17A23: Exam Report

Draw and label a left ventricular pressure volume loop in a normal adult (40% of marks). List the information that can be obtained from this loop (60% of marks).

65% of candidates passed this question.

Many candidates lost marks for poor quality diagrams with inaccurate labelling. An accurate diagram was required. Many answers lacked sufficient detail regarding contractility and afterload.

G3vi / 17A23: Draw and label a LV pressure volume loop in a normal adult (40 marks). List information that can be obtained from this loop (60 marks)

Definition

A loop that depicts the changes in LV pressure & volume throughout the cardiac cycle

Construction

  • Heart isolated in a lab
  • Measure P changes for different volumes
  • @SYSTOLE: measure P for different volumes during contraction whilst preventing outflow
  • @DIASTOLE: gradually fill heart & measure P at each volume before contraction
  • Plotted on P vs V to generate a loop
Construction

Information Obtained

Stroke Volume

    • EDV – ESV
    • ~70mL

Stroke Work

  • Work = force x distance = pressure x volume
  • ∴area inside PV loop is the external work performed by LV to expel blood for that cardiac cycle

Ejection Fraction

Ejection Fraction
    • ~50 – 70%

LV EDV

  • Represents preL
  • ~120mL

LV ESV

  • Implications for LAP
  • ↑ESV = ↓gradient for blood to move from LA → LV
  • ∴LA will require ↑P to move blood

Peak Pressure

  • = SBP
  • The maximum pressure achieved in systole

LV EDPVR (Elastance)

  • Slope of line A → B
  • Shows ELASTANCE (lusitropy) of ventricle
  • Elastance = ∆P / ∆V
  • Compliance may be inferred (1 / Elastance)
  • Towards the end it curves up (1 compliance) because LV difficult to overfill. Sarcomere length cannot extend beyond 2.2µm

LV ESPVR

  • Reflects CONTRACTILITY
  • Starts just after 0mL
  • Intersects D
  • Straight line
  • Intersects PEAK PRESSURE & C
  • The angle it makes with C = CONTRACTILITY

Afterload

  • Gradient of the line from LV EDV → D
  • The angle it makes with SV = afterL
  • Steeper slope = ↑afterL
Contractivity