2005A16 Briefly explain the principles of doppler ultrasound used to measure cardiac output.

Definition

·  Volume of blood ejected by LV per time

·  = HR x SV

Normal value

·  70mL/kg/min

·  5L/min in adult

Definition

·  Wave of compression and rarefaction through a medium >20kHz (probes 2-15MHz)

Probe

·  Curvilinear array of piezoelectric crystals

Piezoelectric effect

·  Emission: alternating current (AC) -> crystal oscillation -> US pressure wave

·  Detection: reflected US pressure wave -> oscillation -> AC transduction

Doppler effect

·  Frequency at detector ↑/↓ if relative motion of source towards/away from detector

·  Allows calculation of velocity

Doppler equation

V = (∆F x C) /2F₀cosθ

·  ∆F: change in frequency from emission to detection

·  C: speed of sound

·  2x: due to round trip of ultrasound wave

·  F₀: emitted frequency

·  cosθ: correct for incident angle

LVOT surface area (LVOT-SA)

·  Parasternal long axis view

·  B-mode (image)

·  Area = π x (diameter/2)²

LVOT velocity-time integral (LVOT-VTI)

·  Apical 5 chamber view

·  Pulse-wave doppler

·  Align beam with LVOT centre

·  Obtain VTI = stroke distance

Cardiac output calculation

·  Heart rate x LVOT-SA x LVOT-VTI

Pros

·  Non-invasive

·  Provides other info r.e. heart

Cons

·  Training ++

·  Operator error prone

·  Windows impaired if obese, COPD, IPPV

Measurement

·  Similar to TTE

·  Probe in oesophagus, facing descending aorta

·  Upper body correction factor: estimated by demographics (age, sex, height, weight)

·  VTI: ultrasound probe

·  Aortic surface area: second ultrasound probe, or estimated by demographics

Pros

·  Relatively non-invasive

·  Can be continuous

·  Minimal training required

Cons

·  Potentially false assumptions of correction factor

·  Operator error prone (e.g. not square with aorta)