# G3iii: Define myocardial contractility & describe dP/dT, ESPVR, Ejection Fraction

## Defintion

**Contractility** = the intrinsic ability of myocardial fibres to shorten, independent of preL & afterL

*(it augments force of myocyte contraction via SLIDING FILAMENT)*

- ∆ inotropy are caused by cellular mechanisms that regulate the interaction between actin & myosin, independent of ∆ sarcomere length
- ↑CONTRACTILITY causes:

**↑ Active Tension**→ so that for any given preL sarcomere length, there is more tension because the inotropy**augments the force**of the contraction

**↑ Rate of Shortening**→ ↑ inotropy will ↑rate of shortening for any given afterload because of ↑rate cross-bridge turnover*e. V*_{MAX}

## Indices of Contraction

- Papillary Muscle Force = GOLD STANDARD
- All other indices depend on HR/preL/afterL
**dP/dT**=_{MAX}**maximum rate**in ΔP in LV during isovolumetric contraction- It is a pre-ejection index
**independent**of afterL*(because AoV is shut)*but is dependent on preL - It is an
**invasive**measurement done in a cardiac cath lab

- It is a pre-ejection index

**Ejection fraction**: the % of EDV ejected from the LV with each contraction

- Normally
**>55%**

- Normally

- EjF affected:
- Minimally by preL if MV/AoV normal
- ↓linearly with ↑ afterL
- Inverse relationship with HR

- May be measured by Doppler wave assessment of MR jet
*(which occurs in IVC**∴**MV jet velocity ∆ is reflecting the LVP ∆)* **ESPVR:**the maximal P generated by the ventricle at any given LV volume- At end of systole the sarcomeres are maximally activated ∴you can measure the P generated in the LV for a given volume

- On the PV loop
- Starts just after 0mL
- Intersects D
*(AoV shut)* - Straight line
- Intersects SBP
*(120)*& DBP*(80)* - Angle of line it makes with C = contractility

- As you ↑/↓ vol, the LVP changes

**P/V = Elastance**

∴slope of this line is elastance, which is _{KA} E_{0} → where the PV loop falls on the ESPVR line

- ↑slope of ESPVR = line moves
**LEFT** - ↓slope of ESPVR = line moves
**RIGHT**

## Factors Influencing Myocardial Contractility

- Contractility depends on:
- Substrate supply
- Integrity of myofilaments
- Coordinated electrical depolarisation
- Metabolic homeostasis
- Coronary blood flow

## ↑ C

- SNS through ↑Ca
^{2+}release

## ↓ C

- PNS through ↓Ca
^{2+}release - Hypercapnia = ↓pH = ↓Ca
^{2+}release - Volatiles = direct – ve inotrope