Echocardiography = a group of interrelated ultrasound applications used to examine the heart and great vessels.
- piezo-electric crystal which expands proportionally when a continuous alternating voltage is applied at 8MHz -> wave is generated by the compression of particles.
- these crystals are able to create ultrasonic waves and also sense how they interact with tissues -> construction of an image.
- Audible sound – frequency 20 and 20,000 Hz
- Ultrasound – >20 kHz
- Clinical U/S – 1 to 10 MHz – because of short wave length it is easily steered, focused and manipulated.
- Wave length – temporal duration between any two peaks or troughs in a cycle.
- Frequency – the number of cycles per second (Hz)
- Velocity = frequency x wavelength
C = f x wavelength
C = speed of sound (m/s)
f = frequency (Hz)
wavelength = (meters)
- Acoustic impedance = the ability to transmit sound (related to density of tissue) – as density increases -> sound travels faster.
- Amplitude (strength) of sound wave = peak pressure (measured in decibels).
- Blood flow measurements required the following information:
(1) cross sectional area of ascending aorta.
(2) transducer placed so beam falls in line parallel to aortic flow.
(3) U/S device integrates measured blood flow velocity over period of ejection to determine average value of RBCs at each heart beat (needs TOE).
(4) Q = average velocity value for each heart beat in aorta x aortic cross-sectional area x HR
V = (cfd/2ft) cos O
V = blood flow velocity
c = speed of sound in body tissue (1540m/s)
fd = doppler frequency
cos O = cosine of angle between sound beam & blood flow.
ft = frequency of transmitted u/s
Cardiac Output measurement:
Q = SV X HR
= (Aortic Area x V x Tej) x HR
Q = cardiac output
Aortic area = cross sectional area
V = velocity for each beat
Tej = time period during ejection
HR = heart rate
(1) 2D anatomical imaging
- used TTE or TOE
- cornerstone of imaging
- other modes use 2D as a reference
- image plane is determined by the axis of the heart not the spine.
- allows the display of structures along a beam as a function of time
- beam is fixed in position and there is a rapid sampling rate -> assessment of thin moving structures (valves)
- one dimensional ‘ice pick’ view
- time (x axis) and depth (y axis)
(3) Doppler techniques
- pulsed wave, continuous wave and colour-flow
Pulsed wave Doppler
- performed with a duplex transducer (2D and Doppler)
- single ultrasound transducer transmits and receives signals
- blood flow velocities of a small volume of blood are obtained at a specific depth
- useful for velocity measurements at specific sites such as the LVOT
Continuous wave Doppler
- transducer with two crystals (one continuously transmitting and one continuously receiving)
- can measure high velocity blood flow (AS)
- able to measure all the velocities along the beam
⇒ blood flow velocities from PW or CW Doppler can be converted into pressure gradients using the simplified Bernoulii equation (change in P = 4V2)
- change in frequency of sound waves with a moving object with colour superimposed
- blue away and red towards transducer(BART).
- information: direction of blood flow, timing of CFD signals, estimation of blood flow velocity and laminar vs turbulent flow differentiation.
- ‘real time’ 3D images
References and Links
- McAlister NH, McAlister NK, Buttoo K. Understanding cardiac “echo” reports. Practical guide for referring physicians. Can Fam Physician. 2006 Jul;52:869-74. PMC1781094.
Chris is an Intensivist and ECMO specialist at the Alfred ICU in Melbourne. He is also a Clinical Adjunct Associate Professor at Monash University. He is a co-founder of the Australia and New Zealand Clinician Educator Network (ANZCEN) and is the Lead for the ANZCEN Clinician Educator Incubator programme. He is on the Board of Directors for the Intensive Care Foundation and is a First Part Examiner for the College of Intensive Care Medicine. He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives.
After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia’s Northern Territory, Perth and Melbourne. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education.
He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. He coordinates the Alfred ICU’s education and simulation programmes and runs the unit’s education website, INTENSIVE. He created the ‘Critically Ill Airway’ course and teaches on numerous courses around the world. He is one of the founders of the FOAM movement (Free Open-Access Medical education) and is co-creator of litfl.com, the RAGE podcast, the Resuscitology course, and the SMACC conference.
His one great achievement is being the father of three amazing children.
On Twitter, he is @precordialthump.
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