ECG Early Warning System (EWS) in Terms of Time-Variant Deformations and Creep-Recovery Strain Tests

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Referring to an earlier stress-strain case study, the objective of this risk management project is to develop the ECG Early Warning System (EWS) based upon time-dependent viscoelastic deformations and observed creep-recovery mechanisms in the cardiac muscle.

The creep-recovery test involves loading a material at constant stress, holding that stress for some length of time and then removing the load. The response of a typical viscoelastic material to this test is shown below.

Strain response to the creep-recovery test
  • First there is an instantaneous straining (IS), followed by an ever-increasing strain over time known as creep strain (CS). Elastic recovery (ER) stage: when unloaded, the elastic strain is recovered immediately. There is then anelastic recovery (AR) – strain recovered over time due to the viscoelastic time memory effect; this anelastic strain may be significant in some materials. A permanent strain (PS) may then be left in the material.
  • For viscoelastic materials, a time-dependent function is used instead of a single value of Young’s modulus, and this is called the Young’s relaxation modulus E(t).
  • The creep compliance function is used to describe creep J(t) behavior and can be related to the Young’s modulus E(t).

This analysis leads to the following CVD risk management chart to be discussed below:

  • Stage 1: The creep deformation is recovered almost entirely when the load is released, i.e. ER >> AR and PS ~ 0.
  • Stage 2: A significant AR effect comparable to CS in terms of both magnitude and duration, whereas PS effect is still negligible.
  • Stage 3: The creep deformation is not recovered when the load is released due to the joint effect of AR and PS comparable to CS.
  • Stage 4: Long-term AR and significant PS effects similar to that of CS are observed.

Summary

Stages 1 and 2 support the following experimental observations:

Cardiac muscle undergoes creep deformation from 2 to 3 % of its original length in 100 s. Large loads that stretch the muscle beyond 15% of its original length produce negligible PS effects, whereas the time course of AR is nearly identical to that of creep.

Stage 3 is characterized by the long-term AR and PS magnitudes comparable to that of CS, the creep deformation suffered under maintained loading is partially recovered when the load is released.

Stage 4 is characterized by the significant level of PS ~ CS, the creep deformation suffered under maintained loading is not recovered when the load is released.

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