroke volume and ejection fraction corresponded to strain and were reduced with afterload elevation.

Conclusions

Longitudinal strain and myocardial work have qualitatively different responses to increased afterload. While moderate changes in afterload cause reductions in strain that can be falsely interpreted as reductions in contractility, myocardial work increases as it incorporates the increased workload at moderately elevated afterload.

Funding Acknowledgement

Type of funding sources: Public Institution(s). Main funding source(s): The Norwegian Health Association

Duration of the preejection phase is less preload dependent and therefore a better marker of acute response to cardiac resynchronization therapy than maximum pressure rise

Funding Acknowledgements

Type of funding sources: Public grant(s) – EU funding. Main funding source(s): EU’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie

Background

There is no consensus on which haemodynamic marker should be used to quantify acute response to cardiac resynchronization therapy (CRT) during implantation of the device. CRT has been shown to acutely reduce left ventricular (LV) end systolic as well as end-diastolic volume (EDV), precluding the use of preload dependent markers such as LV maximum pressure rise (dP/dtmax).

Purpose

As resynchronization will abolish the uncoordinated regional early systolic contractions of the LV, it will shorten the time to maximal pressure rise and aortic valve opening. For this reason, the purpose of this study was to investigate if duration from the time-point of ventricular pacing to dP/dtmax is less preload dependent and a better marker of acute response to CRT than dP/dtmax by comparing how the 2 markers reflected LV function during different CRT configurations.

Methods

LV pressure by micromanometer and volume by sonomicrometry were measured in 6 anaesthetized canines with left bundle branch block. Transient caval constrictions were performed to vary preload. Preload dependency of the 2 markers was compared by normalizing their values and calculating their relations to EDV. In 4 of the animals, biventricular pacing was performed at 3 different pacing sites with variations in atrioventricular delays that provided a range of response to CRT. To correct for acute

changes in preload by CRT, stroke volume (SV) at identical EDV found from transient caval constrictions, were assessed and used as reference to grade improved LV function. Linear regression analysis was used to assess the correlation of both the duration of the preejection phase and dP/dtmax with SV.

Results

The duration of the preejection phase varied less with changes in preload compared to dP/dtmax: the slopes of their relation to EDV were -0.6 ± 0.7 %/ml and 4.8 ± 2.1 %/ml (p = 0.004), respectively. Turning CRT on, acutely reduced EDV from 74 ± 16 to 69 ± 17 ml (p < 0.001) at the best pacing configuration. For the different pacing sites and settings, there was a consistent relation in all animals where the preejection phase shortened as SV increased (average r2 = 0.75) (Figure A). dP/dtmax showed no clear relation to SV (average r2 = 0.22) and included cases with both negative and positive slopes (Figure B).

Conclusions

The duration of the preejection phase correlated with changes in LV function induced by CRT while dP/dtmax performed poorly as preload was changed. Hence, the novel timing parameter was less preload dependent and may be a better marker for assessing acute response to CRT. Abstract Figure.